Wavepackets light beams

A light beam is now being considered that consists of a stream of individual axisymmetric photon wavepackets of narrow linewidth and where the macroscopic breadth of the beam is much larger than the individual photon radius r of Eq. (173). The volume density of the wavepackets is assumed to be uniform in space. Then the mean distance between the centra of the wavepackets becomes... 

As a method to control wavepackets, alternative to the use of ultra-short pulses, I would like to propose use of frequency-modulated light. Since it is very difficult to obtain a well-controlled pulse shape without any chirp, it is even easier to control the frequency by the electro-optic effect and also by appropriate superposition of several continuous-wave tunable laser light beams. 

One of the most important yet simple ideas that ignited excitement about fem-tochemistry is wavepacket interferometry (Salour and Cohen-Tannoudji, 1977 Scherer, et al., 1990, 1991, 1992 Jonas and Fleming, 1995 Weinacht, et al., 1999), an optical form of Ramsey-fringe spectroscopy (Ramsey, 1990). A molecular system is subjected to two identical optical pulses created by splitting one pulse at a beam splitter. The two pulses are called the pump and the probe . The time delay between pump and probe pulses is scanned systematically using an optical delay line. The optical arrangement is very similar to that of a Fourier Transform Spectrometer (Heller, 1990). The difference in the paths traveled by the pump and probe pulses, Ad, before the two pulses are recombined at a second beam splitter corresponds to a time delay, At = Ad/c, where c is the speed of light. 

The idealized wave described by Eq. (3.15a) or (3.15b) continues indefinitely for aU time and all values of y. Any real beam of light must start and stop at some point, and so cannot be described completely by this expression. It can, however, be described by a linear combination of idealized waves with a distribution of frequencies. Such a combination of waves is called a wave group or wavepacket. The details of the distribution function depend on the width and shape of the pulse. This description is essentially the same as using a linear combination of wavefunctions for a localized particle in a box (Sect. 2.2.2) or a harmonic potential well (Sect. 2.2.1 and Chap. 11). 

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