Electron-electron interactions can render an otherwise conducting material insulating, with the insulator-metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator-metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator-metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

We present frequency tunable metamaterial designs at terahertz (THz) frequencies

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...

This paper presents the design, fabrication, and characterization of a real-time voltage-tunable terahertz metamaterial based on microelectromechanical systems and broadside-coupled split-ring resonators. In our metamaterial, the magnetic and electric interactions between the coupled resonators are modulated by a comb-drive actuator, which provides continuous lateral shifting between the coupled resonators by up to 20 μm. For these strongly coupled split-ring resonators, both a symmetric mode and an anti-symmetric mode are observed. With increasing lateral shift, the electromagnetic interactions between the split-ring resonators weaken, resulting in frequency shifting of the resonant modes. Over the entire lateral shift range, the symmetric mode blueshifts bỹ 60 GHz, and the anti-symmetric mode redshifts by~50 GHz. The amplitude of the transmission at 1.03 THz is modulated by 74%; moreover, a 180°phase shift is achieved at 1.08 THz. Our tunable metamaterial device has myriad potential applications, including terahertz spatial light modulation, phase modulation, and chemical sensing. Furthermore, the scheme that we have implemented can be scaled to operate at other frequencies, thereby enabling a wide range of distinct applications.