Controlling the way light interacts with material excitations is at the heart of cavity quantum electrodynamics (QED). In the strong-coupling regime, quantum emitters in a microresonator absorb and spontaneously re-emit a photon many times before dissipation becomes effective, giving rise to mixed light-matter eigenmodes. Recent experiments in semiconductor microcavities reached a new limit of ultrastrong coupling, where photon exchange occurs on timescales comparable to the oscillation period of light. In this limit, ultrafast modulation of the coupling strength has been suggested to lead to unconventional QED phenomena. Although sophisticated light-matter coupling has been achieved in all three spatial dimensions, control in the fourth dimension, time, is little developed. Here we use a quantum-well waveguide structure to optically tune light-matter interaction from weak to ultrastrong and turn on maximum coupling within less than one cycle of light. In this regime, a class of extremely non-adiabatic phenomena becomes observable. In particular, we directly monitor how a coherent photon population converts to cavity polaritons during abrupt switching. This system forms a promising laboratory in which to study novel sub-cycle QED effects and represents an efficient room-temperature switching device operating at unprecedented speed.

In a microcavity, light-matter coupling is quantified by the vacuum-Rabi frequency Omega_R. When Omega_R is larger than radiative and nonradiative 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 (ultrastrong coupling) was predicted when Omega_R becomes comparable to the transition frequency. Here we report 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 antiresonant light-matter coupling and the photon-energy renormalization of the vacuum field

The optical response of the intersubband excitation of multiple two-dimensional electron gases within a semiconductor microcavity has been studied through angle-dependent reflectance measurements. Using a resonator based on total internal reflection, a clear splitting of about 14 meV of the coupled intersubband cavity modes is observed from 10 K to room temperature, with resulting polaritonlike dispersion. The experimental findings are in good agreement with theoretical calculations performed in a transfer-matrix formalism.

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We present an analytic model that explains the self-ordering of quantum nanostructures grown on nonplanar surfaces. Self-limiting growth in these structures results from the interplay among growthrate anisotropy, curvature-induced capillarity, and, for alloys, entropy of mixing effects. Experimental results on self-limiting organometallic chemical vapor deposition on corrugated surfaces are in quantitative agreement with the model. The implications of the self-limiting growth characteristics on the self-ordering of quantum wells, wires, and dots are discussed. [S0031-9007(98)07220-2] PACS numbers: 68.65. + g, 81.10.Bk, 82.65.Dp Two-or three-dimensionally quantum-confined semiconductor structures have attracted much attention because of their interesting physical properties and potential device applications [1]. To overcome limitations in size and interface quality related to traditional lithography techniques, many efforts have been devoted to study their formation during the epitaxial process [2]. This can be accomplished if a suitable driving force is introduced to yield the desired lateral heterostructure patterning. A widely used approach in this direction is to exploit self-ordering processes on planar surfaces, as for strained-induced Stranski-Krastanow growth of quantum dots (QDs) [3,4]. Such techniques have the advantage that self-ordering is achieved without any surface patterning prior to growth; however, they suffer from a limited control on uniformity and deposition site due to the intrinsic random nature of the nucleation process.Self-ordering of nanostructures on nonplanar surfaces has the potential for solving these problems, as the corrugated surface can provide a template for the nucleation sites. In fact, organometallic chemical vapor deposition (OMCVD) and molecular beam epitaxy (MBE) on substrates patterned with corrugations (see Fig. 1) [5,6] or with pyramidal patterns [7] have been successfully employed to fabricate uniform arrays of quantum wires (QWRs) and QDs. Despite the accurate structural control demonstrated with this approach, the understanding of the self-limiting growth mechanism on such corrugated surfaces has been essentially phenomenological [8]. Existing models can, in fact, predict only constant growth rates of thick layers on mm-sized facets, depending on their orientation and environment, as a result of gas-phase and surface diffusion [9,10]. The growth behavior of facets in the 10-nm scale, relevant to the self-ordering of quantum nanostructures, cannot be explained with such models, since facet-size dependent surface diffusion fluxes should be invoked to account for the self-limiting growth [11].In this Letter we address the self-limiting growth of a corrugated surface, and establish a model that quantitatively describes the self-ordering of quantum wells (QWs), QWRs, and QDs on such patterned substrates.The formation of surface patterns during growth relies on lateral gradients in the surface chemical potential m. Considering, for simplicity, variations in only one dim...

Gate-voltage control of interedge tunneling at a split-gate constriction in the fractional quantum Hall regime is reported. Quantitative agreement with the behavior predicted for out-of-equilibrium quasiparticle transport between chiral Luttinger liquids is shown at low temperatures at specific values of the backscattering strength. When the latter is lowered by changing the gate voltage, the zero-bias peak of the tunneling conductance evolves into a minimum, and a nonlinear quasiholelike characteristic emerges. Our analysis emphasizes the role of the local filling factor in the split-gate constriction region.

We demonstrate that the emission characteristics of site-controlled InGaAs/GaAs single quantum dots embedded in photonic crystal slab cavities correspond to single confined excitons coupled to cavity modes, unlike previous reports of similar systems based on self-assembled quantum dots. By using polarization-resolved photoluminescence spectroscopy at different temperatures and a theoretical model, we show that the exciton-cavity interaction range is limited to the phonon sidebands. Photon-correlation and pump-power dependence experiments under nonresonant excitation conditions further establish that the cavity is fed only by a single exciton.

Remarkable nonlinearities in the differential tunneling conductance between fractional quantum Hall edge states at a constriction are observed in the weak-backscattering regime. In the ν = 1/3 state a peak develops as temperature is increased and its width is determined by the fractional charge. In the range 2/3 ≤ ν ≤ 1/3 this width displays a symmetric behavior around ν = 1/2. We discuss the consistency of these results with available theoretical predictions for inter-edge quasiparticle tunneling in the weak-backscattering regime.. Notably I T vanishes when the bias voltage V T , with V T labeling the potential difference between the two edges, tends to zero [5].In the opposite limit of weak backscattering the quantum Hall fluid is weakly perturbed by the QPC constriction. In this case the inter-edge tunneling current (again, at ν = 1/q) consists of Laughlin quasiparticles of charge e * = νe that scatter between the edges through the quantum Hall fluid (see Fig.1 panel (a)). At T=0 the quasiparticle tunneling rate is predicted to grow at low voltages as I T ∝ V (2ν−1) T in contrast to the electron-tunneling case discussed above. This remarkable nonlinear behavior is removed at finite temperature.When V T falls below a critical value V T,max of the order of k B T /e * , the tunneling current reverts to the linear ohmic behavior I T ∝ V T . In the differential tunneling characteristics