Laser heated high density fluids probed by coherent anti-Stokes Raman spectroscopy

We describe experiments integrating coherent anti-Stokes Raman spectroscopy (CARS), laser-heating and diamond-anvil cell (DAC) technologies for probing molecular vibrations of transparent molecular fluids at the pressure-temperature conditions of energetic detonation and interiors of giant planets. In these experiments, we use a microfabricated metal toroid and a cw Nd:YLF laser to conductively heat a surrounding transparent sample in a DAC. The laser is operated at a TEM01* mode to match the shape of the toroid and thus produces a uniform heating area. The CARS probe utilizes two pulsed lasers with similar cavity lengths: a commercial narrow-band mode-locked Q-switched Nd:YAG laser and a homebuilt broadband dye laser. A strong CARS signal is then produced from the sample in the central region of laser-heated metal toroid where the two laser pulses spatially and temporally coincide. In this article, we demonstrate that this technique is capable of producing high quality vibrational spectra from nitrogen fluid above 2000 K and 13 GPa, where the application of spontaneous Raman spectroscopy is limited because of intense thermal radiation. (C) 2005 American Institute of Physics.