InGaAs-GaAs Quantum-dot Lasers Research Articles

Dependence of Linewidth Enhancement Factor on Duty Cycle in InGaAs–GaAs Quantum-Dot Lasers

The linewidth enhancement factor (alpha-factor) of InGaAs-GaAs quantum-dot (QD) lasers is studied as a function of the current pulse duty cycle. In this letter, we demonstrate the impact that current pulse duty cycle has on measured -factor in QD lasers due to increased thermal effects. Below threshold, while comparable quantum-well lasers blue-shift under all conditions, the QD lasers demonstrate an unusual sensitivity of chirp to duty cycle of the current, with a blue-shift at low duty cycles, and a red-shift at high duty cycles and dc current, which may lead to negative -factor. The assumption of Gaussian gain distribution and linear gain shift with temperature is used to analyze the -factor at different duty cycle currents. The result agrees well with the positive, zero, and negative alpha-factors reported for QD lasers. The effect of duty cycle is quite significant and can shift the alpha-factor substantially going from pulse to dc. This is due to the low thermal conductivity of QD regions and increased thermal effects in those devices.

Carrier dynamics and high-speed modulation properties of tunnel injection InGaAs-GaAs quantum-dot lasers

We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In/sub 0.4/Ga/sub 0.6/As-GaAs-AlGaAs heterostructures, which show that at room temperature, injected electrons preferentially occupy the excited states in the dots and states in the barriers layers. The relaxation time of these carriers to the dot ground state is >100 ps. This leads to large gain compression in quantum-dot (QD) lasers and limits the attainable small-signal modulation bandwidth to /spl sim/ 5-7 GHz. The problem can be alleviated by tunneling "cold" electrons into the lasing states of the dots from an adjoining injector layer. The design, growth, and steady-state and small-signal modulation characteristics of tunnel injection In/sub 0.4/Ga/sub 0.6/As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by three-pulse DTS measurements, are /spl sim/ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/I/sub th/ /spl sim/ 7 is f/sub -3 dB/ = 23 GHz and the gain compression factor for this frequency response is /spl epsiv/ = 8.2 /spl times/ 10/sup -16/ cm/sup 3/. The differential gain obtained from the modulation data is dg/dn /spl cong/ 2.7 /spl times/ 10/sup -14/ cm/sup 2/ at room temperature. The value of the K-factor is 0.205 ns and the maximum intrinsic modulation bandwidth is 43.3 GHz. Analysis of the transient characteristics with appropriate carrier and photon rate equations yield modulation response characteristics identical to the measured ones. The Auger coefficients are in the range /spl sim/ 3.3 /spl times/ 10/sup -29/ cm/sup 6//s to 3.8 /spl times/ 10/sup -29/ cm/sup 6//s in the temperature range 15/spl deg/C<T<85/spl deg/C, determined from large-signal modulation measurements, and these values are smaller than those measured in separate confinement heterostructure QD lasers. The measured high-speed data are comparable to, or better than, equivalent quantum-well lasers for the first time.

Continuous-wave low-threshold performance of 1.3-/spl mu/m InGaAs-GaAs quantum-dot lasers

The understanding of material quality and luminescence characteristics of InGaAs-GaAs quantum dots (QD's) is advancing rapidly. Intense work in this area has been stimulated by the recent demonstration of lasing from a QD active region at the technologically important 1.3-/spl mu/m wavelength from a GaAs-based heterostructure laser. Already, several groups have achieved low-threshold currents and current densities at room temperature from In(Ga)As QD active regions that emit at or close to 1.3 /spl mu/m. In this paper, we discuss crystal growth, QD emission efficiency, and low-threshold lasing characteristics for 1.3-/spl mu/m InGaAs-GaAs QD active regions grown using submonolayer depositions of In, Ga, and As. Oxide-confinement is effective in obtaining a low-threshold current of 1.2 mA and threshold-current density of 19 A/cm/sup 2/ under continuous-wave (CW) room temperature (RT) operation. At 4 K, a remarkably low threshold-current density of 6 A/cm/sup 2/ is obtained.

Low-threshold oxide-confined 1.3-μm quantum-dot laser

Data are presented on low threshold, 1.3-/spl mu/m oxide-confined InGaAs-GaAs quantum dot lasers. A very low continuous-wave threshold current of 1.2 mA with a threshold current density of 28 A/cm/sup 2/ is achieved with p-up mounting at room temperature. For slightly larger devices the continuous-wave threshold current density is as low as 19 A/cm/sup 2/.

Theoretical calculation of lasing spectra of quantum-dot lasers: effect of homogeneous broadening of optical gain

A theoretical calculation is presented for the effect of homogeneous broadening of optical gain on lasing spectra of quantum-dot lasers. Based on a coupled set of rate equations considering both the size distribution of quantum dots and a series of longitudinal cavity modes, we show that dots with different energies start lasing independently due to their spatial localization when the gain spectrum is a delta-like function, and that the dot ensemble contributes to a narrow-line lasing collectively under large homogeneous broadening. The result explains quite excellently the experimental lasing spectra found in self-assembled InGaAs-GaAs quantum-dot lasers.

Enhanced modulation bandwidth (20 GHz) of In/sub 0.4/Ga/sub 0.6/As-GaAs self-organized quantum-dot lasers at cryogenic temperatures: role of carrier relaxation and differential gain

Measurements of the threshold current, slope efficiency and optical modulation characteristics of self-assembled InGaAs-GaAs quantum-dot lasers have been made in the temperature range of 20-200 K in order to understand the carrier dynamics in these devices. The dc characteristics of these devices showed a region of almost temperature independent threshold current up to 85 K (T/sub 0/=670 K) with a maximum slope efficiency at 150 K. The maximum measured bandwidth increased from 5 GHz at room temperature to 20 GHz at 80 K. This is consistent with the bandwidth being limited by carrier relaxation time through electron-hole scattering.