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Nonlinear Optical Spectrometer Disclosure Number: IPCOM000077882D
Original Publication Date: 1972-Oct-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 35K

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A system has been devised whereby Raman-active excitations can be studied without using Raman scattering techniques.

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Nonlinear Optical Spectrometer

A system has been devised whereby Raman-active excitations can be studied without using Raman scattering techniques.

It is well known that the material excitations which are Raman active such as optical phonons, Landau level excitations plasmons, etc., contribute to the nonlinear optical susceptibility, Chi(3)/[-omega(3), omega(1), Omega(1), - omega(2)], which describes mixing of the form omega(3)=2Omega(1)-omega(2). In particular, when the difference frequency Delta omega=omega(1)-omega(2) is near the frequency omega(0) of such a material excitation, then the latter excitation contributes resonantly to Chi(3)/. The imaginary part of Chi(3) at resonance is proportional to the coherent Raman scattering cross section, which describes the stimulated Raman scattering from such an excitation.

Thus, an alternate method to Raman scattering as a means of studying Raman-active excitations is now available. By irradiating a sample with two laser beams having respective frequencies omega(1) and omega(2) and studying the omega(3)=2omega(1)-omega(2) mixing as a function of Delta omega, the resonant contributions of the sample excitations to Chi(3)/ may be studied. Furthermore, certain large momentum excitations, such as optical phonons far from the center of the Brillouin zone, which are not accessible by ordinary Raman scattering, may contribute resonantly to this type of optical mixing.

A nonlinear optical spectrometer for accomplishing the above-noted alternate to Raman scattering is shown in Fig. 1. To obtain two tunable frequencies omega(1) and omega(2), a single nitrogen-laser NL pumps two tunable dye lasers. This method is chosen for its high-repetition rate, although other tunable coherent sources may be used. The single output beam of the nitrogen laser is split into two beams by beam splitter M1, beam 1 reflecting from M1 through lens L1 to dye cell DC1 of a laser having Brewster angle windows, and beam 2 reflecting from 1008 reflecting mirror M through lens L2 to a dye cell DC2 of a second dye laser. Each dye cell laser has a diffraction grating DG that serves as the end reflector of its respective laser, an intracavity beam-expanding telescope FET for each lase...