This work is financially supported by National Research Foundation grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2018M3D1A1058998, NRF-2015R1A5A1037668) funded by the Ministry of Science and ICT (MSIT), Republic of Korea.

1. Fabrication of the dielectric metasurface
<ol>
	<li>Precleaning of a fused silica substrate
	<ol>
		<li>Prepare a double-side polished, fused silica substrate (length: 2 cm; width: 2 cm; thickness: 500 &#956;m).</li>
		<li>Immerse the fused silica substrate in 50 mL of acetone and conduct the sonication process for 5 min at 40 kHz.</li>
		<li>Immerse the substrate in 50 mL of 2-propanol (IPA) and conduct the sonication process for 5 min at 40 kHz.</li>
		<li>Rinse the substrate with the IPA and blow nitrogen (N2) gas to dry the substrate before the evaporation of the IPA.</li>
	</ol>
	</li>
	<li>Deposition of a-Si:H by PECVD
	<ol>
		<li>Locate the prepared substrates on a zig inside the load lock chamber of the PECVD system.</li>
		<li>On PECVD software, set the chamber temperature to 300 &#176;C and set the radio frequency power to 800 W.</li>
		<li>Set the SiH4 gas flow rate to 10 sccm and the H2 gas flow rate to 75 sccm.</li>
		<li>Set the process pressure to 25 mTorr. Click the Start button to start the deposition process, which takes ~300 s.</li>
	</ol>
	</li>
	<li>Formation of the Cr etching mask
	<ol>
		<li>Load the sample obtained from step 1.2.4 on the sample holder of the spin coater. Release poly(methyl methacrylate) (PMMA) A2 on the sample using a filter-mounted 5 mL syringe and start the coating process with a rotation speed of 2,000 rpm for 1 min. 
		NOTE: Released PMMA should cover the whole substrate; otherwise, the spin-coated film will not be uniform.</li>
		<li>Transfer the sample from the sample holder to a hot plate, and bake the sample with a hot plate at 180 &#176;C for 5 min. Then, cool the sample at room temperature for 1 min.</li>
		<li>Load the sample on the sample holder of the spin coater. Release E-spacer on the sample, using a 1 mL pipette, and start the coating process with a rotation speed of 2,000 rpm for 1 min. 
		NOTE: Released E-spacer should cover the whole substrate; otherwise, the spin-coated film will not be uniform.</li>
		<li>Load and fix the sample on the zig for EBL. Put the zig into the EBL chamber and, then, load it into the main chamber.</li>
		<li>On the EBL console, push the Isolation button and, then, the FC button. Set the magnification to its maximum value using the magnification knob.</li>
		<li>Turn the Zero check button on. Turn the Beam current knob to set the beam current value to 50 pA. Turn the Zero check button off.</li>
		<li>Push the Reference button to move the stage to the reference position. Turn the Blank button off.</li>
		<li>Set the magnification value to 100,000 using the magnification knob. Adjust the focus and stigmatism knobs to obtain the clearest image in the EBL display. Turn the Blank button on.</li>
		<li>On the computer connected to the EBL console, run the Linux terminal. Move the current location to the folder that has the .gds file, using the cd command.</li>
		<li>Enter gds2cel to convert the .gds file to a .cel file and wait until it finishes. Enter job to run the main software.</li>
		<li>Click the Chip size modification menu. Select 600 &#956;m x 600 &#956;m and 240,000 dots. Click Save and then Exit.</li>
		<li>Click the Pattern data creation menu. Enter ps in the command window to load the pattern .cel file generated from step 1.3.10. Enter i in the command window and click the pattern to magnify the pattern image.</li>
		<li>Enter sd in the command window and 3 to set the dose time to 3 &#956;s. Enter sp in the command window and 1,1 to set the exposing pitch to a normal condition. Enter pc in the command window and a filename to create a .ccc file. Click the center of the pattern.</li>
		<li>Enter cp in the command window and click the pattern to apply the exposing conditions from step 1.3.13. Enter sv in the command window and a filename to create a .con file. Enter q in the command window to exit the Pattern data creation menu.</li>
		<li>Click the Exposure menu. Enter i and the .con filename from step 1.3.14. Enter e and click the Exposure button to start the exposing process. 
		NOTE: The exposing time depends on the pattern area and density. General metasurface patterns of a 300 &#956;m x 300 &#956;m area take ~3 h.</li>
		<li>When the exposing process finishes, turn the Isolation button off. Push the EX button to move the stage.</li>
		<li>Unload the sample from the chamber after finishing the exposure. Immerse the sample in 50 mL of deionized (DI) water for 1 min to remove the E-spacer.</li>
		<li>Prepare 10 mL of methyl isobutyl ketone (MIBK):IPA = 1:3 solution in a beaker surrounded by ice. Immerse the sample into the MIBK:IPA = 1:3 solution for 12 min. Then, rinse the sample with the IPA and blow N2 gas to dry the sample.</li>
		<li>Load and fix the sample on the holder of the electron beam evaporator. Mount the holder inside the chamber of the evaporator.</li>
		<li>Load a graphite-crucible-containing piece-type Cr inside the evaporation chamber.</li>
		<li>On the software of the electron beam evaporator, click the Chamber pumping button to create a vacuum on the inside of the chamber, and lower the pressure to 3 x 10-6 mTorr.</li>
		<li>Select Chromium in the material section and click the Material button to apply it. Click the E-beam shutter button to open the source shutter. Click the High voltage and the Source button, in that order.</li>
		<li>Click the upward arrow button to increase the electron beam power slowly, and repeat this until the deposition rate reaches 0.15 nm/s. 
		NOTE: One click per 5 s is slow enough.</li>
		<li>Click the Zero button to reset the thickness gauge. Click the Main shutter button to open the main shutter. When the thickness gauge reaches 30 nm, click the Main shutter button to close the main shutter. 
		NOTE: Deposition time can be easily calculated from the deposition rate. A 30 nm-thick deposition takes ~200 s, in the condition used here.</li>
		<li>Click the E-beam shutter button to close the source shutter. Click the downward arrow button to decrease the electron beam power slowly, and repeat this until the power reaches 0. 
		NOTE: One click per 5 s is slow enough.</li>
		<li>Click the Source and, then, the High voltage button. Wait for 15 min to cool the chamber. Click the Vent button to vent the chamber and unload the sample from the holder.</li>
		<li>Immerse the sample in 50 mL of acetone for 3 min. Conduct the sonication process for 1 min at 40 kHz. Rinse the sample with IPA and blow N2 gas to dry the sample.</li>
	</ol>
	</li>
	<li>Etching process of a-Si:H
	<ol>
		<li>Spread thermal glue to the back of the sample. Attach the sample on the zig and load the zig on the etching system.</li>
		<li>On the software, set the chlorine (Cl2) gas flow rate to 80 sccm and the hydrogen bromide (HBr) gas flow rate to 120 sccm. Set the source power to 500 W and the bias to 100 V. Click the Start button to start the etching process for 100 s.</li>
		<li>Unload the sample and remove the thermal glue with a dustproof wiper.</li>
		<li>Immerse the sample in 20 mL of Cr etchant for 2 min and in 50 mL of DI water for 1 min. Rinse the sample with DI water and blow N2 gas to dry the sample.</li>
	</ol>
	</li>
	<li>Obtaining the scanning electron microscope image of the fabricated metasurface
	<ol>
		<li>Load the sample on the sample holder of the spin coater, release the E-spacer on the sample using a 1 mL pipette, and start the coating process with a rotation speed of 2,000 rpm for 1 min.</li>
		<li>Fix the sample on the sample holder of the scanning electron microscope (SEM), using carbon tape. Put the holder in the load lock chamber of the SEM and create a vacuum in the load lock chamber.</li>
		<li>Transfer the holder from the load lock chamber to the main chamber. Turn the electron beam on with a 15 kV acceleration voltage.</li>
		<li>Move the stage to a 1 cm working distance. Find the metasurface by moving the stage horizontally. Adjust the stigmatism and focal length until the image becomes clear.</li>
		<li>Capture the images.</li>
		<li>Turn off the electron beam. Move the stage to the extraction position. Transfer the holder from the main chamber to the load lock chamber.</li>
		<li>Vent the load lock chamber and unload the sample.</li>
		<li>Immerse the sample into 50 mL of DI water for 1 min to remove the E-spacer. Blow N2 gas to dry the sample.</li>
	</ol>
	</li>
</ol>
2. Optical characterization of the dielectric metasurface
NOTE: Direct radiation of a laser can damage eyes. Avoid direct eye exposure and wear the appropriate laser safety glasses.
<ol>
	<li>Mount a 635 nm-wavelength laser on the optical table (Figure 1a). Turn the laser on and wait for 10 min to stabilize the beam power.</li>
	<li>Adjust the horizontal and vertical alignment of the laser using an alignment screen both near and far from the laser.</li>
	<li>Place a neutral density filter in front of the laser. Mount the first convex lens behind the neutral density filter. Place an iris at the back focal plane of the convex lens to remove noise.</li>
	<li>Mount the second convex lens with twice the focal length from the first convex lens. Place a linear polarizer behind the second convex lens. Place a right-handed circular polarizer behind the linear polarizer.</li>
	<li>Mount the third convex lens behind the circular polarizer. Mount the fabricated metasurface on the holder. Locate the metasurface at the back focal plane of the convex lens. 
	NOTE: The laser beam should be incident from the substrate to the patterned area.</li>
	<li>Place a thick white paper screen, which has a 1 cm-diameter hole in the center, behind the metasurface. Mount a protractor on the optical table aligning the origin with the metasurface.</li>
	<li>Measure the power of the three diffracted beams, which are three bright spots on the screen, using a power meter. 
	NOTE: If the laser beam power is not maintained at a constant, calculate the average beam power over a period of time.</li>
	<li>Replace the right-handed circular polarizer with a left-handed circular polarizer. Measure the three diffracted beam powers, using the power meter.</li>
	<li>Remove the left-handed circular polarizer. Measure three diffracted beam powers, using the power meter.</li>
	<li>Decrease the laser beam power, using the neutral density filter to allow a CCD measurement. Place the right-handed circular polarizer. Capture the three diffracted beam profiles using the CCD. 
	NOTE: Weaker laser beam power is preferred, to prevent CCD damage. A beam power of 300 &#956;W has been used in this work.</li>
	<li>Replace the right-handed circular polarizer with the left-handed circular polarizer. Capture the three diffracted beam profiles, using the CCD.</li>
	<li>Remove the left-handed circular polarizer. Capture the three diffracted beam profiles, using the CCD.</li>
	<li>Replace the 635 nm-wavelength laser with a 532 nm-wavelength laser.</li>
	<li>Repeat steps 2.2 to 2.12.</li>
</ol>

<ol>
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The authors have nothing to disclose.

Some fabrication steps should be conducted carefully, to generate a metasurface that is the same as the original design. In the resist development process, a low-temperature solution is usually preferred. The standard condition is room temperature, but the reaction speed can be slowed down by decreasing the solution temperature to 0 &#176;C. Although the corresponding reaction time becomes longer, a finer pattern can be obtained than with standard conditions. The reaction time control is also easy owing to the low reaction speed. Another critical step for a fine pattern is drying IPA after the resist development. The N2 gas moves and evaporates the rest of the IPA on the sample. Some amount of IPA does not move, creating randomly distributed islands. If the IPA islands are formed and then evaporated, the sample will be damaged. Therefore, to minimize IPA island formation, strong blowing is better than weak blowing, unless the substrate is broken by the strong air flow. An appropriate power of the sonication is helpful to clearly peel off the thin film. After the lift-off step, it is possible to check whether the thin film clearly peels off or not by using a conventional optical microscope. Fortunately, if any Cr thin film remains on the patterned area, it is possible to remove the residue by an additional sonication process. This is a considerable advantage of the Cr mask, because masks made of other materials, such as gold, are extremely difficult to remove once the residue dries.
EBL is an effective method to fabricate nanoscale structures, but this method suffers from a low throughput, impinging large-scale manufacturing. One way to improve productivity is by making master molds, using EBL, and printing the pattern using the master mold. This method is called nanoimprint lithography. Although the fabrication of the mold using EBL takes a long time, the result is that patterns can be transferred in a short time, using the mold that can be used repeatedly. Moreover, it is also possible to transfer the pattern onto a flexible substrate by modifying the printing processes.
In this work, we present a detailed process for the fabrication of dielectric metasurfaces. The method is not limited to the application of beam splitters; other metasurface applications, such as lenses, holograms, and optical cloaks, can be realized via this method. Compared to plasmonic metasurfaces, dielectric metasurfaces provide a much higher efficiency at visible wavelengths due to low optical losses of dielectrics. Hence, this protocol can pave the way to study and develop practical metasurfaces.

Metasurfaces consisting of two-dimensional subwavelength antenna arrays have demonstrated many promising optical functionalities, such as achromatic lenses1,2, holograms3,4,5,6, and optical cloaks7. Conventional bulky optical components can be replaced with ultrathin metasurfaces while maintaining the original functionalities. For example, a beam splitter is an optical device used to separate an incident beam into two beams. Typical beam splitters are made by combining two triangular prisms. Since their interface characteristics determine the beam splitting properties, it is hard to reduce the physical size without functional degradation. On the other hand, ultrathin beam splitters can be realized with metasurfaces encoded with a one-dimensional linear phase gradient8,9. The thickness of metasurfaces is less than their working wavelengths, and separation properties can be controlled by the phase distribution.
We designed a metasurface beam splitter which can generate equal-intensity beams regardless of the incident polarization states10. This characteristic comes from a Fourier hologram. Due to the image of two white spots on a black background, generated hologram from the metasurface is the same as the encoded image. The Fourier hologram does not have a specific focal length, so the encoded image can be observed in the whole space behind the metasurface11. If the same two-spot image is generated behind the metasurface, it also works as a beam splitter. The Fourier hologram by the metasurface creates an inverted image, which is called a twin image, with respect to the orthogonal polarization states. The twin image is typically regarded as noise. However, the two-spot image encoded in this metasurface is origin-symmetric, resulting in a perfect overlap of the original and twin images. Since any polarization states can be represented by a linear combination of right-handed (RCP) and left-handed (LCP) circular polarizations, the device described here shows the polarization-independent functionality.
Here, we present a protocol for the fabrication and optical characterization of dielectric metasurfaces enabling equal-intensity beam generation. The phase distribution of this device is retrieved from the Gerchberg&#8211;Saxton (GS) algorithm, which is generally used for phase-only holograms12. a-Si:H of 300 nm thick is deposited on the fused silica substrate, using PECVD. A Cr mask is defined on the a-Si:H film, using EBL. The mask pattern corresponds to the phase distribution derived from the GS algorithm. ICP-RIE is exploited to etch the a-Si:H film along the Cr mask. The rest of the Cr mask is removed by Cr etchant finalizing the sample fabrication. The optical functionality of the fabricated metasurface is characterized using a customized optical setup. When a laser beam is incident to the metasurface, the transmitted beam is separated into three parts, namely two diffracted beams and one zeroth-order beam. The diffracted beams deviate from an extension of the incident beam path while the zeroth-order beam follows it. To verify the functionality of this device, we measured the beam power, beam profile, and diffracted angle using a power meter, CCD, and protractor, respectively.
All the fabrication processes and materials used are optimized for the target functionality. For visible working frequencies, the individual antenna sizes should be a few hundreds of nanometers, and the material itself should have a low optical loss at visible wavelengths. Only a few kinds of fabrication methods are applicable when defining such small structures. Typical photolithography, as well as direct laser writing, are incapable of the fabrication due to the diffraction limit. Focused ion beam milling can be used, but there are critical issues of gallium contamination, pattern design dependence, and the slow process speed. Practically, EBL is the only way to facilitate the fabrication of metasurfaces working at visible frequencies13.
Dielectrics are usually preferred due to the unavoidable ohmic loss of metals. The optical loss of a-Si:H is low enough for our purpose. Although the optical loss of a-Si:H is not as low as low-loss dielectrics such as titanium dioxide1,4 and crystalline silicon14, the fabrication of a-Si:H is much simpler. Typical evaporation and sputtering processes are not capable of the deposition of an a-Si:H film. PECVD is usually required. During the PECVD process, some hydrogen atoms from SiH4 and H2 gases are trapped among the silicon atoms, resulting in an a-Si:H film. There are two ways to define a-Si:H patterns. One is the deposition of a-Si:H on a patterned photoresist, followed by the lift-off process, and the other is by defining an etching mask on the a-Si:H film, followed by the etching process. The former is well-suited to evaporation processes, but it is not easy to deposit a-Si:H film using evaporation. Hence, the latter is the optimal way to make a-Si:H patterns. Cr is used as the etching mask material because of its high etching selectivity with silicon.

<table><tbody><tr><td>Plasma enhanced chemical vapor deposition</td><td>BMR Technology</td><td>HiDep-SC</td><td></td></tr><tr><td>Electron beam lithography</td><td>Elionix</td><td>ELS-7800</td><td></td></tr><tr><td>E-beam evaporation system</td><td>Korea Vacuum Tech</td><td>KVE-E4000</td><td></td></tr><tr><td>Inductively-coupled plasma reactive ion etching</td><td>DMS</td><td>-</td><td></td></tr><tr><td>Ultrasonic cleaner</td><td>Honda</td><td>W-113</td><td></td></tr><tr><td>E-beam resist</td><td>MICROCHEM</td><td>495 PMMA A2</td><td></td></tr><tr><td>Resist developer</td><td>MICROCHEM</td><td>MIBK:IPA=1:3</td><td></td></tr><tr><td>Conducting polymer</td><td>Showa denko</td><td>E-spacer</td><td></td></tr><tr><td>Chromium etchant</td><td>KMG</td><td>CR-7</td><td></td></tr><tr><td>Acetone</td><td>J.T. Baker</td><td>925402</td><td></td></tr><tr><td>2-propanol</td><td>J.T. Baker</td><td>909502</td><td></td></tr><tr><td>Chromium evaporation source</td><td>Kurt J. Lesker</td><td>EVMCR35D</td><td></td></tr><tr><td>Collimated laser diode module</td><td>Thorlabs</td><td>CPS-635</td><td>wavelength: 635 nm</td></tr><tr><td>ND:YAG laser</td><td>GAM laser</td><td>GAM-2000</td><td>wavelength: 532 nm</td></tr><tr><td>power meter</td><td>Thorlabs</td><td>S120VC</td><td></td></tr><tr><td>CCD Camera</td><td>INFINITY</td><td>infinity2-2M</td><td></td></tr><tr><td>ND filter</td><td>Thorlabs</td><td>NCD-50C-4-A</td><td></td></tr><tr><td>Linear polarizer</td><td>Thorlabs</td><td>LPVISA100-MP2</td><td></td></tr><tr><td>Lens</td><td>Thorlabs</td><td>LB1676</td><td></td></tr><tr><td>Iris</td><td>Thorlabs</td><td>ID25</td><td></td></tr><tr><td>Circular polarizer</td><td>Edmund optics</td><td>88-096</td><td></td></tr><tr><td>sample holder</td><td>Thorlabs</td><td>XYFM1</td><td></td></tr><tr><td>PECVD software</td><td>BMR Technology</td><td>HIDEP</td><td></td></tr></tbody></table>

demonstration of equal-intensity beam generation by dielectric metasurfaces

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated amorphous silicon (a-Si:H) is deposited on the fused silica substrate, using plasma-enhanced chemical vapor deposition (PECVD). Typical amorphous silicon deposited by evaporation causes severe optical loss, impinging the operation at visible frequencies. Hydrogen atoms inside the amorphous silicon thin film can reduce the structural defects, improving optical loss. Nanostructures of a few hundreds of nanometers are required for the operation of metasurfaces in the visible frequencies. Conventional photolithography or direct laser writing is not feasible when fabricating such small structures, due to the diffraction limit. Hence, electron beam lithography (EBL) is utilized to define a chromium (Cr) mask on the thin film. During this process, the exposed resist is developed at a cold temperature to slow down the chemical reaction and make the pattern edges sharper. Finally, a-Si:H is etched along the mask, using inductively coupled plasma&#8211;reactive ion etching (ICP-RIE). The demonstrated method is not feasible for large-scale fabrication due to the low throughput of EBL, but it can be improved upon by combining it with nanoimprint lithography. The fabricated device is characterized by a customized optical setup consisting of a laser, polarizer, lens, power meter, and charge-coupled device (CCD). By changing the laser wavelength and polarization, the diffraction properties are measured. The measured diffracted beam powers are always equal, regardless of the incident polarization, as well as wavelength.

The measurement results show the polarization-independent functionality of the device presented here (Figure 1). Measured beam powers of diffraction orders of m = &#177; 1 are equal regardless of the incident polarization state (i.e., RCP, LCP, and linear polarization). Since any arbitrary polarization states can be decomposed by the linear combination of RCP and LCP, the device&#8217;s functionality can be maintained, regardless of polarization states. The diffraction angles are 24&#176; and 28.5&#176; for the wavelengths of 532 nm and 635 nm, respectively, and the angles can be controlled by changing the encoded hologram.
Diffraction efficiency is defined by the ratio of the diffracted beam powers (m = &#177; 1) to the incident beam power. The device presented here consists of same-sized nanorods with different orientations based on the geometric-phase-resulting broadband operation (Figure 2). Theoretically, the efficiency should be higher than 20% for both wavelengths. However, the measured diffraction efficiencies are 18.3% at &#955; = 532 nm and 9.1% at &#955; = 635 nm. The discrepancy mainly comes from the beam size being larger than the metasurface itself. The measured profiles of the zeroth-order beam clearly show that the incident beam size is larger than the metasurface (i.e., the excess portion of the incident beam directly goes to the power meter without interacting with the metasurface, reducing the diffraction efficiency) (Figure 3).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59066/59066fig1.jpg" /> 
Figure 1: Diffracted beam power measurement. (a) The optical setup for the laser illumination. The next two panels show the measured diffracted beam power (b) at &#955; = 532 nm and (c) at 635 nm. Since the laser beam power was not consistent, the measured beam powers are calculated by time-averaging recorded values. The error bars in the figure represent the maximum and minimum values during the recording time. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59066/59066fig2.jpg" /> 
Figure 2: SEM images of the fabricated metasurface. (a) Top view of the metasurface. (inset) Geometry of the unit cell: length (L) = 150 nm, width (W) = 80 nm, height (H) = 300 nm, and pitch (P) = 240 nm. (b) Perspective view of the metasurface. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59066/59066fig3.jpg" /> 
Figure 3: Captured beam profiles by CCD taken 14 cm behind the metasurface. The beam diameters can be estimated at ~3 mm at &#955; = 532 nm and ~5 mm at &#955; = 635 nm. The corresponding beam diverging angles are approximately 2.5&#176; and 4.1&#176;, respectively. The captured beam profiles have laser speckles, but they can be removed by diffusers1 or Dammann grating3,15. This figure has been modified from Yoon et al.10. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59066/59066fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces at JoVE.com

Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

A protocol for the fabrication and optical characterization of dielectric metasurfaces is presented. This method can be applied to the fabrication of not only beam splitters, but also of general dielectric metasurfaces, such as lenses, holograms, and optical cloaks.

The fabrication and characterization protocol for a metasurface beam splitter, enabling equal-intensity beam generation, is demonstrated. Hydrogenated ...

demonstration-equal-intensity-beam-generation-dielectric

Nanyang Technological University

Research

JoVE Journal

Engineering

6.2K Views.  Pohang University of Science and Technology (POSTECH). Dit protocol biedt een gedetailleerde fabricagemethode om metasurfaces met een hoog rendement te realiseren, een van de belangrijkste kwesties op het gebied van metasurfaceonderzoek. Vergeleken met andere methoden om atomaire laagdepositie te testen, is deze techniek een goedkope en snelle fabricagemethode om hoogrendementsmetasurfaces te realiseren die werken op zichtbare golflengten. Dit protocol kan worden toegepast om algemene metasurfaces zoals lenzen, hologrammen en optische klokken te ...

Video: Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

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 ...

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical1 and experimental demonstration2 of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical3, near IR4, mid IR5 , THz6 , mm-wave7 , microwave8 and radio9 bands. Applications include perfect lenses10, sensors11, telecommunications12, invisibility cloaks13 and filters14,15. We have recently developed single band16, dual band17 and broadband18 THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers19. In our MM absorber the THz radiation is absorbed in a thickness of ~ &lambda;/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Metamaterials (MM),  artificial materials engineered to have properties that may not be found in  nature, have been widely explored since the first ...

The Generation of Higher-order Laguerre-Gauss Optical Beams for High-precision Interferometry

Thermal noise in high-reflectivity mirrors is a major impediment for several types of high-precision interferometric experiments that aim to reach the standard quantum limit or to cool mechanical systems to their quantum ground state. This is for example the case of future gravitational wave observatories, whose sensitivity to gravitational wave signals is expected to be limited in the most sensitive frequency band, by atomic vibration of their mirror masses. One promising approach being pursued to overcome this limitation is to employ higher-order Laguerre-Gauss (LG) optical beams in place of the conventionally used fundamental mode. Owing to their more homogeneous light intensity distribution these beams average more effectively over the thermally driven fluctuations of the mirror surface, which in turn reduces the uncertainty in the mirror position sensed by the laser light.
We demonstrate a promising method to generate higher-order LG beams by shaping a fundamental Gaussian beam with the help of diffractive optical elements. We show that with conventional sensing and control techniques that are known for stabilizing fundamental laser beams, higher-order LG modes can be purified and stabilized just as well at a comparably high level. A set of diagnostic tools allows us to control and tailor the properties of generated LG beams. This enabled us to produce an LG beam with the highest purity reported to date. The demonstrated compatibility of higher-order LG modes with standard interferometry techniques and with the use of standard spherical optics makes them an ideal candidate for application in a future generation of high-precision interferometry.

Large laser-interferometers are being constructed to create a new type of astronomy based on gravitational waves. Their sensitivities, as for many other high-precision experiments, are approaching fundamental noise limits such as the atomic vibration of their components. We are pioneering technologies to overcome these limits using novel laser beam shapes.

Thermal noise in high-reflectivity mirrors is a major impediment for several  types of high-precision interferometric experiments that aim to reach the  ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

Shaping the Amplitude and Phase of Laser Beams by Using a Phase-only Spatial Light Modulator

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser radiation. The method is based on the coherent sum of two uniform waves, previously encoded into a phase-only spatial light modulator (SLM) by spatial multiplexing of their phases. Here, the interference process is carried out by spatial filtering of light frequencies at the Fourier plane of certain imaging system. The correct implementation of this method allows arbitrary phase and amplitude information to be retrieved at the output of the optical system.
It is an on-axis, rather than off-axis encoding technique, with a direct processing algorithm (not an iterative loop), and free from coherent noise (speckle). The complex field can be exactly retrieved at the output of the optical system, except for some loss of resolution due to the frequency filtering process. The main limitation of the method might come from the inability to operate at frequency rates higher than the refresh rate of the SLM. Applications include, but are not limited to, linear and non-linear microscopy, beam shaping, or laser micro-processing of material surfaces.

We show how to encode the complex field of laser beams by using a single phase element. A common-path interferometer is employed to mix the phase information displayed into a phase-only spatial light modulator to finally retrieve the desired complex field pattern at the output of an optical imaging system.

The aim of this article is to visually demonstrate the utilization of an interferometric method for encoding complex fields associated with coherent laser ...

Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms

The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption display in the form of ultrathin and almost flat optical devices. Compared to the conventional holographic technique with spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher image resolution and larger field of visibility for holographic images. Here, a protocol is reported for the fabrication and optical characterization of optical metaholograms that are sensitive to the spin and direction of incident light. The metasurfaces are composed of hydrogenated amorphous silicon (a-Si:H), which has large refractive index and small extinction coefficient in the entire visible range resulting in high transmittance and diffraction efficiency. The device produces different holographic images when the spin or direction of incident light are switched. Therefore, they can encode multiple types of visual information simultaneously. The fabrication protocol consists of film deposition, electron beam writing and subsequent etching. The fabricated device can be characterized using a customized optical setup that consists of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).

We present a protocol for fabrication of spin- and direction-multiplexed visible metaholograms, then conduct an optical experiment to verify their function. These metaholograms can easily visualize encoded information, so they can be used for projective volumetric display and information encryption.

The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption ...

Fabrication of a Fabry-P&#233;rot Etalon for Application in Trace-Gas Spectroscopy

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are used for their high sensitivity as well as their exceptional filtering capability. However, air-spaced etalons with high finesse are usually built by specialized facilities. Their production requires a clean room, special glass handling, and coating machinery, meaning commercially available FPEs are sold for a high price. In this article, a new and cost-effective method to fabricate fiber-coupled FPEs with standard photonic laboratory equipment is presented. The protocol should serve as a step-by-step guide for the construction and characterization of these FPEs. We hope this&#160;will enable researchers to conduct fast and cost-effective prototyping of FPEs for various fields of application. The FPE, as presented here, is used for spectroscopic applications. As shown in the representative results section via proof of principle measurements of water vapor in ambient air, this FPE has a finesse of 15, which is sufficient for the photothermal detection of trace concentrations of gases.

Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon  

This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are ...

Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces

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