8,161 research outputs found
Ultrafast and octave-spanning optical nonlinearities from strongly phase-mismatched cascaded interactions
Cascaded nonlinearities have attracted much interest, but ultrafast
applications have been seriously hampered by the simultaneous requirements of
being near phase-matching and having ultrafast femtosecond response times. Here
we show that in strongly phase-mismatched nonlinear frequency conversion
crystals the pump pulse can experience a large and extremely broadband
self-defocusing cascaded Kerr-like nonlinearity. The large cascaded
nonlinearity is ensured through interaction with the largest quadratic tensor
element in the crystal, and the strong phase-mismatch ensures an ultrafast
nonlinear response with an octave-spanning bandwidth. We verify this
experimentally by showing few-cycle soliton compression with noncritical
cascaded second-harmonic generation: Energetic 47 fs infrared pulses are
compressed in a just 1-mm long bulk lithium niobate crystal to 17 fs (under 4
optical cycles) with 80% efficiency, and upon further propagation an
octave-spanning supercontinuum is observed. Such ultrafast cascading is
expected to occur for a broad range of pump wavelengths spanning the near- and
mid-IR using standard nonlinear crystals.Comment: resubmitted, revised version, accepted for Phys. Rev. Let
Imprints of a Primordial Preferred Direction on the Microwave Background
Rotational invariance is a well-established feature of low-energy physics.
Violations of this symmetry must be extremely small today, but could have been
larger in earlier epochs. In this paper we examine the consequences of a small
breaking of rotational invariance during the inflationary era when the
primordial density fluctuations were generated. Assuming that a fixed-norm
vector picked out a preferred direction during the inflationary era, we explore
the imprint it would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients. We suggest that it is natural to expect
that the imprint on the primordial power spectrum of a preferred spatial
direction is approximately scale-invariant, and examine a simple model in which
this is true.Comment: 7 pages, no figures; v5: Corrections, as well as use of more standard
convention, in section I
Investigation of atomic oxygen-surface interactions related to measurements with dual air density explorer satellites
For a number of candidate materials of construction for the dual air density explorer satellites the rate of oxygen atom loss by adsorption, surface reaction, and recombination was determined as a function of surface and temperature. Plain aluminum and anodized aluminum surfaces exhibit a collisional atom loss probability alpha .01 in the temperature range 140 - 360 K, and an initial sticking probability. For SiO coated aluminum in the same temperature range, alpha .001 and So .001. Atom-loss on gold is relatively rapid alpha .01. The So for gold varies between 0.25 and unity in the temperature range 360 - 140 K
Scalar Representations and Minimal Flavor Violation
We discuss the representations that new scalar degrees of freedom (beyond
those in the minimal standard model) can have if they couple to quarks in a way
that is consistent with minimal flavor violation. If the new scalars are
singlets under the flavor group then they must be color singlets or color
octets. In this paper we discuss the allowed representations and renormalizable
couplings when the new scalars also transform under the flavor group. We find
that color \bar{3} and 6 representations are also allowed. We focus on the
cases where the new scalars can have renormalizable Yukawa couplings to the
quarks without factors of the quark Yukawa matrices. The renormalizable
couplings in the models we introduce automatically conserve baryon number.Comment: 18 pages, 2 figures V2: Lepton MFV protection of baryon number
discusse
Dark Matter, Baryon Asymmetry, and Spontaneous B and L Breaking
We investigate the dark matter and the cosmological baryon asymmetry in a
simple theory where baryon (B) and lepton (L) number are local gauge symmetries
that are spontaneously broken. In this model, the cold dark matter candidate is
the lightest new field with baryon number and its stability is an automatic
consequence of the gauge symmetry. Dark matter annihilation is either through a
leptophobic gauge boson whose mass must be below a TeV or through the Higgs
boson. Since the mass of the leptophobic gauge boson has to be below the TeV
scale one finds that in the first scenario there is a lower bound on the
elastic cross section of about 5x10^{-46} cm^2. Even though baryon number is
gauged and not spontaneously broken until the weak scale, a cosmologically
acceptable baryon excess is possible. There is tension between achieving both
the measured baryon excess and the dark matter density.Comment: 23 pages, 5 figures; revised version, typos removed, references
added, discussion expande
Development of a simulator for studying simplified lunar escape systems
Design and development of lunar escape system simulator for investigation of lunar escape problems and simplified manual guidance and control for lunar escape vehicle
Trispectrum versus Bispectrum in Single-Field Inflation
In the standard slow-roll inflationary cosmology, quantum fluctuations in a
single field, the inflaton, generate approximately Gaussian primordial density
perturbations. At present, the bispectrum and trispectrum of the density
perturbations have not been observed and the probability distribution for these
perturbations is consistent with Gaussianity. However, Planck satellite data
will bring a new level of precision to bear on this issue, and it is possible
that evidence for non-Gaussian effects in the primordial distribution will be
discovered. One possibility is that a trispectrum will be observed without
evidence for a non-zero bispectrum. It is not difficult for this to occur in
inflationary models where quantum fluctuations in a field other than the
inflaton contribute to the density perturbations. A natural question to ask is
whether such an observation would rule out the standard scenarios. We explore
this issue and find that it is possible to construct single-field models in
which inflaton-generated primordial density perturbations have an observable
trispectrum, but a bispectrum that is too small to be observed by the Planck
satellite. However, an awkward fine tuning seems to be unavoidable.Comment: 15 pages, 3 figures; journal versio
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