Wednesday, December 17, 2014

Abstract-Trajectory-based analysis of low-energy electrons and photocurrents generated in strong-field ionization




Jinlei Liu, Wenbo Chen, Bin Zhang, Jing Zhao, Jianhua Wu, Jianmin Yuan, and Zengxiu Zhao


The three-dimensional classical-trajectory Monte Carlo method is employed to investigate low-energy photoelectron spectra in above-threshold ionization by strong laser fields. By connecting the tunneling coordinates with the final momentum-energy spectra, we identify the effects of the Coulomb potential on electron trajectories and the final energy spectra. In addition, we verify that the photoelectron spectra, depending on the energy region, can be controlled with a two-color laser pulse by varying the phase delay. The modulations of the electron spectra and yields as well as the generated photocurrents reveal the connections among them, which support our previous work [Phys. Rev. Lett. 109, 243002 (2012)] on terahertz wave generation from two-color laser pulses.
DOI: http://dx.doi.org/10.1103/PhysRevA.90.063420
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  • Published 17 December 2014
  • Received 11 November 2014
©2014 American Physical Society

Tuesday, December 16, 2014

Abstract-Boosting terahertz generation in laser-field ionized gases using a sawtooth wave shape



Broadband ultrashort terahertz (THz) pulses can be produced using plasma generation in a noble gas ionized by femtosecond two-color pulses. Here we demonstrate that, by using multiple-frequency laser pulses, one can obtain a waveform which optimizes the free electron trajectories in such a way that they reach the highest velocity at the electric field extrema. This allows to increase the THz conversion efficiency to the percent level, an unprecedented performance for THz generation in gases. Besides the analytical study of THz generation using a local current model, we perform comprehensive 3D simulations accounting for propagation effects which confirm this prediction. Our results show that THz conversion via tunnel ionization can be greatly improved with well-designed multicolor pulses.

Abstract-Terahertz Spectroscopy of 2, 4 Dinitrotoluene over a Wide Temperature Range (7-245 K)




J. Phys. Chem. A, Just Accepted Manuscript
DOI: 10.1021/jp5052134
Publication Date (Web): December 16, 2014
Copyright © 2014 American Chemical Society


Previous THz spectroscopy of the TNT explosive precursor, 2, 4-dinitrotoluene (DNT), has been restricted to room temperature (apart from one set of data at 11 K). Here, for the first time, we investigate the spectrum as the temperature is systematically varied - from 7 K to 245 K. Many new features appear in the spectrum on cooling below room temperature. As well as the five absorption lines observed previously, we observe five additional lines. In addition, a new room-temperature line at 8.52 THz (281 cm-1) is observed. Six of the lines red-shift with temperature and four of them blue-shift. The blue shift is explained by interplay between intra-molecular and inter-molecular hydrogen bonds. The variation in line width and line intensity with temperature is not systematic, although a conspicuous decrease in line intensity with temperature is observed in all cases. Modeling with hybrid PBE0 and TPSSh functionals helps identify absorption modes.


Abstract-Terahertz magneto-optical spectroscopy of two-dimensional hole and electron systems



N. KamarajuW. PanU. EkenbergD. M. GvozdićS. Boubanga-TombetP. C. UpadhyaJ. RenoA. J. TaylorR. P. Prasankumar

http://arxiv-web3.library.cornell.edu/abs/1412.5058

We have used terahertz (THz) magneto-optical spectroscopy to investigate the cyclotron resonance in high mobility two-dimensional electron and hole systems. Our experiments reveal long-lived (~20 ps) coherent oscillations in the measured signal in the presence of a perpendicular magnetic field. The cyclotron frequency extracted from the oscillations varies linearly with magnetic field for a two-dimensional electron gas (2DEG), as expected. However, we find that the complex non-parabolic valence band structure in a two-dimensional hole gas (2DHG) causes the cyclotron frequency and effective mass to vary nonlinearly with the magnetic field, as verified by multiband Landau level calculations. This is the first time that THz magneto-optical spectroscopy has been used to study 2DHG, and we expect that these results will motivate further studies of these unique 2D nanosystems.

Abstract-One-way absorption of terahertz waves in rod-type and multilayer structures containing polar dielectrics



Andriy E. Serebryannikov, Shunji Nojima, and Ekmel Ozbay

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.235126

One-way absorption can be obtained at terahertz frequencies in low-profile rod-type and multilayer dielectric structures with broken spatial inversion symmetry, which contain either a rod layer or an ultrathin homogeneous layer made of a polar dielectric. Perfect absorption for one of the two opposite incidence directions and perfect reflection for the other one are observed at the edge of the polaritonic gap in a wide range of the incident angle variation, when the thickness of the entire structure is of the order of the incident wavelength. Moreover, this regime appears in a wide frequency range, in which the forward-to-backward absorption contrast is strong. The exploited mechanism is connected with the parameter adjustment that enables the location of the polaritonic gap of the polar dielectric, of which the lossy part of the structure is made, inside the stop band arising due to the periodicity of the lossless part of the structure that is made of a nondispersive dielectric. It also exploits absorption enhancement in the lossy part by backing it with the highly reflecting lossless part, which has alternating stop and pass bands.
DOI: http://dx.doi.org/10.1103/PhysRevB.90.235126
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  • Published 16 December 2014
  • Received 14 May 2014
  • Revised 26 November 2014

©2014 American Physical Society