Mondays at 4:00 PM
Thursdays at 3:45 PM
Coffee and Cookies 15 minutes prior to the colloquium in the lobby by the colloquium location

Locations vary, please see specific dates for location

Winter Quarter Colloquium Chair: Gerald Seidler

Thursday January 7
PAA - Ronald Geballe Auditorium, Rm. A102
Daniel Solli (UCLA)
Title:Real-Time Measurements, Long Tails, and Rogue Events
Abstract: Rare, extreme events wield tremendous influence in many systems. Natural disasters, pandemics, stock market crashes, and freak ocean waves provide striking examples. In some cases, brief, cataclysmic events may be the key factors that shape the long-term evolution of a system. In ultrafast science, extreme events and other transient phenomena are also significant. However, most conventional measurement techniques are unable to record short, non-repetitive events because they do not operate in real time. We have developed a novel technique that enables real-time spectral acquisition over sustained intervals, opening a new window onto the world of rapid, transient phenomena. Using this method, we have recently discovered optical rogue waves—rare, brief pulses of intense light analogous to freak ocean waves. Our measurements show that optical rogue waves follow probability distributions characterized by long tails: extreme events are rare, but more common than expected from Gaussian statistics. Testifying to the potential of extreme-value science for applications, we have harnessed rogue waves to produce an enhanced source of white light. Real-time measurements also hold great promise for studying dynamic processes in other contexts, including biological systems. For example, real-time interrogation has the potential to offer new insights into the mysteries of protein folding, an area of critical importance in basic science and medicine.

Monday January 11
Alipasha Vaziri (Howard Hughes Medical Institute)
PAA - Ronald Geballe Auditorium, Rm. A102
Title:Unlocking the Power of Non-linear and Ultrafast Optics in Biology: Applications to New Tools for Bio-imaging and Neuroscience
Abstract: In the recent years from the intersection of physics and biology, significant advancements in life sciences have emerged. This development has been fueled by two main drivers. On one hand many biological fields such as neuroscience are currently limited by the available tools; hence the development of new physical techniques and methods have enabled new biological discoveries. On the other hand, a physics-based approach to addressing biological questions can lead to an understanding of biological problems on a more fundamental level. In this context I will discuss our recent application of non-linear and ultrafast optics to the development of new techniques in functional and structural bio-imaging and report on two advancements; one in optical super-resolution imaging and another in subcellular optical control of neuronal activity with high temporal precession.

A general feature of most of the super-resolution imaging techniques based on photo-activated localization microscopy (PALM) has been that the imaging depth is limited to a fraction of an optical wavelength. However, to study whole cells, the extension of these methods to a 3D-super-resolution technique is required. We have overcome this limitation by using a two-photon illumination technique called temporal focusing in which the spectral properties of the pulse are used to control its axial intensity distribution in space. Using temporally focused beams we have demonstrated super-resolution imaging in 3D over an axial range of ~10μm in various biological samples.

The combination of genetic tools and optics have for the first time provided a method for cell type specific initiation of neuronal response in vitro and in vivo with a wide range of neurobiological applications. However in almost all studies widefield light sources combined with linear excitation of molecules have been used leading to a spatially unlocalized activation of a large neuronal population. Moreover, excitation via two-photon scanning which usually provides higher optical localization has proven to be challenging for the excitation of Channelrhodopsin, the most widely used light gated ion-channel. Using two-photon “sculpted light” activation of Channelrhodopsin we have demonstrated targeted, single cell and subcellular specific optogenetic initiation of neuronal response. The unprecedented spatial and temporal resolution of this method has a wide range of applications in fundamental neuroscience questions that could have not been addressed until now by the current methods. The extension of light sculpting to more general spatial light distributions in 3D will have further number of important applications in structural and functional bio-imaging.

Thursday January 14
PAA - Rm. A114
Michael Rust (Harvard)
Title:Revealing Biological Mechanisms with Mathematical Modeling and Fluorescence Microscopy
Abstract: In order to understand the mechanisms that give rise to biological function, we must move beyond an inventory of the components involved towards a quantitative picture including both how those components interact dynamically and how they are spatially organized within a cell.  I will describe my efforts to obtain this kind of understanding in a model circadian clock system derived from photosynthetic cyanobacteria.    At the core of this clock are three interacting proteins that generate an autonomous ~24 hour rhythm in phosphorylation of one of the components.  Remarkably, this nonlinear oscillator can be reconstituted in a test tube using only purified proteins.  I will show how a simple mathematical model using kinetic parameters constrained by experimental measurements can quantitatively explain both the origin of stable oscillations and the ability of the oscillator to phase shift in response to a stimulus.  I will conclude by discussing fluorescence techniques I developed as a graduate student for tracking the motion of proteins and resolving their structure in cells at length scales below the diffraction limit of light.  A fusion of these optical methods with mathematical modeling and biochemistry will allow us to bring our understanding of this circadian clock and other biological systems out of the test tube and into the complex environment of a living cell. 

Thursday January 21                    
PAA - A118
Xiaodong Xu (Cornell)
Title: Optically probing Electron Spin and charge in solid state nanostructures
Abstract: The optical control of spin and charge are central elements of optically-driven spintronics and optoelectronics. When electrons are confined in solid state nanostructures or at nanoscale interfaces, it opens up opportunities to discover novel physics and to engineer nanoscale devices that miniaturize spintronics and optoelectronics to the single atom level. In this talk, I will present our recent progress in spintronics and optoelectronics, based on quantum dot (QD) nanostructures and graphene field effect transistors, respectively.

A single spin trapped inside a semiconductor QD is a promising candidate for spin-based electronics and quantum logic. One crucial requirement is a long quantum coherence time. However, the electron spin coherence time deteriorates rapidly due to hyperfine coupling with the nuclear environment in III-V materials. We successfully overcome this obstacle by suppressing the nuclear spin fluctuations while coherently manipulating a single electron spin trapped inside a single QD by coherent population trapping spectroscopy. The discovered dynamic nuclear spin polarization feedback process can enhance the quantum coherence of an electron spin by three orders of magnitude compared to its thermal value.

Graphene, a single atomic membrane formed by carbon atoms, is an interesting potential optoelectronic material with unusual electronic, optical, and thermal properties. There is tremendous interest in graphene-based optoelectronic devices. However, due to the unique Dirac cone electronic structure, the physical mechanism giving rise to the optoelectronic responses of zero-bandgap graphene remains elusive. By designing a novel graphene single-bilayer interface junction, we use a scanning photocurrent microscopy (SPM) to demonstrate that the photo-thermoelectric effect dominates the photocurrent generation. We show that this SPM technique can be used as a local probe of the density of states at novel interface nanostructures.

Monday January 25
Charlie Falco (University of Arizona)
PAA - Ronald Geballe Auditorium, Rm. A102
Title:Metallic Ultra- Thin Films and Nanostructure
Abstract: Developments in ultra-high vacuum technology have made possible the sequential deposition of two or more elements with great regularity and little interdiffusion at the interfaces, resulting in our ability to grow high quality films as thin as a single atomic monolayer.  In a few metallic systems we have achieved long range structural coherence in all three dimensions, allowing us to study new collective magnetic excitations, unusual electronic properties at surfaces and interfaces, elastic property anomalies, dimensionality crossover effects in superconductivity, and x-ray optical properties.

In this talk I will give an introduction to nanostructured materials, along with a description of the Molecular Beam Epitaxy (MBE) process that allows us to grow and characterize metallic ultra-thin films.  Then, following a review of the relevant aspects of magnetism, I will describe several examples of interesting and useful magnetic phenomena exhibited by these materials.

Research supported by U.S. DOE DE-FG02-93ER45488 and ONR/DARPA N00014-02-01-0627.

Thursday January 28
PAA - Rm. A-118
Keji Lai (Stanford)
Title: Microwave microscopy of local conductivity – from nano-devices to phase separation in complex materials
Abstract: The microscopic electrical properties of novel materials are of fundamental importance in nanoscale science and technology. Using shielded cantilever probes and ultra-sensitive microwave electronics, the local dielectric constant and conductivity at 1GHz can be imaged with a spatial resolution (<100nm) less than one millionth of the free space wavelength. We have demonstrated simultaneous topographic and microwave imaging on semiconductor devices, phase change materials, and graphene in different modalities.

A cryogenic version (2 – 300K) of the microwave microscope has also been implemented to study the emergence of microscopic phase separation in colossal magnetoresistive manganites. With increasing magnetic fields, the filamentary metallic domains percolating through the insulating background align preferentially along certain crystal axes of the substrate, suggesting the anisotropic elastic strain as the dominant physical interaction. The microwave images also reveal drastically different domain structures between the zero-field-cool and field-cool processes, consistent with the macroscopic transport measurements.

Monday February 1
PAA - Ronald Geballe Auditorium, Rm. A102
Naomi Ginsburg (UC Berkeley)
Title:Ultrafast physics in photosynthesis: Mapping sub-nanometer energy flow
Abstract: In the first picoseconds of photosynthesis, photoexcitations of chlorophyll molecules are passed through a network of chlorophyll-binding proteins to a charge transfer site, initiating the conversion of absorbed energy to chemical fuels. The remarkably high quantum efficiency of this energy transfer relies on near-field coupling between adjacent chlorophyll molecules and their interaction with protein phonon modes. Using two-dimensional electronic spectroscopy, we track the time-evolution of energy flow in a chlorophyll-protein complex, CP29, found in green plants. The results from these nonlinear four-wave mixing experiments elucidate the role of CP29 as a light harvester and energy conduit by drawing causal relationships between the spatial and electronic configurations of its chlorophyll molecules. Through independent control of experimental light pulse polarizations, we have furthermore developed a technique to determine the relative angles between the transition dip ole moments responsible for energy transfer. This work not only yields tools for structural and spectral molecular characterization, but also deepens our understanding of how photosynthetic systems have evolved to optimize the conversion of light to biomass.

Thursday February 4
PAA - Rm. A-118
David Schuster (Yale)
Title: Hybrid Quantum Information Processing with Circuit QED
Abstract: Quantum computing represents an enormous challenge with the competing requirements of fast manipulation, long storage, and long distance transport of fragile quantum states. Individually these goals have been realized with nanosecond manipulations (quantum circuits), coherence times measured in seconds (atomic ions/nuclear spins), and entanglement transported over kilometers (linear optics). Yet thus far no system has achieved all of the necessary components simultaneously. Just as classical computers have evolved to make use of magnetic, charge, and optical technologies, perhaps the ultimate realization of a quantum computer will also involve hybrid quantum systems. I will describe how superconducting circuits can be used to manipulate single photons, and how they can act as a universal quantum bus to interface with many other physical systems. This hybrid approach can both improve prospects for quantum information science as well as illuminate new physics of the component systems. As a specific example, I will show how superconducting circuits can be coupled to mesoscopic spin ensembles which might serve as a high fidelity quantum memory and also provide a means to access broadband, low temperature (millikelvin), ultra low power (attowatt) electron spin resonance.

Monday February 8
PAA - Ronald Geballe Auditorium, Rm. A102
Paul Wiggins (Massachusetts Institute of Technology)
Title: Structuring a bacterial chromosome
Abstract: The bacterial chromosome is condensed into a compact DNA-protein complex called the nucleoid. It is the nucleoid, not naked DNA, which is the substrate for all genetic processes from gene expression and DNA repair to chromosome replication. The physical structure of chromosomes has functional consequences: It affects gene regulation from the simplest prokaryotes to multicellular organisms. Nucleoid organization also plays a poorly understood yet central role in chromosome segregation. In spite of its biological significance, little is known about the mechanisms that physically organize the chromosome on a cellular-scale. In this talk I will describe work that combines biophysics with more traditional genetics and cell biology techniques to probe the mechanisms of nucleoid organization.

In spite of the common assumption that the interphase chromosome is well-modeled by an unstructured polymer, measurements of the locus positions reveal that the E.coli chromosome is precisely organized into a filament with a linear order. The vast majority of genetic loci are positioned in the cell with a precision of 10% of the cell length, with the exception of loci close to the replication terminus. The measured dependence of the precision of inter-locus distance on genomic distance singles out intra-nucleoid interactions as the mechanism responsible for chromosome organization. From the magnitude of this variance, we infer the existence of an as-yet uncharacterized higher-order DNA organization in bacteria. We demonstrate that both the stochastic and average structure of the nucleoid is captured by a fluctuating elastic filament model. The analysis of mutant strains reveals that the poorly structured terminus region plays a central but unexpected role in the organization of the entire nucleoid filament.

Thursday Feb 11
3:45 PM
PAA - Rm. A-118
Qiang Lin (CalTech)
Title:Manipulating Motion with Light: Cavity Optomechanics in Nanophotonic Structures
Abstract: Optical control of mechanical motion underlies a variety of applications ranging from laser cooling to optical tweezing, playing a critical role across many multidisciplinary fields extending from atomic physics to molecular biology. In this talk, I will discuss our recent progress in enhancing the relatively weak force exerted by photons through specially designed coupled-disk nano-optomechanical structures. The dramatically enhanced optical force introduces strong nonlinear coupling between the mechanical and optical degrees of freedom of nanophotonic cavities, enabling precise engineering of the optomechanical properties and sensitive probing of the mechanical motion through the optical wave dynamics. This provides a new testbed to study fundamental physics such as quantum optomechanical dynamics at the mesoscopic scale, and also allows to realize novel optical functionalities unprecedented by conventional approaches. I will give a brief overview of the current state of optomechanical research, then will move on to discuss our work on how to use this type of force to excite regenerative mechanical oscillation, cool the mechanical thermal Brownian motion, optically modify the mechanical rigidity, and to induce EIT-like coherent mixing of optomechanical excitations, as well as how to utilize these cavity-optomechanical effects for on-chip photonic signal processing such as broadband wavelength routing, fast optical switching, and other applications.

Thursday Feb 18
3:45 PM
PAA - Rm. A-118
Alan Bristow (JILA, UC Boulder)
Title: Coherent light-matter interactions: shining new light on nanotechnology
Abstract: Lasers have revolutionized science and technology, enabling better understanding of light-matter interactions in a variety of media and nanostructures. Ultrafast pulses provide a snapshot of the fundamental electronic dynamics and dephasing in a material, with strong implications for device performance. Tailoring the light field in such experiments allows for coherent control of the light-matter interaction. I demonstrate optical two-dimensional Fourier-transform spectroscopy, which uses phase control of a multi-pulse excitation sequence to isolate the competing excitation pathways and disorder effects in GaAs quantum wells. Separation of these effects is important to questions relating to many-body physics. I also demonstrate quantum interference control of transient currents injected into silicon, which are detected by terahertz emission. This technique shows a novel means of making unstrained and unbiased bulk silicon into an optoelectronic material. Coherent control of light-matter interactions in real or artificial atoms and molecules will benefit the characterizing and functionality of future nanotechnologies.

Monday February 22
PAA - Ronald Geballe Auditorium, Rm. A102
Anand Bhattacharya
Title: Digital Synthesis: A pathway to new materials and novel collective states
Abstract: The complex oxides of the perovskite family have a very diverse range of collective states, with properties that range from High-T_C superconductivity to half metallic ferromagnetism and multiferroicity. These properties arise as a result of correlations and interplay between their electronic, magnetic and lattice degrees of freedom. Using molecular beam epitaxy based techniques, these materials can often be brought together in heterostructures with atomically sharp interfaces. Such interfaces may provide an opportunity for the correlated degrees of freedom in these materials to "reconstruct" in novel ways, giving rise to collective states that might be distinct from those found in either constituent. We have explored this idea in the context of the manganites. LaMnO₃ and SrMnO₃, both antiferromagnetic insulators, are end members of the La_1-xSr_xMnO3 phase diagram which includes a highly spin-polarized ferromagnetic metal and a variety of antiferromagnets. We have synthesized superlattices of (LaMnO3)_p/(SrMnO3)_q where p, q are integers. In this talk, I will discuss the properties of these "digital manganites" where, depending upon the choice of p and q, we can obtain a ferromagnetic metal, interfacial ferromagnetism, and superlattices with enhanced Néel temperatures. We will discuss these results in light of the interfacial effects that may be responsible for the properties of these digital materials.

Monday Mar 1
PAA - Ronald Geballe Auditorium, Rm. A102
Omer Blaes (UCSB)
Title: TBA
Abstract: TBA

Monday Mar 8
PAA - Ronald Geballe Auditorium, Rm. A102
Kai-Mei Fu (HP Labs)
Title: TBA
Abstract: TBA

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