|
Large-aperture harmonic generation is rapidly becoming a standard practice in laser-fusion research. At present, harmonic generation is commonly done with bare nonlinear crystals--that is, crystal plates not protected from atmospheric degradation, and not relieved of Fresnel loss, by anti-reflection windows and index-matching fluid. This situation has been dictated by the recently-observed, aperture-dependent nonlinear loss in harmonic generation cells using layers of conventional index-matching fluids (Cargille 5610 and FC104). While the overall cost of using bare crystals can be tolerated at the small apertures (10-15 cm) in use at present, for the 74-cm harmonic crystal arrays to be used on the Nova laser, loss-free and degradation-free performance is required. In this paper, results will be presented from a study of the 0.53-pm and 1.06-pm high-intensity transmission properties of 10 organic fluids: benzonitrile, benzene, xylene, tetrachloroethylene, acetonitrile, decahydronaphtalene, Cargille 5610, FC104, poly-chlorotrifluoroethylene (Halocarbon 56) and poly-bromotrifluoroethylene (BFC57). We present measurements of the threshold for stimulated Raman scattering (SRS) in 8 of the fluids in a standardized 8-cm, longitudinal geometry. In the two polymeric fluids, SRS was not detected at input intensities up to the superbroadening thresholds (43 and 62 GW/cm2, respectively). Suppression of SRS in these two fluids is achieved by fluorination and polymerization of the parent tetrachloroethylene molecule. Results will also be related from tests in the required transverse geometry, using 8-cm aperture, 0.532-pm pulses of up to 3 GW/cm2. The aforementioned loss mechanism in Cargille 5610 is identified to be transverse stimulated Raman scattering. Finally, we describe a successful demonstration of loss-free, damage-free performance at an aperture-intensity product of 25 GW/cm in Halocarbon 56 fluid.
|
