Kebin Fan

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

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.

A dual band terahertz metamaterial absorber

We present the design, fabrication and characterization of a dual band metamaterial absorber which experimentally shows two distinct absorption peaks of 0.85 at 1.4 THz and 0.94 at 3.0 THz. The dual band absorber consists of a dual band electric-field-coupled (ELC) resonator and a metallic ground plane, separated by an 8 µm dielectric spacer. Fine tuning of the two absorption resonances is achieved by individually adjusting each ELC resonator geometry.

Terahertz metamaterials on free-standing highly-flexible polyimide substrates

We have fabricated resonant terahertz metamaterials on free-standing polyimide substrates. The low-loss polyimide substrates can be as thin as 5.5??m yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. Our results provide a path forward for creating multi-layer non-planar metamaterials at terahertz frequencies.

Metamaterials on Paper as a Sensing Platform

There is increasing interest in the development of cost-effective, practical, portable, and disposable diagnostic devices suited to on-site detection and analysis applications, which hold great promise for global health care,[1,2] environmental monitoring,[3] water and food safety,[4] as well as medical and threat reductions.[5] Lab-on-a-chip (LOC) devices, which scale single or multiple lab processes down to chip format (millimeters to a few square centimeters in size), facilitated by micro- and nanoscale technologies have attracted significant attention because of their small sample volume requirements and excellent portability.[6] Various LOC devices have been designed and fabricated in the past two decades, most of which involve a lithography-based patterning process on a solid or elastomeric substrate, such as glass or plastic, for a variety of functionalities that include sample preparation,[7] microfluidic mixing,[8] biochemical reactions,[9] and analysis.[10]

Flexible metamaterial absorbers for stealth applications at terahertz frequencies

We have wrapped metallic cylinders with strongly absorbing metamaterials. These resonant structures, which are patterned on flexible substrates, smoothly coat the cylinder and give it an electromagnetic response designed to minimize its radar cross section. We compare the normal-incidence, small-beam reflection coefficient with the measurement of the far-field bistatic radar cross section of the sample, using a quasi-planar THz wave with a beam diameter significantly larger than the sample dimensions. In this geometry we demonstrate a near-400-fold reduction of the radar cross section at the design frequency of 0.87 THz. In addition we discuss the effect of finite sample dimensions and the spatial dependence of the reflection spectrum of the metamaterial.

Performance enhancement of terahertz metamaterials on ultrathin substrates for sensing applications

We design, fabricate, and characterize split-ring resonator (SRR) based planar terahertz metamaterials (MMs) on ultrathin silicon nitride substrates for biosensing applications. Proof-of-principle demonstration of increased sensitivity in thin substrate SRR-MMs is shown by detection of doped and undoped protein thin films (silk fibroin) of various thicknesses and by monitoring transmission changes using terahertz time-domain spectroscopy. SRR-MMs fabricated on thin film substrates show significantly better performance than identical SRR-MMs fabricated on bulk silicon substrates paving the way for improved biological and chemical sensing applications.

Microwave and terahertz wave sensing with metamaterials

We have designed, fabricated, and characterized metamaterial enhanced bimaterial cantilever pixels for far-infrared detection. Local heating due to absorption from split ring resonators (SRRs) incorporated directly onto the cantilever pixels leads to mechanical deflection which is readily detected with visible light. Highly responsive pixels have been fabricated for detection at 95 GHz and 693 GHz, demonstrating the frequency agility of our technique. We have obtained single pixel responsivities as high as 16,500 V/W and noise equivalent powers of 10(-8) W/Hz(1/2) with these first-generation devices.

Metamaterial Silk Composites at Terahertz Frequencies

www.MaterialsViews.com C O M Metamaterial Silk Composites at Terahertz Frequencies M U N IC By Hu Tao , Jason J. Amsden , Andrew C. Strikwerda , Kebin Fan , David L. Kaplan , Xin Zhang , * Richard D. Averitt , * and Fiorenzo G. Omenetto * A IO N Silk has been a highly desired and widely used textile since its fi rst appearance in ancient China. [ 1 , 2 ] Glossy and smooth, silk is favored not only by fashion designers, but also tissue engineers because it is mechanically tough but degrades harmlessly inside the body, offering new opportunities as a highly robust and biocompatible material substrate. [ 3 , 4 ] Since silk fi lms are optically transparent, it is possible to create a new collection of optical elements such as lenses and diffractive gratings, by 2D and/or 3D patterning of the silk fi lms. [ 5 , 6 ] Furthermore, silk fi broin has been proven to be a biologically favorable carrier that enables bio-dopants such as enzymes and proteins to maintain their functionality. [ 7 , 8 ] This opens the door to a new class of biophotonic devices that could potentially be implanted into the human body to monitor interactions between specifi c targets and embedded dopants. In addition to manipulating the silk fi lms and embedding appropriate dopants, it is desirable to incorporate resonant electromagnetic structures with the silk fi lms. This would enable hybrid silk-based sensors that couple bio-functionality with an easily measured electromagnetic response that changes in response to the local environment. Metamaterials are resonant sub-wavelength electromagnetic composites typically consisting of highly conducting metals. Importantly, metamaterials provide the means to design and control both the effective electric permittivity ( ε ) and magnetic permeability ( μ ). The power of metamaterials lies in the fact that it is possible to construct materials with a user designed electromagnetic response (often not available with naturally occurring materials) at a precisely controlled target frequency [ 9 ]

Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs.

We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ~20-160 kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.

Frequency tunable terahertz metamaterials using broadside coupled split-ring resonators

We present frequency tunable metamaterial designs at terahertz (THz) frequencies using broadside-coupled split ring resonator (BC-SRR) arrays. Frequency tuning, arising from changes in near field coupling, is obtained by in-plane horizontal or vertical displacements of the two SRR layers. For electrical excitation, the resonance frequency continuously redshifts as a function of displacement. The maximum frequency shift occurs for displacement of half a unit cell, with vertical displacement resulting in a shift of 663 GHz (51% of f0) and horizontal displacement yielding a shift of 270 GHz (20% of f0). We also discuss the significant differences in tuning that arise for electrical excitation in comparison to magnetic excitation of BC-SRRs.