Switchable Transparency

Can we control the optical properties (e.g. Transparency, beam deflection power, focal length) of materials using cheap and environmentally benign components?

Can this be the key to solve the issue of photovoltaics concentrator tracking? (see Vision)

Could this be used in other fields like Lidar Scanning, Adaptive ilumination design, Privacy screens, Optical instrument design?

These questions began, lately, to creep in my mind when I suddenly realized that butter, wax, oil are everyday examples of materials having a natural optical switching property. Opaque when solid they become transparent when molten. In general most mixtures of multiple “greasy” and “oily” compounds exhibit this property. The theory is fascinating and is relevant to multiple fields like astronomical observation, fog formation, atmospheric turbidity, free space laser data links….

At this point I realized that this can be done in multiple ways (and is several commercial applications too!), Liquid lenses are ubiquitous in cell phones, switchable transparency optical cells are at the base of e-readers, microfluidic offers a path for reconfigurable optical circuits….

But we followed a different path!

Together with my student we ended up mixing wax, alcohol and silicone (PDMS) obtaining a cheap rubbery material exhibiting a strong optical switching.

Figure 3: Local switching of the PDMS/Wax compound caused by a concentrated light beam

Figure 4: Global switching for a heated (50 Celsius) sample.

The process is stable and fully repeatable with a sample cost of few cents per cubic centimeter.

Applications?

An obvious application is that of a fail-safe laser shutter (a small pipe filled with this material will be transparent while heated by an electric coil, loss of power will lead to quick cooling and beam cut-off). A more sophisticate control will allow for a low cost laser beam attenuator with dynamical control.

An important application can be in the non-mechanical tracking of concentrator solar systems as described in a talk at SPIE Optics and Photonics 2014 in San Diego.

Essentially a light trap (a box with all but one reflecting walls and a solar cell on one side) can be used as a concentrator allowing light to enter from the top transparent face (Fig. a below). However, radiation escaping back through the open surface prevents the system from working.

If you add a few lenses you could reduce the light entrance areas, and maybe “blank out” the unused portion of the top entrance (Fig. b below). However, as the sun moves (relatively speaking!) your focal points will “travel” during the day…. And so you don’t really know where to place the openings…. And you are back to square one! This is the curse of solar concentration…. You need to track a moving source!

But, if you have a material that can “sense” the concentrated solar beam and become transparent ONLY where it is needed, it will “track” the (traveling) entrance point of the concentrated beam… (Fig. C, D and E below)

And this is exactly the material we have!

 The fist test with a 3d printed lightrap (enjoying my Makerbot!) demonstrated the proof of concept.

Figure 5: The light trap with a top PDMS/Wax optical switch and a detail of the 3d printed light trap.