One way to identify molecules with mirror versions – a property known as chirality – is to look at how they scatter light waves. The handedness is imprinted on the direction the waves vibrate, or their polarisation. But current techniques for measuring polarisation involve using multiple lenses and other optical elements like beam-splitters and filters, which can degrade the image quality.
Now Reza Khorasaninejad of Harvard University and his colleagues have come up with a single nanotechnology lens that can do the same job. The lens is made from a layer of titanium dioxide that has been etched by a beam of electrons into rows of pillars just 600 nanometres high, sitting on top of an ordinary sheet of glass.
In a row, each rectangular pillar is at an angle to the one before it, so that the orientation of the pillars along the line seems to rotate clockwise or anticlockwise. Alternating rows twist in opposite directions, creating two side-by-side images without the need for bulky optics. “We have huge control over the light shaping,” says Khorasaninejad. “The weight, size and compactness of the structure is very small.”
To test out the lens, the team took a picture of a Chrysina gloriosa, a beetle whose shell is known to reflect left-handed light (above). In the future, Khorasaninejad says they hope to improve the resolution of the lens to let them pick out left- from right-handed molecules, making it useful for developing safe drugs
The vast majority of biologically active compounds, ranging from amino acids to essential nutrients such as glucose, possess intrinsic handedness. This in turn gives rise to chiral optical properties that provide a basis for detecting and quantifying enantio-specific concentrations of these molecules. However, traditional chiroptical spectroscopy and imaging techniques require cascading of multiple optical components in sophisticated setups. Here, we present a planar lens with an engineered dispersive response, which simultaneously forms two images with opposite helicity of an object within the same field-of-view. In this way, chiroptical properties can be probed across the visible spectrum using only the lens and a camera without the addition of polarizers or dispersive optical devices. We map the circular dichroism of the exoskeleton of a chiral beetle, Chrysina gloriosa, which is known to exhibit high reflectivity of left-circularly polarized light, with high spatial resolution limited by the numerical aperture of the planar lens. Our results demonstrate the potential of metasurfaces in realizing a compact and multifunctional device with unprecedented imaging capabilities
SOURCES- New Scientist, Nanoletters