All material generates fluctuating electromagnetic (EM) evanescent field on its surface due to charge/current fluctuations, which are thermally agitated (thermal noise) or non-thermally excited (excess noise or shot noise). In general, current fluctuation (or noise) in a nanoscale region of the material carries key information of the local thermal/nonequilibrium phenomenon taking place at the given point [1]. In this seminar, I will describe an unprecedented novel microscope, called terahertz (THz) scanning noise microscope (Fig. 1(a)), which maps ultrahigh frequency (15-30 THz, 1 THz= 10¹² Hz) current fluctuation with nanoscale spatial resolution. THz SNoiM is demonstrated to be a powerful and unique experimental tool for studying local nonequilibrium dynamics in a variety of material systems [2-4]. The example shown in Fig. 1(b) is the first direct visualization of hot electrons in a GaAs/AlGaAs quantum well (QW) device, which reveals the fundamental nature of non-local hot electron energy dissipation [4]. Physical processes probed by SNoiM are inaccessible with any other currently known methods. Particularly, it has been demonstrated [5] that conventional near-field microscope with external light excitation is unable to image the same phenomena that are visualized by SNoiM.

Reference:
[1]   S. Komiyama, J. Appl. Phys. 125, 010901 (2019).
[2]   S. Komiyama et al., arXiv: 1601.00368.
[3]   Q. Weng et al., Nano Lett. 18, 4220 (2018).
[4]   Q. Weng et al., Science 360, 775 (2018).
[5]   Q. Weng et al., Appl. Phys. Lett. 114, 153101 (2019).