G. Biasiol

Sub-cycle switch-on of ultrastrong light-matter interaction

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.

Signatures of the ultrastrong light-matter coupling regime

Aji A. Anappara, Simone De Liberato, 3 Alessandro Tredicucci, ∗ Cristiano Ciuti, Giorgio Biasiol, Lucia Sorba, and Fabio Beltram NEST CNR-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa (Italy) Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot Paris 7 and CNRS, UMR 7162, Bâtiment Condorcet, 75013 Paris (France) Laboratoire Pierre Aigrain, Ecole Normale Supérieure and CNRS, UMR 8551, 75005 Paris (France) Laboratorio Nazionale TASC CNR-INFM, Area Science Park, SS 14 Km 163.5, Basovizza, I-34012 Trieste (Italy) (Dated: March 10, 2009)

Selective metal electrodeposition through doping modulation of semiconductor surfaces

We demonstrate selective electrodeposition of magnetic layers on doped semiconductors resulting in a self-aligned pattern which replicates the doping pattern in the semiconductor surface. A Schottky barrier forms at the interface between a semiconductor substrate and the electrolyte, which upon application of a cathodic potential is biased in the forward (reverse) direction for n- or p-type semiconductors, respectively. Electron transfer from an n-type semiconductor is thus possible, while breakdown of the Schottky barrier would be necessary for deposition on a p-type substrate. The process will thus be spatially selective on a lateral modulation of the substrate doping. As an example we demonstrate the deposition of Co on GaAs.

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.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.

Antenna-coupled microcavities for enhanced infrared photo-detection

We demonstrate mid-infrared detectors embedded into an array of double-metal nano-antennas. The antennas act as microcavities that squeeze the electric field into thin semiconductor layers, thus enhancing the detector responsivity. Furthermore, thanks to the ability of the antennas to gather photons from an area larger than the devices physical dimensions, the dark current is reduced without hindering the photo-generation rate. In these devices, the background-limited performance is improved with a consequent increase of the operating temperature. Our results illustrate how the antenna-coupled microcavity concept can be applied to enhance the performances of infrared opto-electronic devices.

Structure and Formation Mechanisms of AlGaAs V-Groove Vertical Quantum Wells Grown by Low-Pressure Organometallic Chemical Vapor Deposition

The structure of AlGaAs vertical quantum well (VQW) structures grown by low‐pressure organometallic chemical vapor deposition on V‐grooved GaAs substrates was analyzed as a function of growth temperature and Al mole fraction using transmission electron microscopy and atomic force microscopy (AFM). The low‐pressure growth yields several, extremely narrow (a few nm wide) branches of Ga‐enriched VQWs at the bottom of the grooves. The variation in Al content across the VQW was evaluated by measuring the AlGaAs oxide thickness on a cleaved edge of the structure using AFM in air. The transmission electron microscopy analysis demonstrates that the different VQW branches originate from distinct nanofacets that self‐order at the bottom of the V‐groove, probably due to facet‐induced segregation of group III species.

Self Ordering and Confinement in Strained InGaAs/AlGaAs V-Groove Quantum Wires Grown by Low-Pressure Organometallic Chemical Vapor Deposition

The structure and low temperature luminescence properties of compressively strained InGaAs/AlGaAs quantum wire (QWR) arrays grown by low-pressure organometallic chemical vapor deposition on V-grooved substrates are reported. The strain gives rise to quasi-periodic undulations of the wire facets along the wire axis, resulting in ordered chains of quantum dotlike structures. Low-temperature photoluminescence shows efficient emission from the wires with narrow (as low as 9.8 meV) linewidths and relatively high intensities. At high excitation densities, several quasi-one-dimensional QWR subbands appear as a result of bandfilling, presenting virtually no energy shifts (<2 meV), even when several (⩾3) subbands are filled.

Self-limiting growth of GaAs surfaces on nonplanar substrates

The growth of GaAs epitaxial layers by organometallic chemical vapor deposition on top of V-grooved substrates is found to exhibit a self-limiting behavior. As in the case of the self-limiting growth of AlGaAs on similar patterned substrates, the self-limiting GaAs profile exhibits characteristic crystallographic nanofacets. However, these facets are considerably broader than in typical, self-limiting AlGaAs profiles obtained at similar growth temperatures. Atomic force microscopy in air reveals the three-dimensional structure of the self-limiting GaAs surfaces, showing monolayer steps on the central (100) nanofacets and quasiperiodic modulation caused by step bunching on the side {311}A nanofacets. The width of the self-limiting GaAs V grooves can be reduced to less than 10 nm at sufficiently low growth temperatures, thus providing useful templates for growing, e.g., self ordered InGaAs/GaAs quantum wires.

Magnetotransport in high-g-factor low-density two-dimensional electron systems confined inIn0.75Ga0.25As∕In0.75Al0.25Asquantum wells

We report magneto-transport measurements on high-mobility two-dimensional electron systems (2DESs) confined in In_0.75Ga_0.25As/In_0.75Al_0.25As single quantum wells. Several quantum Hall states are observed in a wide range of temperatures and electron densities, the latter controlled by a gate voltage down to values of 1.10^11 cm^-2. A tilted-field configuration is used to induce Landau level crossings and magnetic transitions between quantum Hall states with different spin polarizations. A large filling factor dependent effective electronic g-factor is determined by the coincidence method and cyclotron resonance measurements. From these measurements the change in exchange-correlation energy at the magnetic transition is deduced. These results demonstrate the impact of many-body effects in tilted-field magneto-transport of high-mobility 2DESs confined in In_0.75Ga_0.25As/In_0.75Al_0.25As quantum wells. The large tunability of electron density and effective g-factor, in addition, make this material system a promising candidate for the observation of a large variety of spin-related phenomena.

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.