Laser-assisted tip positioning for thermal near-field microscopy

Thermal near-field microscope probes thermally excited evanescent waves (wavelength: 8-25 mu m) 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 mu m 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.