Picosecond inverse Raman spectroscopy and sum‐frequency generation in potassium dihydrogenphosphate crystals

Experiments in Raman amplification spectroscopy monitor the intensity change of one laser beam under the perturbing influence of another in a Raman‐active medium. Accompanying such changes one frequently observes the presence of other, non‐Raman, processes arising from non‐linear susceptibility terms χ (2) and χ (3) , in addition to linear changes caused by species absorption and fluorescence. One such non‐Raman process observed during a Raman study of potassium dihydrogenphosphate (KDP) is sum‐frequency generation. Although the subject of second‐harmonic/sum‐and difference‐frequency generation is well understood, the indirect quantitative measurement of such processes by means of the measured power loss of one or other of the incident beams has remained an area hitherto unexplored. This paper redresses this omission by comparing experimentally the directly measured sum‐frequency power generated in a KDP frequency‐doubling crystal with the power loss of the probe beam transmitted through the crystal. The results, viewed in an overall perspective, are very promising and the experiments demonstrate the value of the pump‐probe technique as a means of investigating non‐linear optical mixing processes, with the added advantage of an experimental set‐up which may be adapted, with the minimum of adjustments, to accommodate simultaneous investigations of several other spectroscopic processes occurring within the same system. These studies provide an interesting and important route for the further understanding and utilization of the non‐linear processes occurring in optical crystals such as KDP.