The Institute of Industrial Science at the University of Tokyo develop spectroscopic measurement technique for previously unmeasurable thermally excited evanescent waves

The surfaces of materials are covered by thermally excited evanescent waves induced by the thermal fluctuations of electron motions and lattice vibrations. Although these electromagnetic waves carry important information, including temperatures (lattice and electron temperatures) at the material surface, they are also subject to diverse fluctuations in wavenumber components that can readily nullify each other near the surface and experience attenuation within distances of less than 100 nm. Postgraduate Student Ryoko Sakuma from the School of Engineering at the University of Tokyo (at the time of research), Project Assistant Professor Kuan-Ting Lin (at the time of research), and Professor Yusuke Kajihara of the Institute of Industrial Science at the University of Tokyo have jointly developed a spectroscopic technique for measuring thermally excited evanescent waves with nanoscale resolution. The results were published in Scientific Reports.

The research group has previously developed a passive near-field microscope designed for scattering thermally excited evanescent waves using metal probe tips with diameters of less than 50 nm. These waves were detected using a highly sensitive detector, CSIP (charge-sensitive infrared phototransistor), within a confocal optical system, achieving the detection of thermally excited evanescent waves with a spatial resolution of 20 nm.

However, the limitations of conventional passive near-field microscopes hindered spectral measurements, precluding a comprehensive assessment of the detailed dynamics of surface materials. In this study, a grating-type spectroscopic optical system was integrated into the passive near-field microscopy setup, aiming to facilitate spectroscopic measurements of thermally excited evanescent waves. The introduction of the spectroscopic optical system at room temperature resulted in the generation of background noise from optical elements through radiation, burying the detection signal within the noise. Therefore, the spectroscopic optical system was incorporated within a cryostat operating at 4.2 K to mitigate the issue.

In measurement experiments, the research group focused on dielectrics (gallium nitride (GaN) and aluminum nitride (AlN)) exhibiting surface phonon resonance, particularly at the primary wavelengths of thermally excited evanescent waves. By varying the measurement wavelengths, they observed attenuation curves, depicting the relationship between distance from the surface and near-field signal, at each wavelength. Notably, they discovered highly distinctive differences in the attenuation curves between cases close to and far from the surface phonon resonance wavelength.

For example, during the measurement of the attenuation curve at a wavelength near 14 micrometers, a signal characteristic of the thermally excited evanescent wave theory was obtained for AlN (resonance wavelength: 11.8 micrometers), despite its surface phonon resonance wavelength being distant. The signal exhibited attenuation at a few tens of nanometers. Conversely, for GaN (resonance wavelength: 14.1 micrometers), characterized by a closely situated surface phonon resonance wavelength, the signal was only observed at the same wavelength as the resonance wavelength. However, the observed attenuation distance was several hundred nanometers, significantly longer than predicted by the theory. This result strongly suggests that only surface phonon polaritons exist in the wavelength band close to the surface phonon resonance wavelength. It indicates a notable absence of high-frequency thermal fluctuations with diverse wave numbers in this range.

These findings offer new insights, implying the necessity to revise the theory surrounding thermally excited evanescent waves. This departure from the foundational theory, which posits thermal fluctuations existing across all wavelengths with short attenuation distances, prompts a reevaluation of the existing theoretical framework. The distinctly clear signals obtained at high resolution (20 nm) from GaN and AlN, materials commonly used in power semiconductors, underscore the potential application of this measurement technique in assessing thermal excitation noise within micro-devices of power semiconductors, which has significant implications for contributing to the optimization of device design.