An Introduction to the Physics of Classical Brillouin Spectroscopy

CeneraUy the intensity I(q, ffl) of scattered hght per sohd angle and frequency interval is proportional to the space and hme Fourier transform Fr,t A of the autocorrelahon funchon A of the ophcal polarizabihty fluctuahons ai i(f , t) [2] [Eq. (1)], as is expressed by Eq. (2). 

The subscripts s and i denote scattered and incident hght, respectively, and refer to the appropriate ophcal polarizahon directions of the hght waves here [3]. Omitting, for simphcity, the tensor properties of Sa we see the spectral power density Is i(q, cu) to be proportional to the mean square fluctuation component a(q) at frequency co [Eq. (3)]. 

All kinds of excitahons, e.g., phonons, excitons, spin waves, may contribute to a(q). Higher-order processes such as mulh-phonon interachons may be in- 

Assuming a certain elementary excitation characterized by an extensive parameter y/(q,co), a conjugated force F(q, t ), and a susceptibility (q, ) [Eq. 

The relationship may be extended to include several, say s, coupled modes ij/y To obtain the resulting field of generalized forces, one has to add the contributions of these s modes [Eq. (7)]. 

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