T. Schwarzl

Midinfrared surface-emitting PbSe/PbEuTe quantum-dot lasers

Midinfrared laser emission from self-organized PbSe quantum dots in a high-finesse vertical-cavity surface-emitting laser structure is reported. The structure was grown by molecular-beam epitaxy and consists of high reflectivity epitaxial EuTe/PbEuTe Bragg mirrors with a PbSe/Pb1−xEuxTe quantum-dot superlattice as the active region. Narrow laser emission at 4.2–3.9 μm induced by optical pumping is achieved at temperatures up to 90 K. The observation of simultaneous two-mode emission indicates a width of the inhomogeneously broadened PbSe dot gain spectrum of about 18 meV.

Above-room-temperature mid-infrared lasing from vertical-cavity surface-emitting PbTe quantum-well lasers

Above-room-temperature operation of vertical-cavity surface-emitting lasers emitting in the mid-infrared is reported. The stimulated emission is generated in PbTe quantum wells embedded in two-wavelength microcavities by optically pumping with fs laser pulses. The spectrum of the laser modes is broadened and blue-shifted due to dynamic band filling. The intensity of the mid-infrared emission and the laser threshold depends on the energy of the microcavity resonance. At a wavelength of 3.1 μm, laser operation is obtained up to a temperature of 65 °C, limited by nonradiative recombination processes.

Epitaxial Bragg mirrors for the mid-infrared and their applications

Bragg interference mirrors consisting of stacks of dielectric layers with an optical thickness of a quarter wavelength are of great importance for optoelectronic device applications. For the mid-infrared spectral range mirrors with high reflectivity stop bands are fabricated from combinations of Pb1� xEuxTe/EuTe materials by molecular beam epitaxy on BaF2 substrates. These mirrors designed by the transfer matrix method exhibit reflectivities in excess of 99% by only 3 Bragg mirror layer pairs and very wide stop band regions, reaching a width of up to 60% of the target wavelength. Based on these very efficient mirrors, planar microcavities are demonstrated with an ultra-high effective finesse of up to 1700. Stimulated emission between 3 and 6mm is obtained by optically pumping a vertical-cavity surface-emitting laser containing PbTe quantum wells with Pb1� xEuxTe barriers as active medium embedded between two dielectric Bragg mirrors. Depending on the design of the resonator, pulsed laser operation is observed up to 651C. The enhancement of light absorption in the cavity is used to study the absorption of superlattices containing correlated self-organized PbSe quantum dots. r 2002 Elsevier Science Ltd. All rights reserved.

Size control and midinfrared emission of epitaxial PbTe∕CdTe quantum dot precipitates grown by molecular beam epitaxy

Epitaxial quantum dots with symmetric and highly facetted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. By varying the thickness of the initial 2D layers, the dot size can be effectively controlled between 5 and 25nm, and areal densities as high as 3×1011cm−2 can be achieved. The size control allows the tuning of the quantum dot luminescence over a wide spectral range between 2.2 and 3.7μm. As a result, ultrabroadband emission from a multilayered quantum dot stack is demonstrated, which is a precondition for the development of superluminescent diodes operating in the near infrared and midinfrared.

4.8 μm vertical emitting PbTe quantum-well lasers based on high-finesse EuTe/Pb1−xEuxTe microcavities

Vertical laser emission at 4.8 μm from PbTe quantum wells in high-finesse Pb0.95Eu0.05Te/EuTe microcavity structures at temperatures between 35 and 85 K is reported. The vertical-cavity laser structure was grown by molecular-beam epitaxy on BaF2(111) substrates, and consisted of a 2λ cavity with four 20 nm quantum wells at the cavity antinodes. Laser emission was excited by optical pumping with a pulsed Nd:YVO4 laser. The comparison of the cavity mode positions with envelope function calculations of the quantum-well energy levels indicates that, in this temperature range, lasing is due to transitions between the ground level of the oblique valleys in the conduction and valence bands.

Midinfrared continuous-wave photoluminescence of lead–salt structures up to temperatures of 190 °C

Continuous-wave photoluminescence in the midinfrared for PbSe/PbEuSe and PbTe/PbEuTe multiquantum well structures as well as for PbSe and PbTe bulklike structures, excited by a semiconductor laser diode, is investigated. All samples are grown by molecular-beam epitaxy on BaF2(111) substrates under the same growth conditions. Both for the Te-based systems and for the Se-based systems, it turns out that bulklike structures show photoluminescence up to higher temperatures than multiquantum well structures. In particular, emission spectra from PbTe/PbEuTe multiquantum wells are obtained up to temperatures of 200 K and from PbSe/PbEuSe multiquantum well structures up to 60 °C whereas for bulklike PbSe photoluminescence at temperatures as high as 190 °C is demonstrated.

Midinfrared IV–VI vertical-cavity surface-emitting lasers with zero-, two-, and three-dimensional systems in the active regions

A comparison between IV–VI vertical-cavity surface-emitting midinfrared lasers containing active regions of different dimensionality is presented. Optically pumped laser emission is observed at wavelengths between 3.5 and 4.4 μm. The microcavities consist of high-reflectivity EuTe/PbEuTe Bragg mirrors, with active regions consisting either of a self-organized PbSe/PbEuTe quantum-dot superlattice, PbTe/PbEuTe multiquantum wells, or bulk-like PbTe. For the zero dimensional active medium, laser emission is obtained at temperatures up to 150 K. The results for the lasers with two-dimensional active region are similar to those with the three-dimensional bulk-like active region, for which lasing is observed up to 317 K. The threshold pump intensity is only 4 kW/cm2 at 195 K, and 15.6 kW/cm2 at room temperature.

Ultra-high-finesse IV–VI microcavities for the midinfrared

IV–VI semiconductor-based midinfrared microcavities with very high-quality factors were grown by molecular-beam epitaxy. The structures consist of PbTe/EuTe Bragg mirrors with three to five layer pairs rendering reflectivities in excess of 99.7%. The PbTe resonator layer between the reflectors has an optical length of λ/2, which yields a first-order cavity. The transmission spectra of the microcavities show a very narrow Lorentzian-shaped Fabry–Perot resonance at 1877 cm−1 (λ=5.32 μm) with a full width at half maximum of 0.63 cm−1 78 μeV. This corresponds to an ultrahigh effective cavity finesse of 1700.

Emission properties of 6.7μm continuous-wave PbSe-based vertical-emitting microcavity lasers operating up to 100K

A detailed analysis of midinfrared cw lasing of IV–VI vertical-cavity surface-emitting devices is presented. The structures, based on high-finesse microcavities containing PbSe as active medium, show optically pumped cw laser emission up to temperatures of 100K at a long wavelength of 6.7μm. Stimulated emission with a very narrow beam divergence below 1° and a large temperature tuning range of 70nm is found. The measured linewidth of the laser emission is only 0.6nm, limited by the spectrometer resolution with a strong narrowing with respect to the linewidth of the subthreshold signal. The observed cw output power amounts up to 1.2mW at 85K.

Midinfrared electroluminescence from PbTe/CdTe quantum dot light-emitting diodes

Midinfrared electroluminescence of epitaxial PbTe quantum dots in CdTe with emission in the 2–3 μm wavelength range is demonstrated up to room temperature. The light-emitting diode structures were grown by molecular beam epitaxy with the active PbTe quantum dots embedded in the intrinsic zone of a CdTe/CdZnTe p-i-n junction on GaAs (100) substrates. The current and temperature dependences of the electroluminescence emission are presented. The comparison with photoluminescence measurements shows that midinfrared light-emission from the diodes originates from the quantum dots.