71 research outputs found
Loss of Resolution for the Time Reversal of Waves in Random Underwater Acoustic Channels
In this paper we analyze a time-reversal experiment in a random underwater
acoustic channel. In this kind of waveguide with semi-infinite cross section a
propagating field can be decomposed over three kinds of modes: the propagating
modes, the radiating modes and the evanescent modes. Using an asymptotic
analysis based on a separation of scales technique we derive the asymptotic
form of the the coupled mode power equation for the propagating modes. This
approximation is used to compute the transverse profile of the refocused field
and show that random inhomogeneities inside the waveguide deteriorate the
spatial refocusing. This result, in an underwater acoustic channel context, is
in contradiction with the classical results about time-reversal experiment in
other configurations, for which randomness in the propagation medium enhances
the refocusing.Comment: 31 pages, 11 figure
Quantum interferometry with three-dimensional geometry
Quantum interferometry uses quantum resources to improve phase estimation
with respect to classical methods. Here we propose and theoretically
investigate a new quantum interferometric scheme based on three-dimensional
waveguide devices. These can be implemented by femtosecond laser waveguide
writing, recently adopted for quantum applications. In particular, multiarm
interferometers include "tritter" and "quarter" as basic elements,
corresponding to the generalization of a beam splitter to a 3- and 4-port
splitter, respectively. By injecting Fock states in the input ports of such
interferometers, fringe patterns characterized by nonclassical visibilities are
expected. This enables outperforming the quantum Fisher information obtained
with classical fields in phase estimation. We also discuss the possibility of
achieving the simultaneous estimation of more than one optical phase. This
approach is expected to open new perspectives to quantum enhanced sensing and
metrology performed in integrated photonic.Comment: 7 pages (+4 Supplementary Information), 5 figure
Coupled paraxial wave equations in random media in the white-noise regime
In this paper the reflection and transmission of waves by a three-dimensional
random medium are studied in a white-noise and paraxial regime. The limit
system derives from the acoustic wave equations and is described by a coupled
system of random Schr\"{o}dinger equations driven by a Brownian field whose
covariance is determined by the two-point statistics of the fluctuations of the
random medium. For the reflected and transmitted fields the associated Wigner
distributions and the autocorrelation functions are determined by a closed
system of transport equations. The Wigner distribution is then used to describe
the enhanced backscattering phenomenon for the reflected field.Comment: Published in at http://dx.doi.org/10.1214/08-AAP543 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Local field topology behind light localization and metamaterial topological transitions
We revisit the mechanisms governing the sub-wavelength spatial localization
of light in surface plasmon polariton (SPP) modes by investigating both local
and global features in optical powerflow at SPP frequencies. Close inspection
of the instantaneous Poynting vector reveals formation of optical vortices -
localized areas of cyclic powerflow - at the metal-dielectric interface. As a
result, optical energy circulates through a subwavelength-thick 'conveyor belt'
between the metal and dielectric where it creates a high density of optical
states (DOS), tight optical energy localization, and low group velocity
associated with SPP waves. The formation of bonding and anti-bonding SPP modes
in metal-dielectric-metal waveguides can also be conveniently explained in
terms of different spatial arrangements of localized powerflow vortices between
two metal interfaces. Finally, we investigate the underlying mechanisms of
global topological transitions in metamaterials composed of multiple metal and
dielectric films, i.e., transitions of their iso-frequency surfaces from
ellipsoids to hyperboloids, which are not accompanied by the breaking of
lattice symmetry. Our analysis reveals that such global topological transitions
are governed by the dynamic local re-arrangement of local topological features
of the optical interference field, such as vortices and saddle points, which
reconfigures global optical powerflow within the metamaterial. These new
insights into plasmonic light localization and DOS manipulation not only help
to explain the well-known properties of SPP waves but also provide useful
guidelines for the design of plasmonic components and materials for a variety
of practical applications.Comment: 25 pages, 9 figures, Ch. 8 of Singular and Chiral Nanoplasmonics
(S.V. Boriskina and N.I. Zheludev Eds.) Pan Stanford, Singapore, 201
Optical super-resolution and periodical focusing effects by dielectric microspheres
Optical microscopy is one of the oldest and most important imaging techniques; however, its far-field resolution is diffraction-limited. In this dissertation, we proposed and developed a novel method of optical microscopy with super-resolution by using high- index dielectric microspheres immersed in liquid and placed on the surface of the structures under study. We used barium titanate glass microspheres with diameters of D~2-220 µm and refractive indices n~1.9-2.1 to discern minimal feature sizes ~?/4 (down to ~?/7) of various photonic and plasmonic nanostructures, where ? is the illumination wavelength. We studied the magnification, field of view, and resolving power, in detail, as a function of sphere sizes.
We studied optical coupling, transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index
n=v3, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster’s angle form periodically focused modes with radial polarization and 2D period, where D is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization- dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory.
We expanded the concept of periodically focused modes to design a practical device for ultra-precise contact-mode laser tissue-surgery, with self-limiting ablation depth for potential application in retina surgery. By integrating arrays of dielectric spheres with infrared hollow waveguides and fibers, we fabricated prototypes of the designs and tested them with an Er:YAG laser. Furthermore, we proposed another design based on conical arrays of dielectric spheres to increase the coupling efficiency of the probe
Enhancing the Resolution of Imaging Systems by Spatial Spectrum Manipulation
Much research effort has been spent in the 21st century on superresolution imaging techniques, methods which can beat the diffraction limit. Subwavelength composite structures called ``metamaterials had initially shown great promise in superresolution imaging applications in the early 2000s, owing to their potential for nearly arbitrary capabilities in controlling light. However, for optical frequencies they are often plagued by absorption and scattering losses which can decay or destroy their interesting properties. Similar issues limit the application of other superresolution devices operating as effective media, or metal films that can transfer waves with large momentum by supporting surface plasmon polaritons. In this dissertation, new methods of mitigating the loss of object information in lossy and noisy optical imaging systems are presented. The result is an improvement in the upper bound on lateral spatial resolution. A concentration is placed on metamaterial and plasmonic imaging systems, and the same methods are subsequently adapted to more conventional far-field imaging systems. First, through numerical simulation it is shown that a lossy metamaterial lens has degraded imaging performance which can be partially compensated by deconvolution post-processing of the resultant image. This post-processing procedure is then shown to emulate a physical process called plasmon injection, which has been previously implemented to effectively remove the losses in a plasmonic metamaterial. Next, a more realistic scenario is considered; a thin film of silver acting as a near-field plasmonic ``superlens. In this case, methods are implemented to model incoherent light propagation so that the image can be reconstructed using only intensity data, removing the need for phase measurement. The same procedure from above is followed, and the resolution is enhanced. To push the resolution further, a spatial filtering method called active convolved illumination is developed to overcome the resolution limit set by the noise floor of the system. Finally, the spatial filtering methods are applied to more a more conventional far-field imaging system. Supported by experiment, the lateral resolution of a low numerical aperture imaging system is improved by blocking photons at the Fourier plane. For coherent light, a diffractive superlens is designed which uses the same principles from the above theory, except it encodes the high spatial frequency waves into propagating waves via a diffraction grating. The result is lateral resolution performance that surpasses similar previously published devices by 10 nm at a wavelength more than 80 nm longer
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