Fine tuning photons

Last month I did a rough cut and paste overview (using papers available via google scholar) on the topic of rectennas or nantennas finishing on the topic of the possible capture of infrared light for power generation. Now an article published last month in Science (Vol. 332 no. 6030 pp. 702-704 ) has demonstrated the capture of infrared radiation using nantennas constructed using silicon based semiconductors. While this is a significant advance, hold the champers. The fabricated device, while useful as a sensor, only converts ~0.01 % of the absorbed photons into a photocurrent over the wavelengths for which it is tuned. The authors suggest some methods to increase this to 2%. What is significant is that because it uses mature silicon technology devices could be fairly rapidly mass produced  IF the overall performance can be increased.
It's not solar nanvarna yet but… stay tuned!

Photodetection with Active Optical Antennas  (free pdf)

Mark W. Knight, Heidar Sobhani, Peter Nordlander and Naomi J. Halas

Nanoantennas are key optical components for light harvesting; photodiodes convert light into a current of electrons for photodetection. We show that these two distinct, independent functions can be combined into the same structure. Photons coupled into a metallic nanoantenna excite resonant plasmons, which decay into energetic, “hot” electrons injected over a potential barrier at the nanoantenna-semiconductor interface, resulting in a photocurrent. This dual-function structure is a highly compact, wavelength-resonant, and polarization-specific light detector, with a spectral response extending to energies well below the semiconductor band edge.

More coverage at the Royal Society of Chemistry.

There's something about the mention of hot electrons that gets the chemist in me a bit excited.

Rectify this

Below is a cut and paste “review” of the idea behind light rectification as an alternative method of deriving electrical power from solar energy. To preempt comments, this is not a magic bullet solution, it is some way yet before it may be useful (if ever) and I am not suggesting that “all our energy problems are over hip hip hooray”. However, IF advances keep apace, the possibility of being able to mass print such a device as envisioned below may be a welcome development.

Rectification is part of the process of converting an AC signal into a DC signal. Simple half wave rectification uses a diode that allows the forward going current to pass and blocks the reverse going current. Additional components are then used to “smooth” the resulting signal. Full rectification uses a bridge rectifier (essentially a collection of diodes) arranged in such a way that both the input forward and reverse currents appear at the output of the circuit in the same direction. Again, when smoothed this becomes a DC current.

Rectification is not a new technology. The simple “cats whisker” rectifier (a piece of wire in contact with a semiconducting crystal) of simple crystal radio sets – perhaps a novelty for the Nintendo generation – was the heart of the detector.

Rectification of microwaves is achievable at high power and is one of the technologies behind schemes to transmit solar power from space to ground based receivers. It has also been demonstrated as a means of wireless transmission.

In theory, the same principle should be applicable to shorter wavelength radiation, if some fundamental limitations can be overcome.