Spatial chirp

The spatial and temporal modifications of the laser profile during propagation are calculated for the case of a negatively chirped 42-fs pulse with 5-mJ energy. The gas medium is a 9-mm Ne gas jet with a density of 1.4xl018cm 3. 

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. 

Equations (7)-(9) are well suited for numerical evaluation with arbitrary functions defining the spatial and temporal distributions of the laser pulse. In addition, the system (7)-(9) is rather convenient for analytical treatment. In particular, one can develop further perturbative expansion of (7)-(8) in terms of the fine structure constant a. In the leading order, this yields nonrelativistic formulas which agree with those formerly derived by us in [19]. For unchirped laser signals (i.e., t) = 0) these reduce further to the result of [31] by expanding all quantities in powers of the laser intensity I(r,t). Extensive numerical tests carried out by us for various forms of the chirp pulse en-... 

In the experiments described here, two separate techniques have been used for interferometric characterization of the shocked material s motion frequency domain interferometry (FDI) [69, 80-81] and ultrafast 2-d spatial interferometric microscopy [82-83]. Frequency domain interferometry was used predominantly in our early experiments designed to measure free surface velocity rise times [70-71]. The present workhorse in the chemical reaction studies presented below is ultrafast interferometric microscopy [82], This method can be schematically represented as in Figure 6. A portion of the 800 nm compressed spectrally-modified pulse from the seeded, chirped pulse amplified Ti sapphire laser system (Spectra Physics) was used to perform interferometry. The remainder of this compressed pulse drives the optical parametric amplifier used to generate tunable fs infrared pulses (see below). 

---