Few-cycle laser pulses

As demonstrated for CS2, the pump-and-probe technique with CMI is a promising tool for investigating how a nuclear wavepacket evolves in real time. Considering that few-cycle laser pulses are now becoming available, we are now entering into a new era when it will be possible to see molecules in intense laser fields in real time with highest temporal resolution [34,35]. 

S. DeSilvestri et al., Few-cycle pulses by external compression, in Few-Cycle Laser Pulses Generation and Its Application, ed. by P.X. Kartner. Topics Appl. Phys., vol. 95 (Springer, Berlin, 2004)... 

LA. Walmsley, Chtiracterization of ultrashoit opticed pulses in the few-cycle regime using spectral phase interferometry for direct electric field reconstruction, in Few Cycle Laser Pulse Generation and Its Applications, ed. by F.X. Kartner (Springer, Heidelbeig, 2004), p. 265... 

In order to learn about the true quantum efficiency of photogeneration one therefore has to study the photoinduced charge generation mechanism at faster time scales. Pump probe spectroscopy utilising a few optical-cycle laser pulses (5-6 fs) in the visible spectral range with broadband frequency conversion techniques [89] now makes it possible to study extremely fast optically-initiated events with unprecedented time resolution. Such a setup was used to time-resolve the kinetics of the charge transfer process from a polymer chain to a fullerene moiety in thin films of poly[2-methoxy, 5-(3, 7 -dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) and [6,6]-phenyl C6i butyric acid methyl ester (PCBM). Solutions prepared from 1 wt% solutions of toluene on thin quartz substrates were studied. 

Two scenarios for H-bond dynamics will be addressed below First, IR laser control of HT between different tautomers in the electronic ground state second, H-bond breaking by combining IR pre-excitation with a second UV pulse which switches the electronic state. In both cases short few-cycle IR pulses will play an important role. 

An approach to achieving H-bond fragmentation in a selective manner is shown in Fig. 4.8(b). It consists of using a few-cycle IR pulse followed by a UV laser... 

The study confirmed that the action of the precursors of the main pulse has to be carefully considered in the ultrashort intense laser pulse interactions. Nevertheless, a regime of rather stable propagation [34] of a laser pulse of tens of femtoseconds was found in a broad window of the laser intensity/medium density diagram, in which (1) the ionization occurs in one or a few optical cycles, avoiding effects of self-phase modulation and defocusing for most of the... 

In the results discussed so far, the external laser field has been approximated by a monochromatic plane wave. This is a reasonable approximation for pulses with a length down to, say, eight cycles which covers most NSDI experiments. Recently, however, laser pulses of only a few cycles length have become the tools of choice in laser-atom physics [44],... 

In conclusion, it is worth reiterating that the anomalous absorption effects described here may be manifest in any experiments that employ sufficiently high-intensity broadband radiation. To this extent, anomalies may be observable in experiments not specifically involving USES light. In particular, the continued advances in techniques of laser pulse compression have now resulted in the production of femtosecond pulses only a few optical cycles in duration (Knox et al. 1985 Brito Cruz et al. 1987 Fork et al. 1987) which necessarily have a very broad frequency spread, as the time/energy uncertainty principle shows. Thus, mean-frequency absorption may have a wider role to play in the absorption of femtosecond pulses. If this is correct, it raises further questions over the suitablity of absorption-based techniques for their characterization. 

Figure4.11 Selective H-bond breakingin FHF by ph otodetac h m ent of the anionusing few-cycle IR+UV laser pulses, (a)-(d) are sequential snapshots ofthe wave packet moving in the bound anionic and repulsive neutral...

Figure 4.13 Comparison of traditional cw UVor IR-rUV laser pulses (a), versus few-cycle IR-tUV laser pulses in an oriented ensemble of FHF" molecules (b) or a randomly oriented ensemble (c).

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