Polaritonic TeraHertz Devices

THz band remains the last region of electromagnetic spectrum which does not have wide application in modern technology due to lack of solid state source of THz radiation: compact, reliable and scalable. Fundamental objection preventing creation of such source is small rate of spontaneous emission of the THz photons: according to the Fermi Golden rule this rate is about tens of inverse milliseconds, while lifetime of the charge carrier in the solid typically lies in picoseconds range due to the efficient interaction with phonons.

The rate of spontaneous emission of THz photons can be increased by application Purcell effect, but even in this case cryogenic temperature is required for the operation of solid state THz devices. Further increase of emission rate can be achieved via bosonic stimulation, when THz radiative transition occurs into the quantum state, in which condensate of bosons is formed. Such situation can be realized for transition between upper and lower polariton state in semiconductor
microcavity in polariton lasing regime, in the case when parity excitonic parts of upper and lower polariton states is made different. In this case emission of THz radiation characterized by substantial quantum efficiency can be achieved even at room temperature. Further, pronounced non-linear properties of polaritonic systems lead to various non-linear effects in coupled system of polaritons and THz photons, what open the way for development of novel class of solid state devices, like compact THz short pulse generators, THz switch, and THz detectors.

Terahertz radiation sources and detectors

The realization of efficient terahertz (THz) radiation sources and detectors is one of the important objectives of modern applied physics. This has wide ranging applications in biology, medicine, security and non-destructive in-depth imaging.

THz radiation can be generated in semiconductor microcavities through exciton polariton lasing, where a radiative THz transition between microcavity polariton branches is allowed. A special design of quantum well microcavity is required, which provides mixing of bright and dark quantum well excitons.

The POLATER (Polaritonic teraHertz devices) project produced a wide range of results underpinning theoretical advances in the above areas. This has included the development of detailed theory and design for radiative THz transitions in quantum microcavities and THz intersub-band polaritons as well as the fabrication of a set of microcavities with effective polaritonic THz radiative transitions.

Researchers modelled, built and theoretically investigated microcavities of various designs. POLATER investigated GaAs, InAs as well as GaN structures. An analysis of the polaritonic THz active area, based on quantum wells, quantum wires and quantum dots with broken symmetry has been completed. The team developed and fabricated a novel type of cavity based on cylindrical Tamm plasmons.

Researchers fabricated and characterised a multi-quantum well structure for polaritonic emitters based on InAs monolayers. The structure demonstrates a pronounced superradiant mode that can be used for the development of THz emitters.

POLATER designed polaritonic cascade structures. An important outcome here is the development of the Bosonic Cascade Laser theory, where each polariton injected to the device emits several THz photons.

The public view of THz has been largely influenced by a number of articles in the press citing its ability to image through clothing and detect explosives or weapons. However, the potential applications of THz light are far more wide-ranging. THz radiation can provide a direct probe to reveal the fundamental properties and operation of life through, for example, the study of conformational changes in protein structures, protein hydration shells and DNA. The creation of inexpensive, reliable, scalable and portable sources and detectors of THz radiation would ensure their widespread exploitation in the future.