Electrical control of polariton coupling in intersubband microcavities

We demonstrate the external control of the coupling between the intersubband transition and the photonic mode of a GaAs/AlGaAs microcavity with multiple quantum wells embedded. By electrical gating, the charge density in the wells can be lowered, thereby quenching the intersubband polaritons and reverting the system to uncoupled excitations. The angle-dependent reflectance measurements are in good agreement with theoretical calculations performed in the transfer matrix formalism. The experiment shows the prospects offered by intersubband microcavities through manipulation of the system ground state

Charge transport studies on Si nanopillars for photodetectors fabricated using vapor phase metal-assisted chemical etching.

Si nanopillars (SiNPLs) were fabricated using a novel vapor phase metal-assisted chemical etching (V-Mace) and nanosphere lithography. The temperature dependent current–voltage (I–V) characteristics have been studied over a broad temperature range 170–360 K. The SiNPLs show a Schottky diode-like behavior at a temperature below 300 K and the rectification (about two orders of magnitude) is more prominent at temperature < 210 K. The electrical properties are discussed in detail using Cheung’s and Norde methods, and the Schottky diode parameters, such as barrier height, ideality factor, series resistance, are carefully figured out and compared with different methods. Moreover, the light sensitivity of the SiNPLs has been studied using I–V characteristics in dark and under the illumination of white light and UV light. The SiNPLs show fast response to the white light and UV light (response time of 0.18 and 0.26 s) under reverse bias condition and the mechanism explained using band diagram. The ratio of photo-to-dark current shows a peak value of 9.8 and 6.9 for white light and UV light, respectively. The Si nanopillars exhibit reflectance < 4% over the wavelength region 250–800 nm with a minimum reflectance of 2.13% for the optimized sample. The superior light absorption of the SiNPLs induced fast response in the I–V characteristics under UV light and white light. The work function of the SiNPLs in dark and under illumination has been also studied using Kelvin probe to confirm the light sensitivity

Tunnel-assisted manipulation of intersubband polaritons in asymmetric coupled quantum wells

The authors report the external control of the polariton ground state by manipulating the coupling between the intersubband transition and the photonic mode of a GaAs∕AlGaAs microcavity. The vacuum-field Rabi splitting is varied by means of charge transfer between the energetically-aligned ground subbands of asymmetric tunnel-coupled quantum wells. The authors propose the use of this structure concept for implementing ultrafast modulation of intersubband polaritons

Light-matter excitations in the ultra-strong coupling regime

In a microcavity, light-matter coupling is quantified by the vacuum Rabi frequency $\Omega_R$. When $\Omega_R$ is larger than radiative and non-radiative loss rates, the system eigenstates (polaritons) are linear superposition of photonic and electronic excitations, a condition actively investigated in diverse physical implementations. Recently, a quantum electrodynamic regime (ultra-strong coupling) was predicted when $\Omega_R$ becomes comparable to the transition frequency. Here we report unambiguous signatures of this regime in a quantum-well intersubband microcavity. Measuring the cavity-polariton dispersion in a room-temperature linear optical experiment, we directly observe the anti-resonant light-matter coupling and the photon-energy renormalization of the vacuum field

Coupling of THz radiation with intervalence band transitions in microcavities

The strong coupling of THz radiation and material excitations can improve the quantum efficiency of THz emitters. In this paper, we investigate THz polaritons and antipolaritons based on valence band transitions, which allow TE coupling in a simple configuration. The approach can improve the quantum efficiency of THz based devices based on TE mode in the strong coupling regime of THz radiations and intervalence bands transitions in a GaAs/AlGaAs quantum wells. A Nonequilibrium Many Body Approach for the optical response beyond the Hartree-Fock approximation is used as input to the effective dielectric function formalism for the polariton/antipolariton problem. The energy dispersion relations in the THz range are obtained by adjusting the full numerical solutions to simple analytical expressions, which can be used for non specialists in a wide number of new structures and material systems. The combination of manybody and nonparabolicity at high densities leads to dramatic changes in the polariton spectra in a nonequilibrium configuration, which is only possible for intervalence band transitions

Beyond the Jaynes-Cummings model: circuit QED in the ultrastrong coupling regime

In cavity quantum electrodynamics (QED), light-matter interaction is probed at its most fundamental level, where individual atoms are coupled to single photons stored in three-dimensional cavities. This unique possibility to experimentally explore the foundations of quantum physics has greatly evolved with the advent of circuit QED, where on-chip superconducting qubits and oscillators play the roles of two-level atoms and cavities, respectively. In the strong coupling limit, atom and cavity can exchange a photon frequently before coherence is lost. This important regime has been reached both in cavity and circuit QED, but the design flexibility and engineering potential of the latter allowed for increasing the ratio between the atom-cavity coupling rate and the cavity transition frequency above the percent level. While these experiments are well described by the renowned Jaynes-Cummings model, novel physics is expected in the ultrastrong coupling limit. Here, we report on the first experimental realization of a superconducting circuit QED system in the ultrastrong coupling limit and present direct evidence for the breakdown of the Jaynes-Cummings model.Comment: 5 pages, 3 figure

Cavity Quantum Electrodynamics in the Ultrastrong Coupling Regime

We revisit the mathematical formulation of the famous Jaynes-Cummings-Paul Hamiltonian, which describes the interaction of a two-level atom with a single mode of an electromagnetic cavity reservoir. We rigorously show that under the condition of Ultrastrong coupling between the atom and cavity, in which the transition frequency is comparable to the coupling frequency, the bosonic field operators undergo non-sinusoidal time variations. As a result, the well-known solutions to the Jaynes-Cummings-Paul model are no longer valid, even when the rotating wave approximation is not used. We show how a correct mathematical solution could be found instead.Comment: 13 pages, 7 figure

Switching ultrastrong light–matter coupling on a subcycle scale

Intersubband cavitypolaritons in a quantum wellwaveguide 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 cavitypolaritons directly in the time domain. Future perspectives of room-temperature subcycle control of ultrastrong electron–photon interaction are discussed

Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity

This work was financially supported by the Research Fund of the University of Iceland, the Icelandic Research Fund, grant no. 163082-051, and the Icelandic Instruments Fund. HSG acknowledges support from Ministry of Science and Technology of Taiwan, under grant no. 103-2112-M-002-003-MY3.We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence

Light-matter interaction in intersubband microcavities

The research work presented in this thesis is focused on the study of the optoelectronic coupling between the intersubband excitation in a two-dimensional electron gas (2DEG) and the resonant photonic mode of a planar semiconductor microcavity, in which the 2DEGs are embedded. When a generic electronic excitation interacts resonantly with a discrete cavity mode, a strong-coupling regime arises if the interaction strength of the electron-photon system (vacuum-field Rabi energy) is larger than the damping rates. This condition has been demonstrated in diverse research fields: from atomic physics to organic/semiconductor excitons coupled to a planar microcavity, to superconductor qubits coupled to microwave transmission lines. In semiconductor physics, the strong coupling results in the formation of quasi-particles termed cavity polaritons, which are the linear superposition of light and matter excitations. In 2003, the strong coupling of intersubband transitions in doped quantum wells with confined photons, and the corresponding formation of `intersubband cavity polaritons', were experimentally observed up to room temperature. In contrast to other strongly coupled systems, intersubband microcavities are more appealing due to the unique possibility of externally controlling light-matter interaction. The manipulation of polariton coupling hinges on the principle that the intensity of intersubband absorption in the active region can be controlled either through the carrier density modulation or by altering the oscillator strength of the transition. Owing to the large oscillator strength and relatively low-energy of the transition, in intersubband microcavities the vacuum-field Rabi splitting can be a significant fraction of the intersubband transition energy. Such a regime of light-matter interaction was predicted theoretically and termed as the `ultrastrong coupling regime'. The investigation of the optoelectronic coupling is here conducted in two different directions: (i) exploring suitable means for the external manipulation of intersubband cavity polaritons, (ii) realizing the conditions for observing the ultrastrong coupling regime of light-matter interaction. The devices employed in the investigation are multiple quantum well active structures embedded in intersubband microcavities - based either on dielectric mirrors or on plasmon mode resonators. The results presented in this thesis contain various experimental realizations of the external control of polariton coupling in a solid-state device, with unprecedented modulation depth and speed. Moreover the first experimental observation of the ultrastrong coupling of light-matter interaction is also reported. These are fundamental steps towards the generation of the photon pairs from vacuum fluctuations in a quantum electrodynamical scheme analogous to the well known dynamic Casimir effect, which is yet to be realized experimentally