Novel light source in nanostructures

This is the second and concluding part of the article on nanostructures.

QUANTUM ENGINEERING using ultrathin semiconductor layers grown by molecular beam epitaxy (MBE) allows one to design and observe quantum phenomena on a nanometer (billionth of a meter) scale. The quantum cascade laser is a brilliant outcome of the rapidly emerging field of nanoscience and technology. The wavelength tunability of QCL: The InAlAs/InGaAs active regions can be engineered to emit at any wavelength between 4 and 25 microns, coupled with the fact that it is capable of operating in what is, compared with other laser technologies, a high temperature (i.e., room temperature) makes it ideal for gas sensing applications. Further, due to the high optical power output, it can detect lower concentrations of gases under scrutiny.

Seeing infrared

Most trace gases of importance have observable absorption features in the mid infrared range, called the molecular jingerprint region of the spectrum. Two large regions of good transparency are found around 3 to 5 micrometer and from 7.5 to 13 micrometer, the two so-called atmospheric windows. This can be advantageously used for detection of many trace gases and vapours down to a few parts-perbillion in volume. These include chemicals such as CO, CH3, N2O, S02, HCI, and HNO3, as well as many organic compounds that are important in pollution detection, atmospheric chemistry and industrial process monitoring and control applications.

The principles of gas-sensing by the QCL is rather straightforward. The QCL generates light at a very specific frequency. This light is passed through a gas mixture and is detected by a sensor on the other side. Gases absorb light at particular wavelengths characteristic of the molecular structure of the gas and strong absorption lie in the mid infrared. As the laser light source is designed to emit at the gas absorption line, absorption of optical power indicates the presence of the gas and can also indicate the concentration of the gas.

The high sense-ability of QCL makes it an important tool for a wide-ranging real-world applications like:

Remote sensing: (over a range from hundreds of meters to a few miles) of chemicals such as toxic gases, vapours emanating from industrial smokestacks, landfills and other hazardous waste sites.

Point sensors: - monitoring of automobile emissions on the entrance and exit ramps of highways, etc., combustion and catalytic converter diagnostics.

Law enforcement: detection of explosives and of illicit drug production sites.

Military applications: sensors for biological toxins and toxic gases such as nerve and mustard gas, heat sensors such as the type used in infrared surface-to-air missiles. Highpower (a few W) mid-infrared semiconductor lasers, emitting at selected wavelengths in the 3-5 micrometers window, could find use in military countermeasures such as blinding the sensor of a heat- seeking missile.

Non-invasive medical diagnostics: The laser is useful for example, to detect in a patient's breath molecules characteristic of ulcer formation. Similarly, more powerful QC lasers will one day be used for blood glucose monitoring, without actually drawing blood from the patient.

Application in space explorations: Recently, at the Jet Propulsion Laboratory of Caltech, USA, measurements of the concentration of CH4 and N20 in the earth's atmosphere were made from ground level to the stratosphere (- 70, 000 ft) using a 7.95 micrometer wavelength QCL.

QCL is also envisioned to find important applications in space explorations in the form of a QC laser spectrometer. Space agencies believe that there are wide ranging and immediate applications to Mars, Titan, Venus and Europa missions. For example, it could be operated on a descending or penetrating probe, lander, rover, or aerobot. It would consume only a few watts of power, and weigh less than one kilogram. QC laser spectrometer would directly access the wavelength region of strong vibration-rotation spectral lines. It would be able to measure concentrations of several planetary gases such as H20, CH4, CO, C02, C2H2, HCN, C2H6, C2N2, HCN, OCS, H2S, and S02, and numerous stable isotopes. In this way, QCL spectrometer would make important contributions to atmospheric photochemistry and transport, mineralogical and biological experiments. Similarly, it would be useful for respiratory or hazardous gas monitoring for human exploration of the solar system. Improvements on the design and performance are being reported quite regularly in various research journals. Electrons in the QCL described so far make a diagonal transition from one quantum well to another while emitting photons. In another type of QCL operation electrons make a vertical transition from a higher subband to a lower subband within the same quantum well. The active regions in those cases contain only two quantum wells (the second well is to extract the electrons after the optical transitions in the first well).

At the Central Laboratories of Thomson-CSF, Paris, France, researchers have succeeded in making QCL where the quantum wells are created by sandwiching wafers of galliumarsenide (GaAs) and aluminum gallium arsenide (A1GaAs). These axe the most common lowcost semiconductor compounds and are technologically mature materials. Additionally, the crystal structure of GaAs matches perfectly with that of A1GaAs offering significant flexibility in the QC' laser design. Several other laboratories, notably in Vienna, Austria and Sheffield, U.K. have also made great progress in this direction.

There are also recent reports in the literature on type-II QCLs that combine the advantages of a cascade design and the interband transitions of the conventional diode laser. In this design, the conduction band of one semiconductor overlaps the valence band of the adjacent semiconductor. An electron tunneling between a quantum well in the conduction band of indium arsenide (InAs) and the valence band of indium gallium antimonide (InGaSb) emit a photon and then is re-injected in the next successively connected active region, and so on. The advantage of this laser is the higher efficiency as compared to the standard QCL.

T.Chakraborty

Institute of Mathematical Sciences

Chennai 600113

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