The Everhart Lecture Series is a forum to encourage interdisciplinary interaction among graduate students and faculty, to share ideas about recent research developments, problems and controversies, and to recognize the exemplary presentation and research abilities of Caltech's graduate students. Lecturers discuss scientific topics at a level suitable for graduate students and faculty from all fields while addressing current research issues.
Each fall, three graduate-student lecturers are selected to present their work as part of the Everhart Lecture Series based on each student's:
Speakers receive a $500 honorarium and recognition at graduation. Streaming video recordings of previous Everhart lectures can be viewed online at the Caltech Today Theater.
Applications are currently being accepted for the 2010 Everhart Lecture Series.
Complete an application form along with a brief, sealed letter of recommendation. Letters should comment on the nominee's research and speaking skills, as well as state how long and in what capacity you have known the nominee. Feel free to nominate a graduate student regardless of whether they are presently completing an application. Self-nominations are also allowed.
Applications must include six copies of the following:
Applications will be accepted through
Send applications and nominations to:
Everhart Lecture Series
c/o Gregory Ogin
MC 103-33
Please email the ELS committee chair, Gregory Ogin, for inquiries.
All enrolled graduate students may apply, including those not planning to graduate this year.
Making an Escape: Neural Control and Biomechanics of Flight Initiation in Drosophila
In response to an approaching predator, the fruit fly Drosophila melanogaster quickly initiates flight. This rapid take-off is believed to be a reflex coordinated by a pair of giant interneurons (giant fibers, GFs). However, it has been difficult to evoke escapes in wild-type flies using only visual stimuli, and thus flight initiation behavior in the unrestrained wild-type fly is poorly described. We have taken advantage of recent advances in high-speed videography to capture video sequences of fruit fly flight initiation at the temporal resolution of 6,000 frames per second. Three-dimensional kinematic analysis of take-off sequences indicates that flies can produce two different kinds of takeoff. During voluntary takeoffs, early wing elevation leads to slower and more stable initial flight. In contrast, during visually elicited escapes, the wings are pulled down close to the body during the takeoff jump, resulting in fast but unsteady flights in which the fly rotates rapidly about all three of its body axes. Thus, we find that the two types of Drosophila flight initiation result in different flight performances once the fly is airborne, and that these performances are distinguished by a trade-off between speed and stability. We also found that flies can use visual information to plan a jump directly away from a looming threat. This is surprising, given the simple architecture of the giant fiber pathway thought to mediate escape. We found that approximately 200 ms before take-off, flies begin a series of postural adjustments that determine the direction of their escape. These movements position their center of mass so that leg extension will push them away from the expanding visual stimulus. These preflight movements are not the result of a simple feed-forward motor program because their magnitude and direction depend on the flies' initial postural state. Furthermore, flies plan a take-off direction even in instances when they choose not to jump. This sophisticated motor program is evidence for a form of rapid, visually mediated motor planning in a genetically accessible model organism.
Looking Beyond the Cosmological Horizon
The Universe is 13.7 billion years old. This finite age implies that our observable Universe is bounded by a cosmological horizon; light from the other side of this horizon has not yet had time to reach us. Our observable Universe was remarkably homogenous in its infancy, and the standard explanation for this initial homogeneity is that a period of extremely rapid expansion immediately after the Big Bang pushed all remnants of the Universe’s violent birth outside our cosmological horizon, leaving our observable Universe smooth and featureless. This cosmic growth spurt is called inflation, and while inflation successfully explains several properties of the Universe, it creates as many questions as it answers. What caused inflation? How long did it last? What was there before inflation? Inflation hides the answers to these questions beyond our cosmological horizon.
I will present a brief history of the Universe, focusing on the evidence for inflation. I will then show how a puzzling asymmetry in the cosmic microwave background can be interpreted as a signature of pre-inflationary remnants lurking beyond the cosmological horizon. The average amplitude of temperature fluctuations in the cosmic microwave background in one hemisphere of the sky is significantly larger than the average amplitude in the opposite hemisphere. My collaborators and I have shown that structure outside the horizon can generate this asymmetry. I will conclude by showing how future probes of the cosmic microwave background will further test the theory of inflation in general and our proposed origin of the asymmetry in particular.
Rolling Out the Solar Carpet: Microwire Solar Cells in Flexible Polymer Layers
Despite high public demand for clean energy alternatives to eliminate our addiction to greenhouse gas emitting fossil fuels, the price of these technologies relative to oil and coal has prevented their widespread implementation. Solar energy in particular has enormous potential as a carbon-free resource but is several times the cost of electricity produced from coal. One of the main reasons for this is that photovoltaics of practical efficiency require high-quality, pure semiconductor materials as their absorber layers. In a typical planar junction solar cell, a photogenerated electron or hole created deep within the material must be able to travel all the way to the junction without recombining so that it can be collected to produce current. Radial junction wire array solar cells, on the other hand, have the potential to decouple the directions of light absorption and charge carrier collection so that a semiconductor with a carrier diffusion length shorter than its absorption depth (i.e. – a lower quality, cheaper material) can still effectively produce current. In this case light would be absorbed along the longer axial dimension of the semiconductor wires while the resulting electrons or holes would be collected along the much shorter radial dimension in a massively parallel array resembling carpet fibers on a microscale, hence the term “solar carpet.”
In working towards the creation of inexpensively processable radial junction solar cells, highly ordered Si wire arrays grown on a single crystal wafer were transferred into a transparent, flexible polymer matrix. Arrays of vertically oriented Si wires were grown on Si wafers by photolithographically patterning a catalyst metal onto an oxide buffer layer, followed by vapor-liquid-solid (VLS) growth with SiCl4 gas. The wires were then cast in polydimethylsiloxane (PDMS), a low cost polymer, which was subsequently cured and mechanically separated from the substrate. The resulting wire/polymer composite layers were highly flexible while still maintaining the array alignment and fidelity. Furthermore, the single crystal growth substrate was demonstrated to be reusable by chemically regenerating the patterned oxide template and electrodepositing fresh catalyst metal into it. This scheme has the potential to enable the economic, roll-to-roll processing of wire array solar cells by incorporating inorganic semiconductor material grown on a recyclable substrate into an inexpensive, flexible organic layer. Preliminary studies on the current-voltage characteristics of these Si wire array/polymer composite films have been conducted by employing them as photoelectrochemical solar cells.
Looking for more examples? Past Everhart lectures can be found here.
Sponsored by the Graduate Student Council and the Caltech Student Affairs