Recursos de colección

Caltech Authors (143.226 recursos)

Repository of works by Caltech published authors.

Group = Kavli Nanoscience Institute

Mostrando recursos 1 - 20 de 417

  1. Hyper-Selective Plasmonic Color Filters

    Fleischman, Dagny; Sweatlock, Luke A.; Murakami, Hirotaka; Atwater, Harry
    The subwavelength mode volumes of plasmonic filters are well matched to the small size of state-of-the-art active pixels (~ 1 µm) in CMOS image sensor arrays used in portable electronic devices. Typical plasmonic filters exhibit broad (> 100 nm) transmission bandwidths. Dramatically reducing the peak width of filter transmission spectra would allow for the realization of CMOS hyperspectral imaging arrays, which demand the FWHM of transmission peaks to be less than 30 nm. We find that the design of 5 layer metal-insulator-metal-insulator-metal structures gives rise to multi-mode interference phenomena that suppresses spurious transmission features gives rise to a single narrow...

  2. Daytime radiative cooling using near-black infrared emitters

    Kou, Jun-long; Jurado, Zoila; Chen, Zhen; Fan, Shanhui; Minnich, Austin J.
    Recent works have demonstrated that daytime radiative cooling under direct sunlight can be achieved using multilayer thin films designed to emit in the infrared atmospheric transparency window while reflecting visible light. Here, we demonstrate that a polymer-coated fused silica mirror, as a near-ideal blackbody in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight (8.2 °C) and at night (8.4 °C). Its performance exceeds that of a multilayer thin film stack fabricated using vacuum deposition methods by nearly 3 °C. Furthermore, we estimate the cooler has an average net cooling...

  3. Monod-Wyman-Changeux Analysis of Ligand-Gated Ion Channel Mutants

    Einav, Tal; Phillips, Rob
    We present a framework for computing the gating properties of ligand-gated ion channel mutants using the Monod-Wyman-Changeux (MWC) model of allostery. We derive simple analytic formulas for key functional properties such as the leakiness, dynamic range, half-maximal effective concentration ([EC_(50)]), and effective Hill coefficient, and explore the full spectrum of phenotypes that are accessible through mutations. Specifically, we consider mutations in the channel pore of nicotinic acetylcholine receptor (nAChR) and the ligand binding domain of a cyclic nucleotide-gated (CNG) ion channel, demonstrating how each mutation can be characterized as only affecting a subset of the biophysical parameters. In addition, we...

  4. Dynamics of Photo-excited Hot Carriers in Hydrogenated Amorphous Silicon Imaged by 4D Electron Microscopy

    Liao, Bolin; Najafi, Ebrahim; Li, Heng; Minnich, Austin J.; Zewail, Ahmed H.
    The dynamics of charge carriers in amorphous semiconductors fundamentally differ from those in crystalline semiconductors due to the lack of long-range order and the high defect density. Despite intensive technology-driven research interests and the existence of well-established experimental techniques, such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We observe an unexpected regime of fast diffusion immediately after photoexcitation along with...

  5. Excitonic Effects in Emerging Photovoltaic Materials: A Case Study in Cu_2O

    Omelchenko, Stefan T.; Tolstova, Yulia; Atwater, Harry A.; Lewis, Nathan S.
    Excitonic effects account for a fundamental photoconversion and charge transport mechanism in Cu_2O; hence, the universally adopted “free carrier” model substantially underestimates the photovoltaic efficiency for such devices. The quasi-equilibrium branching ratio between excitons and free carriers in Cu_2O indicates that up to 28% of photogenerated carriers during photovoltaic operation are excitons. These large exciton densities were directly observed in photoluminescence and spectral response measurements. The results of a device physics simulation using a model that includes excitonic effects agree well with experimentally measured current–voltage characteristics of Cu_2O-based photovoltaics. In the case of Cu_2O, the free carrier model underestimates the...

  6. Interfacing broadband photonic qubits to on-chip cavity-protected rare-earth ensembles

    Zhong, Tian; Kindem, Jonathan M.; Rochman, Jake; Faraon, Andrei
    Ensembles of solid-state optical emitters enable broadband quantum storage and transduction of photonic qubits, with applications in high-rate quantum networks for secure communications and interconnecting future quantum computers. To transfer quantum states using ensembles, rephasing techniques are used to mitigate fast decoherence resulting from inhomogeneous broadening, but these techniques generally limit the bandwidth, efficiency and active times of the quantum interface. Here, we use a dense ensemble of neodymium rare-earth ions strongly coupled to a nanophotonic resonator to demonstrate a significant cavity protection effect at the single-photon level—a technique to suppress ensemble decoherence due to inhomogeneous broadening. The protected Rabi...

  7. Single-mode dispersive waves and soliton microcomb dynamics

    Yi, Xu; Yang, Qi-Fan; Zhang, Xueyue; Yang, Ki Youl; Vahala, Kerry
    Dissipative Kerr solitons are self-sustaining optical wavepackets in resonators. They use the Kerr nonlinearity to both compensate dispersion and to offset optical loss. Besides providing insights into nonlinear resonator physics, they can be applied in frequency metrology, precision clocks, and spectroscopy. Like other optical solitons, the dissipative Kerr soliton can radiate power in the form of a dispersive wave through a process that is the optical analogue of Cherenkov radiation. Dispersive waves typically consist of an ensemble of optical modes. A limiting case is demonstrated in which the dispersive wave is concentrated into a single cavity mode. In this limit,...

  8. Coherent ultra-violet to near-infrared generation in silica ridge waveguides

    Oh, Dong Yoon; Yang, Ki Youl; Fredrick, Connor; Ycas, Gabriel; Diddams, Scott A.; Vahala, Kerry J.
    Short duration, intense pulses of light can experience dramatic spectral broadening when propagating through lengths of optical fibre. This continuum generation process is caused by a combination of nonlinear optical effects including the formation of dispersive waves. Optical analogues of Cherenkov radiation, these waves allow a pulse to radiate power into a distant spectral region. In this work, efficient and coherent dispersive wave generation of visible to ultraviolet light is demonstrated in silica waveguides on a silicon chip. Unlike fibre broadeners, the arrays provide a wide range of emission wavelength choices on a single, compact chip. This new capability is...

  9. Diamond optomechanical crystals

    Burek, Michael J.; Cohen, Justin D.; Meenehan, Seán M.; El-Sawah, Nayera; Chia, Cleaven; Ruelle, Thibaud; Meesala, Srujan; Rochman, Jake; Atikian, Haig A.; Markham, Matthew; Twitchen, Daniel J.; Lukin, Mikhail D.; Painter, Oskar; Lončar, Marko
    Cavity-optomechanical systems realized in single-crystal diamond are poised to benefit from its extraordinary material properties, including low mechanical dissipation and a wide optical transparency window. Diamond is also rich in optically active defects, such as the nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers, which behave as atom-like systems in the solid state. Predictions and observations of coherent coupling of the NV electronic spin to phonons via lattice strain have motivated the development of diamond nanomechanical devices aimed at the realization of hybrid quantum systems in which phonons provide an interface with diamond spins. In this work, we demonstrate diamond optomechanical crystals...

  10. Stability of slender inverted flags and rods in uniform steady flow

    Sader, John E.; Huertas-Cerdeira, Cecilia; Gharib, Morteza
    Cantilevered elastic sheets and rods immersed in a steady uniform flow are known to undergo instabilities that give rise to complex dynamics, including limit cycle behaviour and chaotic motion. Recent work has examined their stability in an inverted configuration where the flow impinges on the free end of the cantilever with its clamped edge downstream: this is commonly referred to as an ‘inverted flag’. Theory has thus far accurately captured the stability of wide inverted flags only, i.e. where the dimension of the clamped edge exceeds the cantilever length; the latter is aligned in the flow direction. Here, we theoretically...

  11. Silicon-on-insulator-based complementary metal oxide semiconductor integrated optoelectronic platform for biomedical applications

    Mujeeb-U-Rahman, Muhammad; Scherer, Axel
    Microscale optical devices enabled by wireless power harvesting and telemetry facilitate manipulation and testing of localized biological environments (e.g., neural recording and stimulation, targeted delivery to cancer cells). Design of integrated microsystems utilizing optical power harvesting and telemetry will enable complex in vivo applications like actuating a single nerve, without the difficult requirement of extreme optical focusing or use of nanoparticles. Silicon-on-insulator (SOI)-based platforms provide a very powerful architecture for such miniaturized platforms as these can be used to fabricate both optoelectronic and microelectronic devices on the same substrate. Near-infrared biomedical optics can be effectively utilized for optical power harvesting...

  12. Ordering and dimensional crossovers in metallic glasses and liquids

    Chen, David Z.; An, Qi; Goddard, William A., III; Greer, Julia R.
    The atomic-level structures of liquids and glasses are amorphous, lacking long-range order. We characterize the atomic structures by integrating radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: Cu_(46)Zr_(54), Ni_(80)Al_(20), Ni_(33.3)Zr_(66.7), and Pd_(82)Si_(18). Resulting cumulative coordination numbers (CN) show that metallic liquids have a dimension of d=2.55±0.06 from the center atom to the first coordination shell and metallic glasses have d=2.71±0.04, both less than 3. Between the first and second coordination shells, both phases crossover to a dimension of d=3, as for a crystal. Observations from discrete atom center-of-mass position counting are corroborated by...

  13. Gate-Variable Mid-Infrared Optical Transitions in a (Bi_(1-x)Sb_x)_2Te_3 Topological Insulator

    Whitney, William S.; Brar, Victor Watson; Ou, Yunbo; Shao, Yinming; Davoyan, Artur R.; Basov, Dimitri N.; He, Ke; Xue, Qi-Kun; Atwater, Harry A.
    We report mid-infrared spectroscopy measurements of ultrathin, electrostatically gated (Bi_(1-x)Sb_x)_2Te_3 topological insulator films, in which we observe several percent modulation of transmittance and reflectance as gating shifts the Fermi level. Infrared transmittance measurements of gated films were enabled by use of an epitaxial lift-off method for large-area transfer of topological insulator films from infrared-absorbing SrTiO3 growth substrates to thermal oxidized silicon substrates. We combine these optical experiments with transport measurements and angle-resolved photoemission spectroscopy to identify the observed spectral modulation as a gate-driven transfer of spectral weight between both bulk and 2D topological surface channels and interband and intraband channels....

  14. Epitaxy: Programmable Atom Equivalents Versus Atoms

    Wang, Mary X.; Seo, Soyoung E.; Gabrys, Paul A.; Fleischman, Dagny; Lee, Byeongdu; Kim, Youngeun; Atwater, Harry A.; Macfarlane, Robert J.; Mirkin, Chad A.
    The programmability of DNA makes it an attractive structure-directing ligand for the assembly of nanoparticle (NP) superlattices in a manner that mimics many aspects of atomic crystallization. However, the synthesis of multilayer single crystals of defined size remains a challenge. Though previous studies considered lattice mismatch as the major limiting factor for multilayer assembly, thin film growth depends on many interlinked variables. Here, a more comprehensive approach is taken to study fundamental elements, such as the growth temperature and the thermodynamics of interfacial energetics, to achieve epitaxial growth of NP thin films. Both surface morphology and internal thin film structure...

  15. Nonlinear damping and dephasing in nanomechanical systems

    Atalaya, Juan; Kenny, Thomas W.; Roukes, M. L.; Dykman, M. I.
    We present a microscopic theory of nonlinear damping and dephasing of low-frequency eigenmodes in nanomechanical and micromechanical systems. The mechanism of the both effects is scattering of thermally excited vibrational modes off the considered eigenmode. The scattering is accompanied by energy transfer of 2ℏω_0 for nonlinear damping and is quasielastic for dephasing. We develop a formalism that allows studying both spatially uniform systems and systems with a strong nonuniformity, which is smooth on the typical wavelength of thermal modes but is pronounced on their mean free path. The formalism accounts for the decay of thermal modes, which plays a major...

  16. Predicting energy production for multijunction photovoltaics: Effects of spectral variation and cumulative irradiance

    Warmann, Emily C.; Atwater, Harry A.
    Variation of the incident spectrum under real-world illumination conditions can degrade the performance of series connected multijunction solar cells that are optimized for performance under the AM1.5D standard spectrum. Current approaches to correct for this factor and estimate energy production for deployed systems require large amounts of field data and are not useful for evaluating prospective designs. We present a set of 20 spectra that capture the average spectral composition for direct normal irradiance at different cumulative irradiance levels. Combined with the NSRDB information on the frequency of cumulative irradiance levels at locations across the United States, these 20 spectra...

  17. Wave propagation in one-dimensional microscopic granular chains

    Lin, Wei-Hsun; Daraio, Chiara
    We employ noncontact optical techniques to generate and measure stress waves in uncompressed, one-dimensional microscopic granular chains, and support our experiments with discrete numerical simulations. We show that the wave propagation through dry particles (150 μm radius) is highly nonlinear and it is significantly influenced by the presence of defects (e.g., surface roughness, interparticle gaps, and misalignment). We derive an analytical relation between the group velocity and gap size, and define bounds for the formation of highly nonlinear solitary waves as a function of gap size and axial misalignment.

  18. Millivolt modulation of plasmonic metasurface via ionic conductance

    Thyagarajan, Krishnan; Sokhoyan, Ruzan; Zornberg, Leonardo Z.; Atwater, Harry A.
    We report here and experimentally demonstrate an actively controlled gatetunable plasmonic metasurface operating in the visible region of the electromagnetic spectrum, where strikingly the operating voltages for reflectance modulation are much less than 1V. The electrically tunable metasurface consists of inverse dolmen structures (iDolmen) patterned on silver and chromium on a quartz substrate and subsequently covered with a 5 nm thin layer of Al2O_3 followed by a 110 nm indium tin oxide (ITO) layer, which acts as a transparent electrode. Our designed structures show up to 78 percent change in reflection upon applying small voltages (less than 1V). We explain...

  19. Measurement of the Energy-Band Relations of Stabilized Si Photoanodes Using Operando Ambient Pressure X-ray Photoelectron Spectroscopy

    Richter, Matthias Hermann; Lichterman, Michael Frankston; Hu, Shu; Crumlin, Ethan J.; Mayer, Thomas; Axnanda, S.; Favaro, Marco; Drisdell, Walter; Hussain, Z.; Brunschwig, Bruce; Lewis, Nathan S.; Liu, Z.; Lewerenz, Hans Joachim
    The energy-band relations and electronic properties for the light absorber/protection-layer stack of TiO_2-stabilized Si photoanodes have been determined by ambient pressure x-ray synchrotron radiation photoelectron spectroscopy under an applied potential (operando), from single core-level emission lines. The experiments have also been complemented with laboratory-based monochromatic XPS data. Electrochemical parameters are additionally derived directly from x-ray photoemission data, and a method is presented to derive interface-state densities from such operando data.

  20. Measurement of the Energy-Band Relations of Stabilized Si Photoanodes Using Operando Ambient Pressure X-ray Photoelectron Spectroscopy

    Richter, Matthias Hermann; Lichterman, Michael Frankston; Hu, Shu; Crumlin, Ethan J.; Mayer, Thomas; Axnanda, S.; Favaro, Marco; Drisdell, Walter; Hussain, Z.; Brunschwig, Bruce; Lewis, Nathan S.; Liu, Z.; Lewerenz, Hans Joachim
    The energy-band relations and electronic properties for the light absorber/protection-layer stack of TiO_2-stabilized Si photoanodes have been determined by ambient pressure x-ray synchrotron radiation photoelectron spectroscopy under an applied potential (operando), from single core-level emission lines. The experiments have also been complemented with laboratory-based monochromatic XPS data. Electrochemical parameters are additionally derived directly from x-ray photoemission data, and a method is presented to derive interface-state densities from such operando data.

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