Reconfigurable Metasurfaces
Recently, optical metasurfaces, composed of planar arrays of subwavelength-scale resonators, have emerged as an ultrathin and scalable alternative to conventional free-space optical elements. In particular, dielectric metasurfaces have enabled highly-efficient flat waveplates, beam deflectors, and lenses, by virtue of their low non-radiative losses and precisely engineered Mie-type optical modes. However, metasurfaces typically have fixed properties post-fabrication, limiting their potential applications.
Our group is addressing this issue through the design of reconfigurable all-dielectric metasurfaces that can be tuned via electric field or temperature. For instance, we create metasurface-based dynamic-zoom lenses and tunable-polarization synthesizers which could find use in contemporary imaging or depth-sensing applications.
Electrically-actuated varifocal lens using a liquid-crystal-embedded Semiconductor Metasurface
In one application, our group applies low-loss all-dielectric metasurfaces integrated with liquid crystals (LCs) to realize ultrathin electrically tunable lenses. This is accomplished through the careful design of resonant amorphous silicon (aSi) metasurfaces encapsulated in a nematic LC cell. The former supports localized resonant modes typical of regular semiconductor nanoarrays and the latter behaves as an anisotropic dielectric medium with voltage-dependent orientation angle. By means of the field-dependent LC, the local phase response of the aSi resonators can tuned between 0 and 2 by application of a low (10V) AC voltage across the LC cell, yielding real-time changes the spatial phase profile of scattered light and resulting focal length. These resonant phase modulations are independent from phase accumulations due to the LC cell thickness, therefore enabling varifocal components that rival the tuning and optical-quality of traditional tunable LC-lenses while being only a fraction of the thickness.
Figure 1: Voltage-tunable lenses based on Liquid-crystal-embedded metasurfaces. (a) Schematic of a varifocal LC-metalens. Incident light is focused into a spot with electrically-tunable focal length upon transmission through the LC-embedded metasurface. (b) Metasurface unit cell diagram.
Based on this concept, our group has experimentally demonstrated voltage-actuated varifocal lenses and active bifocal imagers.
Figure 2: Experimental demonstrations of voltage-actuated varifocal tuning and bifocal imaging with a LC-metalens. Scanning electron microscope images of the fabricated varifocal metalens, color coded according to its distinct resonator geometries. (b) Measured on-axis intensity of the metalens, revealing a continuous shift in focal distance with applied voltage. (c-f) Demonstration of experimental electrical-switching between two focal planes using the LC-metalens. Images of a standard resolution target at the two metalens focal planes, shown for ‘off’ (c,d) and ‘on’ (e,f) voltages.
We are currently exploring new design schemes and innovative material platforms to address limitations of numerical aperture, chromatic aberrations, and visible-light absorption in these devices – paving the way towards ultrathin distortion-free red-green-blue (RGB) zoom lenses.
Thermally-Reconfigurable Ge-metasurfaces
Our group also engineers metasurface-based polarization modulators. For instance, we have utilized the high thermo-optic coefficient of germanium (Ge) to demonstrate resonant Ge-metasurface-based polarization state generators that can be controlled by heat. This is accomplished through the design of a Ge-based anisotropic resonant metasurface (ARM) which supports a spectrally-sharp (high-Q) resonance that can be excited by one of the principal linear polarizations of incident light. Enabled by the high thermo-optic coefficient of germanium, the central frequency of the high-Q mode can be adjusted by almost its bandwidth within a 100◦C window. Due to the anisotropic nature of the mode, light that is initially linearly polarized becomes elliptically polarized upon transmission through the ARM, with a polarization state that can be widely tuned by heating the sample.
Figure 3: Thermally-reconfigurable metasurface-based polarization states synthesizer. (a) An SEM image of the fabricated metasurface (b) Experimental temperature-dependent mid-IR transmittance spectra of the metasurface, with Q-factor near 70. (c) Schematic depicting the temperature-tunable polarization conversion of light transmitted through the resonant Ge-metasurface.
Selected Publications
[1] Bosch, M., Shcherbakov, M.R., Won, K., Lee, H.-S., Kim, Y., Shvets, G. (2021). Nano Letters 21(9), 3849–3856
[2] Bosch, M., et al. “Polarization states synthesizer based on a thermo-optic dielectric metasurface.” Journal of Applied Physics 126.7 (2019).
[2] Jung, Minwoo, et al. “Polarimetry using graphene-integrated anisotropic metasurfaces.” ACS Photonics 5.11 (2018): 4283-4288.
[3] Dutta-Gupta, S., et al. Electrical tuning of the polarization state of light using graphene-integrated anisotropic metasurfaces. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2090), 20160061 (2017).