8 - Techniques: general considerations

Summary

The purpose of spectroscopy as applied to solid state physics is the investigation of the (complex) response as a function of wavevector and energy; here, in the spirit of optical spectroscopy, however, we limit ourselves to the response sampled at the zero wavevector, q=0 limit. Any spectroscopic system contains four major components: a radiation source, the sample or device under test, a detector, and some mechanism to select, to change, and to measure the frequency of the applied electromagnetic radiation. First we deal with the various energy scales of interest. Then we comment on the complex response and the requirements placed on the measured optical parameters. In the following sections we discuss how electromagnetic radiation can be generated, detected, and characterized; finally we give an overview of the experimental principles.

Energy scales

Charge excitations which are examined by optical methods span an enormous spectral range in solids. The single-particle energy scales of common metals such as aluminum – the bandwidth W, the Fermi energy ε_F, together with the plasma frequency ħω_p – all fall into the 1–10 eV energy range, corresponding to the visible and ultraviolet parts of the spectrum of electromagnetic radiation. In band semiconductors like germanium, the bandwidth and the plasma frequency are similar to values which are found in simple metals; the single-particle bandgap ε_g ranges from 10⁻¹ eV to 5 eV as we go from small bandgap semiconductors, such as InSb, to insulators, such as diamond.