Abstract-Photo-induced terahertz near-field dynamics of graphene/InAs heterostructures

Ziheng Yao, Vyacheslav Semenenko, Jiawei Zhang, Scott Mills, Xiaoguang Zhao, Xinzhong Chen, Hai Hu, Ryan Mescall, Thomas Ciavatti, Stephen March, Seth R. Bank, Tiger H. Tao, Xin Zhang, Vasili Perebeinos, Qing Dai, Xu Du, and Mengkun Liu

Fig. 1 Schematic of the near field optical-pump THz-probe (n-OPTP) setup, equally capable of performing optical pump near-field THz emission (n-OPTE) experiments. ①: 800 nm, 300 mW pump pulses pass through the ITO then go through the center of, while parallel to, the THz beam ②. Optical pump ① and THz probe ② are focused onto the AFM tip apex using an off-axis parabolic mirror. ③: THz gate (detection) beam. Tip scattered THz signals can be mapped out in the time domain by modifying the arrival time of THz gate beam ③ to the THz photoconductive antenna detector, thus changing the delay between ② and ③ (t1). Changing the delay between ① and ③ (t2) by modifying arrival time of pump pulse ① probes the photo-excited ultrafast dynamics of the sample (see main text), where in this work, t1 is fixed at the peak position of the scattered THz electric field while t2 is varied to yield n-OPTE (with THz probe ② blocked) and n-OPTP (with THz probe ② unblocked) measurements.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-10-13611

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (𝜆/6000$\lambda /6000$), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely “screens” the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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-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-Terahertz Dispersion Characteristics of Super-aligned Multi-walled Carbon Nanotubes and Enhanced Transmission through Subwavelength Apertures

Yue Wang, Guangwu Duan, Liying Zhang, Lihua Ma, Xiaoguang Zhao,  Xin Zhang

https://www.nature.com/articles/s41598-018-20118-5

The terahertz (THz) dielectric properties of super-aligned multi-walled carbon nanotube (MWCNT) films were characterized in the frequency range from 0.1 to 2.5 THz with terahertz time-domain spectroscopy. The refractive index, effective permittivity, and conductivity were retrieved from the measured transmission spectra with THz incident wave polarized parallel and perpendicular to the orientation of carbon nanotubes (CNTs), and a high degree of polarization dependence was observed. The Drude-Lorentz model combined with Maxwell-Garnett effective medium theory was employed to explain the experimental results, revealing an obvious metallic behavior of the MWCNT films. Moreover, rectangular aperture arrays were patterned on the super-aligned MWCNT films with laser-machining techniques, and the transmission measurement demonstrated an extraordinarily enhanced transmission characteristic of the samples with incident wave polarized parallel to the orientation of the CNTs. Surface plasmon polaritons were employed to explain the extraordinarily enhanced transmission with high accuracy, and multi-order Fano profile was applied to model the transmission spectra. A high degree of agreement was exhibited among the experimental, numerical, and theoretical results.

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-Terahertz-driven Luminescence and Colossal Stark Effect in CdSe:CdS Colloidal Quantum Dots

Brandt Pein, Wendi Chang, Harold Young Hwang, Jennifer M Scherer, Igor Coropceanu, Xiaoguang Zhao, Xin Zhang, Vladimir Bulovic, Moungi G. Bawendi, and Keith A. Nelson

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b01837?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Optical properties of colloidal semiconductor quantum dots (QDs), arising from quantum mechanical confinement of charge, present a versatile testbed for the study of how high electric fields affect the electronic structure of nanostructured solids. Studies of quasi-DC electric field modulation of QD properties have been limited by electrostatic breakdown processes under high externally applied electric fields, which have restricted the range of modulation of QD properties. In contrast, here we drive CdSe:CdS core:shell QD films with high-field THz-frequency electromagnetic pulses whose duration is only a few picoseconds. Surprisingly, in response to the THz excitation we observe QD luminescence even in the absence of an external charge source. Our experiments show that QD luminescence is associated with a remarkably high and rapid modulation of the QD bandgap, which changes by more than 0.5 eV (corresponding to 25% of the unperturbed bandgap energy). We show that these colossal energy shifts can be explained by the quantum confined Stark effect even though we are far outside the regime of small field-induced shifts in electronic energy levels. Our results demonstrate a route to extreme modulation of material properties and to a compact, high-bandwidth THz detector that operates at room temperature.

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-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-Broadband extraordinary terahertz transmission through super-aligned carbon nanotubes film

Yue Wang, Xiaoguang Zhao, Guangwu Duan, and Xin Zhang

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-14-15730

We experimentally demonstrate the extraordinary transmission of THz waves through super-aligned multi-walled carbon nanotube (MWCNT) films with one-dimensional arrays of sub-wavelength rectangular gratings in the broad frequency range from 0.2 to 2.5 THz. To achieve this, two kinds of MWCNT films (1 μm and 3 μm in thickness) were fabricated by drawing from a sidewall of super-aligned nanotube arrays synthesized by low pressure chemical vapor deposition. The measured complex refraction index of the film exhibits highly anisotropic transmission of THz waves through the MWCNTs. The anisotropy depends not only on the polarization direction of the THz waves but also on the orientation of the MWCNT gratings. We found that the resonantly extraordinary THz transmission originated from the surface plasmon polaritons supported by periodically patterned carbon nanotube gratings. Our experimental results may provide important insights for emerging THz plasmonic devices based on carbon nanotubes.

© 2016 Optical Society of America

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