Twisted nanoscale semiconductors manipulate light in new way

A new photonic effect, chiroptical third-harmonic Mie scattering, was discovered by University of Bath (UK) and University of Michigan (Ann Arbor, MI) researchers working with inorganic chiral materials. This effect can be tapped to speed the discovery and development of new medicines, as well as for quantum optics and quantum computing.

The inspiration for this work came from the many original inorganic chiral materials that Nicholas Kotov, a professor of chemical sciences and engineering at the University of Michigan, is developing within his lab.

Many nonbilaterally symmetric molecules, such as DNA, in biological systems typically are found oriented in only one direction, forming either a right-handed or left-handed curl. Chirality describes the direction a molecule twists, and chiral compounds tend to be optically active.

It turns out that helices of cadmium telluride (CdTe)—a semiconductor that resembles short segments of twisted ribbon—are made by self-assembly of chiral nanoparticles, in a manner similar to organic molecules.

“This process was inspired by the self-assembly of the biological chiral molecule into helices, such as DNA,” says Kotov. “The difference is that nanoparticles are thousands of times heavier and not monodispersed—yet chirality works. Semiconductor nanohelices are similar to some biological structures.”

Novel discovery

When Ventsislav Valev, a professor of physics at the University of Bath, came across these chiral particles, he was fascinated and immediately convinced they must have novel and unexpected properties.

Among these properties, the researchers discovered that when circularly polarized light at a certain wavelength (around 1100 nm) illuminates CdTe nanohelices, light at the third harmonic streams out on the opposite side from where the particles are illuminated.

In other words: When illuminated with red light, small semiconductor helices generate new light that’s blue and elliptically polarized (see figure). “This blue light is emitted in a specific direction, which makes it easy to collect and analyze,” notes Kotov. “The trifecta of unusual optical effects drastically reduces the noise that other nanoscale molecules and particles in biological fluids may cause.”

“The intensity of third-harmonic light depends on the direction of circularly polarized light and on the chirality of the nanohelix (which way the helix twists). To the best of our knowledge, it has never been observed before in any materials.”

There were many surprises along the way. Because of the way the nanohelices assemble, Valev and Kotov initially thought there might be tiny electric dipoles along the nanohelices that would scatter light in all directions—Rayleigh scattering. “So we set out to measure the third-harmonic scattering signal at the right angle from the direction of illumination,” Valev explains. “This is a good geometry because it only measures scattered light away from the direction of illumination and there are few artefacts.”

For months, they failed to record any appreciable signals. They wondered: Could it be that these materials don’t have the properties we were hoping for? “But then we tried measuring along the direction of propagation and all of a sudden there was a huge signal,” Valev says.

They immediately thought it must be an artefact, and what they were seeing was either the 1100 nm laser or some third-harmonic generation beam from one of the optical components.

Then, after checking and rechecking everything, “we finally were forced to acknowledge that this was not Rayleigh but the much more directional Mie scattering we were observing,” Valev says. “This is because the nanohelices were interacting with light not as separate building blocks, but as a single large structure. No wonder we couldn’t measure much signal at 90°! It’s quite cool that the effect is so directional. From the point of view of detection, this makes everything quite ‘straightforward,’ literally.”

Also, the volume within which they can probe for chirality is tiny—meaning the new effect is compatible with requirements for high-throughput chemistry applications.

“High-throughput chemistry relies on preparing many—sometimes thousands—of chemical mixtures simultaneously,” says Valev. “These are prepared on standard microplates the size of a chocolate bar. When there are thousands of wells, each one needs to be miniscule (microliters or smaller). Characterizing chirality in such wells is challenging for existing techniques and this is where our effect could find applications.”

It's fast (proceeds at the speed of light), scalable (with multiple laser beams), and needs only tiny volumes to be illuminated (<1 microliter). “Of course, more research is needed to assess the limits of applicability but, if successful, the technique could one day help in the development of new pharmaceuticals, most of which are chiral today,” Valev notes.

Kotov was surprised by how strong the effect is. “The circular dichroism in typical biological molecules is measured in millidegrees (one thousandth of a degree),” he says. “This effect on semiconductor nanohelices is a thousand times stronger (degrees).”

This method “offers a potent tool for drug discovery as a part of high-throughput screening arsenal, especially for large drugs with molecular mass comparable to proteins,” Kotov adds. He also anticipates applications of this effect for quantum optics and optical computing.

What’s next? “Understanding how the nanohelices assemble from chiral particles give us a path to a wide range of nanohelices,” says Kotov. “I’d like to see what it would take to produce blue third-harmonic light with polarization above 45 degrees. It will be a game-changer for optical devices and facilitate their applications in high-throughput screening.”