2,130 research outputs found
Acoustic confinement and Stimulated Brillouin Scattering in integrated optical waveguides
We examine the effect of acoustic mode confinement on Stimulated Brillouin
Scattering in optical waveguides that consist of a guiding core embedded in a
solid substrate. We find that SBS can arise due to coupling to acoustic modes
in three different regimes. First, the acoustic modes may be guided by total
internal reflection; in this case the SBS gain depends directly on the degree
of confinement of the acoustic mode in the core, which is in turn determined by
the acoustic V-parameter. Second, the acoustic modes may be leaky, but may
nevertheless have a sufficiently long lifetime to have a large effect on the
SBS gain; the lifetime of acoustic modes in this regime depends not only on the
contrast in acoustic properties between the core and the cladding, but is also
highly dependent on the waveguide dimensions. Finally SBS may occur due to
coupling to free modes, which exist even in the absence of acoustic
confinement; we find that the cumulative effect of coupling to these
non-confined modes results in significant SBS gain. We show how the different
acoustic properties of core and cladding lead to these different regimes, and
discuss the feasibility of SBS experiments using different material systems
Optimizing optical Bragg scattering for single-photon frequency conversion
We develop a systematic theory for optimising single-photon frequency
conversion using optical Bragg scattering. The efficiency and phase-matching
conditions for the desired Bragg scattering conversion as well as spurious
scattering and modulation instability are identified. We find that third-order
dispersion can suppress unwanted processes, while dispersion above the fourth
order limits the maximum conversion efficiency. We apply the optimisation
conditions to frequency conversion in highly nonlinear fiber, silicon nitride
waveguides and silicon nanowires. Efficient conversion is confirmed using full
numerical simulations. These design rules will assist the development of
efficient quantum frequency conversion between multicolour single photon
sources for integration in complex quantum networks.Comment: 9 pages, 14 figure
Gap soliton formation by nonlinear supratransmission in Bragg media
A Bragg medium in the nonlinear Kerr regime, submitted to incident
cw-radiation at a frequency in a band gap, switches from total reflection to
transmission when the incident energy overcomes some threshold. We demonstrate
that this is a result of nonlinear supratransmission, which allows to prove
that i) the threshold incident amplitude is simply expressed in terms of the
deviation from the Bragg resonance, ii) the process is not the result of a
shift of the gap in the nonlinear dispersion relation, iii) the transmission
does occur by means of gap soliton trains, as experimentally observed [D.
Taverner et al., Opt Lett 23 (1998) 328], iv) the required energy tends to zero
close to the band edge.Comment: 5 figures, submitted to EuroPhysics Letter
The Zero Age Main Sequence of WIMP burners
We modify a stellar structure code to estimate the effect upon the main
sequence of the accretion of weakly interacting dark matter onto stars and its
subsequent annihilation. The effect upon the stars depends upon whether the
energy generation rate from dark matter annihilation is large enough to shut
off the nuclear burning in the star. Main sequence WIMP burners look much like
protostars moving on the Hayashi track, although they are in principle
completely stable. We make some brief comments about where such stars could be
found, how they might be observed and more detailed simulations which are
currently in progress. Finally we comment on whether or not it is possible to
link the paradoxically young OB stars found at the galactic centre with WIMP
burners.Comment: 4 pages, 3 figs. Matches published versio
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