Interband cascade laser

Like QCLs, ICLs employ the concept of bandstructure engineering to achieve an optimized laser design and reuse injected electrons to emit multiple photons.

These processes typically occur on a much slower time scale than the longitudinal optical phonon interactions that mediates the intersubband relaxation of injected electrons in mid-IR QCLs.

The use of interband transitions allows laser action in ICLs to be achieved at lower electrical input powers than is possible with QCLs.

[4] ICLs operating in cw mode at ambient temperature are able to achieve lasing at much lower input powers than competing mid-IR semiconductor laser technologies.

In addition to producing light, the layered epitaxial structure must also act as a waveguide so that the cascade stages amplify guided optical modes.

The key feature that enables the realization of cascading within an interband diode is the so-called "type-II", or broken-gap, band alignment between InAs and GaSb.

[6] When the device is biased, excess electrons and holes are generated and flow into the active region, where they recombine and emit light.

In this design, the GaInSb hole QW is sandwiched between two InAs electron QWs, which are in turn surrounded by two AlSb barrier layers.

This arrangement maximizes the optical gain by increasing the spatial overlap between the electron and hole wavefunctions that are nominally separated in different layers.

They must also double as rectifying barriers for the opposite type of carrier in order to prevent inter-stage leakage currents.

To maximize the InAs/AlSb superlattice miniband width, the InAs layer thicknesses are varied across the injector so that their ground state energies nearly align when the device is biased.

An additional feature that differentiates the ICL from all other laser diodes is its provision for electrically-pumped operation without a p-n junction.

Nevertheless, it is highly advantageous to dope certain layers in each cascade stage as a means of controlling the active electron and hole densities, via a design technique called "carrier rebalancing.

"[5] While the most favorable combination of electron and hole populations depends on the relative strengths of various free carrier absorption and Auger recombination processes, the studies done thus far indicate that the ICL performance is optimal when at threshold the two concentrations are roughly equal.

For longer-wavelength operation, advantages include the much higher thermal conductivity of bulk InAs as compared to a short-period InAs/AlSb superlattice, as well as a much thinner cladding layer due its larger index contrast with the active region.

[3] The figure on the right shows the performance characteristics of narrow ridge-waveguide interband cascade lasers at room temperature operating in cw mode.

[8] Specifically, the figure shows plots of the amount of power emitted by lasers with different ridge widths for a given injection current.

A distributed-feedback ICL,[10] designed for the excitation of methane gas, was developed at NASA Jet Propulsion Laboratory and included as an instrument on the tunable laser spectrometer on the Curiosity rover that was sent to explore the environment of Mars.

Band alignment of and lattice constant of materials used in interband cascade laser
Schematic of overall epitaxial structure for laser grown on GaSb. The microscope image shows four of the thin-layer cascade stages. This image was taken using transmission electron microscopy .
Band diagram of a single stage in a typical interband cascade laser. The cascade stage is divided into an active region , electron injector, and hole injector. The groups of quantum wells that constitute each region are indicated. The subband extrema energies and corresponding squared wavefunctions are plotted for those subbands most relevant to the device transport and laser action.
Light-current characteristics in continuous-wave mode at room temperature for narrow ridge-waveguide interband cascade lasers with several different ridge widths (w) as indicated in the figure. At the maximum output power, the beam quality is within ≈2 times the diffraction limit for all the ridges. The cw lasing wavelength of these ICLs span from 3.6 to 3.9 μm in temperature range from 20 to 115 °C (as shown in inset). Additional details can be found from Ref. 8.