Search

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO2) thin films during a light-induced insulator-to-metal transition (IMT). Time-resolved scanning near-field optical microscopy (Tr-SNOM) is used to study VO2 across a wide parameter space of infrared frequencies, picosecond time scales, and elevated steady-state temperatures with nanoscale spatial resolution. Room temperature, steady-state, phonon enhanced nano-optical contrast reveals preexisting "hidden" disorder. The observed contrast is associated with inequivalent twin domain structures. Upon thermal or optical initiation of the IMT, coexisting metallic and insulating regions are observed. Correlations between the transient and steady-state nano-optical textures reveal that heterogeneous nucleation is partially anchored to twin domain interfaces and grain boundaries. Ultrafast nanoscopic dynamics enable quantification of the growth rate and bound the nucleation rate. Finally, we deterministically anchor photoinduced nucleation to predefined nanoscopic regions by locally enhancing the electric field of pump radiation using nanoantennas and monitor the on-demand emergent metallicity in space and time.

One route to create tunable metamaterials is through integration with "on-demand" dynamic quantum materials, such as vanadium dioxide (VO2). This enables new modalities to create high performance devices for historically challenging applications. Indeed, dynamic materials have often been integrated with metamaterials to imbue artificial structures with some degree of tunability. Conversely, metamaterials can be used to enhance and extend the natural tuning range of dynamic materials. Utilizing a complementary split ring resonator array deposited on a VO2 film, we demonstrate enhanced terahertz transmission modulation upon traversing the insulator-to-metal transition (IMT) at ~340 K. Our complementary metamaterial increases the modulation amplitude of the original VO2 film from 42% to 68.3% at 0.47 THz upon crossing the IMT, corresponding to an enhancement of 62.4%. Moreover, temperature dependent transmission measurements reveal a redshift of the resonant frequency arising from a giant increase of the permittivity of the VO2 film. Maxwell-Garnett effective medium theory was employed to explain the permittivity change upon transitioning through the IMT. Our results highlight that symbiotic integration of metamaterial arrays with quantum materials provides a powerful approach to engineer emergent functionality.

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO

In the past few decades, electromagnetic metamaterial absorbers have attracted tremendous attention due to near unity absorption of incident electromagnetic waves over a desired frequency range determined by the metamaterial inclusions as opposed to the constituent material properties. Importantly, metamaterial absorbers enable numerous potential applications which include wave manipulation, terahertz and infrared imaging, energy harvesting, radiative cooling, and chemical detection. To understand the underlying physics of metamaterial absorbers, various theoretical models have been developed. However, these models are seemingly conceptually unrelated, each yielding a distinct set of equations and conclusions. This paper reviews four prevalent theoretical approaches which include effective medium theory, transmission line modelling, coupled mode theory, and interference theory. We show that each of the four theoretical approaches provides an understanding of metamaterial absorbers from different points-of-view, each with distinct advantages and limitations. Moreover, the four theoretical models are interconnected and we discuss that, quite generally, impedance matching is the crucial condition for perfect absorption.

Terahertz spectroscopy is used to investigate the coupling between metamaterial resonators and the c-axis Josephson plasma resonance in superconducting La1.85Sr0.15CuO4. The redshift of the plasma resonance frequency is indicative of strong coupling.

We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near-unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell coupling is negligible. However, for sufficiently small interlayer spacing ($\stackrel{l}{\ensuremath{\sim}\phantom{{}_{x}}}20\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}\mathrm{m}$), coupling between the metamaterial layer and the ground plane is non-negligible. Therefore, near-field interactions must be taken into account for a full understanding of the electromagnetic response (EMR). Interference theory is often used to model the EMR of MMPAs analytically, in which interlayer coupling between the metamaterial and ground plane is usually neglected, resulting in a predicted blueshift of the absorption peaks in comparison to experiment. To account for near-field coupling, we incorporate correction terms into the analytical interference model by taking into account the effective interlayer capacitance and inductance. This results in good agreement between interference theory and experiment (and full-wave numerical simulations). Our findings demonstrate that interlayer coupling is an important design parameter for ultrathin MMPAs.

Terahertz perfect absorbers represent an essential photonic component for detecting, modulating, and manipulating terahertz radiation. We utilize single-layer H-shaped all-silicon arrays to demonst...

Metamaterial absorbers are typically comprised of a layer of split-ring resonators and a ground plane with a dielectric spacer layer that provides structural support and in which absorbed energy is deposited. We address the question “What happens to the absorption if the spacer layer is removed?” through the design, fabrication, and characterization of a terahertz metamaterial absorber with air as the spacer layer. Reflection based terahertz time-domain spectroscopy was employed to measure the absorption and interference theory was used to interpret the results. The surface current in the gold ground plane and split-ring resonator layer is solely responsible for the absorption in the form of joule heating. In comparison to dielectric spacer layer absorbers, the quality factor is increased by a factor of ∼3. The electric field is highly concentrated in the volume between split-ring resonator layer and the ground plane offering the potential for novel sensing application if materials can be incorporated into this region (e.g. with microfluidics). In the spirit of this possibility, simulations of the absorption have been performed. The variation of the real part of the permittivity of the spacer material results in an absorption peak frequency shift, while a change in the imaginary part affects the quality factor and amplitude. Ultimately, the high quality factor and the absence of the spacer material provide the air-spacer metamaterial absorber with unique advantages for sensing applications.

The efficient modulation and control of ultrafast signals on-chip is of central importance in terahertz (THz) communications and a promising route toward sub-diffraction limit THz spectroscopy. Two-dimensional (2D) materials may provide a platform for these endeavors. We explore this potential, integrating high-quality graphene p-n junctions within two types of planar transmission line circuits to modulate and emit picosecond pulses. In a coplanar stripline geometry, we demonstrate electrical modulation of THz signal transmission by 95%. In a Goubau waveguide geometry, we achieve complete gate-tunable control over THz emission from a photoexcited graphene junction. These studies inform the development of on-chip signal manipulation and highlight prospects for 2D materials in THz applications., Comment: 11 pages, 9 figures, including supplementary materials

We present a single-layer H-shaped all-silicon array exhibiting tunable ultra-broadband terahertz wave absorption. Experiment and simulation reveal near unity absorption at ~1 THz, with a bandwidth of ~913 GHz for ≥90% absorbance. The absorption is optically tunable, exhibiting a resonance frequency blueshift by 420 GHz, while the peak absorbance remains over 99%. The dynamic response upon optical excitation depends upon the penetration depth of the pump beam in silicon, as demonstrated from simulation taking into account the depth dependence of the carrier concentration in the all-silicon metamaterial perfect absorber. The results unveil the tuning mechanisms in optically tunable all-silicon metamaterials and metasurfaces, guiding the development of functional all-dielectric functional terahertz devices, such as switches and modulators.

Manipulating the phase of electromagnetic radiation is of importance for applications ranging from communication to imaging. Here, real-time reconfigurable phase response and group delay of a tunable terahertz metamaterial consisting of dual-layer broadside coupled split-ring resonators is demonstrated. Utilizing electrostatic comb-drive actuators, the metamaterial resonant frequency is tuned by changing the lateral distance between the two layers which modifies the transmission amplitude and phase spectrum. The phase modulation is approximately 180\ifmmode \mathring{}\else \r{}\fi{} in the vicinity of the resonant frequency. In addition, remarkable modulation in the group delay of transmitted pulses (from \ensuremath{-}7 to 3 ps) is evaluated based on the measured frequency response using the convolution method when the lateral distance is changed from 0 to $24\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. A two-port resonator model, derived from coupled-mode theory and supported by finite-element full-wave simulations, reveals the underlying physics of the modulation. Specifically, the coupling factor between the two layers plays a critical role, the tuning of which provides a route for structure design and optimization. The capability of tuning the phase response and group delay enables applications, such as phase compensation and group-delay equalization at terahertz frequencies.

In this paper, we present a polarization insensitive air-spacer triple band absorber. With the fabricated absorber, the three near unity-absorption peaks can be fine controlled with different structure designs, which have a great advantage in variety applications. Also, with air instead of the traditional dielectric layer, the absorber will exhibit an extremely high quality factor (Q) compare with the one in dielectric layer. Moreover, with the air-spacer triple band absorber structure, the coupling between the metamaterials layer and the ground plane is very strong. As a result, the near-field interaction should be considered in the interference theory. With the coupling effect included, the correctional interference theory can well predict both the numerical and experiment results of the metamaterial perfect absorber. Our findings demonstrate that the capacitance increase and the inductance decrease are the causes of the difference between the interference theory and the simulation and experiment results especially for circles metamaterials structures with thin air-spacer in metamaterial perfect absorber.

Metamaterial absorbers typically consist of a metamaterial layer, a dielectric spacer layer, and a metallic ground plane. We have investigated the dependence of the metamaterial absorption maxima on the spacer layer thickness and the reflection coefficient of the metamaterial layer obtained in the absence of the ground plane layer. Specifically, we employ interference theory to obtain an analytical expression for the spacer thickness needed to maximize the absorption at a given frequency. The efficacy of this simple expression is experimentally verified at terahertz frequencies through detailed measurements of the absorption spectra of a series of metamaterials structures with different spacer thicknesses. Using an array of split-ring resonators (SRRs) as the metamaterial layer and SU8 as the spacer material we observe that the absorption peaks redshift as the spacer thickness is increased, in excellent agreement with our analysis. Our findings can be applied to guide metamaterial absorber designs and understand the absorption peak frequency shift of sensors based on metamaterial absorbers.

We present a detailed analysis of the conditions that result in unity absorption in metamaterial absorbers to guide the design and optimization of this important class of functional electromagnetic composites. Multilayer absorbers consisting of a metamaterial layer, dielectric spacer, and ground plane are specifically considered. Using interference theory, the dielectric spacer thickness and resonant frequency for unity absorption can be numerically determined from the functional dependence of the relative phase shift of the total reflection. Further, using transmission line theory in combination with interference theory we obtain analytical expressions for the unity absorption resonance frequency and corresponding spacer layer thickness in terms of the bare resonant frequency of the metamaterial layer and metallic and dielectric losses within the absorber structure. These simple expressions reveal a redshift of the unity absorption frequency with increasing loss that, in turn, necessitates an increase in the thickness of the dielectric spacer. The results of our analysis are experimentally confirmed by performing reflection-based terahertz time-domain spectroscopy on fabricated absorber structures covering a range of dielectric spacer thicknesses with careful control of the loss accomplished through water absorption in a semiporous polyimide dielectric spacer. Our findings can be widely applied to guide the design and optimization of the metamaterial absorbers and sensors.

Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to material characterization and identification. We present a reconfigurable terahertz metasurface quarter-wave plate consisting of electromechanically actuated microcantilever arrays. Our anisotropic metasurface enables tunable polarization conversion through 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 quarter-wave plate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

In this paper, we present a three-dimensional (3D) terahertz metamaterial perfect absorber (MPA) with a high quality factor. The absorption response of the proposed structure is analyzed and optimized using coupled mode theory and numerical simulations. Subsequently, we fabricate the 3D MPA by employing the multi-layer electroplating process and characterize it using terahertz time domain spectroscopy. A quality factor of 20 is verified by the experimental results, which agrees well with the simulation. The high-Q MPA has potential applications in chemical and biological sensing.

We designed, fabricated, and characterized a metamaterial perfect absorber at terahertz (THz) frequency range utilizing air as the dielectric material. Due to the avoidance of the loss usually introduced by the dielectric material, there was a three times improvement of the quality factor. Also, with metamaterials fabricated on a free-standing silicon nitride membrane, high sensitivity can be achieved compared with traditional metamaterials on thick substrate. Moreover, the absence of the dielectric materials presents the special opportunity to use liquid or gas as the dielectric layer for permittivity characterization or sensing and detection purposes

Author(s): Schalch, Jacob S; Post, Kirk; Duan, Guangwu; Zhao, Xiaoguang; Kim, Young Duck; Hone, James; Fogler, Michael M; Zhang, Xin; Basov, Dmitri N; Averitt, Richard D

This paper reports a mechanically tunable terahertz metamaterial based on microscale electrostatic actuators. The device consists of an array of suspended cantilevers, forming LC resonators with underlying capacitive pads. The applied voltage can pull the cantilever downwards to increase the capacitance value in the resonator, and thus, redshift the resonant frequency. In our preliminary experiments, the resonant frequency can be tuned from 1.105 THz to 1.06 THz under a 40 V voltage; at the same time, the transmission amplitude can be modulated by approximately 15 dB at the resonant frequencies. The results enables the implementation of tunable terahertz devices, including modulators, polarizers and waveplates, employing conventional micro-electromechanical structures.

Ordered microporous carbons containing dispersed platinum nanoparticles were fabricated and chosen as suitable models to investigate micro-structure development and hydrogen transport properties of zeolite-templated carbons. X-ray photoelectron spectroscopy analysis revealed that the enhanced heat of adsorption is related to the narrow micro-channels templated from the zeolite and the presence of certain CO groups on the carbon. The lack of a well-defined and intense rotational transition line and the persistent broad H2 recoil spectrum in neutron scattering results suggests a distribution of binding sites. Most interestingly, hydrogen diffusion occurs on two time scales, consisting of a fast liquid-like jump diffusion on the timescale of picoseconds along with an even faster bulk-like diffusion. The liquid-like motion is characterized by a diffusion constant of (2.1 ± 0.3) × 10−8 m2/s with an activation energy of ca. 77 K; both values indicate somewhat lower mobility than similar dynamics of H2 on nanotubes, activated carbon XC-72, or Grafoil, yet greater mobility than that of bulk liquid. These unusual characteristics for hydrogen in carbons are believed to arise from the network of narrow pores in this zeolite-templated image of the zeolite. In fact, the diffusion constants of the templated carbons are extremely similar to those measured for zeolite 13X.

We present a comprehensive investigation of a continuously tunable metamaterial perfect absorber operating at terahertz frequencies. In particular, we investigate a three-layer absorber structure consisting of a layer of split ring resonators and a metallic ground plane, with a central layer consisting of a mechanically tunable air-spaced layer. The absorber was characterized using terahertz time-domain spectroscopy in reflection (at normal incidence) as a function of spacer thickness from 0 to 1000 μm. Our experimental measurements reveal the detailed evolution of the absorption bands as a function of spacing, in excellent agreement with analysis using interference theory and simulation. Our Fabry-Perot-like structure provides an avenue for achieving massive tunability in metamaterial absorber devices.

We present a three-dimensional terahertz metamaterial perfect absorber (MPA) that exhibits a high quality factor and is polarization insensitive. The unit cell is composed of two orthogonally oriented copper stand-up split ring resonators deposited on a copper ground plane with capacitive gaps in free space away from the substrate. Near unity (99.6%) absorption at ∼1.65 THz is experimentally obtained in excellent agreement with simulation results. The quality factor is ∼37, which is quite large for a terahertz MPA because of reduced material losses in the all-metal structure. According to simulation results, the MPA is insensitive to the polarization of the incident wave, and more than 90% absorption can be achieved for angles of incidence up to 60° for both TE and TM polarized incident THz waves.

We investigate the undocumented B meson decay, B{sup +} {yields} {Psi}(2S){omega}K{sup +}. The data were collected with the BaBar detector at the SLAC PEP-II asymmetric-energy e{sup +}e{sup -} collier operating at the {gamma}(4S) resonance, a center-of-mass energy of 10.58 GeV/c{sup 2}. The {gamma}(4S) resonance primarily decays to pairs of B-mesons. The BaBar collaboration at the PEP-II ring was located at the SLAC National Accelerator Laboratory and was designed to study the collisions of positrons and electrons. The e{sup -}e{sup +} pairs collide at asymmetric energies, resulting in a center of mass which is traveling at relativistic speeds. The resulting time dilation allows the decaying particles to travel large distances through the detector before undergoing their rapid decays, a process that occurs in the in the center of mass frame over extremely small distances. As they travel through silicon vertex trackers, a drift chamber, a Cerenkov radiation detector and finally an electromagnetic calorimeter, we measure the charge, energy, momentum, and particle identification in order to reconstruct the decays that have occurred. While all well understood mesons currently fall into the qq model, the quark model has no a priori exclusion of higher configuration states such as qqqq which has led experimentalists and theorists alike to seek evidence supporting the existence of such states. Currently, there are hundreds of known decay modes of the B mesons cataloged by the Particle Data Group, but collectively they only account for approximately 60% of the B branching fraction and it is possible that many more exist.