Recursos de colección

Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

Group = Kavli Nanoscience Institute

Mostrando recursos 1 - 20 de 423

  1. Atomic-scale Structural and Chemical Characterization of Hexagonal Boron Nitride Layers Synthesized at the Wafer-Scale with Monolayer Thickness Control

    Lin, Wei-Hsiang; Brar, Victor W.; Jariwala, Deep; Sherrott, Michelle C.; Tseng, Wei-Shiuan; Wu, Chih-I; Yeh, Nai-Chang; Atwater, Harry A.
    Hexagonal boron nitride (h-BN) is a promising two-dimensional insulator with a large band gap and low density of charged impurities that is isostructural and isoelectronic with graphene. Here we report the chemical and atomic-scale structure of CVD-grown wafer-scale (~25 cm2) h-BN sheets ranging in thickness from 1-20 monolayers. Atomic-scale images of h-BN on Au and graphene/Au substrates obtained by scanning tunneling microscopy (STM) reveal high h-BN crystalline quality in monolayer samples. Further characterization of 1-20 monolayer samples indicates uniform thickness for wafer-scale areas; this thickness control is a result of precise control of the precursor flow rate, deposition temperature and...

  2. Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

    Liao, Bolin; Zhao, Huan; Najafi, Ebrahim; Yan, Xiaodong; Tian, He; Tice, Jesse; Minnich, Austin J.; Wang, Han; Zewail, Ahmed H.
    As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphorus (BP) has attracted significant research interest owing to its unique electronic and optoelectronic properties, including its widely tunable bandgap, polarization-dependent photoresponse and highly anisotropic in-plane charge transport. Despite extensive study of the steady-state charge transport in BP, there has not been direct characterization and visualization of the hot carriers dynamics in BP immediately after photoexcitation, which is crucial to understanding the performance of BP-based optoelectronic devices. Here we use the newly developed scanning ultrafast electron microscopy (SUEM) to directly visualize the motion of photoexcited...

  3. Use of Supramolecular Assemblies as Lithographic Resists

    Lewis, Scott M.; Fernandez, Antonio; DeRose, Guy A.; Hunt, Matthew S.; Whitehead, George F. S.; Lagzda, Agnese; Alty, Hayden R.; Ferrando-Soria, Jesus; Varey, Sarah; Kostopoulos, Andreas K.; Schedin, Fredrik; Muryn, Christopher A.; Timco, Grigore A.; Scherer, Axel; Yeates, Stephen G.; Winpenny, Richard E. P.
    A new resist material for electron beam lithography has been created that is based on a supramolecular assembly. Initial studies revealed that with this supramolecular approach, high-resolution structures can be written that show unprecedented selectivity when exposed to etching conditions involving plasmas.

  4. Towards an efficient nanophotonic platform integrating quantum memories and single qubits based on rare-earth ions

    Hasan, Zameer U.; Hemmer, Philip R.; Lee, Hwang; Migdall, Alan L.; Zhong, Tian; Kindem, Jonathan M.; Bartholomew, John G.; Rochman, Jake; Craiciu, Ioana; Miyazono, Evan; Faraon, Andrei
    The integration of rare-earth ions in an on-chip photonic platform would enable quantum repeaters and scalable quantum networks. While ensemble-based quantum memories have been routinely realized, implementing single rare-earth ion qubit remains an outstanding challenge due to its weak photoluminescence. Here we demonstrate a nanophotonic platform consisting of yttrium vanadate (YVO) photonic crystal nanobeam resonators coupled to a spectrally dilute ensemble of Nd ions. The cavity acts as a memory when prepared with spectral hole burning, meanwhile it permits addressing of single ions when high-resolution spectroscopy is employed. For quantum memory, atomic frequency comb (AFC) protocol was implemented in a...

  5. Snowflake Topological Insulator for Sound Waves

    Brendel, Christian; Peano, Vittorio; Painter, Oskar; Marquardt, Florian
    We show how the snowflake phononic crystal structure, which has been realized experimentally recently, can be turned into a topological insulator for sound waves. This idea, based purely on simple geometrical modifications, could be readily implemented on the nanoscale.

  6. Superconducting qubits on silicon substrates for quantum device integration

    Keller, Andrew J.; Dieterle, Paul B.; Fang, Michael; Berger, Brett; Fink, Johannes M.; Painter, Oskar
    We present the fabrication and characterization of transmon qubits formed from aluminum Josephson junctions on two different silicon-based substrates: (i) high-resistivity silicon (Si) and (ii) silicon-on-insulator (SOI). Key to the qubit fabrication process is the use of an anhydrous hydrofluoric vapor process which removes silicon surface oxides without attacking aluminum, and in the case of SOI substrates, selectively removes the lossy buried oxide underneath the qubit region. For qubits with a transition frequency of approximately $5$GHz we find qubit lifetimes and coherence times comparable to those attainable on sapphire substrates ($T_{1,\text{Si}} = 27\mu$s, $T_{2,\text{Si}} = 6.6\mu$s; $T_{1,\text{SOI}} = 3.5\mu$s, $T_{2,\text{SOI}} = 2.2\mu$s). This qubit fabrication process in principle permits co-fabrication of...

  7. High-Resolution X-ray Photoelectron Spectroscopy of Chlorine-Terminated GaAs(111)A Surfaces

    Traub, Matthew C.; Biteen, Julie S.; Michalak, David J.; Webb, Lauren J.; Brunschwig, Bruce S.; Lewis, Nathan S.
    Oxide-terminated and Cl-terminated GaAs(111)A surfaces have been characterized in the As and Ga 3d regions by high-resolution, soft X-ray photoelectron spectroscopy. The Cl-terminated surface, formed by treatment with 6 M HCl(aq), showed no detectable As oxides or As^0 in the As 3d region. The Ga 3d spectrum of the Cl-terminated surface showed a broad, intense signal at 19.4 eV and a smaller signal at 21.7 eV. The Ga 3d peaks were fitted using three species, one representing bulk GaAs and the others representing two chemical species on the surface. The large peak was well-fitted by the bulk GaAs emission and...

  8. Microresonator Brillouin gyroscope

    Li, Jiang; Suh, Myoung-Gyun; Vahala, Kerry
    Optical-based rotation sensors have revolutionized precision, high-sensitivity inertial navigation systems. At the same time these sensors use bulky optical fiber spools or free-space resonators. A chip-based, micro-optical gyroscope is demonstrated that uses counterpropagating Brillouin lasers to measure rotation as a Sagnac-induced frequency shift. Preliminary work has demonstrated a rotation-rate measurement that surpasses prior micro-optical rotation-sensing systems by over 40-fold.

  9. A comparison of the chemical, optical and electrocatalytic properties of water-oxidation catalysts for use in integrated solar-fuel generators

    Sun, Ke; Moreno-Hernandez, Ivan A.; Schmidt, William C.; Zhou, Xinghao; Crompton, J. Chance; Liu, Rui; Saadi, Fadl H.; Chen, Yikai; Papadantonakis, Kimberly M.; Lewis, Nathan S.
    The in situ optical properties and electrocatalytic performance of representative catalysts for the oxygen-evolution reaction (OER) have been considered together to evaluate system-level effects that accompany the integration of OER catalysts into a solar-fuel device driven by a tandem-junction light absorber with a photoanode top cell, i.e., a design that requires incident light to be transmitted through the OER catalyst before reaching a semiconducting light absorber. The relationship between the overpotential and optical transmission of the catalysts determined the attainable efficiencies for integrated solar-fuel devices as well as the optimal band gaps for the photoanode in such devices. The systems...

  10. Unidirectional vertical emission from photonic crystal nanolasers

    Kim, Se-Heon; Lee, Yong-Hee; Huang, Jingqing; Scherer, Axel
    Here, we emphasize the importance of using a bottom reflector to achieve unidirectional vertical emission from an ultra-small light emitter. Specifically, we have considered a photonic crystal slab nanocavity laser placed in the vicinity of a perfect mirror. By choosing near one-wavelength distance between the bottom reflector and the cavity, over 80% of photons generated inside the laser cavity can funnel into a small divergence angle of ±30°. It is also interesting to observe that the natural radiation rate (∼1/Q factor) of the nanocavity mode can be modified by varying the gap size, which is analogous to the famous cavity...

  11. 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...

  12. 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...

  13. 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...

  14. 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...

  15. 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...

  16. 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...

  17. 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 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 as a dispersive wave through a process that is the optical analogue of Cherenkov radiation. Dispersive waves typically consist of an ensemble of optical modes. Here, a limiting case is studied in which the dispersive wave is concentrated into a single cavity mode. In this limit, its interaction with...

  18. 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...

  19. 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...

  20. Patterned photostimulation via visible-wavelength photonic probes for deep brain optogenetics

    Segev, Eran; Reimer, Jacob; Moreaux, Laurent C.; Fowler, Trevor M.; Chi, Derrick; Sacher, Wesley D.; Lo, Maisie; Deisseroth, Karl; Tolias, Andreas S.; Faraon, Andrei; Roukes, Michael L.
    Optogenetic methods developed over the past decade enable unprecedented optical activation and silencing of specific neuronal cell types. However, light scattering in neural tissue precludes illuminating areas deep within the brain via free-space optics; this has impeded employing optogenetics universally. Here, we report an approach surmounting this significant limitation. We realize implantable, ultranarrow, silicon-based photonic probes enabling the delivery of complex illumination patterns deep within brain tissue. Our approach combines methods from integrated nanophotonics and microelectromechanical systems, to yield photonic probes that are robust, scalable, and readily producible en masse. Their minute cross sections minimize tissue displacement upon probe implantation....

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