Coherence dynamics of excitons and continuum excitations in InP

The coherent response of excitons and low-energy continuum excitations has been determined in InP at 5 K using spectrally resolved four-wave mixing em ploying 40 fs pulses in a self-diffraction geometry. The excitons were stud ied under simultaneous excitation with free carriers of density 10(15)<N<10 (17) cm(-3). Excellent agreement between experiment and a simple theory bas ed on excitation induced dephasing is obtained for an exciton dephasing rat e which varies as N-alpha with alpha approximate to 0.5 in the density rang e. The sublinear dependence is suggestive of carrier-exciton screening effe cts; however, a variation in exciton oscillator strength and/or phase-space filling effects may also be contributing to the density dependence of the four-wave-mixing signal. For the continuum excitations we spectrally resolv e the influence of LO phonons on continuum dephasing. Below the threshold f or LO-phonon emission by electrons we determine a dephasing rate of similar to 8 ps(-1) at N=5 x 10(16) cm(-3); for states just above the threshold fo r LO-phonon emission, the dephasing rate is >13 ps(-1) our resolution Limit . [S0163-1829(99)06024-5].