Irákleion, Greece
Irákleion, Greece

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Christmann G.,University of Cambridge | Tosi G.,University of Cambridge | Tosi G.,Autonomous University of Madrid | Tosi G.,University of New South Wales | And 9 more authors.
New Journal of Physics | Year: 2014

When pumped nonresonantly, semiconductor microcavity polaritons form Bose-Einstein condensates that can be manipulated optically. Using tightly-focused excitation spots, radially expanding condensates can be formed in close proximity. Using high time resolution streak camera measurements we study the time dependent properties of these macroscopic coherent states. By coupling this method with interferometry we observe directly the phase locking of two independent condensates in time, showing the effect of polariton-polariton interactions. We also directly observe fast spontaneous soliton-like oscillations of the polariton cloud trapped between the pump spots, which can be either dark or bright solitons. This transition from dark to bright is a consequence of the change of sign of the nonlinearity which we propose is due to the shape of the polariton dispersion leading to either positive or negative polariton effective mass. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Anton C.,Autonomous University of Madrid | Liew T.C.H.,Nanyang Technological University | Tosi G.,Autonomous University of Madrid | Martin M.D.,Autonomous University of Madrid | And 6 more authors.
Applied Physics Letters | Year: 2012

We present a time-resolved study of the logical operation of a polariton condensate transistor switch. Creating a polariton condensate (source) in a GaAs ridge-shaped microcavity with a non-resonant pulsed laser beam, the polariton propagation towards a collector, at the ridge edge, is controlled by a second weak pulse (gate), located between the source and the collector. The experimental results are interpreted in the light of simulations based on the generalized Gross-Pitaevskii equation, including incoherent pumping, decay, and energy relaxation within the condensate. © 2012 American Institute of Physics.


Anton C.,Autonomous University of Madrid | Liew T.C.H.,Nanyang Technological University | Tosi G.,Autonomous University of Madrid | Martin M.D.,Autonomous University of Madrid | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We present a time-resolved study of energy relaxation and trapping dynamics of polariton condensates in a semiconductor microcavity ridge. The combination of two nonresonant, pulsed laser sources in a GaAs ridge-shaped microcavity gives rise to profuse quantum phenomena where the repulsive potentials created by the lasers allow the modulation and control of the polariton flow. We analyze in detail the dependence of the dynamics on the power of both lasers and determine the optimum conditions for realizing an all-optical polariton condensate transistor switch. The experimental results are interpreted in the light of simulations based on a generalized Gross-Pitaevskii equation, including incoherent pumping, decay, and energy relaxation within the condensate. © 2013 American Physical Society.


Christmann G.,University of Cambridge | Tosi G.,University of Cambridge | Tosi G.,Autonomous University of Madrid | Berloff N.G.,University of Cambridge | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Optically pumping high-quality semiconductor microcavities allows the spontaneous formation of polariton condensates, which can propagate over distances of many microns. Tightly focused pump spots here are found to produce expanding incoherent bottleneck polaritons, which coherently amplify the ballistic polaritons and lead to the formation of unusual ring condensates in space. This quantum liquid is found to form a remarkable sunflowerlike spatial ripple pattern, which arises from self-interference with Rayleigh-scattered coherent polariton waves in the Cerenkov regime. © 2012 American Physical Society.


Anton C.,Autonomous University of Madrid | Liew T.C.H.,Nanyang Technological University | Cuadra J.,Autonomous University of Madrid | Martin M.D.,Autonomous University of Madrid | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We study the dynamics of polariton condensate wave trains that propagate along a quasi-one-dimensional waveguide. Through the application of tuneable potential barriers the propagation can be reflected and multiple reflections used to confine and store a propagating state. Energy-relaxation processes allow the delayed relaxation into a long-living coherent ground state. Aside from the potential routing of polariton condensate signals, the system forms an AND-type logic gate compatible with incoherent inputs. © 2013 American Physical Society.


Gao T.,FORTH IESL | Gao T.,University of Crete | Anton C.,Autonomous University of Madrid | Liew T.C.H.,Nanyang Technological University | And 9 more authors.
Applied Physics Letters | Year: 2015

Spin-selective spatial filtering of propagating polariton condensates, using a controllable spin-dependent gating barrier, in a one-dimensional semiconductor microcavity ridge waveguide is reported. A nonresonant laser beam provides the source of propagating polaritons, while a second circularly polarized weak beam imprints a spin dependent potential barrier, which gates the polariton flow and generates polariton spin currents. A complete spin-based control over the blocked and transmitted polaritons is obtained by varying the gate polarization. © 2015 AIP Publishing LLC.


Toma K.,AIT Austrian Institute of Technology | Descrovi E.,Polytechnic University of Turin | Toma M.,AIT Austrian Institute of Technology | Ballarini M.,Polytechnic University of Turin | And 8 more authors.
Biosensors and Bioelectronics | Year: 2013

A new approach to signal amplification in fluorescence-based assays for sensitive detection of molecular analytes is reported. It relies on a sensor chip carrying a one-dimensional photonic crystal (1DPC) composed of two piled up segments which are designed to increase simultaneously the excitation rate and the collection efficiency of fluorescence light. The top segment supports Bloch surface waves (BSWs) at the excitation wavelength and the bottom segment serves as a Bragg mirror for the emission wavelength of used fluorophore labels. The enhancement of the excitation rate on the sensor surface is achieved through the resonant coupling to BSWs that is associated with strong increase of the field intensity. The increasing of collection efficiency of fluorescence light emitted from the sensor surface is pursued by using the Bragg mirror that minimizes its leakage into a substrate and provides its beaming toward a detector. In order to exploit the whole evanescent field of BSW, extended three-dimensional hydrogel-based binding matrix that is functionalized with catcher molecules is attached to 1DPC for capturing of target analyte from a sample. Simulations supported by experiments are presented to illustrate the design and determined the performance characteristics of BSW-enhanced fluorescence spectroscopy. A model immunoassay experiment demonstrates that the reported approach enables increasing signal to noise ratio, resulting in about one order of magnitude improved limit of detection (LOD) with respect to regular total internal reflection fluorescence (TIRF) configuration. © 2012 Elsevier B.V.


Muller A.,IMT Bucharest | Neculoiu D.,Polytechnic University of Bucharest | Konstantinidis G.,FORTH IESL | Deligeorgis G.,CNRS Laboratory for Analysis and Architecture of Systems | And 5 more authors.
IEEE Electron Device Letters | Year: 2010

This letter describes the manufacture and characterization of surface acoustic wave (SAW) devices on GaN/Si devoted to applications above the 5-GHz frequency range. The SAW structures consist of two face-to-face interdigitated transducers (IDTs), placed at different distances. Using a TiAu metallization, 80-nm-thick and advanced e-beam lithographical techniques with IDTs with fingers and spacings 200 nm wide have been obtained on the GaN layer. On wafer measurement of the S parameters have demonstrated the operation at approximately 5.6 GHz. The frequency response of the devices is explained in detail. © 2006 IEEE.


Koumakis N.,FORTH IESL | Koumakis N.,University of Crete | Petekidis G.,FORTH IESL | Petekidis G.,University of Crete
Soft Matter | Year: 2011

Steady and oscillatory rheology was utilized to study the mechanical response of colloidal glasses and gels with particular emphasis in their yielding behaviour. We used a suspension of hard sphere colloidal particles with short-range depletion attractions induced by the addition of non-adsorbing linear polymer. While high volume fraction hard sphere glasses exhibit a single yield point due to cage breaking, attraction dominated glasses show a two-step yielding reflecting bond and cage breaking, respectively. Here we investigated the yielding behaviour of frustrated colloid-polymer systems with equal attraction strength and range, varying the particle volume fraction, φ, spanning the region from an attractive glass (φ = 0.6) to a low volume fraction (φ = 0.1) attractive gel. Yielding throughout this range, probed both by oscillatory and steady shear, is found to remain a two step process until very low φ's. The first yield strain related with in-cage or inter-cluster bond braking remains constant for φ > 0.3 while the second yield strain, attributed to braking of cages or clusters into smaller constituents, increases as volume fraction is decreased due to enhancement of structural inhomogeneities in the gel. Steady shear tests indicated distinct shear rate regimes: At steady state, low and intermediate shear rates create denser or smaller flowing clusters, whereas high rates may lead to complete break-up into independent particles. When the range of attraction was increased, both yield strains increased scaling with the range of attraction and accompanied structural changes. Finally, ageing leads to an overall strengthening of both the gel and the attractive glass accompanied by an enhancement of the second stress overshoot in steady shear, while the attractive glass also becomes more brittle. © 2011 The Royal Society of Chemistry.


Koumakis N.,FORTH IESL | Pamvouxoglou A.,FORTH IESL | Pamvouxoglou A.,University of Crete | Poulos A.S.,FORTH IESL | And 2 more authors.
Soft Matter | Year: 2012

The effects of particle softness and the role of the outer shell mechanics on the linear viscoelasticity and yielding behaviour of colloidal glasses are critically assessed using three different model colloidal particles: (i) sterically stabilized PMMA particles with model hard sphere interactions, (ii) core-shell microgels with a deformable PNIPAM outer shell and (iii) ultra-soft star-like micelles with inter-penetrable multi-arms. The volume fraction dependence of the elastic modulus and the yield stress reflects the softness of the effective inter-particle potential. The yield strain exhibits distinct non-monotonic volume fraction dependence for hard spheres below close packing whereas for both soft particles it increases above close packing due to particle softness. Stress overshoots in start-up shear show a common increase with shear rate in all systems. However, the stress overshoots are significantly stronger in star-like micelles due to transient arm entanglements. In relation with that similar stress peaks are detected within the period of the large amplitude oscillatory shear only in star-like micelles. Finally, we discuss the scaling exponents for the G′ and G′′ decrease at large oscillatory strain amplitudes and their relation with steady shear stress. © 2012 The Royal Society of Chemistry.

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