Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Area of Science:

• Spectroscopy
• Electron Paramagnetic Resonance (EPR)
• Microwave Engineering

Background:

• Loop-gap resonator (LGR) technology has been advanced for W-band (94 GHz) applications.
• Traditional Electron Paramagnetic Resonance (EPR) often relies on magnetic field modulation.

Purpose of the Study:

• To adapt LGR technology for W-band EPR.
• To investigate microwave frequency modulation (FM) as an alternative to magnetic field modulation.
• To assess the performance and sensitivity of W-band EPR with FM.

Main Methods:

• Frequency translation techniques were used to adapt Q-band (35 GHz) EPR bridge components to W-band.
• A cylindrical TE011 mode cavity was developed for 100 kHz field modulation compatibility.
• Microwave FM was implemented using a varactor-coupled 59 GHz oscillator, with detection at fundamental and harmonic frequencies.
• A spin-label study was conducted to evaluate sensitivity under overmodulation conditions.

Main Results:

• W-band EPR spectra (absorption and dispersion) were successfully obtained using microwave FM at 50 kHz.
• Second and third harmonic detection (100 and 150 kHz) significantly reduced noise compared to fundamental frequency detection.
• The "transfer of modulation" principle was validated, showing microwave FM transferred to spin modulation at higher harmonics.
• The LGR exhibited a loaded Q-value of 90 and a resonator efficiency parameter of 9.3 G/W.

Conclusions:

• W-band EPR utilizing an LGR is a promising configuration for microwave FM experiments.
• Microwave FM, especially with harmonic detection, offers advantages in noise reduction for W-band EPR.
• The developed system demonstrates the feasibility of advanced modulation techniques in high-frequency EPR.