Metamaterials open up various exotic means to control electromagnetic waves and among them polarization manipulations with metamaterials have attracted intense attention. As of today, static responses of resonators in metamaterials lead to a narrow-band and single-function operation. Extension of the working frequency relies on multilayer metamaterials or different unit cells, which hinder the development of ultra-compact optical systems. In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface. Before the phase transition, VO2 behaves as a semiconductor and the metasurface operates as a quarter-wave plate at 0.468 THz. After the transition to metal phase, the quarter-wave plate operates at 0.502 THz. At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light. This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terahertz meta-devices.

Electric field effect in electronic double layer transistor (EDLT) configuration with ionic liquids as the dielectric materials is a powerful means of exploring various properties in different materials. Here, we demonstrate the modulation of electrical transport properties and extremely high mobility of two-dimensional electron gas at LaAlO3/SrTiO3 (LAO/STO) interface through ionic liquid-assisted electric field effect. With a change of the gate voltages, the depletion of charge carrier and the resultant enhancement of electron mobility up to 19 380 cm(2)/(V s) are realized, leading to quantum oscillations of the conductivity at the LAO/STO interface. The present results suggest that high-mobility oxide interfaces, which exhibit quantum phenomena, could be obtained by ionic liquid-assisted field effect.

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We use ionic liquid-assisted electric field effect to tune the carrier density in an electron-doped cuprate ultrathin film and cause a two-dimensional superconductor-insulator transition (SIT). The low upper critical field in this system allows us to perform magnetic field (B)-induced SIT in the liquid-gated superconducting film. Finite-size scaling analysis indicates that SITs induced both by electric and magnetic field are quantum phase transitions and the transitions are governed by percolation effects -quantum mechanical in the former and classical in the latter case. Compared to the hole-doped cuprates, the SITs in electron-doped system occur at critical sheet resistances (R c ) much lower than the pair quantum resistance R Q =h/(2e) 2 =6.45 kΩ, suggesting the possible existence of fermionic excitations at finite temperature at the insulating phase near SITs. Two-dimensional superconductor-insulator quantum phase transitions in an electron-doped cuprate Supplemental Material

SU-8 has great potential in low cost ultra-thick high aspect ratio MEMS applications. Although a broad range of thickness (from m to mm) can be obtained by spin coating, the works about the sidewall profile and dimension control of SU-8 microstructures have not been published in detail. This paper describes the detailed investigations on the effects of processing parameters such as UV wavelength and exposure dose on dimensional change and sidewall profile of SU-8 microstructures. The optimized processing parameters for SU-8 structures with the thickness from 10 to 360 xm are presented.