Optical microscope breaks Abbe’s limit

Ashley N. Rice, ashley.rice@photonics.com

Combining electronic excitation and optical detection has allowed an optical microscope to break Abbe’s diffraction limit. The new nanoscale imaging method could lead to enhanced biosensors by offering insight into how light and complex photonic materials interact.

Researchers at FOM Institute AMOLF (Institute for Atomic and Molecular Physics) in Amsterdam, King’s College London and the Institute of Photonic Sciences (ICFO) in Barcelona broke Ernst Abbe’s specification for the resolution limit of a diffraction-limited microscope using a novel technique called angle-dependent cathodoluminescence imaging spectroscopy.

Natural photonic crystals – nanostructures composed of two materials with a different refractive index arranged in a regular pattern with exotic optical properties – are what create the iridescent colors found in certain species of beetles, birds, butterflies and opal gemstones. These optical properties can be mimicked in fabricated photonic crystals, thanks to recent advances in nanofabrication techniques.

Working with 30-nm spatial resolution, the international team explored the finer details of photonic crystals at a resolution more than 10 times smaller than light’s diffraction limit, providing more insight into how light interacts with matter to produce, for instance, the visible iridescence seen on butterfly wings.

A dorsal view of a male butterfly. Natural photonic crystals make the iridescent colors found in butterflies, beetles and opals, and a new nanoscale imaging method allows scientists to study how light and complex photonic materials interact to engineer photonic crystals for biosensors and more. Courtesy of Wikimedia Commons.

“We were thrilled in the lab to observe the finer details of the photonic crystals that were simply inaccessible before,” said Dr. Riccardo Sapienza of King’s College. “This is very important, as it allows scientists to test optical theories to a new level of accuracy, fully characterize new optical materials and test new optical devices.”

The researchers fabricated a 2-D photonic crystal by etching a hexagonal pattern of holes in an ultrathin silicon nitride membrane. The crystal inhibits light propagation for certain colors of light, which leads to strong reflection of those colors. By leaving out one hole, a very small cavity can be defined where the surrounding crystal acts as a mirror for the light, making it possible to strongly confine it within a crystal defect cavity.

Angle-dependent cathodoluminescence imaging spectroscopy is based on a geological method in which visible light is emitted by a luminescent material when it is hit by an electron beam released by an electron gun. The technique was modified by professor Albert Polman’s team at AMOLF to explore nanophotonic materials.

“Each time a single electron from the electron gun reaches the sample surface, it generates a burst of light, as if we had placed a fluorescent molecule at the impact location,” Sapienza said. “Scanning the electron beam, we can visualize the optical response of the nanostructure, revealing features 10 times smaller than ever done before.”

“For years, we have been struggling with scanning near-field probes and positioning of nanolight sources,” said ICFO professor Niek van Hulst. “Now the scanning e-beam provides a local broadband dipolar light source that readily maps all localized fields inside a photonic crystal cavity.”

This is the first demonstration of 30-nm instrument resolution, Polman said, but he added that 10 nm should be achievable in the future.

“Our research provides a fundamental insight into light at the nanoscale and, in particular, helps in understanding how light and matter interact,” Sapienza said. “This is the key to advance nanophotonic science, and it can be useful to design novel optical devices like enhanced biosensors for health care, more efficient solar cells and displays, or novel quantum optics and information technologies.”

The instrument will be brought to market this fall by startup company Delmic. The development was funded by a Valorization Grant from Technology Foundation STW and the FOM Foundation.

The findings were reported in Nature Materials (doi: 10.1038/nmat3402).