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THz Quantum Cascade Amplier

Currently, most commercial THz spectrometers are time-domain spectrometers (TDS), where THz pulses are generated on antennas by a photocurrent created from a pulsed laser. The detection scheme uses a similar antenna where carriers are generated by the same pulsed laser. The advantage of this apparatus is that the detection scheme is synchronous: the receiver is only “on” when the THz electric field is incident, and this results in a very high signal-to-noise ratio of approximately 50 dB. The disadvantage is that the THz pulses have only micro-Watts of output power, thus the apparatus will only penetrate thin or transparent materials. The major competing THz technology is that of quantum cascade (QC) lasers, which generate radiation with tens of mWatts of power. However, the power advantage of QC lasers is lost by the lack of sensitive detection techniques, and hence they are not used commercially. Until now researchers have tried to combine the technologies of THz-TDS and QCs but the two geometries have proven very difficult to integrate, with antenna emitters in particular proving incompatible with integration.

However, a new geometry emerged in 2010: the so-called the lateral photo-Dember effect that can be used to generate broadband THz pulses. The effect is quite simple, relying on the different mobilities of holes and electrons in a semiconductor which create a changing dipole under photoexcitation to generate THz pulses. We believe that this effect has great potential because it is flexible and its geometry is compatible with integration and quantum cascade lasers. Using the lateral photo-Dember effect will provide an elegant means of coupling a THz pulse into the QC structure, directly, with great efficiency. We intend to exploit this effect and generate THz pulses directly on the facet of a QC cavity and amplify them in the QC waveguide. Therefore we will combine the high output power of quantum cascade lasers with the detection sensitivity and broadband nature of state-of-the-art time-domain technology. It is a game-changing approach that is, according to all indications, absolutely feasible. It is very rare to propose such a potentially high impact research route, which is at the same time such low risk!

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