Micro-resonator Probe for THz Near-field Imaging Beyond the Diffraction Limit

Terahertz (THz) device research and studies of THz phenomena in solid state systems require detection of THz waves and signals on the scale of few microns. These measurements present a major technological problem caused by diffraction of THz waves. The diffraction limit prevents the use of the recently developed THz spectroscopic instrumentation for studies of objects smaller than approximately a wavelength. Near-field surface probing methods have shown potential solutions in overcoming the diffraction limit. However all the existing THz near-field techniques exhibit another fundamental limitation due to significant perturbations in the electric field caused by the near-field probe. The probe invasiveness and a non-uniform frequency response across the THz spectrum prevent the use of the existing near-field probes for mapping of electric field distribution in THz devices. In addition, THz near-field imaging systems with spatial resolution better than ~1/20 of a wavelength suffer from a severe reduction in sensitivity.To mitigate these problems and to allow high spatial resolution studies with THz waves we propose to develop a THz imaging and spectroscopy system with a novel near-field probe. The probe concept exploits the non-invasive nature of the electro-optic detection method and utilizes an optical micro-resonator to enhance the detection sensitivity. The proposed electro-optic micro-resonator will be integrated into a fibre-coupled near-field probe. It will allow THz wave and signal probing with a spatial resolution of ~5 microns (~1/100 of the wavelength) and it will offer full spectroscopic capabilities in the THz range (0.1-2.0 THz). The novelty of this approach is in exploiting the optical cavity resonance for electro-optic detection of THz waves by an extremely small near-field probe. The goal of this research programme is to develop and build the THz near-field probing system and apply it in device research on the sub-wavelength scale. The proposed technology will expand the spectrum of THz studies to micrometre-scale objects. It will aid in the progress of THz device research and will facilitate studies of THz phenomena in physics, materials science and other disciplines.