Stretching graphene using polymeric micro-muscles

The control of strain in two-dimensional materials opens exciting perspectives for the engineering of their electronic properties. While this expectation has been validated by artificial-lattice studies, it remains elusive in the case of atomic lattices. Remarkable results were obtained on nanobubbles and nano-wrinkles, or using scanning probes; microscale strain devices were implemented exploiting deformable substrates or external loads. These devices lack, however, the flexibility required to fully control and investigate arbitrary strain profiles. Here, we demonstrate a novel approach making it possible to induce strain in graphene using polymeric micrometric artificial muscles (MAMs) that contract in a controllable and reversible way under an electronic stimulus. Our method exploits the mechanical response of poly-methyl-methacrylate (PMMA) to electron-beam irradiation. Inhomogeneous anisotropic strain and out-of-plane deformation are demonstrated and studied by Raman, scanning-electron and atomic-force microscopy. These can all be easily combined with the present device architecture. The flexibility of the present method opens new opportunities for the investigation of strain and nanomechanics in two-dimensional materials

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

Tuning a distributed feedback laser with a coupled microcavity

We show that a distributed-feedback terahertz quantum cascade laser can be tuned with a coupled microcavity by anti-crossing of the respective eigenfrequencies. In this proof-of-concept experiment, a tuning range of 20 GHz is obtained, in good agreement with a simple finite element model, which shows that the tuning is determined by the coupling strength between the resonators. The concept could be applied to any laser cavity, but becomes progressively more attractive the lower the emission frequency. (c) 2010 Optical Society of Americ

Finite size effects in surface emitting Terahertz quantum cascade lasers

We analyze surface-emitting distributed feedback resonators for Terahertz quantum cascade lasers fabricated from double-metal waveguides. We explain the influence on resonances and surface-emission properties of the finite length and width of the gratings in connection with absorbing boundary conditions, and show that, contrary to the infinite case, the modes on either side of the photonic band-gap have finite surface losses. The lateral design of the resonator is shown to be important to avoid transverse modes of higher order and anti-guiding effects. Experimental findings are indeed in excellent agreement with the simulations. Both modeling and fabrication can easily be applied to arbitrary gratings, of which we discuss here a first interesting example. (c) 2009 Optical Society of Americ

Ligand signature in the membrane dynamics of single TrkA receptor molecules

The neurotrophin receptor TrkA (also known as NTRK1) is known to be crucially involved in several physio-pathological processes. However, a clear description of the early steps of ligand-induced TrkA responses at the cell plasma membrane is missing. We have exploited single particle tracking and TIRF microscopy to study TrkA membrane lateral mobility and changes of oligomerization state upon binding of diverse TrkA agonists (NGF, NGF R100E HSANV mutant, proNGF and NT-3). We show that, in the absence of ligands, most of the TrkA receptors are fast moving monomers characterized by an average diffusion coefficient of 0.47 μm^2/second; about 20% of TrkA molecules move at least an order of magnitude slower and around 4% are almost immobile within regions of about 0.6 mm diameter. Ligand binding results in increased slow and/or immobile populations over the fast one, slowing down of non-immobile trajectories and reduction of confinement areas, observations that are consistent with the formation of receptor dimeric and oligomeric states. We demonstrate that the extent of TrkA lateral mobility modification is strictly ligand dependent and that each ligand promotes distinct trajectory patterns of TrkA receptors at the cell membrane (ligand ‘fingerprinting’ effect). This ligand signature of receptor dynamics results from a differential combination of receptor-binding affinity, intracellular effectors recruited in the signalling platforms and formation of signalling and/or recycling endosome precursors. Thus, our data uncover a close correlation between the initial receptor membrane dynamics triggered upon binding and the specific biological outcomes induced by different ligands for the same receptor

High-intensity interminiband terahertz emission from chirped superlattices

Electroluminescence at lambdasimilar to69 mum (4.3 THz) is reported from interminiband transitions in quantum-cascade structures with superlattice active regions. Spontaneous emission gives a low-temperature linewidth of 2 meV (0.48 THz) with linear light-current characteristics observed up to high-current densities (625 A/cm(2)), resulting in record output powers of 500 pW. Devices operate up to above liquid-nitrogen temperature, with both emission wavelength and current-voltage characteristics in good agreement with theoretical predictions. (C) 2002 American Institute of Physics

Terahertz quantum-cascade lasers based on an interlaced photon-phonon cascade

A THz (lambdasimilar to80 mum) quantum-cascade laser utilizing alternating photon- and phonon-emitting stages has been developed to achieve efficient extraction of electrons from the lower laser level. Thermal backfilling of electrons is drastically reduced leading to an operation up to 95 K and a weak temperature dependence of the power versus current slope efficiency. The threshold current density is 280 A cm(-2) at 6 K and increases to 580 A cm(-2) at 90 K. Peak output powers of 10 mW at 30 K and 4 mW at 80 K are obtained. (C) 2004 American Institute of Physics

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

InAs/InP/InSb Nanowires as Low Capacitance n-n Heterojunction Diodes

Nanowire diodes have been realized by employing an axial heterojunction between InAs and InSb semiconductor materials. The broken-gap band alignment (type III) leads to a strong rectification effect when the current-voltage (I-V) characteristic is inspected at room temperature. The additional insertion of a narrow InP barrier reduces the thermionic contribution, which results in a net decrease of leakage current in the reverse bias with a corresponding enhanced rectification in terms of asymmetry in the I-V characteristics. The investigated diodes compare favorably with the ones realized with p-n heterostructured nanowires, making InAs/InP/InSb devices appealing candidates to be used as building blocks for nanowire-based ultrafast electronics and for the realization of photodetectors in the THz spectral range

Single-mode operation of terahertz quantum cascade lasers with distributed feedback resonators

Distributed feedback terahertz quantum-cascade lasers emitting at 4.34 and 4.43 THz are presented. Mode selection is based on a complex-coupling scheme implemented into the top-contact layer by a combination of wet chemical etching and ohmic-contact deposition. Single-mode emission stable at all injection currents and operating temperatures is shown, with a side-mode suppression ratio exceeding 20 dB. Peak output powers of up to 1.8 mW are obtained at low temperatures. (C) 2004 American Institute of Physics