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

Summary

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.

Abstract

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).

Introduction

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 manipulation^6,7,8. Optical metasurfaces that have appropriately-designed scatterers can modulate the spectrum, wavefront and polarization of light. Early optical metasurfaces were mainly fabricated using noble meta....

Protocol

1. Device fabrication

NOTE: Figure 1 shows the fabrication process of a-Si:H metasurfaces¹⁷.

1. Prepare a fused silica wafer piece (size = 2 cm x 2 cm, thickness = 500 µm) as a substrate. Rinse the substrate with acetone and isopropyl alcohol (IPA) then blow nitrogen gas over the substrate to dry it.
2. Deposit a 380 nm thick a-Si:H film on the substrate using plasma-enhanced chemical vapor deposition (PECVD) with the following settings: chamber temperature = 300 °C; radio frequency power = 800 W; gas flow rate = 10 sccm for SiH₄ and 75 sccm for ....

Results

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.......

Discussion

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 .......

Materials

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