A nonoptical tip–sample distance control method for near-field scanning optical microscopy using impedance changes in an electromechanical system

Abstract

We present a nonoptical shear-force feedback method to regulate tip–sample distance for near-field scanning optical microscopy. In the shear force setup, the dither piezo and the attached fiber tip form an electromechanical system, whose power dissipation on resonance is sensitive to the change in damping force as the tip approaches and interacts with the sample. At the frequencies of interest (∼10–100 kHz), the change in power dissipation is conveniently manifested as a change in the electrical impedance of the dither piezo. We demonstrate that tip–sample distance feedback control can be achieved by measuring this change in dither piezo impedance. The sensitivity is currently ∼0.5 Å. This new technique is compared to other methods currently used for distance control in near-field scanning optical microscopy.

A low-temperature scanning near-field optical microscope for photoluminescence at semiconductor structures

“…The tip-sample distance is controlled by the shear force measured using a vibrating quartz fork with the fiber tip glued on one side. The fork is operated close to its resonance frequency, and the damping is detected using a lock-in amplifier by measuring the dissipated power [2,3]. When the tip approaches the sample surface, changes in the lock-in signal by about 10% at 300 K and by up to 80% at 90 K are observed, providing a reliable input parameter for feedback operation.…”

Potentiometry with the acoustic near field microscope: A new method for microscopy of surface potentials

“…Furthermore, the optical detection of the cantilever motion, which is a standard technique for the AFM, is not necessary at all for the SNAM. Therefore an increasing number of technical applications of the near field acoustic microscope has been developed recently (for example, ''Nanoswing'', ''Needle Sensor'', ''Beetle SPM'' [10], tactile and non-tactile sensors for micro-machining [11,12] as well as sensors for near-field optical microscopy [13]). …”

Tuning fork shear-force feedback

“…These techniques have the disadvantage that additional stray light is brought into the vicinity of the aperture, disturbing the measurement of the near-field optical signal; moreover, accurate alignment of the external optics with respect to the probe is necessary. An alternative method is to use piezoelectric materials which generate a piezoelectric voltage proportional to the amplitude of the oscillation [4][5][6][7]. Based on this idea, the use of crystalline quartz tuning forks for detecting the probe's amplitude was demonstrated recently by Karraı¨ [4].…”