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Stefano Longhi

Publications and source records attributed to Stefano Longhi.

At least 217 records · Page 12Linked to original sources
arXiv2011

Photonic realization of the relativistic Kronig-Penney model and relativistic Tamm surface states

Photonic analogues of the relativistic Kronig-Penney model and of relativistic surface Tamm states are proposed for light propagation in fibre Bragg gratings (FBGs) with phase defects. A periodic sequence of phase slips in the FBG realizes the relativistic Kronig-Penney model, the band structure of which being mapped into the spectral response of the FBG. For the semi-infinite FBG Tamm surface states can appear and can be visualized as narrow resonance peaks in the transmission spectrum of the grating.

quant-ph↗
arXiv2010

Zitterbewegung of optical pulses in nonlinear frequency conversion

Pulse walk-off in the process of sum frequency generation in a nonlinear $χ^{(2)}$ crystal is shown to be responsible for pulse jittering which is reminiscent to the Zitterbewegung (trembling motion) of a relativistic freely moving Dirac particle. An analytical expression for the pulse center of mass trajectory is derived in the no-pump-depletion limit, and numerical examples of Zitterbewegung are presented for sum frequency generation in periodically-poled lithium niobate. The proposed quantum-optical analogy indicates that frequency conversion in nonlinear optics could provide an experimentally accessible simulator of the Dirac equation.

physics.optics↗
arXiv2010

Field-induced decay of quantum vacuum: visualizing pair production in a classical photonic system

The phenomenon of vacuum decay, i.e. electron-positron pair production due to the instability of the quantum electrodynamics vacuum in an external field, is a remarkable prediction of Dirac theory whose experimental observation is still lacking. Here a classic wave optics analogue of vacuum decay, based on light propagation in curved waveguide superlattices, is proposed. Our photonic analogue enables a simple and experimentally-accessible visualization in space of the process of pair production as break up of an initially negative-energy Gaussian wave packet, representing an electron in the Dirac sea, under the influence of an oscillating electric field.

quant-ph↗
arXiv2010

Bloch oscillations in tight-binding lattices with defects

Defects in tight-binding lattices generally destroy the onset of Bloch oscillations (BOs) and the periodic self-imaging of the wave packet due to the lack of an equally-spaced Wannier-Stark ladder spectrum. Here it is shown that localized and extended defects in the lattice can be engineered to be transparent for BOs. Such lattices are synthesized from the defect-free lattice by the technique of intertwining operators generally employed in supersymmetric quantum mechanics. The energy spectrum of the synthesized lattices differs from the Wannier-Stark ladder of the defect-free lattice because of the missing of pairs of resonances in the ladder, thus ensuring the persistence of BOs.

cond-mat.other↗
arXiv2010

Periodic wave packet reconstruction in truncated tight-binding lattices

A class of truncated tight-binding Hermitian and non-Hermitian lattices with commensurate energy levels, showing periodic reconstruction of the wave packet, is presented. Examples include exact Bloch oscillations on a finite lattice, periodic wave packet dynamics in non-Hermtian lattices with a complex linear site-energy gradient, and self-imaging in lattices with commensurate energy levels quadratically-varying with the quantum number.

cond-mat.other↗
arXiv2010

Photonic realization of the relativistic Dirac oscillator

A photonic realization of the Dirac oscillator (DO), i.e. of the relativistic extension of the quantum harmonic oscillator, is proposed for light propagation in fiber Bragg gratings. Transmission spectra clearly show the existence of electron and positron bound states of the DO, corresponding to resonance modes above and below the Bragg frequency, as well as the asymmetry of the energy spectrum for electron and positron branches.

physics.optics↗
arXiv2010

Klein tunneling of light in fiber Bragg gratings

A photonic analogue of Klein tunneling (KT), i.e. of the exotic property of relativistic electrons to pass a large repulsive and sharp potential step, is proposed for pulse propagation in a nonuniform fiber Bragg grating with an embedded chirped region. KT can be simply observed as the opening of a transmission window inside the grating stop band, provided that the impressed chirp is realized over a length of the order of the analogue of the Compton wavelength.

physics.optics↗
arXiv2010

PT-symmetric laser-absorber

In a recent work, Y.D. Chong et al. [Phys. Rev. Lett. {\bf 105}, 053901 (2010)] proposed the idea of a coherent perfect absorber (CPA) as the time-reversed counterpart of a laser, in which a purely incoming radiation pattern is completely absorbed by a lossy medium. The optical medium that realizes CPA is obtained by reversing the gain with absorption, and thus it generally differs from the lasing medium. Here it is shown that a laser with an optical medium that satisfies the parity-time $(\mathcal{PT})$ symmetry condition $ε(-\mathbf{r})=ε^*(\mathbf{r})$ for the dielectric constant behaves simultaneously as a laser oscillator (i.e. it can emit outgoing coherent waves) and as a CPA (i.e. it can fully absorb incoming coherent waves with appropriate amplitudes and phases). Such a device can be thus referred to as a $\mathcal{PT}$-symmetric CPA-laser. The general amplification/absorption features of the $\mathcal{PT}$ CPA-laser below lasing threshold driven by two fields are determined.

quant-ph↗
arXiv2010

Invisibility in non-Hermitian tight-binding lattices

Reflectionless defects in Hermitian tight-binding lattices, synthesized by the intertwining operator technique of supersymmetric quantum mechanics, are generally not invisible and time-of-flight measurements could reveal the existence of the defects. Here it is shown that, in a certain class of non-Hermitian tight-binding lattices with complex hopping amplitudes, defects in the lattice can appear fully invisible to an outside observer. The synthesized non-Hermitian lattices with invisible defects possess a real-valued energy spectrum, however they lack of parity-time (PT) symmetry, which does not play any role in the present work.

quant-ph↗
arXiv2010

Optical realization of relativistic non-Hermitian quantum mechanics

Light propagation in distributed feedback optical structures with gain/loss regions is shown to provide an accessible laboratory tool to visualize in optics the spectral properties of the one-dimensional Dirac equation with non-Hermitian interactions. Spectral singularities and PT symmetry breaking of the Dirac Hamiltonian are shown to correspond to simple observable physical quantities and related to well-known physical phenomena like resonance narrowing and laser oscillation.

quant-ph↗
arXiv2010

Transmission of Light in Crystals with different homogeneity: Using Shannon Index in Photonic Media

Light transmission in inhomogeneous photonic media is strongly influenced by the distribution of the diffractive elements in the medium. Here it is shown theoretically that, in a pillar photonic crystal structure, light transmission and homogeneity of the pillar distribution are correlated by a simple linear law once the grade of homogeneity of the photonic structure is measured by the Shannon index, widely employed in statistics, ecology and information entropy. The statistical analysis shows that the transmission of light in such media depends linearly from their homogeneity: the more is homogeneous the structure, the more is the light transmitted. With the found linear relationship it is possible to predict the transmission of light in random photonic structures. The result can be useful for the study of electron transport in solids, since the similarity with light in photonic media, but also for the engineering of scattering layers for the entrapping of light to be coupled with photovoltaic devices.

physics.optics↗
arXiv2010

Geometric potential and transport in photonic topological crystals

We report on the experimental realization of an optical analogue of a quantum geometric potential for light wave packets constrained on thin dielectric guiding layers fabricated in silica by the femtosecond laser writing technology. We further demonstrate the optical version of a topological crystal, with the observation of Bloch oscillations and Zener tunneling of purely geometric nature.

physics.optics↗
arXiv2006

Stopping and Time Reversal of Light in Dynamic Photonic Structures via Bloch Oscillations

It is theoretically shown that storage and time-reversal of light pulses can be achieved in a coupled-resonator optical waveguide by dynamic tuning of the cavity resonances without maintaining the translational invariance of the system. The control exploits the Bloch oscillation motion of a light pulse in presence of a refractive index ramp, and it is therefore rather different from the mechanism of adiabatic band compression and reversal proposed by Yanik and Fan in recent works [M.F. Yanik and S. Fan, Phys. Rev. Lett. 92, 083901 (2004); Phys. Rev. Lett. 93, 173903 (2004)].

physics.optics↗
arXiv2006

Bound states in the continuum in a single-level Fano-Anderson model

Bound states in the continuum (BIC) are shown to exist in a single-level Fano-Anderson model with a colored interaction between the discrete state and a tight-binding continuum, which may describe mesoscopic electron or photon transport in a semi-infinite one-dimensional lattice. The existence of BIC is explained in the lattice realization as a boundary effect induced by lattice truncation.

quant-ph↗
arXiv2006

Gap solitons in metamaterials

Electromagnetic localization and existence of gap solitons in nonlinear metamaterials, which exhibit a stop band in their linear spectral response, is theoretically investigated. For a self-focusing Kerr nonlinearity, the equation for the electric field envelope with carrier frequency in the stop band - where the magnetic permeability $μ(ω)$ is positive and the dielectric permittivity $ε(ω)$ is negative - is described by a nonlinear Klein-Gordon equation with a dispersive nonlinear term. A family of standing and moving localized waves for both electric and magnetic fields is found, and the role played by the nonlinear dispersive term on solitary wave stability is discussed.

physics.optics↗
arXiv2006

Non-exponential decay via tunneling in tight-binding lattices and the optical Zeno effect

An exactly-solvable model for the decay of a metastable state coupled to a semi-infinite tight-binding lattice, showing large deviations from exponential decay in the strong coupling regime, is presented. An optical realization of the lattice model, based on discrete diffraction in a semi-infinite array of tunneling-coupled optical waveguides, is proposed to test non-exponential decay and for the observation of an optical analog of the quantum Zeno effect.

quant-ph↗
arXiv2006

Optical Zener-Bloch oscillations in binary waveguide arrays

Zener tunneling in a binary array of coupled optical waveguides with transverse index gradient is shown to produce a sequence of regular or irregular beam splitting and beam recombination events superimposed to Bloch oscillations. These periodic or aperiodic Zener-Bloch oscillations provide a clear and visualizable signature in an optical system of coherent multiband dynamics encountered in solid-state or matter-wave systems

physics.optics↗