Terahertz field enhancement by bull’s eye antenna for force-detected electron paramagnetic resonance measurements

A bull's eye antenna (BEA) consisting of subwavelength concentric periodic grooves is known to show extraordinary enhancement of electromagnetic waves at its central aperture. In this study, we designed and characterized BEA structures working at terahertz (THz) frequencies to increase the oscillating magnetic field intensity at a sample mounted on the aperture. With dimensional parameters optimized, we show a THz field enhancement by one order, enabling substantial sensitivity improvement in THz force-detected electron paramagnetic resonance measurements.