Abstract-An air-spaced terahertz metamaterial perfect absorber

Guangwu Duan, Jacob Schalch, Xiaoguang Zhao, Jingdi Zhang, Richard Averitt, Xin Zhang,



https://www.sciencedirect.com/science/article/pii/S0924424717317648

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 (SRR) 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 SRR 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.

Abstract-Ultrafast terahertz spectroscopy study of a Kondo insulating thin-film SmB 6 : Evidence for an emergent surface state

Jingdi Zhang, Jie Yong, Ichiro Takeuchi, Richard L. Greene, and Richard D. Averitt

https://journals.aps.org/prb/accepted/6b07fYbfWfc1334e19c50870d090a278ce468c67d

We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ~T*=20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1me. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20K, indicative of another channel opening up in the low energy electrodynamics. Taken together, these results suggest the onset of a surface state well below the crossover temperature (100K) after long-range coherence of the f-electron Kondo lattice is established.

Abstract-Electromechanically tunable metasurface transmission waveplate at terahertz frequencies

Xiaoguang Zhao, Jacob Schalch, Jingdi Zhang, Huseyin R. Seren, Guangwu Duan, Richard D. Averitt, and Xin Zhang

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-3-303

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$\sim 230\mathrm{GHz}$ 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.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Analysis of the thickness dependence of metamaterial absorbers at terahertz frequencies

Guangwu Duan, Jacob Schalch, Xiaoguang Zhao, Jingdi Zhang, Richard D. Averitt, and Xin Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-3-2242

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 resonator (SRR) 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.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-A three-dimensional all-metal terahertz metamaterial perfect absorber

Meng Wu,   Xiaoguang Zhao,   Jingdi Zhang, Jacob Schalch, Guangwu Duan,

http://aip.scitation.org/doi/abs/10.1063/1.4996897#

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.

Abstract-Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films

Elsa Abreu, Stephanie N. Gilbert Corder, Sun Jin Yun, Siming Wang, Juan Gabriel Ramirez, Kevin West, Jingdi Zhang, Salinporn Kittiwatanakul, Ivan K. Schuller, Jiwei Lu, Stuart A. Wolf, Hyun-Tak Kim, Mengkun Liu, Richard D. Averitt

(Submitted on 19 Jan 2017)

https://arxiv.org/abs/1701.05531

Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the electronic oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in the metallic state of both materials. The low energy Drude response of V2O3 appears more susceptible than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold increase in V2O3 across the phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other complex materials.

Abstract-Terahertz saturable absorption in superconducting metamaterials

George R. Keiser, Jingdi Zhang, Xiaoguang Zhao, Xin Zhang, and Richard D. Averitt
https://www.osapublishing.org/josab/abstract.cfm?uri=josab-33-12-2649

We present a superconducting metamaterial saturable absorber at terahertz frequencies. The metamaterial was designed to have a resonant absorption peak at 0.5 THz for 𝑇=10  K$T=10\mathrm{K}$. The absorber consists of an array of split ring resonators (SRRs) etched from a 100 nm YBa2Cu3O7${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$film. A polyimide spacer layer and gold ground plane are placed above the SRRs using the metamaterial tape concept, creating a reflecting perfect absorber. Increasing either the temperature or incident electric field (𝐸$E$) decreases the superconducting condensate density and corresponding kinetic inductance of the SRRs. This alters the impedance matching in the metamaterial, broadening the resonance and reducing the peak absorption. At low electric fields, the experimental absorption was optimized near 80% at 𝑓=0.47  THz$f=0.47\mathrm{THz}$ for 𝑇=10  K$T=10\mathrm{K}$ and decreased to 20% for 𝑇=70  K$T=70\mathrm{K}$. For 𝐸=40  kV/cm$E=40\mathrm{kV}/\mathrm{cm}$ and 𝑇=10  K$T=10\mathrm{K}$, the peak absorption was 70%, decreasing to 40% at 200 kV/cm, corresponding to a modulation of 43%.

© 2016 Optical Society of America

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Abstract-Voltage-tunable dual-layer terahertz metamaterials

• Xiaoguang Zhao
• , Kebin Fan
• , Jingdi Zhang
• , George R Keiser
• , Guangwu Duan
• , Richard D Averitt
•  & Xin Zhang

http://www.nature.com/articles/micronano201625?WT.feed_name=subjects_physics

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 by ~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.

Abstract-Terahertz Saturable Absorption in Superconducting Metamaterials

George R. Keiser, Jingdi Zhang, Xiaoguang Zhao, Xin Zhang, Richard D. Averitt
https://export.arxiv.org/abs/1601.02496


We present a superconducting metamaterial saturable absorber at terahertz frequencies. The absorber consists of an array of split ring resonators (SRRs) etched from a 100nm YBaCu3O7 (YBCO) film. A polyimide spacer layer and gold ground plane are deposited above the SRRs, creating a reflecting perfect absorber. Increasing either the temperature or incident electric field (E) decreases the superconducting condensate density and corresponding kinetic inductance of the SRRs. This alters the impedance matching in the metamaterial, reducing the peak absorption. At low electric fields, the absorption was optimized near 80% at T=10K and decreased to 20% at T=70K. For E=40kV/cm and T=10K, the peak absorption was 70% decreasing to 40% at 200kV/cm, corresponding to a modulation of 43%.

Abstract-Terahertz radiation-induced sub-cycle field electron emission across a split-gap dipole antenna

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Jingdi Zhang^1,2,a), Xiaoguang Zhao^3,a), Kebin Fan³, Xiaoning Wang³,Gu-Feng Zhang¹, Kun Geng², Xin Zhang^3,b) and Richard D. Averitt^1,2,b)
 HIDE AFFILIATIONS
1 Department of Physics, University of California, San Diego, La Jolla, California 92093, USA
2 Department of Physics, Boston University, Boston, Massachusetts 02215, USA
3 Department of Mechanical Engineering, Boston University, Boston, Massachusetts 02215, USA
a) J. Zhang and X. Zhao contributed equally to this work.
b) Authors to whom correspondence should be addressed. Electronic addresses:xinz@bu.edu and raveritt@ucsd.edu
Appl. Phys. Lett. 107, 231101 (2015); http://dx.doi.org/10.1063/1.4936841
http://scitation.aip.org/content/aip/journal/apl/107/23/10.1063/1.4936841


We use intense terahertz pulses to excite the resonant mode (0.6 THz) of a micro-fabricated dipole antenna with a vacuum gap. The dipole antenna structure enhances the peak amplitude of the in-gap THz electric field by a factor of ∼170. Above an in-gap E-field threshold amplitude of ∼10 MV/cm⁻¹, THz-induced field electron emission is observed as indicated by the field-induced electric current across the dipole antenna gap. Field emission occurs within a fraction of the driving THz period. Our analysis of the current (I) and incident electric field (E) is in agreement with a Millikan-Lauritsen analysis where log (I) exhibits a linear dependence on 1/E. Numerical estimates indicate that the electrons are accelerated to a value of approximately one tenth of the speed of light.