Abstract-Terahertz pulse generation by two-color laser fields with circular polarization

C. Tailliez, A. Stathopulos, S. Skupin, D. Buožius, I. Babushkin, V. Vaičaitis, L. Bergé

https://arxiv.org/abs/2007.08191

We study the influence of the polarization states of femtosecond two-color pulses ionizing gases on the emitted terahertz radiation. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and an (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent on the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated to longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors.

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

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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.

Abstract-Boosting Terahertz Generation in Laser-Field Ionized Gases Using a Sawtooth Wave Shape

P. González de Alaiza Martínez, I. Babushkin, L. Bergé, S. Skupin, E. Cabrera-Granado, C. Köhler, U. Morgner, A. Husakou, and J. Herrmann

Phys. Rev. Lett. 114, 183901 – Published 6 May 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.183901

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 acquire the largest drift velocity. This allows us to increase the THz conversion efficiency to 2%, an unprecedented performance for THz generation in gases. In addition to 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.

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

P. González de Alaiza Martínez, I. Babushkin, L. Bergé, S. Skupin, E. Cabrera-Granado, C. Köhler, U. Morgner, A. Husakou, J. Herrmann

(Submitted on 16 Dec 2014)

http://arxiv-web3.library.cornell.edu/abs/1412.4995

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.