Intersubband transitions are the electronic transitions between the quantum confined two-dimensional electronic subbands in semiconductor quantum wells . While they are very important for infrared semiconductor devices, such quantum cascade lasers  and quantum well infrared detectors (QWIPs) , intersubband transitions are also siege of very interesting physical phenomena. One of them is the ultra-strong light-matter coupling regime, first predicted by Ciuti et al. , where the light-matter coupling strength becomes a sizable fraction of the energy of the material excitation. I will discuss our realizations of the ultra-strong coupling, where highly doped quantum wells were combined with plasmonic patch microcavities . The later provide almost perfect overlap between the cavity mode and the electronic polarization, allowing the observation of the ultra-strong coupling from the THz to the Mid-Infrared region, up to room temperature .
Our plasmonic cavities are actually very beneficial for quantum infrared detectors, such as QWIPs, as they not only enhance the light-matter interaction, but also greatly improve the photon collection of the detector owe to an antenna effect . The latter allows a strong suppression of the thermally induced detector dark current, allowing the microcavity-coupled QWIPs to operate up to room temperature, with a sensitivity enhanced in a heterodyne detection mode . I will also discuss how the change of the plasmonic resonators could bring further improvement of the detector performance, but it could also be very interesting for the study of novel regimes of ultra-strong coupling .
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