2,286 research outputs found
High-Q-factor Al [subscript 2]O[subscript 3] micro-trench cavities integrated with silicon nitride waveguides on silicon
We report on the design and performance of high-Q integrated optical micro-trench cavities on silicon. The microcavities are co-integrated with silicon nitride bus waveguides and fabricated using wafer-scale silicon-photonics-compatible processing steps. The amorphous aluminum oxide resonator material is deposited via sputtering in a single straightforward post-processing step. We examine the theoretical and experimental optical properties of the aluminum oxide micro-trench cavities for different bend radii, film thicknesses and near-infrared wavelengths and demonstrate experimental Q factors of > 10[superscript 6]. We propose that this high-Q micro-trench cavity design can be applied to incorporate a wide variety of novel microcavity materials, including rare-earth-doped films for microlasers, into wafer-scale silicon photonics platforms
Integrated TiO2 resonators for visible photonics
We demonstrate waveguide-coupled titanium dioxide (TiO2) racetrack resonators
with loaded quality factors of 2x10^4 for the visible wavelengths. The
structures were fabricated in sputtered TiO2 thin films on oxidized silicon
substrates using standard top-down nanofabrication techniques, and passively
probed in transmission measurements using a tunable red laser. Devices based on
this material could serve as integrated optical elements as well as passive
platforms for coupling to visible quantum emitters.Comment: 4 pages, 3 figure
Subwavelength grating metamaterial waveguides and ring resonators on a silicon nitride platform
We propose and demonstrate subwavelength grating (SWG) metamaterial
waveguides and ring resonators on a silicon nitride platform for the first
time. The SWG waveguide is engineered such that a large overlap of 53% of the
Bloch mode with the top cladding material is achieved, demonstrating excellent
potential for applications in evanescent field sensing and light amplification.
The devices, which have critical dimensions greater than 100 nm, are fabricated
using a commercial rapid turn-around silicon nitride prototyping foundry
process using electron beam lithography. Experimental characterization of the
fabricated device reveals excellent ring resonator internal quality factor
(2.11x10^5) and low propagation loss (~1.5 dB/cm) in the C-band, a significant
improvement of both parameters compared to silicon based SWG ring resonators.
These results demonstrate the promising prospects of SWG metamaterial
structures for silicon nitride based photonic integrated circuits.Comment: 12 pages, 7 figures, submitted to Laser & Photonics Reviews for
publicatio
Stimulated Brillouin scattering in tellurite-covered silicon nitride waveguides
Stimulated Brillouin scattering (SBS), a coherent nonlinear effect coupling
acoustics and optics, can be used in a wide range of applications such as
Brillouin lasers and tunable narrowband RF filtering. Wide adoption of such
technologies however, would need a balance of strong Brillouin interaction and
low optical loss in a structure compatible with large scale fabrication.
Achieving these characteristics in scalable platforms such as silicon and
silicon nitride remains a challenge. Here, we investigate a scalable Brillouin
platform combining low loss SiN and tellurium oxide (TeO)
exhibiting strong Brillouin response and enhanced acoustic confinement. In this
platform we measure a Brillouin gain coefficient of 8.5~mW,
exhibiting a twenty fold improvement over the largest previously reported
Brillouin gain in a SiN platform. Further, we demonstrate cladding
engineering to control the strength of the Brillouin interaction. We utilized
the Brillouin gain and loss resonances in this waveguide for an RF photonic
filter with more than 15 dB rejection and 250 MHz linewidth. Finally, we
present a pathway by geometric optimization and cladding engineering to a
further enhancement of the gain coefficient to 155~mW, a
potential 400 times increase in the Brillouin gain coefficient
Identifying Basal Ganglia Divisions in Individuals Using Resting-State Functional Connectivity MRI
Studies in non-human primates and humans reveal that discrete regions (henceforth, “divisions”) in the basal ganglia are intricately interconnected with regions in the cerebral cortex. However, divisions within basal ganglia nuclei (e.g., within the caudate) are difficult to identify using structural MRI. Resting-state functional connectivity MRI (rs-fcMRI) can be used to identify putative cerebral cortical functional areas in humans (Cohen et al., 2008). Here, we determine whether rs-fcMRI can be used to identify divisions in individual human adult basal ganglia. Putative basal ganglia divisions were generated by assigning basal ganglia voxels to groups based on the similarity of whole-brain functional connectivity correlation maps using modularity optimization, a network analysis tool. We assessed the validity of this approach by examining the spatial contiguity and location of putative divisions and whether divisions’ correlation maps were consistent with previously reported patterns of anatomical and functional connectivity. Spatially constrained divisions consistent with the dorsal caudate, ventral striatum, and dorsal caudal putamen could be identified in each subject. Further, correlation maps associated with putative divisions were consistent with their presumed connectivity. These findings suggest that, as in the cerebral cortex, subcortical divisions can be identified in individuals using rs-fcMRI. Developing and validating these methods should improve the study of brain structure and function, both typical and atypical, by allowing for more precise comparison across individuals
CMOS-compatible 75 mW erbium-doped distributed feedback laser
On-chip, high-power, erbium-doped distributed feedback lasers are demonstrated in a CMOS-compatible fabrication flow. The laser cavities consist of silicon nitride waveguide and grating features, defined by wafer-scale immersion lithography and an erbium-doped aluminum oxide layer deposited as the final step in the fabrication process. The large mode size lasers demonstrate single-mode continuous wave operation with a maximum output power of 75 mW without any thermal damage. The laser output power does not saturate at high pump intensities and is, therefore, capable of delivering even higher on-chip signals if a stronger pump is utilized. The amplitude noise of the laser is investigated and the laser is shown to be stable and free from self-pulsing when the pump power is sufficiently above threshold. Efficient, low noise, narrow band and stable on-chip lasers are essential for a variety of important applications, ranging from integrated analog photonics and microwave generation to coherent communications and light detection and ranging (LiDAR). Recently, two methods have shown the greatest promise for delivering high performance, integrated silicon-compatible lasers; namely, hybrid integration of silicon-on-oxide (SOI) waveguides with III-V semiconductor gain media [1-3], and erbium/ ytterbium-doped glasses on silico
The Sloan Digital Sky Survey Reverberation Mapping Project: Technical Overview
The Sloan Digital Sky Survey Reverberation Mapping project (SDSS-RM) is a
dedicated multi-object RM experiment that has spectroscopically monitored a
sample of 849 broad-line quasars in a single 7 deg field with the SDSS-III
BOSS spectrograph. The RM quasar sample is flux-limited to i_psf=21.7 mag, and
covers a redshift range of 0.1<z<4.5. Optical spectroscopy was performed during
2014 Jan-Jul dark/grey time, with an average cadence of ~4 days, totaling more
than 30 epochs. Supporting photometric monitoring in the g and i bands was
conducted at multiple facilities including the CFHT and the Steward Observatory
Bok telescopes in 2014, with a cadence of ~2 days and covering all lunar
phases. The RM field (RA, DEC=14:14:49.00, +53:05:00.0) lies within the CFHT-LS
W3 field, and coincides with the Pan-STARRS 1 (PS1) Medium Deep Field MD07,
with three prior years of multi-band PS1 light curves. The SDSS-RM 6-month
baseline program aims to detect time lags between the quasar continuum and
broad line region (BLR) variability on timescales of up to several months (in
the observed frame) for ~10% of the sample, and to anchor the time baseline for
continued monitoring in the future to detect lags on longer timescales and at
higher redshift. SDSS-RM is the first major program to systematically explore
the potential of RM for broad-line quasars at z>0.3, and will investigate the
prospects of RM with all major broad lines covered in optical spectroscopy.
SDSS-RM will provide guidance on future multi-object RM campaigns on larger
scales, and is aiming to deliver more than tens of BLR lag detections for a
homogeneous sample of quasars. We describe the motivation, design and
implementation of this program, and outline the science impact expected from
the resulting data for RM and general quasar science.Comment: 25 pages, submitted to ApJS; project website at http://www.sdssrm.or
A surface mooring for air–sea interaction research in the Gulf Stream. Part II : analysis of the observations and their accuracies
Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 39 (2013): 450–469, doi:10.1175/JTECH-D-12-00078.1.A surface mooring was deployed in the Gulf Stream for 15 months to investigate the role of air–sea interaction in mode water formation and other processes. The accuracies of the near-surface meteorological and oceanographic measurements are investigated. In addition, the impacts of these measurement errors on the estimation and study of the air–sea fluxes in the Gulf Stream are discussed. Pre- and postdeployment calibrations together with in situ comparison between shipboard and moored sensors supported the identification of biases due to sensor drifts, sensor electronics, and calibration errors. A postdeployment field study was used to further investigate the performance of the wind sensors. The use of redundant sensor sets not only supported the filling of data gaps but also allowed an examination of the contribution of random errors. Air–sea fluxes were also analyzed and computed from both Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parameterization and using direct covariance measurements. The basic conclusion is that the surface buoy deployed in the Gulf Stream to support air–sea interaction research was successful, providing an improved 15-month record of surface meteorology, upper-ocean variability, and air–sea fluxes with known accuracies. At the same time, the coincident deployment of mean meteorological and turbulent flux sensors proved to be a successful strategy to certify the validity of the bulk formula fluxes over the midrange of wind speeds and to support further work to address the present shortcomings of the bulk formula methods at the low and high wind speeds.The National Science Foundation
(Grant OCE04-24536) funded this work, as part
of the CLIVAR Mode Water Dynamics Experiment
(CLIMODE). The Vetlesen Foundation is also acknowledged
for the early support of S. Bigorre.2013-09-0
Molecular Adaptation to Folivory and the Conservation Implications for Madagascar’s Lemurs
The lemurs of Madagascar include numerous species characterized by folivory across several families. Many extant lemuriform folivores exist in sympatry in Madagascar’s remaining forests. These species avoid feeding competition by adopting different dietary strategies within folivory, reflected in behavioral, morphological, and microbiota diversity across species. These conditions make lemurs an ideal study system for understanding adaptation to leaf-eating. Most folivorous lemurs are also highly endangered. The significance of folivory for conservation outlook is complex. Though generalist folivores may be relatively well equipped to survive habitat disturbance, specialist folivores occupying narrow dietary niches may be less resilient. Characterizing the genetic bases of adaptation to folivory across species and lineages can provide insights into their differential physiology and potential to resist habitat change. We recently reported accelerated genetic change in RNASE1, a gene encoding an enzyme (RNase 1) involved in molecular adaptation in mammalian folivores, including various monkeys and sifakas (genus Propithecus; family Indriidae). Here, we sought to assess whether other lemurs, including phylogenetically and ecologically diverse folivores, might show parallel adaptive change in RNASE1 that could underlie a capacity for efficient folivory. We characterized RNASE1in 21 lemur species representing all five families and members of the three extant folivorous lineages: (1) bamboo lemurs (family Lemuridae), (2) sportive lemurs (family Lepilemuridae), and (3) indriids (family Indriidae). We found pervasive sequence change in RNASE1 across all indriids, a dN/dS value \u3e 3 in this clade, and evidence for shared change in isoelectric point, indicating altered enzymatic function. Sportive and bamboo lemurs, in contrast, showed more modest sequence change. The greater change in indriids may reflect a shared strategy emphasizing complex gut morphology and microbiota to facilitate folivory. This case study illustrates how genetic analysis may reveal differences in functional traits that could influence species’ ecology and, in turn, their resilience to habitat change. Moreover, our results support the body of work demonstrating that not all primate folivores are built the same and reiterate the need to avoid generalizations about dietary guild in considering conservation outlook, particularly in lemurs where such diversity in folivory has probably led to extensive specialization via niche partitioning
- …