Light, Antennas, and Holograms

Optical AntennaIn a previous post I talked about the use of nanoscale antennas for solar power collection. In this post I want to mention a few other ideas which relate to our new-found ability to manufacture extremely small-scale structures using processes in nanotechnology.

Technology is getting to the point where we can manufacture structures on various substrates that are only a few nanometers in size. Certainly it is now very easy to layer conducting elements on silicon which are smaller than a micron in length. In 2011 silicon technology reached the 22 nanometer length scale for CMOS processing. This corresponds to half the distance between memory cells in a memory cell array. We are expecting to get to 11 nanometers by 2015. Optical light has wavelengths between 400 and 650 nanometers and the wavelength of infra-red light is around 1 micron. Therefore we are able to build structures where by comparison the wavelength of light seems rather large.

Light is electromagnetic radiation and is exactly like radio waves, microwaves and X-rays. The distinguishing factor is the wavelength. Just as we can build antennas to pick up radio stations, we can now build antennas to pick up light waves. The only difference is that while a radio antenna might be many meters in size, a light antenna needs to be on the order of one micron. What this means is that we can build conducting structures on a suitable substrate that can resonate with the electrical and magnetic fields that are present in light radiation; light can be converted to and from electrical currents in very tiny wires. As I have mentioned before, this opens up the possibility of being able to directly convert sunlight into electrical power by a more efficient antenna structure rather than by using the photoelectric effect that is employed by conventional solar cells. The main challenge for this direction of research is the construction of suitably tiny electronic components (principally rectifying elements) to convert the high frequency (terahertz) currents into DC which can be of use in a conventional power supply.

Now that we can build structures smaller than light’s wavelength, I can see that many interesting possibilities emerge. If conducting structures are built on the order of nanometers, they may resonate with incoming light radiation in useful ways. Because antennas work for transmitting and receiving, if driven by a suitable electric current, nano-antennas will emit light. As I have already mentioned, we could build fractal antenna structures that can absorb light over a wider bandwidth than a conventional dipole antenna, harvesting more solar power for energy collection. Here however, I would like to consider the light production possibilities of these microscopic structures. Before I get into it, I would like to say that light can be produced also by light emitting diodes and lasers and these light emitting semiconductor structures can be made very small. Therefore I talk here about technology which could use any of these techniques. In addition, I also consider reflective properties of surfaces which have nano-sized reflective and non-reflective regions. Essentially I am interested in light production, absorption and reflection when the size scale gets very tiny.

In radio and radar there is the concept of a phased array. This is a configuration of antennas which are simultaneously used to receive or transmit electromagnetic energy, where the phase of the signal to each antenna is controlled to obtain desired effects. The end result of this is that constructive or destructive interference of the waves is produced in various places in space, and so some measure of control over the 3D structure of the strength of signal away from the actual antenna is produced. One common application is beam forming, in which a beam of transmitted energy is directed along a particular path, or at least a pattern of preferred directions of energy radiation is produced. If the antenna is used to receive a signal using a phased array, the direction of “listening” is controllable at the receiver. This can be useful in scanning the surroundings for signal sources and creating a picture of the radiating environment within a certain frequency band. Examples might be found in radio astronomy or in military applications.

It is interesting to consider how advances in nanotechnology can allow this phased array idea to be applied to applications using energy at the wavelength of light. We could imagine that we could create steerable light radiators where the actual direction of the light beam coming from a flat panel emitter is controllable by electronics, with some directions being preferred. We could also produce an odd kind of scanning camera without a lens by beam forming for optical reception. The planar receptor panel would contain many antennas whose signal was added together in a controllable phase relationship to steer the preferred direction of light reception in a dynamic way. By scanning it in 2D we could build up a picture of the environment at whatever wavelength we might choose and hence sample the whole light field. The wavelength band (which correlates to the color of the light) would be determined by the sizes of the structures that were switched in and out of the receiving circuit. Of course this complex capability would rely on us solving some very hard problems of high frequency control.

Another interesting, and I think fairly realistic, application of nanoscale fabrication in the light-related arena is the development of holographic 3D displays which do not require 3D glasses and do not use lenticular patterns. A real 3D object reflects light rays that head off from each point on its surface in different directions. When you view that object, you capture only the rays that pass through the lens of your eye. When you move your head and look from a different direction, you capture a different set of those rays, and this gives us the ability to see around objects and perceive the 3D form, including all the complexities of surface reflection, specularities, etc.

For a long time we have had the capability of producing holograms with film which allow us to see realistic 3D structures when viewing a 2D surface under the right lighting conditions. In a hologram, what is happening is that the illumination of the real object by laser light during the original image capture leads to a pattern of constructive and destructive interference at the surface of the film. This creates a set of banding patterns on the film. When the film is viewed and illuminated (often with laser light), these patterns cause the light to reflect off the developed film surface in different directions which correspond to the directions of the light that would have come from the real object. This process is quite hard to understand. I believe a good appreciation of it can come from reading Richard Feynman’s excellent book “Quantum Electrodynamics – The Strange Theory of Light and Matter“. Essentially, light reflects from electrons in the electron cloud of a metal. If a mirror-like surface is interrupted with light-absorbing gaps and the gaps are spaced at the right distance apart (on the order of nanometers) to make a reflective optical grating, then interesting effects are obtained. One main effect is that the angle of incidence and reflection are not necessarily related, and in fact the light can be sent off in any direction. Indeed it can also be any color, depending on the pattern of the reflecting region, because some patterns will cause destructive interference and some constructive at various wavelengths if the surface is illuminated by white light.

My main suggestion is to control this process on panels built with steerable light emitters (or reflectors). Instead of the normal pixels of a TV screen where light just gets emitted forwards in a kind of sine-squared profile, imagine if we could steer the light independently from each pixel and control its wavelength. If we could do this we would be able to create very rich displays which have similar properties to the old-school laser-based photographic holograms and allow realistic 3D images to be built in rich color and depth. It may not be necessary to have fine control over each region of a sub-micron scale emitter – one can imagine that the control would be on the order of a normal pixel size – but within each pixel there would have to be some method by which the phase of the nano-scale emitters was coordinated to produce the right color or beam forming. This could perhaps be done with either light antennas, or with tiny laser diodes.

I have thrown around some ideas here about light antennas. I am sure that there are many more applications. It is an exciting area to combine nanotechnology fabrication processes with the properties of light, and I hope that engineers can explore some of these interesting possibilities in the future.