Optical Fiber Probes Molecular Bonds

Visualization of the fiber-in-fiber-out process for optical spectroscopy measurement. (Source: Liu Group, UCR)

Shrinking a light beam to a nanometer-sized point to spy on quantum-scale light-matter inter­actions and retrieving the infor­mation is not easy. Now, engineers at the Univer­sity of California, Riverside, have developed a new technology to tunnel light into the quantum realm at an unpre­cedented effi­ciency. A team led by Ruoxue Yan, an assistant professor of chemical and environ­mental engineering, and Ming Liu, an assistant professor of electrical and computer engineering, describe the world’s first portable, inexpensive, optical nano­scopy tool that integrates a glass optical fiber with a silver nanowire condenser. The device is a high-effi­ciency round-trip light tunnel that squeezes visible light to the very tip of the condenser to interact with molecules locally and send back information that can decipher and visualize the elusive nanoworld.

In the early 1990s, Eric Betzig, the 2014 Nobel laureate in chemistry, made substantial improvements to earlier prototypes in imaging per­formance and reliability. Since then, near-field scanning optical micro­scopy, as the technique is known, has been used to reveal the nanoscale details of many chemical, biological, and material systems. Unfor­tunately, almost another half-century later, this technique is still esoteric and used by few. “Sending light through a tiny pinhole a thousand-times smaller than the diameter of a strand of human hair is no piece of cake,” Liu said. “Only a few in a million photons, or light particles, can pass the pinhole and reach the object you want to see. Getting a one-way ticket is already challenging; a round-trip ticket to bring back a meaningful signal is almost a daydream.”

Scientists have made endless efforts to improve this chance. While the most sophis­ticated probes today allow only one in 1,000 photons to reach the object, the new device delivers half the photons to the tip. “The key of the design is a two-step sequential focusing process,” Yan said. “In the first step, the wavelength of the far-field light slowly increases as it travels down a gradually thinning optical fiber, without changing its frequency. When it matches the wave­length of the electron density wave in the silver nanowire lying on top of the optical fiber, boom! All energy is transferred to the electron density wave and starts to travel on the surface of the nanowire instead.” In the second step of the focusing process, the wave gradually condenses to a few nanometers at the tip apex.

The device, a tiny silver needle with light coming off the tip “is sort of like Harry Potter’s wand that lights up a tiny area,” explained Sanggon Kim, the doctoral student who carried out the study. Kim used the device to map out the frequency of molecular vibra­tions that allow one to analyze chemical bonds that hold atoms together in a molecule. This tip-enhanced Raman spectro­scopy imaging is the most challenging branch of near-field optical microscopy, because it deals with very weak signals. It usually requires bulky, million-dollar equipment to concen­trate light and tedious pre­paration work to get super-resolution images.

With the new device, Kim achieved 1-nanometer resolution on a simple portable equipment. The invention could be a powerful analytical tool that promises to reveal a new world of infor­mation to researchers in all disciplines of nano­science. “The integ­ration of a fiber-nanowire assembly with tip-enhanced Raman spectro­scopy coupled with a scanning tunneling microscope enables the collection of high-resolution chemical images in a simple and elegant setup, placing this tool at the forefront of optical imaging and spectro­scopy. We are proud of this achieve­ment and its impact on chemical research. We are even more encouraged by its potential application in a wide array of disciplines such as bio­logical and materials research, which will further scientific advancement,” said Lin He, acting deputy division director for the National Science Foun­dation Division of Chemistry that in part funded the research. (Source: UC Riverside)

Reference: S. Kim et al.: High external-efficiency nanofocusing for lens-free near-field optical nanoscopy, Nat. Phot. online 10 June 2019; DOI: 10.1038/s41566-019-0456-9

Link: Dept. of Chemical and Environmental Engineering, University of California, Riverside, USA

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