Y.-C. Xin

Growth mechanisms of highly mismatched AlSb on a Si substrate

We describe the growth mechanisms of highly mismatched (Δao∕ao=13%) defect-free AlSb on Si(001) substrates. Nucleation occurs during the first few monolayers of AlSb deposition by crystalline quantum dot formation. With continued growth, the islands coalesce into a bulk material with no vertically propagating defects. Strain energy from the AlSb∕Si interface is dissipated by crystallographic undulations in the zinc-blende lattice, as confirmed by high-resolution transmission electron microscopy (TEM) images. Reciprocal space analysis of the TEM images corroborates a crystallographic rotation associated with the undulations. The resulting AlSb material is >98% relaxed according to x-ray diffraction analysis.

InAs quantum-dot GaAs-based lasers grown on AlGaAsSb metamorphic buffers

Self-assembled InAs quantum-dot lasers grown by molecular-beam epitaxy using an AlGaAsSb metamorphic buffer layer on a GaAs substrate are reported. The resulting quantum-dot ensemble has a density >3×1010/cm2 and a ground-state transition ranging from 1.46 to 1.63 μm. Pulsed, room-temperature operation generates lasing from the first excited state transition at wavelengths ranging from 1.27 to 1.34 μm. The minimum threshold current density (304 A/cm2) is achieved for a 7.7 mm cavity with cleaved, uncoated facets.

Optical gain and absorption of quantum dots measured using an alternative segmented contact method

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm-1 at nominal gain values below 2 cm-1, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm-1 at a gain less than 1 cm-1. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described

Reconfigurable quantum dot monolithic multi-section passive mode-locked lasers

We investigate the dynamical response of a quantum dot photonic integrated circuit formed with a combination of eleven passive and active gain cells operating when these cells are appropriately biased as a multi-section quantum dot passively mode-locked laser. When the absorber section is judiciously positioned in the laser cavity then fundamental frequency and harmonic mode-locking at repetition rates from 7.2GHz to 51GHz are recorded. These carefully engineered multi-section configurations that include a passive wave-guide section significantly lower the pulse width up to 34% from 9.7 to 6.4 picoseconds, as well increase by 49% the peak pulsed power from 150 to 224 mW, in comparison to conventional two-section configurations that are formed on the identical device under the same average power. In addition an ultra broad operation range with pulse width below ten picoseconds is obtained with the 3rd-harmonic mode-locking configuration. A record peak power of 234 mW for quantum dot mode-locked lasers operating over 40 GHz is reported for the first time.

Quantum dot lasers based on a stacked and strain-compensated active region grown by metal-organic chemical vapor deposition

We demonstrate an InAs∕GaAs quantum dot (QD) laser based on a strain-compensated, three-stack active region. Each layer of the stacked QD active region contains a thin GaP (Δao=−3.8%) tensile layer embedded in a GaAs matrix to partially compensate the compressive strain of the InAs (Δao=7%) QD layer. The optimized GaP thickness is ∼4MLs and results in a 36% reduction of compressive strain in our device structure. Atomic force microscope images, room-temperature photoluminescence, and x-ray diffraction confirm that strain compensation improves both structural and optical device properties. Room-temperature ground state lasing at λ=1.249μm, Jth=550A∕cm2 has been demonstrated.

Strain compensation technique in self-assembled InAs/GaAs quantum dots for applications to photonic devices

We report the strain compensation (SC) technique for a stacked InAs/GaAs self-assembled quantum dot (QD) structure grown by metalorganic chemical vapour deposition (MOCVD). Several techniques are used to investigate the effect of the SC technique: the high-resolution x-ray diffraction (XRD) technique is used to quantify the reduction in overall strain, atomic force spectroscopy is used to reveal that the SC layer improves the QD uniformity and reduces the defect density and photoluminescence characterization is used to quantify the optical property of stacked InAs QDs. In addition, experimental and mathematical evaluation of reduction in the strain field in the compensated structure is conducted. We identify two types of strain in stacked QD samples, homogeneous and inhomogeneous strain. XRD spectra indicate that vi > 36% reduction in the homogeneous strain can be accomplished. Inhomogeneous strain field is investigated by studying the strain coupling probability as a function of the spacer thickness, indicating that 19% reduction in inhomogeneous strain within SC structures has been evaluated. Next, device application of SC techniques including lasers and modulators is reported. Room temperature ground-state lasing from 6-stack InAs QDs with GaP SC is realized at a lasing wavelength of 1265 nm with a threshold current density of 108 A cm−2. The electro-optic (EO) properties of 1.3 µm self-assembled InAs/GaAs QDs are investigated. The linear and quadratic EO coefficients are 2.4 × 10−11 m V−1 and 3.2 × 10−18 m2 V−2, respectively, which are significantly larger than those of GaAs bulk materials. Also, the linear EO coefficient is almost comparable to that of lithium niobate.

Selective area growth of InAs quantum dots formed on a patterned GaAs substrate

We describe the growth and characterization of InAs quantum dots (QDs) on a patterned GaAs substrate using metalorganic chemical vapor deposition. The QDs nucleate on the (001) plane atop GaAs truncated pyramids formed by a thin patterned SiO2 mask. The base diameter of the resulting QDs varies from 30 to 40nm depending on the size of the mask. With specific growth conditions, we are able to form highly crystalline surface QDs that emit at 1.6μm under room-temperature photopumped conditions. The crystalline uniformity and residual strain is quantified in high-resolution transmission electron microscopy images and high-resolution x-ray reciprocal space mapping. These strained QDs may serve as a template for selective nucleation of a stacked QD active region.

Optical absorption cross section of quantum dots

We have measured the modal optical absorption spectrum of a three-layer system of InAs quantum dots in a slab waveguide geometry, observing distinct absorption peaks for the ground and excited states. The spectrally integrated absorption cross section for the ground and first excited states are determined to be σ0 = (0.43 ± 0.1) × 10−15 and (0.92 ± 0.2) × 10−15 cm2 eV, respectively. Assuming that the spectral shapes are determined primarily by the inhomogeneous size distribution of dots the Gaussian linewidths are 16 and 19 meV for the ground and first excited state transitions, respectively. The peak ground state absorption cross section is 1.1 × 10−14 cm2. The ground state spectrally integrated cross section estimated by a theory with the envelope function overlap integral taken to be unity is 0.40 × 10−15 cm2 eV, in agreement with the measured value. We conclude that on the basis of the spectrally integrated cross section there is no evidence for a substantial reduction in the strength of the fundamental light–matter interaction in dots compared with systems of higher dimensionality.

Cavity design and characteristics of monolithic long-wavelength InAs/InP quantum dash passively mode-locked lasers.

By extending the net-gain modulation phasor approach to account for the discrete distribution of the gain and saturable absorber sections in the cavity, a convenient model is derived and experimentally verified for the cavity design of two-section passively mode-locked quantum dash (QDash) lasers. The new set of equations can be used to predict functional device layouts using the measured modal gain and loss characteristics as input. It is shown to be a valuable tool for realizing the cavity design of monolithic long-wavelength InAs/InP QDash passively mode-locked lasers.

Compact Optical Generation of Microwave Signals Using a Monolithic Quantum Dot Passively Mode-Locked Laser

Microwave signal generation from the saturable absorber of a monolithic quantum dot passively mode-locked laser is presented. We observe a differential efficiency of 33% that measures the optical-to-RF power conversion. An optimum extraction efficiency of the saturable absorber of about 86% is also found. To assess the stability of the device, the mode-locking operation regime of the quantum dot device is analyzed and compared to the quantum well system. Our findings confirm that quantum dot mode-locked lasers are suitable candidates for the optical generation of RF signals in a compact and efficient semiconductor device.