Laser-assisted tip positioning for thermal near-field microscopy

Abstract

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.