The Microscopic Origin of Efficiency Droop in LEDs
11-20-17
Marco Bernardi, Assistant Professor of Applied Physics and Materials Science, and his colleagues’ semiconductor research has shown that the coupling between electrons and thermal vibrations may be sapping energy from Light-emitting diodes—or LEDs. "Our work shows for the first time that the ever-present interaction between electrons with lattice vibrations can, by itself, explain why excited electrons can leak out of the active layer and account for inefficiencies in GaN LEDs," Professor Bernardi says. [Caltech story]
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Marco Bernardi
Studying Entropy in Metallic Glasses
10-10-17
Brent Fultz, Barbara and Stanley R. Rawn, Jr., Professor of Materials Science and Applied Physics, and colleagues have pinpointed that arrangement of atoms is the main source of an increase in entropy during the glass transition. One persistent mystery about metallic glasses occurs at the so-called "glass transition." A cold metallic glass is hard and brittle, but when it is heated past a certain point—the glass transition—it becomes soft. [Caltech story]
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Brent Fultz
MatSci
Reflective Nanostructures
07-13-17
Andrei Faraon, Assistant Professor of Applied Physics and Materials Science, and colleagues have discovered how to use computer-chip manufacturing technologies to create the kind of reflective materials that make safety vests, running shoes, and road signs appear shiny in the dark. The new technology uses surfaces covered by a metamaterial consisting of millions of silicon pillars, each only a few hundred nanometers tall. By adjusting the size of the pillars and the spacing between them, Faraon can manipulate how the surface reflects, refracts, or transmits light. [Caltech story]
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Andrei Faraon
"Hot" Electrons Move Faster Than Expected
06-15-17
For the first time, Marco Bernardi, Assistant Professor of Applied Physics and Materials Science, and colleagues have been able to directly observe the ultrafast motion of electrons immediately after they are excited with a laser—and found that these electrons diffuse into their surroundings much faster and farther than previously expected. "Our work shows the existence of a fast transient that lasts for a few hundred picoseconds, during which electrons move much faster than their room-temperature speed, implying that they can cover longer distances in a given time when manipulated with lasers," says Professor Bernardi. "This non-equilibrium behavior could be employed in novel electronic, optoelectronic, and renewable energy devices, as well as to uncover new fundamental physics." [Caltech story]
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Marco Bernardi