Research

Sub-wavelength structures can set the absorption, reflection and thermal emission of a surface almost independently of what the surface is made of. Selective absorbers, radiative coolers and structural colour have all been demonstrated this way. The obstacle is manufacturing: the best-performing structures are still written by electron-beam lithography, which does not scale to the areas that solar modules, building surfaces or wafer-level optics need.

We work on nanostructures that can be made without lithography — disordered scattering layers, self-assembled masks, nanowire forests — and on the optics that follow from them. Devices we have built include a single-material near-infrared selective absorber based on a refractive-index-tunable Tamm plasmon structure, a photon sieve on a III-V nanowire forest, double-sided anti-reflection nanostructures on convex lenses, and large-area disordered light-harvesting layers for photovoltaic modules.

The same process variation that makes nanofabrication hard to control also makes each device unique. We read that uniqueness optically and use it for authentication, in silk fibres, liquid crystals and edible materials.

Wearable devices, robotic skin and curved image sensors all need optoelectronic devices on surfaces that are not flat. Thinning a device makes it bendable but usually costs responsivity, and interconnects fail long before the active layer does. Separately, conventional image sensors digitise and transmit every pixel of every frame; in most applications the processor then throws almost all of it away.

On the first problem we use lateral NIPIN phototransistors, which keep their junctions in the plane of the film and therefore tolerate bending, and transfer printing to move finished devices onto flexible substrates. On the second we use ferroelectric-gated phototransistors that hold a weight in the pixel itself, so part of the processing happens where the light lands.

We also build the systems these devices go into, including a wireless, battery-free patch-type tissue oximeter that stays cool enough to wear outdoors by combining radiative cooling with the sensor.