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 employing 40 fs pulses in a self-diffraction geometry. The excitons were studied under simultaneous excitation with free carriers of density 10(15)<N<10(17) cm(-3). Excellent agreement between experiment and a simple theory based on excitation induced dephasing is obtained for an exciton dephasing rate which varies as N-alpha with alpha approximate to 0.5 in the density range. The sublinear dependence is suggestive of carrier-exciton screening effects; 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 resolve the influence of LO phonons on continuum dephasing. Below the threshold for 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 for LO-phonon emission, the dephasing rate is >13 ps(-1) our resolution Limit. [S0163-1829(99)06024-5].