Ph.D Thesis


Ph.D StudentIshay Yakir
SubjectDesign of Novel Micro Resonators Structures for
Ultra-Sensitive Electron Spin Resonance
DepartmentDepartment of Chemistry
Supervisor PROF. Aharon Blank
Full Thesis textFull thesis text - English Version


Abstract

Electron spin resonance (ESR) is a spectroscopic technique that measures the

properties of electron spins in paramagnetic materials by subjecting them to

a combination of static and Radio Frequency (RF) magnetic fields. The static

field splits the energy levels of the spins and the RF field induces transitions between

these two levels. Subsequently, the response of the paramagnetic material to this

excitation results in the production of secondary RF magnetic fields (i.e., the

ESR signal) that are picked up and processed. ESR has many applications

in a variety of scientific fields, ranging from chemistry, biology, and materials

science, through to physics. One significant drawback of conventional ESR,

however, is its relatively low sensitivity, compared to other spectroscopic

techniques. The sensitivity of ESR is related to the minimal number of

spins it can detect, and conventionally many billions of spins are required to

obtain a measurable signal. This research is focused on developing methods

to improve the sensitivity of ESR for spectroscopy and imaging applications.

Arguably, the most dominant element affecting the sensitivity is the

microwave resonator used to pick up the ESR signal generated by the

electrons’ spins. Accordingly, we managed to improve the resonator’s performance by

focusing on three decisive elements. First, we have developed

methods for the design of optimal surface resonators that enhance spin sensitivity.

 These methods relied on: (1) a deep study of the mutual effect

of surface resonator complex topologies on critical resonator’s properties,

such as resonance frequency, Q-factor and effective volume. (2) Advanced

numerical design techniques that employed a newly developed Method of Moments

(MoM) solver to accurately solve surface resonators having

particular and small geometric features, where strong microwave fields are

generated. Our MoM solver relies on efficient methods

developed to model, solve and analyze surface resonators configurations giving rise to

considerable high condition numbers of the resulting matrix

system. (3) An implementation of efficient genetic algorithm that uses our MoM

solver to find new structures of surface resonators, optimizing existing ones, and to

allow for better future designs. These methods was validated both numerically and

experimentally. The experiments obtained the resonator’s scattering (S11) parameters,

as well as providing direct mappings of the resonator’s magnetic field components

using unique ESR microimaging methods.

Second, we managed to enhance the resonator’s performance by developing advanced

coupling methods to the surface resonators employed in

ESR. We present a significant improvement in coupling capabilities

achieved by new surface resonator configurations, and specify approaches

for enhancing the coupling mechanism based on: (1) a gradual coupling via

conductive/inductive-coupled resonators, (2) by employing Multi Loop-Multi

Gap surface resonators, and (3) by using a pair of microstrip lines as an

alternative coupling machineries.

Third, we employed superconducting surface resonators with high Q-factors that

significantly improved spin sensitivity with respect to their normal conductors

counterparts. The design of these resonators was achieved

thanks to modifications of our MoM solver − adapting it to the physical properties of

superconducting surfaces. Results associated with novel

and ultra-sensitive superconducting surface resonators are presented.