The Quantum Cascade Laser

Since these ultrathin layers, called quantum wells, have a size comparable to the electron's De Broglie wavelength, they quantize the electrons energy levels in the direction perpendicular to the plane of the layers. When inside a quantum well, an electron can jump from one state to the other by discrete steps, sometimes emitting photons. The spacing between the energy levels depends on the width of the well, and increases as the well size is decreased. As a consequence, the emission wavelength depends now on the layer thicknesses and not anymore on the bandgap of the constituent materials, leading to a broad frequency flexibility of this device. This has indeed allowed the manufacturing of lasers with emission wavelengths ranging from 2.7 to 250 micrometers using combinations of commonly used III-V semicnductors as InGaAs/AlInAs/InP, GaAs/AlGaAs and InAs/AlSb.

Of course, having an intersubband transition is still far away from obtaining a working QCL. A QCL is way more sophisticated and one needs quantum engineering in order to achieve all the requirements needed to create a Quantum Cascade Laser. These are:

1. Electrical stability at the operating point
2. Population inversion and gain obtained by electron lifetime engineering
3. Low loss optical resonator to match gain value and allow laser oscillation

The following figure shows the quantum well schematic that allowed to achieve lasing emission in the first QCL demonstration in 1994 at Bell Labs by J.Faist.

A QCL is composed by several periods of an elemental structure based on two region: the active region and the relaxation/injection region. The active region can be seen as a standard three level system where each level is a confined state. The lifetime of the transition from level 3 to 2 has to be longer than lifetime of the electrons in level 2 if one wants to achieve population inversion, an essential condition for obtaining gain. Following the active region, comes the injection/relaxation region, whose purpose is to transport the electrons to the following active region down the cascade.

The operating principle of the quantum cascade laser can be summarized as follows:

1. under the right applied bias, an electron will be injected via resonant tunneling in the upper state of the active region and relax to the lower state eventually emitting a photon
2. from here the electron will be transported through the relaxation region and then injected into the upper state of the following period where it will again be able to emit a photon
3. if the population inversion is ensured, as the current is increased gain will match the losses and the device will reach threshold, converting the injected electrons into photons by stimulated emission. The emitted power will depend linearly on the injected current.

In this way, each electron involved in the process can actually generate at most N photons, with N number of periods of the structure, when cascading across the structure.

Compared to conventional (interband) lasers, the quantum cascade laser differs by the following important points:

• As mentioned above, the same semiconductor material system whose technology is very well mastered can be used to manufacture lasers operating across the whole mid-infrared and THz ranges
• It is based on a cascade of identical stages (typically 20-50), allowing one electron to emit many photons, emitting more optical power (electron recycling)
• Due to its unipolar nature it does not suffer from surface recombination
• Due to ultrafast electron lifetimes, QCLs are intrinsically high speed devices and can be modulated to extremely high frequencies.

Today, QCLs are finding their space into modern technology due to their compactness, high power and peculiar frequency range. Despite the progress, though, several challenges are still open and pursued by the scientific community:

• Room Temperature operation for THz QCLs: although mid-infrared QCLs are extremely robust and can operate at temperature higher than 20 Celsius degrees, the THz QCLs struggle to lase above 200 K. This is due to an extreme sensitivity to phonons population inside the system, which hinders radiative emission at relatively high temperature. Solution are still being studied in order to obtain a room temperature THz QCL and consequently to achieve a huge technological upgrade
• New material systems: as said above, the QCL is freed from the choice of the bandgap and it can be based on several material systems. A huge effort is always spent on researching new possible material systems, in particular Si/SiGe, which would then enable the possibility to integrate coherent light sources into Si photonic devices [4]
• Quantum cascade frequency combs: recently our lab demonstrated that broadband quantum cascade lasers display phase coherence between the modes due to an ultrafast internal third order non linearity [5]. Such lasers can be then used as frequency combs. This can have a tremendous impact on Mid-IR laser spectroscopy leveraging on the very small footprint of the QCL and its unique characteristics of spectral agility, rapidity and high resolution. THz devices are particularly promising for ultra-wide bandwidth as our group already demonstrated octave-spanning emission from heterogeneous QCL [6]

Further details and introductory talks can be found in the links at the bottom of this page. If you want to get deep into research, instead, look at all the things we are doing in our labs.

References

[1] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho " Quantum Cascade Laser", Science, 264, 553, (1994)

[2] J. Faist, "Quantum cascade lasers", Oxford (2014).

[3] M. I. Amanti, et al. "Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage", New J. Phys. 11, (2009)

[4] external page FLASH

[5] A. Hugi, et al. "Mid-infrared frequency comb based on a quantum cascade laser", Nature 492, 229–33 (2012).

[6] M. Rösch, et al. "Octave-spanning semiconductor laser", Nat. Photonics 9, 42–47 (2014).