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Absorption Saturation is Tailorable in the Strong Light-Matter Coupling Regime

R. Colombelli, Mathieu Jeannin

Published Dec 31, 2020• 0 citations
OpenAlex

Abstract

It is well known that optical absorption saturation of intersubband transitions in doped semiconductor quantum wells is independent of the introduced doping in the absence of a cavity. When inserting the system in a resonator, we show that this remains valid only in the weak light-matter coupling regime. In the strong light-matter coupling regime instead, we demonstrate that absorption saturation is no more doping independent and it is instead tailorable. Based on this unified formalism for saturation in weak and strong coupling, we provide designs for semiconductor saturable absorption (SESAM) mirrors and bistable systems operating in the mid-infrared range of the electromagnetic spectrum and with extremely low saturation intensities. Countering intuition, we show that the most suitable region to exploit low saturation intensities is not the ultra-strong coupling regime, but is instead at the onset of strong light-matter coupling.

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Physical Review B•Aug 9, 2005•126 citations
It is well known that optical absorption saturation of intersubband transitions in doped semiconductor quantum wells is independent of the introduced doping in the absence of a cavity. When inserting the system in a resonator, we show that this remains valid only in the weak light-matter coupling regime. In the strong light-matter coupling regime instead, we demonstrate that absorption saturation is no more doping independent and it is instead tailorable. Based on this unified formalism for saturation in weak and strong coupling, we provide designs for semiconductor saturable absorption (SESAM) mirrors and bistable systems operating in the mid-infrared range of the electromagnetic spectrum and with extremely low saturation intensities. Countering intuition, we show that the most suitable region to exploit low saturation intensities is not the ultra-strong coupling regime, but is instead at the onset of strong light-matter coupling.
We propose a unified description of intersubband absorption saturation for quantum wells inserted in a resonator, both in the weak and strong light-matter coupling regimes. We demonstrate how absorption saturation can be engineered. In particular, we show that the saturation intensity increases linearly with the doping in the strong coupling regime, while it remains doping independent in weak coupling. Hence, countering intuition, the most suitable region to exploit low saturation intensities is not the ultrastrong coupling regime, but is instead at the onset of the strong light-matter coupling. We further derive explicit conditions for the emergence of bistability. This Letter sets the path toward, as yet, nonexistent ultrafast midinfrared semiconductor saturable absorption mirrors (SESAMs) and bistable systems. As an example, we show how to design a midinfrared SESAM with a 3 orders of magnitude reduction in saturation intensity, down to ≈5 kW cm^{-2}.
We demonstrate that a ionising transition can be strongly coupled to a photonic resonator, leading to the appearance of a discrete optically active polaritonic resonance below the ionisation threshold. The matter component of such hybrid excitation corresponds to a bound exciton created by light-matter interaction. Applying our theory to the case of bound-to-continuum transitions in microcavity-embedded doped quantum wells, we show how those strong-coupling features can be exploited as a novel gauge to tune both optical and electronic properties of semiconductor heterostructures.
We present a full quantum theory for the dissipative dynamics of an optical cavity in the ultra-strong light-matter coupling regime, in which the vacuum Rabi frequency is comparable to the electronic transition frequency and the anti-resonant terms of the light-matter coupling play an important role. In particular, our model can be applied to the case of intersubband transitions in doped semiconductor quantum wells embedded in a microcavity. The coupling of the intracavity photonic mode and of the electronic polarization to the external, frequency-dependent, dissipation baths is taken into account by means of quantum Langevin equations in the input-output formalism. Observable spectra (reflection, absorption, transmission and electroluminescence) are calculated analytically in the case of a time-independent vacuum Rabi frequency.
We analyze the optical modulation of an interband resonant light pulse by intersubband resonant light pulses in a three-level quantum well system. The origin of the third order self-saturation optical nonlinearity resulting in the modification of the interband light absorption is assumed to lie in the carrier density modulation induced by the interband and intersubband transitions. The optical nonlinearity can be optimized to achieve the maximum modulation by applying control light pulses shorter than the intersubband energy relaxation time but longer than the intersubband phase relaxation time. We have also experimentally investigated the material system suitable for optical communication regime and have observed that the In0.53Ga0.47As/AlAs0.56Sb0.44 quantum well lattice matched to InP yielding an intersubband transition as short as 1.4 μm is the most suitable system for the proposed modulation scheme.
The problem of saturation of intersubband absorption and optical rectification in asymmetric quantum wells is discussed theoretically using the density matrix formalism. The effects of the electron-electron interaction are taken into account. It is shown that near the resonance the spectral shapes of the nonlinear optical absorption coefficient and the nonlinear optical rectification coefficient are very similar. In the absence of the electron-electron interaction the spectral responses are nearly Lorentzian. The electron-electron interaction plays an important role in heavy doped systems. In structures with large spatial separation of the ground and the excited states wave functions, the depolarization effect is small and the absorption spectrum is affected mainly by the electrostatic Coulomb interaction. This interaction shifts the absorption peak to higher energy by an amount depending on the intensity of the incident light and leads to the significant distortion (asymmetry) of the line.
A quantum well (QW), with only three transitions, which are all dipole allowed because of the QW asymmetry, is investigated when driven coherently by two infrared optical fields. One field couples the $\ensuremath{\mid}1⟩\ensuremath{-}\ensuremath{\mid}2⟩$ and $\ensuremath{\mid}2⟩\ensuremath{-}\ensuremath{\mid}3⟩$ transitions simultaneously, while a second field couples the $\ensuremath{\mid}1⟩\ensuremath{-}\ensuremath{\mid}3⟩$ transition. The absorption linear response is computed allowing the illustration of some features due to quantum coherence. Without the second field, the absorption of a weak probe is that of a strongly saturated system. As the coupling strength of the second field is increased the absorption strength of a weak probe increases, by over an order of magnitude factor in absorption strength compared to the case in the absence of the second field. The results are shown to be due to an optically mediated, coherent population trapping effect.