Laser power effects in SERS spectroscopy at thin metal films

SERS spectroscopy requires the illumination of delicate metal structures wi th intense laser fight. In the present paper, the effect of laser illuminat ion on these structures is investigated for silver island films and silver films over alumina particles in focused laser beams and at the tip of fiber -optic SERS sensors, Less than proportional increases of SERS intensities w ith increasing laser powers indicate a reduction of SERS enhancement above intensities of 4.5 kW/cm(2) at the focus and around 54 kW/cm(2) at the end face of fiber SERS sensors. The reduction of SERS enhancement has been attr ibuted to irreversible changes of optical properties of the metal structure s at elevated temperatures by comparison with transmission measurements. An ti-Stokes/Stokes ratios of SERS band intensities already show noticeable he ating at laser powers of about I mW and also indicate changes in optical pr operties above a few milliwatts.