Showing papers by "ICFO – The Institute of Photonic Sciences published in 2014"

Toward Ultimate Nanoplasmonics Modeling

Abstract: Advances in the field of nanoplasmonics are hindered by the limited capabilities of simulation tools in dealing with realistic systems comprising regions that extend over many light wavelengths. We show that the optical response of unprecedentedly large systems can be accurately calculated by using a combination of surface integral equation (SIE) method of moments (MoM) formulation and an expansion of the electromagnetic fields in a suitable set of spatial wave functions via fast multipole methods. We start with a critical review of volume versus surface integral methods, followed by a short tutorial on the key features that render plasmons useful for sensing (field enhancement and confinement). We then use the SIE-MoM to examine the plasmonic and sensing capabilities of various systems with increasing degrees of complexity, including both individual and interacting gold nanorods and nanostars, as well as large random and periodic arrangements of ∼1000 gold nanorods. We believe that the present results and methodology raise the standard of numerical electromagnetic simulations in the field of nanoplasmonics to a new level, which can be beneficial for the design of advanced nanophotonic devices and optical sensing structures.

Evidence for a Bose-Einstein condensate of excitons

Abstract: We report compelling evidence for a “gray” condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the “gray” condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.

Efimov trimers under strong confinement

Abstract: Confined ultracold gases exhibit a baffling property: restricting the motion of the atoms causes fewer clusters to form, not more. Researchers theoretically show that strong confinement can be used to engineer more stable structures.

Dynamic molecular monitoring of retina inflammation by in vivo Raman spectroscopy coupled with multivariate analysis.

Abstract: Retinal tissue is damaged during inflammation in Multiple Sclerosis. We assessed molecular changes in inflamed murine retinal cultures by Raman spectroscopy. Partial Least Squares-Discriminant analysis (PLS-DA) was able to classify retina cultures as inflamed with high accuracy. Using Multivariate Curve Resolution (MCR) analysis, we deconvolved 6 molecular components suffering dynamic changes along inflammatory process. Those include the increase of immune mediators (Lipoxygenase, iNOS and TNFα), changes in molecules involved in energy production (Cytochrome C, phenylalanine and NADH/NAD+) and decrease of Phosphatidylcholine. Raman spectroscopy combined with multivariate analysis allows monitoring the evolution of retina inflammation. Raman spectroscopy analysis of the Retinal Ganglion Cell layer of the retina. (A) Design of the analysis of the Ganglion cell layer (GCL) and Retinal Nerve Fiber Layer (RNFL) of the retina based in the physical properties of laser light and anatomical structure of retinal layers. (B) Examples of raw Raman spectra from representative retina sample after 10 hours incubation time (black) and LPS treated retina sample after 10 hours incubation time (red) and 12 hours incubation time (blue). (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cold bosons in optical lattices: a tutorial for Exact Diagonalization

Abstract: Exact diagonalization techniques are a powerful method for studying many-body problems Here, we apply this method to systems of few bosons in an optical lattice, and use it to demonstrate the emergence of interesting quantum phenomena like fragmentation and coherence Starting with a standard Bose-Hubbard Hamiltonian, we first revise the characterization of the superfluid to Mott insulator transitions We then consider an inhomogeneous lattice, where one potential minimum is made much deeper than the others The Mott insulator phase due to repulsive on-site interactions then competes with the trapping of all atoms in the deep potential Finally, we turn our attention to attractively interacting systems, and discuss the appearance of strongly correlated phases and the onset of localization for a slightly biased lattice The article is intended to serve as a tutorial for exact diagonalization of Bose-Hubbard models

Terahertz Depolarization Effects in Colloidal TiO2 Films Reveal Particle Morphology

Abstract: Films of colloidal TiO2 nanoparticles are widely used in photovoltaic and photocatalytic applications, and the nature of electrical conductivity in such materials is therefore of both fundamental and practical interest. The conductive properties of colloid TiO2 films depend strongly on their morphology and deviate greatly from the properties of the bulk material. We report ultrafast photoconductivity studies of films consisting of sintered TiO2 particles of very different sizes performed using time-resolved terahertz spectroscopy. Remarkably, identical photoconductivity spectra are observed for films of particles with diameters of tens and hundreds of nanometers, respectively. The independence of photoconductivity on particle size directly demonstrates that the terahertz photoconductive response of colloidal TiO2 films is not affected by carrier backscattering at particle boundaries as has previously been concluded, but rather by depolarization fields resulting from the spatial inhomogeneities in the diel...

SERS Platforms of Plasmonic Hydrophobic Surfaces for Analyte Concentration: Hierarchically Assembled Gold Nanorods on Anodized Aluminum

Abstract: Effi cient and homogeneous surface-enhanced Raman scattering (SERS) substrates are usually prepared using lithographic approaches, physical evaporation, or in situ chemical reduction. However, these approaches are time-consuming, expensive, and very diffi cult to upscale. Alternatively, template-assisted approaches using colloidal suspensions of preformed nanoparticles have become more popular because of their low cost, fast production, and ability to be scaled up easily. One of the limitations of these methods is the dimensions of the structured surfaces. In this context, a new method for designing low-cost, up-scalable surface patterns that match building block dimensionality based on anodization of aluminum, enabling a hierarchical organization of anisotropic nanoparticles, is presented. The proposed new technology starts with anodized aluminum oxide with regular parallel linear periodicities. To produce a highly effi cient plasmonic surface, gold nanorods are assembled into parallel lines where the long axes of the Au rods are also oriented along the substrate lines, thus inducing reproducible tip-to-tip plasmonic coupling with the corresponding generation of highly active hotspots. Additionally, this advanced material presents an inherent hydrophobicity that can be exploited as a method for concentration of analytes on the surface. SERS detection is demonstrated with benzenethiol and 2-naphtoic acid.

Yb-Fiber-Laser-Pumped Continuous-Wave Frequency Conversion Sources from the Mid-Infrared to the Ultraviolet

Abstract: We describe a wide range of versatile, practical and high-power sources of coherent continuous-wave (cw) radiation from the near- to mid-infrared (mid-IR) to the visible and ultraviolet (UV) developed in our laboratory by exploiting second-order nonlinear frequency conversion in novel quasi-phase-matched (QPM) and birefringent materials pumped by the Yb-fiber laser at 1064 nm. By exploiting optical parametric oscillators in combination with internal and external single-pass harmonic generation in QPM nonlinear materials of MgO:PPLN and MgO:sPPLT, and birefriengent crystal of BiB3O6, we have generated stable, high-power cw radiation covering broad spectral regions from ~4 μm in the mid-IR, to 532 nm in the visible, down to 355 nm in the UV. The developed sources can deliver total output powers of up to 17.5 W in the near- to mid-IR, 13 W in the visible, and 68 mW in the UV, at exceptional efficiencies, with excellent passive stability, high spectral and spatial beam quality, in compact, portable and practical design. We also demonstrate successful realization of cw fiber-laser-pumped Ti:sapphire lasers enabled by the development of simplified high-power cw green sources at 532 nm. The described techniques represent a highly versatile, practical, and effective approach for the extension of fiber laser technology across the entire UV to mid-IR spectrum, and ultimately the THz spectral range, also offering the potential for further power scaling with the increase in the fiber laser pump power.

Weak value amplification: a view from quantum estimation theory that highlights what it is and what isn't

Abstract: Weak value amplification (WVA) is a concept that has been extensively used in a myriad of applications with the aim of rendering measurable tiny changes of a variable of interest. In spite of this, there is still an on-going debate about its true nature and whether is really needed for achieving high sensitivity. Here we aim at solving the puzzle, using some basic concepts from quantum estimation theory, highlighting what the use of the WVA concept can offer and what it can not. While WVA cannot be used to go beyond some fundamental sensitivity limits that arise from considering the full nature of the quantum states, WVA can notwithstanding enhance the sensitivity of real detection schemes that are limited by many other things apart from the quantum nature of the states involved, i.e. technical noise. Importantly, it can do that in a straightforward and easily accessible manner.

Small-molecule detection in thiol-yne nanocomposites via surface-enhanced Raman spectroscopy.

Abstract: Surface-enhanced Raman spectroscopy (SERS) is generally performed on planar surfaces, which can be difficult to prepare and may limit the interaction of the sensing surface with targets in large volume samples. We propose that nanocomposite materials can be configured that both include SERS probes and provide a high surface area-to-volume format, i.e., fibers. Thiol–yne nanocomposite films and fibers were fabricated using exposure to long-wave ultraviolet light after the inclusion of gold nanoparticles (AuNPs) functionalized with thiophenol. A SERS response was observed that was proportional to the aggregation of the AuNPs within the polymers and the amount of thiophenol present. Overall, this proof-of-concept fabrication of SERS active polymers indicated that thiol–yne nanocomposites may be useful as durable film or fiber SERS probes. Properties of the nanocomposites were evaluated using various techniques including UV–vis spectroscopy, μ-Raman spectroscopy, dynamic mechanical analysis, differential scan...

Observation of topological structures in photonic quantum walks.

Abstract: Phases of matter with nontrivial topological order are predicted to exhibit a variety of exotic phenomena, such as robust localized bound states in 1D systems, and edge states in 2D systems, which are expected to display spin helicity, immunity to backscattering, and weak antilocalization. In this Letter, we present an experimental observation of topological structures generated via the controlled implementation of two consecutive noncommuting rotations in photonic discrete-time quantum walks. The second rotation introduces valleylike Dirac points in the system, allowing us to create the nontrivial topological pattern. By choosing specific values for the rotations, it is possible to coherently drive the system between topological sectors characterized by different topological invariants. We probe the full topological landscape, demonstrating the emergence of localized bound states hosted at the topological boundaries, and the existence of extremely localized or delocalized non-Gaussian quantum states. Our results pave the way for the study of valley polarization and applications of topological mechanisms in robust optical-device engineering.

Lectures on dynamical models for quantum measurements

Abstract: In textbooks, ideal quantum measurements are described in terms of the tested system only by the collapse postulate and Born's rule. This level of description offers a rather flexible position for the interpretation of quantum mechanics. Here we analyse an ideal measurement as a process of interaction between the tested system S and an apparatus A, so as to derive the properties postulated in textbooks. We thus consider within standard quantum mechanics the measurement of a quantum spin component ŝz by an apparatus A, being a magnet coupled to a bath. We first consider the evolution of the density operator of S + A describing a large set of runs of the measurement process. The approach describes the disappearance of the off-diagonal terms ("truncation") of the density matrix as a physical effect due to A, while the registration of the outcome has classical features due to the large size of the pointer variable, the magnetization. A quantum ambiguity implies that the density matrix at the final time can be...

On the Robustness of Quantum Simulators

Abstract: Philipp Hauke,1, ∗ Fernando M. Cucchietti,1, 2 Luca Tagliacozzo,1 Maciej Lewenstein,1, 3 and Ivan Deutsch4, 5 ICFO – Institut de Ciencies Fotoniques, Parc Mediterrani de la Tecnologia, 08860 Castelldefels, Spain Barcelona Supercomputing Center (BSC-CNS), Edificio NEXUS I, Campus Nord UPC, Gran Capitan 2-4, 08034 Barcelona, Spain ICREA – Institucio Catalana de Recerca i Estudis Avancats, Lluis Companys 23, E-08010 Barcelona, Spain Center for Quantum Information and Control (CQuIC), University of New Mexico, Albuquerque NM 87131 Department of Physics and Astronomy, University of New Mexico, Albuquerque NM 87131 (Dated: September 30, 2011)

Optical signatures of a fully dark exciton condensate

Abstract: We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; ii) the polarization of the dark condensate can be obtained from the blueshift dependence on the probe photon polarization as well as from the Faraday effect. All these effects result from carrier exchanges between dark and bright states.

Storage of telecom photons in a doped crystal interfaced via frequency up-conversion

Abstract: We report on an experiment demonstrating storage of single-photonlevel light pulses at telecommunication wavelength by frequency up-converting them to the visible range to be resonant with a solid-state optical memory based on a Pr:Y2SiO5 crystal. We convert the telecom photons at 1570 nm to 606 nm using a periodically-poled potassium titanyl phosphate nonlinear waveguide. The maximum device efficiency is inferred to be η dev = 22±1% with a signal to noise ratio exceeding 1 for single-photon-level pulses with durations of up to 560 ns. The converted light is then stored in the crystal using the atomic frequency comb scheme with storage and retrieval efficiencies exceeding ηAFC = 20% for predetermined storage times of up to 5μs. The retrieved light is time delayed from the noisy conversion process allowing us to measure a signal to noise ratio exceeding 100 with telecom single-photon-level inputs. These results represent the first demonstration of single-photon-level optical storage interfaced with frequency up-conversion. PACS numbers: 03.67.Hk,42.50.Gy,42.50.Md,42.65.Wi

Nonlinear THz conductivity in graphene

Abstract: We report the nonlinear THz conductivity of graphene. The heating of charge carriers by strong THz pulses results in a reduction of the high-frequency conductivity of graphene, in spite of reduced scattering for high-energy carriers.

Practical height correction for diffuse optical spectroscopy to account for curved tissue surfaces

Abstract: Non-contact broadband diffuse optical spectroscopy that is meant for longitudinal study of superficial tumor models suffers from systematic errors due to tissue surface. We propose a practical heig ...

Optical spin squeezing: bright beams as high-flux entangled photon sources

Abstract: Polarization squeezing implies multipartite pairwise entanglement, in analogy with spin squeezing. Our experiment demonstrates polarization entanglement in a squeezed state and is an ultra-bright narrowband source of entangled photon pairs, suitable for interaction with atoms.

Cold bosons in optical lattices: correlations, localization, and fragmentation

Abstract: Bosons in optical lattices may exhibit interesting quantum phenomena like fragmentation and coherence. Applying direct diagonalization to a small system, we explore different aspects of such setup: We show that, for a microscopic sample, the superfluid to Mott transition can be characterized by ground state overlaps with analytic trial states for both regimes. We also study a variant of the Mott transition in an inhomogeneous lattice, where one potential minimum is made much deeper than the others. Finally, we turn our attention to attractively interacting systems, and discuss the appearance of strongly correlated phases and the onset of localization for a slightly biased lattice.

Observation of Topological Structures in Photonic Quantum Walks

Abstract: We present an experimental observation of topological structure in 1D photonic discrete-time quantum walks. We probe the full topological landscape demonstrating emergence of localized bound states, and existence of extremely (de)localized non-Gaussian quantum states.

Few-cycle, broadband, mid-infrared parametric oscillator pumped by a 20-fs Ti:sapphire laser

Abstract: We report a broadband mid-IR femtosecond OPO tunable across 2179-3732 nm, pumped by 20-fs pulses at 790 nm, generating idler pulses of 4.3 optical cycles (33 fs) at 2282 nm, with high stability and beam-quality.

Inherent resistivity of graphene to strong THz fields

Abstract: The THz conductivity of graphene at high driving THz fields was characterized using nonlinear ultrafast THz spectroscopy. We found that efficient carrier heating by strong THz signals leads to increased effective carrier scattering time. However, counter-intuitively, the heating also results in reduced high-frequency conductivity.

Resonances in graphene plasmons

Abstract: Graphene is used as a novel, versatile plasmonic material The most common way to implement resonant light-plasmon coupling is to etch graphene into periodic nanostructures, which is invasive Here, we study a non-invasive way to engineer graphene plasmon resonance, based on periodic doping profiles The plasmon resonances are calculated by performing numerical simulations In addition, we report on simulations of near-field resonant coupling between a dipole and graphene plasmons Finally, preliminary results on the experimental realization of graphene plasmon resonances are reported This study demonstrates the potential to exploit graphene plasmons for extreme energy confinement which could lead to strong nonlinear effects

Imaging deep and clear in thick inhomogeneous samples

Abstract: Acquisition of images deep inside large samples is one of the most demanded improvements that current biology applications ask for. Absorption, scattering and optical aberrations are the main difficulties encountered in these types of samples. Adaptive optics has been imported form astronomy to deal with the optical aberrations induced by the sample. Nonlinear microscopy and SPIM have been proposed as interesting options to image deep into a sample. Particularly, light-sheet microscopy, due to its low photo bleaching properties, opens new opportunities to obtain information for example in long time lapses for large 3D imaging. In this work, we perform an overview of the application of adaptive optics to the fluorescence microscopy in linear and non-linear modalities. Then we will focus in the light-sheet microscopy architecture of two orthogonal optical paths which implies new requirements in terms of optical correction. We will see the different issues that appear in light-sheet microscopy particularly when imaging large and non-flat samples. Finally, we will study the problem of the isoplanetic patches.

Observation of Topological Structure in Photonic Quantum Walks

Abstract: We present an experimental observation of topological structure in 1D photonic discrete-time quantum walks. We probe the full topological landscape demonstrating emergence of localized bound states, and existence of extremely (de)localized non-Gaussian quantum states.

Entanglement and Coherence

Abstract: We describe how to exploit the relationship between entanglement of two-photon states and the coherence of each photon. We consider the generation of different quantum states and the observation of Anderson localization with partially coherent light.

Lectures on dynamical models for quantum measurements

Abstract: In textbooks, ideal quantum measurements are described in terms of the tested system only by the collapse postulate and Born's rule. This level of description offers a rather flexible position for the interpretation of quantum mechanics. Here we analyse an ideal measurement as a process of interaction between the tested system S and an apparatus A, so as to derive the properties postulated in textbooks. We thus consider within standard quantum mechanics the measurement of a quantum spin component $\hat s_z$ by an apparatus A, being a magnet coupled to a bath. We first consider the evolution of the density operator of S+A describing a large set of runs of the measurement process. The approach describes the disappearance of the off-diagonal terms ("truncation") of the density matrix as a physical effect due to A, while the registration of the outcome has classical features due to the large size of the pointer variable, the magnetisation. A quantum ambiguity implies that the density matrix at the final time can be decomposed on many bases, not only the one of the measurement. This quantum oddity prevents to connect individual outcomes to measurements, a difficulty known as the "measurement problem". It is shown that it is circumvented by the apparatus as well, since the evolution in a small time interval erases all decompositions, except the one on the measurement basis. Once one can derive the outcome of individual events from quantum theory, the so-called "collapse of the wave function" or the "reduction of the state" appears as the result of a selection of runs among the original large set. Hence nothing more than standard quantum mechanics is needed to explain features of measurements. The employed statistical formulation is advocated for the teaching of quantum theory.

Terahertz carrier dynamics in graphene and graphene nanostructures

Abstract: Photoexcited charge carriers in 2D graphene and in 1D graphene nanostructures were studied with optical pump-THz probe spectroscopy. We find efficient hot-carrier multiplication in 2D graphene, and predominantly free carrier early-time response in 1D nanostructures.

Efimov trimers under strong confinement

Abstract: The dimensionality of a system can fundamentally impact the behaviour of interacting quantum particles. Classic examples range from the fractional quantum Hall effect to high temperature superconductivity. As a general rule, one expects confinement to favour the binding of particles. However, attractively interacting bosons apparently defy this expectation: while three identical bosons in three dimensions can support an infinite tower of Efimov trimers, only two universal trimers exist in the two dimensional case. We reveal how these two limits are connected by investigating the problem of three identical bosons confined by a harmonic potential along one direction. We show that the confinement breaks the discrete Efimov scaling symmetry and destroys the weakest bound trimers. However, the deepest bound Efimov trimer persists under strong confinement and hybridizes with the quasi-two-dimensional trimers, yielding a superposition of trimer configurations that effectively involves tunnelling through a short-range repulsive barrier. Our results suggest a way to use strong confinement to engineer more stable Efimov-like trimers, which have so far proved elusive.