Interband cascade laser (ICL)
A mid-infrared semiconductor laser that cascades several type-II interband active stages in series, so each injected electron can emit more than one photon. ICLs run continuous-wave at room temperature across roughly 3–6 µm with drive powers of a fraction of a watt, and are the usual source for 3–4 µm gas sensing.
An interband cascade laser (ICL) is a semiconductor laser that combines two ideas: photons are emitted by electron–hole recombination across a gap, as in an ordinary laser diode, and several active stages are connected in series, as in a quantum cascade laser, so an electron that recombines in one stage is recycled into the next. Proposed by Rui Q. Yang in 1994, it is grown on GaSb substrates from the InAs/GaSb/AlSb family and emits across roughly 3–6 µm in the mid-wave infrared, with room-temperature continuous-wave operation well established in the 3–4.5 µm range. Single-mode DFB ICLs typically deliver a few to tens of milliwatts CW; their distinguishing figure is low electrical drive power, often around 0.1–0.5 W at the operating point, an order of magnitude below a typical QCL.
Active region
Each stage contains a type-II "W" quantum well: two thin InAs electron wells on either side of a GaInSb hole well, bounded by AlSb barriers, so that electron and hole wavefunctions overlap despite the broken-gap alignment of InAs and GaSb. The lasing transition is interband, from the InAs conduction states to the GaInSb valence states, and its energy is tuned by layer thickness rather than set by a bulk bandgap. After recombination, the electron tunnels out of the valence band into the next stage through a semimetallic InAs/GaSb interface, and electron and hole injectors (chirped superlattices) carry carriers to the next active well.
Because the transition is interband, the upper-state lifetime is limited by Auger and radiative recombination, around a nanosecond, instead of the picosecond phonon scattering that governs intersubband transitions in a QCL. The longer lifetime lowers the threshold current density, reported in the range of about 100–300 A/cm² at room temperature for good devices. Auger recombination is the main loss mechanism and rises steeply with temperature, which is why W-well design and the "carrier rebalancing" of electron and hole densities through heavier injector doping were key steps in lowering threshold.
Voltage and stage count
Each stage drops at least one photon energy of voltage. At 3.5 µm the photon energy is
so a 5-stage ICL needs at least 5 × 0.354 = 1.77 V at threshold, with a few hundred millivolts more from series resistance and injector alignment. Typical designs use 3–10 stages, compared with 20–50 in a QCL; a 30-stage QCL at 4.6 µm has a minimum voltage of 8.1 V. Fewer stages and lower current give ICLs their low power consumption, at the cost of lower maximum output than high-power QCLs. Cascading also means the differential quantum efficiency can exceed 100% in photons per electron, though the wall-plug efficiency remains well below that of near-infrared diodes.
Applications
The 3–4 µm window contains strong C–H stretch absorption, so ICLs are used for tunable diode laser absorption spectroscopy of methane near 3.27 µm, ethane near 3.34 µm, and formaldehyde near 3.6 µm, as well as HCl and other species. The low drive power allows battery-operated and airborne instruments with thermoelectric cooling only. ICL frequency combs, and ICL detectors built from the same cascaded structure, are active research areas.
Pitfalls
Output power falls and threshold rises quickly with temperature, so ICLs are usually run on a thermoelectric cooler near room temperature. Beyond about 5–6 µm the performance declines, and at longer wavelengths QCLs are the stronger choice. As with other narrow-linewidth semiconductor lasers, optical feedback from windows and gas cells produces spectral noise and mode hops, so angled or wedged optics are used in the beam path.
Common questions
What is the difference between an ICL and a QCL?
An ICL uses interband transitions between electrons and holes in type-II quantum wells, with a few cascaded stages and low voltage; a QCL is unipolar, using intersubband transitions within the conduction band and many stages. ICLs need less power and cover 3–4 µm well; QCLs deliver more power and extend from about 4 µm into the terahertz.
Why can't a conventional diode laser cover 3–4 µm?
Mid-infrared type-I diodes on GaSb do reach about 3–3.5 µm, but Auger recombination grows rapidly as the bandgap narrows, and their room-temperature performance falls off beyond about 3 µm. The ICL's cascading and W-well design recover gain in this range.
References: R. Q. Yang, "Infrared laser based on intersubband transitions in quantum wells," Superlattices Microstruct. 17, 77 (1995); I. Vurgaftman et al., "Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption," Nat. Commun. 2, 585 (2011).