Dynamic response and relaxation oscillations in random lasers

Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112~Received 27 August 2001; revised manuscript received 2 November 2001; published 2 January 2002!We experimentally investigate the dynamic response of random lasers using picosecond optical pumping. Itis found that the turn-on time of a random laser is less than 100 ps. Above the lasing threshold, the emissionpulse is shortened dramatically. The dynamics of each individual lasing mode is different. Lasing starts andgets to a maximum value at different times. In addition, lasing in different modes lasts for different periods oftime, and the frequency of some lasing modes also shifts with time. Relaxation oscillations of lasing modes areobserved for a random laser. The theoretical simulations based on the semiclassical laser theory and finitedifferent time domain reproduce most of the experimental observations and provide an understanding of thedynamic response of a random laser.DOI: 10.1103/PhysRevB.65.041103 PACS number~s!: 42.25.Bs, 71.55.Jv, 42.55.2f