G. C. Dente

High power and high brightness from an optically pumped InAs/InGaSb type-II midinfrared laser with low confinement

We report on optically pumped semiconductor lasers emitting near 3.8 μm that exhibit high power and low output divergence. The lasers incorporate multiple InAs/InGaSb/InAs type-II wells imbedded in an InGaAsSb waveguide that is designed to absorb the pump emission. When operated at 85 K, 0.25 mm×2.5 mm broad area devices produce >5 W of peak power under long pulse conditions. Moreover, these extremely bright devices exhibit a fast axis divergence of only ∼15° full width at half maximum (FWHM), coupled with a slow axis divergence of ∼6° FWHM. The first is due to the reduced optical confinement in the transverse direction, while the latter is attributed to the suppression of filament formation, which is another beneficial consequence of the low optical confinement.

High performance optically pumped antimonide lasers operating in the 2.4–9.3μm wavelength range

We provide an update on the further development of optically pumped semiconductor lasers based on the InAs∕InGaSb∕InAs type-II quantum wells. We show increased power generation, as well as the inherent flexibility to produce devices that can emit at any wavelength in the ∼2.4μm to ∼9.3μm range with consistently high photon-to-photon conversion rates.

Optically pumped integrated absorber 3.4 μm laser with InAs-to-InGaAsSb type-II transition

We report optically pumped lasing at λ∼3.4 μm from an integrated absorber structure in which the electrons confined in the InAs quantum wells recombine with holes in adjacent InGaAsSb layers to provide the gain. This type-II laser exhibits an estimated photon-to-photon conversion rate of ∼24% at 85 K. The self-consistent empirical pseudopotential method calculations suggest that Coulomb attraction can lead to a strong enhancement in carrier overlap, and the resulting small shift in transition energy is consistent with that observed.

Optically Pumped Midinfrared Laser With Simultaneous Dual-Wavelength Emission

We demonstrate optically pumped semiconductor lasers that are capable of simultaneously emitting at two different midinfrared wavelengths. The wavelengths can be independently chosen and designed into the heterostructure. The epitaxial III-V antimonide structure employs two sets of type-II quantum wells in a waveguide that is partitioned by a thin electrical barrier that is transparent to the pump radiation. Two-color devices emitting at wavelengths as far apart as ~4.0 and ~5.4 mum are reported.

Midinfrared, optically pumped, unstable resonator lasers

The authors describe high-brightness, broad area midinfrared semiconductor lasers. These devices were fabricated in the authors’ laboratory using a commercial solid-source molecular beam epitaxial system. The laser structures incorporated 14 type-II quantum wells embedded in thick waveguide/absorber regions composed of In0.2Ga0.8As0.18Sb0.82. The optically pumped devices achieved higher brightness operation as unstable resonators. Each unstable resonator was realized by polishing a diverging cylindrical mirror at one of the facets. For an unstable resonator semiconductor laser operating at ∼4.6μm, near 84K, and at a peak power of 6.7W, the device was observed to be nearly diffraction limited at 25 times threshold. In comparison, a standard Fabry-Perot laser was observed to be many times diffraction limited when operated under similar conditions.

Wavelength tuning limitations in optically pumped type-II antimonide lasers

In this paper, we examine the wavelength tuning limitations of type-II antimonide lasers containing InAs∕InGaSb∕InAs quantum wells. Wavelength tuning is accomplished by varying the thickness of the InAs electron wells while keeping all else fixed. In principle, these wells can be tuned from λ≈2.5μm out to far IR wavelengths by increasing the thickness of the InAs layers. However, a practical upper limit of λ≈9.5μm is set due to the high waveguide losses awg and the diminishing modal overlap with the gain at longer wavelengths. The waveguide losses grow as awg∝λ3.44 and are attributable to free carrier absorbance. In order for the long-IR laser devices to achieve threshold, they must continually band fill, spectrally tuning to shorter wavelengths, until the laser gain exceeds the losses, which occurs near 9.5μm.

Ultralow Beam Divergence and Increased Lateral Brightness in Optically Pumped Midinfrared Laser

An optically pumped edge-emitting semiconductor laser emitting near 4.1 μm was designed with weak transverse mode confinement resulting in an exceptionally large transverse optical mode size. Consequently, a laser device is reported that exhibits a fast-axis divergence angle of ~4.2° full-width at half-maximum (FWHM). More notably, a large reduction of the lateral axis divergence is also observed as a result of the increased lateral coherency. The reduced confinement factor and differential gain serves to suppress lateral filamentation. Despite the higher threshold and lower power, a lateral divergence angle of ~3.2° FWHM and increased lateral brightness is achieved.

High-peak power from optically-pumped mid-IR semiconductor lasers

We present work on the high-peak power pumping of optically pumped mid-IR semiconductor lasers. The lasers incorporated 14 type-II InAs/InGaSb/InAs quantum wells (QW). Thick quaternary absorber layers (InGaAsSb) surrounded the QWs, which allowed a large fraction of pump light to be absorbed. The devices were optically pumped with the output of a passively Q-switched Ho:YAG laser at λ = 2.09 μm. The Ho:YAG maximum output power was ~90 kW; this allows the optically pumped semiconductor lasers (OPSLs) to be pumped at several thousand times above threshold. Emission from the QW was observed near 4.1 μm. As the pump power was increased, the QW spectra were observed to broaden and eventually saturate. Under low power pumping conditions, the OPSL pulse tracked the Ho:YAG pulse, which has a 16-ns full-width half-maximum. As the pump power was increased, the OPSL pulse duration increased and the pulse eventually split into two peaks. This may be due to a large increase in the free-carrier absorbance rates in the bulk-like quaternary absorber. Emission from the quaternary was observed near 2.2 μm and its intensity, with respect to the QW intensity, increased significantly as the pump power was increased. This indicates that at the higher pump powers, large fractions of photogenerated carriers are reservoired in the thick absorber layers. The maximum OPSL single-ended peak power was 490 W. This is the highest reported peak power from a mid-IR semiconductor laser. A rate equation model describing the time evolution of the carriers in the QW, the stored carrier population in the reservoir, and the photon population gives good agreement with the data.

The antiguiding parameter in mid-infrared optically pumped semiconductor lasers

We describe measurements of the antiguiding parameter, α, for several optically pumped semiconductor lasers. The two W lasers, incorporated 14 type-II quantum wells (QWs) and operated at wavelengths of ~3.5 and ~4.5 μm. The lasers displayed low antiguiding factors of ~1.0. We attribute the low αs for the W lasers to the higher QW gain as well as to inhomogeneous broadening induced by the 14 QWs. The differing well widths and the independent optical pumping of the wells, leads to a net gain spectrum that is symmetrical about the gain peak. This symmetry, in turn, leads to small differential index shifts at the gain peak; the result of the small differential index and large differential gain is low antiguiding.

Intersubband photoluminescence in InAs quantum wells

We conduct a study of photoluminescence in a series of InAs quantum wells with asymmetric barriers that are designed to generate emission from intersubband transitions near 4 μm wavelength. The results show that optical pumping of the barrier layers can be used to transfer carriers into the upper electron state in the InAs wells to produce photoluminescence.