Saturday, February 17, 2018

Abstract-Departure from BCS response in photoexcited superconducting NbN films observed by terahertz spectroscopy

M. Šindler, C. Kadlec, P. Kužel, K. Ilin, M. Siegel, and H. Němec

We investigate time-resolved terahertz conductivity of thin superconducting NbN films with various thicknesses upon their excitation by intense femtosecond laser pulses. The recovery dynamics following a complete destruction of the superconducting state occurs via a growth of superconducting islands in the normal-state environment. This is in contrast with previous observations of the recovery upon strong-field terahertz excitation [R. Matsunaga and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012)]. We observe that the density of electronic states in the superconducting islands deviates from the BCS theory predictions on a subnanosecond time scale, while equilibrium terahertz conductivity spectra confirm the standard BCS behavior in the ground state.
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Abstract-Sub-THz Imaging Using Non-Resonant HEMT Detectors

Juan A. Delgado-Notario,  Jesus E. Velazquez-Perez,  Yahya M. Meziani,  Kristel Fobelets,

Plasma waves in gated 2-D systems can be used to efficiently detect THz electromagnetic radiation. Solid-state plasma wave-based sensors can be used as detectors in THz imaging systems. An experimental study of the sub-THz response of II-gate strained-Si Schottky-gated MODFETs (Modulation-doped Field-Effect Transistor) was performed. The response of the strained-Si MODFET has been characterized at two frequencies: 150 and 300 GHz: The DC drain-to-source voltage transducing the THz radiation (photovoltaic mode) of 250-nm gate length transistors exhibited a non-resonant response that agrees with theoretical models and physics-based simulations of the electrical response of the transistor. When imposing a weak source-to-drain current of 5 μA, a substantial increase of the photoresponse was found. This increase is translated into an enhancement of the responsivity by one order of magnitude as compared to the photovoltaic mode, while the NEP (Noise Equivalent Power) is reduced in the subthreshold region. Strained-Si MODFETs demonstrated an excellent performance as detectors in THz imaging

Abstract-Terahertz gyrotrons with quasi-regular cavities

 Ivan V. Osharin,  Ilya V. Bandurkin,  Yury K. Kalynov, Audrey V. Savilov

We describe two ways of the use in high-harmonic gyrotrons of quasi-regular cavities with short irregularities formed by the widening of cavity radius. The first one is a significant improvement of the selectivity of the second-harmonic gyrotrons due to the supression of parasitic near-cutoff fundamental-harmonic waves. The second approach is aimed to solve a typical problem of weakly-relativistic high-harmonics gyrotrons, namely, the use of long cavities ensuring extremely high diffraction Q-factors and a great share of Ohmic losses. We propose a quasi-regular cavity with periodic phase correctors, where a far-from-cutoff axial mode with a decreased diffraction Q-factor is excited in a gyrotron-like regime.

Abstract-Terahertz nonreciprocal isolator based on a magneto-optical microstructure at room temperature

Fei Fan, Chuan-Zhong Xiong, Jie-Rong Chen, and Sheng Jiang Chang

We investigated THz nonreciprocal circularly polarized transmission in thin longitudinally magnetized InSb film, especially focusing on its non-eigen nonreciprocal transmission mechanism at room temperature. Then, based on this effect, we presented a THz isolator for linear polarized waves. The nonreciprocal transmission of the InSb film in this device is converted and enhanced by a pair of orthogonal artificial birefringence gratings. After the optimization, the isolation reaches 24 dB, and the insertion loss is <0.5  dB at room temperature and a low magnetic field.
© 2018 Optical Society of America

Friday, February 16, 2018

Current generated when light hits a material reveals electrons behaving like an elusive particle

Scientists predicted and directly measured electrons in a semimetal. The electrons were behaving like elusive massless particles. Shining a circularly polarized light beam (pink spiral) onto a tantalum-arsenide semimetal (ball-and-stick crystal model) generates an electrical current (green arrow). Remarkably, the direction of the current flow changes by switching the light’s polarization from right-handed to left-handed, proving the handedness of exotic Weyl fermions. Credit: Massachusetts Institute of Technology

A massless particle, a.k.a. Weyl fermion, predicted nearly 100 years ago, has been found in another corner of physics. Electrons in a semimetal can behave like these particles. They are either right-handed or left-handed—they are mirror images like our hands. Theory predicted that Weyl semimetals could produce handedness-dependent electrical current by shining circularly polarized infrared light onto it. Scientists then confirmed and measured this current. Changing from right- to left-handed light switched the direction of the current, meaning they could determine the handedness of these electrons.

The detection of handedness of electrons in a Weyl semimetal opens new experimental possibilities for studying and controlling these elusive massless particles and their quantum weirdness. Their quantum behavior can lead to novel optical phenomena. One example is photocurrents (electrical current induced by ). Another example is detection of photons (quantized packets of light) from the mid-infrared optical spectrum to lower frequencies (terahertz). Infrared detection is vital for night vision and heat imaging. Terahertz detection is useful for package-penetrating devices. In addition, the right- and left-handedness in a semimetal could be used like zeroes and ones in conventional computing. The result? Novel pathways to store and carry data.

An elusive massless particle with charge and spin ½, a.k.a. Weyl fermion, was predicted nearly 100 years ago. It still has not been observed in . However, scientists have predicted and observed electrons in the semimetal tantalum arsenide (TaAs) behaving just like the elusive particle. The  have handedness determined by whether the directions of spin and motion of the particle are parallel or anti-parallel. In other words, the electrons in TaAs make up a novel topological phase called a Weyl semimetal. Therefore, electrons in a Weyl semimetal are the low-energy siblings of Weyl fermions in particle physics. Theory predicted that Weyl semimetals could support significant photocurrents due to the combination of specific symmetry breaking, finite chemical potential, and finite tilts of the Weyl energy spectrum. Recently, a team of scientists from multiple institutions set out to test this theory.
In two publications, the scientists first predicted and then reported the direct optical observation of the induced photocurrent and therefore the handedness of Weyl fermions in the semimetal TaAs. In these experiments, researchers observed for the first time that the photocurrent reaches a maximum value for right circularly polarized light. Switching the light to left circularly polarized minimized the total . These observations will lead to additional experiments, because the theory also suggests that Weyl materials that lack a point of inversion symmetry could be used to develop highly sensitive detectors for mid- and far-infrared light.

Abstract- Lens-assisted quasi-optical THz transmitter employing antenna-integrated triple transit region photodiodes

Vitaly Rymanov,  Peng Lu, Sebastian Dülme,  Andreas Stöhr,

In this paper, a compact lens-assisted quasi-optical THz transmitter, using a 2×2 mm2 planar log-periodic toothed antenna / bow-tie antenna (LPTA/BTA) integrated InP-based waveguide triple transit region (TTR) photodiode chip and an extended highly-resistive silicon (HR-Si) quasi-optical lens, is developed and presented for directive THz beam forming. In order to decrease the optical propagation loss in the passive optical waveguide section (<;1 dB), as well as enable the THz range capability of the integrated TTR-photodiode by optimized impedance matching to the antenna feed and increase the THz beam directivity (>25 dBi) of the developed LPTA/BTA-integrated THz photomixer, different numerical analyses are carried out with respect to the optical waveguide and the RF antenna characteristics, considering the integrated HR-Si quasi-optics. Experimentally, THz operation up to a frequency of about 300 GHz is demonstrated for the fabricated lens-assisted quasi-optical THz transmitters.