Evolutionary Modeling Optimizes Optical Cavities

A new design approach based on an evolutionary algorithm has shown the way to photonic crystal nanocavities (PCNs) that trap light for several nanoseconds — longer than other similarly sized cavities.

Using the evolutionary technique, an international team of scientists said they achieved a PCN quality factor — representing the length of time the nanocavity can hold a photon before it escapes — of 2 × 10⁶. The PCNs could be used in future nanophotonic communications systems to prevent signal “traffic jams,” the researchers said.

Diagram of a light-trapping nanostructure. The top layer shows light confined in the tiny red regions. The second layer is the design generated by an approach that mimics evolutionary biology. The bottom two layers show electron micrographs of the realized nanostructure in silicon. The sharp peak on the left is the trace of the long trapping of light. Courtesy of Fabio Badolato, University of Rochester.

Researchers from the Swiss Federal Institute of Technology of Lausanne (EPFL) designed the nanocavities, which were fabricated at the University of Rochester and tested at the University of Pavia in Italy.

“Ideally, you want to confine light as long as possible and inside a volume as small as possible,” said Vincenzo Savona, an associate professor and researcher at EPFL. "The nanocavities we are optimizing are smaller than the optical wavelength itself (about 1 micrometer) and have a quality factor higher than 1 million, meaning that a photon can go back and forth inside the nanocavity more than 1 million times before escaping."

The researchers developed an algorithm that allows quick computer modeling of hundreds of generations of PCNs, each one integrating the best features of the previous generation, to arrive at an optimal design.

“In the end, we improved all of the most widespread PCN designs by a factor of 10 to 20 in terms of lifetime of the confined photons,” Savona said.

The Rochester researchers now plan to explore biosensing applications for the new nanocavities.

The research was published in two journals, Applied Physics Letters (doi: 10.1063/1.4882860) and Scientific Reports (doi: 10.1038/srep05124).

For more information, visit www.epfl.ch and www.rochester.edu.