In the interest of observing microscopic and nanoscopic objects like viruses and nanoparticles, scientists currently use indirect non-optical methods, like electron microscopy. However, this method and other similar ones do not always yield feasible results, especially for particles that are less than 400 nanometers in size. Aside from that, this method can be time-consuming and it requires expensive equipment.
With these disadvantages, scientists from the Nanyang Technological University in Singapore have recently reported in an issue of Science that they have developed a new optical method of measurement using near infrared light. Prof. Nikolay Zheludev and Dr. Guanghui Yuan from the university's Physical & Mathematical Sciences department have devised calculations that indicate how to measure distances that are as small as 0.025 percent of the wavelength of light.
The researchers used a 100 nanometer-thick film of gold that has over 10,000 slits, which diffracted laser light. As the light was diffracted, "superoscillation" occurred. This is a phenomenon that occurs when a sub-wavelength within a light wave oscillates at a faster rate than the light wave itself. The light then produced two cross-polarized beams-the first one was a superoscillatory "interference pattern" that contained variation, and other was a reference wave. From there, the superoscillation gradient called the "local wavevector" was calculated. It had an extremely narrow width that was 400 times smaller than the set diffraction limit, making it a high resolution optical ruler.
The co-author and postdoctoral fellow at the Centre for Disruptive Photonic Technologies, Dr. Yuan, detailed the simplicity of their work. "Our device is conceptually very simple," he said. "What makes it work is the precise pattern in which the slits are arranged. There are two types of slits within the pattern, oriented at right angles to each other. When polarized laser light strikes the gold film, it creates an interference pattern containing extremely tiny features, much smaller than the wavelength of light."
Other co-author and NTU's The Photonics Institute co-director, Prof. Zheludev, emphasized how big a step this was for methods of optical measurement. "This phase-sensitive technique is a major improvement over previous attempts to use superoscillation for optical measurement," he said. "If the goal is to measure the shortest distances possible, phase superoscillations are much more suitable [than earlier methods] due to their smaller size."
The team hopes to use optical fibers in order to develop a compact version of the device, which they would later on commercialize. They are looking at manufacturing and quality control of small electronic devices as the area of application for it.