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Applied Physics Letters

Two-photon excited photon echo in CdS

F. Hillmann, J. Voigt, H. Redlin

Published Dec 18, 2000• 8 citations
Publisher

Abstract

Time-resolved and integrated degenerated four-wave mixing experiments with two-photon excitation at a wavelength of 600–710 nm on a 30–40 μm thick cadmium sulfide crystal are presented for a temperature range from 5 to 300 K. A photon echo is observed showing quantum beats both at positive and negative delay times τ with a period of 0.7–0.8 ps. The signal decays exponentially with time constants of ≈170 fs at τ>0 and half as large at τ<0, independent of temperature. The results are discussed in terms of the decay of a high-density gas of e–h pairs, excitons and excitonic molecules highly out of equilibrium.

DOI
10.1063/1.1335847

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Time-resolved and integrated degenerated four-wave mixing experiments with two-photon excitation at a wavelength of 600–710 nm on a 30–40 μm thick cadmium sulfide crystal are presented for a temperature range from 5 to 300 K. A photon echo is observed showing quantum beats both at positive and negative delay times τ with a period of 0.7–0.8 ps. The signal decays exponentially with time constants of ≈170 fs at τ>0 and half as large at τ<0, independent of temperature. The results are discussed in terms of the decay of a high-density gas of e–h pairs, excitons and excitonic molecules highly out of equilibrium.
The results of the femtosecond four-wave mixing measurements performed on the CdS crystal at room temperature are reported. The dephasing time T d = 180 fs was obtained. The anti-Stokes shift of the primary photon echo spectrum with respect to the excitation spectrum was observed. The some peculiarities of the coherent responses decay curves indicate that this optical dephasing is due to the exciton-exciton interaction. The stimulated photon echo decay curves appear to have a fast decay component (the time constant of approximately 4.5 ps) in case the energy of exciting pulses exceeds the threshold value of 0.4 μJ.
Femtosecond four-wave mixing coherent specstroscopy in CdS crystal on 790 nm at room temperature is analyzed. It is stated that the optical dephasing time is equal to 180 fs. Some specific features of the coherent responses' decay curves indicate that this optical dephasing is due to exciton-exciton interaction. The stimulated photon echo decay curves appear to have a fast decay component (the time constant of approximately 4.5 ps), in case when the energy of exciting pulses exceeds the threshold value of 0.4 μJ.
Entangled free induction decay (EFID) femtosecond signals are experimentally observed for the first time at a wavelength of 790 nm in a cadmium sulfide (CdS) crystal in the two-photon absorption (TPA) regime upon excitation by two crossed (angle, 60°) laser beams. The sample emitted two EFID signals simultaneously in opposite directions. The signals were diffracted by nonequilibrium electric polarization gratings induced by two laser beams in accordance with the laws of energy and momentum conservation. The possibility of exciting EFID signals in the three-photon absorption regime is discussed.
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.
We have performed time-integrated as well as time-resolved femtosecond degenerate four-wave-mixing measurements on inhomogeneously broadened (Zn,Cd)Se/ZnSe quantum-well structures. Potential fluctuations due to alloy disorder in the ternary wells give rise to localized quasi-zero-dimensional excitons and biexcitons. For time-integrated detection, they manifest themselves by distinct (disorder-renormalized) quantum beats. Time resolving these transients reveals a free-induction-decay-like emission nature of the biexciton in contrast to the photon-echo exciton contribution. Using a five-level scheme, containing both biexcitons and antibound two-exciton states, this striking emission-type change is explained by fluctuation and correlation effects upon the biexciton binding energy. A combination of time-resolved and time-integrated four-wave mixing is thus a powerful tool to gain information about the disorder in a given kind of quantum structure.
Phase relaxation of optical excitations in condensed matter can be studied by coherent four—wave—mixing experiments in the time domain without requiring assumptions on the spectral properties of the material. For inhomogeneously broadened transitions, three laser pulses at times t = 0, t = t21 and t = T with wave vectors k1, k2 and k3, respectively, generate a stimulated photon echo pulse, emitted time—delayed at t = T+t21 in the momentum—matching direction according to k3+k2−k1. As a function of t21 the echo amplitude decays due to scattering events, described by the dephasing time constant T2 [1].