Coherent active Raman spectroscopy

The application of lasers in optical experimental techniques has led to a rapid development of research into the properties of elementary excitations in solids. In addition to the conventional methods of linear crystal optics, Raman scattering of light (RSL) has become one of the principal research methods, as have its various modifications, such as coherent active Raman spectroscopy and others. 

This uitrasonio-opticai technique (or haif-opticai technique [89]) was aiso a hyphenated technique in terms of energy sources viz. thermai and opticai for phonon and photon production, respectiveiy). Thermai surface phonons restrict practical application of the technique owing to their iow scattering efficiency, which results in overly long data collection times (typicaiiy severai hours for a singie spectrum, even with advanced multipass interferometers). Similar to active Raman spectroscopy, coherent acoustic phonons are assumed to be excited by two narrow-line frequency tunable laser beams at different frequencies or by laser pulses of short duration compared to the acoustic period. 

A new coherent chiral Raman spectroscopy that arises when 3m — a) in Eq. (16) is resonant with the angular frequency of a vibration has been proposed by Koroteev [27]. Known as BioCARS, such a spectroscopy could exclusively probe chiral vibrations that are simultaneously Raman and hyper-Raman active [27, 28]. BioCARS has not yet been observed. 

S.A. Akhmanov, A.F. Bunkin, S.G. Ivanov, N.l. Koroteev, Polarization active Raman spectroscopy and coherent Raman elhpsometry. Sov. Phys. JETP 47, 667 (1978)... 

An interesting variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS) (99). If two laser beams, with angular frequencies CO and CO2 are combined in a material, and if cjj — is close to a Raman active frequency of the material, then radiation at a new frequency CJ3 = 2cJ2 — may be produced. Detection of this radiation can be used to characterize the material. Often one input frequency is fixed and the other frequency, from a tunable laser, varied until matches the Raman frequency. CARS has the capabiHty for measurements in flames, plasmas, and... 

Successful applications of fourth-order coherent Raman scattering are presented. Interface-selective detection of Raman-active vibrations is now definitely possible at buried interfaces. It can be recognized as a Raman spectroscopy with interface selectivity. Vibrational sum-frequency spectroscopy provides an interface-selective IR spectroscopy in which the vibrational coherence is created in the IR resonant transition. The two interface-selective methods are complementary, as has been experienced with Raman and IR spectroscopy in the bulk. 

In impulsive multidimensional (1VD) Raman spectroscopy a sample is excited by a train of N pairs of optical pulses, which prepare a wavepacket of quantum states. This wavepacket is probed by the scattering of the probe pulse. The electronically off-resonant pulses interact with the electronic polarizability, which depends parametrically on the vibrational coordinates (19), and the signal is related to the 2N + I order nonlinear response (18). Seventh-order three-dimensional (3D) coherent Raman scattering, technique has been proposed by Loring and Mukamel (20) and reported in Refs. 12 and 21. Fifth-order two-dimensional (2D) Raman spectroscopy, proposed later by Tanimura and Mukamel (22), had triggered extensive experimental (23-28) and theoretical (13,25,29-38) activity. Raman techniques have been reviewed recently (12,13) and will not be discussed here. 

Momentum also plays a role in ordinary spontaneous Raman spectroscopy. When the pump radiation at 532 nm is passed through a sample, the aE term of Eq. (2) produces scattering and, for the first Stokes case shown in Fig. la, the frequency is i si = where is the Raman-active vibration excited in the sample. It should be noted that there is an exchange between the radiation field and molecule not only of energy but also of momentum, represented by the vector ky. The direction and magnitude of k, are determined by the photon-scattering direction, which is random for this spontaneous event. The result is scattering in all directions so that there is no coherent addition of photon amplitudes, as expressed in the summation /(i si) = C8q The net intensity from this inco-... 

Inverse Raman scattering Inverse Raman scattering (IRS) is a coherent process involving stimulated loss at an anti-Stokes-shifted frequency. The term inverse Raman refers to the fact that, at resonance, the probe radiation is attenuated. In spontaneous Raman spectroscopy, on the other hand radiation at Raman-active frequencies would he generated in the course of the experiment. Inverse Raman scattering (IRS) and stimulated Raman gain (SRG) are closely related. While one involves stimulated gain at an anti-Stokes-shifted frequency, the other involves stimulated loss at a Stokes-shifted frequency. 

Photoacoustic Raman spectroscopy (PARS) Photoacoustic Raman spectroscopy (PARS) is again a nonlinear spectroscopic technique. In this technique, selective population of a given energy state of a system (transitions must involve change in polarizability) is amplified using coherent Raman amplification (also known as stimulated Raman scattering). In this process, it is also important that the frequency difference of the two incident laser beams must be adjusted to equal the frequency of Raman-active transition. 

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