Measurement of the Thickness of Deposited Magnesium Fluoride Films by Evanescent-wave Fluorescence: A Critical Test of the General TIRAF Theory

Abstract

We have recently developed a quantitative theory describing the stimulation of fluorescence by an evanescent wave, with a view to the precise measurement of thin films by otherwise conventional light microscope photometry. In our set-up, total internal reflection of a focused laser beam occurs at a glass/water interface and the presence of the evanescent wave is reported by a fluorescent dye dissolved in the water. For convenience, we refer to the method as TIRAF (total internal reflection aqueous fluorescence). We have tested our general TIRAF theory critically by using glass/MgF₂/fluorescein layers such that the evanescent wave established in the glass/MgF₂ interface is, in part, separated from the aqueous fluorescein phase by an accurately and independently measured thickness of deposited MgF₂ film. We show that fluorescence measurements on areas as small as 30 µm² allow precise determination of film thicknesses in the range 2–160 nm for two different decay constants of the evanescent wave.     

Design of atomic mirror for silicon atoms

Abstract

Cook and Hill suggested that the gradient of an optical field can be used to reflect atoms. To reflect atoms, the repulsive dipole force is used, which comes from the interaction between the electric dipole moment of atoms and the evanescent field. The evanescent wave is generated when light is totally reflected internally at the interface of different refractive indices. Later, the way to enhance the evanescent wave with a thin dielectric waveguide has been reported. We designed the atomic mirror for silicon atoms, whose structure enhances the evanescent field that is used to repel silicon atoms. We also set up the equations of motion for silicon atoms and derive trajectories of the atoms reflected by the atomic mirror. Optical intensity, incident angle of the light, and effective detuning are described in terms of controlling the trajectory of the atom.     

Theoretical analysis of the evanescent wave absorption coefficient for multimode fibre-optic evanescent wave absorption sensors

Abstract

In an optical fibre of circular cross section, leaky skew rays transmit power in a different way from trapped and meridional rays. In this paper, a theoretical analysis of the evanescent wave absorption coefficient for leaky skew rays in multimode optical fibre evanescent wave absorption sensors is presented. Further, a comprehensive expression for the effective evanescent wave absorption coefficient is obtained. Some numerical results are given to illustrate this theoretical analysis. This work could be applied to optimize the design of fibre-optic evanescent wave absorption sensors.     

Optics for Neutral Atomic Beams: Reflection and Diffraction of Sodium Atoms by Evanescent Laser Light Fields

Abstract

Realization of the analogues of optical components for the manipulation of atomic de Broglie waves has recently become possible through the use of laser-based techniques. The strong fields generated by laser light have already been used to reflect atoms using travelling evanescent waves, and to diffract atomic beams through small angles using transmission gratings formed by a standing laser light wave in vacuo. Present experiments aim to combine these techniques to produce a reflection diffraction grating formed by a standing evanescent laser-light field, and to exploit the larger diffraction angles made possible theoretically by such a scheme.     

Propagating and evanescent waves associated with inhibited reflection in uniaxial crystals: Extraordinary incidence

Abstract

When an extraordinary wave under inhibited reflection conditions is incident on an interface between a uniaxial crystal and an isotropic medium, the reflected extraordinary wave is evanescent and the polarization of the refracted wave changes from linear to elliptical. In the present paper it is shown that the refracted ray undergoes a shift which is not only longitudinal (as the Goos—Hänchen effect in total reflection in isotropic interfaces) but also transversal. The structure of the evanescent reflected wave is studied and the polarization of the transmitted wave is analysed.     

Design of atomic mirror for silicon atoms

Cook and Hill suggested that the gradient of an optical field can be used to reflect atoms. To reflect atoms, the repulsive dipole force is used, which comes from the interaction between the electric dipole moment of atoms and the evanescent field. The evanescent wave is generated when light is totally reflected internally at the interface of different refractive indices. Later, the way to enhance the evanescent wave with a thin dielectric waveguide has been reported. We designed the atomic mirror for silicon atoms, whose structure enhances the evanescent field that is used to repel silicon atoms. We also set up the equations of motion for silicon atoms and derive trajectories of the atoms reflected by the atomic mirror. Optical intensity, incident angle of the light, and effective detuning are described in terms of controlling the trajectory of the atom.     

Evanescent plane waves and the far field: Resolution of a controversy

Abstract

We show that the evanescent part of the plane wave representation of the free-space dyadic Green function contributes to the far field only along a distinguished axis. The claim of contribution of evanescence in all directions is incorrect as it arises from a flawed procedure, but a limited version of that claim may have merit.     

Cooling neutral particles in multimode cavities without spontaneous emission

Abstract

We discuss a scheme to cool, trap and manipulate an ensemble of polarizable particles moving in the field of a multimode optical cavity via the correlated dynamics of the field and the particle motion. Using a large detuning between the atoms and the field, spontaneous emission plays a negligible role in the dynamics and the cooling scheme only requires a sufficiently large optical dipole moment. Increasing the particle number slows down the cooling process but it can be accelerated using an increasing number of field modes with higher pump amplitudes. For the special case of a two mode ring cavity and assuming small deviations of the particle positions from the potential minima, the frequencies and damping rates of the vibrational excitation modes can be explicitly calculated. We find a rapid damping of the centre-of-mass motion and relatively slow damping rates for the relative particle oscillations. These predictions agree quite well with a quantum treatment of the atomic motion as used for the excitations of a non-interacting Bose gas (at T = 0) inside the cavity field. Due to the position-dependent mode coupling, the cooling process in a multimode configuration in general happens much faster than for a standing wave geometry. These analytical results are confirmed by N-particle simulations of the semiclassical equations and show even enhanced damping due to the anharmonicity of the full potential.     

Three-wave approximation for the modal field inside high-index dielectric rods of hybrid plasmonic waveguides

Abstract

An approximate three-wave model is suggested for describing the modal field inside the high-index dielectric rod of a hybrid plasmonic waveguide. An evanescent wave, an uniform wave and a propagating wave are considered along the direction perpendicular to the metal surface. The superposition of these three waves forms the modal field inside the high-index rod. Through numerical tests, we find that this model is highly valid for a large range of waveguide sizes.     

Application of Mie Scattering of Evanescent Waves to Scanning Tunnelling Optical Microscopy Theory

Abstract

In this paper we present a macroscopic theory for scanning tunnelling optical microscopy where the sample is a grating and the tip is modelled by a dielectric sphere. The sphere is immersed in the near-field above the grating and is excited by the diffracted orders which can be evanescent. The detected signal is supposed to be the diffracted intensity by the sphere which is calculated by using Mie theory. When studying Mie scattering of one evanescent wave we show that the multipolar series is perturbed compared to scattering of a homogeneous wave. Even for a small sphere multipolar terms have to be taken into account. We have then proposed a formula for the intensity leading to calculations of intensity profile and surfaces and to discuss the influence of tip radius on resolution of the images.     

The information content of weakly scattered fields: Implications for near-field imaging of three-dimensional structures

Abstract

The structural information carried by the homogeneous and evanescent components of the scattered field is investigated for the case of a single plane wave, either homogeneous or evanescent, incident on a weakly scattering three-dimensional medium. For homogeneous plane wave incidence, it is shown that, unlike the one-to-one mapping that exists in the case of scattering from thin (i.e. two-dimensional) structures, the evanescent components of the scattered field are related to the three-dimensional Fourier transform of the dielectric susceptibility through a generalized Radon transform. For evanescent plane wave incidence, a reciprocal relationship exists between the homogenous components of the scattered field and the three-dimensional Fourier transform of the susceptibility. Inversion techniques are outlined for these two cases, as well as other experimental modalities, which explicitly require, except for separable media, complete (i.e. multiple view) measurement data. These results have direct bearing on total internal reflection microscopy (TIRM), and they yield insight into the limitations of more general near-field imaging techniques.     

How to catch an atom with single photons

Abstract

We report on trapping a single neutral atom in the standing-wave light field of a high-finesse optical cavity containing one photon on average, a single-photon optical trap, or SPOT for short. This trap has the novel feature that the light field is also used to observe the atom in real time. The oscillatory motion of the trapped atom induces well-resolved oscillations of the light intensity. Periodic structure is visible in the fourth-order intensity correlation function, attributed to long-distance flights of the atom along the standing wave. The finite duration of those flights provides evidence for cavity-mediated cooling of atoms. We discuss the various mechanisms determining the trapping time and compare the results with a quantum-jump Monte Carlo simulation to interpret the observed signals.     

Realization of a mirror magneto-optical trap

Abstract

We present details for construction and the operation of a mirror magneto-optical trap for cooling and trapping rubidium atoms. For trap operation, only four input laser beams are needed in contrast to the normal six for a standard trap. In excess of 10⁸ atoms have been trapped with this arrangement, with the atomic ensemble only ～1mm from the surface of a reflective mirror. This trap is highly suited to studies of magnetic guiding and magnetic manipulation of cold atomic ensembles.     

Asymptotics of evanescence

Abstract

The wave from an isotropic point source in two and three dimensions is separated into its homogeneous (H) and evanescent (E) parts, with respect to a distinguished direction z, and the far field r θ evaluated asymptotically as a function of polar angle θ. Only in the ‘forward needle’ θ = 0 (three dimensions) and the transverse directions θ = π/2 (two and three dimensions) is the E wave of comparable strength to the H wave. Uniform asymptotic approximations (in terms of Bessel functions and Fresnel integrals) accurately interpolate between these directions of significant evanescence and all other angles (where the amplitude of E relative to H decays as 1/r¹/²). The forward needle in three dimensions is analogous to glory scattering.     

Optimization of Lamellar Gratings in Ledatron and Orotron Tubes

Abstract

Values of the geometrical parameters of the lamellar grating that constitutes one mirror of the open resonator forming a Ledatron tube are found which maximize the amplitude of the evanescent wave interacting with the electron slab. Taking into account the finite conductivity of the metal, numerical results are obtained and discussed.     

Spiral hyperlens with enhancements of image resolution and magnification

Abstract

A subwavelength spiral hyperlens that is able to image beyond the diffraction limit is studied. The spiral hyperlens is made from an anisotropic metamaterial with a hyperbolic dispersion relation in which the evanescent wave is converted into a propagating wave. Therefore, the propagating wave can be processed by conventional optical systems outside of the spiral hyperlens. The possibility of using a cylindrical hyperlens for overcoming the diffraction limit has been proven analytically and experimentally. In this study, we designed two types of spiral hyperlenses composed of a spiral periodic stack of silver and alumina multilayers. A spiral hyperlens utilizes the spiral geometry to magnify the objects. In comparison with a cylindrical hyperlens, a spiral hyperlens has improved performance in terms of higher image resolution and better image magnifications. Numerical simulations illustrate that the far-field imaging resolution of cylindrical spiral hyperlens is no greater than 110 nm at 365 nm working wavelength.     

Ultracold metastable calcium ensembles,a medium for matter wave amplification?

Abstract

We propose an experimental implementation of matter wave amplification by optical pumping with metastable calcium atoms. First experimental results indicate that pumping rates can be significantly higher than in previous experimental schemes so that it appears promising that the threshold condition for the generation of degeneracy can be reached.     

Applications of Fiber-Optic Evanescent Wave Spectroscopy

Abstract

The evanescent wave (EW) component of light propagated via fiber-optic waveguides can be used to both sense and transmit information regarding the immediate environment of the fiber's surface. In this article, an outline of the theoretical and practical aspects of this emerging methodology is given, as well as a discussion of the advantages, disadvantages, and limitations of the technique. Examples are given of how EW spectroscopy may be used in the analysis of pharmaceutical systems. Evaluation of attributes of components of EW spectroscopy allows prediction of the future for this rapidly evolving area of photonics.     

Evanescent Wave Mixing on Nonlinear Surfaces

Abstract

The problem of using evanescent fields in nonlinear optics is discussed by employing results on quantization of evanescent waves. It is shown that the peculiar properties of the momentum of evanescent modes can be used to realize non-critical optical frequency mixing. In the first illustration, the case of surface second-harmonic generation is discussed. It is then shown that, in the case of difference-frequency generation, it is possible to generate a ‘completely evanescent mode’ which is ‘trapped’ by the surface, which becomes a two-dimensional waveguide.     

Dynamics and spatial order of atoms confined in an optical lattice

Abstract

This paper discusses how Bragg diffraction can be applied to measure the position spread of atomic wave packets localized in the light-induced periodic potential wells of an optical lattice. We describe an observation method which is solely based on monitoring light powers. No hard-to-access system parameters are referred to. We discuss the application of this method in the observation of unusual atomic vibrational modes with a position spread oscillating at twice the fundamental harmonic frequency. We also discuss the influence of the number of trapped atoms on the lattice geometry. We give experimental evidence for a contraction of the optical lattice as the filling factor is increased.