We report on comparative optical studies of InAs/Al0.44Ga0.56As quantum dots (QDs) grown by molecular beam epitaxy either with or without a thin GaAs interlayer inserted between the AlGaAs barrier and InAs QDs. Emission properties of individual QDs are investigated by micro-photoluminescence spectroscopy using 500-nm-size etched cylindric mesa structures. The single-photon statistics of the QDs of both types, emitting in the red spectral range between 636 and 750 nm, is confirmed by the measurements of the second-order correlation function. A negligibly small exciton fine structure splitting is detected in the majority of the QDs grown with the GaAs interlayer that implies the possibility of generating pairs of entangled photons with high entanglement fidelity.
We report on structural and photoluminescence (PL) studies of wide gap II‐VI laser heterostructures involving the graded index waveguide (GIW) based on short period Zn(Mg)SSe/ZnSe superlattices (SLs) and the active region comprising single or multiple electronically‐coupled CdSe/ZnSe QD sheets. Precise compensation of elastic stresses in the SL waveguide and optimization of the ZnSe/GaAs initial growth stage have resulted in good crystalline quality of the laser structures and reduction of the extended defect density down to 104 cm‐2. Express monitoring of the defect density by using the photoluminescence microscope was supported by transmission electron microscopy studies. PL data have demonstrated efficient transport of nonequilibrium carriers through the GIW SLs to the active region (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A violet-green integrated laser converter with a quantum efficiency up to 25.4% and maximum output pulse power of 154 mW at a wavelength of 543 nm has been fabricated on the basis of the II-VI laser heterostructure comprising an active region with five electronicallycoupled CdSe/ZnSe quantum dot sheets embedded in a Zn(Mg)SSe/ZnSe superlattice graded-index waveguide. A pulse InGaN/GaN laser diode emitting at 416 nm was used as a pumping source of the laser converter.Introduction: Compact semiconductor green lasers (l 530 -550 nm) are strongly demanded for the fabrication of low-cost, high resolution pico-projectors which can be incorporated in smartphones, digital cameras, media players, laptops etc. In spite of significant progress in the development of direct-emitting green InGaN laser diodes (LDs) grown on free-standing GaN substrates (l ¼ 523 -525 nm, CW operation mode, output power of 38 -50 mW, WPE of 2.2 -2.3%) [1,2], alternative ways to obtain semiconductor green lasers are still of great importance because the InGaN LDs demonstrate rather high threshold current density (J th 9 kA/cm 2 [2]) steeply increasing with l.One of the alternative approaches has been the blue-green laser converter composed of a high-efficiency Cd(Zn)Se/ZnMgSSe laser heterostructure optically pumped by the emission of a blue-violet III-N laser, which was proposed and realised in our early works [3,4], where the completely optical design was employed. To realise the next generation of converters using blue-violet III-N LDs as pumping sources two essential obstacles should be overcome. First, the threshold power density of II-VI laser heterostructures has to be reduced via structure design and growth optimisation, and, secondly, high-power blue-violet III-N LDs or high-brightness LEDs should appear on the market, which, in turn, is governed by the progress in III-N LD technology.Recently, utilising II-VI laser heterostructures of conventional design [4], but containing five electronically coupled CdSe/ZnSe quantum dot (QD) planes instead of two, allowed us to achieve the pulse output power in green of 65 mW and the quantum conversion efficiency of 8% for the converter pumped by a commercial blue-violet LD (l exc ¼ 416 nm) [5]. The threshold pulse power in that case was 0.65-0.7 W. This Letter reports on the next step in the realisation of high-efficiency violet-togreen electrically pumped laser converters based on the optimised II-VI laser heterostructures with a superlattice (SL) graded-index waveguide (GIW), which demonstrated significantly reduced threshold power density P th 1.5 kW/cm 2 [6].
This paper reports on theoretical calculations and fabrication by molecular beam epitaxy of wide-gap IIVI heterostructures emitting in the true yellow range (560600 nm) at room temperature. The active region of the structures comprises CdSe quantum dot active layer embedded into a strained Zn1−xCdxSe (x = 0.2−0.5) quantum well surrounded by a Zn(S,Se)/ZnSe superlattice. Calculations of the CdSe/(Zn,Cd)Se/Zn(S,Se) quantum dot quantum well luminescence wavelength performed using the envelope-function approximation predict rather narrow range of the total Zn1−xCdxSe quantum well thicknesses (d ≈ 2−4 nm) reducing eciently the emission wavelength, while the variation of x (0.20.5) has much stronger eect. The calculations are in a reasonable agreement with the experimental data obtained on a series of test heterostructures. The maximum experimentally achieved emission wavelength at 300 K is as high as 600 nm, while the intense room temperature photoluminescence has been observed up to λ = 590 nm only. To keep the structure pseudomorphic to GaAs as a whole the tensile-strained surrounding ZnS0.17Se0.83/ZnSe superlattice were introduced to compensate the compressive stress induced by the Zn1−xCdxSe quantum well. The graded-index waveguide laser heterostructure with a CdSe/Zn0.65Cd0.35Se/Zn(S,Se) quantum dotquantum well active region emitting at λ = 576 nm (T = 300 K) with the 77 to 300 K intensity ratio of 2.5 has been demonstrated.
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