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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. Writing in Applied Physics Letters, 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 so-called quantum wells per period (see the figure, panel d). 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 (see the figure, panels a and b). 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.
Filling the THz Gap
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 ETH physicists can be more easily applied and maintained outside the confines of specialised laboratories -- lifting further the lid of the 'THZ treasure chest'.
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