Search

In this study, we passively analyzed the near-field characteristics of thermally excited evanescent waves, which are radiation waves generated by the local dynamics of materials, including electron motions and lattice vibrations. The thermally excited evanescent waves on aluminium nitride (AlN) and gallium nitride (GaN) were measured using passive spectroscopic scattering-type scanning near-field optical microscopy (s-SNOM) in the wavelength ranges of 10.5–12.2 μm and 14.0–15.0 μm, which include the surface phonon-polariton (SPhP) wavelength of the studied dielectrics. We determined the unique decay characteristics of AlN and GaN, indicating a ten-fold increase in the probe area contributing to the scattering of waves near the SPhP wavelength compared to that in other wavelength ranges. The extended probe area correlated with the polariton decay lengths, indicating that the non-enhanced polaritons around K ~ ω/c were dominant in the scattered waves near the SPhP wavelength. In addition to the conventional passive detection mechanisms for metals, the proposed detection scheme will be a versatile passive detection model in the near future. [ABSTRACT FROM AUTHOR]

Passive scattering-type, scanning near-field optical microscopy (s-SNOM) has been employed to study localized, long-wavelength infrared (LWIR) surface waves without external illumination. Here, we develop a cryogenic passive s-SNOM instrument in a vacuum chamber with 4 K liquid-helium cooling. Notably, the extremely low-temperature environment inside the chamber enables the realization of passive near-field detection with low background thermal noise. The technique mainly utilizes a highly sensitive LWIR confocal optical system and a tuning fork-based atomic force microscope, and the near-field detection was performed at a wavelength of 10.2 ± 0.9 µm. In this paper, we discuss the cryogenic s-SNOM implementation in detail and report the investigation of thermally excited surface electromagnetic fields on a self-heated NiCr wire deposited on SiO2 at a temperature of 5 K. The origin of the surface electromagnetic fields was established to be the thermally excited fluctuating charges of the conduction electrons. The cryogenic s-SNOM method presented herein shows significant promise for application in a variety of spheres, including hot-carrier dissipation in ballistic conductors. [ABSTRACT FROM AUTHOR]

We systematically compare the active and the passive methods for infrared scattering-type scanning near-field optical microscopy (s-SNOM). The active SNOM makes use of IR lasers or incoherent thermal emitters to illuminate a sample, whereas the passive method directly measures extremely weak fluctuating electromagnetic evanescent fields spontaneously generated at the sample surface without any external illumination. For this reason, our specific version of the passive SNOM is called a scanning noise microscope (SNoiM). In thermal equilibrium, the two methods are shown to be similar, both mapping the nanoscale variation of the complex dielectric constant of the sample. We demonstrate that a significant difference between the two methods emerges when the sample is driven out of thermal equilibrium, viz., the active SNOM is insensitive whereas the SNoiM is extremely sensitive to the electron temperature in locally heated nanoregions. [ABSTRACT FROM AUTHOR]

• We have proposed a new tip positioning method for thermal near-field microscope using small thermal spot generated by the focused visible laser. • We have developed a tip positioning consisting of an illumination unit and an xy -stage. • A hot spot with 15 μm and 13 K temperature rise was generated on GaAs substrate. • With the help of the hot spot, we successfully obtained thermal near-field signals without fabricating metal mark patterns. Thermal near-field microscope probes thermally excited evanescent waves (wavelength: 8–25 μm) derived from local surface phenomena of materials with a spatial resolution of less than 100 nm. To detect the evanescent waves efficiently, the tip apex should be positioned to the objective focus within 10 µm accuracy. It is thus necessary to deposit metal patterns on the specimen for accurate tip positioning, which restricts observable specimens. We here propose a new tip positioning method, using focused visible laser beam to generate a small hot spot on a specimen substrate, playing the same role as the metal patterns. We develop a new and light (360 g) tip positioning unit, consisting of an illumination unit and an xy -stage for the illumination unit. The illumination unit has a fiber unit and a small objective lens, which move together in light axis direction. Experimental verification confirms that the proposed tip positioning method is valid for the thermal near-field microscope, which opens the door to probing thermal near-field signals on various samples like soft materials. [ABSTRACT FROM AUTHOR]