Intersubband Cavity Polaritons Research Articles

Mid-Infrared Intersubband Cavity Polaritons in Flexible Single Quantum Well

Strong and ultrastrong coupling between intersubband transitions in quantum wells and cavity photons have been realized in mid-infrared and terahertz spectral regions. However, most previous works employed a large number of quantum wells on rigid substrates to achieve coupling strengths reaching the strong or ultrastrong coupling regime. In this work, we experimentally demonstrate ultrastrong coupling between the intersubband transition in a single quantum well and the resonant mode of photonic nanocavity at room temperature. We also observe strong coupling between the nanocavity resonance and the second-order intersubband transition in a single quantum well. Furthermore, we implement for the first time such intersubband cavity polariton systems on soft and flexible substrates and demonstrate that bending of the single quantum well does not significantly affect the characteristics of the cavity polaritons. This work paves the way to broaden the range of potential applications of intersubband cavity polaritons including soft and wearable photonics.

Terahertz Frequency Combs: Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching (Laser Photonics Rev. 15(6)/2021)

In article number 2000575, Miriam S. Vitiello and co-workers report on an integrated architecture for the generation of high power terahertz (THz) frequency combs (FCs) comprising an ultrafast THz polaritonic reflector, exploiting intersubband cavity polaritons, and a broad bandwidth heterogeneous THz quantum cascade lasers (QCL). It is shown that the polaritonic reflector induces spectral reshaping of the QCL emission and stable FC operation over 38% of the laser operational range.

Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

AbstractQuantum cascade lasers (QCLs) represent a fascinating accomplishment of quantum engineering and enable the direct generation of terahertz (THz) frequency radiation from an electrically biased semiconductor heterostructure. Their large spectral bandwidth, high output powers, and quantum‐limited linewidths have facilitated the realization of THz pulses by active mode‐locking and passive generation of optical frequency combs (FCs) through intracavity four‐wave‐mixing, albeit over a restricted operational regime. Here, an integrated architecture is conceived for the generation of high power (5.5–8.0 mW) THz FCs comprising an ultrafast THz polaritonic reflector, exploiting intersubband (ISB) cavity polaritons, and a broad bandwidth (2.3–3.8 THz) heterogenous THz QCL. By tuning the group‐delay‐dispersion in an integrated geometry, through the exploitation of light‐induced bleaching of the ISB‐based THz polaritons, spectral reshaping of the QCL emission and stable FC operation over an operational range up to 38%, characterized by a single and narrow (down to 700 Hz) intermode beatnote are demonstrated. This concept provides design guidelines for a new generation of compact, cost‐effective, electrically driven chip‐scale FC sources based on ultrafast polariton dynamics, paving the way toward the generation of mode‐locked THz microlasers that can strongly impact a broad range of applications in ultrafast sciences, data storage, high‐speed communication, and spectroscopy.

Ultrafast terahertz saturable absorbers using tailored intersubband polaritons

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.

Layer-resolved absorption of light in arbitrarily anisotropic heterostructures

We present a generalized formalism to describe the optical energy flow and spatially resolved absorption in arbitrarily anisotropic layered structures. The algorithm is capable of treating any number of layers of arbitrarily anisotropic, birefringent, and absorbing media and is implemented in an open access computer program. We derive explicit expressions for the transmitted and absorbed power at any point in the multilayer structure, using the electric field distribution from a 4 $\times$ 4 transfer matrix formalism. As a test ground, we study three nanophotonic device structures featuring unique layer-resolved absorption characteristics, making use of (i) in-plane hyperbolic phonon polaritons, (ii) layer-selective, cavity-enhanced exciton absorption in transition metal dichalcogenide monolayers, and (iii) intersubband-cavity polaritons in quantum wells. Covering such a broad spectral range from the far-infrared to the visible, the case studies demonstrate the generality and wide applicability of our approach.

Saturation and bistability of defect-mode intersubband polaritons

In this article we report about linear and nonlinear optical properties of intersubband cavity polariton samples, where the resonant photonic mode is a defect state in a metallo-dielectric photonic crystal slab. By tuning a single geometric parameter of the resonator, the cavity Q-factor can reach values as large as 85, with a consequent large cooperativity for the light-matter interaction. We show that a device featuring large cooperativity leads to sharp saturation, or even bistability, of the polariton states. This nonlinear dynamics occurs at the crossover between the weak and the strong coupling regimes, where the weak critical coupling concept plays a fundamental role.

Linewidth-narrowing phenomena with intersubband cavity polaritons

Absorption spectra of strongly coupled intersubband cavity polaritons have been measured, using a tunable midinfrared quantum cascade laser, with high angular and spectral resolution. Pronounced linewidth narrowing of the polaritons around the anticrossing was found, with polariton linewidths narrower (4.2 meV) than both the bare intersubband transition linewidth and empty cavity linewidth (6.2 and 6 meV, respectively), at room temperature. This is due to variations in the degree of spatial averaging of the in-plane quantum-well disorder as the polariton's extended coherence length is increased by the photonic coupling over the value corresponding to the bare intersubband transition coherence length.

Semiclassical study of intersubband cavity polaritons: Role of plasmonic and radiative coupling effects

We theoretically discuss the properties of intersubband polaritons supported by a multiple-quantum-well structure embedded in the planar microcavity with dielectric and plasmonic mirrors. A semiclassical approach based on the transfer matrix formalism supplemented by the sheet model and the ``microscopic'' implementation of the effective medium approximation is employed. The presence of the intrasubband transition is taken into account. The obtained results show that in the case of realistic systems exhibiting superstrong coupling regime, radiative coupling between cavity modes can lead to the violation of a commonly used single-mode cavity approximation. It manifests itself as a formation of hybrid polariton modes having a non-negligible admixture of higher photonic modes and/or surface plasmonic modes. The comparison with results predicted by a microscopic quantum approach recently developed is also presented. Performing the above mentioned comparison we pay special attention to the role of intra- and interwell Coulomb couplings.

Terahertz lasing from intersubband polariton-polariton scattering in asymmetric quantum wells

Electric dipole transitions between different cavity polariton branches or between dressed atomic states with the same excitation number are strictly forbidden in centro-symmetric systems. For doped quantum wells in semiconductor microcavities, the strong coupling between an intersubband transition in the conduction band and a cavity mode produces two branches of intersubband cavity polaritons, whose normal-mode energy splitting is tunable and can be in the terahertz region. Here, we show that, by using asymmetric quantum wells, it is possible to have allowed dipolar transitions between different polaritonic branches, leading to the emission of terahertz photons. We present a quantum field theory for such a system and predict that high-efficiency, widely tunable terahertz lasing can be obtained.

Microcavity effect on the pump–probe intersubband response of multiple-quantum-well structures

We study theoretically the coherent pump–probe intersubband response of a multiple quantum well (MQW) embedded in a semiconductor microcavity. An n-type doped MQW structure with two subbands in the conduction band is considered. Self-consistent numerical calculations are performed for realistic systems employing a semiclassical approach based on the transfer matrix formalism and the so-called sheet model. They show that in the strong coupling limit the pumping of the system leads to evolution of the intersubband cavity polariton doublet into a Mollow-type spectrum. By using appropriate angles, both the pump and the probe light can be tuned into resonance with the cavity mode. In this double-resonance case, simultaneously with a dramatic enhancement of the Rabi flopping frequency, a strong selective enhancement of distinct parts of the Rabi sidebands is possible.

Switching ultrastrong light–matter coupling on a subcycle scale

Intersubband cavity polaritons in a quantum well waveguide structure are optically generated within less than one cycle of light by a 12-femtosecond near-infrared pulse. Mid-infrared probe transients trace the nonadiabatic switch-on of ultrastrong light-matter coupling and the conversion of bare photons into cavity polaritons directly in the time domain. Future perspectives of room-temperature subcycle control of ultrastrong electron–photon interaction are discussed.

Intersubband cavity polaritons: The role of higher photonic modes

We present a theoretical analysis of the consequence of coupling between higher photonic modes and intersubband excitations in microcavities with embedded multiple quantum wells (MQWs). The polariton dispersion relations and angle-resolved absorption spectra are calculated numerically using a semiclassical approach based on a transfer-matrix formulation and effective-medium approximation. The obtained results are interpreted employing a multiple coupled harmonic-oscillator model supplemented by the input-output formalism. We show that when the MQW occupies a large fraction of space between cavity mirrors the coupling with higher photonic modes cannot be considered as a negligibly small perturbation in formation of the intersubband cavity polariton modes. When appropriate conditions are fulfilled the coupling between higher photonic modes and so-called ``dark'' intersubband electronic excitations can lead, in accordance with experimental results, to the formation of complex multiple-peaked absorption spectra.

How fast electrons and photons mix: Sub-cycle switching of intersubband cavity polaritons

Intersubband cavity polaritons in a quantum well waveguide structure are optically generated within less than one cycle of light by a femtosecond near-infrared pulse. Mid-infrared probe transients trace the non-adiabatic switch-on of ultrastrong light-matter coupling and the conversion of bare photons into cavity polaritons directly in the time domain.

Stimulated Scattering and Lasing of Intersubband Cavity Polaritons

We present a microscopic theory describing the stimulated scattering of intersubband polariton excitations in a microcavity-embedded two-dimensional electron gas. In particular, we consider the polariton scattering induced by the interaction with longitudinal optical phonons. Our theory demonstrates the possibility of final-state stimulation for the scattering of such composite excitations, accounting for the deviations from ideal bosonicity occurring at high excitation densities. By using GaAs parameters, we predict a quantum degenerate regime and lasing without electronic population inversion in an optical pumping configuration.

Publisher's Note: Quantum theory of electron tunneling into intersubband cavity polariton states [Phys. Rev. B79, 075317 (2009)

Received 25 February 2009DOI:https://doi.org/10.1103/PhysRevB.79.129901©2009 American Physical Society

Quantum theory of electron tunneling into intersubband cavity polariton states

Through a nonperturbative quantum theory, we investigate how the quasielectron excitations of a two-dimensional electron gas are modified by strong coupling to the vacuum field of a microcavity. We show that the electronic dressed states originate from a Fano-type coupling between the bare electron states and the continuum of intersubband cavity polariton excitations. In particular, we calculate the electron spectral function modified by light-matter interactions and its impact on the electronic injection of intersubband cavity polaritons. The domain of validity of the present theoretical results is critically discussed. We show that resonant electron tunneling from a narrow-band injector can selectively excite super-radiant states and produce efficient intersubband polariton electroluminescence.

Photovoltaic probe of cavity polaritons in a quantum cascade structure

The strong coupling between an intersubband excitation in a quantum cascade structure and a photonic mode of a planar microcavity has been detected by angle-resolved photovoltaic measurements. A typical anticrossing behavior, with a vacuum-field Rabi splitting of 16meV at 78K, has been measured, for an intersubband transition at 163meV. These results show that the strong coupling regime between photons and intersubband excitations can be engineered in a quantum cascade optoelectronic device. They also demonstrate the possibility to perform angle-resolved midinfrared photodetection and to develop active devices based on intersubband cavity polaritons.

On the ultrastrong vacuum Rabi coupling of an intersubband transition in a semiconductor microcavity

In this invited paper of the 28th International Conference of the Physics of Semiconductors (ICPS-28), we discuss the peculiar quantum electrodynamical properties of a semiconductor microcavity system, in which a cavity photon mode is strongly coupled to an intersubband transition of a doped quantum well system. In this kind of semiconductor system, it is possible to achieve an unprecedented ultrastrong coupling regime, in which the vacuum Rabi frequency is comparable to the electronic transition frequency. We discuss the anomalous quantum properties of the quantum ground state (a squeezed vacuum) and of the intersubband cavity polariton excitations. We address the role of dissipation and point out some future perspectives concerning the investigation of quantum vacuum radiation effects induced by an ultrafast time modulation of the quantum vacuum.

Quantum cascade intersubband polariton light emitters

We propose the use of a quantum cascade approach to achieve photon emission from intersubband cavity polariton states. Judicious quantum engineering of the electronic band structure and of the microcavity resonator allows one to obtain emission in the strong light–matter coupling regime under electrical excitation. In particular, we show that, using InP-based multiple quantum well structures, large polariton splittings of about 40 meV can be obtained within the mid-infrared range of the electromagnetic spectrum (intersubband transition energy ≈130 meV, corresponding to λ ≈ 9–10 µm). The calculations show that the effect is robust with respect to substantial changes of the electronic doping density or of the linewidth of the intersubband transition.