Novel light source in nanostructures

NANOSCIENCE AND technology are rapidly developing fields of recent times. New materials can be created through the control of matter on the nanometer (one billionth of a meter)-length scale. In the last couple of decades, rapid advances in microfabrication technology allowed laboratories around the world to routinely fabricate ultra-thin structures. The turning point in our understanding of nanometer scale electronic properties was the development of atomically precise heterojunctions with which various nanostructured systems are made.

In 1994, a team of scientists at Bell Laboratories' quantum phenomena and device research department invented an entirely different type of laser known as the quantum cascade laser (QCL). Properties of this new laser depend upon very clever manipulation of electron motion through a system created by molecular beam epitaxy (MBE). This laser has proven to be the most powerful and extremely useful device for a wide range of applications. The purpose of this article is to give a flavour of the fascinating developments on nanostructured light source.

Freedom from band gap slavery

In the case of conventional lasers, the light originates from recombination of electrons and holes across the energy gap that exists between the conduction band and valence band of the crystal. Most importantly, the lasing wavelength is therefore determined by the energy separation (band gap) between the two bands Fig(a)0, and is fixed for a given material.

The QCL is fundamentally different from conventional semiconducting laser because it does not utilize recombination of electron- hole pairs to generate photons. Instead, it takes advantage of two processes known in quantum physics: quantum confinement and tunneling. Quantum confinement allows only discrete energy levels for electrons to hop into. In tunneling electrons burrow through a forbidden energy barrier between allowed states. In QC lasers, the wavelength is essentially determined by quantum confinement that depends on the layer's thickness of the active region rather than by the band gap of the material.Thus there is freedom from band gap slavery. As such, the wavelength can be tailored over a wide range using the same material simply by different designing of the device.

Quantum wells and artificial atoms

The active regions of a QCL Fig (b)l are made out of coupled quantum wells. A quantum well is a layer of semiconductor material, typically several nanometers thick, sandwiched between two thicker layers of slightly different composition. The compositions are chosen such that electrons that carry the current have a little less energy in the quantum well layer than in the barrier layers that sandwich it. Electrons are trapped in the quantum well if they do not have the energy to return to the barrier layers. The barrier layers must be thicker than the quantum wells to keep the electrons from escaping by tunneling.

Quantum wells confine electrons only in one dimension in a thin layer. Electrons move freely in the other two dimensions. They are therefore called two-dimensional electron gas. The last three decades have witnessed several fascinating physical phenomena in this two-dimensional electron gas that include the integer quantum Hall effect (Nobel prize to Klaus von Klitzing in 1985) and the fractional quantum Hall effect (Nobel prize in 1998 to Horst St6rmer, Dan Tsui, and Robert Laughlin). In fact, the field of nanotechnology has reached such a state of the art that these days one can routinely go to the extreme limit and confine electrons in all three dimensions - length, width and height. This leads to systems called quantum dots or, more popularly, artificial atoms, a term coined in 1990 by this author and his colleague Dr. Peter Maksym of the University of Leicester. Because of their atom-like discrete energy levels, quantum-dot lasers are expected to outperform any other higher-dimensional quantum lasers. Naturally, experimental efforts are under way in many laboratories around the world to replace the quantum wells in a QCL by quantum dots.

Electronic waterfall

How does the QCL actually work? Electrons are driven by an applied voltage. The active layers are repeated up to 75 times and when the voltage is applied they form an energy staircase. Electrons are injected through the first barrier to accumulate in the lowest energy state of the narrow quantum well. When an electron tumbles down in energy it hops into the new well spatially translated across the barrier. At each hop down the staircase, energy must be lost. Electrons achieve this loss of energy by emitting photons of only one frequency, which corresponds to the height of the steps. So a quantum of light, the photon, is emitted at each stepas electrons cascade down the staircase.

The key to lasing is to keep the intermediate energy level empty. The device is designed such that the intermediate state empties more quickly than it fills from the top level. This in turn, is enhanced by a further lower well that siphons out the electrons. Electrons do not pile up in the bottom well but flow out into the injector (relaxation) region that separates the active regions. The injector/relaxation regions are cleverly designed to assure efficient filling of electrons in the top energy level Fig. (b)l, and blocking of any electron tunneling out of the top energy level. At the other end of the active region, the injector assures fast extraction of electrons out of the intermediate and low energy levels. Structurally, these injectors consist of multiple layers with quantum wells coupled by very thin barriers . From the injector, electrons again tunnel into an excited state in the well of the next active region, and the whole process repeats a total 25 to 75 times. The photons that are generated in the process are held and directed by a surrounding waveguide material.

Since the very first demonstration in 1994, QC lasers have shown tremendous performance improvements and technological progress. At present they are the only semiconductor lasers operating at and above room temperatures in the 3.4 - 17 Aim wavelength range, with high peak power exceeding 100 mW at 300 K. High power operation of mid-infrared QC lasers at room temperature or even above room temperature were also reported. These high optical powers are a direct consequence of the cascade scheme, because unlike in conventional semiconductor lasers where each electron/hole emits at most one photon, in the QCL, each electron on the average emits many photons (upto the number of active stages), as it cascades down the stairs. Recently, researchers have been able to demonstrate the so-called unity cascade efficiency, which means that within the experimental errors, each injected electron indeed emits one photon per stage. Wide-ranging important applications of this laser will be the topic of the next part of the series.

T. Chakraborty, Institute of Mathematical Sciences, Chennai

(To be continued)

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