252 research outputs found
Nonlinear Impurity Modes in Homogeneous and Periodic Media
We analyze the existence and stability of nonlinear localized waves described
by the Kronig-Penney model with a nonlinear impurity. We study the properties
of such waves in a homogeneous medium, and then analyze new effects introduced
by periodicity of the medium parameters. In particular, we demonstrate the
existence of a novel type of stable nonlinear band-gap localized states, and
also reveal an important physical mechanism of the oscillatory wave
instabilities associated with the band-gap wave resonances.Comment: 11 pages, 3 figures; To be published in: Proceedings of the NATO
Advanced Research Workshop "Nonlinearity and Disorder: Theory and
Applications" (Tashkent, 2-6 Oct, 2000) Editors: P.L. Christiansen and F.K.
Abdullaev (Kluwer, 2001
Optical parametric oscillation with distributed feedback in cold atoms
There is currently a strong interest in mirrorless lasing systems, in which
the electromagnetic feedback is provided either by disorder (multiple
scattering in the gain medium) or by order (multiple Bragg reflection). These
mechanisms correspond, respectively, to random lasers and photonic crystal
lasers. The crossover regime between order and disorder, or correlated
disorder, has also been investigated with some success. Here, we report
one-dimensional photonic-crystal lasing (that is, distributed feedback lasing)
with a cold atom cloud that simultaneously provides both gain and feedback. The
atoms are trapped in a one-dimensional lattice, producing a density modulation
that creates a strong Bragg reflection with a small angle of incidence. Pumping
the atoms with auxiliary beams induces four-wave mixing, which provides
parametric gain. The combination of both ingredients generates a mirrorless
parametric oscillation with a conical output emission, the apex angle of which
is tunable with the lattice periodicity
Polarization of tightly focused laser beams
The polarization properties of monochromatic light beams are studied. In
contrast to the idealization of an electromagnetic plane wave, finite beams
which are everywhere linearly polarized in the same direction do not exist.
Neither do beams which are everywhere circularly polarized in a fixed plane. It
is also shown that transversely finite beams cannot be purely transverse in
both their electric and magnetic vectors, and that their electromagnetic energy
travels at less than c. The electric and magnetic fields in an electromagnetic
beam have different polarization properties in general, but there exists a
class of steady beams in which the electric and magnetic polarizations are the
same (and in which energy density and energy flux are independent of time).
Examples are given of exactly and approximately linearly polarized beams, and
of approximately circularly polarized beams.Comment: 9 pages, 6 figure
Substrate-based atom waveguide using guided two-color evanescent light fields
We propose a dipole-force linear waveguide which confines neutral atoms up to
lambda/2 above a microfabricated single-mode dielectric optical guide. The
optical guide carries far blue-detuned light in the horizontally-polarized TE
mode and far red-detuned light in the vertically-polarized TM mode, with both
modes close to optical cut-off. A trapping minimum in the transverse plane is
formed above the optical guide due to the differing evanescent decay lengths of
the two modes. This design allows manufacture of mechanically stable
atom-optical elements on a substrate. We calculate the full vector bound modes
for an arbitrary guide shape using two-dimensional non-uniform finite elements
in the frequency-domain, allowing us to optimize atom waveguide properties. We
find that a rectangular optical guide of 0.8um by 0.2um carrying 6mW of total
laser power (detuning +-15nm about the D2 line) gives a trap depth of 200uK for
cesium atoms (m_F = 0), transverse oscillation frequencies of f_x = 40kHz and
f_y = 160kHz, collection area ~ 1um^2 and coherence time of 9ms. We discuss the
effects of non-zero m_F, surface interactions, heating rate, the substrate
refractive index, and the limits on waveguide bending radius.Comment: 12 pages, 4 figures, revtex, submitted to Phys. Rev. A Replaced:
final version accepted by PRA v.61 Feb 2000. (2 paragraphs added
Nonlinear localized waves in a periodic medium
We analyze the existence and stability of nonlinear localized waves in a
periodic medium described by the Kronig-Penney model with a nonlinear defect.
We demonstrate the existence of a novel type of stable nonlinear band-gap
localized states, and also reveal an important physical mechanism of the
oscillatory wave instabilities associated with the band-gap resonances.Comment: 4 pages, 5 figure
Atom-optics hologram in the time domain
The temporal evolution of an atomic wave packet interacting with object and
reference electromagnetic waves is investigated beyond the weak perturbation of
the initial state. It is shown that the diffraction of an ultracold atomic beam
by the inhomogeneous laser field can be interpreted as if the beam passes
through a three-dimensional hologram, whose thickness is proportional to the
interaction time. It is found that the diffraction efficiency of such a
hologram may reach 100% and is determined by the duration of laser pulses. On
this basis a method for reconstruction of the object image with matter waves is
offered.Comment: RevTeX, 13 pages, 8 figures; minor grammatical change
Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser
G. A. Turnbull, P. Andrew, M. J. Jory, William L. Barnes, and I. D. W. Samuel, Physical Review B, Vol. 64, article 125122 (2001). "Copyright © 2001 by the American Physical Society."We present an experimental study of the emission characteristics and photonic band structure of a distributed feedback polymer laser, based on the material poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. We use measurements of the photonic band dispersion to explain how the substrate microstructure modifies both spontaneous and stimulated emission. The lasing structure exhibits a one-dimensional photonic band gap around 610 nm, with lasing occurring at one of the two associated band edges. The band edge (frequency) selection mechanism is found to be a difference in the level of output coupling of the modes associated with the two band edges. This is a feature of the second-order distributed feedback mechanism we have employed and is clearly evident in the measured photonic band structur
Thermalisation of a two-dimensional photonic gas in a 'white-wall' photon box
Bose-Einstein condensation, the macroscopic accumulation of bosonic particles
in the energetic ground state below a critical temperature, has been
demonstrated in several physical systems. The perhaps best known example of a
bosonic gas, blackbody radiation, however exhibits no Bose-Einstein
condensation at low temperatures. Instead of collectively occupying the lowest
energy mode, the photons disappear in the cavity walls when the temperature is
lowered - corresponding to a vanishing chemical potential. Here we report on
evidence for a thermalised two-dimensional photon gas with freely adjustable
chemical potential. Our experiment is based on a dye filled optical
microresonator, acting as a 'white-wall' box for photons. Thermalisation is
achieved in a photon number-conserving way by photon scattering off the
dye-molecules, and the cavity mirrors both provide an effective photon mass and
a confining potential - key prerequisites for the Bose-Einstein condensation of
photons. As a striking example for the unusual system properties, we
demonstrate a yet unobserved light concentration effect into the centre of the
confining potential, an effect with prospects for increasing the efficiency of
diffuse solar light collection.Comment: 15 pages, 3 figure
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