D. Ouyang

Exciton relaxation and dephasing in quantum-dot amplifiers from room to cryogenic temperature

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.

Linewidth enhancement factor in InGaAs quantum dot amplifiers

We report systematic measurements of the linewidth enhancement factor (LEF) in an electrically pumped InGaAs quantum-dot (QD) amplifier in the temperature range from 50 K to room temperature. At injection currents below transparency, the value of the linewidth enhancement factor of the ground-state interband (excitonic) transition is between 0.4 and 1, and increases with increasing carrier density. Additionally, we investigate the spectral dependence of the LEF by tuning the wavelength of our optical probe from below resonance with the ground state of the QDs up to resonance with the first optically active excited-state transition. We find a decrease of the LEF with increasing photon energy at all investigated temperatures.

Excited-state gain dynamics in InGaAs quantum-dot amplifiers

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.

Ultrafast carrier dynamics and dephasing in InAs quantum-dot amplifiers emitting near 1.3-μm-wavelength at room temperature

The carrier dynamics in an electrically pumped InAs quantum-dot amplifier emitting near 1.3-μm-wavelength at room temperature is measured with femtosecondtime resolution performing a pump–probe and a four-wave mixing experiment resonant to the dot ground–state transition. In contrast to the dynamics of the absorption bleaching over hundreds of picoseconds, an ultrafast gain recovery is measured, promising for high-speed applications of strongly confined InAs dots. Moreover, a dephasing time of 220 fs is measured in the absorption and of 150 fs in the gain case. This latter value is more than three times longer than our previous finding on less-confined quantum dots [P. Borri et al., Appl. Phys. Lett. 76, 1380 (2000)] indicating that the strong confinement can indeed lower the homogeneous broadening under electrical injection.

High performance narrow stripe quantum-dot lasers with etched waveguide

1.3 µm range narrow stripe (8 µm) quantum-dot (QD) lasers processed in deep-mesa geometry, etched through the waveguide, demonstrate high external differential efficiency (50%) and a low threshold current density (<130 A cm−2) superior to the shallow mesa devices. This opens a new way for cost-efficient fabrication of distributed feedback and photonic crystal QD devices.

Self-induced transparency in InGaAs quantum-dot waveguides

We report the experimental observation and the theoretical modeling of self-induced-transparency signatures such as nonlinear transmission, pulse retardation and reshaping, for subpicosecond pulse propagation in a 2-mm-long InGaAs quantum-dot ridge waveguide in resonance with the excitonic ground-state transition at 10 K. The measurements were obtained by using a cross-correlation frequency-resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.

Impact of the mesa etching profiles on the spectral hole burning effects in quantum dot lasers

We investigated the impact of mesa etching profiles on the emission spectra of In(Ga)As quantum dot ridge waveguide lasers grown by metal-organic chemical vapour deposition. The mesa etching was terminated: (i) well before the waveguide, (ii) directly at the waveguide, (iii) after the waveguide forming tilted sidewalls and (iv) after the waveguide forming vertical sidewalls. We found a dramatic impact of the etching profiles on the spectral intensity modulation of the longitudinal modes. The spectral hole burning effect due to the Fabry–Perot cavity resonances causes strong modulation of the lasing spectrum, if the etching profile is terminated at the waveguide, or when the mesa sidewalls are tilted. In addition, deep-etched-through mesas with vertical sidewalls demonstrate extra spectral features induced by the high Q-factor modes originating due to the total internal reflection at the vertical sidewalls. In contrast, no intensity modulation is found in the shallow mesa devices, due to the weak effective refractive index step. The present results indicate extended opportunities for the emission spectrum control characteristic of quantum dot lasers.

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

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.

Mode-locked quantum dot lasers for picosecond pulse generation

In this work we present a detailed study of picosecond optical pulse generation using high-repetition rate mode-locked quantum dot lasers. MOCVD-grown quantum dot lasers emitting at 1.1μm and MBE-grown quantum dot lasers emitting at 1.3μm are investigated. Passive mode-locking at 10GHz, 18GHz and 36GHz with pulse widths in the 6-12ps range are reported. Hybrid mode-locking is demonstrated at 10GHz, showing a significant improvement in the RF spectral characteristics when compared with passive mode-locking. A timing jitter of 600fs (2.5MHz to 50MHz) is measured in the 18GHz passively mode-locked laser. Autocorrelation techniques are used to characterise the high repetition rate mode-locked lasers as well as the time-bandwidth product of the optical pulses. Fourier-transform limited pulses are obtained from passively mode-locked QD lasers.

Large Modal Gain of InAs/GaAs Quantum Dot Lasers

The gain spectra of QD lasers are measured using the electrical stripe length method. Large modal gain of >20 cm -1 per QD sheet is found. The spectra reflect the theoretically predicted change of the carrier distribution function from non-thermal to thermal with increasing temperature.