Electromagnetic fields and transmission properties in tapered hollow metallic waveguides
TL;DR: The electromagnetic spatital distributions are analyzed and it is shown that all modes run continuously from a propagating through a transition to an evanescent region and the value of the attenuation increases as the distance from the cone vertex and the cone angle decrease.
Abstract: We analyze the electromagnetic spatital distributions and address an important issue of the transmission properties of spherical transverse-electric (TE) and transverse-magnetic (TM) eigenmodes within a tapered hollow metallic waveguide in detail. Explicit analytical expressions for the spatital distributions of electromagnetic field components, attenuation constant, phase constant and wave impedance are derived. Accurate eigenvalues obtained numerically are used to study the dependences of the transmission properties on the taper angle, the mode as well as the length of the waveguide. It is shown that all modes run continuously from a propagating through a transition to an evanescent region and the value of the attenuation increases as the distance from the cone vertex and the cone angle decrease. A strict distinction between pure propagating and pure evanescent modes cannot be achieved. One mode after the other reaches cutoff in the tapered hollow metallic waveguide as the distance from the cone vertex desreases.
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TL;DR: In this article, an open hollow cone that can optically guide and focus a laser light into a small beam spot is proposed and the shaping and focusing effects are numerically demonstrated by finite-difference time-domain simulations.
Abstract: An open hollow cone that can optically guide and focus a laser light into a small beam spot is proposed. The shaping and focusing effects are numerically demonstrated by finite-difference time-domain simulations. The results reveal that the cone-focused laser originates from the multi-beam interference and squeeze of two cone side walls acting like oblique targets. Moreover, the influences of oblique incident angle of laser light, cone angle and cone tip size on the focusing properties are discussed and the optimum cone geometry with cone angle of 16° and tip size of 4.5 wavelengths to achieve high energy concentration at the cone tip is also presented. It is shown that with an open hollow cone a laser light can be focused into a tiny highly localized beam spot of 1 µm diameter and keeps propagating forward for a distance of about 8 wavelengths with a tiny spot diameter remaining almost unchanged. The intensity almost increases up to twentyfold in a 1 µm focal spot.
5 citations
20 Nov 2007
TL;DR: In this paper, a deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser, and a 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression.
Abstract: An integrated experiment relevant to fast ignition. A Cu-doped deuterated polymer spherical shell target with an inserted hollow Au cone is imploded by a six-beam 900-J, 1-ns laser. A 10-ps, 70-J laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of Cu K{sub {alpha}} fluorescence by comparison with a Monte Carlo model. The electrons are estimated to carry about 15% of the laser energy. Collisional and Ohmic heating are modeled, and Ohmic effects are shown to be relatively unimportant. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is calculated in the model. Enhanced scattering by instability-induced magnetic fields is suggested. An extension of this fluor-based technique to measurement of coupling efficiency to the ignition hot spot in future larger-scale fast ignition experiments is outlined.
3 citations
TL;DR: The procedure comprises three steps: hydrogenation of Ge-doped SiO2 fibers to increase photosensitivity, recording of Bragg gratings with ultraviolet light to achieve modulation of refractive index, and chemical etching.
Abstract: We present a method of fabricating Ge-doped SiO2 fibers with corrugations around their full circumference for a desired length in the longitudinal direction. The procedure comprises three steps: hydrogenation of Ge-doped SiO2 fibers to increase photosensitivity, recording of Bragg gratings with ultraviolet light to achieve modulation of refractive index, and chemical etching. Finite-length, radially corrugated fibers may be used as couplers. Corrugated tapered fibers are used as high energy throughput probes in scanning near-field optical microscopy.
2 citations
TL;DR: In this article, a highly focused beam with spot radius of around 1 μm and enhanced light intensity is obtained by using an open hollow cone, where two important parameters characterizing the cone-focusing effect are introduced, which are opening angle and cone tip size.
Abstract: A highly focused beam with spot radius of around 1 μm and enhanced light intensity is obtained by using an open hollow cone. Two-dimensional and three-dimensional finite-difference time-domain simulations are performed for clarifying light propagation characteristics and the cone geometry dependence of the cone-focused laser light. Two important parameters characterizing the cone-focusing effect are introduced, which are opening angle and cone tip size. By changing these parameters, the laser intensification, field spatial distribution at the cone tip and the near-field transmission can be controlled. Understanding this is quite important in designing the optimum cone shape for fast ignition and determining the basis for using hollow cone targets as devices for generating high energy charged particles.
1 citations
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TL;DR: It is found that the existence of most of the modes is limited to certain frequencies and geometries, i.e., the solutions have a cutoff in the complex propagation constant plane.
Abstract: Motivated by problems in scanning near-field optical microscopy, we discuss light propagation in circular dielectric waveguides with finite aluminum cladding. In order to understand the origin of the different solutions, optical modes are first investigated for the dielectric waveguide with infinite aluminum cladding and for the aluminum cylinder. For aluminum a plasma dispersion law is assumed, leading to complex dielectric constants with negative real parts and to generally complex propagation constants. The dependence of the dispersion on the geometry and on the frequency is discussed for the various kinds of modes. We find that the existence of most of the modes is limited to certain frequencies and geometries, i.e., the solutions have a cutoff in the complex propagation constant plane. Contrary to dielectric waveguide theory, where cutoff describes the abrupt transition from propagating to evanescent modes, no other solution is generated when cutoff of a mode is reached. Surface modes and other kinds of modes, such as guided or bulk modes, can either couple between each other or transform into each other.
370 citations
TL;DR: In this article, it is shown that the laser light is optically guided inside the conical target and focused at the tip of the cone, where the convergence of hot electrons to the head of a cone is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields.
Abstract: The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.
185 citations
TL;DR: In this article, the surface plasmon (SP) excitation around C-and H-shaped ridge nanoapertures made in silver film is studied. But the near-field collimation of ridge aperture is found completely destroyed.
Abstract: Finite difference time domain computations is used to study surface plasmon (SP) excitation around C- and H-shaped ridge nanoapertures made in silver film. The SP enhances optical transmission, in addition to the transmission mechanism of the waveguide propagation mode and Fabry-Perot-like resonance. However, the near-field collimation of ridge aperture is found completely destroyed. On the other hand, using a bowtie-shaped aperture with sharp ridges made in silver, the loss of near-field collimation can be recovered. A super resolution optical spot with full width half magnitude as small as 12nm×16nm is achieved due to the resonant SP excitation localized at the tips of bowtie. Much higher field enhancement is also obtained compared to the bowtie aperture made in chromium.
150 citations
Journal Article•
144 citations
TL;DR: Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed, and progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers is addressed.
Abstract: Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed in this article. We focus mainly on NSOM, nano-scale spectroscopy with NSOM, probe technology of NSOM, and study of nano-structured metallic surface with NSOM. First, we follow developments of aperture NSOM and apertureless NSOM, and then address progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers. Recent achievement of nano-scale Raman and IR spectroscopy will be introduced. Finally, research on nano-optic elements using surface plasmon polariton with NSOM is introduced as an example of NSOM applications to nano-structured metallic surfaces.
93 citations