Microwave spectroscopy


Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter.

In molecular physics

In the field of molecular physics, microwave spectroscopy is commonly used to probe the rotation of molecules.

In condensed matter physics

In the field of condensed matter physics, microwave spectroscopy is used to detect dynamic phenomena of either charges or spins at GHz frequencies and energy scales in the µeV regime. Matching to these energy scales, microwave spectroscopy on solids is often performed as a function of temperature and/or magnetic field.
Spectroscopy traditionally considers the frequency-dependent response of materials, and in the study of dielectrics microwave spectroscopy often covers a large frequency range. In contrast, for conductive samples as well as for magnetic resonance, experiments at a fixed frequency are common, but frequency-dependent measurements are also possible.

Probing charges in condensed matter physics

For insulating materials, probing charge dynamics with microwaves is a part of dielectric spectroscopy.
Amongst the conductive materials, superconductors are a material class that is often studied with microwave spectroscopy, giving information about penetration depth, energy gap, and quasiparticle dynamics.
Another material class that has been studied using microwave spectroscopy at low temperatures are heavy fermion metals with Drude relaxation rates at GHz frequencies.

Probing spins in condensed matter physics

Microwaves impinging on matter usually interact with charges as well as with spins, with the charge response typically much stronger than the spin response. But in the case of magnetic resonance, spins can be directly probed using microwaves. For paramagnetic materials, this technique is called electron spin resonance and for ferromagnetic materials ferromagnetic resonance.
In the paramagnetic case, such an experiment probes the Zeeman splitting, with a linear relation between the static external magnetic field and the frequency of the probing microwave field. A popular combination, as implemented in commercial X-band ESR spectrometers, is approximately 0.3 T and 10 GHz for a typical material with electron g-factor close to 2.