Laser power effects in SERS spectroscopy at thin metal films

SERS spectroscopy requires the illumination of delicate metal structures with intense laser fight. In the present paper, the effect of laser illumination 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 with 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 attributed to irreversible changes of optical properties of the metal structures at elevated temperatures by comparison with transmission measurements. Anti-Stokes/Stokes ratios of SERS band intensities already show noticeable heating at laser powers of about I mW and also indicate changes in optical properties above a few milliwatts.