Detecting Molecules with Squeezed Light

Nanodiscs act as microresonators, trapping infrared photons and generating polaritons. When illuminated with infrared light, the discs concentrate light in a volume thousands of times smaller than is possible with standard optical materials. (Source: Harvard SEAS)

Researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS have developed a new technique to squeeze infrared light into ultra-confined spaces, gene­rating an intense, nanoscale antenna that could be used to detect single bio­molecules. The researchers harnessed the power of polaritons, particles that blur the distinc­tion between light and matter. This ultra-confined light can be used to detect very small amounts of matter close to the polari­tons. For example, many hazardous substances, such as formal­dehyde, have an infrared signa­ture that can be magnified by these antennas. The shape and size of the polari­tons can also be tuned, paving the way to smart infrared detectors and bio­sensors.

“This work opens up a new frontier in nano­photonics,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics. “By coupling light to atomic vibra­tions, we have concen­trated light into nano­devices much smaller than its wavelength, giving us a new tool to detect and mani­pulate molecules.” Polaritons are hybrid quantum mechanical particles, made up of a photon strongly coupled to vibrating atoms in a two-dimen­sional crystal. “Our goal was to harness this strong inter­action between light and matter and engineer polari­tons to focus light in very small spaces,” said Michele Tamagnone, post­doctoral fellow in Applied Physics at SEAS.

The researchers built nano-discs – the smallest about 50 nano­meters high and 200 nano­meters wide – made of two-dimen­sional boron nitride crystals. These materials act as micro­resonators, trapping infrared photons and generating polari­tons. When illu­minated with infrared light, the discs were able to concen­trate light in a volume thousands of times smaller than is possible with standard optical materials, such as glass. At such high concen­trations, the researchers noticed something curious about the behavior of the polari­tons: they oscil­lated like water sloshing in a glass, changing their oscil­lation depending on the frequency of the incident light.

“If you tip a cup back-and-forth, the water in the glass oscil­lates in one direction. If you swirl your cup, the water inside the glass oscillates in another direction. The polaritons oscil­late in a similar way, as if the nano-discs are to light what a cup is to water,” said Tamagnone. Unlike tradi­tional optical materials, these boron nitride crystals are not limited in size by the wave­length of light, meaning there is no limit to how small the cup can be. These materials also have tiny optical losses, meaning that light confined to the disc can oscil­late for a long time before it settles, making the light inside even more intense.

The researchers further concen­trated light by placing two discs with matching oscil­lations next to each other, trapping light in the 50-nanometer gap between them and creating an infrared antenna. As light concen­trates in smaller and smaller volumes, its intensity increases, creating optical fields so strong they can exert measurable force on nearby particles. “These light-induced forces serve also as one our detec­tion mechanisms,” said Antonio Ambrosio, a principal scientist at Harvard’s Center for Nanoscale Systems. “We observed this ultra-confined light by the motion it induces on an atomically sharp tip connected to a canti­lever.” A future challenge for the Harvard team is to optimize these light nano-concen­trators to achieve inten­sities high enough to enhance the inter­action with a single molecule to detectable values. (Source: Harvard SEAS)

Reference: M. Tamagnone et al.: Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures, Sci. Adv. 4, eaat7189 (2018); DOI: 10.1126/sciadv.aat7189

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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