A computer simulation of a one-dimensional cavity wave in a 200nm nanowire
engineering research at the University of Pennsylvania demonstrates
that polaritons have increased coupling strength when confined to
nanoscale semiconductors. This represents a promising advance in the
field of photonics: smaller and faster circuits that use light rather
research was conducted by assistant professor Ritesh Agarwal,
postdoctoral fellow Lambert van Vugt and graduate student Brian Piccione
of the Department of Materials Science and Engineering in Penn’s School
of Engineering and Applied Science. Chang-Hee Cho and Pavan Nukala,
also of the Materials Science department, contributed to the study.
Their work was published in the journal Proceedings of the National Academy of Sciences.
are quasiparticles, combinations of physical particles and the energy
they contribute to a system that can be measured and tracked as a single
unit. Polaritons are combinations of photons and another quasiparticle,
excitons. Together, they have qualities of both light and electric
charge, without being fully either.
exciton is a combination of a an electron, which has negative charge
and an electron hole, which has a positive charge. Light is an
oscillating electro-magnetic field, so it can couple with the excitons,”
Agarwal said. “When their frequencies match, they can talk to one
another; both of their oscillations become more pronounced.”
light-matter coupling strength is a key factor in designing photonic
devices, which would use light instead of electricity and thus be faster
and use less power than comparable electronic devices. However, the
coupling strength exhibited within bulk semiconductors had always been
thought of as a fixed property of the material they were made of.
Agarwal’s team proved that, with the proper fabrication and finishing techniques, this limit can be broken.
you go from bulk sizes to one micron, the light-matter coupling
strength is pretty constant,” Agarwal said. “But, if you try to go below
500 nanometers or so, what we have shown is that this coupling strength
difference is a function of one of nanotechnology’s principle
phenomena: the traits of a bulk material are different than structures
of the same material on the nanoscale.
you’re working at bigger sizes, the surface is not as important. The
surface to volume ratio — the number of atoms on the surface divided by
the number of atoms in the whole material — is a very small number,”
Agarwal said. “But when you make a very small structure, say 100
nanometers, this number is dramatically increased. Then what is
happening on the surface critically determines the device’s properties.”
researchers have tried to make polariton cavities on this small a
scale, but the chemical etching method used to fabricate the devices
damages the semiconductor surface. The defects on the surface trap the
excitons and render them useless.
cadmium sulfide nanowires are self-assembled; we don’t etch them. But
the surface quality was still a limiting factor, so we developed
techniques of surface passivation. We grew a silicon oxide shell on the
surface of the wires and greatly improved their optical properties,”
The oxide shell fills the electrical gaps in the nanowire surface, preventing the excitons from getting trapped.
also developed tools and techniques for measuring this light-matter
coupling strength,” Piccione said. “We’ve quantified the light-matter
coupling strength, so we can show that it’s enhanced in the smaller
Being able to quantify this increased coupling strength opens the door for designing nanophotonic circuit elements and devices.
stronger you can make light-matter coupling, the better you can make
photonic switches,” Agarwal said. “Electrical transistors work because
electrons care what other electrons are doing, but, on their own,
photons do not interact with each other. You need to combine optical
properties with material properties to make it work.”
research was supported by the Netherlands Organization for Scientific
Research Rubicon Programme, the U.S. Army Research Office, the National
Science Foundation, Penn’s Nano/Bio Interface Center and the National
Institutes of Health.