411 research outputs found
Revealing Carrier-Envelope Phase through Frequency Mixing and Interference in Frequency Resolved Optical Gating
We demonstrate that full temporal characterisation of few-cycle
electromagnetic pulses, including retrieval of the carrier envelope phase
(CEP), can be directly obtained from Frequency Resolved Optical Gating (FROG)
techniques in which the interference between non-linear frequency mixing
processes is resolved. We derive a framework for this scheme, defined Real
Domain-FROG (ReD-FROG), as applied to the cases of interference between sum and
difference frequency components and between fundamental and sum/difference
frequency components. A successful numerical demonstration of ReD-FROG as
applied to the case of a self-referenced measurement is provided. A
proof-of-principle experiment is performed in which the CEP of a single-cycle
THz pulse is accurately obtained and demonstrates the possibility for THz
detection beyond the bandwidth limitations of electro-optic sampling.Comment: 7 pages, 4 figures. To be submitted for publication in Optics
Express, January 201
The time resolved measurement of ultrashort THz-band electric fields without an ultrashort probe
The time-resolved detection of ultrashort pulsed THz-band electric field
temporal profiles without an ultrashort laser probe is demonstrated. A
non-linear interaction between a narrow-bandwidth optical probe and the THz
pulse transposes the THz spectral intensity and phase information to the
optical region, thereby generating an optical pulse whose temporal electric
field envelope replicates the temporal profile of the real THz electric field.
This optical envelope is characterised via an autocorrelation based FROG
measurement, hence revealing the THz temporal profile. The combination of a
narrow-bandwidth, long duration, optical probe and self-referenced FROG makes
the technique inherently immune to timing jitter between the optical probe and
THz pulse, and may find particular application where the THz field is not
initially generated via ultrashort laser methods, such as the measurement of
longitudinal electron bunch profiles in particle accelerators.Comment: 7 pages, 3 figures, submitted to AP
Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4-xNdxTi3O12 films
Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4-xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization 2P(r) of 12 muC/cm(2) and coercive field E-c of 120 kV/cm in a (118)-oriented film; and 2P(r)=40 muC/cm(2), E-c=50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction. (C) 2003 American Institute of Physics
Multi-Channel Transport in Disordered Medium under Generic Scattering Conditions
Our study of the evolution of transmission eigenvalues, due to changes in
various physical parameters in a disordered region of arbitrary dimensions,
results in a generalization of the celebrated DMPK equation. The evolution is
shown to be governed by a single complexity parameter which implies a deep
level of universality of transport phenomena through a wide range of disordered
regions. We also find that the interaction among eigenvalues is of many body
type that has important consequences for the statistical behavior of transport
properties.Comment: 19 Pages, No Figure
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
Tilted Pulse-Front Phase-matching in Three Dimensions:Overcoming The Cherenkov Angle Restrictions
We consider the non-linear generation of THz with tilted pulse-fronts in three dimensions and show that, contrary to the widely held expectations, coherent phase matching can be obtained for pulse-front tilt angles other the Cherenkov angle
Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I.Observational Results
We present optical spectra and emission-line ratios of 12 Narrow-Line Seyfert
1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A
common feature in the EUV continuum of active galactic nuclei is the big blue
bump (BBB), generally associated with thermal accretion disk emission. While
Galactic absorption prevents direct access to the EUV range, it can be mapped
by measuring the strength of a variety of forbidden optical emission lines that
respond to different EUV continuum regions. We find that narrow emission-line
ratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and
[SII]6716,6731 indicate no significant difference between NLS1s and Broad-Line
Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy
distributions of their ionizing EUV - soft X-ray continua are similar. The
relative strength of important forbidden high ionization lines like [NeV]3426
compared to HeII4686 and the relative strength of [FeX]6374 appear to show the
same range as in BLS1 galaxies. However, a trend of weaker
F([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared to
BLS1s. To recover the broad emission-line profiles we used Gaussian components.
This approach indicates that the broad Hbeta profile can be well described with
a broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component
(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typical
range of normal BLS1s. The emission-line flux that is associated with the broad
component in these NLS1s amounts to at least 60% of the total flux. Thus it
dominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the
Astrophys.Journa
Optically Selected BL Lacertae Candidates from the Sloan Digital Sky Survey Data Release Seven
We present a sample of 723 optically selected BL Lac candidates from the SDSS
DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample
constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac
candidate has a high-quality SDSS spectrum from which we determine
spectroscopic redshifts for ~60% of the objects. Redshift lower limits are
estimated for the remaining objects utilizing the lack of host galaxy flux
contamination in their optical spectra; we find that objects lacking
spectroscopic redshifts are likely at systematically higher redshifts.
Approximately 80% of our BL Lac candidates match to a radio source in
FIRST/NVSS, and ~40% match to a ROSAT X-ray source. The homogeneous
multiwavelength coverage allows subdivision of the sample into 637 radio-loud
BL Lac candidates and 86 weak-featured radio-quiet objects. The radio-loud
objects broadly support the standard paradigm unifying BL Lac objects with
beamed radio galaxies. We propose that the majority of the radio-quiet objects
may be lower-redshift (z<2.2) analogs to high-redshift weak line quasars (i.e.,
AGN with unusually anemic broad emission line regions). These would constitute
the largest sample of such objects, being of similar size and complementary in
redshift to the samples of high-redshift weak line quasars previously
discovered by the SDSS. However, some fraction of the weak-featured radio-quiet
objects may instead populate a rare and extreme radio-weak tail of the much
larger radio-loud BL Lac population. Serendipitous discoveries of unusual white
dwarfs, high-redshift weak line quasars, and broad absorption line quasars with
extreme continuum dropoffs blueward of rest-frame 2800 Angstroms are also
briefly described.Comment: 24 pages, 14 figures, 8 tables. Accepted for publication in A
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
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