Ultramicroscopy 42-44 (1992) 345-350 ~ ~ y North-Holland Mechanical and thermal effects of laser irradiation on force microscope cantilevers O. Marti, A. Ruf, M. Hipp, H. Bielefeldt, J. Colchero and J. Mlynek Fakultiit fiir Physik, Uni~,ersith't Konstanz, W-7750 Konstanz, Germany Received 12 August 1991 In an optical lever set-up one or two modulated laser beams of 0,1 to 6 mW modulation amplitude at a wavelength of 670 nm were focused at uncoated and gold-coated microfabricated cantilevers. The motion of the levers was analyzed by an optical lever set-up. The mechanical resonance (30 to 60 kHz) of the cantilevers was excited by the modulated light both in air and under vacuum conditions (10 6 mbar), The measured resonance frequencies and the width of the resonances were identical to the values found by exciting the cantilevers by piezo ceramics. At low frequencies under vacuum conditions, we found an increase of the oscillation amplitude with decreasing frequency. The time constant of this increase is of the order of 5 ms. At the resonance frequency of uncoated cantilevers light pressure effects dominate thermal effects; the resonance is thus excited by light pressure. Gold-coated cantilevers, however, are driven by the bimetal effect, even above 10 kHz. A possible application of the light pressure effects is the use of a modulated light beam in the attractive mode operation of a scanning force microscope to excite the cantilever oscillation. 1. Introduction 2. Theoretical considerations Scanning force microscopy (SEM) [1-3] has proved to be a valuable tool for investigation of a vast array of different systems, such as biological samples [4], magnetic structures [5] and local charges [6]. Many scanning force microscopes to- day use some kind of optical detection for the deflection of force sensing cantilevers [2,3]. Opti- cal detection is simple and efficient for both static and dynamic measurements of forces. The investigation of samples by weak forces is often limited by the noise in the detection system. The interaction of light with force sensing cantilevers is usually neglected, although a report on the photothermal excitation of a cantilever resonance has been published [7]. A better understanding of the physics of the force detection will help to improve the accuracy and usefulness of scanning force microscopes. The interaction of light with the cantilever is through mechanical and thermal processes. The input power Iin is partially reflected (Iref = r/in), where r is the reflectance coefficient and par- tially absorbed according to l(z) = (1 -r)li.e vz (1) where y is the absorption coefficient. Since the thickness of the cantilevers is of the order of the wavelength of the light part of the light (I t .... ) is transmitted; this intensity also includes the light which was diffracted by the cantilever. The me- chanical momentum transferred by reflected pho- tons to the cantilever is given by p = 2hk cos a, (2) 0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved