This work was financially supported by the National Research Foundation (NRF) grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290) funded by the Ministry of Science and ICT of the Korean government. I.K. acknowledges the NRF Global Ph.D. fellowship (NRF-2016H1A2A1906519) funded by the Ministry of Education of the Korean government.

1. Device fabrication
NOTE: Figure 1 shows the fabrication process of a-Si:H metasurfaces17.
<ol>
	<li>Prepare a fused silica wafer piece (size = 2 cm x 2 cm, thickness = 500 &#181;m) as a substrate. Rinse the substrate with acetone and isopropyl alcohol (IPA) then blow nitrogen gas over the substrate to dry it.</li>
	<li>Deposit a 380 nm thick a-Si:H film on the substrate using plasma-enhanced chemical vapor deposition (PECVD) with the f...

<ol>
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The a-Si:H metasurfaces were fabricated in three major steps: a-Si:H thin film deposition using PECVD, precise EBL, and dry etching. Among these steps, the EBL writing process is the most important. First, the pattern density on metasurfaces is quite high, so the process requires precise control over the electron dose (energy) and scanning parameters such as number of dots per unit area. The development condition should also be chosen carefully. The density of the pattern is very high, so when the development process is ...

Optical metasurfaces composed of sub-wavelength nanostructures have enabled many interesting optical phenomena, including optical cloaking1, negative refraction2, perfect light absorption3, color filtering4, holographic image projection5, and beam manipulation6,7,8. Optical metasurfaces that have appropriately-designed scatterers can modulate the spectrum, wavefront and polarization of light. Early optical metasurfaces were mainly fabricated using noble metals (e.g., Au, Ag) due to their high reflectivity and ease of nanofabrication, but they have high Ohmic losses, so the metasurfaces have low efficiency at short visible wavelengths.
Development of nanofabrication techniques for dielectric materials that have low losses in visible light (e.g., TiO29, GaN10, and a-Si:H11) has enabled realization of highly efficient flat optical devices with optical metasurfaces. These devices have applications in optics and engineering. One intriguing application is optical holography for projective volumetric display and information encryption. Compared to conventional holograms that use spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher resolution of holographic images and larger field of visibility.
Recently, encoding of multiple holographic information in a single-layered metahologram device has been achieved. Examples include metaholograms that are multiplexed in spin12,13, orbital angular momentum14, incident light angle15, and direction16. These efforts have overcome the critical shortcoming of metaholograms, which is a lack of design freedom in a single device. Most conventional metaholograms could only produce single encoded holographic images, but multiplexed device can encode multiple holographic images in real time. Hence, the multiplexed metahologram is a crucial solution platform towards real holographic video display or multifunctional anticounterfeiting holograms.
Reported here are protocols to fabricate spin- and direction-multiplexed all-dielectric visible metaholograms, then to optically characterize them13,16. To encode multiple visual information in a single metasurface device, metaholograms are designed which show two different holographic images when the spin or direction of incident light are changed. To fabricate highly efficient holographic images in a manner comparable with CMOS technology, a-Si:H is used for the metasurfaces and dual magnetic resonances and antiferromagnetic resonances induced inside them are exploited. The fabrication protocol consists of film deposition, electron beam writing, and etching. The fabricated device is characterized using a customized optical setup composed of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).

<table border="0" cellpadding="0" cellspacing="0" style="width:881px;" width="880">
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			<td height="21" style="height:21px;width:327px;">Aceton</td>
			<td style="width:187px;">J.T. Baker</td>
			<td style="width:201px;">925402</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Beam splitter</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">CCM1-BS013/M</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Chromium etchant</td>
			<td style="width:187px;">KMG</td>
			<td style="width:201px;">Cr-7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Chromium evaporation source</td>
			<td style="width:187px;">Kurt J. Lesker</td>
			<td style="width:201px;">EVMCR35D</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Clamp</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">CP175</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Conducting polymer</td>
			<td style="width:187px;">Showa denko</td>
			<td style="width:201px;">E-spacer</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Diode laser</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">CPS635</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">E-beam evaporation system</td>
			<td style="width:187px;">Korea Vacuum Tech</td>
			<td style="width:201px;">KVE-E4000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">E-beam resist</td>
			<td style="width:187px;">Microchem</td>
			<td style="width:201px;">495 PMMA A2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Electron beam lithography</td>
			<td style="width:187px;">Elionix</td>
			<td style="width:201px;">ELS-7800</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Half-wave plate</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">AHWP05M-600</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Inductively-coupled plasma reactive ion etching</td>
			<td style="width:187px;">DMS</td>
			<td style="width:201px;">-</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Iris</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">SM1D12</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Isopropyl alcohol</td>
			<td style="width:187px;">J.T. Baker</td>
			<td style="width:201px;">909502</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Kinematic mirror mount</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">KM100/M</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Lens</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">LB1630</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Lens Mount</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">LMR2/M</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Linear polarizer</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">GTH5-A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Mirror</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">PF10-03-G01</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Neutral density filter</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">NDC-50C-4</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Plasma enhanced chemical vapor deposition</td>
			<td style="width:187px;">BMR Technology</td>
			<td style="width:201px;">HiDep-SC</td>
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			<td height="21" style="height:21px;width:327px;">Post</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">TR75/M</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Post holder</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">PH75E/M</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Quarter-wave plate</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">AQWP10M-580</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Resist developer</td>
			<td style="width:187px;">Microchem</td>
			<td style="width:201px;">MIBK:IPA=1:3</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Rotational mount</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">RSP1/M</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">Scanning electron microscopy</td>
			<td style="width:187px;">Hitachi</td>
			<td style="width:201px;">Regulus8100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">XY translation mount</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">XYF1/M</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">1-inch adapter</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">AD11F</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:327px;">1-inch lens mount</td>
			<td style="width:187px;">Thorlabs</td>
			<td style="width:201px;">CP02/M</td>
			<td></td>
		</tr>
	</tbody>
</table>

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).

The a-Si:H metasurfaces enable high cross-polarization efficiency and can be fabricated using a method (Figure 1) that is compatible with CMOS; this trait may enable scalable fabrication and near-future commercialization. The SEM image shows the fabricated a-Si:H metasurfaces (Figure 2). Furthermore, a-Si:H has a larger refractive index than TiO2 and GaN, so even with low aspect ratio nanostructure of around 4.7, an a-SiH meta-hologram with high diffr...

Watch this Scientific Journal Video about Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms at JoVE.com

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

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 ...

Multifunctional meta surface holograms that can produce multiple holographic images, are key flat optical devices for real life ultra compact holographic display application. To generate a multiplex metal hologram, we use a straightforward approach in which two different holographic images can be rendered, depending on the speed of light, and the propagation traction. To understand their multiplex metal hologram setup, feature demonstration of both the fabrication process, and optical characterization, and measurement steps are equally important.Demonstrating the procedure with the Inki Kim will be Dasol Lee, a PhD student from my laboratory. To characterize the pattern deposition by scanning electron microscopy, drop a conductive polymer onto the substrate, and spin code at 2000 revolutions per minute for one minute. Fix the substrate onto the sample holder with carbon tape, and press the air button to vent the load lock chamber.Place the holder onto the holding rod of the load lock chamber, and press EVAC to evacuate the load lock chamber. To set the stage height and tilting angle, set the Z sensor to eight millimeters, and the T sensor to zero degrees. Press the OPEN button to open the load lock chamber door, and press the holding rod to transfer the holder to the main scanning electron microscope chamber.Pull out the rod, and press the CLOSE button. Check the vacuum state, and press execute to remove carbon, or dust in the electron gun with an instant high voltage. Blink on in the microscope software to turn on the electron gun with an accelerating voltage of five kilovolts, and click beam alignment to adjust the beam alignment to precisely locate the electron beam in the center position.Use a stage controller to locate the beam in the center, and click aperture alignment to adjust the aperture, and stigma alignments to make a circular electron beam. Use a stigma controller to make a stable beam just scan on the same spot, and capture scanning electron images with an appropriate focus and stigmata adjustment. When all of the images have been acquired, click OFF to turn off the electron beam, and click Home to return the stage to it's original position.When the stage is in position, open the door of the main chamber, and push the rod to pick up the sample holder. Press the air button to vent the load lock chamber, and unload the holder. For optical characterization, using a spin multiplexed metal hologram, attach a diode laser module to an adapter that can be plugged into a one inch optical mount, and use a post and a postholder to adjust the height of the diode laser.Use a clamp to fix the position, and use a one inch rotational mount to assemble the half wave plate. Place the plate in front of the laser module to rotate the linearly polarized light, and mount two mirrors onto individual one inch kinematic mounts. To align the direction of initial beam, place an alignments disc in front of the laser, and set the laser height.Adjust the two mirrors so that the beam bends twice at 90 degrees in alternating directions, and position the alignment disc near the second mirror. Rotate the knobs to align the light in the center to adjust the angle of the first mirror, and move the alignment disc far from the second mirror. Then rotate the knobs to align the light in the center to adjust the angle of the second mirror.Repeat the alignment until the light passes through the center of the alignment disc in both places, and place a neutral density filter behind the mirror to control the intensity of light. Place an iris behind the neutral density filter to control the diameter of incident light. Mount a linear polarizer, and a quarter wave plate on its own rotational mount, and place a linear polarizer, and a quarter wave plate behind the iris to make a circularly polarized light.Attach the fabricated metal surface to a plate with a hole. Mount the plate on the XY translation mount for rectangular optics, and adjust the XY translation mount so that light is directed to the pattern in the sample. Place a lens after the metal surface, and adjust the position of the lens to be placed at the focal length.Then place a charge coupled device camera after the lens to capture a hologram image. For optical characterization, using a direction multiplexed meta hologram. Place a beam splitter between the quarter wave plate, and the XY translation mount to split the beam into two directions.Then place another beam splitter between the XY translation mount and the lens. Place two mirrors so that the beam bends twice at 90 degrees and alternating directions, and adjust the beam to be directed into the second beam splitter. Finally, align the light so that the beam radiates the sample correctly in the opposite direction, and place another lens at 90 degrees to the right of the first beam splitter.Then place a charge coupled device camera to capture a hologram image from the opposite direction. The best way to produce clear spin, and direction where to place two holographic images is to precisely aligned multiple optical components. In this scanning electron microscopy image, fabricated hydrogenated amorphous silicon metal surfaces, can be observed.A spin multiplexed metal hologram can switch the projected holographic images by simply flipping the handedness of the incident circularly polarized light. Different metal surfaces can produce different responses, depending on whether the light is circularly polarized to the left or to the right. As a result, depending on the input beam polarization states, the information technology University, and Rho laboratory holographic images can be switched in real time with high fidelity.A direction multiplex meta hologram can switch the projected holographic images by changing the incident light direction. For instance, if the light comes in the forward direction from the substrate side, the holographic Rho lab images can be observed. If the light comes in the backward direction from the meta surface side, the Holographic Information Technology University images, can be visualized.Because the clearness of the predicted images is very sensitive to the polarization, and direction of the instant light, the component alignment is particularly important for producing clear holographic images. We plan to develop an active metal hologram that combines this multiplexed metal hologram with an active materials platform to easily change the project, the hologram images by external stimuli.

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Research

JoVE Journal

Engineering

5.6K visualizações.  Pohang University of Science and Technology (POSTECH). Hologramas multifuncionais de meta-superfície que podem produzir múltiplas imagens holográficas, são dispositivos ópticos planos chave para aplicação de tela holográfica ultra compacta da vida real. Para gerar um holograma de metal multiplex, usamos uma abordagem simples na qual duas imagens holográficas diferentes podem ser renderizadas, dependendo da velocidade da luz e da tração de propagação. Para entender sua configuração de holograma de metal multiplex, a demonstração d...