Abstract
In most applications and research areas the measurements of ultrashort laser pulse shape and duration are very important pulse characterization parameters. For this purpose the intensity autocorrelation functions based on second harmonic generation (SHG) are most widely used. However, the measurements of higher-order intensity autocorrelation functions are significantly more sensitive to the laser pulse shape in comparison to the case when SHG is used. Thus, the third-order and fifth-order correlation functions, unlike second-order ones, are sensitive to the asymmetry of the investigated laser pulse [1]. Moreover, for third-harmonic generation (THG) measurements, unlike SHG, there is no need for the noncentrosymmetric crystals because all materials have nonvanishing coefficients of third-order nonlinear susceptibility. Unfortunately, in isotropic normally dispersive media, such as air and other gases, phase-matching of direct THG usually can not be achieved. However, it was recently demonstrated [2] that in such media THG can be phase-matched through six-wave mixing (SWM) when five fundamental photons are mixed together to create a third harmonic (TH) one at frequency ω′3 = 4ω′1 – ω″1 where ω′1 and ω″1 are the frequencies of fundamental photons taken from different laser beams (see Fig. 1 a). Thus, the variation of time delay τ between two pump pulses gives the autocorrelation signal at TH frequency where I1(t) and I2(t) are the intensities of two pump beams and I3(t) is the intensity of TH signal. Note that by applying this process to pulse autocorrelation measurements one can register the background-free fifth-order autocorrelation function which is a far more sensitive that the third-order one. Therefore, in this paper we present the results of measurements of G(5)(τ) and demonstrate that by using noncollinear SWM one can obtain intensity autocorrelation functions that allow to find the pulse-shape and duration of femtosecond laser pulses.
© 2015 IEEE
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