Quantum-dot microlasers based on whispering gallery mode resonators.

The subject of this paper is microlasers with the emission spectra determined by the whispering gallery modes. Owing to the total internal reflection of light on the sidewalls, a high Q-factor is achieved until the diameter is comparable to the wavelength. The light emission predominantly occurs in the plane of the structure, which facilitates the microlaser integration with other elements. We focus on microdisk lasers with various types of the In(Ga)As quantum dots (QDs). Deep localization of charge carriers in spatially separated regions suppresses the lateral diffusion and makes it possible to overcome the undesirable effect of non-radiative recombination in deep mesas. Thus, using conventional epitaxial structures and relatively simple post-growth processing methods, it is possible to realize small microlasers capable of operating without temperature stabilization at elevated temperatures. The low sensitivity of QDs to epitaxial and manufacturing defects allows fabricating microlasers using III-V heterostructures grown on silicon.

Virtual cavity in distributed Bragg reflectors.

We show theoretically and experimentally that distributed Bragg reflector (DBR) supports a surface electromagnetic wave exhibiting evanescent decay in the air and oscillatory decay in the DBR. The wave exists in TM polarization only. The field extension in the air may reach several wavelengths of light. Once gain medium is introduced into the DBR a novel class of diode lasers, semiconductor optical amplifiers, light-emitting diodes, etc. can be developed allowing a new type of in-plane or near-field light outcoupling. To improve the wavelength stability of the laser diode, a resonant cavity structure can be coupled to the DBR, allowing a coupled state of the cavity mode and the near-field mode. A GaAlAs-based epitaxial structure of a vertical-cavity surface-emitting laser (VCSEL) having an antiwaveguiding cavity and multiple GaInAs quantum wells as an active region was grown and processed as an in-plane Fabry-Pérot resonator with cleaved facets. Windows in the top stripe contact were made to facilitate monitoring of the optical modes. Three types of the optical modes were observed in electroluminescence (EL) studies under high current densities > 1 kA/cm2. Mode A with the longest wavelength is a VCSEL-like mode emitting normal to the surface. Mode B has a shorter wavelength, emitting light at two symmetric lobes tilted with respect to the normal to the surface in the direction parallel to the stripe. Mode C has the shortest wavelength and shifts with a temperature at a rate 0.06 nm/K. Polarization studies reveal predominantly TE emission for modes A and B and purely TM for mode C in agreement with the theory. Spectral position, thermal shift and polarization of mode C confirm it to be a coupled state of the cavity mode and near-field DBR surface-trapped mode.

Room-temperature yellow-orange (In,Ga,Al)P-GaP laser diodes grown on (n11) GaAs substrates.

We report room temperature injection lasing in the yellow-orange spectral range (599-605 nm) in (AlxGa1-x)0.5In0.5P-GaAs diodes with 4 layers of tensile-strained InyGa1-yP quantum dot-like insertions. The wafers were grown by metal-organic vapor phase epitaxy side-by-side on (811), (211) and (322) GaAs substrates tilted towards the <111> direction with respect to the (100) surface. Four sheets of GaP-rich quantum barrier insertions were applied to suppress leakage of non-equilibrium electrons from the gain medium. Laser diodes having a threshold current densities of ~7-10 kA/cm2 at room temperature were realized for both (211) and (322) surface orientations at cavity lengths of ~1mm. Emission wavelength at room temperature ~600 nm is shorter by ~8 nm than previously reported. As an opposite example, the devices grown on (811) GaAs substrates did not show lasing at room temperature.

Electrically pumped InGaAs/GaAs quantum well microdisk lasers directly grown on Si(100) with Ge/GaAs buffer.

In this work we report, to the best of our knowledge, the first quantum well electrically-pumped microdisk lasers monolithically deposited on (001)-oriented Si substrate. The III-V laser structure was epitaxially grown by MOCVD on silicon with an intermediate MBE-grown Ge buffer. Microlasers with an InGaAs/GaAs quantum well active region were tested at room temperature. Under pulsed injection, lasing is achieved in microlasers with diameters of 23, 27, and 31 µm with a minimal threshold current density of 28 kA/cm2. Lasing spectrum is predominantly single-mode with a dominant mode linewidth as narrow as 35 pm.

Slow passage through thresholds in quantum dot lasers.

A turn on of a quantum dot (QD) semiconductor laser simultaneously operating at the ground state (GS) and excited state (ES) is investigated both experimentally and theoretically. We find experimentally that the slow passage through the two successive laser thresholds may lead to significant delays in the GS and ES turn ons. The difference between the turn-on times is measured as a function of the pump rate of change ɛ and reveals no clear power law. This has motivated a detailed analysis of rate equations appropriate for two-state lasing QD lasers. We find that the effective time of the GS turn on follows an ɛ^{-1/2} power law provided that the rate of change is not too small. The effective time of the ES transition follows an ɛ^{-1} power law, but its first order correction in ln(ɛ) is numerically significant. The two turn ons result from different physical mechanisms. The delay of the GS transition strongly depends on the slow growth of the dot population, whereas the ES transition only depends on the time needed to leave a repellent steady state.

Hybrid InGaAs quantum well-dots nanostructures for light-emitting and photo-voltaic applications.

Hybrid quantum well-dots (QWD) nanostructures have been formed by deposition of 7-10 monolayers of In0.4Ga0.6As on a vicinal GaAs surface using metal-organic chemical vapor deposition. Transmission electron microscopy, photoluminescence and photocurrent analysis have shown that such structures represent quantum wells comprising three-dimensional (quantum dot-like) regions of two kinds. At least 20 QWD layers can be deposited defect-free providing high gain/absorption in the 0.9-1.1 spectral interval. Use of QWD media in a GaAs solar cell resulted in a photocurrent increment of 3.7 mA cm(-2) for the terrestrial spectrum and by 4.1 mA cm(-2) for the space spectrum. Diode lasers based on QWD emitting around 1.1 µm revealed high saturated gain and low transparency current density of about 15 cm(-1) and 37 A cm(-2) per layer, respectively.

Mode selection in InAs quantum dot microdisk lasers using focused ion beam technique.

Optically pumped InAs quantum dot microdisk lasers with grooves etched on their surface by a focused ion beam are studied. It is shown that the radial grooves, depending on their length, suppress the lasing of specific radial modes of the microdisk. Total suppression of all radial modes, except for the fundamental radial one, is also demonstrated. The comparison of laser spectra measured at 78 K before and after ion beam etching for a microdisk of 8 µm in diameter shows a sixfold increase of mode spacing, from 2.5 to 15.5 nm, without a significant decrease of the dominant mode quality factor. Numerical simulations are in good agreement with experimental results.

Control of emission spectra in quantum dot microdisk/microring lasers.

Focused ion beam is applied to quantum dot based microresonators to form pits or groove on their surface. The emission spectra of the resonators based lasers are significantly thinned out after the ion beam milling, and one or two modes become dominant instead of a group of modes having comparable intensities. The linewidth of the lasing mode is kept unchanged, whereas the lasing threshold demonstrates an insignificant growth.