Current Term Colloquium Videos

February 10, 2017

Yi-Hsin Liu , NASA Goddard Space Flight

Title: Magnetic Reconnection in Plasmas  (Video)

Abstract: Magnetic reconnection is the process whereby a change in topology of magnetic field lines allows for a rapid conversion of magnetic energy into thermal and kinetic energy of the surrounding plasma. This physical process plays a key role in many astrophysical and laboratory contexts, ranging from magnetospheric substorms, solar eruptions, sawtooth crashes in fusion devices and potentially the super-flares in Crab Nebula. The recently launched Magnetospheric Multiscale (MMS) mission provides an unprecedented view of the three-dimensional fine-scale structure of magnetic reconnection. It offers the amazing opportunity to combine key insights from numerical models with high quality in-situ measurements that promise to greatly increase our understanding of this crucial physical process.

A fundamental and long-standing mystery has been why the reconnection energy conversion rate is remarkably efficient and near-universal across a wide spectrum of plasma regimes. In this talk, I demonstrate for the first time how the reconnection geometry determines the energy release, setting an upper limit for the energy conversion rate that is insensitive to the details of the system. This model is in excellent qualitative and quantitative agreement with the universal fast rate observed in disparate systems. I will also present new insights into the complex three-dimensional nature of magnetic reconnection revealed by means of petascale supercomputing, and will interpret my results in the context of MMS. Finally, I describe the next steps we need to take to answer some key open questions in magnetic reconnection.

February 3, 2017

Jack Sankey, McGill University

Title: Toward Optically Defined Micromechanical Systems (Video)

Abstract: Mechanical systems are ubiquitous throughout society, from oscillators in timekeeping devices to accelerometers and electronic filters in automobiles and cell phones. They also represent an indispensable set of tools for fundamental science, providing a means of sensing atomic-scale forces and masses or even the minuscule spacetime distortions from passing gravitational waves. In the field of optomechanics, we exploit the forces exerted by radiation to gain a new level of control over these systems at all size scales.

In this talk I will discuss our group's efforts to create delicate micromechanical systems and then influence their motion with extremely low levels of light. To this end, we have fabricated nanogram-scale "trampolines" having extraordinarily low damping parameters, ringing for six minutes when struck, and record force sensitivities below 20 attonewtons at room temperature. These trampolines also have excellent optical properties and are well-suited for optomechanical applications, notably providing access to a regime in which an average of a single cavity photon could significantly alter the trampoline's damping parameter (and, as a result, its trajectory). I will also discuss plans to apply an optical spring to these and related systems to control frequency, improve mechanical performance, and define the shape and mass of a mechanical mode. 

January 27, 2017

Andrew Fitzpatrick, Boston University

Title: Effective Field Theory and the Phenomenology of Dark Matter Direct Detection (Video)

Abstract: Direct detection searches for dark matter have made significant advances in their level of sensitivity and are expected to improve further in the near future.  The results of these experiments are often interpreted in a limited context of certain kinds of interactions between dark matter and standard matter, but the full range of possible interactions is much richer.  We discuss the theoretical and experimental motivation for considering a broad range of possible structure in the dark matter content of the universe, and a framework for systematically describing this wider range of possibilities.  We describe results from on-going work bringing together this framework with nuclear physics results in order to provide a complete `dictionary’ for the experimental predictions in this larger parameter space.

January 13, 2017

Mark Trodden

Title: Pushing Einstein's Boundaries: Gravitational Approaches to the Challenges of Modern Cosmology  (Video)

Abstract: Einstein’s general theory of relativity (GR) is one of the most successful and well-tested physical theories ever developed. Nevertheless, modern cosmology poses a range of questions, from the smallest scales to the largest, that remain currently unresolved by GR coupled to the known energy and matter contents of the universe. This raises the logical possibility that GR may require modification on the relevant scales.

I will discuss the status of some modern approaches to alter GR to address cosmological problems. We shall see that these efforts are extremely theoretically constrained, leaving very few currently viable approaches. Meanwhile, observationally, upcoming missions promise to constrain allowed departures from GR in exciting new ways, complementary to traditional tests within the solar system. I will finish by describing some promising very recent ideas.

January 6, 2017

Eric R. Fossum

Title: Quanta Image Sensor: Every Photon Counts  (Video)

Abstract: About 10 years after the invention of the CMOS image sensor at the NASA Jet Propulsion Laboratory in the early 1990’s, I was asked to write a book chapter on the future of digital still cameras. I proposed a binary, photon-counting image sensor now called the Quanta Image Sensor. It was a sort of wild idea at the time, but somewhat surprisingly, it now seems technically feasible. In this talk a brief review of the CMOS image sensor and a few fundamental physical principles behind its operation will be given. The Quanta Image Sensor will then be introduced. Conceptually it consists of perhaps one billion specialized pixels called “jots” that are read out at perhaps 1000 frames per second. Each jot is sensitive enough to count a single photoelectron. Recent progress at Dartmouth in achieving the QIS will be presented. We have shown the feasibility of making small (visible light) arrays that can count single photoelectrons at room temperature without the use of avalanche multiplication with good accuracy. The devices, implemented in a 65nm technology-node, backside-illuminated, CMOS image sensor foundry, also feature dark current less than 0.1e-/s at room temperature. The jot device and high speed readout electronics will be discussed, as well as the possible paradigm shift in image capture for scientific and consumer imaging that can now be envisioned and enabled.