Inter-band Cascade Laser (ICL)

1. Theory

Definition: An ICL is a mid-IR semiconductor laser in which electrons and holes recombine across the gap in each of several cascaded stages that are connected by interband tunnelling, which combines the long carrier lifetime of an interband laser with the cascading of a quantum cascade laser.
Type-II broken gap: In InAs/GaSb the conduction band of InAs lies below the valence band of GaSb, so an electron can pass from the valence band of one stage into the conduction band of the next by interband tunnelling; the injectors act as rectifying barriers, so no p-n junction is needed.
Stage: An electron injector, an active region with a “W” structure (a GaInSb hole well between two InAs electron wells, enclosed by AlSb barriers, which raises the electron-hole overlap and the gain), and a hole injector.
Wavelength: Set mainly by the thickness of the InAs electron wells; room-temperature continuous-wave operation from about 3 $μ$ m to about 6 $μ$ m, and lasing up to 11.2 $μ$ m at low temperature.
No phonon bottleneck: The carrier lifetime is set by interband Auger, radiative and Shockley-Read-Hall recombination, which are slower than the optical-phonon scattering that empties the lower level of a quantum cascade laser.
Materials: InAs, GaSb, AlSb and their alloys, closely lattice-matched (about 6.1 Å), grown by molecular beam epitaxy on GaSb or InAs substrates.
Cladding: GaSb-based devices use a thick InAs/AlSb superlattice cladding, which has a low thermal conductivity and is difficult to grow; InAs-based devices use heavily doped InAs, with a hybrid superlattice inner cladding, for longer wavelengths.

2. Working Principle

Cascade: The same current flows through all the stages, and each electron can produce a photon in every stage, so the gain of the stages adds: $N_{c} Γ_{1} g (n_{t h}) = α_{i} + α_{m}$ .
Carrier balance: Heavy n-type doping of the electron injectors balances the electron and hole densities in the active wells, which lowers the threshold current and power considerably.
Threshold current density: Lower than early estimates because the Auger coefficient is smaller, with values of 100 to 200 $A/c m^{2}$ in the laboratory (about 100 $A/c m^{2}$ at 300 K near 3.7 $μ$ m in GaSb-based devices).
Low power consumption: Threshold power density of about 0.3 $kW/c m^{2}$ against 11 $kW/c m^{2}$ for quantum cascade lasers in one comparison, and lasing at input powers as low as 29 mW.
Output: About 0.5 W continuous wave at room temperature and a wall-plug efficiency of 15 to 18%, less than the several watts of the best quantum cascade lasers.
Single mode and tuning: Distributed-feedback ICLs give single-mode output of tens of milliwatts, with a temperature tuning of about 0.4 nm/K and a current tuning of 0.016 nm/mA in 4.5 $μ$ m devices operating up to 180 K.
Related devices: Interband cascade LEDs, VCSELs, photodetectors and frequency combs use the same stage design.

3. Applications

Trace-gas sensing: Absorption spectroscopy of methane, carbon dioxide, carbon monoxide, formaldehyde and carbonyl sulfide; a 3.27 $μ$ m distributed-feedback ICL for methane was used on the Curiosity rover.
Portable and battery-powered spectroscopy: Low power consumption and little heat dissipation.
Breath analysis and combustion diagnostics: Mid-infrared absorption sensing.
Free-space optical communication: A 4.2 $μ$ m ICL with an interband cascade photodetector reached 16 Gbit/s over 2 m (research and development).
Frequency combs: Self-starting, low-power ICL combs for dual-comb spectroscopy at 3 to 4 $μ$ m (research and development).

Name	Formula	Related Formulae	Variables/Constants
Threshold condition and threshold current density of the cascade	$N_{c} Γ_{1} g (n_{t h}) = α_{i} + α_{m}$	$J_{t h} = \frac{q d _{s}}{η _{i}} [A n_{t h} + B n_{t h}^{2} + C n_{t h}^{3}]$ $n_{t h} \approx p_{t h}$ $α_{m} = \frac{1}{L} ln \frac{1}{R}$	$N_{c} : n u mb er o f s t a g es o f t h e c a sc a d e (ab o u t 5 t o 10 in an in t er ban d c a sc a d e l a ser, a g ain s t 30 t o 40 in a q u an t u m c a sc a d e l a ser)$ $Γ_{1} : co n f in e m e n t f a c t or o f o n e s t a g e an d g (n) : ma t er ia l g ain o f i t s a c t i v e w e l l s a t t h e c a r r i er d e n s i t y n$ $n_{t h}, p_{t h} : e l ec t r o n an d h o l e d e n s i t i es in t h e a c t i v e w e l l s a t t h r es h o l d, w hi c h a r e f o u n d t o g i v e t h e b es t p er f or man ce w h e n t h ey a r e ab o u t e q u a l$ $α_{i}, α_{m} : in t er na l an d mi r r or l osses, α_{m} b e in g t h e s am e e x p r ess i o n a s in t h e F ab r y - P er o t l a ser p a g e$ $J_{t h} : t h r es h o l d c u r r e n t d e n s i t y, w hi c h i s t h e s am e in e v er y s t a g e b ec a u se t h e s t a g es a r e in ser i es$ $d_{s} : t hi c k n ess o f t h e a c t i v e w e l l s o f o n e s t a g e$ $A, B, C : S h oc k l ey - R e a d - H a l l, r a d ia t i v e an d A ug er r eco mbina t i o n coe f f i c i e n t s (t h e A ug er coe f f i c i e n t p r o v e d m u c h l o w er t han e a r l y es t ima t es)$ $η_{i} : inj ec t i o n e f f i c i e n cy$ $L, R : c a v i t y l e n g t h an d m e an p o w er r e f l ec t an ce o f t h e f a ce t s$ $(t h e r e l a t i o n s o f t hi s r o w an d o f t h e p o w er an d v o l t a g e r o w s a r e t h e d o u b l e - h e t er os t r u c t u r e an d q u an t u m - c a sc a d e r e l a t i o n s a ppl i e d t o s t a g es in ser i es, w r i tt e n f or t hi s p a g e, n o t t h e p u b l i s h e d I C L m o d e l s)$
Emission wavelength of the W active region	$λ \approx \frac{h c}{E _{e 1} - E _{h 1}}$	$λ [μ m] \approx \frac{1.24}{( E _{e 1} - E _{h 1} ) [ e V ]}$	$λ : e mi ss i o n w a v e l e n g t h, w hi c h i s se t main l y b y t h e t hi c k n ess o f t h e I n A s e l ec t r o n w e l l s$ $E_{e 1} : e n er g y o f t h e l o w es t e l ec t r o n s u bban d o f t h e I n A s w e l l s$ $E_{h 1} : e n er g y o f t h e hi g h es t h o l e s u bban d o f t h e G a I n S b w e l l$ $(t h e t r an s i t i o n i s b e tw ee n an e l ec t r o n s u bban d an d a h o l e s u bban d o f t h e t y p e - I I s t a c k an d n o t a cr oss t h e ban d g a p o f o n e b u l k ma t er ia l, an d t h e o v er l a p o f t h e tw o w a v e f u n c t i o n s i s r ai se d b y t h e W s ha p e)$ $h, c : P l an c k co n s t an t an d s p ee d o f l i g h t$
Output power and wall-plug efficiency	$P = \frac{h ν}{q} η_{i} \frac{N _{c} α _{m}}{α _{i} + α _{m}} (I - I_{t h})$	$η_{w p} = \frac{P}{I V}$	$P : o pt i c a l p o w er o u t o f t h e f a ce t s$ $h ν : p h o t o n e n er g y$ $N_{c} : n u mb er o f s t a g es, b ec a u se o n e e l ec t r o n c an e mi t a p h o t o n in e a c h s t a g e$ $η_{i} : inj ec t i o n e f f i c i e n cy$ $α_{i}, α_{m} : in t er na l an d mi r r or l osses$ $I, I_{t h} : c u r r e n t an d t h r es h o l d c u r r e n t$ $η_{w p} : w a l l - pl ug e f f i c i e n cy, t h e r a t i o o f t h e o pt i c a l p o w er t o t h e e l ec t r i c a l p o w er I V (ab o u t 15 t o 18 % a t r oo m t e m p er a t u r e in t h e b es t co n t in u o u s - w a v e d e v i ces)$
Threshold power of the cascade	$P_{t h} = I_{t h} V_{t h}, V_{t h} \approx N_{c} (\frac{h ν}{q} + Δ V_{s})$	$p_{t h} = \frac{P _{t h}}{A _{d}}$	$P_{t h} : e l ec t r i c a l p o w er a t t h r es h o l d, w hi c h i s l o w in an in t er ban d c a sc a d e l a ser b ec a u se b o t h I_{t h} an d t h e n u mb er o f s t a g es N_{c} a r e s ma l l$ $V_{t h} : v o l t a g e a t t h r es h o l d (ab o u t 6 V a t r oo m t e m p er a t u r e in a 12 - s t a g e d e v i ce o f 2005)$ $Δ V_{s} : a dd i t i o na l v o l t a g e d r o p o f a s t a g e in t h e inj ec t or s an d t h e in t er f a ces$ $p_{t h} : t h r es h o l d p o w er d e n s i t y o v er t h e a r e a A_{d} o f t h e d e v i ce (ab o u t 0.3 k W / c m^{2} f or in t er ban d an d 11 k W / c m^{2} f or q u an t u m c a sc a d e l a ser s in o n e co m p a r i so n)$

Important Questions

Ques: Why does an interband cascade laser need much less electrical power than a quantum cascade laser?

In a quantum cascade laser the electron makes a transition between two subbands of the conduction band, and the upper level has a lifetime of about 1 ps because optical-phonon scattering also empties it, so a large current has to be driven through 30 to 40 stages and a voltage of at least about 10 V is needed. In an interband cascade laser the carriers recombine across the gap, and their lifetime is set by the much slower radiative, Auger and Shockley-Read-Hall recombination, so a smaller current is needed to keep the population. Only about 5 to 10 stages are used, with about 6 V at threshold in a 12-stage device. The result is a threshold power density of about 0.3 kW/cm $^{2}$ against 11 kW/cm $^{2}$ in one comparison, and a reported input power at room temperature that is nearly two orders of magnitude lower than that of a quantum cascade laser. The price is a lower maximum power. Reference: J. R. Meyer, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt and I. Vurgaftman, The Interband Cascade Laser, Photonics 7(3), 75 (2020); I. Vurgaftman et al., Nature Communications 2, 585 (2011), DOI 10.1038/ncomms1595; Wikipedia, Interband cascade laser (accessed October 2026); University of Oklahoma dissertation, High-performance InAs-based interband cascade lasers.

Ques: How do the electrons get from one stage of an interband cascade laser to the next?

The stages are made of InAs, GaSb and AlSb, a material combination with a type-II broken-gap alignment in which the bottom of the InAs conduction band lies below the top of the GaSb valence band. An electron that has recombined with a hole in the active region of one stage is removed from the valence band by the hole injector and passes from this valence band into the conduction band of the electron injector of the next stage by interband tunnelling, an elastic process, after which it is injected into the active region of the next stage. The same interband tunnelling also generates the holes needed there, so each stage is supplied with both carriers although only electrons are supplied by the contacts. The injectors act as rectifying barriers, so no p-n junction is needed. Reference: R.Q. Yang, “Infrared laser based on intersubband transitions in quantum wells,” Superlattices Microstruct. 17, 77 (1995), the original proposal; Wikipedia, Interband cascade laser (accessed October 2026); J. R. Meyer et al., Photonics 7(3), 75 (2020).

Ques: Why is the active region of an interband cascade laser a W-shaped quantum well?

In the type-II alignment the electrons are confined in the InAs layer and the holes in the GaInSb layer, so a simple type-II well has a small overlap of the two wavefunctions and a low gain. The W structure places a GaInSb hole well between two InAs electron wells, with AlSb barriers on the outside, so that the electron wavefunction spreads over both sides of the hole well and overlaps it more, which raises the optical gain. Nearly all designs have used it since about 2004. The emission wavelength is set mainly by the thickness of the InAs electron wells. Reference: Wikipedia, Interband cascade laser (accessed October 2026); J. R. Meyer et al., Photonics 7(3), 75 (2020).

Ques: What is carrier rebalancing, and why is the electron injector heavily n-doped?

The electrons and the holes in the active wells of a cascade are generated internally by interband tunnelling and are not supplied independently, and the earlier devices suffered from an imbalance between the electron and the hole densities in the active wells, which raised the threshold. Heavy n-type doping of the electron injectors, with silicon, adds electrons to the active wells until the two densities are about equal at threshold, which lowered the threshold current and power densities substantially compared with earlier devices, and made room-temperature continuous-wave operation possible with very low input power. Reference: I. Vurgaftman et al., Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption, Nature Communications 2, 585 (2011), DOI 10.1038/ncomms1595; Wikipedia, Interband cascade laser (accessed October 2026).

Ques: What are the best continuous-wave output power and wall-plug efficiency of an interband cascade laser at room temperature?

The sources disagree slightly. The Wikipedia article gives a maximum room-temperature continuous-wave power of nearly 0.5 W and a wall-plug efficiency of nearly 15%, with an operating temperature up to 118 $^{\circ}$ C at 3.7 $μ$ m. The 2020 review by the Naval Research Laboratory group gives more than 500 mW and up to 18%. The review is the more likely to be correct and current, because it is written by the Naval Research Laboratory group that demonstrated the first room-temperature continuous-wave device in 2008, and is the more recent source, and the efficiency depends on the device, the wavelength and the heat sink, so the two values can both be true of different devices. A fair statement is therefore about 0.5 W and 15 to 18%, which is well below the several watts of the best quantum cascade lasers and their wall-plug efficiencies of about 21% continuous wave (27% pulsed). Reference: Wikipedia, Interband cascade laser (accessed October 2026); J. R. Meyer et al., Photonics 7(3), 75 (2020).

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