M. Dallner

Shortened injector interband cascade lasers for 3.3- to 3.6-μm emission

Interband cascade lasers (ICLs) are very attractive light sources to cover the important spectral range from 3.3 to 3.6 µm, which is of major interest for hydrocarbon detection. We present ICLs emitting in this wavelength range that have a notably shortened injector region. Compared to a well-established reference design, the total length of one cascade is reduced by about 16% to only 63 nm. The benefits are a reduction of the number of heterointerfaces, a higher intensity of the optical mode in the active region, and easier strain compensation. Threshold current densities of devices using the new injector design are cut in half throughout the temperature range of operation, compared to devices with the reference design. Deeply etched ridge waveguide devices processed from structures with 14 cascades yielded maximum operation temperatures of 73°C in pulsed operation. Peak output powers exceed 100 mW at a heat-sink temperature of 15°C.

InAs-based interband-cascade-lasers emitting around 7 μm with threshold current densities below 1 kA/cm2 at room temperature

Interband cascade lasers (ICLs) grown on InAs substrates with threshold current densities below 1 kA/cm2 are presented. Two cascade designs with different lengths of the electron injector were investigated. Using a cascade design with 3 InAs quantum wells (QWs) in the electron injector, a device incorporating 22 stages in the active region exhibited a threshold current density of 940 A/cm2 at a record wavelength of 7 μm for ICLs operating in pulsed mode at room temperature. By investigating the influence of the number of stages on the device performance for a cascade design with 2 QWs in the electron injector, a further reduction of the threshold current density to 800 A/cm2 was achieved for a 30 stage device.

Type-II quantum wells with tensile-strained GaAsSb layers for interband cascade lasers with tailored valence band mixing

Optical properties of modified type II W-shaped quantum wells have been investigated with the aim to be utilized in interband cascade lasers. The results show that introducing a tensely strained GaAsSb layer, instead of a commonly used compressively strained GaInSb, allows employing the active transition involving valence band states with a significant admixture of the light holes. Theoretical predictions of multiband k·p theory have been experimentally verified by using photoluminescence and polarization dependent photoreflectance measurements. These results open a pathway for practical realization of mid-infrared lasing devices with uncommon polarization properties including, for instance, polarization-independent midinfrared light emitters.

On the modified active region design of interband cascade lasers

Type II InAs/GaInSb quantum wells (QWs) grown on GaSb or InAs substrates and designed to be integrated in the active region of interband cascade lasers (ICLs) emitting in the mid infrared have been investigated. Optical spectroscopy, combined with band structure calculations, has been used to probe their electronic properties. A design with multiple InAs QWs has been compared with the more common double W-shaped QW and it has been demonstrated that it allows red shifting the emission wavelength and enhancing the transition oscillator strength. This can be beneficial for the improvements of the ICLs performances, especially when considering their long-wavelength operation.

Submonolayer Uniformity of Type II InAs/GaInSb W-shaped Quantum Wells Probed by Full-Wafer Photoluminescence Mapping in the Mid-infrared Spectral Range

The spatial uniformity of GaSb- and InAs substrate-based structures containing type II quantum wells was probed by means of large-scale photoluminescence (PL) mapping realized utilizing a Fourier transform infrared spectrometer. The active region was designed and grown in a form of a W-shaped structure with InAs and GaInSb layers for confinement of electrons and holes, respectively. The PL spectra were recorded over the entire 2-in. wafers, and the parameters extracted from each spectrum, such as PL peak energy position, its linewidth and integrated intensity, were collected in a form of two-dimensional spatial maps. Throughout the analysis of these maps, the wafers’ homogeneity and precision of the growth procedure were investigated. A very small variation of PL peak energy over the wafer indicates InAs quantum well width fluctuation of only a fraction of a monolayer and hence extraordinary thickness accuracy, a conclusion further supported by high uniformity of both the emission intensity and PL linewidth.

InAs-based distributed feedback interband cascade lasers

Single mode emission of InAs-based interband cascade lasers using lateral metal gratings that provide distributed feedback is presented. A double ridge configuration was developed, where the grating is placed above the active region while a second, slightly wider ridge ensures current confinement in the active region. Side mode suppression ratios above 30 dB have been obtained, and single mode emission was observed at wavelengths around 6 μm in continuous-wave operation up to a temperature of −10 °C. Current- and temperature-tuning rates of 0.011 nm/mA and 0.50 nm/ °C, respectively, have been found, and a total tuning range of 6.5 nm has been covered by one device. As a reference, ridge waveguide devices without a grating made from the same material were able to emit in pulsed mode up to 49 °C and in continuous-wave operation at temperatures up to 0 °C around 6 μm.

Interband cascade lasers for the mid-infrared spectral region

Interband cascade lasers are mid-infrared semiconductor lasers that are very promising for low power consumption applications in the 3-6 μm spectral range. Therefore, they are ideal for gas sensing of hydrocarbons. Combining interband transitions utilized in diode lasers with a cascading scheme widely exploited in intersubband transition based quantum cascade lasers, interband cacscade lasers can operate at threshold current densities around or below 100 Acm-2 at room temperature. Distributed feedback interband cascade lasers emit at room temperature continuous wave output powers in the mW range and above, and their side mode suppression ratio can well exceed 20 dB.

Recent advances in GaSb-based structures for mid-infrared emitting lasers: spectroscopic study

There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade lasers. There are presented their crucial optical properties and the related current challenges with respect to the device performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used (photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level calculations based on effective mass approximation and kp theory. Eventually, the potential for further material optimization and prospects for the improved device performances are also discussed.

DFB lasers for sensing applications in the 3.0-3.5 um wavelength range

There are two particularly promising approaches to reach laser emission in the 3.0 - 3.5 μm wavelength range with application grade performance; GaSb based laser structures using GaInAsSb / AlGaInAsSb type-I quantum well (QW) active region, as well as type-II interband cascade (IC) material have been investigated and corresponding results are discussed in this paper. We also present different techniques for the fabrication of spectrally monomode distributed feedback (DFB) lasers for sensing applications in the targeted wavelength range. Based on the different waveguide designs of the two material approaches, different concepts to achieve monomode emission were applied: lateral metal gratings were used for type-I laser structures, vertical sidewall gratings for ICL designs. The fabrication procedure, including growth of the laser structures by molecular beam epitaxy, device processing and characterization, are described in the following. DFB emission under continuous wave (cw) operation was achieved up to room temperature (RT) in the target wavelength range. Sidemode suppression ratios (SMSRs) exceed 30dB for the fabricated devices and mode-hop free monomode tuning ranges of several nanometers are demonstrated.

Influence of carrier concentration on properties of InAs waveguide layers in interband cascade laser structures

We present a characterization of doped InAs layers in interband cascade lasers exploiting the plasmon-enhanced waveguiding. Fast differential reflectance was employed in order to identify the plasma-edge frequency via the Berreman effect and shown as an advantageous method when compared to other types of measurements. The carrier concentration was then derived and compared with the nominal doping densities. The emission properties of the investigated structures were studied by means of photoluminescence (PL). Its full-width at half-maximum and integrated intensity were extracted from PL spectra and analyzed in the function of the doping density (carrier concentration). The PL linewidth was found to be independent of the carrier concentration indicating an insignificant contribution of doping to the structural properties deterioration. The PL intensity decay with the carrier concentration suggests being dominated by Auger recombination losses.