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arXiv:2012.15633 · Physical Review Letters

Unified Description of Saturation and Bistability of Intersubband Transitions in the Weak and Strong Light-Matter Coupling Regimes

Mathieu Jeannin, Jean‐Michel Manceau, R. Colombelli

Published Oct 27, 2021• 24 citations
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Abstract

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}.

DOI
10.1103/physrevlett.127.187401

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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}.
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.
Ultrafast optical switches are one of the crucial components in next generation all-optical communication networks. One promising candidate is the optical switch based on saturable intersubband (ISB) absorption in doped InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQW). We employ the density matrix theory and modified optical Bloch equations with the relaxation-time approximation to simulate the time evolution of optical nonlinearities in quantum well (QW) structures. The absorption saturation characteristics are derived afterwards. The theoretical estimates are used to interpret the corresponding experimental results. Furthermore, several impact factors related to dynamic and saturation characteristics of the materials are discussed theoretically. The studies provide useful clues to optimize the QW material and device design.
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.
In the past few years advance in nanofabrication techniques have made it possible to make well-like compound semiconductor structures of low nanometer size.In these structures,the state of the electron in the direction perpendicular to the length of the well is quantized with sufficiently large separation in energy levels and the motion of the electron parallel to the length is that of a quasi-free electron.One of the most remarkable properties of these one-dimensional electron system is that the optical transitions between the size-quantized subbands are feasible.The result shows there is a large optical nonlinearities in asymmetric quantum wells.The problem of optical saturation of intersubband absorption has recently attracted considerable attention.The problem is very important to the development of superlattice devices such as fast infrared detectors and(modulators) because one needs to know the range of input optical intensities for which linear operation is(possible.) The analytical expression of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained in Pschl-Teller wells using the density matrix formalism taking into account the(intrasubband) relaxation.Since there are two adjustable parameters κ and λ in Pschl-Teller wells,the shapes of the wells will change with the change of the parameters κ and λ,accordingly the linear and the third-order nonlinear optical absorption coefficients will change with the parameters κ and λ.It is shown that the optical absorption coefficient depends on the parameter,and the optical intensity strongly. The absorption saturation will occur with I increase.Numerical results are presented for a typical AlGaAs/GaAs Pschl-Teller well.
We have investigated intersubband absorptions between the conduction ground state and the first excited state of two AlxGa1−xAs/GaAs/AlxGa1−xAs multiple quantum well structures with x=0.3 and 85 Å well width, and with x=0.57 and 96 Å well width. Small-signal measurements show absorption peaks at 10.45 and 10.15 μm, respectively. Under an intense resonant excitation from a pulsed CO2 laser, saturation of the intersubband absorption occurred. The saturation intensity is estimated to be 340±120 kW/cm2 for the first sample and 375±120 kW/cm2 for the second. From these values, we have deduced subband decay times of the order of 10.6±3.5 ps for the first sample and 15.5±5 ps for the second.
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.