New technique could improve optical devices

Understanding the source and orientation of light in light-emitting thin films — now possible with energy-momentum spectroscopy — could lead to better LEDs, solar cells, and other devices that use layered nanomaterials.

PROVIDENCE, R.I. [Brown University] — A multi-university research team has used a new spectroscopic method to gain a key insight into how light is emitted from layered nanomaterials and other thin films.

The orientation of light emission
The angular distribution of light emission from monolayer

MoS₂, left, closely matches the theoretical calculations for
in-plane oriented emitters, right, indicating that light
emission from MoS₂ originates from in-plane oriented
emitters. Credit: Zia lab/Brown University

The technique, called energy-momentum spectroscopy, enables researchers to look at the light emerging from a thin film and determine whether it is coming from emitters oriented along the plane of the film or from emitters oriented perpendicular to the film. Knowing the orientations of emitters could help engineers make better use of thin-film materials in optical devices like LEDs or solar cells.

The research, published online on March 3 in Nature Nanotechnology, was a collaborative effort of Brown University, Case Western Reserve University, Columbia University, and the University of California–Santa Barbara.

The new technique takes advantage of a fundamental property of thin films: interference. Interference effects can be seen in the rainbow colors visible on the surface of soap bubbles or oil slicks. Scientists can analyze how light constructively and destructively interferes at different angles to draw conclusions about the film itself — how thick it is, for example. This new technique takes that kind of analysis one step further for light-emitting thin films.

“The key difference in our technique is we’re looking at the energy as well as the angle and polarization at which light is emitted,” said Rashid Zia, assistant professor of engineering at Brown University and one of the study’s lead authors. “We can relate these different angles to distinct orientations of emitters in the film. At some angles and polarizations, we see only the light emission from in-plane emitters, while at other angles and polarizations we see only light originating from out-of-plane emitters.”

The researchers demonstrated their technique on two important thin-film materials, molybdenum disulfide (MoS2) and PTCDA. Each represents a class of materials that shows promise for optical applications. MoS2 is a two-dimensional material similar to graphene, and PTCDA is an organic semiconductor. The research showed that light emission from MoS2 occurs only from in-plane emitters. In PTCDA, light comes from two distinct species of emitters, one in-plane and one out-of-plane.

Rashid Zia
"If you were making an LED using these layered materials
and you knew that the electronic excitations were
happening across an interface, then there's a specific way
you want to design the structure to get all of that light out
and increase its overall efficiency."

Once the orientation of the emitters is known, Zia says, it may be possible to design structured devices that maximize those directional properties. In most applications, thin-film materials are layered on top of each other. The orientations of emitters in each layer indicate whether electronic excitations are happening within each layer or across layers, and that has implications for how such a device should be configured.

“If you were making an LED using these layered materials and you knew that the electronic excitations were happening across an interface,” Zia said, “then there’s a specific way you want to design the structure to get all of that light out and increase its overall efficiency.”

The same concept could apply to light-absorbing devices like solar cells. By understanding how the electronic excitations happen in the material, it could be possible to structure it in a way that coverts more incoming light to electricity.

“One of the exciting things about this research is how it brought together people with different expertise,” Zia said. “Our group’s expertise at Brown is in developing new forms of spectroscopy and studying the electronic origin of light emission. The Kymissis group at Columbia has a great deal of expertise in organic semiconductors, and the Shan group at Case Western has a great deal of expertise in layered nanomaterials. Jon Schuller, the study’s first author, did a great job in bringing all this expertise together. Jon was a visiting scientist here at Brown, a postdoctoral fellow in the Energy Frontier Research Center at Columbia, and is now a professor at UCSB.”

Other authors on the paper were Sinan Karaveli (Brown), Theanne Schiros (Columbia), Keliang He (Case Western), Shyuan Yang (Columbia), Ioannis Kymissis (Columbia) and Jie Shan (Case Western). Funding for the work was provided by the Air Force Office of Scientific Research, the Department of Energy, the National Science Foundation, and the Nanoelectronic Research Initiative of the Semiconductor Research Corporation.

by Kevin Stacey

Brown to lead multi-university quantum metamaterials research

Through a new Multidisciplinary University Research Initiative (MURI) awarded by the Air Force Office of Scientific Research, Brown will lead an effort to study new optical materials and their interactions with light at the quantum scale. The initiative, which includes six other top universities, will receive $4.5 million over three years, with a possible two-year extension.

Harnessing the power of light at the quantum scale could clear the way for superfast optical microprocessors, high-capacity optical memory, securely encrypted communication, and untold other technologies. But before any of these potential applications sees the light of day, substantial obstacles must be overcome — not the least of which is the fact that the wavelength of light is larger than quantum-scale objects, limiting the range of possible light-matter interactions.

Rashid Zia
"This program will bring together ten groups and 40-plus
researchers... to help answer questions that we couldn't
have imagined a short time ago. We are very optimistic
about where this will lead."

Rashid Zia, the Manning Assistant Professor of Engineering, will lead the team in addressing these challenges. He spoke recently with science writer Kevin Stacey.

What are you hoping to accomplish with this MURI?
We’re trying to help define an emerging field. The title of the MURI is “Quantum Metaphotonics and Quantum Metamaterials.” Ultimately what we’re trying to do is expand the range of materials and light-matter interactions available for quantum optics.

The field of metamaterials has already expanded the range of optical materials and phenomena available at larger, classical scales. People are doing things with metamaterials that we couldn’t have imagined before. For example, researchers are making metamaterials with negative refractive indices, which can literally bend light backward around objects. Others have used metamaterials to make lenses that can image things smaller than the diffraction limit of traditional lenses. What we’re doing now is asking what happens when we bring these metamaterials down to the scale of quantum emitters — the level of things that can emit a single photon at a time.

Can you talk a bit about the challenges involved in doing this?
When you talk about the way light interacts with matter at the quantum level, the types of interactions and the strength of those interactions are limited by a size mismatch. The optical wavelength is something like 100 times larger than a quantum emitter. For example, a quantum dot — a small bit of semiconductor we can use as a light emitter — is 5 to 10 nanometers. The wavelength of light is on the order of 500 to 1,000 nanometers. The problem is that the quantum dot doesn’t know there’s a wave. It can’t see the spatial variation of the light wave, just its local variation in time. So we need to shrink the wavelength of light to increase our interactions. Or we might increase the wavelength to collectively interact with many quantum emitters. And hopefully we can learn something fundamental about the nature of light that opens up new ways of manipulating these interactions. Those are the types of things we’ll be addressing.

In quantum optics we’re limited in part by the kinds of materials we can use. One of the common materials for quantum optics today is the nitrogen vacancy defect in diamond, so-called diamond NV centers. As you can imagine, diamond is not the cheapest or most scalable technology. The challenge posed for us is how to use the semiconductor materials we use for electronics and extend their optical properties with metamaterial designs, so we can perform quantum optics at wavelengths and with materials commonly used in telecommunications today.

How does the research you’re doing in your lab at Brown fit in?
It’s usually assumed that all light-matter interactions at visible frequencies result from the push-pull forces exerted by electric fields. These are called electric dipole transitions. One of the things we do in my lab is study things that aren’t electric dipoles — for example, magnetic dipoles. Because of the size mismatch we just discussed, it’s often assumed that magnetic dipole transitions are around 100,000 times less likely to happen than electric dipole transitions. In other words, it’s assumed that light emission from magnetic dipoles simply doesn’t happen. But the fact is we see magnetic dipole emission every day from the lanthanide ions that are commonly found in fluorescent lights. What we’ve been able to do is quantify the magnetic nature of light.

We just published a paper on this in Nature Communications. Basically, we demonstrated a way to tell how light was emitted, and rather than simply counting the number of photons a system generates, we can tell you which fraction of them came from electric dipoles and which fraction came from magnetic dipoles. This helps us understand fundamental properties about quantum emitters, the source of this light. It might also help us access higher-order light-matter interactions, enabling new ways to modulate light or to trap energy in optical excitations and get it out when you want, which could be useful for things like optical memory.

Who else is involved in this work?
The team includes people who are world-class experts in different areas. Nader Engheta at Penn, Nicholas Fang at MIT, and Xiang Zhang at UC–Berkeley are experts in metamaterials. Harry Atwater at CalTech and Mark Brongersma at Stanford are experts in plasmonics, which is the science of using metal structures to enhance light-matter interactions. Shanhui Fan and Jelena Vuckovic at Stanford are experts in quantum optics. Seth Bank at UT–Austin and Arto Nurmikko and me here at Brown, work on quantum emitters.

It’s really an exciting project. Over the next five years, this program will bring together 10 groups and 40-plus researchers with complementary expertise to help answer questions that we couldn’t have imagined a short time ago. We are very optimistic about where this will lead.

Quantifying the magnetic nature of light emission

In collaboration with The Institute of Photonics Sciences (ICFO) in Barcelona, Brown School of Engineering researchers in the lab of Rashid Zia ’01, Manning Assistant Professor of Engineering, have just published an article in in Nature Communications. Visiting scholar Tim Taminiau from the lab of Niek van Hulst at ICFO worked alongside Brown engineering graduate student Sinan Karaveli to demonstrate how the natural magnetic dipole transitions in lanthanide ions can be used to access optical-frequency magnetic fields.

Authors: Tim H. Taminiau, Sinan Karaveli, Niek F. van Hulst, and Rashid Zia

Examining how light emission is distributed in energy-
and momentum-space can reveal fundamental
information about optical transitions. This image
shows an energy-momentum spectrum of europium ions.
Following bright emissions lines, you may notice
several points where the contrast inverts - these
changes are direct visualizations of the opposite
symmetries of electric and magnetic dipoles transitions.

Abstract:
Tremendous advances in the study of magnetic light-matter interactions have recently been achieved using man-made nanostructures that exhibit and exploit an optical magnetic response. However, naturally occurring emitters can also exhibit magnetic resonances in the form of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic nature of light emission using energy- and momentum-resolved spectroscopy, and leverage a pair of spectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuum fluctuations in the electric and magnetic fields at the nanometre scale. These results reveal a new tool for nano-optics: an atomic-size quantum emitter that interacts with the magnetic component of light.

To access the full article, please go to:
http://www.nature.com/ncomms/journal/v3/n7/full/ncomms1984.html

Zia honored at White House

Assistant Professor Rashid Zia was honored with a Presidential Early Career Award for Scientists and Engineers (PECASE) at the White House last week. President Obama honored 100 winners with the PECASE, the highest award bestowed by the U.S. Government upon scientists and engineers in the early stages of their independent research careers.

More about the award from the White House blog.

Zia wins PECASE award

Rashid Zia, assistant professor of engineering, has been named one of this year’s winners of the Presidential Early Career Awards for Scientists and Engineers. The White House named the latest group of winners this week.

The PECASE award recognizes outstanding scientists and engineers who, early in their careers, show exceptional potential for leadership at the frontiers of knowledge. It is the highest honor bestowed by the U.S. government on scientists and engineers beginning their independent careers. In addition to the recognition, Zia is expected to receive $200,000 annually over five years to support his research.

More of the Brown press release here:
http://news.brown.edu/pressreleases/2009/07/pecase