Light: harvesting with transformation *optics and sensing with a photovoltaic bracelet

There’s a good general description (although it’s still quite technical and challenging) of nanoplasmonics in a Jan. 6, 2014 news release on EurekAlert (later in this posting I have an item about a practical application for photovoltaics),

The control of light is vital to many applications, including imaging, communications, sensing, cancer treatment, and even welding processes for automobile parts. Transformation optics is an emerging field that has revolutionized our understanding of how to control light by constituting an effectively curved electromagnetic space. This revolutionary strategy not only revisits the fundamental physics of light-matter interactions, but also renders trivial the design of optical functions that may otherwise be difficult or virtually impossible, such as an “invisibility cloak,” which could only previously be found in science fiction. When compared with ray optics, the new transformation optics technique provides a picture that is equally intuitive, but that is much more accurate in its description of the wave nature of light by using the electric and magnetic field lines as its basis. Therefore, the validity of this method is not restricted to the macroscopic regime, but can also be extended to the subwavelength scale. In a recent review paper published by SCIENCE CHINA Information Sciences, Yu Luo and colleagues from Imperial College London illustrate how the general capabilities of the transformation optics technique can be used to treat the subwavelength fields that occur in plasmonic systems and review the latest developments in transformation optics as applied to nanophotonics.

Here’s a more detailed description of the difficulties and the solution (transformation optics) from the news release,

In plasmonics, metallic structures with sharp corners can trap light into nanometric volumes, thus giving rise to strong near-field enhancements. This effect can be used to detect single molecules, generate high harmonic signals, and even improve absorption in photovoltaic devices. Further developments using these techniques need to be guided by accurate and versatile theoretical modeling. However, modeling of this type can be difficult, because various aspects associated with the sharp plasmonic structures can hinder provision of accurate and convenient solutions to the problem at hand. First, the size of the sharp metallic point structure is normally much smaller than that of the device overall, which makes it difficult to create meshes for numerical simulations. Second, the strong contrast in the dielectric functions at the metal-dielectric interfaces leads to slow convergence of the field expansions. Yu Luo and colleagues deploy the theory of transformation optics to circumvent these problems. Their idea is to transform a complex plasmonic system with little intrinsic geometrical symmetry into a canonical structure with translational or rotational symmetry, which is then relatively easy to study using conventional theory. For example, two touching nanowires can be transformed into two flat metal surfaces that are separated by a gap, and a sharp metal edge can be related to a periodic array of metal slabs. Other structures that can be studied using transformation optics include pairs of metallic nanospheres, asymmetric core-shell structures and rough metal surfaces. In fact, using transformation optics techniques, we could reverse engineer the optical properties of complex plasmonic nanostructures and redesign these structures based on the requirements of the desired applications.

And then, there’s what seems to be a plea for more researchers in the field,

Practical issues with the realization of plasmonic devices, such as the effects of edge rounding at sharp boundaries on the local field enhancement and resonance properties, can also be considered theoretically using transformation optics and provide useful guidance for the fabrication of these devices. In particular, the necessary conditions are highlighted for both broadband light absorption effects and large field enhancements. Experimental evidence for phenomena that have been predicted by transformation optics has also been reviewed, indicating potential applications in biosensing and broadband solar photovoltaics. These studies demonstrate the accuracy and versatility of transformation optics methods and are expected to encourage more researchers to enter this field. [emphasis mine]

Honestly, I don’t understand nanoplasmonics very well even after reading the description but there’s enough accessible information in the news release to help me achieve a better understanding. For those who want to further explore this latest work in trransformative optics and nanoplasmonics, here’s a link to and a citation for the paper,

Harvesting light with transformation optics by LUO Yu, ZHAO RongKuo, FERNANDEZ-DOMINGUEZ Antonio I., MAIER Stefan A., & PENDRY John B… Sci China Inf Sci, 2013, 56(12): 120401(13).

This paper is open access as of Jan. 8, 2014.

Photovoltaic bracelet/brooch

On to the other ‘light’ topic mentioned earlier. John Brownlee has written about June, a photovoltaic bracelet, which tells you how much sun exposure you’ve had, in a Jan. 7, 2014 article for Fast Company (aka Co-Design; Note: Links have been removed),

… Meet the June, a bedazzling, Bluetooth-connected bracelet that tells you how much sun you’re getting. But don’t dismiss the June just because you’re not worried about the SPF. This is the future of wearables. [emphasis mine]

… fashionably designed wearable that measures exposure to the sun. Made by Netatmo and designed by Louis Vuitton and Harry Winston collaborator Camille Toupet, the June syncs over Bluetooth to a paired iPhone, where an app tells you how much sun you’re getting based upon readings from the bracelet’s photovoltaic gem, and then recommends sunglasses, a hat or a specific sunscreen based upon the measurements. It costs $100,

Lily Hay Newman in a Jan. 8, 2014 posting on Slate’s future tense blog challenges the notion that June is the “future of wearables,”

… it really only does one thing: It measures sun exposure. It’s a single-use device that syncs to a single-use app. Perhaps it foreshadows a world where we each customize our array of wearable sensors by picking and choosing among single-focus gadgets from day to day. Which sensors we want and how we want to look would both play a part in dictating how we dressed and accessorized. Wearables certainly would be a lot more attractive if they weren’t crammed with maximal functionality. But this is also wildly inefficient, and previous technologies haven’t evolved this way. Cameras, MP3 players, calculators, notebooks, calendars, phones, and everything else eventually collapsed into smartphones: one device. No matter how attractive a sensor-turned-bracelet is, there’s a limit to how many wearables one person can actually, you know, wear.

She also notes that June is being marketed to women primarily and suggests that wearables offer an opportunity to change how technology is marketed (Note:Llinks have been removed),

Since the aesthetic direction of wearables is still undetermined, and is currently dictated by the tech inside, the devices present a good opportunity to move away from traditional, often reductive, male and female marketing, which can be particularly blatant in tech. Example: the EPad Femme tablet for women. Alternate example: The Honda Fit She’s. It’s a tall order, but balancing form and function is the crux of the uncertainty in wearables right now.

I recommend reading both articles (Brownlee’s contains a June promotional video). For the curious here’s what the bracelet looks like (from the June webpage),

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June  can also be worn as a brooch; the Netatmo website’s June webpage states,

Versatile, JUNE can be worn as a bracelet or as a brooch.

I haven’t been able to find a product launch date other than it will be ‘sometime in 2014’.

* Removed an extra preposition ‘with’ that preceded the word optics.

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