Optical Microscope Exceeds Diffraction Limit

An optical microscope that combines electronic excitation and optical detection has broken Abbe’s diffraction limit. The nanoscale imaging method offers insight into how light and complex photonic materials interact and could lead to enhanced biosensors and more efficient displays and solar cells.

Researchers at the FOM Institute AMOLF 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 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 butterflies, birds, beetles and opal gemstones.

Dorsal view of male butterfly that was captured in Peru and is stored in the Toulouse Museum. (Image: Wikimedia Commons)

Advancements in nanofabrication techniques now make it possible to fabricate photonic crystals with accurately engineered optical properties.

Working with 30-nm spatial resolution, the team was able to explore 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.

“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 two-dimensional 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 so-called “crystal defect cavity.”

(a) New nano-microscope. The electron beam locally excites the photonic crystal, and the emitted light is collected. (b) To ensure efficient light collection, a parabolic mirror is accurately aligned by a piezoelectric mirror stage, which was designed and built at AMOLF. (Image: AMOLF)

The technique, 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 nanophotonics 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.”

“We demonstrate for the first time an instrument resolution of 30 nm, but I think in the future 10 nm is achievable,” Polman said.

Images taken using the new microscope developed by scientists at the FOM Institute AMOLF, King’s College and ICFO. Lightwaves propagate back and forth within the cavity but cannot escape. The distance between the holes is 330 nm, and the resolution of the measurement is 30 nm. (Image: AMOLF)

“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 the Aug. 19 issue of Nature Materials.

For more information, visit: www.erbium.nl or www.kcl.ac.uk.