Laser Spectroscopy

The department for Materials Research operates a high resolution laser spectrometer for measurements of solid-state systems modified by ion-beam irradiation and other external influences. This system includes a narrow-band tunable continuous wave dye-laser of high output power for the selective excitation of electronic transitions in optical probes that have usually been doped into the solid state system. Specifically, ions of the rare earth elements are employed as solid state probes, since their transitions are very well defined in energy even in a solid matrix. The probe ions sense the electric field of their closer vicinity via the Stark effect. In turn, the local environment of the solid around the probe ion may imprint its configuration onto the probe's optical properties. We aim at clarifying the interdependence of a probe's local modified environment and its altered optical spectrum.

 Scheme: Neodymium-Yttrium-Vanadate laser

With its high laser output power and the highest reasonably achievable sensitivity of its fluorescence detection path, the laser spectrometer is especially tailored for two photon absorption experiments. The typical two photon setup is shown in the scheme: A green, frequency-doubled Neodymium-Yttrium-Vanadate laser pumps the dye-laser. Its output beam enters the sample chamber through a vacuum window that is tilted to its Brewster angle to minimize loss in power. The beam is then tightly focused into the sample under investigation. The fraction of the beam that is transmitted through the sample is recollimated by another lens and exits the chamber passing a second Brewster window. This exiting beam is thereafter used for wavelength and power measurements. At certain wavelengths that correspond to the spectrum of the rare-earth ion used as probe, the illuminating laser light induces two-photon absorption. The fluorescence light is consequently emitted omnidirectional during deexcitation. For reasons of energy conservation, the fluorescence emitted by a single-photon deexcitation posesses exactly half the wavelength of the illuminating laser light plus some usually very small redshift brought about by radiationless decays. A carefully maximized fraction of the fluorescence light is collimated and separated from non-resonant stray light by passing it through a set of interference filters. The filtered fluorescence is then integrally detected by a sensitive, cooled photomultiplier tube. A plot of fluorescence intensity vs. the wave number of exciting laser light recorded in this way is displayed for the case of trivalent gadolinium ions in gadolinium gallium garnet.