Abstract
We study a novel method of quasi-phase-matching for third harmonic generation in a gas cell using the periodic modulation of the gas pressure and thus of the third order nonlinear coefficient in the axial direction created by an ultrasound wave. Using a comprehensive numerical model we describe the quasi-phase matched third harmonic generation of UV (at 266 nm) and VUV pulses (at 133 nm) by using pump pulses at 800 nm and 400 nm, respectively, with pulse energy in the range from 3 mJ to 1 J. In addition, using chirped pump pulses, the generation of sub-20-fs VUV pulses without the necessity for an external chirp compensation is predicted.
Highlights
Third harmonic generation (THG) in gases is a method with one of the simplest setup allowing to generate picosecond and femtosecond pulses in the UV and VUV spectral ranges
In this paper we numerically studied quasi-phase matching of THG in a cell filled with a noble gas in which, by a piezoelectric transducer, an ultrasound wave is generated, changing periodically the gas pressure and the third-order nonlinear coefficient in the axial direction
In this scheme quasi-phase matching is realized by choosing an appropriate ultrasound frequency in the range from 22 kHz to 300 kHz compensating the phase-mismatch between the pump pulse and its third harmonic (TH)
Summary
Third harmonic generation (THG) in gases is a method with one of the simplest setup allowing to generate picosecond and femtosecond pulses in the UV and VUV spectral ranges. For intensities below the ionization threshold THG cannot take place for pump beams focused into a gas cell because the emitted third harmonic (TH) before the focus cancels the one emitted after the focus [1, 2] due to destructive interference. In the case of frequency conversion in solid nonlinear crystals an alternative approach, quasi-phase matching (QPM), is used. It exploits a periodic modulation of the nonlinear susceptibility to correct the linear phase mismatch [18, 19]. For an ultrasound wave-number approximately equal to the module of the linear phase mismatch between the fundamental and the generated TH QPM can be realized, greatly increasing the conversion efficiency. Due to the normal dispersion of the gas the generated VUV pulse at the output is nearly unchirped and does not need any additional chirp compensation stage
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