355 research outputs found
Blue Light Second Harmonic Generation In The Organic Crystal Ortho-Dicyanovinyl-Anisole
Type I phase matched second harmonic generation (SHG) in the organic crystal ortho-Dicyanovinyl-anisole (DIVA) has been measured for a range of near infrared fundamental wavelengths (855-960 nm). Turning curves for type I phase matched SHG were derived from measured refractive index values and show noncritical phase matching at 860. Measured type I phase matched SHG is in close agreement with the calculated results and gives effective SHG coefficients ranging from 1.9 to 5.9 times as large as d(32) Of potassium niobate. (C) 1997 American Institute of Physics
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Resolving Power of Visible to Near-Infrared Hybrid -Ta/NbTiN Kinetic Inductance Detectors
Kinetic Inductance Detectors (KIDs) are superconducting energy-resolving
detectors, sensitive to single photons from the near-infrared to ultraviolet.
We study a hybrid KID design consisting of a beta phase tantalum (-Ta)
inductor and a NbTiN interdigitated capacitor (IDC). The devices show an
average intrinsic quality factor of 4.3 1.3
. To increase the power captured by the light sensitive inductor,
we 3D-print an array of 150150 m resin micro lenses on the
backside of the sapphire substrate. The shape deviation between design and
printed lenses is smaller than 1m, and the alignment accuracy of this
process is m and
m. We measure a resolving power for 1545-402 nm that is limited to 4.9 by
saturation in the KID's phase response. We can model the saturation in the
phase response with the evolution of the number of quasiparticles generated by
a photon event. An alternative coordinate system that has a linear response
raises the resolving power to 5.9 at 402 nm. We verify the measured resolving
power with a two-line measurement using a laser source and a monochromator. We
discuss several improvements that can be made to the devices on a route towards
KID arrays with high resolving powers.Comment: 11 pages, 9 Figues, Journal Pape
Pointing calibration of GroundBIRD telescope using Moon observation data
Understanding telescope pointing (i.e., line of sight) is important for
observing the cosmic microwave background (CMB) and astronomical objects. The
Moon is a candidate astronomical source for pointing calibration. Although the
visible size of the Moon (\ang{;30}) is larger than that of the planets, we
can frequently observe the Moon once a month with a high signal-to-noise ratio.
We developed a method for performing pointing calibration using observational
data from the Moon. We considered the tilts of the telescope axes as well as
the encoder and collimation offsets for pointing calibration. In addition, we
evaluated the effects of the nonuniformity of the brightness temperature of the
Moon, which is a dominant systematic error. As a result, we successfully
achieved a pointing accuracy of \ang{;3.3}. This is one order of magnitude
smaller than an angular resolution of \ang{;36}. This level of accuracy
competes with past achievements in other ground-based CMB experiments using
observational data from the planets.Comment: 18 pages, 17 figures, 3 table
220 fs Er-Yb:glass laser mode-locked by a broadband low-loss Si/Ge saturable absorber
We demonstrate femtosecond performance of an ultra-broadband
high-index-contrast saturable Bragg reflector consisting of a
silicon/silicon-dioxide/germanium structure that is fully compatible with CMOS
processing. This device offers a reflectivity bandwidth of over 700 nm and
sub-picosecond recovery time of the saturable loss. It is used to achieve
mode-locking of an Er-Yb:glass laser centered at 1540 nm, generating 220 fs
pulses, with the broadest output spectrum to date
Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots
We report on the impulsive generation of coherent optical phonons in
CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe
experiments using femtosecond laser pulses were performed by tuning the laser
central energy to resonate with the absorption edge of the nanocrystals. We
identify two longitudinal optical phonons, one longitudinal acoustic phonon and
a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the
optical phonons as a function of the laser central energy exhibits a resonance
that is well described by a model based on impulsive stimulated Raman
scattering. The phases of the coherent phonons reveal coupling between
different modes. At low power density excitations, the frequency of the optical
coherent phonons deviates from values obtained from spontaneous Raman
scattering. This behavior is ascribed to the presence of electronic impurity
states which modify the nanocrystal dielectric function and, thereby, the
frequency of the infrared-active phonons
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