Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to materials characterization and identification. We present a reconfigurable terahertz metasurface quarter-waveplate consisting of electromechanically actuated micro-cantilever arrays. Our anisotropic metasurface enables tunable polarization conversion cantilever actuation. Specifically, voltage-based actuation provides mode selective control of the resonance frequency, enabling real-time tuning of the polarization state of the transmitted light. The polarization tunable metasurface has been fabricated using surface micromachining and characterized using terahertz time domain spectroscopy. We observe a ~230 GHz cantilever actuated frequency shift of the resonance mode, sufficient to modulate the transmitted wave from pure circular polarization to linear polarization. Our CMOS-compatible tunable quarterwaveplate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

with engineered refractive index and impedance, [ 12 ] which has paved the way toward perfect absorbers (PA).Perfect absorption is made possible by simultaneously minimizing the transmission and refl ection of a MM through maximized losses and impedance matching, respectively. [ 13,14 ] Quite recently, resonant PAs have been experimentally demonstrated in various bands of the electromagnetic spectrum with prominent examples in the microwave, [ 15,16 ] THz, [ 17,18 ] infrared, [19][20][21] and visible. [22][23][24][25] Dynamic modulation of MMs has enabled signal modulators, switches, and spatial light modulators. Many modulation and tuning mechanisms have been proposed and applied to control both the strength and resonance frequency of a MM electromagnetic response including optical excitation, [ 7,26,27 ] mechanical actuation, [ 8,9,28 ] thermal or electrical control. [ 6,[29][30][31][32] Many tunable MM perfect absorber schemes have been proposed and some have been demonstrated lately that utilized the aforementioned methods. [33][34][35][36][37][38] In this paper, we present a proof of concept for an optically tunable perfect absorber at THz frequencies with multiple optically modulated absorption bands. We achieved up to 97% and 92% maximum internal absorption with modulation depths of 38% and 91% in the LC and dipole resonance modes, respectively. Design, Fabrication, and Operation PrinciplesOur PA is composed of a planar split ring resonator (SRR) array above a conductive ground plane layer separated with a polyimide dielectric material ( Figure 1 a). Si islands whose conductivity can be tuned via photo-excitation are placed in the capacitive gaps of the SRRs and allow for optical control of the SRR resonance as discussed below (Figure 1 b) (See Supporting Information for the device fabrication). The fi rst generation device presented here has a ∼10 µm active thickness on a 500 µm sapphire substrate.The presence of the ground plane assures negligible transmission. Changes to the SRR dimensions and spacer thickness allow for specifi cation of effective resonant permittivity and permeability providing impedance matching that, with the transmission minimized, results in a large absorption. [ 14,17 ] The effective permittivity arises from the SRR's fundamental resonance mode (the LC mode) that is due to SRR's Development of tunable, dynamic, and broad bandwidth metamaterial designs is a keystone objective for metamaterials research, necessary for the future viability of metamaterial optics and devices across the electromagnetic spectrum. Yet, overcoming the inherently localized, narrow bandwidth, and static response of resonant metamaterials continues to be a challenging endeavor. Resonant perfect absorbers have fl ourished as one of the most promising metamaterial devices with applications ranging from power harvesting to terahertz imaging. Here, an optically modulated resonant perfect absorber is presented. Utilizing photo-excited free carriers in silicon pads placed in the capacitive gaps of split ring reson...

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobility thereby damping the plasmonic response. We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices.

We present our recent progress on a highly flexible tunable perfect absorber at terahertz frequencies. Metamaterial unit cells were patterned on thin GaAs patches, which were fashioned in an array on a 10µm polyimide substrate via semiconductor transfer technique, and the backside of the substrate was coated with gold film as a ground plane. Optical-pump THz-probe reflection measurements show that the absorptivity can be tuned up to 25% at 0.78THz and 40% at 1.75THz through photo-excitation of free carriers in GaAs layers in presence of 800nm pump beam. Our flexible tunable MM perfect absorber has potential applications in energy harvesting, THz modulation and even camouflages coating.

Microfabricated Lamellar grating interferometers (LGI) require fewer components compared to Michelson interferotemeters and offer compact and broadband Fourier transform spectrometers (FTS) with good spectral resolution, high speed and high efficiency. This study presents the fundamental equations that govern the performance and limitations of LGI based FTS systems. Simulations and experiments were conducted to demonstrate and explain the periodic nature of the interferogram envelope due to Talbot image formation. Simulations reveal that the grating period should be chosen large enough to avoid Talbot phase reversal at the expense of mixing of the diffraction orders at the detector. Optimal LGI grating period selection depends on a number of system parameters and requires compromises in spectral resolution and signal-to-bias ratio (SBR) of the interferogram within the spectral range of interest. New analytical equations are derived for spectral resolution and SBR of LGI based FTS systems.