M.Sc Thesis

M.Sc StudentSorkin Anton
SubjectUltra Sensitive Miniature Resonator for Electron Spin
Resonance at mm Frequencies
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Aharon Blank
Full Thesis textFull thesis text - English Version


Electron spin resonance (ESR) is a spectroscopic method based on the Zeeman Effect (splitting of spectral lines due to external static magnetic field). The concept behind ESR is quite similar to that of nuclear magnetic resonance (NMR); yet, the difference lies first and foremost in that NMR focuses on the spectra of nuclei (which have magnetic moment through a property called spin) whilst ESR focuses on the spin of unpaired electrons. This means that the experimental requirements are very different:  For example, different time scales (milliseconds in NMR, nanoseconds-microseconds in ESR), operating frequencies (NMR ~60-1000 MHz, ESR ~1-100 GHz), and spin sensitivity (ESR having up to six orders of magnitude greater sensitivity).

ESR has numerous applications in various fields of science, such as imaging oxygen in and around cells, imaging of semiconductor devices, measurement of radiation defects (in bones, teeth etc.), study of chemical reactions and even quantum computing (future use). Some of these applications (especially quantum computing) require very high spin sensitivity, however commercial ESR systems provide very limited sensitivity of ~ 108 spins at best.

In a typical ESR experiment, the sample is subjected to a strong static magnetic field (that is responsible to the Zeeman Effect) and in addition to a time varying magnetic field (commonly in the microwave frequencies), that induces the transitions between the Zeeman levels.  In order to optimize spin sensitivity the coupling of the sample to the microwave signal should be enhanced by means of a slow travelling wave structure (TWS) or a resonator. 

In this work, we designed, produced and tested ultra-sensitive resonators for ESR that are aimed at improving upon the current state-of-the-art in the field in terms of spin sensitivity.  The sensitivity in ESR is proportional to the frequency and quality factor of the resonator, and inversely proportional to the temperature and its volume.  Hence getting high spin sensitivity would necessitate increasing the operating frequency and the quality factor; while decreasing the operational temperature.  However, above ~100 GHz microwave technology is not developed enough in terms of high power sources and low noise amplifiers.  We identified that the optimal sensitivity should be at the W-band (~95 GHz). Thus, there is a need to design a resonator operating at W-band that would be as small as possible and suitable for low temperatures (~4 K) experiments.

It is anticipated that a miniature resonator with typical microwave magnetic field mode size of few mm operating at low temperatures in the W-band range, would have spin sensitivity that approaches single electron spin for 1 hour of signal averaging, for some specific very favorable samples.  Such capabilities would have wide range of applications in science and technology.