Abstract

Excitons bound to neutral donors in direct gap semiconductors are known to have remarkably large oscillator strengths (f ≠ 75). This leads to extremely fast radiative lifetime (τ ≠ 50 ps). We report here the observation of large resonant degenerate four-wave mixing signals near the l₂ bound exciton in high purity (~10¹⁵/cc) CdS platelets. All the incident beams propagate in the forward direction. An electrooptically modulated cw dye laser operating single mode at 487 nm is used as the optical source. Optical pulses of 2 μs in duration are produced at a repetition rate of 10 kHz. The two incident beams responsible for generating the grating in the sample are mechanically modulated at frequencies ν₁ = 1527 Hz and ν₂ = 1349 Hz. Phase sensitive detection is used to detect the four-wave mixing signals at ν₁ + ν₂. Scattering efficiently larger than 1% for peak input power levels of ~5 mW in each beam is observed. The beam diameter on the sample is ~40 μm. Resonance enhancement of the four-wave mixing signal is observed as the laser is scanned across the bound exciton resonance. An anomalously large broadening of the four-photon signal is observed as the incident intensity is increased. The possible role of light trapping is discussed.

© 1985 Optical Society of America