Terahertz Lasers Become Hot

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. (Source: Faist group, ETHZ)

Terahertz (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 Department of Physics reported now the realization of a THz quantum cascade laser that operates at a tempera­ture of 210 K. 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.

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, which are designed such that suitable electronic transitions take place in them.

QCLs have been proposed in 1971, but they were first demons­trated only in 1994, by Faist and colleagues, then working at Bell Labo­ratories. 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 was achieved.

Reaching 200 K was an impressive feat. That tempera­ture, however, is just below the mark where cryogenic techniques could be replaced with thermo­electric cooling. That the record temperature did not move since 2012 also meant that some sort of ‘psychological barrier’ started to go up – many in the field started to accept that THz QCLs would always have to operate in con­junction with a cryogenic cooler. The ETH team has now broken down that barrier. Now, they present a thermo­electrically cooled THz QCL, operating at tempera­tures of up to 210 K. Moreover, the laser light emitted was strong enough that it could be measured with a room-tempera­ture detector. This means that entire setup worked without cryogenic cooling, further strengthening the potential of the approach for practical appli­cations.

Bosco, Franckié and their co-workers managed to remove the cooling barrier due to two related achievements. First, they used in the design of their QCL stacks the simplest unit structure possible, based on two quantum wells per period. This approach has been known to be a route to higher tempera­tures of operation, but at the same time this two-well design is also extremely sensitive to smallest changes in the geometry of the semi­conductor structures. Optimizing performance relative to one parameter can lead to degra­dation relative to another. With systematic experimental optimi­zation being not a viable option, they had to rely on numerical modelling.

This is the second area where the group has made substantial progress. In recent work, they have established that they can accurately simulate complex experi­mental QCL devices, using an approach known as non­equilibrium Green’s function model. The calcu­lations have to be carried out on a powerful computer cluster, but they are suffi­ciently efficiently that they can be used to search systema­tically for optimal designs. The group’s ability to accurately predict the properties of devices – and to fabricate devices according to precise speci­fications – gave them the tools to realize a series of lasers that consistently work at tempera­tures that could be reached with thermo­electrical cooling. And the approach is by no means exhausted. Ideas for pushing the operational temperature further up exist in the Faist group, and preliminary results do look promising.

The first demons­tration of a terahertz quantum cascade laser operating without cryogenic cooling constitutes an important step towards filling the THz gap, which has long existed between the mature techno­logies for microwave and infrared radiation. With no moving parts or circulating liquids involved, the sort of thermo­electrically cooled THz QCLs now introduced by the physicists can be more easily applied and maintained outside the confines of specialised labora­tories – lifting further the lid of the THZ treasure chest. (Source: ETHZ)

Reference: L. Bosco et al.: Thermoelectrically cooled THz quantum cascade laser operating up to 210 K, Appl. Phys. Lett. 115, 010601 (2019); DOI: 10.1063/1.5110305

Link: Quantum Optoelectronics Group, Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland

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