Laser chirp

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. 

Fig. 4. The shift and broadening of the muonium ls-2s line is simulated in (a) and (a) for various values of power Pw, size ar, pulse length r and laser chirp <5...

Adiabatic passage can result in a robust population transfer if one uses adiabatic variations of at least two effective parameters of the total laser fields. They can be the amplitude and the detuning of a single laser (chirping) or the amplitudes of two delayed pulses [stimulated Raman adiabatic passage (STIRAP) see Ref. 69 for a review]. The different eigenenergy surfaces are connected to each other by conical intersections, which are associated with resonances (which can be either zero field resonances or dynamical resonances appearing beyond a threshold of the the field intensities). The positions of these intersections determine the possible sets of paths that link an initial state and the... 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

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-... 

The purpose of this work is to demonstrate that the techniques of quantum control, which were developed originally to study atoms and molecules, can be applied to the solid state. Previous work considered a simple example, the asymmetric double quantum well (ADQW). Results for this system showed that both the wave paeket dynamics and the THz emission can be controlled with simple, experimentally feasible laser pulses. This work extends the previous results to superlattices and chirped superlattices. These systems are considerably more complicated, because their dynamic phase space is much larger. They also have potential applications as solid-state devices, such as ultrafast switches or detectors. 

First, let us consider a selective and complete excitation in a three-level problem by quadratically chirping the laser frequency as shown in Fig. 28 [42] (the field parameter F is the laser frequency oa). The energy separation CO23... 

Figure 29. Complete excitation from 11 > to 2 > by one period of frequency chirping in the case of the three-level model. Upper part-time variation of the population. Middle part-time variation of laser frequency. Bottom part-envelope of the laser pulse. Taken from Ref. [42].

The laser frequency is a combination of three linear chirping ... 

In the above numerical examples the held parameter F is taken to be the laser frequency and the nonadiabatic transition used is the Landau-Zener type of curve-crossing. The periodic chirping method, however, can actually be more... 

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