1,947 research outputs found
Natural Regulation of Energy Flow in a Green Quantum Photocell
Manipulating the flow of energy in nanoscale and molecular photonic devices
is of both fundamental interest and central importance for applications in
light harvesting optoelectronics. Under erratic solar irradiance conditions,
unregulated power fluctuations in a light harvesting photocell lead to
inefficient energy storage in conventional solar cells and potentially fatal
oxidative damage in photosynthesis. Here, we show that regulation against these
fluctuations arises naturally within a two-channel quantum heat engine
photocell, thus enabling the efficient conversion of varying incident solar
spectrum at Earth's surface. Remarkably, absorption in the green portion of the
spectrum is avoided, as it provides no inherent regulatory benefit. Our
findings illuminate a quantum structural origin of regulation, provide a novel
optoelectronic design strategy, and may elucidate the link between
photoprotection in photosynthesis and the predominance of green plants on
Earth.Comment: 17 pages, 4 figure
Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube PN junction photodiodes
Ultrafast photocurrent measurements are performed on individual carbon
nanotube PN junction photodiodes. The photocurrent response to sub-picosecond
pulses separated by a variable time delay {\Delta}t shows strong photocurrent
suppression when two pulses overlap ({\Delta}t = 0). The picosecond-scale decay
time of photocurrent suppression scales inversely with the applied bias VSD,
and is twice as long for photon energy above the second subband E22 as compared
to lower energy. The observed photocurrent behavior is well described by an
escape time model that accounts for carrier effective mass.Comment: 8 pages Main text, 4 Figure
Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube PN junction photodiodes
Ultrafast photocurrent measurements are performed on individual carbon
nanotube PN junction photodiodes. The photocurrent response to sub-picosecond
pulses separated by a variable time delay {\Delta}t shows strong photocurrent
suppression when two pulses overlap ({\Delta}t = 0). The picosecond-scale decay
time of photocurrent suppression scales inversely with the applied bias VSD,
and is twice as long for photon energy above the second subband E22 as compared
to lower energy. The observed photocurrent behavior is well described by an
escape time model that accounts for carrier effective mass.Comment: 8 pages Main text, 4 Figure
Quieting a noisy antenna reproduces photosynthetic light harvesting spectra
Photosynthesis is remarkable, achieving near unity light harvesting quantum
efficiency in spite of dynamic light conditions and noisy physiological
environment. Under these adverse conditions, it remains unknown whether there
exists a fundamental organizing principle that gives rise to robust
photosynthetic light harvesting. Here, we present a noise-canceling network
model that relates noisy physiological conditions, power conversion efficiency,
and the resulting absorption spectrum of photosynthetic organisms. Taking
external light conditions in three distinct niches - full solar exposure, light
filtered by oxygenic phototrophs, and under sea water - we derive optimal
absorption characteristics for efficient solar power conversion. We show how
light harvesting antennae can be finely tuned to maximize power conversion
efficiency by minimizing excitation noise, thus providing a unified theoretical
basis for the experimentally observed wavelength dependence of light absorption
in green plants, purple bacteria, and green sulfur bacteria
Competing Channels for Hot-Electron Cooling in Graphene
We report on temperature-dependent photocurrent measurements of high-quality dual-gated monolayer graphene p−n junction devices. A photothermoelectric effect governs the photocurrent response in our devices, allowing us to track the hot-electron temperature and probe hot-electron cooling channels over a wide temperature range (4 to 300 K). At high temperatures (T > T[superscript *]), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures (T < T[superscript *]), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This nonmonotonic temperature dependence can be understood as resulting from the competition between two hot-electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions that dominates at high temperatures. Gate control in our high-quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature T[superscript *] depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphene’s photoresponse.United States. Air Force Office of Scientific Research (Grant FA9550-11-1-0225)David & Lucile Packard Foundation (Fellowship
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