11,998 research outputs found
Coupling ideality of integrated planar high-Q microresonators
Chipscale microresonators with integrated planar optical waveguides are
useful building blocks for linear, nonlinear and quantum optical devices. Loss
reduction through improving fabrication processes has resulted in several
integrated micro resonator platforms attaining quality (Q) factors of several
millions. However only few studies have investigated design-dependent losses,
especially with regard to the resonator coupling section. Here we investigate
design-dependent parasitic losses, described by the coupling ideality, of the
commonly employed microresonator design consisting of a microring resonator
waveguide side-coupled to a straight bus waveguide. By systematic
characterization of multi-mode high-Q silicon nitride microresonator devices,
we show that this design can suffer from low coupling ideality. By performing
full 3D simulations to numerically investigate the resonator to bus waveguide
coupling, we identify the coupling to higher-order bus waveguide modes as the
dominant origin of parasitic losses which lead to the low coupling ideality.
Using suitably designed bus waveguides, parasitic losses are mitigated, and a
nearly unity ideality and strong overcoupling (i.e. a ratio of external
coupling to internal resonator loss rate > 9) are demonstrated. Moreover we
find that different resonator modes can exchange power through the coupler,
which therefore constitutes a mechanism that induces modal coupling, a
phenomenon known to distort resonator dispersion properties. Our results
demonstrate the potential for significant performance improvements of
integrated planar microresonators, achievable by optimized coupler designs.Comment: 8 pages, 3 figures, 1 tabl
Initial data for Einstein's equations with superposed gravitational waves
A method is presented to construct initial data for Einstein's equations as a
superposition of a gravitational wave perturbation on an arbitrary stationary
background spacetime. The method combines the conformal thin sandwich formalism
with linear gravitational waves, and allows detailed control over
characteristics of the superposed gravitational wave like shape, location and
propagation direction. It is furthermore fully covariant with respect to
spatial coordinate changes and allows for very large amplitude of the
gravitational wave.Comment: Version accepted by PRD; added convergence plots, expanded
discussion. 9 pages, 9 figure
Factorizing twists and R-matrices for representations of the quantum affine algebra U_q(\hat sl_2)
We calculate factorizing twists in evaluation representations of the quantum
affine algebra U_q(\hat sl_2). From the factorizing twists we derive a
representation independent expression of the R-matrices of U_q(\hat sl_2).
Comparing with the corresponding quantities for the Yangian Y(sl_2), it is
shown that the U_q(\hat sl_2) results can be obtained by `replacing numbers by
q-numbers'. Conversely, the limit q -> 1 exists in representations of U_q(\hat
sl_2) and both the factorizing twists and the R-matrices of the Yangian Y(sl_2)
are recovered in this limit.Comment: 19 pages, LaTe
Numerical simulations of neutron star-black hole binaries in the near-equal-mass regime
Simulations of neutron star-black hole (NSBH) binaries generally consider
black holes with masses in the range , where we expect to find
most stellar mass black holes. The existence of lower mass black holes,
however, cannot be theoretically ruled out. Low-mass black holes in binary
systems with a neutron star companion could mimic neutron star-neutron (NSNS)
binaries, as they power similar gravitational wave (GW) and electromagnetic
(EM) signals. To understand the differences and similarities between NSNS
mergers and low-mass NSBH mergers, numerical simulations are required. Here, we
perform a set of simulations of low-mass NSBH mergers, including systems
compatible with GW170817. Our simulations use a composition and temperature
dependent equation of state (DD2) and approximate neutrino transport, but no
magnetic fields. We find that low-mass NSBH mergers produce remnant disks
significantly less massive than previously expected, and consistent with the
post-merger outflow mass inferred from GW170817 for moderately asymmetric mass
ratio. The dynamical ejecta produced by systems compatible with GW170817 is
negligible except if the mass ratio and black hole spin are at the edge of the
allowed parameter space. That dynamical ejecta is cold, neutron-rich, and
surprisingly slow for ejecta produced during the tidal disruption of a neutron
star : . We also find that the final mass of the remnant
black hole is consistent with existing analytical predictions, while the final
spin of that black hole is noticeably larger than expected -- up to for our equal mass case
Large second harmonic generation enhancement in SiN waveguides by all-optically induced quasi phase matching
Integrated waveguides exhibiting efficient second-order nonlinearities are
crucial to obtain compact and low power optical signal processing devices.
Silicon nitride (SiN) has shown second harmonic generation (SHG) capabilities
in resonant structures and single-pass devices leveraging intermodal phase
matching, which is defined by waveguide design. Lithium niobate allows
compensating for the phase mismatch using periodically poled waveguides,
however the latter are not reconfigurable and remain difficult to integrate
with SiN or silicon (Si) circuits. Here we show the all-optical enhancement of
SHG in SiN waveguides by more than 30 dB. We demonstrate that a Watt-level
laser causes a periodic modification of the waveguide second-order
susceptibility. The resulting second order nonlinear grating has a periodicity
allowing for quasi phase matching (QPM) between the pump and SH mode. Moreover,
changing the pump wavelength or polarization updates the period, relaxing phase
matching constraints imposed by the waveguide geometry. We show that the
grating is long term inscribed in the waveguides, and we estimate a second
order nonlinearity of the order of 0.3 pm/V, while a maximum conversion
efficiency (CE) of 1.8x10-6 W-1 cm-2 is reached
Probing the loss origins of ultra-smooth integrated photonic waveguides
On-chip optical waveguides with low propagation losses and precisely
engineered group velocity dispersion (GVD) are important to nonlinear photonic
devices such as soliton microcombs. Yet, despite intensive research efforts,
nonlinear integrated photonic platforms still feature propagation losses orders
of magnitude higher than in standard optical fiber. The tight confinement and
high index contrast of integrated waveguides make them highly susceptible to
fabrication induced surface roughness. Therefore, microresonators with
ultra-high Q factors are, to date, only attainable in polished bulk
crystalline, or chemically etched silica based devices, that pose however
challenges for full photonic integration. Here, we demonstrate the fabrication
of silicon nitride () waveguides with unprecedentedly smooth
sidewalls and tight confinement with record low propagation losses. This is
achieved by combining the photonic Damascene process with a novel reflow
process, which reduces etching roughness, while sufficiently preserving
dimensional accuracy. This leads to previously unattainable \emph{mean}
microresonator Q factors larger than for tightly confining
waveguides with anomalous dispersion. Via systematic process step variation and
two independent characterization techniques we differentiate the scattering and
absorption loss contributions, and reveal metal impurity related absorption to
be an important loss origin. Although such impurities are known to limit
optical fibers, this is the first time they are identified, and play a tangible
role, in absorption of integrated microresonators. Taken together, our work
provides new insights in the origins of propagation losses in
waveguides and provides the technological basis for
integrated nonlinear photonics in the ultra-high Q regime
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
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