Near-field microscopy of spontaneous THz radiaion

We have developed a THz near-field microscope with an ultrahighly sensitive detector, CSIP (charge-sensitive infrared phototransistor). In our microscope, scattered photons by a tungsten probe are collected by an objective of a confocal THz microscope and are finally detected by the CSIP. To suppress the far-field background, the probe vertically modulated and the signal is demodulated the signal with a lock-in amplifier. The microscope probes spontaneous evanescent field on samples derived from local phenomena with 60 nm spatial resolution and the signal origin from metals is revealed to be thermal charge/current fluctuations. The intensity of passive near-field signal is very well consistent with Bose-Einstein distribution, which corresponds to the sample temperature. We also demonstrate nano-thermometry by probing passive near-field signals on a biased NiCr pattern. The obtained signals correspond to the local temperature and the result shows that the inner side of the line curve is much brighter than outer side. It can be easily interpreted by Kirchhoff’s law. The spatial resolution of the thermometry (60 nm) cannot be experimentally achieved by any other optical thermometry. This demonstration strongly suggests that our microscope is very well suited for real-time temperature mapping of complicated circuit patterns, and others like bio-samples.