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Spectroscopy method could lead to better optical devices

Photonics Spectra
May 2013

PROVIDENCE, R.I. ­– A new spectroscopy method takes advantage of a fundamental property of thin films – interference – and could help optical devices like LEDs and solar cells make better use of these materials.

The technique, called energy-momentum spectroscopy, was developed by a multi-university research team to gain insight into how light is emitted from layered nanomaterials and other thin films. The method allows investigators 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.

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 – devised by scientists at Brown University working with colleagues from Case Western Reserve and Columbia universities, and the University of California, Santa Barbara – 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 a lead author of the study. “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 technique was demonstrated on molybdenum disulfide (MoS2), a 2-D material similar to graphene, and PTCDA, an organic semiconductor; each represents a class of materials promising for optical applications. 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.

Once the emitters’ orientations are known, Zia said, it may be possible to design structured devices that maximize those directional properties. Thin-film materials often 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.

“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 research was published in Nature Nanotechnology (doi: 10.1038/nnano.2013.20).

1. The additive process whereby the amplitudes of two or more overlapping waves are systematically attenuated and reinforced. 2. The process whereby a given wave is split into two or more waves by, for example, reflection and refraction of beamsplitters, and then possibly brought back together to form a single wave.
thin film
A thin layer of a substance deposited on an insulating base in a vacuum by a microelectronic process. Thin films are most commonly used for antireflection, achromatic beamsplitters, color filters, narrow passband filters, semitransparent mirrors, heat control filters, high reflectivity mirrors, polarizers and reflection filters.
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