R. L. Sellin

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.

Subpicosecond high-power mode locking using flared waveguide monolithic quantum-dot lasers

Ultrashort pulse, high-power mode locking is demonstrated in InGaAs quantum dot lasers using a flared waveguide laser incorporating a narrow waveguide sections acting as both a mode filter and saturable absorber. 24GHz mode locking with 780fs pulses and 500mW peak powers are demonstrated at an emissions wavelength of 1.3μm.

Reversed truncated cone composition distribution of In0.8Ga0.2As quantum dots overgrown by an In0.1Ga0.9As layer in a GaAs matrix

We present cross-sectional scanning tunneling microscopy results of self-organized In0.8Ga0.2As quantum dots covered by an In0.1Ga0.9As film inside a GaAs matrix prepared by metalorganic chemical vapor deposition. From images of quantum dots with atomic resolution, we determine a spatial distribution of the In composition within the dots with a shape of a reversed truncated cone. The wetting layer and the overgrown In0.1Ga0.9As layer show vertical intermixing.

Over 30-fold enhancement of light extraction from free-standing photonic crystal slabs with InGaAs quantum dots at low temperature

Highly efficient extraction of photoluminescence is observed from two-dimensional photonic crystal slabs employing InGaAs quantum dots as active material. The introduction of quantum dots reduces diffusion of carriers and thereby suppresses the surface recombination at the air-hole sidewalls of the photonic crystal. Around the normalized frequency of 0.7, over thirty-fold enhancement of the photoluminescence extraction is achieved at 78 K, indicating strong coupling to leaky modes of the free-standing photonic crystal slab. In addition, when the photoluminescence spectra overlaps with a photonic band gap, enhanced light extraction originating from the photonic band gap is observed experimentally.

Direct modulation and mode locking of 1.3 μm quantum dot lasers

We report 7 GHz cut-off frequency, 2.5 and 5 Gb s−1 eye pattern measurements upon direct modulation of 1.3 μm quantum dot lasers grown without incorporating phosphorus in the layers. Passive mode-locking is achieved from very low frequencies up to 50 GHz and hybrid mode-locking is achieved up to 20 GHz. The minimum pulse width of the Fourier-limited pulses at 50 GHz is 3 ps, with an uncorrelated timing jitter below 1 ps. The lasers are optimized for high frequency operation by a ridge waveguide design that includes etching through the active layer and ridge widths down to 1 μm. The far-field shape for 1 μm is close to circular with a remaining asymmetry of 1.2.

Radiation hardness of InGaAs/GaAs quantum dots

The interaction between point defects in the matrix and excitons localized in self-organized InGaAs/GaAs quantum dots is investigated for structures irradiated by protons. The exciton ground state is demonstrated to be unaffected by radiation doses up to 1014 p/cm2. The close proximity of radiation-induced defects leads to a strong nonmonotonous temperature dependence of the luminescence yield: Carriers are lost via tunneling from excited quantum dot states to irradiation-induced defects below ∼100 K, whereas at higher temperatures, carriers escape to the barrier and are captured by defects.

1.24 μm InGaAs/GaAs quantum dot laser grown by metalorganic chemical vapor deposition using tertiarybutylarsine

Metalorganic chemical vapor deposition of GaAs-based laser diodes, using self-organized InGaAs quantum dots (QDs), emitting at >1.24 μm is demonstrated. The environment-friendly alternative precursor tertiarybutylarsine is used as a substitute for arsenic hydride. The active region contains ten closely stacked InGaAs QD layers embedded in a GaAs matrix. Lasing emission at such long wavelengths was achieved by overgrowing the In0.65Ga0.35As QDs with a thin In0.2Ga0.8As film. The application of an in situ annealing step leading to the evaporation of plastically relaxed defect clusters is shown to be decisive for the laser performance. A transparency current density of 7.2 A/cm2 per QD layer and an internal quantum efficiency of 75% were achieved at room temperature.

Alternative-precursor metalorganic chemical vapor deposition of self-organized InGaAs/GaAs quantum dots and quantum-dot lasers

Metalorganic chemical vapor deposition of laser diodes based on triple stacks of self-organized InxGa1−xAs/GaAs quantum dots (QDs) as active medium using the alternative precursor tertiarybutylarsine (TBAs) is reported. Epitaxy of monodispersed QDs using TBAs is demonstrated. Due to the high cracking efficiency of TBAs at low temperatures, the crucial growth parameters V/III ratio and temperature can be tuned almost independently. Ridge-waveguide QD lasers show a transparency current of 29.7 A/cm2—equivalent to 9.9 A/cm2 per QD layer—an internal quantum efficiency of 91.4%, and an internal optical loss of 2.2 cm−1.