Nonlocal feedback in nonlinear systems

A shifted or misaligned feedback loop gives rise to a two-point nonlocality that is the spatial analog of a temporal delay. Important consequences of this nonlocal coupling have been found both in diffusive and in diffractive systems, and include convective instabilities, independent tuning of phase and group velocities, as well as amplification, chirping and even splitting of localized perturbations. Analytical predictions about these nonlocal systems as well as their spatio-temporal dynamics are discussed in one and two transverse dimensions and in presence of noise.Comment: 13 pages, to be published in EPJ

Signal amplification and control in optical cavities with off-axis feedback

We consider a large class of optical cavities and gain media with an off-axis external feedback which introduces a two-point nonlocality. This nonlocality moves the lasing threshold and opens large windows of control parameters where weak light spots can be strongly amplified while the background radiation remains very low. Furthermore, transverse phase and group velocities of a signal can be independently tuned and this enables to steer it non mechanically, to control its spatial chirping and to split it into two counter-propagating ones.Comment: 4 pages, 4 picture

Optical control of scattering, absorption and lineshape in nanoparticles

We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response

Evaluation of E. M. fields and energy transport in metallic nanoparticles with near field excitation

We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error

Dataset on coherent control of fields and induced currents in nonlinear multiphoton processes in a nanosphere

We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts

Enhancing ultraviolet spontaneous emission with a designed quantum vacuum

We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 − 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 × 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 × 103 for emission within a cone with a 60º semi-angle. This effect is due to both an enhancement of the field at the emitter’s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers

Coherent control of radiation patterns of nonlinear multiphoton processes in nanoparticles

We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts

Scattering of light with angular momentum from an array of particles

Understanding the scattering properties of various media is of critical importance in many applications, from secure high-bandwidth communications to extracting information about biological and mineral particles dissolved in sea water. In this paper we demonstrate how beams carrying orbital angular momentum can be used to detect the presence of symmetric or chiral subsets of particles in disordered media. Using a generalized Mie theory, we calculate analytical expressions for quasimonochromatic structured light scattered by dilute distributions of micro- and nanoparticles. These allow us to determine the angular momentum of the scattered field as a function of the angular momentum of the incident beam and of the spatial distributions of scattering particles. Our numerical results show that we can distinguish structured from random distributions of particles, even when the number density of ordered particles is a few percent of the total istribution. We also find that the signal-to-noise ratio, in the forward direction, is equivalent for all orders of the Laguerre-Gaussian modes in relatively dense (but still dilute) distributions of particles smaller than the beam waist and the Rayleigh range of the beam

Plasmon modes in single gold nanodiscs

Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc

The effect of oxidation on the far-field scattering of aluminium patch antennae from visible to UV

In this paper we evaluate the effect of oxidation on the total power scattered in the far field by a 60nm radius Al sphere in the presence of a substrate comprised of either Al or silica (SiO 2). Using an effective medium approach to model the Al particle with an outer layer of alumina (Al 2O 3), we find that the UV peak of total energy scattered in the far field shifts towards longer wavelengths for volume fractions of Al 2O 3 up to 20%. When particles with these volume fractions are held above an Al substrate, enhancement of two orders of magnitudes of the far-field power radiated by a dipole in the gap can be observed. For larger volume fractions of Al 2O 3, the total intensity of light scattered is significantly reduced