Harmonic chirp

The Wigner distributions of harmonics obtained with laser intensity of 1 x 1015 W/cm2 are shown in Fig. 8.3. In this case, the harmonics are negatively chirped at the leading edge of the chirp-free pulses as shown in Fig. 8.3a. So the positively chirped laser pulses can compensate for the negative harmonic chirp, as shown in Fig. 8.3b. 

The layout of the experimental set-up is shown in Figure 8-3. The laser source was a Ti sapphire laser system with chirped pulse amplification, which provided 140 fs pulses at 780 nm and 700 pJ energy at a repetition rate of 1 kHz. The excitation pulses at 390 nm were generated by the second harmonic of the fundamental beam in a 1-nun-thick LiB305 crystal. The pump beam was focused to a spot size of 80 pm and the excitation energy density was between 0.3 and 12 ntJ/crn2 per pulse. Pump-... 

In this section, we present the first experimental evidence of the destructive interference (DI) and the constructive interference (Cl) in a mixed gas of He and Ne, which prove the validity of the method. The observed interference modulation is, as discussed in Sect. 4.2, attributed to the difference between the phases of the intrinsically chirped harmonic pulses from He and Ne, which leads to the novel method for broadband measurement of the harmonic phases and for observing the underlying attosecond electron dynamics. 

A number of new ideas have been put forward recently. A X7 enhancement of the generated second harmonic power was obtained by ultraviolet patterning to produce a chirped periodic nonlinear susceptibility on a 60 pm wide channel waveguide [41 ]. A complete analysis of theory and experiment is given, illustrating the potential of this new technique, very competitive with the contact electrode method developed earlier [42], and first introduced without the chirping and with an enhancement factor of about 1.5 to 2 by the same group [43,44]. 

Control of High Harmonic Generation Processes Using Chirped and Self-Guided Femtosecond Laser Pulses... 

Fig. 8.3. Wigner distribution of harmonics after propagating through a 0.7-mm long neon medium of density 2 x 1018 cm 3. Laser intensity applied is 5 x 101BW/cm2. The laser intensity and laser pulse duration used in four different cases are a 1 x 101BW/cm2, chirp-free 25 fs, b positively chirped 35 fs from a, c 5 x 1015 W/cm2, chirp free 25fs, and d negatively chirped 100fs from c...

Fig. 8.8. Harmonic spectra from Ne driven by femtosecond laser pulses with different laser chirp at the gas jet position of z = —18 mm. The sign of the pulse duration refers to the sign of applied laser chirp...

The analysis of the spatial distribution of high harmonics can provide detailed information on the spatial behavior of harmonics. The differences between the harmonic fields generated with PC and NC pulses of 42 fs can be seen in Fig. 8.11, where we have plotted the near field of the harmonics around H61 as a function of frequency and radial coordinate. The PC case (upper panel in Fig. 8.11) shows a diffuse spectral profile near the axis and a sharper one between 40 pm and 70 pm. On the contrary, the NC pulse produces a clean spectral profile in the compact central region extending up to about 40 pm. The present result demonstrates an optimization of the generation volume for our specific conditions of initial laser chirp. 

---