Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi
Ente Cassa di Risparmio di Firenze (No. 2014.0770A2202.8861)

1. Fabrication of Ultrahigh Factor of Quality Microspheres
<ol>
	<li>Strip about 1-2 cm of a standard single-mode (SMF) silica fiber off its acrylic coating using an optical stripper.</li>
	<li>Clean the stripped part with acetone and cleave it.</li>
	<li>Introduce the cleaved tip in one arm of a fusion splicer and produce a series of electric arc discharges using the splicer controller. Select &#34;manual operation&#34; from the splicer controller menu, set the values for arc power level and arc duration to 60 and 800 msec, respectively; select &#34;arc&#34; and push the bottom &#34;+&#34;.</li>
	<li>Once a sphere is taking shape, stop, rotate the fiber by 90&#176; and repeat step 1.3.</li>
	<li>Repeat step 1.3 at least 4 times to obtain a microsphere of about 160 &#956;m. Repeat 16 times to obtain a microsphere of about 260 &#956;m. 
	Note: The electric arch discharges will produce the high melting temperature needed to fuse the silica glass. The surface tension will draw a spheroid from the mollified fiber tip; the size of the spheres is directly proportional to the number of arc shots, saturating at a diameter of about 350 &#956;m, as it can be seen in Figure 119. The rotation ensures a spherical shape of the resonator.</li>
</ol>
2. Drawing a Tapered Fiber
Note: The tapered fiber is also needed for coupling light into the microresonators. The size of the microsphere will determine the waist of the taper. For sphere diameters larger than 125 &#956;m, the diameter of the taper can be of about 3-4 &#956;m. For smaller ones, the diameter of the taper should be smaller, say 1-2 &#956;m. In order to keep losses at low level and to have just one mode in the tapered section (the fundamental one), the tapering has to be adiabatic (gradual transition from thick to thin diameter). The typical total length of the adiabatic tapered section is about 2 cm. Figure 2 shows the home-made device for pulling the fiber and Figure 3A shows a microphoto of a typical waist zone.
<ol>
	<li>Strip 3-4 cm of a standard single-mode (SMF) silica fiber off its acrylic coating using an optical stripper, and connect the fiber ends to a laser (input) and a power meter (output). Be sure that the stripped zone is approximately in the middle of the fiber, not at one end. Use a bare fiber terminator in order to be able to connect the fiber ends to the laser and power meter. Place the laser and the power meter on top of the work bench.</li>
	<li>Place the stripped fiber inside a short alumina cylinder, and the coated ends of the fiber into two translation stages that actuate simultaneously during the pulling process.</li>
	<li>Heat the alumina cylinder (that acts as an oven) by an oxygen-butane flame up to a temperature close to a melting point of silica (about 2,100 &#176;C).</li>
	<li>Infer the adiabaticity of the taper from the observation of the transmission of a laser light operating at 635 nm. Check that at the output a homogeneous circular spot is preserved while tapering, indicating that no mode scrambling occurs. Stop pulling and retire the flame when the transmitted power stops oscillating, and is constant over time.</li>
	<li>Glue the tapered fiber into a microscope glass slide shaped in the form of a U to accommodate the taper (See Figure 3B). Use a microscope glass slide of dimensions 76x26x1.2 mm.</li>
</ol>
3. Fabrication of Small Microspheres
Note: Small microspheres with diameters below the size of a standard fiber clad require previous tapering of the fiber. The minimum diameter obtained using this method is about 25 &#956;m.
<ol>
	<li>By following section 2, draw a tapered fiber, pulling till it breaks.</li>
	<li>Follow all steps of section 1 (fabrication of UHQ microspheres) but in step 1.3, modify the values on the splicer controller as follows: arc power 20, arc duration 1,200 msec.</li>
</ol>
4. Coupling Light into the Microsphere
Note: We use the taper to couple light into the microsphere and measure the resonances of the microresonator.
<ol>
	<li>4.1.&#160;&#160; &#160;Prepare a T shaped PVC/aluminum holder with a channel in the middle. Fix the residual fiber stem of the microsphere with a piece of scotch magic or paper adhesive tape into holder. Clamp the holder with two screws into a translation stage with piezoelectric actuators and a positioning resolution of 20 nm.</li>
	<li>Fix the taper glued to the glass slide into another translation stage with the slide plane positioned perpendicular to the microsphere fiber stem. Splice the ends of the taper to terminated fiber cables. Connect one end to the tunable diode laser and the other one to an InGaAs photodiode detector.</li>
	<li>Use a microscope tube with long working distance (&#62;20 mm) to control the gap between the taper and microsphere. In order to monitor the system in the other direction place a mirror at 45&#176; with respect to the tube direction so that the position the taper relative to the equator of the microsphere can be controlled.
	<ol>
		<li>Position the equator of the microsphere in contact with the tapered fiber.</li>
	</ol>
	</li>
	<li>Turn on the laser and check the transmission spectrum of the microsphere-taper system in an oscilloscope.
	<ol>
		<li>Tune the CW laser operating at 1,550 nm until resonances appear. The resonances can be identified as Lorentzian shaped dips in the spectrum.</li>
	</ol>
	</li>
	<li>Measure the resonance linewidth (full width half maximum of the Lorentzian shaped dip). Calculate the Q factor as the frequency of the pump divided by the resonance linewidth.</li>
	<li>Reduce/increase the gap between the sphere and the taper, changing both resonance width and depth for increasing/decreasing the coupling efficiency.</li>
</ol>
5. Stimulated Raman Scattering
<ol>
	<li>Insert an erbium doped fiber amplifier (EDFA) between the CW laser operating at 1,550 nm and the attenuator. The EDFA works in the wavelength range of 1,530-1,570 nm. Note: This will boost the laser power, reaching a maximum output power of 2 W. Nonlinear effects need high input powers. Figure 4 shows a sketch of the experimental set-up.</li>
	<li>Connect one end of the taper with terminated fiber cables to a 3 dBm splitter. Connect one of the splitter output fibers to the optical spectrum analyzer and the other one to a photo detector that is connected to the oscilloscope.</li>
	<li>Tune the laser from high to low frequencies until a resonance with a thermal drift comparable to the wavelength scan speed of the laser is found. When the thermal self-locking20 is achieved a broadening of the resonance can be seen on the oscilloscope.</li>
	<li>Check the output power transmitted through the taper into an optical spectrum analyzer. Increase the power until the Raman laser line appears. It is detuned from the pump wavelength at about 13.5 THz.</li>
</ol>

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The authors declare that they have no competing financial interests. D.F. is a PhD student at the University of Parma.

Microspheres are compact and efficient nonlinear oscillators and they are very easy to fabricate and handle. Tapered fibers can be used for coupling and extracting the light in/from the resonator. Resonance contrast up to 95% and Q factors of about 3 x 108 can be obtained.
The main limitation of these fabrication techniques is mass production and integration. Cleanliness of the fibers is critical to both microspheres and tapers, and so is humidity. Both devices must be kept in dry environment for a long lasting laboratory life. Very thin tapers are fragile; great care should be taken when coupling. Regarding the Q factor, the microsphere size can be critical. In microspheres with diameters ranging from 50 to 500 &#956;m, Q&#39;s in excess of 1010 have been demonstrated in vacuum23. The intrinsic Q of a microsphere is determined by contributions from several types of losses: intrinsic curvature losses (Qrad), Raman Scattering and Rayleigh scattering losses on residual surface inhomogeneity (the latter are size dependent, the lower the diameter the higher the losses22), intrinsic material losses, and losses introduced by surface contaminants. Qrad-1 vanishes with increasing size: it decreases faster than R-5/2.24 Our microspheres that range from diameters of 25 to 250 &#956;m have Q factors several magnitudes below the ultimate vacuum value of Q. The Q factors obtained ranged from 5 x 106 up to 3 x 108.
Other methods used for fabrication of microspheres involve the use of CO2 or butane/N2O torch. In all procedures, the surface tension will draw the melted silica into a spheroid. Here, the choice of the instrument for melting the fiber is only economical. CO2 lasers are expensive, torches or splicers are present in all labs using fibers. Tapers could also be fabricated by hydrofluoric acid (HF) erosion of the glass cladding and core. This method is extremely long; about 5 hours are needed for thinning a 125 &#956;m fiber to a 4 &#956;m taper. Another drawback is the lack of adiabaticity, HF will erode all glass at the same rate.
Tapers should show low losses; otherwise it will be difficult to observe nonlinear effects. Coupling efficiency is also very important. The gap between the taper and the microsphere will determine the coupling regime. By direct change of the gap, and/or slight detuning from the resonance, the nonlinear effect can be either enhanced or decreased.
WGMR can pave the way to non-classical light generation for quantum computing applications. Atoms can be trapped near their surfaces for quantum electrodynamics experiments and taper fibers will allow an efficient transport in challenging environments. SRS and SARS can be used as radiation for spectroscopic measurements and also for active sensing as it has been recently proved25.

Whispering gallery mode resonators (WGMR) show two unique properties, a long photon lifetime and small mode volume that allow the reduction of the threshold of nonlinear phenomena1-3. Whispering gallery modes are optical modes that are confined at the dielectric air interface by total internal reflection. The small mode volume is due to the high spatial confinement whereas the temporal confinement is related to the quality factor Q of the cavity. WGMR can have different geometries and there are different fabrication techniques suitable for obtaining high Q resonators4-6 Surface tension cavities such as silica microspheres exhibit near atomic scale roughness, which translates in high quality factors. Both types of confinement significantly reduce the threshold for nonlinear effects due to the strong energy buildup inside the WGMR. It also allows continuous wave (CW) nonlinear optics.
WGMR can be described using the quantum numbers n, l, m and their polarization state, in a strong analogy with the hydrogen atom7. The spherical symmetry allows the separation in radial and angular dependencies. The radial solution is given by Bessel functions, the angular ones by the spherical harmonics8.
Silica glass is centrosymmetric and, therefore, second order phenomena related to &#935;(2) interactions are forbidden. At the surface of the microsphere, the inversion of symmetry is broken and &#935;(2) phenomena can be observed1. However, phase matching conditions for second-order frequency generation are more problematic than the equivalent in third order frequency generation, especially because the wavelengths involved are quite different and the role of dispersion can be quite important. The second order interactions are extremely weak. The generated power scales with Q3 whereas for a third order interaction the generated power scales with Q4.9 For that reason, the focus of this work is third order optical non-linear susceptibility &#935;(3) interactions such as Stimulated Raman Scattering (SRS) and Stimulated Antistokes Raman Scattering (SARS), being SARS the less explored interaction10,11. Chang12 and Campillo13 pioneered the studies of nonlinear phenomena using droplets of highly nonlinear materials as WGMR but the pump laser was pulsed instead of CW. Silica microspheres14,10 and microtoroids15 provided more stable and robust platforms compared to the micro-droplets, gaining much of the attention in the last decades. Particularly, silica microspheres are very easy to fabricate and handle.
SRS is a pure gain process that can be easily achieved in silica WGMR14,15, since reaching a threshold is enough. In this case, the high circulating intensity inside the WGMR guarantees Raman lasing, but for parametric oscillations is not sufficient. In these cases, efficient oscillations require phase and mode matching, energy and momentum conservation law and a good spatial overlap of all resonant modes to be fulfilled16-18. This is the case for SARS and FWM in general.

<table><tbody><tr><td>Optical Fiber</td><td>Corning</td><td>SMF28</td><td></td></tr><tr><td>Fiber coating stripper</td><td>Thorlabs</td><td>T06S13</td><td>Available from other vendors as well</td></tr><tr><td>Fiber cleaver</td><td>Fitel</td><td>S325A</td><td>Available from other vendors as well</td></tr><tr><td>Fusion splicer</td><td>Furakawa</td><td>S177A-1R</td><td>Available from other vendors as well</td></tr><tr><td>Butane and Oxygen Gas</td><td>n/a</td><td></td><td>any vendor</td></tr><tr><td>Microscope tube</td><td>Navitar</td><td>Zoom 6000</td><td>Modular Kit</td></tr><tr><td>CCD camera</td><td>n/a</td><td>N/A</td><td>any will fit</td></tr><tr><td>Monitor</td><td>n/a</td><td>N/A</td><td>any monitor is valid</td></tr><tr><td>3-Axis Stage</td><td>PI Instruments, Thorlabs, Melles</td><td></td><td></td></tr><tr><td>Assorted posts and mounts</td><td>Thorlabs</td><td></td><td>Available from other vendors as well</td></tr><tr><td>Polarization control</td><td>Thorlabs</td><td>FPC030</td><td>Available from other vendors as well</td></tr><tr><td>Attenuator</td><td>Throlabs</td><td>VOA50</td><td></td></tr><tr><td>Photodiode</td><td>Thorlabs</td><td>PDA400</td><td>discontinued, replaced by PDA10CS-EC</td></tr><tr><td>Oscilloscope</td><td>Tektronix</td><td>DPO7104</td><td></td></tr><tr><td>Optical spectrum analyzer</td><td>Ando</td><td>AQ6317B</td><td></td></tr><tr><td>Erbium Doped Fiber Amplifier</td><td>IPG Photonics</td><td>EAD-2K-C</td><td></td></tr><tr><td>Tunable Laser</td><td>Yenista</td><td>TUNICS</td><td></td></tr></tbody></table>

stimulated stokes and antistokes raman scattering in microspherical whispering gallery mode resonators

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

The Q factors of the microspheres fabricated following the protocol described above are in excess of 108 (Figure 5) for large diameters (&#62;200 &#956;m) and in excess of 106 for small diameters (&#60; 50 &#956;m). Resonance contrast above 95% (close to critical coupling) can be easily observed. For high circulating intensities, the following nonlinear effects in the infrared region can be observed: stimulated Raman scattering (SRS), cascaded SRS21, stimulated anti-Stokes Raman scattering (SARS) and four wave mixing (FWM) and degenerated FWM. The Raman gain amplifies in equal way the light travelling in forward and backward direction, creating standing waves for SRS and cascaded SRS. FWM pairs are travelling waves. An example of measurements can be seen in Figures 6 and 7.
Figure 6 shows two SRS lines separated by 100 nm (1,608 nm and 1,708 nm) and in the vicinity of the pump a cascaded four-photon parametric process based on electronic Kerr nonlinearity of the medium, for a microsphere of about 50 &#956;m of diameter, pumped at 1,546.6 nm. In this case FWM is degenerated, two photons of the pump generate a signal and idler photon. Similar results were obtained by pumping a microsphere of about 98 &#956;m of diameter at 1,551 nm (Figure 7). Here, a Raman comb can be seen centered at 1,666.2 nm, and secondary lines can be seen in the vicinity of the pump with low efficiency (degenerated FWM). Also, the anti-Stokes line is centered at 1,451 nm, and two symmetric sidebands are separated by 10 nm. In this case, the pump and Stokes fields are sufficiently built-up, but the efficiency of SARS is hindered by the phase mismatch due to cross phase modulation (XPM) among the interactive fields (pump, stokes and anti-stokes). In the case of perfect phase matching, the Stokes and anti-Stokes components will mirror each other.
SARS is always detected in presence of SRS, and never in the absence of SRS, in agreement with the theory of Bloembergen and Shen22. SARS intensity is specially enhanced when SARS frequency is resonant with a cavity mode and phase matched with the pump and the SRS signal. Figure 8 is an example. It shows an SRS and SARS line separated by 90 nm (1,629 nm and 1,459 nm, respectively) and other SRS lines centered at 1,613.8 nm, 1,645.6 nm, 1,710 nm, 1,727.6 nm and 1,745.8 nm. Figure 9 shows a case of perfect phase matching for a microsphere of 65 &#956;m diameter pumped at 1572 nm. The stokes line is centered at 1640 nm and the Antistokes is centered at 1490 nm (separation in frequency of about 347 cm-1).
<img alt="Figure 1" src="/files/ftp_upload/53938/53938fig1.jpg" /> 
Figure 1. Microsphere dimensions. Size of the microspheres produced at the tip of a standard 125 &#956;m telecom fiber, as a function of the arc shots in a fiber fusion splicer. This figure has been modified from [18]. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53938/53938fig2.jpg" /> 
Figure 2. Drawing a taper fiber. Experimental set-up for drawing a tapered fiber. The fiber is held by two fiber clamps, which are located on a sliding block, on top of two rails. The structure is portable. On block is connected to a screw that pulls the fiber apart. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53938/53938fig3.jpg" /> 
Figure 3. A taper fiber. (A) Optical micrograph of a taper waist. The green color is due to interference effects, and its homogeneity indicates the homogeneity in thickness along the tapered section. (B) The taper glued to its U-shaped glass support. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53938/53938fig4.jpg" /> 
Figure 4. Experimental set-up: the signal from a tunable diode laser (TDL) is amplified by an EDFA and, after passing an attenuator and a polarizer, is launched into the WGMR by means of a tapered fiber. The output signal is split and sent into an optical spectrum analyzer (OSA) and to a photodiode to monitor the signal into an oscilloscope. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53938/53938fig5.jpg" /> 
Figure 5. Resonances. WGM resonance of a silica sphere with a diameter of 250 &#956;m coupled to a 4 &#956;m waist tapered fiber. Red line is the best fit using a Lorentz function. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53938/53938fig6.jpg" /> 
Figure 6. Nonlinear spectrum of small microspheres. Experimental spectrum of FWM and cascaded Raman lines in a microsphere of 50 &#956;m diameter. The pump is the peak at 1,546.6 nm, cascaded FWM peaks are the symmetrical lines that appear close to the pump (separation of 13 nm), whereas cascaded Raman lines separated at about 13.5 THz (or about 100 nm) from the pump and from themselves (1,608 nm first line, 1,708 nm second line). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/53938/53938fig7.jpg" /> 
Figure 7. Nonlinear spectrum of ultrahigh Q microspheres. Experimental spectrum of SARS, FWM in the vicinity of the pump and SRS comb in a microsphere of 98 &#956;m diameter. The pump is the peak centered at 1,551 nm, degenerated FWM is seen close to pump. The SRS line separated by 100 nm is centered at 1,646 nm and the corresponding SARS is centered at 1,451.5 nm. The two symmetric lines in the vicinity of the SARS line are degenerated FWM. The Raman comb is centered at 1,666.2 nm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/53938/53938fig8.jpg" /> 
Figure 8. Cavity enhanced SARS Spectrum. Experimental spectrum of SARS in a microsphere of 40 &#181;m diameter. The pump is centered at 1,539.4 nm, the SRS line is centered at 1,629.6 nm and the corresponding SARS is centered at 1,459 nm. The other SRS lines are centered at 1,613.8 nm, 1,645.6 nm, 1,710 nm, 1,727.6 nm and 1,745.8 nm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" src="/files/ftp_upload/53938/53938fig9.jpg" /> 
Figure 9. Perfect Phase Matched SRS-SARS. Experimental spectrum of SARS and SRS perfectly phase matched with a SARS-SRS intensity ratio close to 1. The microsphere diameter is 65 &#956;m. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53938/53938fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators at JoVE.com

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS).

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres ...

stimulated-stokes-antistokes-raman-scattering-microspherical

Nanyang Technological University

Research

JoVE Journal

Engineering

11.2K Views.  Enrico Fermi Center.  Efficiënte productie van niet-lineaire verschijnselen in verband met de derde orde optische non-lineaire gevoeligheid Χ (3) interacties in drievoudig resonante silica microsferen wordt in dit document. De interacties gemeld hier zijn: gestimuleerde Raman Scattering (SRS), en vier wave mixing processen bestaande uit gestimuleerde Anti-Stokes Raman Scattering (SARS). 

Video: Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Rejection of Fluorescence Background in Resonance and Spontaneous Raman Microspectroscopy

Raman spectroscopy is often plagued by a strong fluorescent background, particularly for biological samples. If a sample is excited with a train of ultrafast pulses, a system that can temporally separate spectrally overlapping signals on a picosecond timescale can isolate promptly arriving Raman scattered light from late-arriving fluorescence light. Here we discuss the construction and operation of a complex nonlinear optical system that uses all-optical switching in the form of a low-power optical Kerr gate to isolate Raman and fluorescence signals. A single 808 nm laser with 2.4 W of average power and 80 MHz repetition rate is split, with approximately 200 mW of 808 nm light being converted to &lt; 5 mW of 404 nm light sent to the sample to excite Raman scattering. The remaining unconverted 808 nm light is then sent to a nonlinear medium where it acts as the pump for the all-optical shutter. The shutter opens and closes in 800 fs with a peak efficiency of approximately 5%. Using this system we are able to successfully separate Raman and fluorescence signals at an 80 MHz repetition rate using pulse energies and average powers that remain biologically safe. Because the system has no spare capacity in terms of optical power, we detail several design and alignment considerations that aid in maximizing the throughput of the system. We also discuss our protocol for obtaining the spatial and temporal overlap of the signal and pump beams within the Kerr medium, as well as a detailed protocol for spectral acquisition. Finally, we report a few representative results of Raman spectra obtained in the presence of strong fluorescence using our time-gating system.

We discuss the construction and operation of a complex nonlinear optical
system that uses ultrafast all-optical switching to isolate Raman from fluorescence
signals. Using this system we are able to successfully separate Raman and fluorescence
signals utilizing pulse energies and average powers that remain biologically safe.

Raman spectroscopy is often plagued by a strong fluorescent background, particularly for biological samples. If a sample is excited with a train of ...

Bioengineering

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 ...

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.

The paper describes a method for producing extreme nanowires by melt infiltration into carbon nanotubes and how 1D systems may be characterized and investigated using Resonance Raman Spectroscopy to determine vibrational and optical excitation energies.

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the ...

High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as aqueous solutions and biomaterials, with fast acquisition times. Here, we discuss the assembly and operation of a Brillouin spectrometer that uses stimulated Brillouin scattering (SBS) to measure stimulated Brillouin gain (SBG) spectra of water and lipid emulsion-based tissue-like samples in transmission mode with &lt;10 MHz spectral-resolution and &lt;35 MHz Brillouin-shift measurement precision at &lt;100 ms. The spectrometer consists of two nearly counter-propagating continuous-wave (CW) narrow-linewidth lasers at 780 nm whose frequency detuning is scanned through the material Brillouin shift. By using an ultra-narrowband hot rubidium-85 vapor notch filter and a phase-sensitive detector, the signal-to-noise-ratio of the SBG signal is significantly enhanced compared to that obtained with existing CW-SBS spectrometers. This improvement enables measurement of SBG spectra with up to 100-fold faster acquisition times, thereby facilitating high spectral-resolution and high-precision Brillouin analysis of soft materials at high speed.

We describe the construction of a rapid continuous-wave-stimulated-Brillouin-scattering (CW-SBS) spectrometer. The spectrometer employs single-frequency diode-lasers and an atomic vapor notch-filter to acquire transmission spectra of turbid/non-turbid samples with high spectral-resolution at speeds up to 100-fold faster than those of existing CW-SBS spectrometers. This improvement enables high-speed Brillouin material analysis.

Recent years have witnessed a significant increase in the use of spontaneous Brillouin spectrometers for non-contact analysis of soft matter, such as ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Implementation of a Nonlinear Microscope Based on Stimulated Raman Scattering

Stimulated Raman scattering (SRS) microscopy uses near-infrared excitation light; therefore, it shares many multi-photon microscopic imaging properties. SRS imaging modality can be obtained using commercial laser-scanning microscopes by equipping with a non-descanned forward detector with proper bandpass filters and lock-in amplifier (LIA) detection scheme. A schematic layout of a typical SRS microscope includes the following: two pulsed laser beams, (i.e., the pump and probe directed in a scanning microscope), which must be overlapped in both space and time at the image plane, then focused by a microscope objective into the sample through two scanning mirrors (SMs), which raster the focal spot across an x-y plane. After interaction with the sample, transmitted output pulses are collected by an upper objective and measured by a forward detection system inserted in an inverted microscope. Pump pulses are removed by a stack of optical filters, whereas the probe pulses that are the result of the SRS process occurring in the focal volume of the specimen are measured by a photodiode (PD). The readout of the PD is demodulated by the LIA to extract the modulation depth. A two-dimensional (2D) image is obtained by synchronizing the forward detection unit with the microscope scanning unit. In this paper, the implementation of an SRS microscope is described and successfully demonstrated, as well as the reporting of label-free images of polystyrene beads with diameters of 3 &#181;m. It is worth noting that SRS microscopes are not commercially available, so in order to take advantage of these characteristics, the homemade construction is the only option. Since SRS microscopy is becoming popular in many fields, it is believed that this careful description of the SRS microscope implementation can be very useful for the scientific community.

In this manuscript, the implementation of a stimulated Raman scattering (SRS) microscope, obtained by the integration of an SRS experimental set-up with a laser scanning microscope, is described. The SRS microscope is based on two femtosecond (fs) laser sources, a Ti-Sapphire (Ti:Sa) and synchronized optical parametric oscillator (OPO).

Stimulated Raman scattering (SRS) microscopy uses near-infrared excitation light; therefore, it shares many multi-photon microscopic imaging properties. ...

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional &#960;-conjugate Systems

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of &#946;-carotene in toluene solution. A C=C stretch band of &#946;-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of &#960;-conjugate systems from simple molecules to complex materials.

Details of signal generation and optimization, measurement, data acquisition, and data handling for a femtosecond time-resolved near-IR stimulated Raman spectrometer are described. A near infrared stimulated Raman study on the excited-state dynamics of &#946;-carotene in toluene is shown as a representative application.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near ...

Chemistry

Multiplex Chemical Imaging Based on Broadband Stimulated Raman Scattering Microscopy

Stimulated Raman scattering (SRS) microscopy is a nonlinear optical technique for label-free chemical imaging. This analytical tool delivers chemical maps at high speed, and high spatial resolution of thin samples by directly interrogating their molecular vibrations. In its standard implementation, SRS microscopy is narrowband and forms images with only a single vibrational frequency at a time. However, this approach not only hinders the chemical specificity of SRS but also neglects the wealth of information encoded within vibrational spectra. 
These limitations can be overcome by broadband SRS, an implementation capable of extracting a vibrational spectrum per pixel of the image in parallel. This delivers hyperspectral data that, when coupled with chemometric analysis, maximizes the amount of information retrieved from the specimen. Thus, broadband SRS improves the chemical specificity of the system, allowing the quantitative determination of the concentration of the different constituents of a sample. Here, we report a protocol for chemical imaging with broadband SRS microscopy, based on a home-built SRS microscope operating with a custom differential multichannel-lock-in amplifier detection. It discusses the sample preparation, alignment of the SRS apparatus, and chemometric analysis. By acquiring vibrational Raman spectra, the protocol illustrates how to identify different chemical species within a mixture, determining their relative concentrations.

We present a protocol to acquire chemical images with broadband stimulated Raman scattering (SRS) microscopy. Based on an SRS microscope that operates with differential multichannel-lock-in detection, the protocol describes the sample preparation, adjustment of the SRS apparatus, and chemometrics to disentangle different constituents of chemically heterogeneous samples.

Stimulated Raman scattering (SRS) microscopy is a nonlinear optical technique for label-free chemical imaging. This analytical tool delivers chemical maps ...

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

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