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Terahertz (THz) radiation is a bit like a treasure chest that resists being opened fully. Residing in the electromagnetic spectrum between the infrared and microwave regions, THz radiation combines a range of properties that are ideal with a view to applications. It provides a window to unique spectroscopic information about molecules and solids, it can penetrate non-conducting materials such as textiles and biological tissue, and it does so without ionising—and hence damaging—the object, or subject, under study. This opens up intriguing prospects for non-invasive imaging and non-destructive quality control, among other applications. But whereas there is no shortage in ideas for potential uses, their implementation is hampered by a lack of practical technologies for generating and detecting THz radiation.
Therefore the excitement as Lorenzo Bosco, Martin Franckié and colleagues from the group of Jérôme Faist at the Institute for Quantum Electronics of ETH Zurich reported now the realization of a THz quantum cascade laser that operates at a temperature of 210 K (-63 °C). That is the highest operational temperature achieved so far for this type of device. More importantly, this is the first time that operation of such a device has been demonstrated in a temperature regime where no cryogenic coolants are needed. Instead, Bosco et al. used a thermoelectric cooler, which is much more compact, cheaper and easier to maintain than cryogenic equipment. With this advance, they removed the main obstacles on the route to various practical applications.
a) The thermoelectrically cooled laser box with the laser mounted on top of a Peltier element (white square), allowing operation between 195 K and 210.5 K with the laser emitting vertically through the window in the top lid. b) The laser chip as mounted in the laser box, contacted with thin gold wires bonded on top of several laser ridges. c) Schematic of one laser ridge; the horizontal lines show the quantum-well structure formed by layered semiconductors. The ridge (150 micrometres wide) is sandwiched between thin layers of copper. d) Conduction band edge (white lines) tilted by the applied operation bias, with the electron density resolved in energy shown in colour. The electrical bias drives electrons through the non-radiative transitions indicated by the dashed arrow. This pumps the state in the thin well, which becomes more populated than the state in the wider well indicated by the green arrow, allowing for net stimulated emission of terahertz photons.
A Cascade Towards Applications
Quantum cascade lasers (QCLs) have long been established as a natural concept for THz devices. Like many lasers that are widely used as sources of light in the visible-to-infrared frequency region, QCLs are based on semiconductor materials. But compared to typical semiconductor lasers used, for instance, in barcode readers or laser pointers, QCLs operate according to a fundamentally different concept to achieve light emission. In short, they are built around repeated stacks of precisely engineered semiconductor structures (see the figure, panel c), which are designed such that suitable electronic transitions take place in them (panel d). QCLs have been proposed in 1971, but they were first demonstrated only in 1994, by Faist and colleagues, then working at Bell Laboratories (US). The approach has proved its value in a board range of experiments, both fundamental and applied, mainly in the infrared region. The development of QCLs for THz emission has made substantial advances, too, starting from 2001. Widespread use has been hindered though by the requirement for cryogenic coolants -- typically liquid helium -- which adds substantial complexity and cost, and makes devices large and less mobile. Progress towards operation of THz QCLs at higher temperatures got essentially stuck seven years ago, when operation of devices at around 200 K (-73 °C) was achieved.