Abstract
We study the excitation of electron currents in a transparent cell of sub-millimeter size filled by an atomic gas and illuminated by an intense two-color femtosecond laser pulse. The pulse consists of a strong fundamental component and its second harmonic of low intensity, both circularly polarized. We show that for a sufficiently small 20μm-size interaction volume the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous. This coherent dipole plasma oscillation results in a remarkably efficient conversion of the electron energy into that of radiation emitted in the terahertz frequency domain. Simultaneously, strong quasi-static electric fields of maximal strength Em ≃ 10MV/cm are shown to exist inside the plasma during several hundred femtoseconds after the ionizing two-color laser pulse has gone.
Highlights
Electromagnetic radiation in the terahertz (THz) frequency domain is known to have a considerable potential for utilization both in fundamental experimental physics and in various applications
We show that for a sufficiently small 20 μm-size interaction volume the plasma oscillation excited by asymmetric ionization is almost spatially homogeneous
Optical rectification in crystals [6] provides the currently highest convergence efficiency from optical or infrared (IR) to THz radiation at frequencies ν < 3 THz. Such sources can deliver THz pulses of 10 μJ energy and 1 MV/cm electric field strength. Their main limitations are the relatively narrow spectral width determined by material absorption and the intensity damage threshold of crystals which becomes restrictive at high repetition rates of pump lasers
Summary
Electromagnetic radiation in the terahertz (THz) frequency domain is known to have a considerable potential for utilization both in fundamental experimental physics and in various applications. Such sources can deliver THz pulses of 10 μJ energy and 1 MV/cm electric field strength Their main limitations are the relatively narrow spectral width determined by material absorption and the intensity damage threshold of crystals which becomes restrictive at high repetition rates of pump lasers. Our results show a relatively high energy conversion which appears more efficient for a smaller interaction volume and indicate the presence of strong long-living quasi-static electric fields On this basis, we suggest that the application of small gas-filled cells or thin gas beams where a two-color laser field can induce an almost homogeneous oscillating dipole, may lead to a much higher IR-to-THz energy conversion efficiency than that presently achieved in the regime of filamentation. The longitudinal cell size has a direct effect on the plasma response
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