In EPR spectroscopy, the microwave resonator has 2 functions: it (i) converts microwave power into microwave magnetic fields that excite the transitions between the electron spin states and it (ii) converts the oscillating magnetic flux, whose source is the precessing spin polarization, into a travelling electromagnetic wave that can be measured by the detection electronics. For this purpose, most conventional EPR spectrometers working at <50 GHz use standing wave resonators, whose volume must be ≈ λ3 mw, where λmw is the wavelength of the microwave. If the volume of the available sample is significantly smaller than this, the filling factor and the sensitivity of the spectrometer become small. One approach to boost the sensitivity consists in reducing the size of the resonator by designing it in the form of sub-wavelength resonant structures. The aim of this project is to develop optimized sub-wavelength resonators for specific applications in other projects of this priority program. Depending on the requirements of these individual projects, we will design suitable structures, simulate them using finite element software and manufacture them, using lithographic techniques. Testing will be done in collaboration with the projects that will use the resonators.
Broadband EPR in microresonators
EPR microresonators provide an enormous sensitivity boost for small samples. Another specific property of microresonators,compared to classical cavity resonators is their large bandwidth. This is an important asset for experiments that require short switching times of the exciting microwave field, fast modulation of amplitude, frequency or phase, or irradiation with multiple frequency components and / or short dead times. In the present project, we will develop the experimental techniques that utilize this potential for the detection of broad lines and rapidly decaying signals. In addition, we will also develop related broadband techniques that further increase the overall sensitivity of microresonators by using concurrent data acquisition for all spins in the resonator bandwidth. This additional sensitivity boost also exploits the large bandwidth of microresonators and compensates the sensitivity reduction due to the low quality factor. The techniques developed in this project will be useful for many other projects in this priority programme that use microresonators for sensitive detection of signals from small samples.