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Lasers & Sources

Carrier-rebalanced interband cascade lasers require very low input powers

Design innovations have dramatically improved the threshold input power and all other performance characteristics of interband cascade lasers operating in the midwave-IR.
13 March 2012, SPIE Newsroom. DOI: 10.1117/2.1201202.004124
Antimonide interband cascade laser (ICL)

The recent availability of midwave-IR (i.e., 3–6μm) semiconductor lasers capable of emitting narrow spectral lines at ambient or thermoelectric-cooler temperatures (above −20°C) has spawned the development of a new generation of chemical sensing systems designed to exploit the prevalence of strong mid-IR spectroscopic signatures. Widespread use of these sensors is expected in such applications as greenhouse gas monitoring, control of combustion and other industrial processes, sensing of chemical and biological agents, and leak detection. Although the required laser output powers tend to be rather modest (≤1mW), minimizing the drive power can be critical because the most attractive systems will be quite compact and often powered by batteries.

Several distinct classes of mid-IR semiconductor lasers are currently being developed. For example, recent advances have substantially extended the spectral range of conventional antimonide-based quantum-well (QW) diodes, although to date room-temperature (RT) continuous-wave (CW) operation has been achieved only up to wavelengths slightly beyond 3μm.1 The most widely publicized approach has been the indium phosphide-based quantum cascade laser (QCL), which employs multiple stages of QWs stacked in series. This configuration splits the usual bands of available quantum states for electrons into subbands. Each electron injected into the device can emit a cascade of photons by making an intersubband transition (emitting one photon) in each stage that it traverses. Although multi-watt RT CW output powers have been generated, QCL threshold current densities (the level at which lasing begins) tend to be high,2, 3 ≈1kA/cm2. Thus far, high performance with high yield is limited by material constraints to wavelengths beyond about 4μm.4 A third alternative is the antimonide interband cascade laser (ICL),5, 6 which combines the interband active transitions of a conventional diode laser with the multiple cascaded stages of a QCL. Previous ICLs demonstrated spectral coverage of at least 2.9–4.2μm, and CW operation up to 72°C.

Figure 1. Continuous wave (CW) output power (left scale) and wallplug efficiency (right scale) as functions of input power at 25°C for narrow-ridge interband cascade lasers (ICLs) emitting at a wavelength of about 3.7μm. The laser cavities had high-reflectance coatings on one facet and were 0.5mm long. Data are shown for three different ridge widths (w), varying from 5 to 11μm.

A distinctive feature of the ICL is that whereas light is generated via radiative recombination of electrons and holes (as in a conventional diode laser), no holes are actually injected into the device. They are instead created internally at carefully designed semimetallic InAs/GaSb (indium arsenide/gallium antimonide) interfaces when an external electric field is applied. Our recent detailed simulations7 of the carrier statistics showed that although this process produces equal densities of electrons and holes, most of the generated electrons remain in the electron injector whereas most of the holes transfer efficiently to the active region via a relatively thin hole injector. Consequently, the hole population in the active QWs has substantially outnumbered the electrons in all previous ICL designs. Because that condition exacerbated the already-deleterious effects of free carrier absorption of light (i.e., internal loss) and Auger non-radiative recombination (which removes carriers), the resulting efficiency and optical gain per unit of injected current density remained far below the structure's ideal capacity.

Figure 2. Temperature dependence of CW threshold current densities (Jth) of narrow-ridge ICLs with five cascade stages. The ridges each had one high-reflectance and one uncoated facet (HR/U) and were 4mm long (Lcav) by 10.9μm wide (sample A) and 10.3μm wide (sample B), respectively. The inset shows CW emission spectra at 25°C.

However, the calculations further suggested that very heavy n-doping of the electron injectors, to the mid-1018cm−3 range, should eliminate the carrier imbalance. Although most of the additional electrons continue to populate the injector, a fraction transfer to the active QWs to roughly equalize the electron and hole populations there. This ‘carrier rebalancing’ maximizes the optical gain per unit current density and also reduces the internal loss because far fewer holes are required to generate the threshold gain.

Redesigned ICLs incorporating carrier rebalancing have, by several key figures of merit, displayed record-setting performance compared to all previous mid-IR semiconductor lasers. For pulsed emission at about 3:7μm, we observed a RT threshold current density as low as 167A/cm2, at a threshold voltage of 2.1V. The corresponding threshold power density of 0.35kW/cm2 is far below all previous ICL results. Performance varies depending on the dimensions of the ‘ridge,’ which is etched into the structure to laterally confine the lasing mode. A narrow ridge displayed CW lasing up to 109°C, and other ridges emitted more than 150mW CW at RT. Figure 1 illustrates the CW output power and wall-plug efficiency (optical output power divided by electrical input power) as functions of input power at 25°C for three 0.5mm-long laser cavities with high-reflection coatings on the back facet. The maximum wall-plug efficiency of 13.5% is only slightly lower than that of the best QCLs emitting at longer wavelengths. The most remarkable finding is that none of the devices in Figure 1 requires more than about 35mW of input power to achieve CW lasing. The lowest RT CW input power reported to date for a QCL is 830mW, for a device with a partially transmitting high-reflectance output facet.8 More typical QCL values, in the 2–5W range, are two orders of magnitude larger than the new ICL result.

Figure 2 shows that such performance can be extended to considerably longer wavelengths as well (about 4.8 and 5.6μm for samples A and B, respectively).9 Even in this spectral range, the RT CW threshold power densities of <1kW/cm2 are more than an order of magnitude lower than the best values (≈12kW/cm2) ever reported for state-of-the-art QCLs. The maximum operating temperatures were 60°C (for sample A) and 48°C (for sample B).

Rebalancing of the hole/electron population ratio in interband cascade lasers has substantially reduced the devices' threshold input powers, to values more than an order of magnitude below state-of-the-art QCLs emitting in the same spectral range. Because most chemical spectroscopy systems do not require high output power, operation near threshold will substantially extend the battery lifetimes and reduce system complexity. We have also demonstrated single-mode ICLs,10 and are working to improve those. Other research will focus on further reducing the current and power thresholds of ICLs and concomitantly increasing their output power and wallplug efficiency. These characteristics should position the new generation of carrier-rebalanced ICLs as the mid-IR lasers of choice for applications requiring compactness, low cost, and low power budgets.

Chul Soo Kim, Chadwick Canedy, William Bewley, Charles Merritt, Igor Vurgaftman, Joshua Abell, Jerry Meyer
Naval Research Laboratory (NRL)
Washington, DC

Chul Soo Kim is a research physicist in the Optical Sciences Division at NRL. His research interests include mid-IR lasers, optoelectronics, nanophotonics, and plasmonic devices.

Chadwick Canedy is with the Optical Sciences Division at NRL, where, along with his collaborators, he develops mid-IR semiconductor lasers for IR countermeasures and biological and chemical sensing applications as well as long-wave IR photodetectors for night vision capabilities.

William Bewley received the PhD degree from the University of California, Santa Barbara, in 1993. Since 1997, he has been with NRL, involved in the characterization of semiconductor lasers and negative-photoluminescent devices.

Charles Merritt has been at NRL since 1987. During that time, he has worked in the areas of energetic materials reaction mechanisms, optical phenomena of aerosol droplets, organic LED characterization, aerosol particle capture technology, and mid-IR semiconductor laser characterization.

Igor Vurgaftman has been with the Optical Sciences Division of NRL since receiving his PhD in electrical engineering from the University of Michigan, Ann Arbor, in 1995. At NRL, he has investigated mid-IR lasers and type-II superlattice photodetectors, among other topics. He is a fellow of the American Physical Society (APS) and the Optical Society of America (OSA).

Joshua Abell has been with the Optical Sciences Division at NRL since receiving his PhD in electrical engineering from Boston University in 2008. At NRL, he has studied the growth by molecular beam epitaxy of GaSb- and InP-based III-V semiconductors.

Jerry Meyer is carrying out basic and applied research at NRL, where he is the head of the Quantum Optoelectronics Section and a senior scientist for Quantum Electronics (Science and Technology). His investigations have focused on semiconductor optoelectronic materials and devices for the IR. He is a fellow of SPIE, APS, OSA, IEEE, and the Institute of Physics.

Mijin Kim
Sotera Defense Solutions, Inc.
Crofton, MD

Mijin Kim has worked with Jerry Meyer's group at NRL since 2002, when she was an NRC Postdoctoral Fellow there, and continuing as a physicist with Sotera Defense Solutions since 2006. Her research interests include optoelectronic devices such as photonic crystals, plasmonics, and mid-IR cascade lasers.

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