The expression “Spectroscopy” covers the set of experimental techniques, which allow determining the optical properties of materials. Spectroscopy uses light to probe matter. For many components – atoms, molecules, crystals, etc, from solid state physics to bio-physics – the use of these techniques allows to study the kind of processes which present an optical signature, i.e. which are active in light absorption or emission. This makes it a very efficient means of material characterization.
Temperature is one of the important parameters that one may vary during optical spectroscopic measurements. Most of emission and absorption lines are broadened due to thermal motion: cooling allows to reduce line widths and to discriminate between lines, which are spectrally close to each other. In addition, many transfer processes between energy levels are thermally activated; by elimination of some of them, a temperature variation modifies the spectrum and the intensity of the emitted light and allows to compare and to measure their efficiencies.
At the DON, we dispose of several devices, which combine different sources to perform optical spectroscopy (lamps XBO, HBO, argon ion lasers, excimere lasers etc.), various detection systems (spectrometers equipped with cooled CCD’s or photomultipliers), and different cryogenic systems (on the base of liquid helium or liquid nitrogen). The whole temperature scale from 2K up to room temperature is accessible. Measuring transmission and absorption spectra allows determining the energy of optical transitions in different materials and thus their density of states. Reflection measurements are useful tools in the case of composite materials, deposited on opaque substrates. Photo-luminescence measurements (PL), i.e. emission measurements under optical excitation, may allow revealing the presence of crystal imperfections or impurities at very low concentrations. Last but not least, photo-luminescence excitation (PLE) spectroscopy is complementary to that of absorption spectroscopy and provides information on relaxation processes between states, excited by the light field and those which re-emit visible light.