New Optical Material Could Potentially Transform Optoelectronic Technologies

The picture shows a layer of phase-transition material SmNiO3 placed on top of a Columbia Engineering School logo. The transparency of the material can be controlled by electron doping under ambient conditions. Pristine SmNiO3 is opaque; partial phase-transition makes the material translucent, and complete phase-transition makes it transparent. (Credit - Nanfang Yu, Columbia Engineering)

Researchers led by Nanfang Yu, assistant professor of applied physics at Columbia Engineering, have found a new phase-transition optical material and demonstrated novel devices that dynamically manipulate light over a much wider wavelength range and with larger modulation amplitude than what was previously possible.

The researchers were from a number of universities, including Purdue, Harvard, Drexel, and Brookhaven National Laboratory. They discovered that samarium nickelate (SmNiO3) can be electrically tuned endlessly between an opaque and a transparent state over an unparalleled wide range of spectrum from the blue in the visible with wavelength of 400 nm to the thermal radiation spectrum in the mid-infrared with wavelength of a few tens of micrometers.

This research is the first to investigate the optical properties of SmNiO3 and the first demonstration of the material in photonic device applications. It is published online today in Advanced Materials.

The performance of SmNiO3 is record-breaking in terms of the magnitude and wavelength range of optical tuning. There is hardly any other material that offers such a combination of properties that are highly desirable for optoelectronic devices. The reversible tuning between the transparent and opaque states is based on electron doping at room temperature, and potentially very fast, which opens up a wide range of exciting applications, such as 'smart windows' for dynamic and complete control of sunlight, variable thermal emissivity coatings for infrared camouflage and radiative temperature control, optical modulators, and optical memory devices.

Nanfang Yu, Assistant Professor, Columbia Engineering

Some of the potential new operations include using SmNiO3's ability in manipulating thermal radiation to construct "intelligent" coatings for thermoregulation and infrared camouflage. These coatings could make people and vehicles seem much colder than they really are, and therefore barely visible under a thermal camera during the night.

The coating could help to decrease the large temperature gradients on a satellite by modifying the relative thermal radiation from its dark and bright side in relation to the sun and extend the service life of the satellite.

Since this phase-transition material can potentially switch between the opaque and transparent states with high speed, it could be used in modulators in optical memory devices, and for free-space optical communication and optical radar.

For a long time, researchers have been eager to build active optical devices that can dynamically manipulate light. These include Boeing 787 Dreamliner's "smart windows," which control - although not completely - the transmission of sunlight, high-data-rate, long-distance fiber optic communications systems where data is "written" into light beams by optical modulators, and rewritable DVD discs on which a laser beam can be used to write and delete data.

Active optical devices are not widely used in daily life, but it has been very difficult to find modern actively tunable optical materials and to prepare suitable device architectures that magnify the effects of such tunable materials.

When Shriram Ramanathan, associate professor of materials science at Harvard, found SmNiO3's enormous tunable electric resistivity at room temperature, Yu made a note of it. The two met at the IEEE Photonics Conference in 2013 and planned to work together.

Yu, with his students, began working with Ramanathan, who is a co-author of this paper. They conducted preliminary optical studies of the phase-transition material, combined the material into nanostructured designer optical interfaces--"metasurfaces"--and developed prototype active optoelectronic devices, such as optical modulators that control a beam of light, and variable emissivity coatings that manipulate the efficiency of thermal radiation.

SmNiO3 is really an unusual material,because it becomes electrically more insulating and optically more transparent as it is doped with more electrons--this is just the opposite of common materials such as semiconductors.

Zhaoyi Li, PhD student, Columbia Engineering

It was observed that doped electrons "lock" into pairs with the electrons originally in the material, a quantum mechanical occurrence referred to as "strong electron correlation," and this impact makes these electrons unavailable to perform electric current and absorb light. Therefore, after electron doping, SmNiO3 thin films that were initially opaque abruptly allow over 70% of visible light and infrared radiation to pass through.

"One of our biggest challenges," Zhaoyi adds, "was to integrate SmNiO3 into optical devices. To address this challenge, we developed special nanofabrication techniques to pattern metasurface structures on SmNiO3 thin films. In addition, we carefully chose the device architecture and materials to ensure that the devices can sustain high temperature and pressure that are required in the fabrication process to activate SmNiO3."

Going forward, Yu and his partners plan to conduct a systematic study to comprehend the basic science of the phase transition of SmNiO3 and to investigate its technological applications. The team will test the intrinsic speed of phase transition and the number of phase-transition cycles the material can handle before it fails.

They will also explore issues relating to technology, including synthesizing smooth and ultra-thin films of the material and formulating nanofabrication methods to combine the material into novel flat optical devices.

This work is one crucial step towards realizing the major goal of my research lab, which is to make an optical interface a functional optical device. We envision replacing bulky optical devices and components with 'flat optics' by utilizing strong interactions between light and two-dimensional structured materials to control light at will. The discovery of this phase-transition material and the successful integration of it into a flat device architecture are a major leap forward to realizing active flat optical devices not only with enhanced performance from the devices we are using today, but with completely new functionalities.

Nanfang Yu, Assistant Professor, Columbia Engineering

Yu's team included Ramanathan, his Harvard PhD student You Zhou, and his Purdue postdoctoral fellow Zhen Zhang, who synthesized the phase-transition material and performed portions of the phase transition experiments (this work started at Harvard and continued when Ramanathan moved to Purdue); Drexel University Materials Science Professor Christopher Li, PhD student Hao Qi, and research scientist Qiwei Pan, who helped make solid-state devices by integrating SmNiO3 with novel solid polymer electrolytes; and Brookhaven National Laboratory staff scientists Ming Lu and Aaron Stein, who assisted in device nanofabrication. Yuan Yang, Assistant Professor of Materials Science and Engineering in the Department of Applied Physics and Applied Mathematics at Columbia Engineering, was consulted during the progress of this study.

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