Effect of light intensity on reflectance and transmittance of a laser beam incident on a gold film

The time variation of the optical reflectance and transmittance of the continuous-wave laser beams at different powers incident onto a gold film grown on a glass substrate has been investigated. The reflectance and transmittance start being time-varied as its power exceeds 10 mW. The observed variation can be attributed to thermal effects of laser illumination above a certain power threshold (about 10 mW), which is dependent on the optical constants and thickness of the gold film. Furthermore, it has been demonstrated that the transmittance of a signal beam whose power is lower than the power threshold can be modulated by a pump light whose power is larger than 10 mW. The time duration of these time-varied phenomena is more than 1 minute, so it is quite different from the transient phenomenon of a pulse laser beam, whose time duration usually is on a picosecond scale. Finally, a quantitative model is given to explain the experimental results.