arXiv subjects

Vincenzo Savona

Publications and source records attributed to Vincenzo Savona.

At least 73 records · Page 4Linked to original sources

arXiv2014

Unconventional photon blockade in doubly resonant microcavities with second-order nonlinearity

It is shown that non-centrosymmetric materials with bulk second-order nonlinear susceptibility can be used to generate strongly antibunched radiation at an arbitrary wavelength, solely determined by the resonant behavior of suitably engineered coupled microcavities. The proposed scheme exploits the unconventional photon blockade of a coherent driving field at the input of a coupled cavity system, where one of the two cavities is engineered to resonate at both fundamental and second harmonic frequencies, respectively. Remarkably, the unconventional blockade mechanism occurs with reasonably low quality factors at both harmonics, and does not require a sharp doubly-resonant condition for the second cavity, thus proving its feasibility with current semiconductor technology.

cond-mat.mes-hall↗

arXiv2013

Long-distance radiative excitation transfer between quantum dots in disordered photonic crystal waveguides

We theoretically investigate the magnitude and range of the photon-mediated interaction between two quantum dots embedded in a photonic crystal waveguide, including fabrication disorder both in the crystal and in the dot positioning. We find that disorder-induced light localization has a drastic effect on the excitation transfer rate - as compared to an ideal structure - and that this rate varies widely among different disorder configurations. Nevertheless, we also find that significant rates of 50 micro-eV at a range of 10 micro-meters can be achieved in realistic systems.

cond-mat.mes-hall↗

arXiv2013

Superfluid-insulator transition in weakly interacting disordered Bose gases: a kernel polynomial approach

An iterative scheme based on the kernel polynomial method is devised for the efficient computation of the one-body density matrix of weakly interacting Bose gases within Bogoliubov theory. This scheme is used to analyze the coherence properties of disordered bosons in one and two dimensions. In the one-dimensional geometry, we examine the quantum phase transition between superfluid and Bose glass at weak interactions, and we recover the scaling of the phase boundary that was characterized using a direct spectral approach by Fontanesi et al. [Phys. Rev. A 81, 053603 (2010)]. The kernel polynomial scheme is also used to study the disorder-induced condensate depletion in the two-dimensional geometry. Our approach paves the way for an analysis of coherence properties of Bose gases across the superfluid-insulator transition in two and three dimensions.

cond-mat.quant-gas↗

arXiv2013

Unconventional photon blockade in coupled optomechanical systems

We show that in laser-driven coupled optomechanical systems, photon antibunching can occur under weak optomechanical coupling, contrarily to common expectation. This unconventional photon blockade originates from destructive quantum interference between different excitation pathways bringing from the ground to two-photon states. Using a quantum open-system approach, we study the antibunching as a function of driving field amplitude, temperature, and pure dephasing rate, and derive optimal values of the system parameters for its occurrence. These values are remarkably close to those characterizing optomechanical systems in some current experimental studies.

quant-ph↗

arXiv2013

Radiative coupling of quantum dots in photonic crystal structures

We derive a general formalism to model the polariton states resulting from the radiation-matter interaction between an arbitrary number of excitonic transitions in semiconductor quantum dots and photon modes in a photonic crystal structure in which the quantum dots are embedded. The Maxwell equations, including the linear nonlocal susceptibility of the exciton transitions in the quantum dots, are cast into an eigenvalue problem, which can be applied to any structure whose photon modes can be computed with reliable accuracy, and in addition naturally allows for disorder effects to be taken into account. We compute realistic photon modes using Bloch-mode expansion. As example systems, we study typical InGaAs quantum dots in a GaAs photonic crystal structures -- an $Ln$ cavity or a $\mathit{W1}$ waveguide. For a single dot, we reproduce known analytical results, while for the two dot case, we study the radiative excitation transfer mechanism and characterize its strength, the dependence on the detuning between quantum dot and photon modes, and the dependence on inter-dot distance. We find in particular that the inter-dot radiative coupling strength can reach $100 μ{eV}$ in a short cavity, and its decay with distance in longer cavities and waveguides is determined by the group velocity of the exchanged photons and their radiative lifetime. We also show that, for an $Ln$ cavity of increasing length, the radiative excitation transfer mechanism is subject to a crossover from a regime where a single photon mode is dominating, to a multi-mode regime -- occurring around $\mathit{n}$ = 150 for the system under study.

cond-mat.mes-hall↗

arXiv2013

Multimode entanglement in coupled cavity arrays

We study a driven-dissipative array of coupled nonlinear optical resonators by numerically solving the Von Neumann equation for the density matrix. We demonstrate that quantum correlated states of many photons can be generated also in the limit where the nonlinearity is much smaller than the losses, contrarily to common expectations. Quantum correlations in this case arise from interference between different pathways that the system can follow in the Hilbert space to reach its steady state under the effect of coherent driving fields. We characterize in particular two systems: a linear chain of three coupled cavities and an array of eight coupled cavities. We demonstrate the existence of a parameter range where the system emits photons with continuous-variable bipartite and quadripartite entanglement, in the case of the first and the second system respectively. This entanglement is shown to survive realistic rates of pure dephasing and opens a new perspective for the realization of quantum simulators or entangled photon sources without the challenging requirement of strong optical nonlinearities.

quant-ph↗

arXiv2012

Effect of hole-shape irregularities on photonic crystal waveguides

The effect of irregular hole shape on the spectrum and radiation losses of a photonic crystal waveguide is studied using Bloch-mode expansion. Deviations from a perfectly circular hole are characterized by a radius fluctuation amplitude and correlation angle. It is found that the parameter which determines the magnitude of the effect of disorder is the standard deviation of the hole areas. Hence, for a fixed amplitude of the radius fluctuation around the hole, those effects are strongly dependent on the correlation angle of the irregular shape. This result suggests routes to potentially improve the quality of photonic crystal structures.

physics.optics↗

arXiv2011

Electromagnetic modes of a disordered photonic crystal

We present a systematic numerical approach to compute the eigenmodes and the related eigenfrequencies of a disordered photonic crystal, characterized by small fluctuations of the otherwise periodic dielectric profile. The field eigenmodes are expanded on the basis of Bloch modes of the corresponding periodic structure, and the resulting eigenvalue problem is diagonalized on a truncated basis including a finite number of Bloch bands. The Bloch-mode expansion is very effective for modeling modes of very extended disordered structures in a given frequency range, as only spectrally close bands must be included in the basis set. The convergence can be easily verified by repeating the diagonalization for an increased band set. As illustrations, we apply the method to the W1 line-defect waveguide and to the L3 coupled-cavity waveguide, both based on a photonic crystal slab with a triangular lattice of circular holes. We compute and characterize the eigenfrequencies, spatial field profiles and radiation loss rates of the localized modes induced by disorder. For the W1 waveguide, we demonstrate that radiation losses, at the bottom of the spatially-even guided band, are determined by a small hybridization with Bloch modes of the spatially-odd band, induced by disorder in spite of their frequency separation. The Bloch-mode expansion method has a very broad range of applications: it can be also used to accurately compute the modes of structures with systematic modulations of the periodic dielectric constant, as in several designs of high-Q cavities.

physics.optics↗

arXiv2010

Fragmentation and the Bose-glass phase transition of the disordered 1D Bose gas

We investigate the superfluid-insulator quantum phase transition in a disordered 1D Bose gas in the mean field limit, by studying the probability distribution of the density. The superfluid phase is characterized by a vanishing probability to have zero density, whereas a nonzero probability marks the insulator phase. This relation is derived analytically, and confirmed by a numerical study. This fragmentation criterion is particularly suited for detecting the phase transition in experiments. When a harmonic trap is included, the transition to the insulating phase can be extracted from the statistics of the local density distribution.

cond-mat.quant-gas↗

arXiv2010

Energy relaxation in the Gross-Pitaevskii equation

We introduce a dissipation term in the Gross-Pitaevskii equation that describes the stimulated relaxation of condensed bosons due to scattering with a different type of particles. This situation applies to Bose-Einstein condensates of quasi-particles in the solid state, such as magnons and excitons. Our model is compatible with the phenomenology of superfluidity: supercurrents are stable up to a critical speed and decay when they are faster.

cond-mat.quant-gas↗

arXiv2010

The linear spectrum of a quantum dot coupled to a nano-cavity

We develop a theoretical formalism to model the linear spectrum of a quantum dot embedded in a high quality cavity, in presence of an arbitrary mechanism modifying the homogeneous spectrum of the quantum dot. Within the simple assumption of lorentzian broadening, we show how the known predictions of cavity quantum electrodynamics are recovered. We then apply our model to the case where the quantum dot interacts with an acoustic-phonon reservoir, producing phonon sidebands in the response of the bare dot. In this case, we show that the sidebands can sustain the spectral response of the cavity-like peak even at moderate dot-cavity detuning, thus supporting recent experimental findings.

cond-mat.other↗

arXiv2009

Resolution of the mystery of counter-intuitive photon correlations in far off-resonance emission from a quantum dot-cavity system

Cavity quantum-electrodynamics experiments using an atom coupled to a single radiation-field mode have played a central role in testing foundations of quantum mechanics, thus motivating solid-state implementations using single quantum dots coupled to monolithic nano-cavities. In stark contrast to their atom based counterparts, the latter experiments revealed strong cavity emission, even when the quantum dot is far off resonance. Here we present experimental and theoretical results demonstrating that this effect arises from the mesoscopic nature of quantum dot confinement, ensuring the presence of a quasi-continuum of transitions between excited quantum dot states that are enhanced by the cavity mode. Our model fully explains photon correlation measurements demonstrating that photons emitted at the cavity frequency are essentially uncorrelated with each other even though they are generated by a single quantum dot.

cond-mat.mes-hall↗

arXiv2009

Superfluid to Bose-glass transition in a 1D weakly interacting Bose gas

We study the one-dimensional Bose gas in spatially correlated disorder at zero temperature, using an extended density-phase Bogoliubov method. We analyze in particular the decay of the one-body density matrix and the behaviour of the Bogoliubov excitations across the phase boundary. We observe that the transition to the Bose glass phase is marked by a power-law divergence of the density of states at low energy. A measure of the localization length displays a power-law energy dependence in both regions, with the exponent equal to -1 at the boundary. We draw the phase diagram of the superfluid-insulator transition in the limit of small interaction strength.

cond-mat.quant-gas↗

arXiv2009

Creation and detection of vortices in polariton condensates

We investigate theoretically the creation, persistence and detection of quantized vortices in nonequilibrium polariton condensates within a stochastic classical field model. The life time of the quantized vortices is shown to increase with the spatial coherence and to depend on the geometry of the condensate. The relation with superfluidity in conventional superfluids is discussed. Three different ways to measure the vorticity of the polariton condensate are proposed.

cond-mat.quant-gas↗

arXiv2008

Stochastic classical field model for polariton condensates

We use the truncated Wigner approximation to derive stochastic classical field equations for the description of polariton condensates. Our equations are shown to reduce to the Boltzmann equation in the limit of low polariton density. Monte Carlo simulations are performed to analyze the momentum distribution and the first and second order coherence when the particle density is varied across the condensation threshold.

cond-mat.other↗

arXiv2008

Polariton parametric photoluminescence in spatially inhomogeneous systems

A general theory of polariton parametric photoluminescence in spatially inhomogeneous systems is developed. The quantum Langevin equations are solved in a generalized Bogoliubov de Gennes approximation. We apply the formalism to the specific case of a disordered microcavity. In this case, we numerically solve the equations for the coherent emission and the photoluminescence. We describe the effect of the exciton and photon disorder on the photoluminescence pattern exhibited in momentum space, finding a good agreement with the experimental observations.

cond-mat.other↗

arXiv2008

Fifteen years of microcavity polaritons

We present an overview of the advances in the physics of polaritons in semiconductor microcavities, starting from their first discovery in 1992. After summarizing the research during the early years, when the basic optical properties were investigated, we describe the important results related to the phenomenon of parametric polariton scattering, highlighting its link to non-classical polariton physics. We conclude with a perspective view of the future research in the domain, focusing on the exciting developments in the direction of polariton quantum collective phenomena and quantum nanodevices

cond-mat.mes-hall↗

arXiv2008

Influence of phonons on the emission spectrum of a quantum dot embedded in a nanocavity

We model the emission spectrum of a quantum dot embedded in a (e.g. photonic crystal) nanocavity, using a semi-classical approach to describe the matter-field interaction. We start from the simple model of a quantum dot as a two-level system, and recover the result expected from cavity quantum electrodynamics. Then, we study the influence of electron-acoustic-phonons interaction. We show that the surrounding semiconductor plays an essential role in the emission spectrum in strong coupling.

cond-mat.other↗