May 14, 2015 - George Barbastathis

Click to Signup Location: Rebecca's Cafe, 275 Grove St., Auburndale, MA 02466
Dinner Reservation Deadline: May 11, 2015 @ 6pm

Compressive phase retrieval

Compressive sensing is a class of image recovery techniques utilizing sparsity priors to recover undersampled signals with high fidelity. This talk is about compressive sensing for phase retrieval from coherent fields and correlation function retrieval from partially coherent fields. For coherent fields, I will discuss in particular the use of intensity priors in the “transport of intensity equation” method, where the phase is obtained by analogy to a lateral pressure potential in a compressible flow. Transport of intensity is especially interesting in the x-ray regime, where standard interferometry is difficult because common sources are spatially partially coherent and beam splitters-combiners are not available; as a convincing example, I will show how the sparsity prior of quasi-constant object density allows successful x-ray phase recovery despite the low coherence. For partially coherent fields, I will describe how by using phase-space (Wigner space) methods and sparsity priors on the number of coherent modes it is possible to retrieve the correlation function, which can still lead to complete characterization of physical objects. 


Read more: May 14, 2015 - George Barbastathis

April 16, 2015 - Tony F. Heinz

Click to Signup Location: Prysm Inc., 45 Winthrop St., Concord, MA 01742
Dinner Reservation Deadline: April 13, 2015 @ 6pm

Optical Properties of Two-Dimensional Materials - Graphene and Beyond

Graphene, a single atomic layer of carbon atoms, has attracted great attention worldwide because of its potential for novel science and technology.  Recently, this interest has expanded to the much wider class of 2D materials that occur as layers of van-der-Waals crystals. While preserving graphene’s flexibility and tunability by external perturbations, atomically thin layers of this broader set of materials provides access to more varied electronic and optical properties, including semiconducting and insulating behavior.

In this talk, we will discuss some of the distinctive optical properties of this emerging class of atomically thin 2D materials.  Graphene has now been investigated across a spectral range from the THz to the UV.  The optical properties reveal much interesting physics and also show strong tunability in response by means of external gating. Recently, atomically thin layers of semiconductors in the family of transition metal dichalcogenides (MX2 where M = Mo, W and X = S, Se, Te) have also been prepared and investigated.  Although weak light emitters in the bulk, at monolayer thickness these materials emit light efficiently. We will describe some of the surprising properties of these systems.


Read more: April 16, 2015 - Tony F. Heinz

March 19, 2015 - Gregg Favalora

Hardware Startups 101:   Entrepreneurship for Engineers

Considering striking out on your own to commercialize a product that has optical, electronic, or otherwise tangible elements? Curious or worried about: fundraising, early product development, term sheets, or sales? Gregg Favalora, entrepreneur in optical systems since 1997, will layer the case study of (3-D display pioneer) Actuality Systems and its eventual exit with lessons and context in key topics that may be new for career engineers. Topics in this high-density talk will be addressed from the standpoint of electro-optical entrepreneurship, and will be a rapid introduction to key topics for further inquiry: what startups are *really* like, frugal prototyping, product definition, getting to First Customer Ship, financing and the basics of Term Sheets, publicity, equity budgets, the Whole Product, accelerators, and Characters to Be Wary Of. You will also be provided with resource lists for useful meetings, books, and key people to “follow.” 


Read more: March 19, 2015 - Gregg Favalora

January 15, 2015 - Jeff Hecht

Half a century of laser weapon development

DARPA launched the Pentagon’s first laser weapon program in early 1959, when it issued a million-dollar contract to TRG Inc. to try to build a laser based on Gordon Gould’s patent application. Ever since, the military has been trying to scale lasers to weapon-level powers. In the early 1960s, they tried to scale laser rods into laser logs, only to run into thermal problems. As that program wound down, AVCO Everett Research Laboratory developed the gas-dynamic CO2 laser, the first laser to produce tens of kilowatts CW. Chemical lasers followed, and MIRACL reached 2 megawatts CW, for a few seconds at a time, by the early 1980s. “Star Wars” produced the Alpha chemical laser, which also reached multi megawatt powers, and was supposed to be scalable to an orbiting laser battle station. The Airborne Laser followed, and for a while it looked like it really would really work. Now a dark horse, the fiber laser, has been shooting down targets at relatively short ranges in the Persian Gulf. This talk will cover the long road to this promising success, why it looks like it might work, and what its limits are.


Read more: January 15, 2015 - Jeff Hecht

February 19, 2015 - Steven Byrnes

The Shockley-Queisser Limit and Its Discontents

In 1961, Shockley and Queisser proved that single-junction solar cells cannot be more than ~34% efficient, under certain assumptions. Many researchers have viewed this theoretical efficiency limit as a challenge to overcome, and have proposed a dizzying variety of techniques to surpass that limit: multijunctions, concentration, multi-exciton generation, intermediate bands, hot electrons, upconversion, downconversion, anisotropic re-radiation, etc.

In the first part of the talk, Dr. Byrnes will give a tutorial on the Shockley-Queisser limit, explaining the steps and assumptions. Then, for each of those limit-surpassing techniques, he will explain how it works, why it can (in principle) beat the S-Q limit, and whether he thinks it is promising for real-world solar cells.



Read more: February 19, 2015 - Steven Byrnes

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