Double-resonance probe for near-field scanning optical microscopy

Abstract

A surface-contact transducer is developed for scanning probe microscopes, whose operating principle is based on the coincidence between the resonance frequency of a 32kHz quartz tuning fork and that of the probe attached to it. This allows the transducer to have a high quality factor and, if the vibration amplitude of the probe tip exceeds that of the tuning fork prongs, materially improves its force sensitivity. The resonance transducer proposed by us has an experimentally verified force sensitivity of 8pN (rms) in the 300Hz frequency band, which is of the same order of magnitude as the sensitivity of atomic force microscope (AFM) cantilever sensors. The manufacture of such transducers equipped with optical-fiber probes for near-field scanning optical microscopy and with tungsten probes for AFM is described as an example.

Near-field scanning optical microscopy using polymethylmethacrylate optical fiber probes

“…These values are quite comparable with those obtained earlier for glass fibers [9]. Attainment of such large quality factors enables us to use the specialized low noise, precise and fast electronics to measure the resonant frequency f res and Q-factor of a tuning fork to control the SNOM operation.…”

“…Image sizes: 6 mm  6 mm and 300 nm  700 nm, respectively. Constant quality factor mode of the SNOM controller electronics [9,10] was used as a feedback source.…”

“…2) so that the working frequency of the latter coincides with the resonant frequency of lateral dithering of the free-standing part of the fiber beam b properly glued onto the tuning fork metal case a. In the paper [9] we experimentally demonstrated that such an assembling enables to preserve a large quality factor of the tuning fork after gluing the glass fiber probe onto it and also presented a theoretical model explaining such preservation. Double resonant operation requires accurate gluing and careful control of the length of the free-standing part of the fiber beam as well as the position where the thin glass connector between the probe and the tuning fork is to be glued.…”

“…Sharpened POF probes were glued onto the tuning fork using the ''double resonance'' principle of operation first introduced in [9] and then successfully used for Scanning Near-field Optical Microscopy of different samples [10,11]. This mode of operation relies specifically on the arrangement of the fiber probe onto the tuning fork (see inset in Fig.…”

“…Double resonant operation requires accurate gluing and careful control of the length of the free-standing part of the fiber beam as well as the position where the thin glass connector between the probe and the tuning fork is to be glued. It was experimentally established that for a standard 125 mm-diameter sharpened glass fiber, the resonance condition holds when the length of a freestanding part of the fiber beam is equal to 4.870.1 mm [9]. Based on the well-known formula expressing the resonant frequencies of the dithering of a cylindrical beam whose one end is clamped and other is free, o ¼ ða 2 n =l 2 Þ ffiffiffiffiffiffiffiffiffiffiffiffi ffi EI=rS p (here I= (1/4)(pr 4 ) is the moment of inertia, l and r are the length and the radius of the cylindrical rod, respectively, E and r are, respectively, Young's modulus and density of the material it is made of, S= pr 2 and a n is a numerical constant characterizing nth resonance: a 1 = 1.875, a 2 =4.694, etc.…”