Mengkun Liu

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.

Silk‐Based Conformal, Adhesive, Edible Food Sensors

An array of passive metamaterial antennas fabricated on all protein-based silk substrates were conformally transferred and adhered to the surface of an apple. This process allows the opportunity for intimate contact of micro- and nanostructures that can probe, and accordingly monitor changes in, their surrounding environment. This provides in situ monitoring of food quality. It is to be noted that this type of sensor consists of all edible and biodegradable components, holding utility and potential relevance for healthcare and food/consumer products and markets.

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]

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.

Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

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.

Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials

Active, widely tunable optical materials have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-devices. Here, we demonstrate that spatially selective defect engineering on the nanometer scale can transform phase-transition materials into optical metasurfaces. Using ion irradiation through nanometer-scale masks, we selectively defect-engineered the insulator-metal transition of vanadium dioxide, a prototypical correlated phase-transition material whose optical properties change dramatically depending on its state. Using this robust technique, we demonstrated several optical metasurfaces, including tunable absorbers with artificially induced phase coexistence and tunable polarizers based on thermally triggered dichroism. Spatially selective nanoscale defect engineering represents a new paradigm for active photonic structures and devices.

Rapid Transfer-Based Micropatterning and Dry Etching of Silk Microstructures

Over the last two decades silk produced by the silkworm Bombyx mori has found new utility as a sustainable material platform for high-technology applications encompassing photonics, electronics and optoelectronics [1-4]. Silk fibers have been used as an FDA approved medical suture material for decades [5] due to their biocompatibility and mechanical properties [6]. These properties, along with the inherent biodegradability of silk, has driven the use of this protein for biological studies [6]. Native silk fibers can be solubilized and reprocessed into an aqueous silk fibroin protein solution [7], which can then be used to generate a multitude of new material formats [5] such as hydrogels [8], foams [9], electrospun mats [10] and sponges [11]. These new forms of silk are finding utility in drug delivery, cell culture and tissue engineering applications. Silk films with excellent optical properties (> 90% transmission in the visible spectrum) [12] are currently being explored for applications in optics and biophotonics [3, 4]. Additionally, the environmentally benign, all-aqueous processing conditions and the chemistry of silk allow bioactive components, such as enzymes to be stabilized in the protein matrix [13].

Cooperative photoinduced metastable phase control in strained manganite films

A major challenge in condensed-matter physics is active control of quantum phases. Dynamic control with pulsed electromagnetic fields can overcome energetic barriers, enabling access to transient or metastable states that are not thermally accessible. Here we demonstrate strain-engineered tuning of La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme photo-susceptibility, where even a single optical pulse can initiate a transition to a long-lived metastable hidden metallic phase. Comprehensive single-shot pulsed excitation measurements demonstrate that the transition is cooperative and ultrafast, requiring a critical absorbed photon density to activate local charge excitations that mediate magnetic-lattice coupling that, in turn, stabilize the metallic phase. These results reveal that strain engineering can tune emergent functionality towards proximal macroscopic states to enable dynamic ultrafast optical phase switching and control.

THz spectroscopy of VO2 epitaxial films: controlling the anisotropic properties through strain engineering

We investigate far-infrared properties of strain engineered vanadium dioxide nanosheets through epitaxial growth on a (100)R TiO2 substrate. The nanosheets exhibit large uniaxial strain leading to highly uniform and oriented cracks along the rutile c-axis. Dramatic anisotropy arises for both the metal-insulator transition temperature, which is different from the structural transition temperature along the cR axis, and the metallic state conductivity. Detailed analysis reveals a Mott-Hubbard like behavior along the rutile cR axis. §contributed equally to this work