Time-resolved infrared spectral photography

Abstract

An intense broadband continuum light pulse of uniform spectral intensity and short time duration is generated. This continuum pulse is then downconverted in frequency to a region of interest, preferably in the infrared region, by applying it as a pump pulse to a molecular or atomic vapor so as to induce stimulated Raman scattering. The resulting Raman Stokes pulse surprisingly tends to have the same spectral bandwidth, intensity uniformity and time duration as the pump continuum pulse. The downconverted continuum pulse (the Raman Stokes pulse) is then used to probe a sample. The sample converts the uniform spectral intensity distribution of the probe pulse into a nonuniform spectral intensity distribution which contains the absorption spectrum of the sample. This spectrum pulse has the same spectral bandwidth and time duration as the Raman Stokes pulse and is finally upconverted in frequency to a region where the spectrum pulse can be conveniently recorded. This is done with a four-wave Raman mixing process in an alkali metal vapor. The vapor is simultaneously pumped with the spectrum pulse and with a second pumping light pulse (or beam) having a frequency in the vicinity of a suitable resonance line of the vapor so as to induce SERS. In the alkali metal vapor, the resulting Raman Stokes pulse and spectrum pulse beat together with the second pumping light pulse to produce an upconverted pulse at a higher frequency band, which surprisingly also tends to have the same spectral bandwidth, intensity distribution (spectrum) and time duration as the spectrum pulse. The upconverted pulse is then recorded either photographically or photoelectrically with conventional spectrographic apparatus.