Using antimonide-based lasers for the mid-IR gap

Quantum cascade lasers (QCLs) have demonstrated RT CW operation at even longer wavelengths (greater than 6 µm), but a void remains in the mid-IR region that several research groups are hoping to fill.

Room-temperature type-I lasers One leader in antimonide-based high-power laser-diode development is the optoelectronics group at Sarnoff Corporation (Princeton, NJ). Teaming up with mid-IR research groups such as the State University of New York (SUNY) at Stony Brook for type-I band structure devices, and the Naval Research Laboratory (NRL) in Washington for type-II W quantum-well (QW) structures, Sarnoff has achieved record performance marks for high-power mid-IR diode lasers (figure 1).

Initially the team achieved RT laser-diode emission at wavelengths exceeding 2 µm using designs containing InGaAsSb active regions and grown on GaSb substrates. However, output power was limited to 10 mW. Increasing the operating wavelength to 2.7 µm was achieved through using InGaAsSb/AlGaAsSb multi-QW structures with an indium content of up to 40% in the wells.

More recently, a collaboration between Sarnoff, SUNY and Princeton Lightwave (a Sarnoff spin-out) has implemented a new design for AlGaAsSb/InGaAsSb/GaSb type-I MQW lasers, demonstrating RT CW operation at 2.5 µm with a power of 1 W. In these MBE-grown structures, compressive strain in the In-rich InGaAsSb QWs was reduced through the addition of As.

Using these new designs, Sarnoff and SUNY have reported 2.7 and 2.8 µm lasers operating at RT with output powers of 500 and 160 mW, respectively (Kim et al. 2003). However, increasing emission to 2.82 µm resulted in a fall in output power to only 50 mW. The decrease in power with wavelength - and also with operating temperature - is caused by the reduction of hole confinement within the structure and the degradation of material, rather than by Auger recombination as was previously thought (the latter is a non-radiative process involving either two electrons and a hole, or one electron and two holes).

The Sarnoff/SUNY team has also demonstrated a 19-emitter laser array exhibiting RT CW operation at 2.3 µm with an output power of 10 W. The 1 cm-wide laser bar contained 19 emitters with 100 µm apertures and 1 mm-long cavities. Peak wall-plug efficiency was 9%, with the differential gain of such GaSb-based QW lasers double that of comparable InP-based devices.

George Kim, technical manager in optoelectronics fabrication at Sarnoff, has revealed that the company is now focusing its efforts on high-power RT CW operation above 3 µm to fill the gap between type-I lasers (which operate at up to 2.8 µm) and QCLs, which now operate down to 4.3 µm (see last paragraph).

One option is to use InGaAs(N)Sb QWs. Incorporating N into various III-V structures decreases the bandgap by more than 100 meV per atomic percentage. As well as increasing the laser wavelength, the incorporation of N reduces compressive strain in the QW and suppresses Auger recombination. Kim and co-workers estimate that adding 1% N to In0.5Ga0.5As0.19Sb0.81 QWs (used in 2.7 µm lasers) could increase the laser wavelength to 3.5 µm, while adding 2% N could result in emission above 4 µm.

The company is positioning itself as a small-production-scale provider of IR laser diodes operating above 1.8 µm. "This will support the needs of researchers or manufacturers in developing mid-IR laser-based systems," said Kim.

Type-II W lasers Diode lasers, even if requiring a heat sink to be able to operate at 250 K, and offering output powers of a few milliwatts, could still fulfill several applications. For example, the detection sensitivity of methane at 3.3 µm is 1.7 ppb, an improvement of more than two orders of magnitude on 600 ppb for the more accessible wavelength of 1.65 µm. Also, high-power mid-IR lasers emitting in the transparent-window regions of the atmosphere have direct applications in secure communications, target designation and laser radar.

Single-stage and interband cascade type-II W devices continue to show the best performance of any semiconductor lasers in the 3-4 µm region, but a number of issues remain to be resolved.

Optically pumped type-II W antimonide lasers operating well above RT in pulsed mode have achieved good CW performance - for example, CW operation at 290 K at 3 µm and 210 K at 5.9 µm. At 77 K, such devices have produced output powers of more than 0.5 W/facet, and power-conversion efficiencies of 12% per facet.

However, manufacturing electrically pumped W lasers is a complex task; carriers must be transported through the cladding and SCH layers before being injected into the active region in an efficient manner. One issue specific to the type-II structure is the result of electrons and holes in different layers of the structure - a typical W laser might have electrons in its InAs wells, with holes in the central InGaSb layer. Carriers must recombine across the InAs/InGaSb interface to produce a mid-IR photon, so the quality of that interface is critical. This issue is one reason for the almost exclusive use of MBE in the growth of antimonide-based IR lasers.

Satisfactory performance in the 3-4 µm region has been achieved from electrically pumped W lasers; in 2000, NRL, Sarnoff and Sensors Unlimited reported 3.25 µm broadened waveguide diode lasers operating in CW mode at up to 195 K. At 78 K, the threshold current density was 63 A/cm2, and 140 mW of CW output power was generated.

NRL s Igor Vurgaftman said: "W lasers now operate in CW mode up to around 200 K with a wavelength of about 3.5 µm. At 78 K the devices have an output power of around 200 mW, together with a threshold current density of 67 A/cm2 and a maximum slope efficiency of 106 mW/A. The device structure, grown by MBE on an n-type GaSb substrate, contains a five-period W active region in the sequence InAs (well), Ga0.75In0.25Sb, InAs (well), Al0.15Ga0.85As0.05Sb0.95."

One such device was used for preliminary experiments to detect methane, which has a strong absorption band around 3.315 µm. With the laser operating at a heat-sink temperature of 110 K, the system could detect methane at a partial pressure of 7 x 10-7 atm in a nitrogen atmosphere (Bewley et al. 2004).

Cascade lasers An alternative approach, using type-II interband cascade structures, has enabled CW operation at up to 214 K in the 3-5 µm range, according to Maxion Technologies and the US Army Research Laboratory. At 80 K, the devices operated in CW mode at 3.4 µm with 23% power-conversion efficiency and a differential external quantum efficiency of 532%.

Rui Yang at the Jet Propulsion Laboratory says that his group has operated mid-IR interband cascade lasers at temperatures up to 217 K in CW mode. In pulsed mode, the lasers operated up to 325 K at 3.2-3.36 µm, and up to 300 K at 4.1 µm.

While the maximum CW operating temperature of type-II antimonide lasers creeps slowly upwards in the 3-4 µm region, QCLs have been able to demonstrate CW RT operation at ever-shorter wavelengths.

Northwestern University researchers have demonstrated RT CW operation from an InP-based intersubband QCL at 6 µm, with an output power of 132 mW at 293 K and 21 mW at 308 K (Yu et al. 2003). Manijeh Razeghi, from Northwestern, says that a paper describing 4.7 µm operation has been accepted, while RT CW operation has already been demonstrated at 4.3 µm.

Further information Type-I , type-II and quantum cascade lasers

Conventional interband diode lasers have a type-I QW structure with electrons and holes localized in the same layer. These devices operate up to around 3 µm, but at longer wavelengths their performance decreases rapidly.

In type-II interband lasers, also termed "W" lasers, electrons and holes are in adjacent layers. Photons are emitted with an energy significantly lower than the bandgap energy of the constituent layers, helping to suppress loss from Auger recombination.

For intersubband QCLs, the lasing transition takes place between two quantized states (sub-bands) of the conduction band. The transitions occur inside several series of coupled QWs with cascaded energy levels, connected by injector regions to supply electrons. It is also possible to fabricate type-II interband cascade lasers.

Further reading W W Bewley et al. 2004 Proc SPIE 5365-29.
J G Kim et al. 2003 Appl. Phys. Lett. 83(10) 1926.
J Yu et al. 2003 Appl. Phys. Lett. 83(13) 2503.

• To find out more information about Sarnoff s commercial mid-IR laser products, contact David Zish at dzish@sarnoff.com.