Non‐linear optics probes of ultra‐violet energy levels of solids at high pressures

In general, one photon absorption and reflectivity spectroscopies are limited to study of excited states having energies for which pressure cell windows are transparent to the probing photons. This provides a serious barrier to making measurements of excited states having energies greater than about 5.5 eV using uv grade Type II diamond windows, 3.5 eV in common type I diamond, or about 7.5 eV using sapphire windows. Non‐linear optical processes on the other hand, can probe states having energies which are sums and or differences of photon energies in beam illuminating the sample. For example, coherent two photon resonances have been used to seek electronic excited states at greater than 6 eV in hydrogen within a type I diamond anvil cell at 37 GPa.In addition to the above mentioned fundamental gain in energy space which can be probed optically, there are a number of secondary advantages in the multiphoton resonance techniques. These have been used to probe details of the exciton prolariton levels in alka...