In most optoelectronic devices, light emission and absorption are considered perturbative. However, the ultra-strong light-matter coupling regime, characterized by highly non-perturbative interactions, has shown potential to alter fundamental material properties, including electrical conductivity, reaction rates, topological order, and nonlinear susceptibility. This research investigates a quantum infrared detector operating within this regime. The detector leverages collective electronic excitations, resulting in renormalized polariton states significantly detuned from bare electronic transitions. The study's purpose is to understand how single-particle fermionic transport is efficiently coupled with intersubband polaritons, which are inherently bosonic collective states. This is crucial for developing semiconductor devices with non-perturbative light interactions. Previous bosonic approaches, while efficient for calculating collective excitation energies and light-matter coupled states, neglect the dynamics of electronic populations, treating them as constants. Moreover, these models overlook the strong detuning of renormalized light-matter coupled states from bare electronic levels responsible for current flow. This research directly addresses this gap by combining experimental and theoretical investigations of semiconductor quantum detectors where a single-particle electronic extractor level interacts with collective light-matter coupled states. A novel quantum theory, free from the limitations of bosonization, explicitly incorporates the fermionic nature of carriers and population dynamics, providing a quantitative explanation of photocurrent magnitude and spectral features. This model connects single-particle subband populations (driving electronic transport) to collective light-induced electronic polarizations through nonlinear Bloch-type equations.

The light-matter interaction strength depends heavily on the electromagnetic environment. Dicke superradiance demonstrates the strong enhancement of emission and absorption in dense emitter ensembles. Recent solid-state demonstrations of this, using a dense two-dimensional electron gas in quantum wells (QWs), show all electrons contributing to a collective intersubband plasmon state. When this plasmon is coupled with a resonant microcavity, reversible energy exchange occurs at the vacuum Rabi frequency (ΩR). This results in two light-matter coupled states (intersubband polaritons) separated by 2ħΩR, observed across various spectral ranges. Collective strong coupling has been shown to alter fundamental material properties and enable novel device functionalities. The precise control over semiconductor epitaxial growth allows tailoring of artificial electronic potentials based on tunnel-coupled QWs, optimizing photo-generated electron extraction, as seen in quantum cascade detectors (QCDs). Microcavity-coupled unipolar devices have been used to explore strong and ultra-strong coupling regimes. However, the efficient coupling of single-particle fermionic transport with intersubband polaritons, intrinsically bosonic, remains a challenge. Existing bosonic models are efficient in predicting energies, but they fail to capture the dynamics of electronic populations and the strong detuning of the renormalized states from the bare electronic levels. This necessitates a new model that accounts for these factors.

The study uses a semiconductor quantum detector with eleven repetitions of four GaAs QWs separated by Al0.35Ga0.65As barriers. Photons are absorbed in a wide QW, exciting electrons from the first to the second level (energy difference E12=108 meV). Photoexcited electrons cascade through levels 3, 4, and 5, transferring to the next structure period. The Fermi level is set just below the last extraction level, ensuring high electron density and strong collective effects. The structure's absorption and photocurrent were characterized in multipass and mesa configurations. The absorption spectrum showed a significant shift from the single-particle transition E21, peaking at the collective electron excitation energy of the intersubband plasmon (E12≈130 meV). The measured photocurrent was further shifted towards the extractor level transition (E31=145 meV), indicating transport driven by collective excitation. An analytical model accurately replicated the experimental data. To achieve strong coupling, the QCD absorbing region was integrated into a resonant double-metal microcavity. The cavity resonance (Ec) was controlled by the width of metallic ridges. The interaction between the cavity and the intersubband plasmon created lower (LP) and upper (UP) polariton states, evidenced by reflectivity spectra showing a Rabi splitting of 22 meV at 80 K (17% of the electronic excitation transition). The system allowed tuning of polariton resonances around the extractor level E13. A microscopic quantum model was developed, employing a Hamiltonian that considers microcavity photon coupling, collective electronic effects, and tunnel coupling between levels 2 and 3. The Hamiltonian includes terms for the electromagnetic resonator, single-particle electron energies, collective electronic polarization (P12), dipole-dipole interactions (causing plasmonic effects and a blue shift of the 1→2 transition), light-matter coupling, and electron tunneling between levels 2 and 3. The system dynamics were calculated using a density matrix approach with relaxation terms, at T=0 K, with no electrons above the Fermi level in the ground state. An external field drives the cavity, coupled with electronic populations (Ni) and coherences (Pij). The mesa structure was modeled by applying the driving field directly to the electronic system, resulting in a set of nonlinear equations similar to semiconductor Bloch equations. The steady-state photocurrent (Iph) was expressed as eN3/τ34, where N3 is the population of the extractor level and τ34 is the relaxation time. The responsivity of the detector was expressed in terms of absorption efficiency, tunneling process characteristic time (τe), tunnel coupling strength (τ), frequency difference between levels 2 and 3 (ω23), and coherence relaxation rate (γ23). The model uses an "Eulerian" picture of QCD transport, assuming current continuity. The absorption efficiency considered the entire absorbing region, while the current is calculated per period. The coherent gain G1(ħω) is a key component of the model, connecting light-matter coupled and collective states to fermionic transport. The model was validated against experimental data from both mesa and cavity configurations, obtaining relaxation rates for populations and coherences. The role of the coherence ρ13 between levels 1 and 3 was confirmed by removing its effect; the resulting photocurrent spectrum significantly differed from experimental results. Bias voltage adjustments allowed varying the transition energy E13, demonstrating the resonant behavior of G1(ħω). The light-induced population N3 arises from two mechanisms: electron promotion to level 2 followed by tunneling to level 3 and direct transfer from level 1 to 3 assisted by collective polarization. The model also includes a comparison of strong and weak coupling regimes.

The experimental results showed a significant blue-shift of both the absorption and photocurrent spectra compared to the single-particle transition energy. This is due to the collective electron excitation of the intersubband plasmon. The photocurrent spectrum was further shifted towards the extractor level transition, which is a clear experimental signature of electronic transport being driven by the collective excitation. The microscopic quantum model, which explicitly accounts for the fermionic nature of carriers and population dynamics, accurately reproduces both the magnitude and spectral features of the experimental photocurrent. The model shows that the photocurrent is maximized when the energy of the upper polariton state is aligned with the extractor transition. The model highlights the role of the coherence ρ13 between levels 1 and 3 in the photocurrent generation process. This coherence is induced by the tunneling contribution in the Hamiltonian. Removing the effect of ρ13 from the model results in a significant deviation from the experimental data. The coherent gain G1(ħω), a crucial term in the model, demonstrates a resonant behavior that strongly influences the spectral shape and intensity of the photocurrent. This function describes an "electronic filter" that links the light-matter coupled and collective states to the fermionic transport. The study compares the high (strong coupling) and low (weak coupling) doping cases, revealing a significant difference in the responsivity. In the strong coupling regime, the responsivity is maximized when the upper polariton energy matches the extractor transition energy. This effect is absent in the weak coupling regime. The coherent gain G1 significantly increases the responsivity in the strong coupling regime, compensating for the decrease in the tunnel gate G1. This increase is linked to the polariton-induced transport, where both G1(ħω) and the absorption efficiency η(ħω) are maximized. The study emphasizes the nonlinear nature of the photocurrent generation process and the necessity of non-perturbative approaches to account for it.

This research reveals a novel mechanism for enhancing photoconductivity in devices through non-perturbative light-matter coupling and collective effects, paving the way for novel optoelectronic devices. The semi-classical model presented here can be extended to a fully quantum version to include vacuum field fluctuations and higher-order correlations. Further generalizations include replacing the extractor level with an electronic continuum to model strongly coupled QWIP structures. The model can be adapted to scenarios with higher absorption and light-matter coupling strengths. This opens up avenues for optimizing energy alignment between the extractor level and collective many-body states, which are not linked to bare electronic levels. The model also allows for exploring the design of confined plasmons and their interaction with single-electron transport. The significant increase in responsivity observed in the strong coupling regime warrants further investigation, as it suggests the possibility of exceeding the maximum responsivity in resonant extraction scenarios. This could lead to detectors with low dark currents and high working temperatures.

This study demonstrates a novel mechanism for enhancing photoconductivity in quantum detectors through ultra-strong light-matter coupling and collective electronic effects. A new microscopic quantum model accurately predicts experimental observations, highlighting the crucial role of coherence between energy levels and the non-linear nature of the photocurrent generation process. This work opens exciting avenues for designing advanced optoelectronic devices with enhanced performance, potentially leading to detectors with low dark currents and high operating temperatures.

The current model utilizes a semi-classical approach, treating the light field as a coherent state. While this approximation provides valuable insights, a fully quantum treatment of the light field would offer a more complete description, particularly regarding vacuum field fluctuations. The study focuses on a specific detector design; further investigations are needed to determine the generalizability of these findings to other device architectures and material systems. The experimental measurements were performed at a specific temperature (80 K). Future studies could investigate the temperature dependence of the observed effects, which might have an influence on the device performance and the efficiency of the light-matter interaction.