Spectral Phase Distribution Retrieval through Coherent Control of Harmonic Generation

The temporal intensity distribution of the third harmonic of a Ti:sapphire laser generated in Xe gas is fully reconstructed from its spectral phase and amplitude distributions. The spectral phases are retrieved by cross correlating the fundamental laser frequency field with that of the third harmonic, in a three laser versus one harmonic photon coupling scheme. The third harmonic spectral amplitude distribution is extracted from its field autocorrelation. The measured pulse duration is found to be in agreement with that expected from lowest order perturbation theory both for unstretched and chirped pulses.

Figure 1

Coupling scheme.

Figure 2

Top view (upper panel) and a side view (lower panel) of the experimental setup. $L\mathrm{\text{:}}f=30\mathrm{cm}$ lens. NLM1, NLM2: nonlinear medium (gas cell) 1 and 2. PH: pinhole. $G\mathrm{\text{:}}600\mathrm{\text{lines}}/\mathrm{mm}$ grating SM1, SM2, SM3: spherical mirrors. $F\mathrm{\text{:}}3\mathrm{mm}$-thick BK7 glass filter. TS: translation stage. Inset: spatial interference fringes of the TH recorded for three different delays (from left to right $\lambda /2$, 0, $\lambda /4$) between the two overlapping pulses. The images are captured by a CCD camera at the far field.

Figure 3

IR-TH cross-correlation trace of: an unstretched pulse (a), of a chirped pulse. The inset portions are expanded areas of the trace from $-60$ to $-50\mathrm{fs}$ and 50 to 60 fs depicting the chirp of the stretched pulse (b), Fourier transform spectra (c). In (c): phase distribution of the unstretcehd pulse (filled circles), phase distribution of the chirped pulse (open circles), TH field amplitude determined from the FT of the TH field autocorrelation (filled area), linear fit (straight line), polynomial fit (dashed line). As zero of the $y$ axis in the cross-correlation traces is taken the THG signal produced at long delays for which the pulses are not overlapping and thus no interference occurs. (a.u. stands for arbitrary units.)

Figure 4

Reconstructed unstretched (filled black area) and chirped (filled gray area) pulse. The inset trace is the field distribution of the stretched pulse reconstructed using the row measured phases (prior polynomial fit).