Ž . Applied Surface Science 144–145 1999 510–513 Method for increasing sensitivity of shear-force distance control for scanning near-field microscopy R. Kantor a, ) , M. Lesnak b , N. Berdunov c , I.V. Shvets a a Physics Department, Trinity College, Dublin 2, Ireland b Physics Department, Technical UniÕersity of OstraÕa, OstraÕa, Poruba, 708 00, Czech Republic c Physics Department, Kazan State UniÕersity, Kazan, 42 0008, Russian Federation Abstract Scanning-near field optical microscopy requires a distance control mechanism. In most cases, it is based on the shear-force detection. In this paper we report how the performance of the shear-force detection based on the most common nonoptical approach, a Quartz tuning fork, can be improved. Our approach is based on exciting oscillations in just one arm of the fork, not two. This approach reduces the response time of the shear-force detection system. We also introduce an ultra-sensitive system with a long free fiber tip. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Shear-force distance control; Scanning near-field miscroscopy 1. Introduction Shear-force detection is commonly used as the key method for distance control in near-field optical microscopy and related techniques. It is based on exciting probe oscillations parallel to the surface and monitoring their dependence on probe–sample sepa- ration. A dissipative interaction between them dumps the oscillations. The detection of the oscillations can be performed optically by an interferrometric method or by measuring the intensity of reflected or shad- owed beam spot. Numerous variations of optical wx detection system have been proposed 1 . More re- cently, various nonoptical methods were introduced as they offer some important advantages over the optical ones. These methods are based on, e.g., wx piezoelectric tuning fork 2 , use of capacitance de- ) Corresponding author. Tel.: q353-1-608-2552; Fax: q353-1- 671-1759; E-mail: kantorr@tcd.ie.or wx tectors 3 , detection of voltage induced in a sec- ondary piezo element glued to the holder or directly wx to the fiber 4 . Method based on tuning fork is becoming the dominant one. However, in compari- son to other methods, it has relatively long time constant of relaxation processes, which limits the speed of shear-force feedback system. In this paper we modified the configuration employing Quartz oscillator to reduce the response time and to increase the sensitivity of the system. 2. Linear model of tuning fork oscillator in stan- dard configuration wx In the original design 2 the fork base is glued to a piezo, which is excited at the main resonance frequency. The probe is glued to one of the arms. In the main mode the arms oscillate with opposite phase, the forces between the two arms are much 0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-4332 98 00850-2