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