(Top) Scanning electron microscopy image of optically switchable chiral THz metamolecules, (Bottom) The purple, blue and tan colors represent the gold meta-atom structures at different layers, with the two silicon pads shown in green. (courtesy of Zhang, et. al) |
A
multi-institutional team of researchers that included scientists with
the U.S. Department of Energy’s Lawrence Berkeley National Laboratory
(Berkeley Lab) has created the first artificial molecules whose
chirality can be rapidly switched from a right-handed to a left-handed
orientation with a beam of light. This holds potentially important
possibilities for the application of terahertz technologies across a
wide range of fields, including reduced energy use for data-processing,
homeland security and ultrahigh-speed communications.
Chirality
is the distinct left/right orientation or “handedness” of some types of
molecules, meaning the molecule can take one of two mirror image forms.
The right-handed and left-handed forms of such molecules, called
“enantiomers,” can exhibit strikingly different properties. For example,
one enantiomer of the chiral molecule limonene smells of lemon, the
other smells of orange. The ability to observe or even switch the
chirality of molecules using terahertz (trillion-cycles-per-second)
electromagnetic radiation is a much coveted asset in the world of high
technology.
“Natural
materials can be induced to change their chirality but the process,
which involves structural changes to the material, is weak and slow.
With our artificial molecules, we’ve demonstrated strong dynamic
chirality switching at light-speed,” says Xiang Zhang, one of the
leaders of this research and a principal investigator with Berkeley
Lab’s Materials Sciences Division.
Working
with terahertz (THz) metamaterials engineered from nanometer-sized gold
strips with air as the dielectric – Zhang and his colleagues fashioned a
delicate artificial chiral molecule which they then incorporated with a
photoactive silicon medium. Through photoexcitation of their
metamolecules with an external beam of light, the researchers observed
handedness flipping in the form of circularly polarized emitted THz
light. Furthermore, the photoexcitation enabled this chirality flipping
and the circular polarization of THz light to be dynamically controlled.
“In
contrast to previous demonstrations where chirality was merely switched
on or off in metamaterials using photoelectric stimulation, we used an
optical switch to actually reverse the chirality of our THz
metamolecules,” Zhang says.
Zhang,
who holds the Ernest S. Kuh Endowed Chair Professor of Mechanical
Engineering at the University of California (UC) Berkeley, where he
also directs the Nano-scale Science and Engineering Center, is one of
three corresponding authors of a paper describing this work in Nature Communications.
The paper is titled “Photoinduced handedness switching in terahertz
chiral metamolecules.” The other corresponding authors are Shuang Zhang
of the University of Birmingham in the United Kingdom, and Antoinette
Taylor of DOE’s Los Alamos National Laboratory.
The
optically switchable chiral THz metamolecules consisted of a pair of
3D meta-atoms of opposite chirality made from precisely structured gold
strips. Each meta-atom serves as a resonator with a coupling between
electric and magnetic responses that produces strong chirality and large
circular dichroism at the resonance frequency.
“When
two chiral meta-atoms of the same shape but opposite chirality are
assembled to form a metamolecule, the mirror symmetry is preserved,
resulting in the vanishing of optical activity,” Zhang says. “From a
different point of view, the optical activity arising from these two
meta-atoms of opposite chirality cancels out each other.”
Schematic shows the chirality switching metamolecule consists of four chiral resonators with fourfold rotational symmetry. An external beam of light instantly reverses the metamolecule’s chirality from right-handed to left-handed. (courtesy of Zhang, et. al) |
Silicon
pads were introduced to each chiral meta-atom in the metamolecule but
at different locations. In one meta-atom, the silicon pad bridged two
gold strips, and in the other meta-atom, the silicon pad replaced part
of a gold strip. The silicon pads broke the mirror symmetry and induced
chirality for the combined metamolecule. The pads also functioned as the
optoelectronic switches that flipped the chirality of the metamolecule
under photoexcitation.
Says
corresponding author Shuang Zhang, “Our scheme relies on the
combination of two meta-atoms with opposite properties, in which one is
functional while the other is inactive within the frequency range of
interest. With suitable design, the two meta-atoms respond oppositely to
an external stimulus, that is, the inactive one becomes functional and
vice versa.”
THz
electromagnetic radiation—also known as T-rays—falls within the
frequency range of molecular vibrations, making it an ideal
none-invasive tool for analyzing the chemical constituents of organic
and non-organic materials. Being able to flip the handedness of chiral
metamolecules and control the circular polarization of THz light could
be used to detect toxic and explosive chemicals, or for wireless
communication and high-speed data processing systems. As most biological
molecules are chiral, including DNA, RNA and proteins, THz-based
polarimetric devices should also benefit medical researchers and
developers of pharmaceutical drugs among others.
“The
switchable chirality we can engineer into our metamaterials provides a
viable approach towards creating high performance polarimetric devices
that are largely not available at terahertz frequencies,” says
corresponding author Antoinette Taylor. “This frequency range is
particularly interesting because it uniquely reveals information about
physical phenomena such as the interactions between or within
biologically relevant molecules. It may enable control of electronic
states in novel material systems, such as cyclotron resonances in
graphene and topological insulators.”
Taylor
and her co-authors say that the general design principle of their
optically switchable chiral THz metamolecules is not limited to
handedness switching but could also be applied to the dynamic reversing
of other electromagnetic properties.
In addition to the corresponding authors, other authors of the Nature Communications paper were Jiangfeng Zhou, Yong-Shik Park, Junsuk Rho, Ranjan Singh, Sunghyun Nam, Abul Azad, Hou-Tong Chen and Xiaobo Yin.
This research was primarily supported by the DOE Office of Science.