Quantum-cascade laser

Quantum cascade lasers (QCLs) are semiconductor lasers that emit in the near- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.

Unlike typical interband semiconductor lasers that emit electromagnetic radiation through the recombination of electron–hole pairs across the material band gap, QCLs are unipolar and laser emission is achieved through the use of intersubband transitions in a repeated stack of semiconductor superlattices, an idea first proposed in the paper "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice" by R.F. Kazarinov and R.A. Suris in 1971.

Intersubband vs. interband transitions

Within a bulk semiconductor crystal, electrons may occupy states in one of two continuous energy bands - the valence band, which is heavily populated with low energy electrons and the conduction band, which is sparsely populated with high energy electrons. The two energy bands are separated by an energy band gap in which there are no permitted states available for electrons to occupy. Conventional semiconductor laser diodes generate light by a single photon of being emitted when a high energy electron in conduction band recombines with a hole in the valence band. The energy of the photon and hence the emission wavelength of laser diodes is therefore determined by the band gap of the material system used.

A QCL however does not use bulk semiconductor materials in its optically active region. Instead it comprises of a periodic series of thin layers of varying material composition forming a superlattice. The superlattice introduces a varying electric potential across the length of the device, meaning that there is a varying probability of electrons occupying different positions over the length of the device. This is referred to as one-dimensional multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of discrete electronic subbands. By suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system which is required in order to achieve laser emission. Since the position of the energy levels in the system is primarily determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of QCLs over a wide range in the same material system.

Additionally, in semiconductor laser diodes, electrons and holes are annihilated after recombining across the band gap and can play no further part in photon generation. However in a unipolar QCL, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted. This process of a single electron causing the emission of multiple photons as it traverses through the QCL structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than semiconductor laser diodes.

The independence of laser operation from the conduction and valence band edge characteristics makes laser operation from indirect bandgap materials such as the Si/SiGe is theoretically possible.

Operating principles

QCLs are typically based upon a three-level system.

Schematic of a three-level laser showing the relevant scattering times between subbands

Rate equations

Assuming the formation of the wavefunctions is a fast process compared to scattering between states, the time independent solutions to the Schrodinger equation may be applied and the system can be modelled using rate equations. Each subband contains a number of electrons $n_{i}$ (where $i$ is the subband index) which scatter between levels with a lifetime $\tau _{if}$, where $i$ and $f$ are the initial and final subband indices. The rate equations of the three laser levels are given by

${\frac {dn_{1}}{dt}}=I_{in}+{\frac {n_{1}}{\tau _{13}}}+{\frac {n_{2}}{\tau _{23}}}-{\frac {n_{3}}{\tau _{31}}}-{\frac {n_{3}}{\tau _{32}}}$

${\frac {dn_{2}}{dt}}={\frac {n_{3}}{\tau _{32}}}+{\frac {n_{1}}{\tau _{12}}}-{\frac {n_{2}}{\tau _{21}}}-{\frac {n_{2}}{\tau _{23}}}$

${\frac {dn_{3}}{dt}}={\frac {n_{2}}{\tau _{23}}}+{\frac {n_{1}}{\tau _{13}}}-{\frac {n_{3}}{\tau _{31}}}-{\frac {n_{3}}{\tau _{32}}}-I_{out}$

In the steady state, the time derivatives are equal to zero and $I_{in}=I_{out}=I$. Under the assumption that absorption processes can be ignored (which is valid at low temperatures), the middle rate equation gives

${\frac {n_{3}}{\tau _{32}}}={\frac {n_{2}}{\tau _{21}}}$

Therefore if $\tau _{32}>\tau _{21}$ then $n_{3}>n_{2}$ and a population inversion will exist. The scattering rates are tailored by suitable design of the layer thicknesses in the superlattice which determine the electron wave functions of the subbands. The scattering rates are heavily dependent upon the overlap and energy spacing between the wavefunctions.

Electron wavefunctions in a three quantum well QCL active region. The upper laser level is shown in bold.

The figure shows the wavefunctions in a three quantum well (3QW) QCL active region and injector. The ground states of the three quantum wells in the active region form the three laser levels.

Growth and fabrication

The QCL consists of alternating layers of two different semiconductors (for example GaAs/AlGaAs or AlInAs/InGaAs) forming a quantum heterostructure. The layers are grown on a substrate using a process called molecular beam epitaxy (MBE), or metalorganic vapour phase epitaxy (MOVPE). The wavelengths that are available from quantum cascade lasers are therefore diverse, giving emission in the range of ] covering the mid-infrared range.

In 1996, Claire Gmachl, a post-doctoral researcher at Bell Laboratories significantly reduced the linewidth of the QCL by incorporating a waveguide into the design, thus amplifying one particular wavelength. Often a structure called a distributed feedback (DFB) is built on top of the laser crystal to prevent it from emitting at other than the desired wavelength.

Applications

The laser's high optical power output, tuning range and room temperature operation make it useful for spectroscopic applications like the remote sensing of environmental gases and pollutants in the atmosphere. It may eventually be used for vehicular cruise control in conditions of poor visibility, collision avoidance radar, industrial process control, and medical diagnostics such as breath analyzers.

External links

• Bell Labs summary
  • Bell Labs technical information
• PhysicsWeb: Quantum Cascade Lasers
• Walter Schottky Institute explanation, with diagrams