Name: demonstration of equal-intensity beam generation by dielectric metasurfaces
Uploaded: 2019-06-07T12:30:00
Description: Watch this Scientific Journal Video about Demonstration of Equal-Intensity Beam Generation by Dielectric Metasurfaces at JoVE.com

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...

<ol>
	<li>Khorasaninejad, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metalenses+at+visible+wavelengths:+Diffraction-limited+focusing+and+subwavelength+resolution+imaging.">Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging.</a> Science. 352 (6290), 1190-1194 (2016).</li><li>Chen, W. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+broadband+achromatic+metalens+for+focusing+and+imaging+in+the+visible.">A broadband achromatic metalens for focusing and imaging in the visible.</a> Nature Nanotechnology. 13 (3), 220-226 (2018).</li><li>Zheng, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metasurface+holograms+reaching+80%+efficiency.">Metasurface holograms reaching 80% efficiency.</a> Nature Nanotechnology. 10 (4), 308-312 (2015).</li><li>Devlin, R. C., Khorasaninejad, M., Chen, W. T., Oh, J., Capasso, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Broadband+high-efficiency+dielectric+metasurfaces+for+the+visible+spectrum.">Broadband high-efficiency dielectric metasurfaces for the visible spectrum.</a> Proceedings of the National Academy of Sciences. 113 (38), 10473-10478 (2016).</li><li>Yoon, G., Lee, D., Nam, K. T., Rho, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pragmatic+Metasurface+Hologram+at+Visible+Wavelength:+The+Balance+between+Diffraction+Efficiency+and+Fabrication+Compatibility.">Pragmatic Metasurface Hologram at Visible Wavelength: The Balance between Diffraction Efficiency and Fabrication Compatibility.</a> ACS Photonics. 5 (5), 1643-1647 (2018).</li><li>Yoon, G., Lee, D., Nam, K. T., Rho, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term="Crypto-Display"+in+Dual-Mode+Metasurfaces+by+Simultaneous+Control+of+Phase+and+Spectral+Responses.">"Crypto-Display" in Dual-Mode Metasurfaces by Simultaneous Control of Phase and Spectral Responses.</a> ACS Nano. 12 (7), 6421-6428 (2018).</li><li>Ni, X., Wong, Z. J., Mrejen, M., Wang, Y., Zhang, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+ultrathin+invisibility+skin+cloak+for+visible+light.">An ultrathin invisibility skin cloak for visible light.</a> Science. 349 (6254), 1310-1314 (2015).</li><li>Khorasaninejad, M., Crozier, K. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silicon+nanofin+grating+as+a+miniature+chirality-distinguishing+beam-splitter.">Silicon nanofin grating as a miniature chirality-distinguishing beam-splitter.</a> Nature Communications. 5, 5386 (2014).</li><li>Zhang, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoscale+beam+splitters+based+on+gradient+metasurfaces.">Nanoscale beam splitters based on gradient metasurfaces.</a> Optics Letters. 43 (2), 267 (2018).</li><li>Yoon, G., Lee, D., Nam, K. T., Rho, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Geometric+metasurface+enabling+polarization+independent+beam+splitting.">Geometric metasurface enabling polarization independent beam splitting.</a> Scientific Reports. 8 (1), 9468 (2018).</li><li>Goodman, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Introduction to Fourier Optics. , (2005).</li><li>Gerchberg, R. W., Saxton, W. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+practical+algorithm+for+the+determination+of+the+phase+from+image+and+diffraction+plane+pictures.">A practical algorithm for the determination of the phase from image and diffraction plane pictures.</a> Optik. 35 (2), 237-246 (1972).</li><li>Yoon, G., Kim, I., Rho, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Challenges+in+fabrication+towards+realization+of+practical+metamaterials.">Challenges in fabrication towards realization of practical metamaterials.</a> Microelectronic Engineering. 163, 7-20 (2016).</li><li>Zhou, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+Silicon+Metasurfaces+for+Visible+Light.">Efficient Silicon Metasurfaces for Visible Light.</a> ACS Photonics. 4 (3), 544-551 (2017).</li><li>Dammann, H., G&#246;rtler, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-efficiency+in-line+multiple+imaging+by+means+of+multiple+phase+holograms.">High-efficiency in-line multiple imaging by means of multiple phase holograms.</a> Optics Communications. 3 (5), 312-315 (1971).</li></ol>

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 reacti...

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; an...

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 ...

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