Squeezing light at the nanoscale

https://www.nanowerk.com/nanotechnology-news/newsid=50446.ph

Nanowerk News) 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.

The research is published in Science Advances ("Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures").

Nano-discs act as micro-resonators, 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. At such high concentrations, the polaritons oscillate like water sloshing in a glass, changing their oscillation depending on the frequency of the incident light. (Image: Harvard SEAS)

"This work opens up a new frontier in nanophotonics," said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, and senior author of the study. "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 and co-first author of the paper.

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 micro-resonators, 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: By Leah Burrows, Harvard John A. Paulson School of Engineering and Applied Sciences

A strange new world of light

By Leah Burrows,

https://www.nanowerk.com/nanotechnology-news/newsid=48524.php (Nanowerk News) There's nothing new thing under the sun -- except maybe light itself.

Over the last decade, applied physicists have developed nanostructured materials that can produce completely new states of light exhibiting strange behavior, such as bending in a spiral, corkscrewing and dividing like a fork.

These so-called structured beams not only can tell scientists a lot about the physics of light, they have a wide range of applications from super-resolution imaging to molecular manipulation and communications.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have developed a tool to generate new, more complex states of light in a completely different way.

A metasurface uses circularly polarized light to generate and control new and complex states of light, such swirling vortices of light. The new tool can be used to not only explore new states of light but also new applications for structured light. (Image: Second Bay Studio/Harvard SEAS)

The research is published in Science.

"We have developed a metasurface which is a new tool to study novel aspects of light," said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper. "This optical component makes possible much more complex operations and allows researchers to not only explore new states of light but also new applications for structured light."

The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialization opportunities.

The new metasurface connects two aspects of light, known as orbital angular momentum and circular polarization (or spin angular momentum). Polarization is direction along which light vibrates. In circularly polarized light, the vibration of light traces a circle. Think about orbital angular momentum and circular polarization like the motion of a planet. Circular polarization is the direction in which a planet rotates on its axis while orbital momentum describes how the planet orbits the sun.

The fact that light can even carry orbital momentum is a relatively recent discovery -- only about 25 years old?-- but it's this property of light which produces strange new states, such as beams in the shape of corkscrews.

A metasurface can generate strange new beams of light that swirl and corkscrew. The black hole in the center of these vortices can be used to image features smaller than half a wavelength of light or move tiny molecules. (Image: Capasso Lab/Harvard SEAS)

Previous research has used the polarization of light to control the size and shape of these exotic beams but the connection was limited because only certain polarizations could convert to certain orbital momentums.

This research, however, significantly expands that connection.

"This metasurface gives the most general connection, through a single device, between the orbital momentum and polarization of light that's been achieved so far," said Robert Devlin, co-first author of the paper and former graduate student in the Capasso Lab.

The device can be designed so that any input polarization of light can result in any orbital angular momentum output -- meaning any polarization can yield any kind of structured light, from spirals and corkscrews to vortices of any size. And, the multifunctional device can be programmed so that one polarization results in one vortex and a different polarization results in a completely different vortex.

"This is a completely new optical component," said Antonio Ambrosio, Principal Scientist at Harvard Center for Nanoscale Systems (CNS) and co-first author of the paper. "Some metasurfaces are iterations or more efficient, more compact versions of existing optical devices but, this arbitrary spin-to-orbital conversion cannot be done with any other optical device. There is nothing in nature as well that can do this and produce these states of light."

One potential application is in the realm of molecular manipulation and optical tweezers, which use light to move molecules. The orbital momentum of light is strong enough to make microscopic particles rotate and move.

"You can imagine, if we illuminate the device with one polarization of light, it will create a force of a particular kind," said Ambrosio. "Then, if you want to change the force, all you need to do is change the polarization of the incoming light. The force is directly related to the design of the device."

Another application is high-powered imaging. The black hole in the center of the vortex, known as the zero-light intensity region, can image features smaller than the diffraction limit, which is usually half of the wavelength of light. By changing the polarization of light, the size of this center region can be changed to focus different-sized features.

But these beams can also shed light on fundamental questions of physics.

"These particular beams are first and foremost of fundamental scientific interest," said Noah Rubin, co-first author of the paper and graduate student in the Capasso Lab. "There is interest in these beams in quantum optics and quantum information. On the more applied side, these beams could find application in free-space optical communication, especially in scattering environments where this is usually difficult. Moreover, it has been recently shown that similar elements can be incorporated into lasers, directly producing these novel states of light. This may lead to unforeseen applications."