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Applied Physics Letters

Complete ground state gain recovery after ultrashort double pulses in quantum dot based semiconductor optical amplifier

S. Dommers, Vasily V. Temnov, U. Woggon, Jordi Gomis‐Brescó, Juan P. Martínez‐Pastor, M. Laemmlin, D. Bimberg

Published Jan 15, 2007• 93 citations
Publisher

Abstract

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.

DOI
10.1063/1.2431789

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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.
We present an analytical description of ultrashort femtosecond pump–probe experiments and investigate the gain response of quantum dot (QD) semiconductor optical amplifiers. The calculation provides a full analytical solution of numerical studies to recent experiments in such structures (Gomis-Bresco et al 2008 Phys. Rev. Lett. 101 256803). Our approach is based on QD Bloch equations, which are analytically evaluated within the third-order temporal perturbation theory (χ (3) level). In particular, we study the influence of the coherence and the population dynamics of two confined QD levels on the gain as a function of the delay time between the pump and the probe pulse. We discuss how to engineer optimal conditions for high-performance QD amplifiers, which are characterized by an ultrafast gain recovery and a pronounced gain depletion.
We numerically investigate the influence of the optical pumping wavelength on the ultrafast gain and phase recovery acceleration of quantum-dot (QD) semiconductor optical amplifiers (SOAs) by solving 1088 coupled rate equations. The temporal variations of the gain and phase recovery response at the ground state (GS) of QDs are calculated at various signal wavelengths when the optical pumping wavelengths at the excited state (ES) of QDs are varied. The phase recovery response is fastest when the wavelength of the signal and pumping beams corresponds to the respective emission wavelength of the GS and the ES in the same size of QDs. The absorption efficiency of the optical pumping beam at the ES is determined by the Lorentzian line shape function of the homogeneous broadening.
The gain recoveries in quantum dot semiconductor optical amplifiers are numerically studied by rate equation models. Similar to the optical pump-probe experiment, the injection of double optical pulses is used to simulate the gain recovery of a weak continuous signal for the QD SOAs. The gain recoveries are fitted by a response function with multiple exponential terms. For the pulses duration of 10 ps, the gain recovery can be described by three exponential terms with the time constants, and for the pulse with the width of 150 fs, the gain recovery can be described by two exponential terms, the reason is that the short pulse does not consume lot of carriers.
We report on the characterization of the ultrafast gain and refractive index dynamics of an InAs/InGaAsP self-assembled quantum dot semiconductor optical amplifier (SOA) operating at 1.55 mum through heterodyne pump-probe measurements with 150 fs resolution. The measurements show a 15 ps gain recovery time at a wavelength of 1560 nm, promising for ultrafast switching at >40 GHz in the important telecommunications wavelength bands. Ultrafast dynamics with 0.2-1.5 ps lifetimes were also found consistent with carrier heating and spectral hole burning. Comparing with previous reports on quantum dot SOAs at 1.1-1.3 mum wavelengths, we conclude that the carrier heating is caused by a combination of free-carrier absorption and stimulated transition processes.
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.
In this work, the ultrafast gain dynamics of a quantum dot (QD)-based semiconductor optical amplifier (SOA) is modeled on the basis of semiconductor Bloch equations that include microscopically calculated nonlinear scattering rates between QD carriers and the surrounding carrier reservoir. This enables us to separately study the dynamics of electrons and holes inside the device as well as the coherent effects related to the fast polarization dynamics. We show that the optical pulse power and the dephasing time of the polarization mainly affect the gain depletion inside the active region, while the electric injection current density and thus the internal carrier dynamics influence the gain recovery. We observe that carrier–carrier scattering is the source of desynchronized behavior of electrons and holes in the recovery dynamics of QD-based SOAs. The amplification of pulse trains in the SOA predicted by our model agrees well with experimental data.