This project was funded by the National Science Foundation BRIGE Award (1221019).

1. Setting up the System
<ol>
	<li>Use cyanoacrylate epoxy to adhere a 9 mm diameter, 1 mm thick red latex rubber cylinder to the front face of one 10 MHz ultrasonic transducer and use cyanoacrylate epoxy to adhere a 9 mm diameter, 1 mm thick red latex rubber cylinder to a 6 mm thick acrylic block which is then epoxied the same way to the reference ultrasonic transducer to act as an acoustic spacer.</li>
	<li>Set up an optical train that has a beam expander being hit by the laser first. Then place a manually adjustable aperture second. Finally use a polarizing beam splitter cube as the third element and place the ultrasonic transducer not in the EFPA prism holder and the transducer in the EFPA prism holder at each output of the non-polarizing beam splitter. 
	Note: The polarizing beam splitter cube is used to ensure a pure, single polarization for excitation as this is critical to the proper functionality of all EFPA techniques.</li>
	<li>Expand the outgoing laser beam using lenses to create a beam expander of at least 3X from the q-switched ND:YAG laser. 
	Note: The beam is intentionally oversized compared to the latex rubber absorber on the sample transducer in order to ensure proper sensor function despite laser light walk off due to refraction through the prism at varying angles of incidence.</li>
	<li>Align the optical train and EFPA prism holder such that the flat side of the mount nearest the prism will be set to a 0&#176; angle using a digital level. This ensures a correct starting point for the angular spectrum data that will be collected during the experiments.</li>
	<li>Connect and power on external devices such as the oscilloscope, stepper motor driver to the computer, the ultrasonic transducers, and XY stage motors. Physically connect the transducer not in the EFPA prism mount to Ch0 and physically connect the transducer in the EFPA prism mount to Ch1 via 50 ohm BNC cables. The software is preprogrammed to recognize acoustic signals from these specific channels.</li>
</ol>
2. EFPA System Initialization and Optical Alignment
<ol>
	<li>Manually adjust the adjustable aperture to block the beam to a 1 mm diameter.</li>
	<li>Start the programming software (e.g., LabVIEW), set the angle to 70&#176; by pressing the green &#34;move&#34; button to move the mount to the angle necessary for 70&#176; excitation at the prism/sample interface.</li>
	<li>Using appropriate laser safety glasses (OD 7+ at 532 nm), look into the prism from the side perpendicular to the laser beam and manually move the stage in the X and Y axes using the hand wheels until the 1 mm laser spot is fluorescently visible on the rubber latex. Ensure that the beam is centered on the latex.</li>
	<li>Expand the manually adjustable aperture to its maximum opening and look at the running front panel of the program to ensure both the laser energy measurement photoacoustic signal from the EFPA prism mount (red line) and the photoacoustic signal from the laser energy measurement side (white line) are visible and are approximately the same amplitude.</li>
	<li>Stop the program by pressing the &#34;STOP&#34; button. 
	Note: If the button is not pressed the prism will have to be manually reset before continuing the testing. Once the initialization protocol has been completed, TIRPAS, PAS/TIRPAS refractometry, or OTPAS can be performed.</li>
</ol>
3. TIRPAS Technique
<ol>
	<li>Place the prism in the plastic prism mount adapter as shown in Figure 3. Next, place 2.5 &#181;l of immersion oil index matched to the type of prism used, onto the center of the prism and sandwich the oil by placing a substrate on top of the oil layer.</li>
	<li>Place 25 &#181;l of sample on the latex rubber connected to the transducer in the EFPA transducer mount as shown in Figure 3 so that it coats the entire surface without bubble formation. The sample can be any optically absorbing material such as a solution of dye, a biological fluid, or an analyte suspended in a solution. No preparation of the sample is needed. Compress the prism mount and tighten the mount together with mounting screws to a set torque of 16.75 g/mm for each screw.</li>
	<li>Select the &#34;Setup&#34; tab and select &#34;Setup&#34; on the drop-down menu.</li>
	<li>Run the program entitled OTPAS thin film analyzer_USB-5133.vi (Supplementary File).</li>
	<li>View the acoustic signal generated by the sample as shown in Figure 4. 
	Note: The angle of incidence can be changed to control the optical penetration depth of the evanescent field to observe thinner or thicker optical sections of the sample.</li>
</ol>
4. PAS/TIRPAS Refractometry
<ol>
	<li>Place the prism in the plastic prism mount adapter as shown in Figure 3. Next, place 2.5 &#181;l of immersion oil index matched to the type of prism used, onto the center of the prism and sandwich the oil by placing a substrate on top of the oil layer.</li>
	<li>Place 25 &#181;l of sample onto the rubber piece connected to the transducer in the EFPA transducer mount as shown in Figure 3. Compress the prism mount and tighten the mount together with mounting screws to a set torque of 16.75 g/mm for each screw.</li>
	<li>Select the &#34;Angular Spectrum&#34; tab and select &#34;Angular Spectrum&#34; on the drop down menu. Next, input the appropriate parameters into the program as shown in Table 1.</li>
	<li>Run the program and wait until the angular spectrum has been completed and the program has ended.</li>
	<li>Right click on the angular spectrum graph and select &#34;Export &#8594; Export data to excel&#34; to save the data and open the .csv file.
	<ol>
		<li>Open this data in a graphing program (e.g., KaleidaGraph), and perform a numerical derivative on it clicking on &#34;Macros&#34; and selecting &#34;Derivative&#34;. Input the appropriate columns to take the derivative on and press &#34;Okay&#34; and the numerical derivative will be computed.</li>
		<li>Graph the numerical derivative vs. angle and select &#34;Curve fit&#34;. Select the &#34;Smooth&#34; option5,18,98 and select the checkbox of the data under &#34;Curve fit selections&#34; to fit to smooth noise from the data. Select the down arrow under &#34;View&#34; and select &#34;Copy curve fit to data window&#34; to extract the curve fit data to another column.</li>
	</ol>
	</li>
	<li>Manually search through the curve fit to find the local minimum and its corresponding angle of incidence that indicates a transition from the PAS to TIRPAS regimes. That minimum corresponds to the measured critical angle, as shown in Figure 5. Using the equation nsample = nprismsin&#952;c, calculate the bulk refractive index of the unknown sample at the wavelength used for laser interrogation. Typical results are shown in Table 1.</li>
</ol>
5. OTPAS
<ol>
	<li>Place 2.5 &#181;l of immersion oil (index matched to type of glass used) on the center of the prism.</li>
	<li>Place the film or substrate to be tested film side up (away from the prism) and ensure that no bubbles are formed during placement. 
	Note: If bubbles form, remove sample film or substrate and reattempt application.</li>
	<li>Place 25 &#181;l of immersion oil on the latex rubber so that the immersion oil coats the entire surface without bubble formation.</li>
	<li>Compress the substrate/film layers as shown in Figure 3. Tighten the mounting screws to a set torque of 16.75 g/mm that must be identical for each screw. 
	Note: Torque wrench in protocol is in oz.-in., therefore 16.75 g/mm ~ 15 oz.-in.</li>
	<li>Select the &#34;Angular Spectrum&#34; tab and select &#34;Angular Spectrum&#34; on the drop down menu. Next, input the appropriate parameters into the program as shown in Table 3.</li>
	<li>Run the program and wait until the angular spectrum has been completed and the program has ended.</li>
	<li>Rerun the test by performing steps 5.1-5.6 using the substrate or film (whichever wasn&#8217;t done previously) as shown in Figure 6.</li>
	<li>Select &#8220;Curve Fitting&#8221; in the drop down box and select the &#8220;Curve fitting&#8221; tab. Next, input the appropriate parameters as shown in Table 5. Select the film scan under &#8220;Sample&#8221;. Select the substrate scan under &#8220;Substrate&#8221;.</li>
	<li>Input the refractive index, polarization, and other options for the scans previously run in steps 5.1-5.6 as shown in Table 4.</li>
	<li>Run the program by selecting &#34;Curve Fitting&#34; in the drop down box and selecting the &#34;Curve Fitting&#34; tab. Observe the refractive index and thickness under &#34;Film RI&#34; and &#34;Thickness of film&#34; shown at the top right of the program&#39;s graphical user interface. Typical data is shown in Figure 7.</li>
	<li>Use the &#34;Batch fit&#34; option to fit many scans at once by inputting the number of scans to batch fit and selecting a .csv file to output the data to and repeat step 5.10. 
	Note: Once the program is run it will fit each set of data and output all refractive index, thickness, and residual values to the .csv. In order for this to work, the scans must be in a numerical list such as scan_001.csv, scan 002.csv, etc.</li>
</ol>

The authors declare that they have no competing financial interests.

The use of devices for the characterization of materials at the nanoscale will undoubtedly continue far into the future in order to produce new materials and combinations of materials to solve difficult and costly societal problems. Many methods such as ellipsometry, atomic force microscopy (AFM), and traditional photoacoustics are currently used to evaluate material properties at the nanoscale despite their inherent limitations due to few alternatives. EFPA shows promise as a consolidated set of techniques that can push beyond the typical limitations of the photoacoustic effect to provide a single instrument to characterize materials at the nanoscale.
The protocol described herein and its associated sub-techniques have the capability to estimate both bulk and thin film refractive indices of a material, film thickness, and have detection capabilities for a variety of biosensing applications. OTPAS specifically has the advantage that it can estimate the thickness and refractive index of nanometer scale films despite the fact that the films themselves do not contain inherently absorbing pigmentation. EFPA as a consolidated set of techniques has a few advantages over techniques such as ellipsometry, atomic force microscopy, and the traditional photoacoustic effect. The first advantage is that EFPA is comparatively inexpensive to set up. A typical EFPA setup can estimate refractive index from both the bulk and surface, can estimate thickness, and can probe absorbing materials and will cost around $5,000 to set up. In contrast, AFM setups cost around $50,000 and cannot estimate optical properties. Secondly, EFPA can estimate these properties within a few minutes without complex interaction such as in ellipsometry where input parameter error can cause differences in results. Finally, EFPA, when compared to the traditional photoacoustic effect, has axial resolution 100X smaller as it can estimate thickness of 200 nm films6 whereas the traditional photoacoustic effect is limited to 7,500 nanometers when using similar lasers and setup82. Conveniently, almost every material has independent and differing surface and bulk refractive index properties as well as absorptive properties, which collectively make for functional differences in their use in optical research systems.
Critical steps within the protocol come down to three primary considerations. First, optical coupling of the sample, whether that be a liquid or a film, is crucial to obtain reliable and repeatable results. Air bubbles within the immersion oil or sample will lead to inaccurate characterization of the thickness, refractive index, and in some cases a complete lack of sensitivity of the transducer to the excited sample owing to the inability of ultrasound to adequately pass through a liquid/air interface due to vastly differing acoustic impedance values. Second, with regard to OTPAS, a sample that is homogeneously thick in the area irradiated by the laser beam in order to provide consistent thickness values during scanning since as of yet EFPA cannot determine thickness differences within the area of excitation. Finally, again with regard to OTPAS, the sample film must be transparent to the wavelength being used. This is a fundamental assumption made in the mathematical equations that describe how OTPAS finds thickness and refractive index values. If absorption is substantial enough that more than 1% of the incident light is absorbed by the film, characterization will not match the real thickness and refractive index values of the film.
Modifications to the EFPA technique can be made with relative ease and other types of components can be substituted in for more effective scans or functionality. The only major issues that have to be addressed when building an EFPA system are the following. First, it is crucial to have a method of automatically rotating a prism mount and acquiring acoustic signals at small (0.05 degree) increments. Second, having a nanosecond pulse width or shorter laser system to excite samples using the photoacoustic effect. Third, having a graphical user interface or program to acquire the data to do mathematical curve fitting and numerical derivatives for estimating thickness and bulk/thin film refractive index.
There are three fundamental limitations to EFPA. The first is that in order to find the bulk refractive index, thin film refractive index, thickness, or detect materials using TIRPAS, the sample must have a refractive index that is less than that of the prism or substrate it is in contact with so that total internal reflection can occur which is the basis of all three of these techniques. The second limitation is that EFPA currently requires physical contact in order to estimate thickness, refractive index, and for detection of properties. The final limitation is that OTPAS, a sub-technique of EFPA, currently requires optically transparent materials to estimate thickness and refractive index of the thin film of interest.
This type of consolidated instrument could find many applications in characterization environments including research cores and industrial environments. Future work regarding EFPA techniques is focused on the improvement of the refractive index accuracy through improvements in instrumentation and translational/rotational stages. Additional advancements will focus around the improvement of signal-to-noise ratio for the detection of smaller quantities of absorbing materials, so as to characterize weakly absorbing materials, as are found most commonly in biological systems.

Advancements in the understanding of optical materials1,3,4,6,7,10,13-16 have provided new insights into the creation of thin film materials for a host of optical devices, including antireflection coatings on lenses, high extinction ratio optical filters, and highly absorbing slab waveguides17. These advancements would not be possible without the use of many characterization techniques, such as ellipsometry4,6,18, contact angle measurement, atomic force microscopy7,11,19, and scanning/transmission electron microscopy, which assist in the iterative improvement of these technologies by providing direct measures or indirect estimates of fundamental optical material properties. Said properties, such as the refractive index, govern how the materials interact with incident photons, which directly affects their function and their use in optical applications. However, each of these techniques has limitations relating to resolution, sample preparation, cost, and complexity, and each generates only a subset of the data needed to fully characterize the material. That being said, a new set of techniques, known as evanescent field-based photoacoustics (EFPA)5,6,15,18,20-49 as shown in Figure 1, has the potential to estimate material properties at the nanoscale in a consolidated set of experiments. EFPA encompasses the sub-techniques of total internal reflection photoacoustic spectroscopy (TIRPAS)23,25,26,33-35,43-45 , photoacoustic spectroscopy/total internal reflection photoacoustic spectroscopy refractometry (PAS/TIRPAS refractometry)18, and optical tunneling photoacoustic spectroscopy (OTPAS)6, and has been used to estimate bulk and thin film refractive index, film thicknesses, as well as to detect absorbing materials at a prism/sample or substrate/sample interface.
In order to understand the EFPA mechanism, one must first understand the concept of photoacoustic spectroscopy (PAS), which refers to the generation of ultrasonic pressure waves by the rapid thermoelastic expansion of a chromophore, following the absorption of an ultra-short (&#60; &#181;sec) pulse of light (Figure 1). Theoretical and mathematical framework for the photoacoustic effect discussed in this paper can be obtained here50-59. The resulting change in pressure can be detected by an ultrasonic microphone or transducer. The photoacoustic effect, originally discovered in 1880 with the invention of Alexander Graham Bell&#39;s photophone, was &#34;rediscovered&#34; in the early 1970s due to advancements in laser and microphone technology, and eventually put into practical use to fill niche applications from biomedical imaging to thin film analysis to non-destructive testing of materials.1,53-57,59-82 This effect can be mathematically described with one-dimensional wave equations, wherein the wave is a simple acoustic source whose pressure (p) varies in both position (x) and time (t):
<img alt="Equation1" src="/files/ftp_upload/54192/54192equation1.jpg" />
with solutions for simple acoustic sources of the form64
<img alt="Equation2" src="/files/ftp_upload/54192/54192equation2.jpg" />
where p is pressure, &#915;=&#945;vs2/Cp where &#945; is the volume thermal expansion coefficient, vs is the speed of sound in the medium, and Cp is the heat capacity at constant pressure, H0 is the radiant exposure of the laser beam, c is the speed of sound in the excited medium, x is length, and t is time. The magnitude of the resulting acoustic wave relies directly upon the optical absorption coefficient of the material, &#181;a, which is the inverse of the optical penetration depth, &#948;, which is in turn a measure of the distance the light travels until it decays to 1/e of its initial optical intensity. While Equation (1) is a general equation for a one dimensional plane wave source, typical absorbers will emit a spherical acoustic wave in three dimensions. Beyond the mathematical description, applications of the photoacoustic effect54 span many imaging modalities such as microscopy, tomography, and even molecular imaging owing to the photoacoustic effect having high sensitivity due to the large optical absorption due to the naturally present chromophore hemoglobin. Other applications of the photoacoustic effect even include the estimation of various thin film properties15,16,20,21,24,26-32,36-39,41,42,56,83,84. However, PAS does have certain limitations: (1) its extensive optical penetration depth eliminates the ability to probe near-field optical properties at surfaces (2) its efficiency of capturing the emitted acoustic energy is low due to spherical propagation of the majority of the energy away from the detector (3) samples must include chromophores in the wavelength regime under consideration.
When combined with evanescent field-based techniques, however, many of these limitations can be ameliorated. The evanescent field occurs when a beam of light undergoes total internal reflection (TIR), as described by Snell&#39;s Law, which effect also allows fiber optic waveguides to guide light large distances (km) for computation and telecommunication applications. In practical applications, the evanescent field is used in a variety of characterization and imaging technologies, including attenuated total reflectance spectroscopy (ATR). Imaging is achieved with high contrast due to the confinement of the light to within the first few hundred nanometers into the sample of interest. The evanescent field takes the form of an exponentially decaying field that extends into the external medium to an optical penetration depth that is typically on the order of the wavelength being used (usually ~500 nm or less) as shown in equations 3 and 4.
<img alt="Equation3" src="/files/ftp_upload/54192/54192equation3.jpg" />
where I is the light intensity in % at a location z from the prism/sample interface, I0 is the initial light intensity in % at the interface, z is distance in nanometers, and &#948;p is the optical penetration depth as shown in equation 4. With such a small optical penetration depth, the evanescent field is able to interact with the environment very close to the interface of the two materials, and well below the optical and acoustic diffraction limits. The optical properties of materials or particles within this range may perturb the field or otherwise alter its generation, which interaction can be detected by a variety of methods3,5,6,10,15,17,18,21,23,25-27,29-47,84-95.
When evanescent techniques are combined with PAS, the photoacoustic waveforms produced can be used to characterize materials or particles interacting with the evanescent field, creating the evanescent-field based photoacoustics (EFPA) family of techniques, as shown in Figure 1. This family includes, but is not limited to, total internal reflection photoacoustic spectroscopy (TIRPAS), optical tunneling photoacoustic spectroscopy (OTPAS), and surface plasmon resonance photoacoustic spectroscopy (SPRPAS). The interested reader should refer to the following references for derivations of the equations used for TIRPAS5,6,18,23,25,26,33-35,43-47, PAS/TIRPAS refractometry18, and OTPAS6. In each case, the photoacoustic effect is generated through a different excitation mechanism than simple transmittance through a prism; for instance, in TIRPAS, the light is evanescently coupled through a prism/substrate/sample interface into the chromophores (which could include the sample material itself, or guest molecules within the sample), whereas in SPRPAS, the primary mode of excitation is instead through the absorption of a surface plasmon, which is a secondary E-M wave created when the energy of the evanescent field is transferred into the electron cloud of a metal layer deposited on the prism surface. This family of techniques was originally invented in the early 1980s by Hinoue et al., and improved upon by T. Inagaki et al. with the invention of SPRPAS, but saw very little development due to technical limitations of the light sources and available detection equipment. More recently, previous investigations have shown that increased sensitivity and utility are possible with modern polyvinylidene fluoride (PVDF) ultrasonic detectors and q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. Specifically, nanosecond-pulsed Nd:YAG lasers result in a 106 fold increase in the peak power, which enables EFPA techniques to become useful tools for evaluating the optical properties of a variety of materials and interfaces5,6,15,18,21-29,31-47,84,96. Additionally, previous work has further shown the capability of such techniques to determine structural information about materials at an interface, which was previously never achievable with traditional photoacoustic spectroscopy (PAS) technologies due to their relatively large penetration depth53,55,57,59,61,62,69,73,75,80,81.
This capability is shown in the protocols that follow under the OTPAS technique; however, on a more fundamental level the three techniques each rely on a different definitive equation, which determines the capabilities of the technology. For example, in TIRPAS, the optical penetration depth of the evanescent field, &#948;&#39;p, primarily drives the resulting acoustic signal intensity to an absorbing sample, and is described by:
<img alt="Equation4" src="/files/ftp_upload/54192/54192equation4.jpg" />
where &#955;1 is the wavelength of light traveling through the prism medium and is defined by the relation &#955;1 = &#955;/n1 wherein n1 is the refractive index of the prism material. Additionally, &#952; refers to the angle of excitation, and n21 refers to the ratio of the refractive indices of each medium and is defined by n21 = n2/n1, wherein n2 is the refractive index of the sample material. The larger the optical penetration depth, the more material is being irradiated. For the photoacoustic effect, the greater the optical penetration depth, the more material is being excited that can produce acoustic waves leading to a larger acoustic signal.
Unlike TIRPAS however, in PAS/TIRPAS refractometry the primary equation is Snell&#39;s law:
<img alt="Equation5" src="/files/ftp_upload/54192/54192equation5.jpg" />
where n1 is the refractive index of the prism, &#952;1 is the angle of incidence at the prism/sample interface, n2 is the refractive index of the sample, and &#952;2 is the angle of the light that is refracted through the second medium. The sensitivity of estimating the refractive index of a material is primarily driven by the accuracy of the estimation of &#952;1. In total internal reflection, which is achieved when &#952;1 is beyond the critical angle which generates an evanescent field, sin &#952;2 = 1 and therefore, Equation 5 reduces to n2 = n1sin&#952;1. (Note: &#952;1= &#952;critical) Knowing the angle at which the numerical derivative (dP/d&#952; where P is the peak to peak voltage of the photoacoustic signal and &#952; is the angle of incidence of the light with the sample) of the photoacoustic signal has a local minima allows for the estimation of &#952;1 that allows the user to solve for n2 and thus estimate the bulk refractive index of a sample as shown in Figure 1.
Finally, in OTPAS, the following equation relates optical transmission in % to photoacoustic peak to peak voltage by:
<img alt="Equation6" src="/files/ftp_upload/54192/54192equation6.jpg" />
where T is the percent optical transmission, p is the peak-to-peak voltage generated by the angular spectrum of a substrate with a film on it, p0 is the peak-to-peak voltage generated by the angular spectrum of a substrate, &#946; is the coupling constant based upon the refractive index of prism and the immersion oil, &#945; is the attenuation factor, and is a factor that includes thickness and refractive index of the sample film within the evanescent field. The sensitivity of this technique to thickness and refractive index is driven by the accuracy of estimating the peak to peak acoustic signal intensities, p and p0 at each angle of incidence in the angular spectrum. It has been shown that &#946; can be directly calculated based upon the refractive indices of the prism and the immersion oil; consequently, it is a straightforward task to calculate the optical transmission at each angle of incidence and to then extract an estimate for the refractive index and thickness of the film through statistical curve fitting analysis. The interested reader should refer to Goldschmidt et al. for more information.5,6
The EFPA system is a photoacoustic based system capable of estimating the thickness, thin film refractive index, bulk refractive index, and generating acoustic signals through optical absorption for detection. The system is comprised of a laser, an optical train to guide the light to the prism/sample and to the laser energy measurement side. The laser energy measurement side is used to normalize the photoacoustic signal to the incident laser energy as shown in Figure 2. The EFPA system is driven by a stepper motor driver to rotate the prism/sample for the angular spectra in PAS/TIRPAS refractometry and OTPAS. The system acquires data through a digital acquisition card and provides a user interface and automated stage control through an in house program.

&#160;
<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="31">
			<td height="31" style="height:31px;width:216px;">100 mm plano convex lens</td>
			<td style="width:71px;">Thorlabs</td>
			<td style="width:119px;">LA1509</td>
			<td style="width:167px;">Plano convex lens for beam expander</td>
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;width:216px;">-30 mm plano concave lens</td>
			<td style="width:71px;">Thorlabs</td>
			<td style="width:119px;">LC2679</td>
			<td style="width:167px;">Plano concave lens for beam expander</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:216px;">10 MHz Ultrasonic transducers</td>
			<td style="width:71px;">Harisonic</td>
			<td style="width:119px;">I31006T</td>
			<td style="width:167px;">Ultrasonic sensors used for photoacoustic detection, both for laser energy measurement and for OTPAS mount</td>
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Immersion oil Type A</td>
			<td rowspan="2" style="width:71px;">Cargille</td>
			<td rowspan="2" style="width:119px;">16482</td>
			<td rowspan="2" style="width:167px;">Index of 1.519 to match that of NBK-7 substrates</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="18">
			<td height="39" rowspan="2" style="height:40px;width:216px;">Natural latex rubber sheet (red)</td>
			<td rowspan="2" style="width:71px;">McMastercarr</td>
			<td rowspan="2" style="width:119px;">86085K11</td>
			<td rowspan="2" style="width:167px;">Index ~1.519 to match that of Type A immersion liquid to act as an optical absorber to measure optical tunneling and laser energy</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="33">
			<td height="33" style="height:33px;width:216px;">Laser goggles</td>
			<td style="width:71px;">VERE</td>
			<td style="width:119px;">53</td>
			<td style="width:167px;">Used to protect eyes from laser light</td>
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Cage mounted non polarizing beam splitter cube</td>
			<td rowspan="2" style="width:71px;">Thorlabs</td>
			<td rowspan="2" style="width:119px;">CM1-BS013</td>
			<td rowspan="2" style="width:167px;">Split laser light so that one half can be measured and one half can be used for excitation</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Cage mounted polarizing beam splitter cube</td>
			<td rowspan="2" style="width:71px;">Thorlabs</td>
			<td rowspan="2" style="width:119px;">CM1-PBS251</td>
			<td rowspan="2" style="width:167px;">Ensure light is polarized before being used for optical tunneling experiments</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Graduated ring activated iris diaphragm</td>
			<td rowspan="2" style="width:71px;">Thorlabs</td>
			<td rowspan="2" style="width:119px;">SM1D12C</td>
			<td rowspan="2" style="width:167px;">Cut beam down to a smaller size for alignment</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="31">
			<td height="31" style="height:31px;width:216px;">Data acquisition card</td>
			<td style="width:71px;">National Instruments</td>
			<td style="width:119px;">USB-5133</td>
			<td style="width:167px;">USB oscilloscope to acquire data</td>
		</tr>
		<tr height="46">
			<td height="46" style="height:46px;width:216px;">Stepper motor driver</td>
			<td style="width:71px;">National Instruments</td>
			<td style="width:119px;">MID-7604</td>
			<td style="width:167px;">Stepper motor driver to drive stepper motors for angular spectra</td>
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Sherline XY stage (14&#8221;)</td>
			<td rowspan="2" style="width:71px;">Sherline</td>
			<td rowspan="2" style="width:119px;">5600-CNC/5610-CNC</td>
			<td rowspan="2" style="width:167px;">Sherline XY stage</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">4-jaw self centering chuck</td>
			<td rowspan="2" style="width:71px;">Sherline</td>
			<td rowspan="2" style="width:119px;">1076/1034</td>
			<td rowspan="2" style="width:167px;">Sherline rotational attachment</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Right angle attachment</td>
			<td rowspan="2" style="width:71px;">Sherline</td>
			<td rowspan="2" style="width:119px;">3701</td>
			<td rowspan="2" style="width:167px;">Right angle attachment to attach rotational mount</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">CNC rotary table</td>
			<td rowspan="2" style="width:71px;">Sherline</td>
			<td rowspan="2" style="width:119px;">8730</td>
			<td rowspan="2" style="width:167px;">Rotary table for holding OTPAS prism/sample</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Surelite I-20 laser system</td>
			<td rowspan="2" style="width:71px;">Continuum</td>
			<td rowspan="2" style="width:119px;">I-20</td>
			<td rowspan="2" style="width:167px;">Q-switched Nd:YAG laser for exciting samples</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">NBK-7 prism</td>
			<td rowspan="2" style="width:71px;">Thorlabs</td>
			<td rowspan="2" style="width:119px;">PS911</td>
			<td rowspan="2" style="width:167px;">Right angle prism for EFPA</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Adjustable torque wrench</td>
			<td rowspan="2" style="width:71px;">Tohnichi</td>
			<td rowspan="2" style="width:119px;">RTD40Z</td>
			<td rowspan="2" style="width:167px;">Adjustable torque wrench to equally tighten down the EFPA mount for each technique to 16.75 g/mm</td>
		</tr>
		<tr height="21">
		</tr>
		<tr height="20">
			<td height="41" rowspan="2" style="height:41px;width:216px;">Digital level</td>
			<td rowspan="2" style="width:71px;">Micromark</td>
			<td rowspan="2" style="width:119px;">84519</td>
			<td rowspan="2" style="width:167px;">Digital level to ensure EFPA prism holder starts at 0 degrees.</td>
		</tr>
		<tr height="21">
		</tr>
	</tbody>
</table>

evanescent field based photoacoustics: optical property evaluation at surfaces

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. Optical property evaluation of thin films and the surfaces of bulk materials is an important step in understanding new optical material systems and their applications. The method presented can estimate thickness, refractive index, and use absorptive properties of materials for detection. This metrology system uses evanescent field-based photoacoustics (EFPA), a field of research based upon the interaction of an evanescent field with the photoacoustic effect. This interaction and its resulting family of techniques allow the technique to probe optical properties within a few hundred nanometers of the sample surface. This optical near field allows for the highly accurate estimation of material properties on the same scale as the field itself such as refractive index and film thickness. With the use of EFPA and its sub techniques such as total internal reflection photoacoustic spectroscopy (TIRPAS) and optical tunneling photoacoustic spectroscopy (OTPAS), it is possible to evaluate a material at the nanoscale in a consolidated instrument without the need for many instruments and experiments that may be cost prohibitive.

Results have been shown for TIRPAS, PAS/TIRPAS refractometry, and OTPAS that are the subtechniques within the EFPA platform. Figure 4 shows a representative TIRPAS acoustic wave generated from an absorbing sample. The bipolar nature of the acoustic wave is characteristic of the TIRPAS technique and indicates that TIRPAS is occurring. This bipolar waveform occurs due to the acoustic reflection at the interface between the sample and the glass substrate due to a large difference in acoustic impedance. For PAS/TIRPAS refractometry Figure 5 and Table 1 was obtained. Figure 5 shows the angular spectrum and numerical derivative obtained for a sample undergoing testing to estimate the bulk refractive index. Table 1 shows the results of using the PAS/TIRPAS refractometry to estimate the bulk refractive index of a Water/PEG/Direct red dye mixture as compared to the bulk refractive index estimate using a standard handheld refractometer. Finally, OTPAS results are shown in Figure 7 and Table 2. Figure 7 shows two figures of the angular scans that are taken during OTPAS. Table 2 shows a comparison between OTPAS and spectroscopic ellipsometry of the same thin film samples.
<img alt="Figure 1" src="/files/ftp_upload/54192/54192fig1.jpg" /> 
Figure 1. Subtechnologies of EFPA. EFPA is currently comprised of three distinct sub-technologies. These technologies are TIRPAS, PAS/TIRPAS refractometry, and OTPAS. Each technique can evaluate materials to derive or determine different properties. TIRPAS detects materials based upon their optical absorption for biosensing purposes, PAS/TIRPAS refractometry evaluates bulk refractive index, and OTPAS evaluates thin film refractive index and thickness. In TIRPAS, the light beyond the critical angle &#952;c creates an evanescent field that can generate an acoustic wave upon interacting with an optical absorber. In PAS/TIRPAS refractometry, both TIRPAS and PAS waveforms are obtained from both evanescent field photoacoustic excitation and traditional photoacoustic excitation. By plotting these two regimes on an angular spectrum graph, the transitional angle can be observed, which can then be used to derive the refractive index. Finally, in OTPAS, a spectrum of acoustic signals are obtained with laser irradiation beyond the critical angle &#952;c for both a thin film on a substrate and a bare substrate. By applying a non-linear curve-fitting algorithm to the data, the thin film thickness and refractive index can be derived. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54192/54192fig2.jpg" /> 
Figure 2. EFPA schematic/photo. Left: To set up EFPA the laser beam must be expanded to overfill the sensing area covered by the latex rubber. The beam should initially be at a 45-degree angle to the prism as shown. Right: Photo of the setup showing the optical train. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54192/54192fig3.jpg" /> 
Figure 3. Sample loading. Samples are loaded with the prism making optical contact via immersion oil to the substrate. In TIRPAS or PAS/TIRPAS refractometry, direct liquid contact is achieved with the sample on the substrate for testing. In OTPAS, the optical coupling through additional immersion oil between the substrate and red latex rubber allows for optical tunneling to occur. The mount is then clamped together using a torque wrench and mounting screws. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54192/54192fig4.jpg" /> 
Figure 4. TIRPAS typical data. TIRPAS waveforms typically have a bipolar acoustic signal appearance that is characteristic of the TIRPAS method. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54192/54192fig5.jpg" /> 
Figure 5. PAS/TIRPAS typical data. Left: Angular spectrum data that is obtained by irradiating the sample at various angles of incidence. Right: Numerical derivative of left figure that reveals a local minima indicating the transition from the PAS to TIRPAS regimes, which in turn corresponds with the position of the critical angle. Reprinted with permission.18 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54192/54192fig6.jpg" /> 
Figure 6. Program flow chart. The program is run in a few iterative steps. The prism mount is set to zero degrees and then parameters are selected before running the program. Then the program is run to acquire an angular spectrum of both a substrate and a film. Finally, a curve is fit to the data to estimate the film refractive index and thickness. Reprinted with permission.6 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/54192/54192fig7.jpg" /> 
Figure 7. OTPAS typical data. Left: This figure shows the angular spectrum scans of an MgF2 film and an N-BK7 substrate respectively. Right: By dividing the MgF2 film angular spectrum scan by the N-BK7 substrate scan and multiplying by a constant factor beta, degree of optical tunneling (%) vs. angle of incidence can be obtained, which allows for the estimation of the refractive index and thickness of the thin film. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/54192fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>PAS/TIRPAS</td>
			<td>Sample 1</td>
			<td>Sample 2</td>
			<td>Sample 3</td>
			<td>Sample 4</td>
			<td>Sample 5</td>
			<td>Atago R-5000</td>
		</tr>
		<tr>
			<td>Direct red/PEG 
			125 &#181;g/ml</td>
			<td>1.395</td>
			<td>1.395</td>
			<td>1.395</td>
			<td>1.395</td>
			<td>1.395</td>
			<td>1.395-1.397</td>
		</tr>
		<tr>
			<td>Direct red/PEG 
			250 &#181;g/ml</td>
			<td>1.39</td>
			<td>1.39</td>
			<td>1.39</td>
			<td>1.39</td>
			<td>1.39</td>
			<td>1.390-1.396</td>
		</tr>
		<tr>
			<td>Direct red/PEG 
			500 &#181;g/ml</td>
			<td>1.388</td>
			<td>1.389</td>
			<td>1.389</td>
			<td>1.389</td>
			<td>1.389</td>
			<td>1.381-1.395</td>
		</tr>
		<tr>
			<td>Direct red/PEG 
			750 &#181;g/ml</td>
			<td>1.382</td>
			<td>1.382</td>
			<td>1.387</td>
			<td>1.387</td>
			<td>1.387</td>
			<td>1.372-1.395</td>
		</tr>
		<tr>
			<td>Myoglobin 
			460 &#181;g/ml</td>
			<td>1.33</td>
			<td>1.329</td>
			<td>1.331</td>
			<td>1.33</td>
			<td>1.331</td>
			<td>1.335</td>
		</tr>
	</tbody>
</table>
Table 1. PAS/TIRPAS results. The following table shows typical results for Direct Red dye with 50% PEG mixed in to raise the refractive index. Reprinted with permission.18
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>Test type&#160;</td>
			<td>Technique&#160;</td>
			<td>Film type&#160;</td>
			<td>Refractive index&#160;</td>
			<td>Thickness (nm)&#160;</td>
		</tr>
		<tr>
			<td>Intrasample&#160;</td>
			<td>OTPAS&#160;</td>
			<td>MgF2 200 nm&#160;</td>
			<td>1.384&#177;0.004&#160;</td>
			<td>203&#177;6&#160;</td>
		</tr>
		<tr>
			<td>Intrasample&#160;</td>
			<td>Ellipsometry&#160;</td>
			<td>MgF2 200 nm&#160;</td>
			<td>1.393&#177;0.001&#160;</td>
			<td>192.4&#177;1.1&#160;</td>
		</tr>
		<tr>
			<td>Intersample&#160;</td>
			<td>OTPAS&#160;</td>
			<td>MgF2 200 nm&#160;</td>
			<td>1.395&#177;0.011&#160;</td>
			<td>220&#177;19&#160;</td>
		</tr>
		<tr>
			<td>Intersample&#160;</td>
			<td>Ellipsometry&#160;</td>
			<td>MgF2 200 nm&#160;</td>
			<td>1.392&#177;0.002&#160;</td>
			<td>195.2&#177;1.8&#160;</td>
		</tr>
	</tbody>
</table>
Table 2. OTPAS results. The following table shows typical results for 200 nm MgF2 thin films in OTPAS vs. spectroscopic ellipsometry. Intrasample refers to testing a single film ten times, whereas intersample refers to testing ten films independently. Reprinted with permission.6
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td># of averages</td>
			<td fo:width="0.6in" style="width:40px">1</td>
			<td>Start angle</td>
			<td fo:width="0.6in" style="width:35px">60</td>
			<td>Warmup (min)</td>
			<td style="width:90px">0</td>
			<td style="width:90px">Refractive index (prism)</td>
			<td>1.519</td>
		</tr>
		<tr>
			<td># scans</td>
			<td>1</td>
			<td>Step size</td>
			<td>0.1</td>
			<td>Save to</td>
			<td>&#8220;yourfilename&#8221;.csv</td>
			<td>Microstep #</td>
			<td>10</td>
		</tr>
		<tr>
			<td>Set Q-switch</td>
			<td>275</td>
			<td>Stop angle</td>
			<td>80</td>
			<td>Laser select</td>
			<td>Surelite</td>
			<td>Velocity (rpm)</td>
			<td>500</td>
		</tr>
		<tr>
			<td>Error tolerance (%)</td>
			<td>5</td>
			<td>Laser startup</td>
			<td>on</td>
			<td>Low pass filter (program)</td>
			<td>1.00 x 107</td>
			<td>Acceleration (RPS)</td>
			<td>200</td>
		</tr>
	</tbody>
</table>
Table 3. Angular spectrum settings for PAS/TIRPAS refractometry. The following table shows the settings necessary for an angular spectrum in PAS/TIRPAS refractometry.
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td># of averages</td>
			<td fo:width="0.6in" style="width:40px">64</td>
			<td>Start angle</td>
			<td fo:width="0.6in" style="width:35px">70</td>
			<td>Warmup (min)</td>
			<td style="width:90px">1</td>
			<td style="width:90px">Refractive index (prism)</td>
			<td>1.519</td>
		</tr>
		<tr>
			<td># scans</td>
			<td>1</td>
			<td>Step size</td>
			<td>0.1</td>
			<td>Save to</td>
			<td>&#8220;yourfilename&#8221;.csv</td>
			<td>Microstep #</td>
			<td>10</td>
		</tr>
		<tr>
			<td>Set Q-switch</td>
			<td>275</td>
			<td>Stop angle</td>
			<td>72</td>
			<td>Laser select</td>
			<td>Surelite</td>
			<td>Velocity (rpm)</td>
			<td>500</td>
		</tr>
		<tr>
			<td>Error tolerance (%)</td>
			<td>5</td>
			<td>Laser startup</td>
			<td>on</td>
			<td>Low pass filter (program)</td>
			<td>1.00 x 107</td>
			<td>Acceleration (RPS)</td>
			<td>200</td>
		</tr>
	</tbody>
</table>
Table 4. Angular spectrum settings for OTPAS. The following table shows the settings necessary for an angular spectrum in OTPAS.
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td style="width:65px">Lower refractive index</td>
			<td>1</td>
			<td style="width:72px">Tolerance</td>
			<td>1.00 x 10-12</td>
			<td style="width:80px">Coupler RI</td>
			<td style="width:83px">1.519</td>
			<td style="width:65px">Substrate data</td>
			<td style="width:83px">Select yourfilename.csv</td>
		</tr>
		<tr>
			<td>Lower thickness</td>
			<td>0 nm</td>
			<td>Refractive index guess</td>
			<td>1.3</td>
			<td>Wavelength</td>
			<td>532 nm</td>
			<td>Multiple files save to</td>
			<td>Select yourfilename.csv</td>
		</tr>
		<tr>
			<td>Upper thickness</td>
			<td>1,000 nm</td>
			<td>Thickness guess</td>
			<td>200 nm</td>
			<td>Polarization</td>
			<td>P polarized</td>
			<td>How many files</td>
			<td># of files you want to fit</td>
		</tr>
		<tr>
			<td>Max iteration</td>
			<td>5,000</td>
			<td>Substrate RI</td>
			<td>1.519</td>
			<td>Film data</td>
			<td>Select yourfilename.csv</td>
			<td>Type of fit</td>
			<td>Single fit/Batch fit</td>
		</tr>
	</tbody>
</table>
Table 5. Curve fitting parameters. The following table shows the curve fitting parameters necessary for correct parameter estimation.
Supplemental Code File: OTPAS thin film analyzer_USB-5133.vi. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54192/OTPAS_Thin_Film_Analyzer_USB-5133.vi" target="_blank">Please click here to download this file.</a>

Watch this Scientific Journal Video about Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces at JoVE.com

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

Here we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. This technique evanescent field-based photoacoustics can be used to create a photoacoustic metrology system to estimate materials&#39; thicknesses, bulk and thin film refractive indices, and explore their optical properties.

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. ...

evanescent-field-based-photoacoustics-optical-property-evaluation-at

Nanyang Technological University

Research

JoVE Journal

Engineering

11.6K Views.  Duquesne University. Here we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. This technique evanescent field-based photoacoustics can be used to create a photoacoustic metrology system to estimate materials' thicknesses, bulk and thin film refractive indices, and explore their optical properties.

Video: Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.
The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 &mu;m) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

High-speed Particle Image Velocimetry Near Surfaces

Multi-dimensional and transient flows play a key role in many areas of science, engineering, and health sciences but are often not well understood. The complex nature of these flows may be studied using particle image velocimetry (PIV), a laser-based imaging technique for optically accessible flows. Though many forms of PIV exist that extend the technique beyond the original planar two-component velocity measurement capabilities, the basic PIV system consists of a light source (laser), a camera, tracer particles, and analysis algorithms. The imaging and recording parameters, the light source, and the algorithms are adjusted to optimize the recording for the flow of interest and obtain valid velocity data. 
Common PIV investigations measure two-component velocities in a plane at a few frames per second. However, recent developments in instrumentation have facilitated high-frame rate (&gt; 1 kHz) measurements capable of resolving transient flows with high temporal resolution. Therefore, high-frame rate measurements have enabled investigations on the evolution of the structure and dynamics of highly transient flows. These investigations play a critical role in understanding the fundamental physics of complex flows.
A detailed description for performing high-resolution, high-speed planar PIV to study a transient flow near the surface of a flat plate is presented here. Details for adjusting the parameter constraints such as image and recording properties, the laser sheet properties, and processing algorithms to adapt PIV for any flow of interest are included.

A procedure for studying transient flows near boundaries using high-resolution, high-speed particle image velocimetry (PIV) is described here. PIV is a non-intrusive measurement technique applicable to any optically accessible flow by optimizing several parameter constraints such as the image and recording properties, the laser sheet properties, and analysis algorithms.

Multi-dimensional and transient flows play a key role in many areas of science, engineering, and health sciences but are often not well understood. The ...

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...