Stimulated Scatterings

Stimulated scatterings occupy a special place in nonlinear optics. They have been investigated ever since the dawn of nonlinear optics and lasers. Devoting only a section to the discussion of this snbject is not likely to do justice to the voluminous work that has been done in this area. The main objective of this section, however, is to outline the fundamental principles involved in these scattering processes in the context of the wave mixing processes discussed previously. 

In this section, we will discuss three exemplary types of stimulated scattering processes  

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Ruhman S, Joly A G and Nelson K A 1987 Time-resolved observations of coherent molecular vibrational motion and the general occurrence of impulsive stimulated scattering J. Chem. Phys. 86 6563-5... 

The preliminary analysis of the growth rate for a hydrogen condensate agrees with the expectations for stimulated scattering, S.C. Moss et al. to be published... 

The experiments discussed in this book are diverse, but they break down into two broad categories (1) resonant infrared methods in which ultrafast IR pulses are tuned to the wavelength of the vibrational transition and (2) Raman methods (in some instances referred to as impulsive stimulated scattering), in which two visible wavelengths have a difference in frequency equal to the vibrational frequency. In some experiments, infrared and Raman techniques are combined in a single measurement. 

Ruhman S, Kohler B, Joly AG, Nelson KA. Intermolecular vibrational motion in CS2 liquid at 165time-resolved impulsive stimulated scattering. Chem Phys Lett 1987 141 16-24. 

Figure 2 Impulsive stimulated scattering generation of material excitations with crossed beams, (a) Crossed optical beams interfere at the sample and produce a spatially periodic driving force on the material with wavelength A. (b) A probe beam incident upon the spatially periodic material response is partially diffracted, with the diffraction efficiency being proportional to the square of the amplitude of the material response.

Figure 3.16 Apparatus utilized by DeMartini and Ducuing to study vibrational relaxation in n-H2 by the use of stimulated Raman scattering. A g-switched ruby laser produces stimulated Raman scattering in the pressure cell. The non-g-switched laser produces a 500-yusec train of spikes, with peak power below threshold for stimulated scattering, for the probing beam (from ref. 137).

Most stimulated scattering experiments are carried out by overlapping spatially and temporally two laser outputs of appropriately tuned frequencies and wave vectors (tui, kj and (coj, k2) to excite coherent phonons (or other Raman-active modes) of the difference frequency and wave vector (Q = oji — coj, Qo = Itj — k2) [37]. The excitation process can be described by the stimulated scattering equation of motion [9] for a damped, nondispersive vibrational mode. 

The impulsive stimulated scattering (ISS) experiment is a stimulated, time domain analog of spontaneous, frequency domain light-scattering (LS) spec-... 

More variations on these themes could be discussed, but the basic idea should be clear. Through absorption, stimulated emission, and stimulated scattering, it should become possible to exert considerable influence on coherent molecular motion in ground and excited electronic states. 

Brown JM, Slutsky LJ, Nelson KA, Cheng L-T (1989) Single-crystal elastic constants for San Carlos Peridot An apphcation of impulsive stimulated scattering. J Geophys Res B94 9485-9492 Carpenter MA, Salje EKH, Graeme-Barber A (1998) Spontaneous strain as a determinant of thermodynamic properties for phase transitions in minerals. Em J Mineral 10 621-692... 

The detailed dynamics and mechanisms of these parametric processes between the two sets of waves (E, E ) and (Eg, EJ are similar to the two-wave stimulated scattering effects discussed in the following section. Some theoretical discussions have also been presented in reference [26]. 

Fig. 6 Experimental setup for observing o-e wave stimulated scattering. Sample is located at the focal plane of the input lens.

I. C. Khoo, R. R. Michael and P. Y. Yan, "Simultaneous Occurrence of Phase Conjugation and Pulse Compression in Stimulated Scatterings in Liquid Crystal Mesophases," IEEE J. Quant. Electron. QE23, 1344 (1987). 

Anti-Stokes shift Difference in frequency between the pump wave and the generated wave in a stimulated scattering interaction when the generated wave is at a higher frequency than the pump wave. 

Stimulated scattering Nonlinear frequency conversion interaction in which the generated wave has exponential gain and energy is transferred to or from the nonlinear medium. 

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