Ground State Gain Research Articles

Transverse mode switching in quantum well-dot lasers triggered by gain saturation

Having investigated narrow-ridge lasers based on a single layer of InGaAs quantum well-dots emitting at the wavelength of 1.06 μm we have shown anomalous transverse multimode lasing not observed in the broad-area lasers. The effect is shown to be caused by the wavelength shift triggered by the ground state gain saturation.

Gain dynamics of quantum dot devices for dual-state operation

Ground state gain dynamics of In(Ga)As-quantum dot excited state lasers are investigated via single-color ultrafast pump-probe spectroscopy below and above lasing threshold. Two-color pump-probe experiments are used to localize lasing and non-lasing quantum dots within the inhomogeneously broadened ground state. Single-color results yield similar gain recovery rates of the ground state for lasing and non-lasing quantum dots decreasing from 6 ps to 2 ps with increasing injection current. We find that ground state gain dynamics are influenced solely by the injection current and unaffected by laser operation of the excited state. This independence is promising for dual-state operation schemes in quantum dot based optoelectronic devices.

Resonant intersubband plasmon induced current in InGaAs quantum wells on GaAs

We present measurements of the current enhancement due to the coupling of two intersubband plasmons in In0.05Ga0.95As quantum wells. With changing bias, an emissive and an absorptive intersubband plasmon mode cross attractively and trapped electrons in the ground state gain enough energy from the plasma wave to be lifted up to the second subband, where they can contribute to the current. This effect can be directly observed as an increase of 33% in the current. A magnetic field applied parallel to the growth direction allows a control of the strength of the intersubband plasmon coupling up to a quenching.

Self-Heating Effect on the Two-State Lasing Behaviors in 1.3-µm InAs–GaAs Quantum-Dot Lasers

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-µm self-assembled InAs–GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light–current (L–I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.

Interconnection between ground state and excited state gain in InAs/GaAs quantum dot semiconductor optical amplifiers

AbstractDifferent energy levels are generally assumed to be associated with ground state (GS) and excited state (ES) gain in semiconductor quantum dot lasers and amplifiers. We present calculations based on an 8 band k ·P Hamiltonian which show that this is not the case. Two distinct absorption bands are calculated due to transitions between the GS electrons and all dot hole states. In addition to the peak due to transitions between GS electron and GS hole levels, a higher energy band is also calculated due to transitions between GS electrons and highly excited dot hole states. We demonstrate that the removal of electron–hole pairs through stimulated emission at the ES gain maximum can result in an amplitude reduction for a probe pulse tuned to the GS gain maximum, due to the existence of these higher energy transitions, and also due to the effects of two photon absorption. We also find that the instantaneous phase change at the ES gain maximum due to dot to dot interband transitions is small compared to the measured value. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experiments and Simulation of Spectrally-Resolved Static and Dynamic Properties in Quantum Dot Two-State Lasing

We investigated the spectrally-resolved static and dynamic properties of quantum-dot lasers. The ground-state quenching and abnormal turn-on delay in quantum-dot two-state lasing behaviors are systematically studied. We found that the more saturated ground-state gain would quench ground-state emission more easily and result in the abnormal turn-on delay. Using a rate-equation model, we have successfully simulated such phenomena by considering the current heating effect.

Zero and Controllable Linewidth Enhancement Factor in p-Doped 1.3 µm Quantum Dot Lasers

We have studied the effects of δ-p-doping on the operating characteristics of quantum dot (QD) lasers. It is well known that increasing δ-p-doping in a laser core increases both the internal loss and threshold current. For QD lasers however, it has the beneficial effects of increasing differential efficiency and saturated ground state gain. A further consequence of increased δ-p-doping is an ultra low linewidth enhancement factor (LEF) that can be tuned through zero and even made negative with increased doping at low injected current densities.

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

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.

Plasmon signatures in high harmonic generation

High harmonic generation in polarizable multi-electron systems is investigated in the framework of multi-configuration time-dependent Hartree–Fock. The harmonic spectra exhibit two cut-offs. The first cut-off is in agreement with the well-established, single active electron cut-off law. The second cut-off presents a signature of multi-electron dynamics. The strong laser field excites nonlinear plasmon oscillations. Electrons that are ionized from one of the multi-plasmon states and recombine to the ground state gain additional energy, thereby creating the second plateau.

Linewidth Study of InAs–InGaAs Quantum Dot Distributed Feedback Lasers

The linewidth of laterally loss-coupled distributed feedback (DFB) lasers based on InAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) is investigated. Narrow linewidth operation of QD devices is demonstrated. A linewidth-power product less than 1.2 MHz /spl middot/ mW is achieved in a device of 300-μm cavity length for an output power up to 2 mW. Depending on the gain offset of the DFB modes from the QD ground state gain peak, linewidth rebroadening or a floor is observed at a cavity photon density of about 1.2-2.4×10/sup 15/ cm/sup -3/, which is much lower than in QW lasers. This phenomenon is attributed to the enhanced gain compression observed in QDs.

High-temperature operating 1.3-μm quantum-dot lasers for telecommunication applications

High-performance 1.3-μm-emitting quantum-dot lasers were fabricated by self-organized growth of InAs dots embedded in GaInAs quantum wells. The influence of the number of quantum-dot layers on the device performance was investigated. Best device results were achieved with six-dot layers. From the length dependence; a maximum ground state gain of 17 cm/sup -1/ for six dot layers could be determined. Ridge waveguide lasers with a cavity length of 400 μm and high-reflection coatings show threshold currents of 6 mA and output powers of more than 5 mV. Unmounted devices can be operated in continuous wave mode up to 85/spl deg/C. A maximum operating temperature of 160/spl deg/C was achieved in pulsed operation for an uncoated 2.5-mm-long ridge waveguide laser.

Ground-state emission and gain in ultralow-threshold InAs-InGaAs quantum-dot lasers

Emission spectra and modal optical gain are investigated in ultralow-threshold MBE-grown InAs-InGaAs quantum dot (QD) structures. The record lowest room-temperature inversion current is found to be /spl sim/13 A cm/sup -2/. The rate-equation model is proposed describing the optical gain related to the ground-state (GS) transitions in QDs. The ground-state gain goes to the maximum value that corresponds to the total inversion of available levels. The gain cross section for the GS emission is estimated as /spl sim/7/spl times/10/sup -15/ cm/sup 2/.

Transition dipole moment of InAs/InGaAs quantum dots from experiments on ultralow-threshold laser diodes

Semiconductor ultralow-threshold InAs quantum-dot lasers are investigated operating at 1230–1250 nm at room temperature (laser threshold range is of 16–83 A/cm2 for ground-state emission). The dependence of gain on current is derived from measurements of the threshold current as a function of the cavity length. The ground-state gain appears at very low current: the inversion threshold of ∼13 A/cm2 is a record low value. Analysis of these data for diodes of different molecular beam epitaxial-grown wafers leads to a squared dipole moment of the transition of ∼9.2×10−57 C2 m2 that corresponds to the length of elementary dipole of ∼0.6 nm.

Gain and linewidth enhancement factor in InAs quantum-dot laser diodes

Amplified spontaneous emission measurements are investigated below threshold in InAs quantum-dot lasers emitting at 1.22 /spl mu/m. The dot layer of the laser was grown in a strained quantum well (QW) on a GaAs substrate. Ground state gain is determined from cavity mode Fabry-Perot modulation. As the injection current increases, the gain rises super-linearly while changes in the index of refraction decrease. Below the onset of gain saturation, the linewidth enhancement factor is as small as 0.1, which is significantly lower than that reported for QW lasers.