Measurement results showing parts of a light wave varying over nanometer scales, much smaller than a wavelength. (Left: wavevector map. Right: phase map.) This “superoscillation” phenomenon can be used to perform ultra-precise optical measurements. Figure credit: Guanghui Yuan.
Microscopes, and other optical devices that use light to visualize objects, suffer from a fundamental limitation known as the diffraction limit: the shortest distances they can reliably measure is around half the wavelength of the light used. For visible light, this limit is around 400 nanometers – tiny by human standards, but substantially larger than many objects scientists are interested in. Viruses and nanoparticles, for instance, range in size from 10 to 100 nanometers.
Now, two scientists at Nanyang Technological University, Singapore, have demonstrated a new way to use near-infrared laser light to measure distances of a nanometer (one billionth of a meter), the shortest distance ever measured using a direct optical method. Professor Nikolay Zheludev and Dr Guanghui Yuan, researchers at NTU’s School of Physical and Mathematical Sciences and Centre for Disruptive Photonic Technologies, reported their breakthrough in a paper published in the journal Science in May 2019.
Zheludev and Yuan performed their measurements by exploiting an optical phenomenon known as superoscillation. The concept of superoscillation first arose in the 1980s from theoretical quantum physics research performed by Yakir Aharonov, an Israeli physicist, and was subsequently extended to optics and other fields by the British physicist Michael Berry. The phenomenon occurs when a complex wave, such as a light wave scattered by an irregular object, contains small regions that oscillate much more rapidly than the rest of the wave.
“Although the concept of superoscillation is profound, our experimental setup is quite simple,” says Dr. Yuan. “We created a 100-nanometer thick gold film with over 10,000 tiny slits cut into it, which functions as a type of diffraction grating. The slits are arranged in a special computer-generated pattern so that when a laser shines on it, the diffracted light exhibits superoscillation.”
Schematic of the superoscillatory optical ruler. A metasurface of 0.040 mm by 0.040 mm area and 100 nanometer thickness generates a superoscillatory diffraction pattern. The phase of the light field contains abrupt variations that serve as the “marks” of the optical ruler. Figure credit: Guanghui Yuan.
The superoscillatory light wave contains regions in which its phase varies much more abruptly than the rest of the wave. By comparing it with a reference light wave, the scientists are able to detect these sharp phase variations, which occur over much distances less than 1/400 times the wavelength of light. This allows the light field to be used as an optical ruler of unprecedented precision.
“This phase-sensitive technique is a major improvement over previous attempts to use superoscillation for optical measurement,” explains Professor Zheludev, who serves as the director of the Centre for Disruptive Photonic Technologies and co-director of the Optoelectronics Research Centre at Southampton University in the UK. “Earlier methods, developed by our research group as well as others, used a type of superoscillation that produces ‘hot spots’ in the light intensity. Those hot spots are easy to detect, but if the goal is to measure the shortest distances possible, phase superoscillations are much better due to their smaller size.”
According to the researchers’ theoretical calculations, devices based on their new method can ultimately measure distances down to 1/4000 the wavelength of light, roughly the size of a single atom.
Professor Zheludev believes their discovery is likely to find applications in industry, such as in the manufacturing and quality control of electronics, where extremely precise optical measurements are required. He notes that although non-optical methods already exist for measuring nanometer distances, they tend to be time-consuming or difficult to use. Scanning electron microscopy, for instance, requires samples to be placed in a vacuum chamber, whereas the present optical technique can be performed in air.
The research team’s next step is to develop a more compact version of their apparatus, based on optical fibers, to be commercialized as a new type of ultra-precise optical ruler.
G. H. Yuan and N. I. Zheludev, Detecting nanometric displacements with optical ruler metrology, Science 364, 771 (2019)