A. Stintz

Optical characteristics of 1.24-μm InAs quantum-dot laser diodes

The optical characteristics of the first laser diodes fabricated from a single-InAs quantum-dot layer placed inside a strained InGaAs QW are described. The saturated modal gain for this novel laser active region is found to be 9-10 cm/sup -1/ in the ground state. Room temperature threshold current densities as low as 83 A/cm/sup 2/ for uncoated 1.24-/spl mu/m devices are measured, and operating wavelengths over a 190-nm span are demonstrated.

Room-temperature operation of InAs quantum-dash lasers on InP [001]

The first self-assembled InAs quantum dash lasers grown by molecular beam epitaxy on InP (001) substrates are reported. Pulsed room-temperature operation demonstrates wavelengths from 1.60 to 1.66 μm for one-, three-, and five-stack designs, a threshold current density as low as 410 A/cm2 for singlestack uncoated lasers, and a distinctly quantum-wire-like dependence of the threshold current on the laser cavity orientation. The maximal modal gains for lasing in the ground-state with the cavity perpendicular to the dash direction are determined to be 15 cm–1 for single-stack and 22 cm–1 for five-stack lasers.

Low-threshold current density 1.3-μm InAs quantum-dot lasers with the dots-in-a-well (DWELL) structure

The wavelength of InAs quantum dots in an In/sub 0.15/Ga/sub 0.85/As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-/spl mu/m. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm/sup -2/ for 7.8-mm cavity length cleaved facet lasers under pulsed operation.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL lasers /spl eta//sub i/ for the dot densities studied.

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.

High-responsivity, normal-incidence long-wave infrared (λ∼7.2 μm) InAs/In0.15Ga0.85As dots-in-a-well detector

Normal incidence InAs/In0.15Ga0.85As dots-in-a-well detectors operating at T=78 K with λp∼7.2 μm and a spectral width (Δλ/λ) of 35% are reported. The peak at 7.2 μm is attributed to the bound-to-bound transitions between the ground state of the dot and the states within the InGaAs well. A broad shoulder around 5 μm, which is attributed to the bound-to-continuum transition, is also observed. Calibrated blackbody measurements at a device temperature of 78 K yield a peak responsivity of 3.58 A/W (Vb=−1 V), peak detectivity=2.7×109 cm Hz1/2/W (Vb=−0.3 V), conversion efficiency of 57% and a gain ∼25.

Characterization of InAs quantum dots in strained InxGa1−xAs quantum wells

The properties of InAs quantum dots placed in a strained InGaAs quantum well are investigated. The structures are grown by solid-source molecular beam epitaxy on GaAs substrates and are characterized using photoluminescence and atomic force microscopy. Emission wavelength and the optical quality of the quantum dots vary with growth temperature and also depend on the position of the dots in the well. A strong dependence of the dot properties on the capping conditions is established. A postgrowth anneal similar to a typical laser cladding growth results in a large photoluminescence (PL) blueshift and reduces the PL intensity by more than two orders of magnitude. It is shown that these dots-in-a-well structures have superior optical properties as compared to conventional InAs dots in a GaAs matrix, and their emission wavelength can be tuned past the technologically important wavelength of 1.3 μm.

Passive mode-locking in 1.3 μm two-section InAs quantum dot lasers

Passive mode locking was achieved at 1.3 μm in oxide-confined, two-section, bistable quantum dot (QD) lasers with an integrated intracavity QD saturable absorber. Fully mode-locked pulses at a repetition rate of 7.4 GHz with a duration of 17 ps were observed under appropriate bias conditions. No self-pulsation accompanied the mode locking. These results suggest that a carefully designed QD laser is a candidate for ultrashort pulse generation.

Cavity Q, mode volume, and lasing threshold in small diameter AlGaAs microdisks with embedded quantum dots

The quality factor (Q), mode volume (V(eff)), and room-temperature lasing threshold of microdisk cavities with embedded quantum dots (QDs) are investigated. Finite element method simulations of standing wave modes within the microdisk reveal that Veff can be as small as 2(lambda/n)(3) while maintaining radiation-limited Qs in excess of 10(5). Microdisks of diameter 2 microm are fabricated in an AlGaAs material containing a single layer of InAs QDs with peak emission at lambda = 1317 nm. For devices with V(eff) ~2 (lambda/n)(3), Qs as high as 1.2x10(5) are measured passively in the 1.4 microm band, using an optical fiber taper waveguide. Optical pumping yields laser emission in the 1.3 microm band, with room temperature, continuous-wave thresholds as low as 1 microW of absorbed pump power. Out-coupling of the laser emission is also shown to be significantly enhanced through the use of optical fiber tapers, with laser differential efficiency as high as xi ~ 16% and out-coupling efficiency in excess of 28%.

Three-color (λp1∼3.8 μm, λp2∼8.5 μm, and λp3∼23.2 μm) InAs/InGaAs quantum-dots-in-a-well detector

We report a three-color InAs/InGaAs quantum-dots-in-a-well detector with center wavelengths at ∼3.8, ∼8.5, and ∼23.2 μm. We believe that the shorter wavelength responses (3.8 and 8.5 μm) are due to bound-to-continuum and bound-to-bound transitions between the states in the dot and states in the well, whereas the longer wavelength response (23.2 μm) is due to intersubband transition between dot levels. A bias-dependent activation energy ∼100 meV was extracted from the Arrhenius plots of the dark currents, which is a factor of 3 larger than that observed in quantum-well infrared photodetectors operating at comparable wavelengths.