Quantum cascade laser (QCL)
A unipolar semiconductor laser using intersubband transitions in a periodic stack of quantum wells. Emits in the mid-infrared and terahertz where conventional interband diode lasers fail.
A quantum cascade laser uses transitions between confined energy states (intersubbands) of the conduction band, rather than transitions across the bandgap between conduction and valence bands. The emission wavelength is set by the energy spacing of the engineered subbands rather than by the material bandgap, allowing operation at wavelengths where no suitable direct-bandgap semiconductor exists.
The active region is a periodic stack of typically 20 – 50 stages. Each stage contains:
- An active region: 2 – 4 quantum wells designed so the upper laser state and lower laser state are spaced by the desired photon energy
- An injector region: a graded superlattice that funnels electrons from the lower state of one stage to the upper state of the next stage via tunneling and phonon-assisted relaxation
Because every electron passes through every stage in series, the internal quantum efficiency can exceed unity — one electron generates photons in an -stage device. The cost is a high voltage requirement (typically 10 – 15 V), since the voltage across each stage adds to the total drive voltage.
Performance characteristics:
| Parameter | Mid-IR QCL (4 – 10 μm) | Terahertz QCL (1 – 5 THz) |
|---|---|---|
| Operating temperature | Room temperature, CW | Cryogenic (< 200 K) typical |
| Wall-plug efficiency | 5 – 27 % CW, 50 % pulsed (cooled) | 1 % |
| Output power | 0.1 – 5 W CW | 0.1 – 1 W |
| Threshold current density | 0.5 – 2 kA/cm² | 0.05 – 0.2 kA/cm² |
| Linewidth enhancement factor | 0.1 – 1 (very low) | Low |
QCLs are the dominant source for:
- Mid-IR molecular spectroscopy (3 – 25 μm covers most vibrational fingerprint absorption bands)
- Trace gas sensing (methane, CO, NO, NO, etc.)
- Infrared countermeasures (atmospheric transmission windows)
- Free-space communication through atmospheric IR windows
- Terahertz imaging and spectroscopy (cryogenic operation limits applications)
DFB and DBR versions are available; external-cavity QCLs achieve nm continuous tuning by rotating an external grating. Unlike interband diode lasers, QCLs have intrinsically narrow gain bandwidth per stage — wide tuning requires multiple stacks designed for different center wavelengths.
The terahertz gap (loosely 0.5 – 10 THz between QCL and conventional electronic source frequency limits) remains an active research area; room-temperature THz operation was a milestone achieved around 2020 – 2021.