Effective-density-matrix approach for intersubband plasmons coupled to a cavity field: Electrical extraction and injection of intersubband polaritons

The main technological obstacle hampering the dissemination of modern optoelectronic devices operating with large light-matter coupling strength $\Omega$ is an in-depth comprehension of the carrier current extraction and injection from and into strongly coupled light-matter states, the so-called polaritonic states. The main challenge lies in modeling the interaction between excitations of a different nature, namely bosonic excitations [the plasmonic intersubband (ISB) excitations] with fermionic excitations (the electrons within the extraction or injection subband). In this work, we introduce a comprehensive quantum framework that encompasses both the ISB plasmonic mode and the extractor and injector mode, with a specific emphasis on accurately describing the coherent nature of transport. This reveals inherent selection rules dictating the interaction between the ISB plasmon and the extraction and injection subband. To incorporate the dynamics of the system, this framework is combined to a density-matrix model and a quantum master equation, which have the key property to distinguish intra- and intersubband mechanisms. These theoretical developments are confronted to experimental photocurrent measurements from midinfrared quantum cascade detectors ($\lambda =10\text{μ}m$) embedded in metal-semiconductor-metal microcavities, operating at the onset of the strong light-matter coupling regime ($2\Omega =9.3\mathrm{meV}$). We are able to reproduce the different features of the photocurrent spectra, notably the relative amplitude evolution of the polaritonic peaks with respect to the voltage bias applied to the structure. These results on extraction allow us to elucidate the possibility to effectively inject electronic excitations into ISB plasmonic states, and thus polaritonic states.