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30Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2019
ENHANCED EMISSION OF INTENSE TERAHERTZ WAVES FROM GAS TARGETS DRIVEN BY BICHROMATIC MIDINFRARED LASER PULSES
S.V. Popruzhenko1, V. Tulsky2, M. Baghery3, U. Saalmann3,
1Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia,
e-mail: sergey.popruzhenko@,gmail.com 2Institute of Physics, University of Rostock, Germany, e-mail: tulskyva@„mail.ru Max Planck Institute for the Physics of Complex Systems, Dresden, Germany
e-mail: [email protected]
Recently, a new method for the generation of intense few-cycle electromagnetic pulses in the terahertz (THz) frequency domain which employs ionization of gases by high-power bichromatic laser fields has been suggested and experimentally verified [1]. It was shown that coherent two-color laser pulses induce asymmetric ionization of a gas medium resulting in the excitation of strong photocurrents which emit broadband THz radiation. Within this method, using a superposition of the fundamental and the second harmonic of infrared (IR) lasers, record values of quasi-static electric fields with amplitudes up to 10MV/cm have been achieved [2]. Further increase of the THz intensity and pulse energy is of demand for a number of applied and fundamental research.
It has been shown in a recent experiment [3] that application of circularly polarized (CP) fundamental and second harmonic may lead to a few-fold increase in the emitted THz energy, compared to conventional schemes with linear polarization. This happens because ionization by CP pulses leads to a much larger value of the photoelectron drift momentum, so that even a small symmetry violation introduced by the second harmonic results in the excitation of a strong net photocurrent. An even greater, order of magnitude enhancement in the IR-to-THz conversion efficiency, has been shown when midinfrared (MIR) two-color fields are applied instead of conventionally used 800-nm Ti:Sa pulses [4].
In this work, we suggest combining the advantages of CP and MIR laser pulses to achieve even higher conversion efficiencies and THz intensities. In order to find conditions maximizing the IR-to-THz conversion, we investigate the dynamics and radiation of electron ionization-created plasma within a simple analytic model based on the strong field approximation (SFA). Photoelectron momentum distributions obtained within the SFA are used then as the initial condition for particle-in-cell (PIC) simulations of the collective plasma dynamics. We propose a setup which employs a small gas volume with a size close to that of the emitted THz wavelength and show that, in combination to CP radiation, this leads to the excitation of a highly coherent electron motion in the gas cell, so that all electrons of the target emit almost as a single particle of a high charge. The highly coherent election motion leads to a very fast radiation damping of the plasma oscillations and therefore restricts the duration of a THz pulse almost by that of the driving bichromatic femtosecond laser pulse. As a result, extremely short single-cycle pulses with a broad spectrum located in the domain 0.1-10THz can be generated.
The proposed scheme is currently under experimental verification.
References
[1] K. Y. Kim et al, Opt. Express 15, 4577 (2007); T.-J.Wang et al, Appl. Phys. Lett. 95, 131108 (2009).
[2] T. I. Oh et al, Appl. Phys. Lett. 105, 041103 (2014).
[3] C. Meng et al, Appl. Phys. Lett. 109, 131105 (2016).
[4] M. Clerici et al, Phys. Rev. Lett. 110, 253901 (2013).
[5] V.A. Tulsky et al., Phys. Rev. A 98, 053415 (2018).
