Narrow Light Beam Research Articles

Thermalization and Bose-Einstein condensation of quantum light in bulk nonlinear media

We study the thermalization and the Bose-Einstein condensation of a paraxial, spectrally narrow beam of quantum light propagating in a lossless bulk Kerr medium. The spatiotemporal evolution of the quantum optical field is ruled by a Heisenberg equation analogous to the quantum nonlinear Schrödinger equation of dilute atomic Bose gases. Correspondingly, in the weak-nonlinearity regime, the phase-space density evolves according to the Boltzmann equation. Expressions for the thermalization time and for the temperature and the chemical potential of the eventual Bose-Einstein distribution are found. After discussing experimental issues, we introduce an optical setup allowing the evaporative cooling of a guided beam of light towards Bose-Einstein condensation. This might serve as a novel source of coherent light.

Discrete-time anti-windup compensation for synchrotron electron beam controllers with rate constrained actuators

By accelerating electrons to relativistic speeds, synchrotrons generate extremely intense and narrow beams of electromagnetic light that are used for academic research and commercial development across a range of scientific disciplines. In order to achieve optimum performance, the stability of the electron beam is a crucial parameter for synchrotrons and is achieved by a beam stabilisation system that is used to control the location of the electron beam and minimise any instability of the electron beam caused by external disturbances. Slew rate limits are common nonlinearities encountered with the actuators in synchrotron feedback systems which can impose significant limitations on the robustness and the performance of the control system. This paper describes an Internal Model Control (IMC) based anti-windup synthesis using an algebraic Riccati equation for a discrete-time control system to compensate against the performance deterioration in the presence of rate constraints. An Integral Quadratic Constraint (IQC) framework is used to analyse the robust stability of the anti-windup augmented closed loop system in the presence of norm-bounded uncertainty. The anti-windup augmented controller is implemented at Diamond Light Source, the UK’s national synchrotron facility and improvements in robustness and performance were achieved with respect to the use of no anti-windup compensation.

The theory of stochastic cosmological lensing

On the scale of the light beams subtended by small sources, e.g. supernovae, matter cannot be accurately described as a fluid, which questions the applicability of standard cosmic lensing to those cases. In this article, we propose a new formalism to deal with small-scale lensing as a diffusion process: the Sachs and Jacobi equations governing the propagation of narrow light beams are treated as Langevin equations. We derive the associated Fokker-Planck-Kolmogorov equations, and use them to deduce general analytical results on the mean and dispersion of the angular distance. This formalism is applied to random Einstein-Straus Swiss-cheese models, allowing us to: (1) show an explicit example of the involved calculations; (2) check the validity of the method against both ray-tracing simulations and direct numerical integration of the Langevin equation. As a byproduct, we obtain a post-Kantowski-Dyer-Roeder approximation, accounting for the effect of tidal distortions on the angular distance, in excellent agreement with numerical results. Besides, the dispersion of the angular distance is correctly reproduced in some regimes.

GPU-accelerated inverse identification of radiative properties of particle suspensions in liquid by the Monte Carlo method

Inverse identification of radiative properties of participating media is usually time consuming. In this paper, a GPU accelerated inverse identification model is presented to obtain the radiative properties of particle suspensions. The sample medium is placed in a cuvette and a narrow light beam is irradiated normally from the side. The forward three-dimensional radiative transfer problem is solved using a massive parallel Monte Carlo method implemented on graphics processing unit (GPU), and particle swarm optimization algorithm is applied to inversely identify the radiative properties of particle suspensions based on the measured bidirectional scattering distribution function (BSDF). The GPU-accelerated Monte Carlo simulation significantly reduces the solution time of the radiative transfer simulation and hence greatly accelerates the inverse identification process. Hundreds of speedup is achieved as compared to the CPU implementation. It is demonstrated using both simulated BSDF and experimentally measured BSDF of microalgae suspensions that the radiative properties of particle suspensions can be effectively identified based on the GPU-accelerated algorithm with three-dimensional radiative transfer modelling.

Light propagation in a homogeneous and anisotropic universe

This article proposes a comprehensive analysis of light propagation in an anisotropic and spatially homogeneous Bianchi I universe. After recalling that null geodesics are easily determined in such a spacetime, we derive the expressions of the redshift and direction drifts of light sources; by solving analytically the Sachs equation, we then obtain an explicit expression of the Jacobi matrix describing the propagation of narrow light beams. As a byproduct, we recover the old formula by Saunders for the angular diameter distance in a Bianchi I spacetime, but our derivation goes further since it also provides the optical shear and rotation. These results pave the way to the analysis of both supernovae data and weak lensing by the large-scale structure in Bianchi universes.

Flat focusing mirror.

The control of spatial propagation properties of narrow light beams such as divergence, focusing or imaging are main objectives in optics and photonics. In this letter, we propose and demonstrate experimentally a flat focusing mirror, based on an especially designed dielectric structure without any optical axis. More generally, it also enables imaging any light pattern in reflection. The flat focusing mirror with a transversal invariance can largely increase the applicability of structured photonic materials for light beam propagation control in small-dimension photonic circuits.

Nondiffractive-nondiffusive beams in complex crystals

The study of spatial dispersion of two-dimensional complex crystals, with periodic modulations of both gain-loss and the refractive index, reveals simultaneous nondiffractive-nondiffusive light propagation. Narrow light beams and light patterns propagate without dispersion while amplified through the complex material. We determine and explore nondiffractive-nondiffusive regimes for collinear and noncollinear propagation.

Swiss-cheese models and the Dyer-Roeder approximation

In view of interpreting the cosmological observations precisely, especially when they involve narrow light beams, it is crucial to understand how light propagates in our statistically homogeneous, clumpy, Universe. Among the various approaches to tackle this issue, Swiss-cheese models propose an inhomogeneous spacetime geometry which is an exact solution of Einstein's equation, while the Dyer-Roeder approximation deals with inhomogeneity in an effective way. In this article, we demonstrate that the distance-redshift relation of a certain class of Swiss-cheese models is the same as the one predicted by the Dyer-Roeder approach, at a well-controlled level of approximation. Both methods are therefore equivalent when applied to the interpretation of, e.g., supernova obervations. The proof relies on completely analytical arguments, and is illustrated by numerical results.

Cyclosis-mediated transfer of H2O2 elicited by localized illumination of Chara cells and its relevance to the formation of pH bands

Cytoplasmic streaming occurs in most plant cells and is vitally important for large cells as a means of long-distance intracellular transport of metabolites and messengers. In internodal cells of characean algae, cyclosis participates in formation of light-dependent patterns of surface pH and photosynthetic activity, but lateral transport of regulatory metabolites has not been visualized yet. Hydrogen peroxide, being a signaling molecule and a stress factor, is known to accumulate under excessive irradiance. This study was aimed to examine whether H2O2 produced in chloroplasts under high light conditions is released into streaming fluid and transported downstream by cytoplasmic flow. To this end, internodes of Chara corallina were loaded with the fluorogenic probe dihydrodichlorofluorescein diacetate and illuminated locally by a narrow light beam through a thin optic fiber. Fluorescence of dihydrodichlorofluorescein (DCF), produced upon oxidation of the probe by H2O2, was measured within and around the illuminated cell region. In cells exhibiting active streaming, H2O2 first accumulated in the illuminated region and then entered into the streaming cytoplasm, giving rise to the expansion of DCF fluorescence downstream of the illuminated area. Inhibition of cyclosis by cytochalasin B prevented the spreading of DCF fluorescence along the internode. The results suggest that H2O2 released from chloroplasts under high light is transported along the cell with the cytoplasmic flow. It is proposed that the shift of cytoplasmic redox poise and light-induced elevation of cytoplasmic pH facilitate the opening of H(+)/OH(-)-permeable channels in the plasma membrane.

Photonic Rutherford scattering: A classical and quantum mechanical analogy in ray and wave optics

Using Fermat's least-optical-path principle, the family of ray trajectories through a special (but common) type of a gradient refractive index lens n(r)=n0+ΔnR/r is solved analytically. The solution gives a ray equation r(ϕ) that is closely related to Rutherford scattering trajectories; we therefore refer to this refraction process as “photonic Rutherford scattering.” It is shown that not only do the classical limits correspond but also the wave-mechanical pictures coincide—the time-independent Schrödingier equation and the Helmholtz equation permit the same mapping between the scattering of massive particles and optical scalar waves. Scattering of narrow beams of light finally recovers the classical trajectories. The analysis suggests that photothermal single-particle microscopy measures photonic Rutherford scattering in specific limits and allows for an individual single-scatterer probing. A macroscopic experiment is demonstrated to directly measure the scattering angle to impact parameter relation, which is otherwise accessible only indirectly in Rutherford-scattering experiments.

Optical Nanoantennas with Tunable Radiation Patterns

We address new optical nanoantenna systems with tunable highly directional radiation patterns. The antenna comprises a regular linear array of metal nanoparticles in the proximity of an interface with high dielectric contrast. We show that the radiation pattern of the system can be controlled by changing parameters of the excitation, such as the polarization and/or incidence angles. In the case of excitation under the total reflection condition, the system operates as a nanoscopic source of radiation, converting the macroscopic incident plane wavefront into a narrow beam of light with adjustable characteristics. We derive also simple analytical formulas which give an excellent description of the radiation pattern and provide a useful tool for analysis and antenna design.

Intense Bessel-like beams arising from pyramid-shaped microtips

We show both numerically and experimentally that intense, narrow, and low-divergence beams of light are produced at the apex of dielectric pyramid-shaped microtips. These beams exhibit a Bessel transverse profile but are narrower than the usual Bessel beam, allowing for a significant enhancement of the light intensity inside the beam. They are generated by axicon-like structures with submicrometric height imprinted in glass by combining optical lithography and chemical etching. The resulting beams are experimentally imaged using fluorescence microscopy, in remarkable agreement with numerical computations.

A bound for the range of a narrow light beam in the near field

We investigate the possibility of light beams that are both narrow and long range with respect to the wavelength. On the basis of spectral electromagnetic field representations, we have studied the decay of the evanescent waves, and we have obtained some bounds for the width and range of a light beam in the near-field region. The range determines the spatial bound of the near field in the direction of propagation. For a number of representative examples we found that narrow beams have a short range. Our analysis is based on the uncertainty relations between spatial position and spatial frequency.

Two-photon polymerisation of novel shapes using a conically diffracted femtosecond laser beam

Sub-micron ring, pillar and wall structures were written by two-photon polymerisation of a sol–gel resin using a femtosecond laser beam which was shaped using internal conical diffraction. The ring structure was written using a demagnified image of the ring-shaped beam which arises in conical diffraction of a narrow light beam. The pillar and wall structures were produced by imaging the Bessel beam formed by conical diffraction in combination with a converging lens.

Beyond Snel's law: Refraction of a nano-beam of light

The refraction of a localized narrow beam is significantly different from that of a plane wave. As the beam width decreases to be in the order of the wavelength, the refraction behavior deviates noticeably from Snel's law, and when the width of a light beam is smaller than about one fifth of the wavelength of the incident light, finite-difference time-domain simulations demonstrate that refraction becomes negligible. That is, the narrow light beam retains its propagation direction even after entering another medium at an oblique angle. The result reveals novel features of nano-beams and may have applications in precise biomedical measurement or micro optical device.

Improved automated monitoring and new analysis algorithm for circadian phototaxis rhythms in Chlamydomonas.

Automated monitoring of circadian rhythms is an efficient way of gaining insight into oscillation parameters like period and phase for the underlying pacemaker of the circadian clock. Measurement of the circadian rhythm of phototaxis (swimming towards light) exhibited by the green alga Chlamydomonas reinhardtii has been automated by directing a narrow and dim light beam through a culture at regular intervals and determining the decrease in light transmittance due to the accumulation of cells in the beam. In this study, the monitoring process was optimized by constructing a new computer-controlled measuring machine that limits the test beam to wavelengths reported to be specific for phototaxis and by choosing an algal strain, which does not need background illumination between test light cycles for proper expression of the rhythm. As a result, period and phase of the rhythm are now unaffected by the time a culture is placed into the machine. Analysis of the rhythm data was also optimized through a new algorithm, whose robustness was demonstrated using virtual rhythms with various noises. The algorithm differs in particular from other reported algorithms by maximizing the fit of the data to a sinusoidal curve that dampens exponentially. The algorithm was also used to confirm the reproducibility of rhythm monitoring by the machine. Machine and algorithm can now be used for a multitude of circadian clock studies that require unambiguous period and phase determinations such as light pulse experiments to identify the photoreceptor(s) that reset the circadian clock in C. reinhardtii.

Role of epidermis in the optics and thermal physics of human skin

Some engineering approaches to the problems of optics and thermophysics of biological tissues have been developed. The light fields both within and beyond biological medium have been studied. The role of the epidermis in the formation of these fields is revealed. It is shown that in the general case epidermis cannot be considered as a spectral filter that only absorbs and does not scatter light. A convenient approximation of the fluence rate in a two-layer (epidermis and dermis) tissue is proposed, which is the sum of two exponential functions of depth in the epidermis. This approximation made it possible to obtain an analytical solution to the problem of heating biological tissues by an external narrow light beam. It is found that epidermis only slightly affects the temperature fields under blue light irradiation. However, when an external source of red light is used, the epidermis works as a heater and can increase several times the dermis temperature in comparison with a single-layer tissue. The reasons for these heating features are discussed. Examples of corresponding calculations are given.

Bloch oscillation and Landau–Zener tunnelling in a modulated optical lattice in a photovoltaic photorefractive crystal

We theoretically study the beam dynamical behaviour in a modulated optical lattice with a quadratic potential in a photovoltaic photorefractive crystal. We find that two different Bloch oscillation patterns appear for the excitation of both broad and narrow light beams. One kind of optical Landau–Zener tunnelling also appears upon the Bloch oscillation and can be controlled by adjusting the parameter of the optical lattice. Unlike the case of linear potential, the energy radiation due to Landau–Zener tunnelling can be confined in modulated lattices of this kind. For high input intensity levels, the Landau–Zener tunnelling is suppressed by the photovoltaic photorefractive nonlinearity and a symmetry breaking of beam propagation from the modulational instability appears.

Double hollow cathode plasma jet-low temperature method for the TiO 2− xN x photoresponding films

The low temperature double hollow cathode plasma jet method for producing photoresponding thin transparent layers of TiO 2− x N x is reported. This technique allows involvement of thermally sensitive materials as supports. Dependence of the properties of the films on the applied RF field power was clearly demonstrated. Produced films were first characterized with the aid of a number of characterization methods (AFM, SEM, XRD, photothermal deflection spectroscopy (PDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–vis), then their photocurrent generation efficiencies upon illumination with spectrally well defined narrow light beams were described. These tests based on the recorded polarization curves revealed a significant dependence of the scanning direction (anodic and/or cathodic) on the E on (photocurrent onset) and an appearance of the photocurrent maxima. Open circuit potential (OCP), cyclic voltammetry and amperometry tests were also performed. The RF power magnitude was identified as the parameter affecting substantially the photoinduced properties of the produced films.

Characterization of Glass Surface Morphology by Optical Coherence Tomography

Optical coherence tomography (OCT) has been applied as a tool to assist glass conservators and researchers in monitoring and predicting the evolution of glass surfaces – the crucial issue in the correct maintenance of historic glass. This technique of examination of the internal structure of objects with infrared light originates from diagnostic medicine but application to any object that weakly absorbs and/or scatters light is straightforward. Since the intensity of the examining light is very low, this remote method is also completely non-invasive. The object is usually penetrated by a narrow beam of light of broad spectrum. The distribution of the backscattering and reflecting centres within the object is extracted by means of interferometry. The resolution achieved is in the range 2–15 μm axially and usually between 15 and 40 μm transversely. In the case of glass, the technique allows the identification and characterization of leached and hydrated glass surface layers and thus helps to describe the condition of so-called unstable glass. Five historic artefacts were examined to show the advantages and limitations of the method.