Abstract-Terahertz spectroscopy from air plasmas created by two-color femtosecond laser pulses: The ALTESSE project

L. Bergé, K. Kaltenecker, S. Engelbrecht, A. Nguyen, S. Skupin, L. Merlat, B. Fischer, B. Zhou, I. Thiele, Peter U. Jepsen,

https://iopscience.iop.org/article/10.1209/0295-5075/126/24001

Terahertz pulses are very popular because of their numerous applications, for example in security. Located between microwaves and optical waves in the electromagnetic spectrum, their spectral domain can now be exploited for molecular spectroscopy using terahertz emission from plasmas formed by femtosecond laser pulses ionizing gases such as air. Downconversion of broadband optical spectra in a plasma produces intense radiation suitable for the detection of suspect materials remotely. The different physical mechanisms involved to create terahertz radiation by laser-matter interaction are reviewed. The new potentialities offered by intense ultrafast lasers allow the acquisition of unique spectral signatures characterizing various materials.

Abstract-Terahertz emission from laser-driven gas plasmas: a plasmonic point of view

I. Thiele, B. Zhou, A. Nguyen, E. Smetanina, R. Nuter, K. J. Kaltenecker, P. González de Alaiza Martínez, J. Déchard, L. Bergé, P. U. Jepsen, and S. Skupin

Fig. 1. Illustrated configurations of THz emission from an ellipsoidal plasma induced by a 2C Gaussian laser pulse (FH in red, SH in purple) with strongly elliptical beam shape propagating along 𝑧$z$. The laser electric field is (a) 𝑦$y$-polarized (along the long axis of the elliptical beam) and (b) 𝑥$x$-polarized (along the short axis of the elliptical beam). The plasma is sketched as a blue tri-axial ellipsoid, and its projections are shown in the respective planes. Experimentally measured forward-emitted THz pulses are presented as white lines, demonstrating a significantly shorter pulse duration for an 𝑥$x$-polarized pulse, which can be attributed to triggering a plasmonic resonance (see Section 3 for details).

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-12-1617

We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gas-plasmas. For two-color femtosecond pump pulses, a plasmonic resonance is shown to broaden the terahertz emission spectra significantly. We identify the resonance as a leaky mode, which contributes to the emission spectra whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. As a direct consequence, such resonances can be controlled by changing the polarization properties of elliptically shaped driving laser pulses. Both experimental results and 3D Maxwell consistent simulations confirm that a significant terahertz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser beam shape as an additional degree of freedom.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz emission from laser-driven gas-plasmas: a plasmonic point of view

I. Thiele, B. Zhou, A. Nguyen, E. Smetanina, R. Nuter, P. González de Alaiza Martínez, K. J. Kaltenecker, J. Déchard, L. Bergé, P. U. Jepsen, S. Skupin

https://arxiv.org/abs/1803.06889

We investigate terahertz generation in two-color laser-induced gas plasmas. Resonances attributed to plasmonic effects are shown to broaden the emission spectra significantly, when electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. We demonstrate that such resonances can be turned on and off by changing the polarization properties of elliptically-shaped driving laser-pulses. Both 3D Maxwell consistent simulations and experimental results confirm that a significant THz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser-beam shape as an additional degree of freedom.

Abstract-Spectral dynamics of THz pulses generated by two-color laser filaments in air: the role of Kerr nonlinearities and pump wavelength

A. Nguyen, P. González de Alaiza Martínez, J. Déchard, I. Thiele, I. Babushkin, S. Skupin,  L. Bergé,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-5-4720

We theoretically and numerically study the influence of both instantaneous and Raman-delayed Kerr nonlinearities as well as a long-wavelength pump in the terahertz (THz) emissions produced by two-color femtosecond filaments in air. Although the Raman-delayed nonlinearity induced by air molecules weakens THz generation, four-wave mixing is found to impact the THz spectra accumulated upon propagation via self-, cross-phase modulations and self-steepening. Besides, using the local current theory, we show that the scaling of laser-to-THz conversion efficiency with the fundamental laser wavelength strongly depends on the relative phase between the two colors, the pulse duration and shape, rendering a universal scaling law impossible. Scaling laws in powers of the pump wavelength may only provide a rough estimate of the increase in the THz yield. We confront these results with comprehensive numerical simulations of strongly focused pulses and of filaments propagating over meter-range distances.

© 2017 Optical Society of America

Full Article  |  PDF Article

Abstract-Spectral dynamics of THz pulses generated by two-color laser filaments in air: The role of Kerr nonlinearities and pump wavelength

A. Nguyen, P. Gonzalez de Alaiza Martinez, J. Dechard, I. Thiele, I. Babushkin, S. Skupin, L. Berge

(Submitted on 17 Nov 2016)

https://arxiv.org/abs/1611.05851

We theoretically and numerically study the influence of both instantaneous and Raman-delayed Kerr nonlinearities as well as a long-wavelength pump in the terahertz (THz) emissions produced by two-color femtosecond filaments in air. Although the Raman-delayed nonlinearity induced by air molecules weakens THz generation, four-wave mixing is found to impact the THz spectra accumulated upon propagation via self-, cross-phase modulations and self-steepening. Besides, using the local current theory, we show that the scaling of laser-to-THz conversion efficiency with the fundamental laser wavelength strongly depends on the relative phase between the two colors, the pulse duration and shape, rendering a universal scaling law impossible. Scaling laws in powers of the pump wavelength may only provide a rough estimate of the increase in the THz yield. We confront these results with comprehensive numerical simulations of strongly focused pulses and of filaments propagating over meter-range distances.