HIGHLY SENSITIVE RADIOFREQUENCY LOCK-IN PHOTODETECTION TECHNIQUE WITHDOUBLE-MODULATION FOR ULTRASHORT-PULSE LASERS

A highly sensitive photodetection system for optical pump-probe experi ments is described. The system is based on the lock-in amplification t echnique with double modulation at high f(0) and low f(1) frequencies (the photodetection system operates at the fixed frequency f(0) = 6.2 MHz). The system is implemented as an attachment for a low-frequency l ock-in amplifier and includes a silicon photodiode loaded to a narrow- bandwidth amplifier and an RF down-converter. The peak-to-peak sensiti vity to small changes in the optical power is determined by the photoc urrent shot noise at the detection frequency f(0) and equals delta P/P similar to 5 x 10(-7) 1/root Hz. The developed technique is used with a Nd:YAG laser in experiments on time-resolved optoacoustic and polar ization spectroscopy.