Role of photostriction and photothemal effects in photon induced deflection of Si microcantilevers

Laser beam (He-Ne - 6328 A0) induced deflection in uncoated, Au, and Al coated Si microcantilevers (MCs) of various dimensions was studied, using an AFM head, to examine the induced stress contribution from photothermal and photostriction phenomenon. The laser beam was made to incident sequentially on front and back surfaces of the MC, by flipping it, and results were compared in terms of deflection magnitude, direction of bending and response time. In case of uncoated MCs, it was observed that the induced deflection was always in the direction of the incident laser beam with a fast response. In contrast, in metal (Au, Al) coated MCs, deflection direction was found to be dependent on laser beam exposure side with two orders of enhanced deflection sensitivity and slow response time. The former is attributed to photostriction effect while the latter is explained on the basis of photothermal effect in bimetallic MCs. Flipping experiments on bimettalic MCs, further revealed that stress contribution from these two source mechanisms crucially depends on MC dimensions and direction of laser exposure.