Ultra stable tuning fork sensor for low-temperature near-field spectroscopy

Physics Department, Swiss Federal Institute of Technology Lausanne.
Ultramicroscopy (Impact Factor: 2.44). 03/2001; 90(2-3):97-101. DOI: 10.1016/S0304-3991(01)00144-9
Source: PubMed


We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.

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    • "By bringing the tapered fibre probe into close proximity with the sample surface, the inevitable diffraction limit encountered in conventional far-field optical methods can be overcome. Since initial demonstrations that crystal quartz tuning forks can be used as sensors for acoustic (Güthner et al ., 1989) and force (Karrai & Grober, 1995) microscopy, applications of this technique have been extended to near-field optical microscopy (Davydov et al ., 1999; Crottini et al ., 2002), atomic force microscopy (Giessibl, 2000; Callaghan et al. , 2002), and electrostatic force microscopy (Seo et al ., 2002; Wang et al ., 2002). Recently, we demonstrated the working principles of the tapping-mode tuning fork near-field scanning optical microscopy (TMTF-NSOM; Tsai & Lu, 1998) and successfully applied this new method to study the physical properties of a single mode optical fibre waveguide (Tsai et al ., 1999), superresolution near-field structures (Tsai & Lin, 2000), and semiconductor lasers (Lu et al ., 2001). "
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    ABSTRACT: We present the results of an experimental and theoretical study on the optimum design of shear-force sensors, used in scanning probe microscopes. We have optimized a configuration consisting of a tuning-fork/fiber-tip assembly, achieving quality factors (Q) exceeding 8000, and have presented a theoretical analysis of the design wherein the force holding the fiber and fork in contact is provided solely by elastic mechanical deformation, which allows full control of the performance of the system. On this basis, we constructed a high-quality-factor configuration with the fiber glued onto the tuning fork.
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