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Physical Review B

Measurement of the ultrafast gain recovery in InGaAs/GaAs quantum dots: Beyond a mean-field description

Paola Borri, Valentina Cesari, W. Langbein

Published Sep 27, 2010• 7 citations
Publisher

Abstract

We measured the ultrafast gain recovery dynamics of the ground-state transition in an ensemble of electrically pumped InGaAs/GaAs quantum dots having a nonequilibrium carrier distribution prepared by an optical prepump pulse. We find that the gain recovery dynamics after optical depletion by the prepump is faster than without prepump, an effect most pronounced at low temperature (15 K) but observable up to room temperature. This finding is not consistent with a mean-field description of the carrier distribution and gives direct evidence that microstates with discrete carrier numbers determine the macroscopic response. The observed dynamics results from a conditional recovery where microstates with an internal relaxation slower than the depletion lead time are selectively suppressed.

DOI
10.1103/physrevb.82.115326

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We measured the ultrafast gain recovery dynamics of the ground-state transition in an ensemble of electrically pumped InGaAs/GaAs quantum dots having a nonequilibrium carrier distribution prepared by an optical prepump pulse. We find that the gain recovery dynamics after optical depletion by the prepump is faster than without prepump, an effect most pronounced at low temperature (15 K) but observable up to room temperature. This finding is not consistent with a mean-field description of the carrier distribution and gives direct evidence that microstates with discrete carrier numbers determine the macroscopic response. The observed dynamics results from a conditional recovery where microstates with an internal relaxation slower than the depletion lead time are selectively suppressed.
The ultrafast gain recovery dynamics of the first excited state (ES) is studied in an electrically pumped InGaAs quantum-dot amplifier at room temperature and compared with the ground-state (GS) gain dynamics. Pump-probe differential transmission experiments are performed in heterodyne detection and the gain dynamics are investigated as a function of injection current. An ultrafast (<200 fs) initial gain recovery of both GS and ES transition is found, promising for optical signal processing at high bit rates. The obtained results suggest the occurrence of a fast recovery of the state occupation mediated by carrier-carrier scattering as long as a reservoir of carriers in the ESs and wetting layer is present.
We measured the gain dynamics at the ground-state transition in an electrically pumped InP/AlGaInP quantum-dot optical amplifier at room temperature by femtosecond differential transmission. The gain shows an ultrafast recovery within 200 fs, even faster than in state-of-the-art InAs/GaAs quantum-dot amplifiers. This finding, likely to be due to the less confined and more closely spaced hole levels in InP dots, is promising for optical signal processing at high bit rates. We furthermore measured the pump-induced refractive index changes and deduced a linewidth enhancement factor similar to the one in InAs/GaAs quantum dots.
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We present temperature-dependent measurements of the dephasing time in the ground-state transition of strongly-confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. At low temperature we measure a dephasing time of several hundred picoseconds. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a sharp zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. We also explore the dephasing time beyond the one exciton occupation, by electrically injecting carriers. Electrical injection into the barrier region results in a dominantly pure dephasing of the excitonic ground-state transition. Once the injected carriers have filled the electronic ground state, additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.
We present an extensive experimental study of the exciton relaxation and dephasing in InGaAs quantum dots (QDs) in the temperature range from 10 K to 295 K. The QDs are embedded in the active region of an electrically pumped semiconductor optical amplifier. Ultrafast four-wave mixing and differential transmission spectroscopy on the dot ground-state transition are performed with a sensitive heterodyne detection technique. The importance of the population relaxation dynamics to the dephasing is determined as a function of injection current and temperature. Above 150 K dephasing processes much faster than the population relaxation are present, due to both carrier-phonon scattering and Coulomb interaction with the injected carriers. Only at low temperatures (<30 K) does population relaxation of multiexcitons in the gain regime fully determine the dephasing.