ABSTRACT
Initially, low-order harmonic generation using relatively long pulses
was aimed to define the conditions of efficient third harmonic
(3H) generation in plasma plumes [1]. The advantages of plasma
harmonics could largely be realizedwith the use of a low-excited and
weakly ionized plasma, because the limiting processes governing the
dynamics of laser frequency conversion would play a minor role in
this case. Attention was drawn to this feature early in the study of
third harmonic generation (THG) in a weakly ionized plasma. An
analysis of low-order (third [2] and fifth [3]) harmonic generation
in such plasmas produced by a laser prepulse on the surface of
a solid target allowed formulating several recommendations with
regard to further advancement toward shorter wavelengths. The 3H
conversion efficiency in that case was≤10−3. Some stronger 3H conversion efficiency was observed in the
spark from the optical breakdown of an atmospheric pressure
gas near a metal surface [4]. The breakdown was produced by
a nanosecond Nd:YAG laser with 250 mJ pulse energy and pulse
duration of 15 ns. The laser that was used for 3H generation had
a pulse duration of 40 ps and energy of 50 mJ. One can note
considerably higher concentration of medium in this case (of the
order of 5 × 1019 cm−3) than moderate ablation plumes used in plasma harmonic experiments (1017-1018 cm−3).
