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…]
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...]
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…]