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rss_1.0 Recursos de colección

Caltech Authors (106,101 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 46,698

1. Impacts of ocean albedo alteration on Arctic sea ice restoration and Northern Hemisphere climate - Cvijanovic, Ivana; Caldeira, Ken; MacMartin, Douglas G.
The Arctic Ocean is expected to transition into a seasonally ice-free state by mid-century, enhancing Arctic warming and leading to substantial ecological and socio-economic challenges across the Arctic region. It has been proposed that artificially increasing high latitude ocean albedo could restore sea ice, but the climate impacts of such a strategy have not been previously explored. Motivated by this, we investigate the impacts of idealized high latitude ocean albedo changes on Arctic sea ice restoration and climate. In our simulated 4xCO_2 climate, imposing surface albedo alterations over the Arctic Ocean leads to partial sea ice recovery and a modest...

2. An In Vitro Enzymatic Assay to Measure Transcription Inhibition by Gallium(III) and H_3 5,10,15-tris(pentafluorophenyl)corroles - Tang, Grace Y.; Pribisko, Melanie A.; Henning, Ryan K.; Lim, Punnajit; Termini, John; Gray, Harry B.; Grubbs, Robert H.
Chemotherapy often involves broad-spectrum cytotoxic agents with many side effects and limited targeting. Corroles are a class of tetrapyrrolic macrocycles that exhibit differential cytostatic and cytotoxic properties in specific cell lines, depending on the identities of the chelated metal and functional groups. The unique behavior of functionalized corroles towards specific cell lines introduces the possibility of targeted chemotherapy. Many anticancer drugs are evaluated by their ability to inhibit RNA transcription. Here we present a step-by-step protocol for RNA transcription in the presence of known and potential inhibitors. The evaluation of the RNA products of the transcription reaction by gel electrophoresis and UVVis spectroscopy provides information...

3. Apodized phase mask coronagraphs for arbitrary apertures II. Comprehensive review of solutions for the vortex coronagraph - Carlotti, A.; Pueyo, L.; Mawet, D.
Context. With a clear circular aperture, the vortex coronagraph perfectly cancels an on-axis point source and offers a 0.9 or 1.75λ/D inner working angle for topological charge 2 or 4, respectively. Current and near-future large telescopes are on-axis, however, and the diffraction effects of the central obscuration, and the secondary supports are strong enough to prevent the detection of companions 10^(-3)–10^(-5) as bright as, or fainter than, their host star. Aims. Recent advances show that a ring apodizer can restore the performance of this coronagraph by compensating for the diffraction effects of a circular central obscuration in a 1D modeling of...

4. On Advanced Estimation Techniques for Exoplanet Detection and Characterization Using Ground-based Coronagraphs - Lawson, Peter R.; Poyneer, Lisa; Barrett, Harrison; Frazin, Richard; Caucci, Luca; Devaney, Nicholas; Furenlid, Lars; Gładysz, Szymon; Guyon, Olivier; Krist, John; Maire, Jérôme; Marois, Christian; Mawet, Dimitri; Mouillet, David; Mugnier, Laurent; Pearson, Iain; Perrin, Marshall; Pueyo, Laurent; Savransky, Dmitry
The direct imaging of planets around nearby stars is exceedingly difficult. Only about 14 exoplanets have been imaged to date that have masses less than 13 times that of Jupiter. The next generation of planet-finding coronagraphs, including VLT-SPHERE, the Gemini Planet Imager, Palomar P1640, and Subaru HiCIAO have predicted contrast performance of roughly a thousand times less than would be needed to detect Earth-like planets. In this paper we review the state of the art in exoplanet imaging, most notably the method of Locally Optimized Combination of Images (LOCI), and we investigate the potential of improving the detectability of faint...

5. Review of small-angle coronagraphic techniques in the wake of ground-based second-generation adaptive optics systems - Mawet, Dimitri; Lawson, Peter; Traub, Wesley; Trauger, John; Serabyn, Eugene; Krist, John; Mennesson, Bertrand; Wallace, James K.
Small-angle coronagraphy is technically and scientifically appealing because it enables the use of smaller telescopes, allows covering wider wavelength ranges, and potentially increases the yield and completeness of circumstellar environment – exoplanets and disks – detection and characterization campaigns. However, opening up this new parameter space is challenging. Here we will review the four posts of high contrast imaging and their intricate interactions at very small angles (within the first 4 resolution elements from the star). The four posts are: choice of coronagraph, optimized wavefront control, observing strategy, and post-processing methods. After detailing each of the four foundations, we will...

6. Hybrid quantum nanophotonic devices for coupling to rare-earth ions - Miyazono, Evan; Hartz, Alex; Zhong, Tian; Faraon, Andrei
With an assortment of narrow line-width transitions spanning the visible and IR spectrum and long spin coherence times, rare-earth doped crystals are the leading material system for solid-state quantum memories. Integrating these materials in an on-chip optical platform would create opportunities for highly integrated light-matter interfaces for quantum communication and quantum computing. Nano-photonic resonators with high quality factors and small mode volumes are required for efficient on-chip coupling to the small dipole moment of rare-earth ion transitions. However, direct fabrication of optical cavities in these crystals with current nanofabrication techniques is difficult and unparallelized, as either exotic etch chemistries or...

7. Tunable and quantum metaphotonics (Presentation Video) - Atwater, Harry A.
Progress in understanding resonant subwavelength structures has fueled an explosion of interest in fundamental processes and nanophotonic devices. The carrier density and optical properties of photonic nanostructures are typically fixed at the time of fabrication, but field effect tuning of the potential and carrier density enables the photonic dispersion to be altered, yielding new approaches to energy conversion and tunable radiative emission. Electrochemical in metals yields tunable resonances and reveals the plasmoelectric effect, a newly-discovered photoelectrochemical potential. Finally, while plasmons are usually described in a classical electromagnetic theory context, under single photon excitation quantum coherent states emerge. We demonstrate entanglement or coherent superposition states of single plasmons using...

8. Energy flows in graphene: hot carrier dynamics and cooling - Song, Justin C. W.; Levitov, Leonid S.
Long lifetimes of hot carriers can lead to qualitatively new types of responses in materials. The magnitude and time scales for these responses reflect the mechanisms governing energy flows. We examine the microscopics of two processes which are key for energy transport, focusing on the unusual behavior arising due to graphene's unique combination of material properties. One is hot carrier generation in its photoexcitation dynamics, where hot carriers multiply through an Auger type carrier–carrier scattering cascade. The hot-carrier generation manifests itself through elevated electronic temperatures which can be accessed in a variety of ways, in particular optical conductivity measurements. Another...

9. The Spitzer Archival Far-Infrared Extragalactic Survey - Hanish, D. J.; Capak, P.; Teplitz, H. I.; Desai, V.; Armus, L.; Brinkworth, C.; Brooke, T.; Colbert, J.; Edwards, L.; Fadda, D.; Frayer, D.; Huynh, M.; Lacy, M.; Murphy, E.; Noriega-Crespo, A.; Paladini, R.; Scarlata, C.; Shenoy, S.
We present the Spitzer Archival Far-InfraRed Extragalactic Survey (SAFIRES). This program produces refined mosaics and source lists for all far-infrared (FIR) extragalactic data taken during the more than six years of the cryogenic operation of the Spitzer Space Telescope. The SAFIRES products consist of FIR data in two wavelength bands (70 and 160 μm) across approximately 180 square degrees of sky, with source lists containing far-infrared fluxes for almost 40,000 extragalactic point sources. Thus, SAFIRES provides a large, robust archival far-infrared data set suitable for many scientific goals.

10. The Cobalt Hydride that Never Was: Revisiting Schrauzer’s “Hydridocobaloxime” - Lacy, David C.; Roberts, Gerri M.; Peters, Jonas C.
Molecular cobalt-dmg (dmg = dimethylglyoxime) complexes are an important class of electrocatalysts used heavily in mechanistic model studies of the hydrogen evolution reaction (HER). Schrauzer’s early isolation of a phosphine-stabilized “[H-Co^(III)(dmgH)_2P(nBu)_3]” complex has long provided circumstantial support for the plausible intermediacy of Co(III)-H species in HER by cobaloximes in solution. Our investigation of this complex has led to a reassignment of its structure as [Co^(II)(dmgH)_2P(nBu)_3], a complex that contains no hydride ligand and dimerizes to form an unsupported Co–Co bond in the solid state. A paramagnetic S = 3/2 impurity that forms during the synthesis of [Co^(II)(dmgH)_2P(nBu)_3] when exposed to...

11. Interannual Variability in the Large-Scale Dynamics of the South Asian Summer Monsoon - Walker, Jennifer M.; Bordoni, Simona; Schneider, Tapio
This study identifies coherent and robust large-scale atmospheric patterns of interannual variability of the South Asian summer monsoon (SASM) in observational data. A decomposition of the water vapor budget into dynamic and thermodynamic components shows that interannual variability of SASM net precipitation (P − E) is primarily caused by variations in winds rather than in moisture. Linear regression analyses reveal that strong monsoons are distinguished from weak monsoons by a northward expansion of the cross-equatorial monsoonal circulation, with increased precipitation in the ascending branch. Interestingly, and in disagreement with the view of monsoons as large-scale sea-breeze circulations, strong monsoons are...

12. Topological polaritons and excitons in garden-variety systems - Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Timothy C. H.
We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.

13. Unraveling bed slope from relative roughness in initial sediment motion - Prancevic, Jeff P.; Lamb, Michael P.
Understanding incipient sediment transport is crucial for predicting landscape evolution, mitigating flood hazards, and restoring riverine habitats. Observations show that the critical Shields stress increases with increasing channel bed slope, and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms, particle interlocking across the channel width, and large bed sediment relative to flow depth (relative roughness). Here we use scaled flume experiments with variable channel widths, bed slopes, and particle densities to separate these effects which otherwise covary in natural streams. The critical Shields stress increased with bed slope for both natural gravel (ρ_s = 2.65 g/cm^3) and...

14. Influence of Organic Structure Directing Agent Isomer Distribution on the Synthesis of SSZ-39 - Dusselier, Michiel; Schmidt, Joel E.; Moulton, Roger; Haymore, Barry; Hellums, Mark; Davis, Mark E.
The aluminosilicate molecular sieve with the AEI framework topology (SSZ-39) is currently of great interest for use in a number of important applications such as exhaust gas NO_x reduction and the methanol-to-olefins reaction. It is likely that advances in the synthesis of this molecular sieve will be needed for applications to proceed. Here, dimethylpiperidine based organic structure directing agents (OSDAs) are used to prepare SSZ-39, and the influence of diastereo- and structural isomeric mixtures on the synthesis of SSZ-39 is reported. Although differences in the rates of molecular sieve formation as well as preferential isomer incorporation occur, the synthesis of...

15. Coronagraphic Imaging of Debris Disks From a High Altitude Balloon Platform - Unwin, Stephen; Traub, Wesley; Bryden, Geoffrey; Brugarolas, Paul; Chen, Pin; Guyon, Olivier; Hillenbrand, Lynne; Krist, John; Macintosh, Bruce; Mawet, Dimitri; Mennesson, Bertrand; Moody, Dwight; Roberts, Lewis C., Jr.; Stapelfeldt, Karl; Stuchlik, David; Trauger, John; Vasisht, Gautam
Debris disks around nearby stars are tracers of the planet formation process, and they are a key element of our understanding of the formation and evolution of extrasolar planetary systems. With multi-color images of a significant number of disks, we can probe important questions: can we learn about planetary system evolution; what materials are the disks made of; and can they reveal the presence of planets? Most disks are known to exist only through their infrared flux excesses as measured by the Spitzer Space Telescope, and through images measured by Herschel. The brightest, most extended disks have been imaged with...

16. Revisiting tectonic corrections applied to Pleistocene sea-level highstands - Creveling, Jessica R.; Mitrovica, Jerry X.; Hay, Carling C.; Austermann, Jacqueline; Kopp, Robert E.
Tectonic displacement contaminates estimates of peak eustatic sea level (and, equivalently, minimum continental ice volumes) determined from the elevation of Quaternary interglacial highstand markers. For sites at which a stratigraphic or geomorphic marker of peak Marine Isotope Stage (MIS) 5e sea level exists, the standard approach for estimating local tectonic uplift (or subsidence) rates takes the difference between the elevation of the local highstand marker and a reference MIS 5e eustatic value, commonly chosen as +6 m, and divides by the age of the marker. The resulting rate is then applied to correct the elevation of all other local observed...

17. Complex apodization Lyot coronagraphy for the direct imaging of exoplanet systems: design, fabrication, and laboratory demonstration - Trauger, John; Moody, Dwight; Gordon, Brian; Krist, John; Mawet, Dimitri
We review the design, fabrication, performance, and future prospects for a complex apodized Lyot coronagraph for highcontrast exoplanet imaging and spectroscopy. We present a newly designed circular focal plane mask with an inner working angle of 2.5 λ/D. Thickness-profiled metallic and dielectric films superimposed on a glass substrate provide control over both the real and imaginary parts of the coronagraph wavefront. Together with a deformable mirror for control of wavefront phase, the complex Lyot coronagraph potentially exceeds billion-to-one contrast over dark fields extending to within angular separations of 2.5 λ/D from the central star, over spectral bandwidths of 20% or...

18. Stratigraphy of Aeolis Dorsa, Mars: Stratigraphic context of the great river deposits - Kite, Edwin S.; Howard, Alan D.; Lucas, Antoine S.; Armstrong, John C.; Aharonson, Oded; Lamb, Michael P.
Unraveling the stratigraphic record is the key to understanding ancient climate and past climate changes on Mars (Grotzinger, J. et al. [2011]. Astrobiology 11, 77–87). Stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers. Therefore, river deposits when placed in stratigraphic order could constrain the number, magnitudes, and durations of the wettest (and presumably most habitable) climates in Mars history. We use crosscutting relationships to establish the stratigraphic context of river and alluvial-fan deposits in the Aeolis Dorsa sedimentary basin, 10°E of Gale crater. At Aeolis...

19. Conceptual study for a sub-pupil instrument having 4 high order adaptive optics paths for parallel multi-wavelength high contrast imaging, and medium resolution spectrometry - Gonte, Frederic Y. J.; Bourget, Pierre; Girard, Julien; Haguenauer, Pierre; Mawet, Dimitri
We present the concept of an instrument that will create 4 circular sub-pupils of 3 m in diameter. Each sub-pupil path will be corrected by a high order adaptive optics system (SR~80% in H) without spider and M2 obstruction. These four independent channels, obviously all pointed towards the same field, allows the possibility of covering totally different parts of the electromagnetic spectrum simultaneously without compromising Signal to Noise Ratio. Each channel can be dedicated to very specialized but complementary purposes: high contrast imaging, pseudo-wide field imaging, high precision multi-color photometry, medium-resolution spectroscopy, polarimetry and sparse-aperture masking.

20. Alternative hypotheses for making the Moon - Stevenson, David
Stevenson replies: I thank these letter writers for their alternative suggestions. Actually, neither is really new, and my failure to mention them—or others—is not because I was unaware of their existence, but rather because of the major challenges that these alternatives must overcome. In the Lagrange point scenario, which is widely known in the community, the challenge is to devise a story in which such bodies naturally arise in the context of a model that explains the planetary system, not just Earth–Moon. It is not sufficient to postulate them. A new paper1 might suggest that the similarity of the impacting...

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