InP-based quantum cascade lasers monolithically integrated onto silicon.

Lasing is reported for ridge-waveguide devices processed from a 40-stage InP-based quantum cascade laser structure grown on a 6-inch silicon substrate with a metamorphic buffer. The structure used in the proof-of-concept experiment had a typical design, including an Al0.78In0.22As/In0.73Ga0.27As strain-balanced composition, with high strain both in quantum wells and barriers relative to InP, and an all-InP waveguide with a total thickness of 8 µm. Devices of size 3 mm x 40 µm, with a high-reflection back facet coating, emitted at 4.35 µm and had a threshold current of approximately 2.2 A at 78 K. Lasing was observed up to 170 K. Compared to earlier demonstrated InP-based quantum cascade lasers monolithically integrated onto GaAs, the same laser structure integrated on silicon had a lower yield and reliability. Surface morphology analysis suggests that both can be significantly improved by reducing strain for the active region layers relative to InP bulk waveguide layers surrounding the laser core.

Progress in high-power continuous-wave quantum cascade lasers [Invited].

Multi-watt continuous-wave room temperature operation with efficiency exceeding 10% has been demonstrated for quantum cascade lasers essentially in the entire mid-wave and long-wave infrared spectral regions. Along with interband cascade lasers, these devices are the only room-temperature lasers that directly convert electrical power into mid- and long-infrared optical power. In this paper, we review the progress in high-power quantum cascade lasers made over the last 10 years. Specifically, an overview of the most important active region, waveguide, and thermal design techniques is presented, and various aspects of die packaging for high-power applications are discussed. Prospects of power scaling with lateral device dimensions for reaching optical power level in the range from 10 W to 20 W are also analyzed. Finally, coherent and spectral beam-combining techniques for very high-power infrared platforms are discussed.

Continuous wave operation of buried heterostructure 4.6 µm quantum cascade laser Y-junctions and tree arrays.

Room-temperature continuous-wave operation for buried heterostructure 4.6 µm quantum cascade laser Y-junctions and tree arrays, overgrown using hydride vapor phase epitaxy, has been demonstrated. Pulsed wall plug efficiency for the Y-junctions with bending radius of 5mm was measured to be very similar to that of single-emitter lasers from the same material, indicating low coupling losses. Comparison between model and experimental data showed that the in-phase mode was dominating for 10mm-long Y-junctions with 5 µm-wide 1mm-long stem and 5 µm-wide branches. Total optical power over 1.5 W was demonstrated for four-branch QCL tree array.

Multiwatt long wavelength quantum cascade lasers based on high strain composition with 70% injection efficiency.

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission near 9 µm, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. Maximum pulsed and continuous wave room temperature optical power of 4.5 and 2 W and wallplug efficiency of 16% and 10%, respectively, were demonstrated for a 3mm by 10 µm laser mounted epi-side down on an AlN/SiC composite submount. Pulsed laser characteristics were shown to be self-consistently described by a simple model based on rate equations using measured 70% injection efficiency for the upper laser level.

Tapered 4.7 µm quantum cascade lasers with highly strained active region composition delivering over 4.5 watts of continuous wave optical power.

A strain-balanced, Al0.7In0.22As/In0.72Ga0.28As/InP quantum cascade laser structure, designed for light emission at 4.7µm using the non-resonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in tapered buried heterostructure geometry and then mounted on AlN/SiC composite submounts using hard solder. A 10 mm long laser with 7.5µm-wide central section tapered up to 20µm at laser facets generated over 4.5W of single-ended CW/RT optical power at 283K. Maximum wallplug efficiency of 16.3% for this laser was reached at 4W level. Reliability of over 2,000h has been demonstrated for an air-cooled system delivering optical power of 3W in a collimated beam with overall system efficiency exceeding 10%.

Optically multiplexed multi-gas detection using quantum cascade laser photoacoustic spectroscopy.

We report high-throughput, nondispersive optical multiplexing of laser beams using a scanning galvanometer. We have utilized this technique for multispecies trace-gas detection using multiple quantum cascade laser photoacoustic spectroscopy. We demonstrate switching from one laser to another in less than 1 s, a performance level needed for a comprehensive multispecies sensor, and a high signal-to-noise ratio detection of five gaseous components, NH(3), NO(2), dimethyl methyl phosphonate (DMMP, a simulant for nerve agents), acetone, and ethylene glycol, in a room air gas mixture containing approximately 3 ppb of NH(3), approximately 8 ppb of NO(2), approximately 20 ppb of DMMP, approximately 30 ppb of acetone, and approximately 40 ppb of ethylene glycol.

Sub-parts-per-billion level detection of dimethyl methyl phosphonate (DMMP) by quantum cascade laser photoacoustic spectroscopy.

The need for the detection of chemical warfare agents (CWAs) is no longer confined to battlefield environments because of at least one confirmed terrorist attack, the Tokyo Subway [Emerg. Infect. Dis. 5, 513 (1999)] in 1995, and a suspected, i.e., a false-alarm of a CWA in the Russell Senate Office Building [Washington Post, 9 February 2006, p. B01]. Therefore, detection of CWAs with high sensitivity and low false-alarm rates is considered an important priority for ensuring public safety. We report a minimum detection level for a CWA simulant, dimethyl methyl phosphonate (DMMP), of <0.5 ppb (parts in 10(9)) by use of a widely tunable external grating cavity quantum cascade laser and photoacoustic spectroscopy. With interferents present in Santa Monica, California street air, we demonstrate a false-alarm rate of 1:10(6) at a detection threshold of 1.6 ppb.

High-sensitivity detection of triacetone triperoxide (TATP) and its precursor acetone.

Triacetone triperoxide (C(9)H(18)O(6), molecular mass of 222.24 g/mol) (TATP) is a powerful explosive that is easy to synthesize using commonly available household chemicals, acetone, and hydrogen peroxide 1 2. Because of the simplicity of its synthesis, TATP is often the explosive of choice for terrorists, including suicide bombers. For providing safety to the population, early detection of TATP and isolation of such individuals are essential. We report unambiguous, high-sensitivity detection of TATP and its precursor, acetone, using room-temperature quantum cascade laser photoacoustic spectroscopy (QCL-PAS). The available sensitivity is such that TATP, carried on a person (at a nominal body temperature of 37 degrees C), should be detectable at some distance. The combination of demonstrated detection of TATP and acetone should be ideal for screening at airports and other public places for providing increased public safety.

High-sensitivity detection of TNT.

We report high-sensitivity detection of 2,4,6-trinitrotoluene (TNT) by using laser photoacoustic spectroscopy where the laser radiation is obtained from a continuous-wave room temperature high-power quantum cascade laser in an external grating cavity geometry. The external grating cavity quantum cascade laser is continuously tunable over approximately 400 nm around 7.3 microm and produces a maximum continuous-wave power of approximately 200 mW. The IR spectroscopic signature of TNT is sufficiently different from that of nitroglycerine so that unambiguous detection of TNT without false positives from traces of nitroglycerine is possible. We also report the results of spectroscopy of acetylene in the 7.3-microm region to demonstrate continuous tunability of the IR source.

Sub-parts-per-billion level detection of NO2 using room-temperature quantum cascade lasers.

We report the sub-parts-per-billion-level detection of NO2 using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of approximately 300 mW tunable over approximately 350 nm centered at 6.25 microm. We demonstrate minimum detection level of approximately 0.5 parts per billion of NO2 in the presence of humidified air.

Improved performance of quantum cascade lasers through a scalable, manufacturable epitaxial-side-down mounting process.

We report substantially improved performance of high-power quantum cascade lasers (QCLs) by using epitaxial-side-down mounting that provides superior heat dissipation properties. We used aluminum nitride as the heatsink material and gold-tin eutectic solder. We have obtained continuous wave power output of 450 mW at 20 degrees C from mid-IR QCLs. The improved thermal management achieved with epitaxial-side-down mounting combined with a highly manufacturable and scalable assembly process should permit incorporation of mid-IR QCLs in reliable instrumentation.