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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Representative Results
  • Discussion
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

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.

Abstract

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–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.

Introduction

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 ....

Protocol

1. Fabrication of the dielectric metasurface

  1. Precleaning of a fused silica substrate
    1. Prepare a double-side polished, fused silica substrate (length: 2 cm; width: 2 cm; thickness: 500 μm).
    2. Immerse the fused silica substrate in 50 mL of acetone and conduct the sonication process for 5 min at 40 kHz.
    3. Immerse the substrate in 50 mL of 2-propanol (IPA) and conduct the sonication process for 5 min at 40 kHz.
    4. Rinse the substrate with the IPA and blow.......

Representative Results

The measurement results show the polarization-independent functionality of the device presented here (Figure 1). Measured beam powers of diffraction orders of m = ± 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’s functionality can be maintained, regardless of polarization states. The diffraction angles are 24° an.......

Discussion

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 °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.......

Acknowledgements

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.

....

Materials

NameCompanyCatalog NumberComments
Plasma enhanced chemical vapor depositionBMR TechnologyHiDep-SC
Electron beam lithographyElionixELS-7800
E-beam evaporation systemKorea Vacuum TechKVE-E4000
Inductively-coupled plasma reactive ion etchingDMS-
Ultrasonic cleanerHondaW-113
E-beam resistMICROCHEM495 PMMA A2
Resist developerMICROCHEMMIBK:IPA=1:3
Conducting polymerShowa denkoE-spacer
Chromium etchantKMGCR-7
AcetoneJ.T. Baker925402
2-propanolJ.T. Baker909502
Chromium evaporation sourceKurt J. LeskerEVMCR35D
Collimated laser diode moduleThorlabsCPS-635wavelength: 635 nm
ND:YAG laserGAM laserGAM-2000wavelength: 532 nm
power meterThorlabsS120VC
CCD CameraINFINITYinfinity2-2M
ND filterThorlabsNCD-50C-4-A
Linear polarizerThorlabsLPVISA100-MP2
LensThorlabsLB1676
IrisThorlabsID25
Circular polarizerEdmund optics88-096
sample holderThorlabsXYFM1
PECVD softwareBMR TechnologyHIDEP

References

  1. Khorasaninejad, M., et al. Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging. Science. 352 (6290), 1190-1194 (2016).
  2. Chen, W. T., et al.

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Metasurface FabricationDielectric MetasurfaceEqual intensity Beam GenerationPECVDPMMAElectron Beam LithographySilicon Photonics

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