Short wavelength quantum cascade lasers

Introduction

Laser technology was revolutionised by the introduction of semiconducting devices, which could be easily integrated with electronic circuits. Semiconductor lasers have found a range of applications including optical fibre communications, DVD drives, laser pointers and survey equipment.

In simple semiconducting lasers electrons are injected into the conduction band by a voltage applied to the device. When one of these electrons falls into the valence band it releases a photon with energy equal to the band gap. This photon can then stimulate the decay of another electron, a process that releases a second photon of the same energy and phase, a process that can lead to a strong coherent emission from the device.

In a simple semiconductor laser a large driving current creates a population inversion between the conduction and valence bands, so a single electron transition generates a photon that causes a cascade of further photon releases. This amplification of the light intensity is the critical process in a laser.

Above: One photon stimulates the decay of neighbouring electrons, which emit photons in phase with the first. As long as there are more electrons in the conduction band than in the valence band, the semiconductor is able to amplify the light, which is the main requirement for laser operation.

A photon of energy equal to the band gap can, however, also promote an electron from the valence to the conduction band, during which process the photon is absorbed, reducing the light intensity. To prevent this process dominating the properties of the device it is necessary to force a population inversion, with more electrons in the conduction band than there are in the valence band. In simple devices this is achieved by driving a large current through the device, such that the conduction band is flooded with electrons, and the valence band is continuously being depopulated.

The disadvantages of these simple devices are that the output wavelength is governed by the semiconductor band gap. In addition, they have a high power consumption, required to maintain the population inversion. A final problem is that their efficiency decreases rapidly with increased temperature, as electron populations are smeared out through the wide band structure of available states (this problem is compounded by the fact that these devices tend to run hot due to their high power consumption!).

These problems can be overcome to some extent by replacing the bulk semiconductor with a nanostructure (e.g. a quantum well, quantum wire or quantum dot). In these systems electrons make transitions between conduction and valence band states in the nanostructure, the modified properties of the nanostructures results in lasers which require reduced operating currents and are less temperature sensitive. However the photon energy is still close to that of the band gap of the semiconductor; it is difficult to obtain very low photon energies corresponding to far infrared wavelengths. To achieve such wavelengths a radically different structure is required, the Quantum Cascade Laser, which utilises the novel properties of semiconductor quantum wells and superlattices.

Quantum wells - confined semiconducting lasers

In thin (nanoscale) semiconductor sheets electron movement perpendicular to the sheet becomes quantised. This leads to the development of quantised energy states, sometimes referred to as 'sub-bands'. Energy differences between these quantised states are a function of the thickness of the semiconductor sheet, and are a lot smaller than the band gap, so laser devices can be made that emit light with wavelengths further into the infra-red (useful for chemical sensors) and also the precise wavelength can easily be selected by varying the well width, this control is not possible in a traditional laser where the band gap is a fixed property of the semiconductor.

Effect of the quantum well on the distribution of the density of states in a semiconductor.

Above: As the width of the semiconductor sheet is reduced below about 50nm the number of possible electronic wave functions starts to fall very quickly, and the density of states becomes step-like. The energy difference between the confined states is inversely proportional to the thickness of the semiconductor sheet.

Superlattices

A single quantum well has a set of quantised energy levels for electron motion perpendicular to the two-dimensional sheet. If we bring a second quantum well up into close proximity with the first, the energy levels of the two wells will interact in much the same way that energy levels of two atoms interact - as more quantum wells are brought together these levels start to form bands. To distinguish them from the intrinsic bands in the semiconductor, these are often referred to as 'mini-bands'. The stack structure of quantum wells is referred to as a 'superlattice', distinguishing it from the crystal lattice of the underlying semiconductor layers.

The energy states in each quantum well can interact if the wells are close enough together, leading to the formation of mini-bands of allowed electron energy states. When this occurs the structure can be called a 'superlattice'.

Above: The energy states in each quantum well can interact if the wells are close enough together, leading to the formation of mini-bands of allowed electron energy states. When this occurs the structure can be called a 'superlattice'.

The mini-bands formed by superlattices are useful because they allow electrons to be transported perpendicular to the plane of the quantum wells. In addition transport is only allowed for energies corresponding to those of the mini-bands. Superlattices form a critical component of Quantum Cascade lasers.

The quantum cascade laser (QCL)

The QCL is fed with electrons by a superlattice. In a cascade laser several active regions with tuned supperlattice mini-bands are stacked one on top of another, so that each electron passing through the device can produce several photons.

Above: In this QCL photons are produced when electrons fall from the n=3 to the n=2 state in the quantum well. Electrons are then quickly removed from the n=2 state by dropping down to the n=1 state and then exit the quantum well into the mini-band of a superlattice. This superlattice then transports the electrons to the next quantum well, injecting them with the correct energy into the n=3 state. Electrons hence cascade down a series of steps, emitting a sequence of photons. The voltage required to operate the device shifts the energies of the states in the quantum wells making up the superlattice. To counteract this each quantum well has a slightly different width, and hence different energy states. The applied voltage brings these states into resonance creating the superlattice with the required mini-bands.

In a conventional laser where the lasing transition is between the conduction and valence band each injected electron can produce at most one photon. However in a QCL a given electron could produce a photon for each stage of the laser. In contrast in a standard laser where the applied voltage is close to that of the lasing transition (the band gap), for a QCL it will approximately equal the lasing energy multiplied by the number of periods. QCLs have higher operating currents than conventional lasers because instead of emitting a photon when dropping between the energy levels in the quantum well the electron can emit a quantum of lattice vibration (a phonon).

Research at Sheffield

Researchers at Sheffield are pushing the boundaries of what is possible with InGaAs quantum wells, developing high performance QCLs operating at very short wavelengths. Recently the group demonstrated lasers working at 3.1um at 20K and 3.6um at room temperature. These devices use 30 repeats of the basic structure to achieve sufficient gain.

References

"Nanoscale Science and Technology", Eds. R.W. Kelsall, I.W. Hamley and M. Geoghegan, published Wiley, 2005.

"InGaAs/AlAsSb/InP quantum cascade lasers operating at wavelengths close to 3um", by D.G. Revin, J.W. Cockburn, M.J. Steer, R.J. Airey, M. Hopkinson, A.B. Krysa, L.R. Wilson and S. Menzel, Applied Phys. Lett. 90 (2007), 021108

Acknowledgements

The author would like to thank Prof David Mowbray (University of Sheffield, Department of Physics and Astronomy) for his assistance in preparing this article (home page, research).

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