Role of many-body effects in the coherent dynamics of excitons in low-temperature-grown GaAs

Abstract

Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs indicate a polarization-dependent nonlinear optical response at the exciton, which we attribute to Coulomb-mediated coupling between excitons and electron-hole pairs simultaneously excited by the broad-bandwidth laser pulses. Strong suppression of the exciton response through screening by carriers injected by a third pump pulse was observed, an effect that is transient due to rapid carrier trapping. Our findings highlight the need to account for the complex interplay of disorder and many-body effects in the design of ultrafast optoelectronic devices using this material.

Role of strain on the coherent properties of GaAs excitons and biexcitons

“…In addition, XXXX polarized spectra for both samples show two vertical streaks (denoted C L , and C H ), which are coherences resulting from MBI between the electron-hole continuum and the X L and X H resonances. 33,46 Selecting XYYX polarization is known to suppress MBI, 47,48 such that in 2DFTS spectra the signals are reduced by a factor of approximately five, the C L and C H features are strongly reduced, the off-diagonal features become similar or weaker than the diagonal features, 23,24 and biexciton features become more prominent. 24,44 The latter effect is more pronounced for the high strain sample.…”

Control of the Urbach band tail and interband dephasing time with post-growth annealing in low-temperature-grown GaAs

“…Our experiments also show an increase in the interband dephasing time (T 2 ) with annealing temperature up to 500 • C. This indicates that arsenic point defects contribute strongly to carrier scattering in weakly-annealed LT-GaAs films. These scattering processes are diminished with annealing due to the reduction in the As i and As Ga point defects as precipitates begin to form, which has been found to occur for T a as low as 300 • C. 11,30 By extending our earlier FWM studies of as-grown films of LT-GaAs 27,28 to lower excitation powers, our experiments reveal a dip in the FWM response in the vicinity of the exciton. This dip is found to be insensitive to laser tuning and disappears when the sample is annealed at 550 • C, indicating that it is tied to the coexistence of band tail and exciton signal contributions, reminiscent of a Fano resonance.…”

“…The above assertion can be verified by carrying out experiments that suppress the exciton feature. In previous FWM experiments on LT-GaAs, the exciton signal was identified as resulting from excitation-induced dephasing (EID), 28 through which non-degenerate Coulomb interactions of excitons with carriers excited on the interband continuum lead to diffraction of the exciton polarization from a grating in the total free carrier population. [32][33][34][35] Such an EID signal is only present at zero delay due to interference of signal contributions from population gratings over a broad range of …”

“…This is due primarily to the need to apply nonlinear optical spectroscopy, 15 which enables the band tail states to be isolated from the strong defect-induced absorption that dominates the linear optical response of LT-GaAs. 20,[27][28][29] Here we report the application of four-wave mixing (FWM) spectroscopy to study the influence of annealing on the optical properties of LT-GaAs. The Urbach energy (E U ) was extracted in our experiments from the low-energy tail of the FWM spectrum for a range of annealing temperatures (T a ).…”

“…For these results, the excitation laser was tuned to 1.520 eV. The FWM signal for the LT-GaAs samples for T a ≤ 500 • C contains two main features: (i) a peak associated with the exciton resonance (at approximately 1.5 eV, 27,28 ); and (ii) a signal originating from the optically-excited interband continuum transitions at higher detection energies. The exciton response is peaked at zero delay, while the peak position of the interband response moves to larger values of interpulse delay as T a is increased.…”