Modified diabolo antennas for broadband enhancement of sub-terahertz oscillating magnetic fields.

Enhancement of the magnetic field components of electromagnetic waves is of particular interest across a wide range of applications such as enhanced magnetic dipole emission and increased sensitivity of magnetic resonance. Diabolo antennas are known to significantly enhance the local intensity of oscillating magnetic fields. Although the enhancement factor is sufficiently high for these purposes, the drawback is a narrow frequency band due to their resonant nature. Here we propose modified diabolo antennas, or nested U-shaped antennas, to expand the working frequency band in the sub-terahertz region. In this study, we investigated the geometrical dependence of nested U-shaped antennas and optimized their shape for practical applications in sub-terahertz electron spin resonance spectroscopy.