Abstract-Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

L.-L. Zhang, W.-M. Wang, D. Wu, R. Zhang, S.-J. Zhang, C.-L. Zhang, Y. Zhang, Z.-M. Sheng, X.-C. Zhang

(Submitted on 19 Oct 2017)

https://arxiv.org/abs/1710.06998

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at ω2/ω1=1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of ω2/ω1=1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.

Abstract-Terahertz emission driven by two-color laser pulses at various frequency ratios

W.-M. Wang, Z.-M. Sheng, Y.-T. Li, Y. Zhang, J. Zhang

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.023844

We present a simulation study of terahertz radiation from a gas driven by two-color laser pulses in a broad range of frequency ratios 

${\omega }_1/{\omega }_0$. Our particle-in-cell simulation results show that there are three series with ${\omega }_1/{\omega }_0=2n,n+1/2,n\pm 1/3$ ($n$ is a positive integer) for high-efficiency and stable radiation generation. The radiation strength basically decreases with the increasing ${\omega }_1$ and scales linearly with the laser wavelength. These rules are broken when ${\omega }_1/{\omega }_0<1$ and much stronger radiation may be generated at any ${\omega }_1/{\omega }_0$. These results can be explained with a model based on gas ionization by two linear-superposition laser fields, rather than a multiwave mixing model.

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Abstract-Terahertz emission from two-plasmon-decay induced transient currents in laser-solid interactions

G.-Q. Liao¹, Y.-T. Li^1,2,a), C. Li¹, S. Mondal³, H. A. Hafez³, M. A. Fareed³, T. Ozaki³, W.-M. Wang^1,2, Z.-M. Sheng^2,4,5,a) and J. Zhang^2,5
 http://scitation.aip.org/content/aip/journal/pop/23/1/10.1063/1.4939605?TRACK=RSS
a) Authors to whom correspondence should be addressed. Electronic addresses:ytli@iphy.ac.cn and zmsheng@sjtu.edu.cn
Phys. Plasmas 23, 013104 (2016); http://dx.doi.org/10.1063/1.4939605

We have studied the generation of terahertz (THz) radiation via the interaction of intense femtosecond laser pulses with solid targets at a small incidence angle. It is found that preplasma with a moderate density gradient can enhance the emission. We also observe saturation of the THz output with the driving laser energy. We find that THz emission is closely related to the 3/2 harmonics of the driving laser. Particle-in-cell simulations indicate that under the present experimental conditions, the THz emission could be attributed to the transientcurrents at the plasma-vacuum interface, mainly formed by the two-plasmon-decay instability.

Abstract-Tunable Circularly Polarized Terahertz Radiation from Magnetized Gas Plasma

W.-M. Wang, P. Gibbon, Z.-M. Sheng, and Y.-T. Li

Phys. Rev. Lett. 114, 253901 – Published 23 June 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.253901

It is shown, by simulation and theory, that circularly or elliptically polarized terahertz radiation can be generated when a static magnetic (B) field is imposed on a gas target along the propagation direction of a two-color laser driver. The radiation frequency is determined by ω2p+ω2c/4−−−−−−−−√+ωc/2, where ωp is the plasma frequency and ωc is the electron cyclotron frequency. With the increase of the B field, the radiation changes from a single-cycle broadband waveform to a continuous narrow-band emission. In high-B-field cases, the radiation strength is proportional to ω2p/ωc. The B field provides a tunability in the radiation frequency, spectrum width, and field strength.

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