Picosecond quantum-beat spectroscopy of bound excitons in CdS

Abstract

We report the first observation of quantum beats in the resonance fluorescence of a bound exciton in a semiconductor, the ionized-donor exciton complex in CdS. Measurements of the polarization behavior and magnetic-field dependence of the beats permitted a detailed investigation of strain eff'ects and magneto-optical parameters of the states. From the damping of the beats, coherence times of the order of several hundreds of picoseconds are deduced. For the neutral-donor and -acceptor bound exciton states, very rapid dephasing with coherence times of the order of 20 ps is found.PACS numbers: 42.50. Md, 71.35.+Z, 78.20.Ls, Phase relaxation is of fundamental importance for the understanding of the dynamical behavior of electronic states. In solids, phase relaxation times in the picosecond and subpicosecond regime require high time resolution, and therefore only nonlinear optical techniques, like photon echo [1,2], transient absorption correlation [3], or four-wave mixing [4], hitherto allowed exploration of the dephasing of continuum electron-hole states and excitons in various semiconductor systems. However, an alternative, linear method to investigate dephasing of quantum states in solids is quantum-beat spectroscopy. This technique has widely been used in studies of atoms and molecules [5], but most recently, by studying the resonance fluorescence of free exciton states in AgBr [6], it was demonstrated to be applicable for solids too. In this technique, a set of nearly degenerate electronic states, e.g., split by a magnetic field, is excited coherently by a short laser pulse with a spectral width larger than the energy splitting of the levels. The beats show up as oscillations in the time-dependent resonance fluorescence from these states. From the damping of the beats, the coherence time Tcoh (also called dephasing time T2) of the states can be deduced, which is defined through l/Tcoh~2/ri + l/r2, where T\ and T2 denote the energy relaxation and pure dephasing times. The beating frequencies are directly related to the energy splitting of the states, which can be varied by the magnetic field, allowing a determination of important magneto-optical parameters like electron and hole g factors. A particular advantage of quantum beats in spontaneous fluorescence is that they are not aff'ected at all by an inhomogeneous broadening of the states. Therefore, small splittings can be measured that are masked in the spectrum and not accessible to conventional spectroscopy. From a fundamental point of view, beats in spontaneous fluorescence originating from the superposition of wave functions have to be distinguished from beating phenomena observed in investigations of exciton states using nonlinear four-wave mixing [7,8], as these are due to interference of coherent polarizations oscillating at different frequencies.In order to be able to use the full potential of quantum-beat spectroscopy, the coherence time of the states investigated should be long, corresponding to a small homogeneous linewidth Fhom'^^^/rcoh-In semiconduc...

Dephasing of acceptor bound excitons in II–VI semiconductors

“…[ 11). With spectrally selective ps-excitation dephasing times of some hundred ps are reported fix bound exciton complexes in semiconductors, determined either by linear quantum beal spectroscopy [2] or by degenerate four-wave mixing [3, 41. Extending the investigations to fs-experiments, a mixture of coherently excited states near the band gap of semiconductors is prepared, leading to complex interference effects [5].…”

Coherent superpositions of three states for phosphorous donors in silicon prepared using THz radiation

“…It allows both extraction of Δ, and may also be used for discrimination between quantum beats and polarisation interference24 between two independent oscillators in situations, where the system is not well understood (not the case for the donors here). Quantum beating where the difference frequency is of the order 1 THz has been observed in semiconductor wells and dots using interband (near-IR) pumps13252627. Terahertz pulses can also induce other quantum coherent effects such as the Autler–Townes splitting28, and ac Stark effect29.…”

Nonlinear quantum beats of propagating polaritons

“…However, the Coulomb interaction between electrons and holes infer the presence of exciton resonances just below the band edge. These levels can give rise to observation of quantum beats in linear resonance fluorescence measurements [2][3][4] as well as in nonlinear four-wave mixing [5][6][7][8][9][10] and induced magnetoabsorption [11]. The quantum beat measurements give information about energy splittings, oscillator strengths, dephasing times, etc.…”