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.

Laser Physics

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

The series of quantum wells and barriers in the superlattice give rise to a series of electronic subbands and by suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system. As with all types of laser, a population inversion is required is order to achieve laser emission. QCLs are typically based upon a three-level system

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

Advantages over interband laser diodes

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 QCL, once an electron has emitted a photon in one period of the superlattice, it is recycled 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 emission wavelength of laser diodes is determined by the band gap of the material system used. However the emission wavelength of QCLs can be tuned over a wide range by changing the layer thicknesses in the semiconductor superlattice. The independence of laser operation from the conduction and valence band edge characteristics allows much greater flexibility of emission wavelengths from conventional semiconductor materials such as the GaAs/AlGaAs material system. Furthermore, laser operation from indirect bandgap materials such as the Si/SiGe material system is theoretically possible.

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.

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 emitted wavelength of the laser is determined primarily by the thickness of the heterolayers and the layer materials itself. This is a great advantage over diode lasers, whose wavelengths depend primarily on the band gap of the material and is therefore restricted. The wavelengths that are available from quantum cascade lasers are therefore diverse, giving emission in the range of 3.5 - 160µm 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.

Intersubband 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 bandgap in which there are no permitted states available for electrons to occupy.

Conventional semiconductor lasers rely on a single photon of light being emitted when a high energy electron from the conduction band "falls" into an unoccupied hole in the valence band. The wavelength of light emitted is therefore strongly dependent upon the energy bandgap of the laser material.

The quantum cascade laser however does not use bulk semiconductor materials in its optically active region. Instead, it consists of a periodic series of thin layers of varying material composition. The varying composition 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 electric subbands. For any given energy, an electron may occupy a number of different subbands, each with a different momentum.

By careful selection of material composition and thickness of each layer in the device, and the applied external electric field, an electric subband minimum in a given period of the device may be aligned with a higher energy subband minimum in the adjacent period. Therefore, an electron may take part in an optical transition between electric subbands in a given period before tunneling into the next period of the structure and performing another optical transition. This process may occur dozens of times for each electron moving through the device, giving high optical power output.

External links

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