Tuesday, May 22, 2018

Abstract-THz pulses over 50 millijoules generated from relativistic picosecond laser-plasma interactions

Guoqian Liao, Yutong Li, Hao Liu, Jie Zhang,


Ultrahigh-power terahertz (THz) radiation sources are essential for many applications, such as nonlinear THz physics, THz-wave based compact accelerators, etc. However, until now none of THz sources reported, whether based upon large-scale accelerators or high power lasers, have produced THz pulses with energies above the millijoule (mJ) barrier. Here we report on the efficient generation of low-frequency (<3 THz) THz pulses with unprecedentedly high energies over 50 mJ. The THz radiation is produced by coherent transition radiation of a picosecond laser-accelerated ultra-bright bunch of relativistic electrons from a solid target. Such high energy THz pulses can not only trigger various nonlinear dynamics in matter, but also open up a new research field of relativistic THz optics 

Abstract-High-Tc Josephson Junctions as Quasiclassical THz Detectors

 Irina Gundareva,  Valery Pavlovskiy,  Yuriy Divin,


[100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions (JJs) with resistances Rn = (50-500) Ω and characteristic voltages IcRn(50 K) of 2.5 mV have been fabricated and their potential as classical terahertz detectors has been evaluated. The dc I-V curves of the JJs at temperatures T above 40 K have been found free of hysteresis and demonstrated high nonlinearity with the second derivatives d2V/dI2 up to 107-108 Ω/A at T = 50 K. The response ΔV(I) at the current I corresponding to the maximum of d2V/dI2 has been observed in a power range of more than four orders of magnitude, and ΔV is directly proportional to the incident power P (square-law detection) at low power, then ΔV ~ P1/2 (linear detection) at higher power. The responses ΔV(V) at low voltages V to external radiation with the frequencies from 0.05 to 3 THz have been experimentally found to be proportional to d2V/dI2 (classical detection) at low frequencies and to dV/dI (Josephson detection) at high frequencies with a crossover near the low-frequency limit fl = (0.9 ± 0.3) THz of the ac Josephson effect in our JJs. Numerical simulations of such quasiclassical detector with Rn = 300 Ω at T = 50 K predict the NEP-values of 5·10-15 W/Hz1/2 at the frequencies up to 1 THz.

Monday, May 21, 2018

Abstract-Uncovering the Connection Between Low-Frequency Dynamics and Phase Transformation Phenomena in Molecular Solids

Michael T. Ruggiero, Wei Zhang, Andrew D. Bond, Daniel M. Mittleman, and J. Axel Zeitler

The low-frequency motions of molecules in the condensed phase have been shown to be vital to a large number of physical properties and processes. However, in the case of disordered systems, it is often difficult to elucidate the atomic-level details surrounding these phenomena. In this work, we have performed an extensive experimental and computational study on the molecular solid camphor, which exhibits a rich and complex structure-dynamics relationship, and undergoes an order-disorder transition near ambient conditions. The combination of x-ray diffraction, variable temperature and pressure terahertz time-domain spectroscopy, ab initio molecular dynamics, and periodic density functional theory calculations enables a complete picture of the phase transition to be obtained, inclusive of mechanistic, structural, and thermodynamic phenomena. Additionally, the low-frequency vibrations of a disordered solid are characterized for the first time with atomic-level precision, uncovering a clear link between such motions and the phase transformation. Overall, this combination of methods allows for significant details to be obtained for disordered solids and the associated transformations, providing a framework that can be directly applied for a wide range of similar systems.
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Abstract-Ultrafast Terahertz Conductivity Probes of Topologically Enhanced Surface Transport Driven by Mid-Infrared Laser Pulses in Bi2Se3

The recent discovery of topology-protected charge transport of ultimate thinness on surfaces of three-dimensional topological insulators (TIs) are breaking new ground in fundamental quantum science and transformative technology. Yet a challenge remains on how to isolate and disentangle helical spin transport on the surface from bulk conduction. Here we show that selective midinfrared femtosecond photoexcitation of exclusive intraband electronic transitions at low temperature underpins topological enhancement of terahertz (THz) surface transport in doped Bi2Se3, with no complication from interband excitations or need for controlled doping. The unique, hot electron state is characterized by conserved populations of surface/bulk bands and by frequency-dependent hot carrier cooling times that directly distinguish the faster surface channel than the bulk. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., γBS/γSS3.80 in equilibrium. These behaviors are absent at elevated lattice temperatures and for high pumpphoton frequencies and uences. The selective, mid-infrared-induced THz conductivity provides a new paradigm to characterize TIs and may apply to emerging topological semimetals in order to separate the transport connected with the Weyl nodes from other bulk bands.

Abstract-Dynamic Terahertz Response in the Dirac Semimetal Cd3As2 Induced by Ultrafast Optical Excitation

Wei Lu, Jiwei Ling, Faxian Xiu, Dong Sun,


Hot electron relaxation and coupling in Cd3As2 is studied by employing ultrafast time-domain THz spectroscopy. The excited carriers enhance the absorption of THz and relax in ps-time scales, the cooling rules consistent with mid-IR spectroscopy.
© 2018 The Author(s)

Abstract-Time-domain terahertz gas spectroscopy using hollow-optical-fiber gas cell

Takashi Katagiri,  Takahiro Suzuki, Yuji Matsuura,


For gas analysis systems based on terahertz time-domain spectroscopy (THz-TDS), relatively complex systems have been proposed to enhance the sensitivity to measure small amounts of gas. This paper proposes a simple system comprising a hollow optical fiber with an inner dielectric layer as a low-volume and long-path gas cell THz-TDS-based gas analysis system. The inner dielectric layer of the fiber not only reduces the transmission loss for THz waves, but also protects the metal layer of the fiber from reactive gases. The proposed dielectric-loaded fiber is fabricated by depositing a metal thin film on a polymer film tube. In a gas analysis experiment conducted using the fiber with a THz-TDS system, separating the absorption peaks of NH3 gas and water vapor initially proved to be difficult, although NH3 gas was successfully detected. However, by improving the frequency resolution via Fourier transform and introducing a longer hollow optical fiber, the two peaks were successfully separated and the system sensitivity improved.
© 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)