FIGURE 1.

Tapping-mode-like distance control for SNOM measurements in liquid. The force sensor, a piezoelectric quartz tuning fork, is aligned parallel to the surface so that the prongs are oscillating normal to the sample. A short SNOM probe (length ∼3 mm) is attached to the lower prong. Though the SNOM probe is immersed ∼2 mm in the solution, the force sensor does not get in contact with the liquid and thus can preserve its high sensitivity. An optical fiber is used to couple light into the SNOM probe by bringing it close to the entrance face of the short fiber.