The mid-IR (MIR) spectral region between 3 and 5μm is of great importance for medical diagnostics, free space communication, and high-sensitivity hydrocarbon detection, among other applications. In particular, tunable laser absorption spectroscopy allows sensitive monitoring of gaseous hydrocarbons such as methane, ethane, and formaldehyde. Because these gases have strong absorption lines in the MIR, this region offers higher sensitivity than more accessible parts of the spectrum.1 We developed an interband cascade laser (ICL) that provides single-mode continuous-wave uncooled MIR emission that should be particularly useful for sensing gaseous hydrocarbons.
To explain our device, we must first discuss how optical gain is achieved in ICLs. Neither conventional semiconductor diode lasers2–4 nor quantum cascade lasers5, 6 have yet been able to entirely cover the entire 3–5μm window when continuous-wave operation at ambient temperatures is required.7 Instead, the ICL,8 which combines salient features of the other two laser types, has emerged as the most promising light source for this wavelength range.9–11
These three types of lasers use different designs of the gain region, each with unique advantages and limitations.7 Diode lasers use transitions between the conduction band (CB) and the valence bands to generate radiation. They show excellent performance in the near-IR, but extending their operation to wavelengths much beyond 3μm is difficult because non-radiative decay rates increase in the MIR. In quantum cascade lasers, the radiative transitions take place between quantized levels in the CB. In this case, the energy offsets in the CB limit the shortest achievable emission wavelength.
Figure 1. Band structure of the cascade in an interband cascade laser (ICL). The conduction band (CB) is in black, valence bands (VBs) in red and blue, and carrier probability densities are drawn in other colors. Carriers from the adjacent electron and hole injectors feed the active quantum well (QW). InAs: Indium arsenide. GaSb: Gallium antimonide. AlSb: Aluminum antimonide.
The ICL design can be seen as a hybrid between those two types of semiconductor lasers. Figure 1 shows the band structure of a typical ICL cascade with the corresponding carrier probability densities. As in a diode laser, gain for the laser light is provided by stimulated recombination of electrons and holes in the active quantum well (QW). By changing the widths of the layers that make up the QW, we can adjust the emission wavelength over a wide range.11 Unlike a diode laser, however, charge carriers are injected not by a p-n junction around the active region but by dedicated injectors. The special band alignment between the electron and hole injector enables the generation of a hole in the latter at the boundary between them by an electron tunneling from the gallium antimonide into the indium arsenide (InAs) layer (see Figure 1). The hole is injected into the active QW region, while the electron goes on into the electron injector. The electron injector consists of an InAs/aluminum antimonide superlattice with decreasing QW thicknesses, which lifts the electron energy with respect to the CB edge. This injects the electron into the next active QW region. This internal carrier creation scheme can result in quantum efficiencies larger than one.12
This laser concept was first developed by Rui Q. Yang in 1995,8 but early ICLs suffered from high threshold current densities that limited their maximum operation temperatures. During the past four years, researchers at different institutions have improved ICL structures remarkably. In a major breakthrough, researchers at the Naval Research Laboratory demonstrated room temperature operation in 2008.13Room temperature lasers are attractive because they are more convenient and less expensive than lasers that must be cooled. In addition, we reduced optical losses by optimizing the doping levels and increasing the thickness of the lower cladding layer, which blocks mode leakage into the substrate. Also, we shortened the injector regions, which increased the mode intensity within the active region. In the active QW, finally, we adjusted the alloy and thickness of the layers to maximize the spatial overlap of the wavefunctions in the upper and lower lasing state.
Figure 2. Scanning electron microscopy (SEM) image of the cross section of a ridge waveguide ICL.
We grew interband cascade lasers in molecular beam epitaxy chambers equipped with two valved cracker cells for both antimony and arsenic. We used different flux rates and soak times during growth to compensate strain in every part of the structure. Lateral waveguiding and spatially localized carrier injection are provided by narrow ridges that we defined by e-beam or optical lithography and then etched into the laser layer by a combination of dry and wet etching. Further processing steps include passivation of the ridges and definition of top and bottom contacts by evaporation. Because thermal resistance is an important parameter for these devices, we electroplated gold to a thickness of approximately 5μm to provide good heat dissipation. Figure 2 shows the cross section of such a ridge waveguide prior to electroplating.
Figure 3 shows the output power characteristics and the spectrum of a 4μm-wide ridge ICL in continuous-wave mode. The threshold power consumption at 15°C is only 132mW, which is lower than any threshold power input ever reported for a quantum cascade laser in this wavelength region.
Figure 3. Output power characteristics of a 4μm-wide ridge ICL at different temperatures. An emission spectrum with a center wavelength around 3.5μm is shown in the inset. a.u.: Arbitrary units.
Figure 4. Emission spectrum of a single-mode ICL. The inset shows an SEM image of the corrugated ridge. SMSR: Sidemode suppression ratio.
For gas sensing, these lasers must emit in a single mode that can be scanned over characteristic absorption lines of a gas. Continous wave operation allows for very narrow linewidth emission compared to pulsed operation, providing better selectivity. One approach to providing single-mode emission incorporates sidewall gratings into a ridge waveguide.14 The gratings provide wavelength-selective feedback. Figure 4 shows the emission spectrum of such a device with a sidemode suppression ratio of more than 25dB. The sidewall grating can be seen in the gap of the contact metallization in the inset scanning electron microscopy picture.
In conclusion, we demonstrated ridge waveguide ICL operation in continuous-wave mode up to room temperature at an emission wavelength of 3.5μm.15 This was possible due to several steps that optimized the waveguide structure, the active region, and, in particular, the laser state wavefunction's spatial overlap. Furthermore, single-mode emission was achieved by adding vertical sidewall gratings.
While ICLs are already high-performance laser sources for the 3–5μm range, there is plenty of room for further development. One direction is certainly the extension of the wavelength range further into the IR. Another important aspect for gas sensing applications is the realization of tunable sources, which let users trace several gases with a single laser.
Expert technical support in sample preparation by A. Wolf, M. Wagenbrenner, S. Handel, S. Kuhn, and T. Steinl is gratefully acknowledged. We also acknowledge fruitful collaborations with nanoplus and would like to explicitly thank L. Nähle, M. von Edlinger, M. Fischer, and J. Koeth. This work has been funded by the European Commission in the context of the Seventh Framework Programme project SensHy (grant 223998).
Sven Höfling, Robert Weih, Adam Bauer, Alfred Forchel, Martin Kamp
Chair for Applied Physics
Sven Höfling received his Diploma from the University of Applied Science Coburg for his investigation of indium gallium nitride LEDs at the Fraunhofer Institute of Applied Solid State Physics, Freiburg. In 2003, he joined his current institution, from which he received a doctorate for his work on single-mode quantum cascade lasers. He has led the Optoelectronic Materials and Devices Group there since 2006.
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