Showing papers in "Journal of Optics in 2009"

Micromachining of photonic devices by femtosecond laser pulses

Abstract: In this paper we review the micromachining of photonic devices in several materials by means of ultrashort laser pulses. The physical mechanisms underlying the refractive index modification and the different laser systems used to induce such modification are discussed. A thorough review of the photonic devices demonstrated with the femtosecond laser microfabrication technique is presented. In particular, this paper is focused on photonic devices based on optical waveguides. The devices are organized into two categories: passive and active devices. In the former category power splitters, directional couplers, interferometers and Bragg gratings are reviewed, while in the latter amplifiers and lasers are discussed. Finally, conclusions and future perspectives of femtosecond laser micromachining in photonics are provided.

Chiral metamaterials: simulations and experiments

Abstract: Electromagnetic metamaterials are composed of periodically arranged artificial structures. They show peculiar properties, such as negative refraction and super-lensing, which are not seen in natural materials. The conventional metamaterials require both negativeand negative μ to achieve negative refraction. Chiral metamaterial is a new class of metamaterials offering a simpler route to negative refraction. In this paper, we briefly review the history of metamaterials and the developments on chiral metamaterials. We study the wave propagation properties in chiral metamaterials and show that negative refraction can be realized in chiral metamaterials with a strong chirality, with neithernor μ negative required. We have developed a retrieval procedure, adopting a uniaxial bi-isotropic model to calculate the effective parameters such as n±, κ, � and μ of the chiral metamaterials. Our work on the design, numerical calculations and experimental measurements of chiral metamaterials is introduced. Strong chiral behaviors such as optical activity and circular dichroism are observed and negative refraction is obtained for circularly polarized waves in these chiral metamaterials. We show that 3D isotropic chiral metamaterials can eventually be realized.

Inorganic and hybrid nanostructures for optical limiting

Abstract: This paper reviews the recent development of inorganic and hybrid nanomaterials for optical limiting applications. The synthesis, testing method, optical limiting property and mechanism of several representative classes of nanomaterial, including carbon nanotubes, silver and gold nanocomposites, and selected other conducting and semiconducting nanomaterials, are introduced separately. The nonlinear optical mechanisms observed in inorganic nanomaterials, i.e. nonlinear scattering, two-photon absorption, free-carrier absorption, etc, are discussed in conjunction with the influence of the material properties and the laser source on the optical limiting performance.

Extrinsic electromagnetic chirality in metamaterials

Abstract: Three- and two-dimensional chirality arising from the mutual orientation of non-chiral planar metamaterial structures and the incident electromagnetic wave (extrinsic chirality) lead to pronounced optical activity, circular dichroism and asymmetric transmission indistinguishable from those seen in media consisting of three- and two-dimensionally chiral molecules (intrinsic chirality).

Geometrodynamics of polarized light: Berry phase and spin Hall effect in a gradient-index medium

Abstract: We review the geometrical-optics evolution of an electromagnetic wave propagating along a curved ray trajectory in a gradient-index dielectric medium. A Coriolis-type term appears in Maxwell equations under transition to the rotating coordinate system accompanying the ray. This term describes the spin–orbit coupling of light which consists of (i) the Berry phase responsible for trajectory-dependent polarization variations and (ii) the spin Hall effect representing polarization-dependent trajectory perturbations. These mutual phenomena are described within universal geometrical structures underlying the problem and are explained by the dynamics of the intrinsic angular momentum carried by the wave. Such close geometrodynamical interrelations illuminate a dual physical nature of the phenomena.

On genuine cross-spectral density matrices

Abstract: For an electromagnetic stochastic beam, the choice of the mathematical structure of the cross-spectral density matrix is limited by the constraint of non-negative definiteness. We present a sufficient condition for building these matrices in such a way that this constraint is automatically satisfied. This allows us to put into evidence that electromagnetic beams can exhibit very peculiar correlation properties, some of which would not be encountered in scalar treatments. These results are illustrated by means of a number of examples.

Nonlinear plasmonics with gold nanoparticle antennas

Abstract: We investigate the nonlinear optical properties of gold nanoparticle pairs. Two excitation beams of frequencies ω1 and ω2 are used to induce nonlinear polarizations at the junction of a particle dimer. Nonlinearities of the second and third order can be controllably induced as a function of the dimer geometry, leading predominantly to second-harmonic generation (SHG), sum frequency generation (SFG) and four-wave mixing (4WM). Due to their center symmetry, dimers with identical particle diameters give rise to a very weak second-order response, without affecting the third-order response. Therefore, a sharp probe functionalized with a symmetric metal dimer acts as a nanoscale photon source emitting narrow-band photons of frequency 2ω1 − ω2. We demonstrate that this source can be employed as a near-field optical probe for high-resolution fluorescence imaging.

Magnetoplasmonic nanostructures: systems supporting both plasmonic and magnetic properties

Abstract: We present in this work our current understanding on magnetoplasmonic structures, that is, systems whose constituents exhibit simultaneously magnetic and plasmonic properties. We analyze both the influence of the plasmon resonance on the magneto-optical properties of the system and the ability of the magnetic field to modulate the plasmon properties. In particular we show how, in magnetoplasmonic systems sustaining localized or propagating surface plasmons, the associated electromagnetic field enhancement gives rise to an enhancement of the magneto-optical activity. On the other hand, we have analyzed the modulation of the propagating surface plasmon polariton wavevector in noble metal/ferromagnet/noble metal trilayers by an external magnetic field. These phenomena can be addressed as new concepts for the development of active plasmonic devices.

Plasmonic components fabrication via nanoimprint

Abstract: A review report on nanoimprinted plasmonic components is given. The fabrication of different metal–dielectric geometries and nanostructured surfaces that support either propagating or localized surface plasmon modes is discussed. The main characteristics and advantages of the nanoimprint technology for the fabrication of various plasmonic structures are outlined. The discussion of plasmonic waveguiding structures focuses on planar waveguides based on metal strips embedded into a dielectric and on profiled metal surfaces. Nanoimprint-based fabrication of two-dimensional nanostructured plasmonic surfaces for enhanced transmission studies and sensor applications is also discussed. Throughout the report, the main fabrication schemes are described, as well as the challenges facing future manufacturing of plasmonic components for device applications.

Freeform LED lens for rectangularly prescribed illumination

Abstract: Freeform lenses are playing a more and more important role in LED secondary optics design. In this study, based on the new light energy mapping relationship, edge ray principle, Snell's law and error control of surface construction, a modified discontinuous freeform lens design method was presented for rectangularly prescribed illumination, with the advantages of a flexible energy mapping relationship, accurate light irradiation control and easier to manufacture. A polymethyl methacrylate (PMMA) discontinuous freeform lens was designed as an example for LED tunnel illumination according to this method. The numerical simulation results demonstrated that the light pattern of the lens was in good agreement with the expected illumination performance when using a point source. Tolerance analyses were also conducted. An extended light source had little effect on the light output efficiency (LOE) of the lens but significantly decreased the effective illumination area. Installation errors had more effect on the uniformity and shape of the light pattern than the LOE of the lens. The tolerances of vertical, horizontal and rotational deviation of this lens were 0.4 mm, 0.4 mm and 2°, respectively.

Harmonically mode-locked semiconductor-based lasers as high repetition rate ultralow noise pulse train and optical frequency comb sources

Abstract: Recent experimental work on semiconductor-based harmonically mode-locked lasers geared toward low noise applications is reviewed. Active, harmonic mode-locking of semiconductor-based lasers has proven to be an excellent way to generate 10 GHz repetition rate pulse trains with pulse-to-pulse timing jitter of only a few femtoseconds without requiring active feedback stabilization. This level of timing jitter is achieved in long fiberized ring cavities and relies upon such factors as low noise rf sources as mode-lockers, high optical power, intracavity dispersion management and intracavity phase modulation. When a high finesse etalon is placed within the optical cavity, semiconductor-based harmonically mode-locked lasers can be used as optical frequency comb sources with 10 GHz mode spacing. When active mode-locking is replaced with regenerative mode-locking, a completely self-contained comb source is created, referenced to the intracavity etalon.

Nonlinear optics and surface relief gratings in alkynyl–ruthenium complexes

Abstract: Generally, the organometallic complexes rich in carbon and containing π-conjugated chains are materials interesting for the study of electronic transfer processes. Currently, the ruthenium–acetylide organometallic complexes form part of the most studied organometallic compounds in nonlinear optics. In this work, we highlight the nonlinear optical properties and the photo-induced structure of new organometallic complexes containing a ruthenium–acetylide donor fragment able to compete with the strongest organic donors. We determine, using various experimental techniques (degenerate four-wave mixing, second and third harmonic generation), the influence of the functionalization of these molecular structures on the improvement of their second-and third-order nonlinear optical properties exploiting in particular the nature of the acceptor fragment and the π-conjugated transmitter. Also we present the dynamics of formation of photo-induced surface relief gratings using a transmission holographic technique and the atomic force microscopy on ruthenium–acetylide complexes containing an azobenzene fragment.

Vulnerability to known-plaintext attack of optical encryption schemes based on two fractional Fourier transform order keys and double random phase keys

Abstract: From the perspective of optical information security, we demonstrate that conventional phase retrieval algorithms, such as the Gerchberg–Saxton algorithm, can be still valid when extended from the Fourier transform domain into the fractional Fourier transform (FRFT) domain. Based on this extension, we propose a method of known-plaintext attack (KPA) on double random phase encoding in the fractional Fourier transform domain (DRPE-FRFT). With this attack, an opponent can access two encryption phase keys with the help of the phase retrieval algorithm in the FRFT domain, and meanwhile obtain the two FRFT order keys by using an exhaustive search. This indicates that the optical encryption scheme based on the DRPE-FRFT scheme is vulnerable to KPA due to the nature of the linearity of FRFT. The validity of this attack is verified by numerical simulations.

Focusing of high order cylindrical vector beams

Abstract: We present the results of calculations of focusing high order cylindrical vector beams in the limit of high numerical aperture. We derive a form of the vectorial diffraction integrals for arbitrary radial and azimuthal mode indices and evaluate these numerically for a number of different modes. We identify combinations of mode indices and lens filling factors that produce focal volume shapes that may be of interest for a number of applications such as optical trapping, two-photon lithography or optical super-resolution. Finally we evaluate the effect of spherical aberration on the focusing.

Polarization properties of chiral fiber gratings

Abstract: Recent experiments (Kopp et al 2007 J. Opt. Soc. Am. B 24 A48) have demonstrated that the polarization sensitivity of chiral fiber gratings depends strongly on the grating symmetry: double-helix fibers are polarization sensitive while single-helix fibers are not. A coupled-mode perturbation theory is developed and used to explain the polarization properties of chiral fiber gratings. Features of the transmission spectrum such as multiple dips in the spectrum and circular dichroism are also derived and attributed to chiral Bragg scattering of the core modes into the cladding modes of the fiber.

Laser microfluidics: fluid actuation by light

Abstract: The development of microfluidic devices is still hindered by the lack of robust fundamental building blocks that constitute any fluidic system. An attractive approach is optical actuation because light field interaction is contactless and dynamically reconfigurable, and solutions have been anticipated through the use of optical forces to manipulate microparticles in flows. Following the concept of an 'optical chip' advanced from the optical actuation of suspensions, we propose in this survey new routes to extend this concept to microfluidic two-phase flows. First, we investigate the destabilization of fluid interfaces by the optical radiation pressure and the formation of liquid jets. We analyze the droplet shedding from the jet tip and the continuous transport in laser-sustained liquid channels. In the second part, we investigate a dissipative light-flow interaction mechanism consisting in heating locally two immiscible fluids to produce thermocapillary stresses along their interface. This opto-capillary coupling is implemented in adequate microchannel geometries to manipulate two-phase flows and propose a contactless optical toolbox including valves, droplet sorters and switches, droplet dividers or droplet mergers. Finally, we discuss radiation pressure and opto-capillary effects in the context of the 'optical chip' where flows, channels and operating functions would all be performed optically on the same device.

A one-dimensional photonic crystal with a superconducting defect layer

Abstract: A one-dimensional dielectric photonic crystal with a complex defect layer, consisting of ultrathin superconducting and dielectric sublayers, is theoretically studied. It is shown that the position of the defect mode and the transmittivity at the defect mode frequency strongly depend on the thickness of the superconducting sublayer as well as on the temperature.

Chalcogenide glass prism based SPR sensor with Ag–Au bimetallic nanoparticle alloy in infrared wavelength region

Abstract: A chalcogenide prism based surface plasmon resonance sensor with Ag–Au bimetallic alloy nanoparticles is proposed. The plasmonic structure based on (chalcogenide) sulfide glass Ge20Ga5Sb10S65, commonly known as 2S2G, allows for a wide range of sensing applications in the infrared wavelength region. Theoretical calculations are carried out to design a device providing an accurate sensing in the infrared range by exploiting the unique optical properties of 2S2G chalcogenide glass. The angular interrogation method is used to analyze the sensor's performance in terms of its intrinsic sensitivity. The intrinsic sensitivity takes both angular shift as well as the angular width of the SPR curve into account. The adequate values of crucial design parameters are predicted in order to achieve the best possible sensing performance. The proposed sensor can be very useful for chemical and bio-sensing applications in the infrared due to the large operating window of chalcogenide materials.

Enhancement of image hiding by exchanging two phase masks

Abstract: Recently, a new image hiding method based on optical interference was proposed. The image is hidden in two pure phase masks. However, the silhouette of the encrypted image can be obtained by using just one of these two masks, which reduces the secrecy of the algorithm. A method for secrecy enhancement based on exchanging the same parts of two masks is proposed. This method can effectively remove the silhouette of the encrypted image constructed using just one mask. Simulations are carried out to demonstrate the validity of this method.

The optical vortex coronagraph

Abstract: An optical vortex coronagraph is a high contrast imaging system which has the potential to completely extinguish light from an on-axis point source, allowing glare-free, high throughput imaging of off-axis targets. An important application is the direct detection of exoplanets orbiting distant stars. The optical physics of the OVC is reviewed, a heuristic argument describing its operation is presented for the first time, and performance limitations are explored.

Preparation of ferroelectric nanoparticles for their use in liquid crystalline colloids

Abstract: In this paper we summarize our many years of experience in the preparation and optimization of stable colloids of ferroelectric nanoparticles dispersed in an isotropic carrier and in a liquid crystal host The colloids are of interest for use in electro-optic devices, photorefractive hybrids and nonlinear optical elements We also outline some of the most interesting features the nanoparticles bring to liquid crystals, along with the potential of these relatively new colloids

Longitudinal optical binding of several spherical particles studied by the coupled dipole method

Abstract: We employed a coupled dipole method (CDM) to study theoretically the interaction among several spherical particles placed into two counter-propagating mutually incoherent Bessel beams. This interaction is mediated by the light scattering among the particles. It has already been demonstrated that, if the intensity of the incident beam is sufficiently high, the scattered light is strong enough to self-arrange the objects in the space. Namely, the counter-propagating and incoherent Bessel beams are extremely useful to be employed because the interaction among the particles via the scattered light is not superimposed by other optical forces coming from the radiation pressure of each beam and axial gradients of the beam intensities. Therefore so-called optical binding between the particles is enhanced and leads to several stable configurations of the particles. We studied these stable configurations using the CDM for various properties of the beams and particles and we also compared these theoretical results with the experimental observations.

Saturable and reverse saturable absorption and nonlinear refraction in nanoclustered Amido Black dye?polymer films under low power continuous wave He?Ne laser light excitation

Abstract: We report an observed transition from a saturable absorption type of behaviour to a reverse saturable absorption one for solid films of a guest?host system constituted by an organic chromophore, Amido Black 10B, embedded in a vinyl polymer, polyvinyl alcohol, and comprising a uniform distribution of aggregated nanoclusters, as studied using the standard Z-scan technique under low intensity continuous wave laser light excitation at 632?nm, while increasing the concentration of the dye content. This is attributed to the presence of higher aggregates of the dye molecules in the sample. Besides this, the samples also displayed complex nonlinear refraction behaviour, yielding a net negative nonlinearity, explained on the basis of a possible, simultaneous occurrence of refractive nonlinearities of different origin, in addition to the obvious effect of absorption. The estimated values of the effective coefficients of nonlinear absorption, nonlinear refraction and third-order nonlinear susceptibility, |?(3)|, compared to those reported for continuous wave laser light excitation, measure up to the highest among them. These nonlinear effects could be the basis for possible applications of this new reverse saturable absorption material, sensitive even to low power excitation, as an efficient material for use in nonlinear optical devices.

A colloid of ferroelectric nanoparticles in a cholesteric liquid crystal

Abstract: The introduction of a minor quantity of ferroelectric nanoparticles into a cholesteric mixture causes a 45% decrease of the driving voltage with both the optical cell quality and the director field in the cell remaining undisturbed. The drop of the driving voltage results from a more than twofold increase of the effective dielectric anisotropy of the nematic matrix, which is the basic component of the cholesteric compound. The results reported clearly show how promising the ferroelectric liquid crystalline (LC) nanocolloids are for various applications. In particular, they offer a unique way for an effective tuning of the dielectric, optical and electro-optic properties of LC materials in a non-synthetic way that is a new direction for the development of advanced anisotropic meso-materials. We suggest that the strong impact of ferroelectric nanoparticles on the properties of the studied chiral nematics is due to the particle's permanent polarization, which produces a giant electric field around it.

Encoded binary diffractive element to form hyper-geometric laser beams

Abstract: A binary diffractive optical element (1500 pixels × 1500 pixels, diameter 4.5 mm) is designed by partial encoding and fabricated by direct electron beam writing in SiO2. Two conjugate hyper-geometric laser modes are generated using the fabricated element. The root-mean-square transverse intensity deviation of the experimental from theoretical diffraction pattern, determined within a circle of radius 1 mm, is below 13%.

POLICRYPS: a liquid crystal composed nano/microstructure with a wide range of optical and electro-optical applications

Abstract: POLICRYPS (an acronym of polymer liquid crystal polymer slices) is a nano/microcomposite structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystal (NLC). The structure is obtained by irradiating a homogeneous syrup of NLC, monomer and curing agent molecules with an interference pattern of UV/visible light under suitable experimental and geometrical conditions; the spatial periodicity can be easily varied from an almost nanometric (200 nm) to a micrometric (15 µm) scale. Where the effect on an impinging reading light beam is concerned, the POLICRYPS can be utilized either in a transmission or a reflection configuration (depending on the geometry and substrate used) with negligible scattering losses, while the effect of spatial modulation of the refractive index (from polymer to NLC values) can be switched on and off by applying an external electric field of the order of few V µm−1. In this paper, we start by reviewing the general features of the POLICRYPS structure, that is the 'recipe' to fabricate it, along with a chemical–diffusive model that indicates the right physical and chemical conditions to make samples exhibiting good morphological, optical and electro-optical properties. We then show some possible utilizations of POLICRYPS with a light beam impinging almost perpendicularly to the structure: a switchable diffraction grating and a switchable optical phase modulator. Furthermore, we put into evidence that POLICRYPS channels can become an array of mirrorless optical micro-resonators for lasing effects. Finally, we report about utilization of POLICRYPS with a light beam impinging parallel to the structure and perpendicular to the channels, demonstrating that, in this case, the structure becomes a tuneable Bragg filter. Performances exhibited in all above applications put the POLICRYPS structure at the top level of the state of art of application oriented research in optics of liquid crystalline composite materials.

Microrheology of complex fluids using optical tweezers: a comparison with macrorheological measurements

Abstract: The increasing interest in the mechanical properties of complex systems at mesoscopic scale has recently fueled the development of new experimental techniques, collectively indicated as microrheology. Unlike bulk-based approaches (macrorheology), these new techniques make use of micrometric probes (usually microspheres) which explore the mechanical properties of the surrounding medium. In this paper we discuss the basic idea of microrheology and we will focus on one specific technique based on optical tweezers (OT). The discussion starts from Newtonian fluids to tackle the more general case of complex fluids, also showing results of these techniques on solutions of a relevant biomolecule: hyaluronic acid (HA). In particular, we study the viscoelastic properties of low molecular weight HA (155?kDa) at low ionic strength over an extended frequency range (0.1?1000?Hz) and in a wide range of concentrations (0.01?20?mg?ml?1), which include both the dilute and semidilute regime. In the concentration range here explored and within the test frequencies covered by our techniques, samples prevalently exhibit a viscous behavior, the elastic contribution becoming significant at the highest concentrations. By comparing OT outcomes to those obtained by a traditional rheometer, we found that they were in good agreement in the overlapping frequency range of the two techniques, thus confirming the reliability of the microrheological approach.

Effective volume holographic structures based on organic?inorganic photopolymer nanocomposites

Abstract: We demonstrate a practical approach for the development of a broad range of nanocomposites based on acrylate polymers and organically capped inorganic nanoparticles (NPs). The submicrometer scale volume patterning of the nanocomposites using holographic photopolymerization was investigated. The specific adjustment of both the material parameters (core?shell of the NP, monomer mixture, concentrations) and the patterning conditions led to materials that were suitable for the fabrication of effective optical diffractive elements and specific functional microdevices with light-emissive and nonlinear optical (NLO) properties. The nanocomposite preparation and properties, their holographic performance and some examples of functional polymer?NP structures are reported.

Oblique section of a paraxial light beam: criteria for azimuthal energy flow and orbital angular momentum

Abstract: Comparison of the intensity distributions in a transverse cross section, orthogonal to the beam propagation axis, and in an oblique section, inclined to the axis by some acute angle, provides information about the transverse energy flows in the beam, in particular, the transverse energy circulation. The beam intensity pattern in an oblique section differs from the 'geometric-optics' expectations, which can be quantitatively described by the corresponding shift of the beam 'centre of gravity'. Measurements of this shift can be used for the study and characterization of the transverse energy flows and for evaluation of the beam orbital angular momentum. Situations in which the intensity distribution in oblique sections plays an important role occur in the beam transformation at a diffraction grating (in non-zero diffraction orders) and even at the beam propagation through a reflecting boundary. An optical vortex generation with the help of a hologram possessing a 'fork' structure supplies a special case of such a situation.

All-optical ultra-compact photonic crystal NOR gate based on nonlinear ring resonators

Abstract: We have proposed an ultra-compact all-optical photonic crystal 'NOR' gate, based on two Kerr nonlinear photonic crystal ring resonators. This logic gate can operate with a bit rate of about 138.9 Gbit s−1. We have employed an Si nanocrystal as the nonlinear material for its appropriate nonlinear properties. The operational principle and properties of our proposed gate has been described by finite difference time domain and plane wave expansion methods.