Skip to main content

Chiral metamaterial produces record optical shift under incremental power modulation

Laser light shows the nanopatterned structure of a chiral metamaterial developed by researchers in the School of Electrical and Computer Engineering at the Georgia Institute of Technology. Credit: Rob Felt, Georgia Tech
Georgia Institute of Technology researchers have demonstrated an optical metamaterial whose chiroptical properties in the nonlinear regime produce a significant spectral shift with power levels in the milliwatt range.

The researchers recently demonstrated properties of their chiral metamaterial, in which they spectrally modified two absorptive resonances by incrementally exposing the material to power intensities beyond its linear optical regime. With a 15 milliwatt change in excitation power, they measured a 10-nanometer spectral shift in the material's transmission resonances and a 14-degree polarization rotation.
The researchers believe that may be the strongest nonlinear optical rotation ever reported for a chiral metamaterial, and is about a hundred thousand times larger than the current record measurement for this type of structure. The research, supported by the National Science Foundation and the Air Force Research Laboratory, was reported February 27 in the journal Nature Communications.
"Nanoscale chiral structures offer an approach to modulating optical signals with relatively small variations in ," said Sean Rodrigues, a Ph.D. candidate who led the research in the laboratory of Associate Professor Wenshan Cai in Georgia Tech's School of Electrical and Computer Engineering. "To see this kind of change in such a thin material makes chiroptical metamaterials an interesting new platform for optical signal modulation."
This modulation of chiroptical responses from metamaterials by manipulating input power offers the potential for new types of active optics such as all-optical switching and light modulation. The technologies could have applications in such areas as data processing, sensing and communications.
Chiral materials exhibit optical properties that differ depending on their opposing circular polarizations. The differences between these responses, which are created by the nanoscale patterning of absorptive materials, can be utilized to create large chiroptical resonances. To be useful in applications such as all-optical switching, these resonances would need to be induced by external tuning - such as variations in power input.
"When you increase the power, you shift the spectrum," Rodrigues said. "In effect, you change the transmission at certain wavelengths, meaning you're changing the amount of light passing through the sample by simply modifying input power." For optical engineers, that could be the basis for a switch.
The material demonstrated by Cai's lab are made by nano-patterning layers of silver - approximately 33 nanometers thick - onto glass substrates. Between the carefully-designed silver layers is a 45-nanometer layer of dielectric material. An elliptical pattern is created using electron beam lithography, then the entire structure is encapsulated within a dielectric material to prevent oxidation.
"It is the engineering of these structures that gives us these chiral optical properties," Rodrigues explained. "The goal is really to take advantage of the discrepancy between one circular polarization versus the other to create the broadband resonances we need."
The material operates in the visible to near-infrared spectrum, at approximately 740 to 1,000 nanometers. The optical rotation and circular dichroism measurements were taken with the beam entering the material at a normal incident angle.
The researchers induced the change in circular dichroism by increasing the optical power applied to the material from 0.5 milliwatts up to 15 milliwatts. While that is comparatively low power for a laser system, it has a high enough energy flux (energy transfer in time) to instigate change.
"The beam size is roughly 40 microns, so it is really focused," said Rodrigues. "We are putting a lot of energy into a small area, which causes the effect to be fairly intense."
The researchers don't yet know what prompts the change, but suspect that thermal processes may be involved in altering the material's properties to boost the circular dichroism. Tests show that the  applications do not damage the metamaterial.
Cai's laboratory has been studying chiral  of different kinds for a variety of optical applications. In June 2015, they reported the realization of one of the long-standing theoretical predictions in nonlinear optical metamaterials: creation of a nonlinear material that has opposite refractive indices at the fundamental and harmonic frequencies of light. Such a material, which doesn't exist naturally, had been predicted for nearly a decade.

Comments

Post a Comment

Popular posts from this blog

This strange mineral grows on dead bodies and turns them blue

If you were to get up close and personal with Ötzi the Iceman – the 5,000-year-old mummy of a  tattooed ,  deep-voiced  man who died and was frozen in the Alps – you’d notice that his skin is flecked with tiny bits of blue. At first, it would appear that these oddly bluish crystal formations embedded in his skin are from freezing to death or some other sort of trauma, but it’s actually a mineral called  vivianite  (or blue ironstone) and it happens to form quite often on corpses left in iron-rich environments. For Ötzi, the patches of vivianite are  from him resting  near rocks with flecks of iron in them, but other cases are way more severe. According to Chris Drudge at Atlas Obscura , a man named John White was buried in a cast iron coffin back in 1861. During those days, coffins often had a window for grieving family members to peer inside even if the lid was closed during the funeral. Sometime after he was buried, that window broke, allow...
Post a Comment
Read more

It's Official: Time Crystals Are a New State of Matter, and Now We Can Create Them

Peer-review has spoken. Earlier this year , physicists had put together a blueprint for how to make and measure time crystals - a bizarre state of matter with an atomic structure that repeats not just in space, but in time, allowing them to maintain constant oscillation without energy. Two separate research teams managed to create what looked an awful lot like time crystals  back in January,  and now both experiments have successfully passed peer-review for the first time, putting the 'impossible' phenomenon squarely in the realm of reality. "We've taken these theoretical ideas that we've been poking around for the last couple of years and actually built it in the laboratory,"  says one of the researchers , Andrew Potter from Texas University at Austin. "Hopefully, this is just the first example of these, with many more to come." Time crystals  are one of the coolest things physics has dished up in recent months, because they point to a...
Post a Comment
Read more

The Dark Side Of The Love Hormone Oxytocin

New research shows oxytocin isn't the anti-anxiety drug we thought it was. Oxytocin, the feel-good bonding hormone released by physical contact with another person, orgasm and childbirth (potentially encouraging  monogamy ), might have a darker side. The  love drug  also plays an important role in intensifying  negative emotional memories  and increasing feelings of fear in future stressful situations, according to a new study. Two experiments performed with mice found that the hormone activates a signaling molecule called extracellular-signal-related kinases (ERK), which has been associated with the way the brain  forms memories   of fear . According to Jelena Radulovic, senior author on the study and a professor at Northwestern University's medical school, ERK stimulates fear pathways in the brain's lateral septum, the region with the highest levels of oxytocin. Mice without oxytocin receptors and mice with even more oxytocin receptors tha...
Post a Comment
Read more