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Conference on Lasers and Electro-Optics

Ultrafast gain recovery dynamics of the excited state in InGaAs quantum dot amplifiers

Stephan Schneider, U. Woggon, Paola Borri, W. Langbein, D. Ouyang, Roman L. Sellin, D. Bimberg

Published Jan 1, 2005• 7 citations
Publisher

Abstract

The gain dynamics in electrically-pumped InGaAs quantum dot amplifiers at 300 K is measured to be in the subpicosecond range for both ground and excited state transitions, promising for all-optical signal processing at >40 GHz repetition rates.
DOI
10.1109/cleo.2005.202237

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Applied Physics Letters•Jan 15, 2007•93 citations
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.
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.
Gain recovery dynamics are studied in electrically pumped quantum dot (QD) based semiconductor optical amplifiers (SOAs) after amplification of double femtosecond laser pulses using ultrafast pump-probe spectroscopy with heterodyne detection. The authors observe a distinct change in gain recovery in the ground state when a significant excited state population is achieved. A complete gain recovery is found when two 150fs pulses with 5ps time delay pass through the SOA in resonance to the ground state under high injection currents of 80–100mA. The obtained results open the way for ultrafast (>200GHz) operation in p-doped QD based SOAs at 1.3μm telecommunications wavelengths.