Researchers working on the next generation of photovoltaic solar cells—cells that convert sunlight directly into electrical current—are looking toward exotic materials (which are expensive) or more common substances, but use subtler methods to extract energy. A new study used a basic semiconductor material, already in use in solar cell research, but made it into a set of wires in a brushlike structure. The key was making the wires’ diameter smaller than the wavelength of light, which exploited a resonant property to extract more energy than expected from the photons. In this way, the researchers achieved efficiencies comparable to normal (planar) solar cells.

In the new study, the researchers determined that two major factors dictated optimal performance: the diameter of the wire, and the conductive properties of the InP [indium phosphide]. From theoretical predictions, wires with diameters significantly less than visible-light wavelengths achieve resonance when the light strikes, vastly increasing the amount of energy that can be absorbed. The new model suggested peak efficiency would be reached with an nanowire diameter of 180 nanometers. [Read more…]

Boosting solar cell efficiency with wires smaller than the wavelength of light


Metamaterials are a fascinating subject, worthy of a blog post at some point (to be written in my copious spare time, of course). If a real material won’t do what you need, you design one. Instead of atoms, you place small dots (known as quantum dots or nanodots) containing semiconducting or metallic atoms in a lattice pattern, making a two-dimensional solid with special electronic properties. A recent paper described the fabrication of a metamaterial built from gold nanodots, which has very precise response to light: when photons of a specific wavelength strike the surface at a given angle, almost nothing gets reflected back, whereas shifting the angle or wavelength slightly produces measurable reflection. The researchers used this metamaterial to make a sensitive molecular sensor, one capable of sniffing out individual proteins.

V. G. Kravets and colleagues demonstrated the detection of tiny masses, on the order of a single biomolecule, using nanoscale optics. They fabricated a material that responded resonantly to light. When a tiny amount of mass was added to the surface, it caused a dramatic change in the amount of reflected light. This enabled the researchers to detect the presence of mass accumulation to the level of 10-15 grams over a millimeter patch—equivalent to detecting a single human skin cell landing on a coffee table. [Read more...]

Foreign contaminant! Detection on molecular level using gold nanodots

(This was my original title for my article, but my editors evidently didn’t like it. I guess I’m too old school. Ahem. Moving right along.)

As you may know, quantum physics shows that matter has both a wavelike and particle-like character. When you combine quantum physics and special relativity, you find that a particle at rest vibrates with a frequency that depends only on its mass: the Compton frequency. For most purposes, the Compton frequency is useless: it’s huge, even for low-mass particles like electrons, and scales up proportionally for more massive particles like protons. However, researchers have figured out a way to access the Compton frequency of cesium atoms by stimulating them with lasers in a particular way. This could lead to more precise atomic clocks, enabling even more detailed measurement of the second of time—and provide a new way to measure the masses of subatomic particles.

Shau-Yu Lan and colleagues exploited advanced techniques to construct an atomic clock based on a single cesium atom, a device capable of dividing the huge natural frequencies of the atom into more manageable quantities. This provided a strong demonstration of the ability to construct clocks based on a single microscopic mass. And, because we already have excellent clocks to compare them with, this can potentially work in the opposite direction, leading to accurate mass measurements in the future. [Read more…]

Straight outta Compton…

Phased arrays consist of multiple antennas, all driven from a single source. By combining the output from all those antennas, you can make the light very directional, or give it a particular shape. Typically, phased arrays use radio light: big radar installations use them, as does the Very Large Array (VLA) of radio telescopes. A lab now has produced a phased array of 4,096 antennas on a single chip, which emit visible light instead—a potentially profound breakthrough in nanoscale photonic devices.

Modern nanoscale technology is now allowing researchers to create phased arrays for optical (visible) light. Jie Sun and colleagues fabricated a phased array of 4,096 microscopic antennas on a single silicon chip. This allowed them to shape the output waveform, so they could transmit an image of the MIT logo by combining the light from each tiny antenna in precise ways—something that could not be done with (say) a similar array of LEDs. Potential applications for this research include biomedical imaging, holography, and laser communications. [Read more…]

4,096 miniature antennas on a chip send shaped light

The orbiting Kepler observatory has been a remarkably successful project since its inception. By watching one small patch of the sky continuously, Kepler has enabled astronomers to discover upward of 2300 possible exoplanets—planets orbiting other stars. While many of those candidates likely are not actually planets, follow-up observations have confirmed 854 exoplanets as of December 28, 2012. The American Astronomical Society meeting, happening as I type this post, is devoting about 30% of its sessions to discussing recent exoplanet discoveries. This is an astoundingly rich field of study!

However, it’s also one that is remarkably accessible. Through the citizen science program Planet Hunters, non-scientists helped discover 42 planet candidates, 15 of which may lie in their system’s habitable zone—the region at which liquid water may exist on the surface.

The Planet Hunters identified 42 exoplanet candidates, including 33 with at least three transits—the more transits we can observe, the more reliable the identification as a planet, and the better the estimates of orbital characteristics. Forty of the potential exoplanets have orbits longer than 100 days, and 9 may have orbital periods greater than 400 days, placing them farther out than most previously identified worlds. [Read more…]

Maybe you could be the one to discover the next Earth

How quickly things can change in science: just a few years ago, we were barely able to talk about the diversity of planetary systems. Now, we are able to distinguish between planets orbiting in tight binary systems from those in wide binaries. Additionally, exoplanets in tight binaries can orbit either both stars together (circumbinary, or Tatooine-like systems) or one of two stars, where the second might be like a Jupiter in the system. In wide binaries, the second star is so far away that it’s barely attached to the system, but a new set of simulations may show that may actually lead to greater instability than experienced by planets in tight binaries.

Nathan A. Kaib, Sean N. Raymond, and Martin Duncan ran extensive computer simulations to model exoplanets residing in wide binary systems. They found that perturbations from other stars outside the binary system had a profound effect on the shape of the system’s orbits. In some cases, planets were ejected from the system entirely or ended up in larger or highly eccentric (elongated) orbits. Based on these results, the researchers argued that some of the observed exoplanet systems with eccentric orbits may actually reside in wide binary systems where we haven’t yet detected the companion stars. [Read more, and watch the video!]

Bad news for some planets in binary star systems?

Our local group of galaxies—known imaginatively as the Local Group—has two huge galaxies: the Milky Way and M31, also known as the Andromeda Galaxy. Both of these galaxies are large enough to have a number of satellites, including the substantial Magellanic Clouds and M33 (Triangulum Galaxy). However, most satellites are dwarf galaxies, very faint and relatively low mass. As a result, a moderately complete census of satellites has proven difficult even for the Milky Way, but what recent observations have found is surprising. In both cases, a number of the satellite galaxies orbit in a single plane, and at least in the case of Andromeda, they orbit in the same direction.

The Pan-Andromeda Archaeological Survey (featuring the diverting acronym PAndAS) was established to provide a high-resolution, large-scale panorama of M31 and its environs. 27 dwarf galaxies that can be unambiguously associated with Andromeda lie within the PAndAS survey region. The astronomers measured the distances and velocities of each of these galaxies, yielding a three-dimensional and dynamical view of the M31 system.

They found 15 of those satellites were arranged along a relatively thin arc from the perspective of Earth, meaning they lie close to a single plane. Further analysis revealed 13 of the 15 galaxies were also moving in a coherent pattern: those “north” of Andromeda were moving away from us, while those “south” were traveling toward us. That indicates a clear rotational pattern; the authors estimated only a 1.4 percent probability of motion like this being random chance. [Read more…]

Why do half of Andromeda’s satellite galaxies orbit in a plane?