Showing papers in "Journal of Optics in 2008"

Plasmonic and Metamaterial Cloaking: Physical Mechanisms and Potentials

Abstract: Artificial materials, metamaterials and plasmonic media have recently received tremendous attention from the scientific communities, media and the general public, following novel ideas and suggestions for their potential use in a variety of applications such as cloaking. Here we briefly review and highlight some of the available solutions for invisibility and cloaking that employ metamaterials and plasmonic materials at various frequencies. We briefly overview some of the different cloaking mechanisms recently proposed in the literature, such as plasmonic cloaking based on scattering cancelation, coordinate-transformation cloaking and anomalous localized resonances for cloaking, in particular providing some details for scattering-cancelation-based plasmonic cloaking. We mention the main analogies and differences among these various approaches and we discuss some possible ideas for realizations and applications of these results, with particular attention to the physical phenomena involved.

Angular momentum of a strongly focused Gaussian beam

Abstract: A circularly polarized paraxial Gaussian laser beam carries ± ¯ h angular momentum per photon as spin, with zero orbital angular momentum. Focusing the beam with a rotationally symmetric lens cannot change this angular momentum flux, yet the focused beam must have spin |Sz| < ¯ h per photon. The remainder of the original spin is converted to orbital angular momentum, manifesting itself as a longitudinal optical vortex at the focus. We investigate the nature of this orbital angular momentum.

Raman spectra of polymethyl methacrylate optical fibres excited by a 532 nm diode pumped solid state laser

Abstract: Polymethyl methacrylate (PMMA) optical fibres are fabricated by a preform drawing process. The Raman spectra of PMMA fibres are recorded using a diode pumped solid state laser emitting at 532 nm and a CCD-spectrograph in the 400‐3800 cm −1 range. The variation of the Raman intensity with the length of the optical fibre is studied. Investigations are carried out on the variation of FWHM of the Raman peak at 2957 cm −1 with the length of the optical fibre and pump power. The differential scattering cross section and gain coefficient of the Raman peak at 2957 cm −1 in PMMA are calculated in relation to that of toluene.

Nondiffracting vortex beams with continuous orbital angular momentum order dependence

Abstract: We introduce a new class of nondiffracting fractional vortex beams that connect Bessel beams of successive order in a smooth transition. The new fractional-order beams preserve the same nondiffracting feature of Bessel beams of integer order, and their orbital angular momentum per photon can take any value in a continuous range. The propagation of the more physically realizable fractional-order Bessel–Gauss beams, i.e. fractional Bessel beams apodized by a Gaussian transmittance, through general ABCD optical systems is studied in detail and is complemented by the experimental generation of several instances of fractional beams which in turn confirms our theoretical predictions.

Electromagnetic coupling and plasmon localization in deterministic aperiodic arrays

Abstract: In this paper we explore the potential of one-dimensional and two-dimensional deterministic aperiodic plasmonic arrays for the design of electromagnetic coupling and plasmon-enhanced, sub-wavelength optical fields on chip-scale devices. In particular, we investigate the spectral, far-field and near-field optical properties of metal nanoparticle arrays generated according to simple deterministic sequences characterized by fractal Fourier spectra. Additionally, we will consider the case of flat Fourier-transform sequences, which reproduce the behavior of purely random systems to an arbitrary degree of accuracy. Based on the coupled dipole approach (CDA) and finite difference time domain (FDTD) simulations, we study the radiative (long-range) and quasi-static (short-range) electromagnetic coupling in deterministic aperiodic plasmon arrays of metal nanoparticles. In addition, we investigate the local field enhancement and the enhancement scaling in periodic and aperiodic arrays with increasing degree of complexity. We believe that the accurate control of electromagnetic coupling and sub-wavelength field enhancement in deterministic aperiodic environments will enable novel nanodevice applications in areas such as field-enhanced nanosensors, engineered SERS substrates and optical nano-antenna arrays.

Propagation of Laguerre?Gaussian beams in nonlocal nonlinear media

Abstract: In this paper, the exact analytical Laguerre–Gaussian (LG) solutions are derived in strongly nonlocal nonlinear media in a cylindrical coordinate system. The comparisons of analytical solutions with numerical simulations of the nonlocal nonlinear Schrodinger equation (NNLSE) show that the analytical LG solutions are in good agreement with the numerical simulations in the case of strong nonlocality. The Gaussian breather and the Gaussian soliton, ring vortex breathers and solitons, necklace-type breathers and solitons can be obtained by treating them as special cases of LG breathers and LG solitons. The area of the dark region across the LG vortex ring breathers and vortex ring solitons can easily be controlled by proper choice of the beam parameters.

Aspects of hologram calculation for video frames

Abstract: This paper presents a modified version of the Fienup algorithm that can compute an image projecting hologram significantly faster. The proposed method, referred to as Fienup with don’t-care (Fidoc), is sufficiently fast to enable real-time hologram calculation for video projection purposes. It achieves high speed and excellent image quality by dispersing noise in ‘don’t-care’ areas around the image. Through simulations it was shown that for the same amount of computation, reconstruction quality is significantly better when using the Fidoc method instead of the simple GS (Gerchberg‐Saxton) and Fienup algorithms.

The thermodynamics of optical étendue

Abstract: The concept of etendue is applied to the propagation of luminescent radiation, and to the transformation of such radiation in absorbing and luminescent media. Central to this analysis is the notion of etendue as a measure of the number of rays in the beam which permits the definition of entropy and transition to the formalism of statistical mechanics. When considered from the statistical viewpoint, etendue conservation along the path of a beam in clear and transparent media then implies the conservation of entropy. The changes in thermodynamic parameters of a beam upon absorption and re-emission can then be determined in terms of the corresponding changes resulting from the addition or removal of photons from the incident and emitted beam. The thermodynamic theory which follows gives the rate of entropy generation in this process. At moderate light intensities, the results resemble the thermodynamics of a two-dimensional gas. The formalism allows an extension to absorption/emission processes where a high-temperature incident light beam is transformed reversibly into low-temperature luminescent radiation, corresponding to a potential increase in the open-circuit voltage of a solar cell.

Exact and geometrical optics energy trajectories in twisted beams

Abstract: Energy trajectories, that is, integral curves of the Poynting (current) vector, are calculated for scalar Bessel and Laguerre–Gauss beams carrying orbital angular momentum. The trajectories for the exact waves are helices, winding on cylinders for Bessel beams and hyperboloidal surfaces for Laguerre–Gauss beams. In the geometrical optics approximations, the trajectories for both types of beam are overlapping families of straight skew rays lying on hyperboloidal surfaces; the envelopes of the hyperboloids are the caustics: a cylinder for Bessel beams and two hyperboloids for Laguerre–Gauss beams.

Simultaneous topography and tomography of latent fingerprints using full-field swept-source optical coherence tomography

Abstract: We demonstrate simultaneous topography and tomography of latent fingerprints using full-field swept-source optical coherence tomography (OCT). The swept-source OCT system comprises a superluminescent diode (SLD) as broad-band light source, an acousto-optic tunable filter (AOTF) as frequency tuning device, and a compact, nearly common-path interferometer. Both the amplitude and the phase map of the interference fringe signal are reconstructed. Optical sectioning of the latent fingerprint sample is obtained by selective Fourier filtering and the topography is retrieved from the phase map. Interferometry, selective filtering, low coherence and hence better resolution are some of the advantages of the proposed system over the conventional fingerprint detection techniques. The present technique is non-invasive in nature and does not require any physical or chemical processing. Therefore, the quality of the sample does not alter and hence the same fingerprint can be used for other types of forensic test. Exploitation of low-coherence interferometry for fingerprint detection itself provides an edge over other existing techniques as fingerprints can even be lifted from low-reflecting surfaces. The proposed system is very economical and compact.

Various dark hollow beams propagating in uniaxial crystals orthogonal to the optical axis

Abstract: The propagation of dark hollow beams with circular, elliptical and rectangular symmetries in uniaxial crystals orthogonal to the optical axis is investigated. Analytical formulae for various dark hollow beams propagating in uniaxial crystals orthogonal to the optical axis are derived, and the propagation properties of various dark hollow beams are studied in detail. We find that the dark hollow beam of circular or non-circular symmetry will spread in the xy plane with the increase in propagation distance, and various hollow beams can keep their initial beam profile almost invariant for a short propagation distance in uniaxial crystals.

Intensity fluctuations and the degree of cross-polarization in stochastic electromagnetic beams

Abstract: An expression is derived for the correlation between intensity fluctuations at pairs of points in a stochastic electromagnetic beam, obeying Gaussian statistics. The new formula, which is of particular interest for the theory of the Hanbury Brown?Twiss effect, applies to beams of any state of coherence and state of polarization. It contains a new 'two-point' degree of polarization whose presence indicates that, in general, the knowledge of the degree of coherence and of the usual degree of polarization are not adequate for determining the correlation between intensity fluctuations.

Nonlinear optical performance of chemically tailored phthalocyanine–polymer films as solid-state optical limiting devices

Abstract: Linear and nonlinear optical (NLO) investigations into the material response of a selection of solid-state phthalocyanine?polymer films are reported. Using the simple method of sequentially deposited spin cast phthalocyanine doped polymer films, one can characterize the resulting NLO response by means of the Z-scan technique with 6?ns pulses at 532?nm. Effective optical coefficients are calculated with a nonlinear absorption based model and their intensity dependence is investigated. When compared with the phthalocyanine solutions, the phthalocyanine?poly(methyl methacrylate) (PMMA) films exhibit excellent nonlinear extinction effects, outperforming corresponding phthalocyanine solutions by 2?3 orders of magnitude. Molecular engineering of metallophthalocyanine compounds, through both axial and peripheral substitutes, has led to an optimization of the resulting NLO response in the nanosecond regime. Mechanistic implications of the optical dissipation for various substituted phthalocyanines are also discussed and examined.

Dynamics and collisions of moving solitons in Bragg gratings with dispersive reflectivity

Abstract: Results of systematic investigation of the stability of moving solitons and soliton–soliton collisions are reported within the framework of the recently proposed model which is a generalization of the standard model of fiber Bragg gratings (BGs). The new model takes into account dispersion of the Bragg reflectivity, which is relevant for the description of novel optical media, such as BGs on photonic wires, Bragg superstructures, periodic composite waveguides, etc. The stability analysis for moving solitons is necessary because experimental methods create only sufficiently fast solitons in fiber BGs. We demonstrate that the dispersive reflectivity expands the stability region for the solitons, while unstable solitons are transformed into persistent breathers. The parameter region which admits the merger of colliding solitons into a single one also gets much broader under the action of the dispersive reflectivity, and new outcomes of the collision become possible, which are absent in the standard BG model: elastic bounce of the solitons and an inelastic reaction generating three solitons out of two.

Performance advances in interferometric optical profilers for imaging and testing

Abstract: Interferometric optical profilers deliver non-contact, fast, full-field measurements with vertical resolution down to a fraction of a nanometer. Over the last decade advancements to these instruments have allowed for the analysis of not only static but also dynamic objects, like cantilevers and other microelectromechanical system devices, moving or vibrating at up to 1 MHz frequencies. Special objectives and illumination allow for imaging and testing of objects enclosed in environmental or protective chambers or immersed in liquids—including biological samples. Advanced analysis of the interference signal allows for the measurement not just of the surface profile but also transparent coatings.

Modulation of the gold particle-plasmon resonance by the metal-semiconductor transition of vanadium dioxide

Abstract: We report experimental observations of relative blue-shifts in the particle?plasmon resonance of gold nanoparticles (Au NPs) covered with a vanadium dioxide (VO2) film as the VO2 material undergoes a semiconductor-to-metal transition at approximately 67??C. Although the extinction spectra of the Au NPs exhibit significant red-shifts in the presence of the surrounding VO2 film as compared to the same particles in air, the key result of this work is the dynamically controlled blue-shift of the Au-NP dipole resonance upon thermal switching of the VO2 overlayer from the semiconducting to the metallic state. We also report on the size and polarization dependence of the extinction spectra for both states, and present Mie theory calculations that confirm in a semi-quantitative way the observed trends in the VO2-induced modulation of the Au-NP plasmon resonance, and their origin in the VO2 dielectric function.

Efficiency improvement and spectral shift of an organic light-emitting device by attaching a hexagon-based microlens array

Abstract: In this paper, we present and analyze the influences of the fill factor and the sag of hexagon-based microlenses on the optical characteristics of an organic light-emitting device (OLED), such as spectral shift, CIE (abbreviation of the French ‘Commission internationale de l’´ eclairage’) coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly on increasing the fill factor of the microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinates decreased linearly on increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 35% and 40%, respectively, by attaching a microlens array having a fill factor of 0.90 and a height ratio of 0.56. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased on increasing the viewing angles, due to the microcavity effect.

The Virgo 3 km interferometer for gravitational wave detection

Abstract: Virgo, designed, constructed and developed by the French-Italian VIRGO collaboration located in Cascina (Pisa, Italy) and aiming to detect gravitational waves, is a ground-based power recycled Michelson interferometer, with 3 km long suspended Fabry-Perot cavities. The first Virgo scientific data-taking started in mid-May 2007, in coincidence with the corresponding LIGO detectors. The optical scheme of the interferometer and the various optical techniques used in the experiment, such as the laser source, control, alignment, stabilization and detection strategies are outlined. The future upgrades that are planned for Virgo from the optical point of view, especially concerning the evolution of the Virgo laser, are presented. Finally, the next generation of the gravitational wave detector (advanced Virgo) is introduced from the point of view of the laser system.

Bidirectional multiwavelength Brillouin fiber laser generation in a ring cavity

Abstract: Bidirectional multiwavelength Brillouin fiber laser (BFL) generation is demonstrated using a 25 km long single-mode fiber as a Brillouin gain medium in a ring cavity. Odd-order Brillouin Stokes waves appear in the backward direction whereas Brillouin pump and the even Stokes orders are in the forward direction with the line spacing 0.16 nm (∼20 GHz) between each two consecutive waves in forward and backward directions. In addition, by a combination of the backward and forward outputs, we have a higher number comb generation of a multiwavelength BFL with the line spacing 0.08 nm (∼10 GHz). The proposed configuration can work at any wavelength which is a benefit to the others.

Efficient and compact diode-side-pumped Nd:YLF laser operating at 1053 nm with high beam quality

Abstract: A very efficient, diode-side-pumped Nd:YLF laser is demonstrated using a compact cavity design based on total internal reflection inside the gain medium. With one pass through the crystal using a single bounce at the pumped face, efficiency in excess of 40% in multimode operation was measured, giving 6.6 W of output power for 16.2 W of pump power. Using two bounces inside the crystal, the beam quality was improved to fundamental mode with 4.2 W of output power for 16.2 W of pump power.

Spatial information transmission beyond a system's diffraction limit using optical spectral encoding of the spatial frequency

Abstract: We propose optical spectral encoding of an object's spatial frequencies as a means of transmitting, through a low-numerical-aperture optical system, spatial information with an instantaneous spatial frequency bandwidth wider than the optical system's diffraction-limited bandwidth. We validate this new superresolution approach experimentally and demonstrate one of its possible practical implementations - wide-field spectrally encoded imaging that is sensitive to nanometre-scale local variations in the microstructure of centimetre-scale samples.

Light-induced manipulation with surface plasmons

Abstract: We review recent advances achieved in the field of surface plasmon-based optical manipulations. We first discuss enhanced optical forces at the surface of a flat metal film and their use for self-organizing a large number of micro-objects. We then show how a suitable engineering of plasmon fields near micro-gold pads enables trapping at a specific location with much weaker laser intensity compared to conventional optical tweezers. This part is illustrated by a series of numerical simulations based on the theory of the Green dyadic. Finally, we show that, beyond their low power requirement, this new generation of integrated optical tweezers offers new perspectives in optical manipulation including parallel trapping with a single beam and controllable selectivity on the object polarizability.

Monitoring of atmospheric trace gases, clouds, aerosols and surface properties from UV/vis/NIR satellite instruments

Abstract: A new generation of UV/vis/near-IR satellite instruments like GOME (since 1995), SCIAMACHY (since 2002), OMI (since 2004), and GOME-2 (since 2006) have allowed one to measure backscattered solar radiance from the Earth with moderate spectral resolution over a large wavelength range (240–790 nm). The SCIAMACHY instrument also includes additional spectral channels in the near-IR. From the measured spectra several important stratospheric and tropospheric trace gases (e.g. O_3, NO_2, OClO, HCHO, SO_2, BrO, H_2O) as well as clouds, aerosols and surface properties can be determined from space. Because of its extended spectral range, the SCIAMACHY instrument also allows the retrieval of greenhouse gases (CO_2, CH_4) and CO in the near-IR. Almost all of the tropospheric trace gases have been observed by these instruments for the first time. From satellite data it is possible to investigate their temporal and spatial variation. Also, different sources can be characterized and quantified. The derived global distributions can serve as input and for the validation of atmospheric models. Here we give an overview of the current status of these new instruments and data products and their recent applications in the investigation of various atmospheric and oceanic phenomena.

Numerical calculation library for diffraction integrals using the graphic processing unit: The GPU-based wave optics library

Abstract: In optics, several diffraction integrals, such as the angular spectrum method and the Fresnel diffraction, are used for calculating scalar light propagation. The calculation result provides us with the optical characteristics of an optical device, the numerical reconstruction image from a hologram, and so forth. The acceleration of the calculation commonly uses the fast Fourier transform; however, in order to analyze a three-dimensional characteristic of an optical device and compute real-time reconstruction from holograms, recent computers do not have sufficient computational power. In this paper, we develop a numerical calculation library for the diffraction integrals using the graphic processing unit (GPU), the GWO library, and report the performance of the GWO library. The GPU chip allows us to use a highly parallel processor. The maximum computational speed of the GWO library is about 20 times faster than a personal computer.

Design and analysis of a multilayer cladding large-mode-area optical fibre

Abstract: We present a large-mode-area optical fibre design consisting of a multilayer cladding. The cladding is formed by alternate high and low index regions specially designed to strip-off higher-order modes, while causing a nominal loss to the fundamental mode. The fibre is analysed by the transfer matrix method and the leakage losses of the modes are calculated. A high differential leakage loss between the first two modes ensures single-mode operation. With the proposed design a fibre with 30 µm core diameter and 0.16 numerical aperture can exhibit single-mode operation at 1550 nm wavelength. Such a large-core fibre can suppress the nonlinear effects and should be useful for high power applications, including fibre lasers and optical communication systems employing DWDM.

A scanning device for VIS–NIR multispectral imaging of paintings

Abstract: We present a scanning device for multispectral imaging of paintings in the 380–2300 nm spectral range (32 VIS+14 NIR bands). The system is based on contact-less and single-point measurement of the spectral reflectance factor. Multispectral images are obtained by scanning the painted surface under investigation. At present the VIS and NIR modules work separately due to the lack of synchronization between them. Measurement campaigns were carried out on several paintings in situ and at the INOA Optical Metrology Laboratory located inside the Opificio delle Pietre Dure in Florence. We report herein on the measurements carried out on a few panel and canvas paintings. Multivariate image analyses (MIAs) were performed and the detected images were analysed by means of the conventional principal component analysis (PCA) and the K-nearest-neighbouring cluster analysis (KNN).

3D structures using surface relief gratings of azobenzene materials

Abstract: We present here a method of step-by-step 3D structure fabrication using holographic formation of surface relief gratings (SRG) in azobenzene-containing layers alternated with spacer layers. The method possesses opportunities that could not be realized by other holographic approaches, especially the variation of grating periods or the shape of the SRG in different layers (hierarchical structures), full flexibility of the structure type and parameters. One of the main problems to be solved is the compatibility of active azobenzene-containing polymers and a material for the spacer layers. We resolved the problem and found active and spacer polymers with excellent compatibility. The materials allow the preparation of films with good optical quality and the inscription of SRG with modulation amplitudes up to 1 µm in reasonable time. The refractive index difference between active and spacer polymers (currently 0.15–0.2) results in appreciable diffraction efficiency of the SRG after the application of the spacer layer. The phase correlation technique between adjacent layers has been developed. The fabrication of multilayer structures has been demonstrated by examples of a combination of linear, tetragonal and hexagonal SRG with three active layers and hierarchical structures.

The all-angle self-collimating phenomenon in photonic crystals with rectangular symmetry

Abstract: We propose a two-dimensional photonic crystal of rectangular symmetry with rather straight iso-frequency lines over the whole first Brillouin zone along theX direction to realize all-angle self-collimation. By tracking the spatial evolution and transmission efficiency of the collimated beam along the propagating direction, it is found that the beam spreading and propagation loss are negligible for incident angles within ±85 ◦ . The self-collimating behaviors in the rectangular and square lattices are compared and the differences between them are investigated. As an application, a Y-shaped beam splitter with transmission efficiencies for both branches as high as 43.8% is constructed by combining two rotated rectangular structures.

Digital color encryption using a multi-wavelength approach and a joint transform correlator

Abstract: We propose a digital color image encryption technique by using a joint transform correlator (JTC) architecture and a wavelength multiplexing operation. In our optical arrangement, the color image to be encrypted is separated into three channels: red, green and blue. One of the JTC apertures contains the input image information corresponding to a determined color channel bonded to a random phase mask, while the other JTC aperture contains the reference random phase key code. Since the speckle size generated by the random phase masks is wavelength dependent, the illuminating wavelength variation will produce a corresponding joint power spectrum (JPS) modification. Consequently, wavelength changes can be used to multiplex the encrypted information associated to each color channel. We sequentially store every JPS in the same medium. We present digital results that confirm our approach.

Phase hologram formation in highly concentrated phenanthrenequinone-PMMA media

Abstract: For phase holographic gratings in layers of polymethylmethacrylate, containing phenanthrenequinone in high concentration (nearly 3 mol%), a discrepancy between experimental (up to 9) and estimated (∼45) magnitudes of the thermal diffusion amplification coefficient has been revealed. Analysis of plausible reasons of the lower experimental efficiency of the diffusion amplification has been carried out. The influence of material deformations on the reflection grating formation process was investigated experimentally. It is shown that thermoactivated amplification of holograms under high phenanthrenequinone concentration and its profound modulation are depressed by the arising density ‘grating’.