Research Bits: March 7

Optical signal processing with acoustic waves
Researchers from Pohang University of Science & Technology (POSTECH) demonstrated an optical-wave signal that can be amplified or canceled using optically driven acoustic waves on a silicon chip.

Optical signal processing using Brillouin scattering, in which acoustic waves scatter light, has been demonstrated in nanophotonic structures. But research so far has only measured simple acoustic wave generation and light scattering. To determine the feasibility of active optical signal processing through optically driven acoustic wave control, the team proposed a method using the interference of acoustic waves generated on a silicon chip.

First, three optical waveguides were fabricated side by side on a silicon chip. Two acoustic waves were interfered with by generating acoustic waves in two optical waveguides using optical forces and controlling the time for them to reach the third optical waveguide.

The researchers observed the amplification and cancelation of a microwave signal between constructive and destructive interferences of acoustic waves on a silicon chip with a contrast greater than 10,000 times. They also demonstrated that the intensity of a pulse signal can be adjusted.

“This is the first demonstration of optical signal processing using acoustic-wave interference in a nanostructure,” said Heedeuk Shin, a processor in the Department of Physics at POSTECH. “Through this research, we present a new direction for optical signal processing and sensing technology, and look forward to new applications in the optomechanical system.”

Optical materials from plants
Researchers from Aalto University, University of Turku, Research Institute of Sweden, and University of British Columbia are looking into how plant biomass, called lignocellulose, can be used for optical applications, potentially replacing commonly used materials like sand and plastics.

Lignocellulose includes cellulose, hemicellulose, and lignin, which are found in nearly every plant. By breaking it down and recombining it in new ways, new materials can be created. The researchers assessed the various manufacturing processes and characteristics needed for optical applications, such as transparency, reflectiveness, UV-light filtering, and structural colors.

“We wanted to map out as comprehensively as possible how lignocellulose could replace the unrenewable resources found in widely used technology, like smart devices or solar cells,” said Jaana Vapaavuori, assistant professor of functional materials at Aalto University. “Through combining properties of lignocellulose, we could create light-reactive surfaces for windows or materials that react to certain chemicals or steam. We could even make UV protectors that soak up radiation, acting like a sunblock on surfaces.”

Kati Miettunen, professor of materials engineering at the University of Turku, added, “We can actually add functionalities to lignocellulose and customize it more easily than glass. For instance, if we could replace the glass in solar cells with lignocellulose, we could improve light absorption and achieve better operating efficiency.”

Lignin and cellulose are often extracted from trees, but the researchers said that the more than a billion tons of biomass waste created by industry and agriculture each year would be a source of valuable material that is currently underutilized.

“There is massive untapped potential in the leftovers of lignocellulose from other industries,” said Vapaavuori.

The researchers are still engaged in exploring the materials that can be created from lignocellulose and building prototypes. At Aalto University, scientists have developed light fibers and light-reactive fabrics.

Vapaavuori noted that there are two main ways scale up and commercialization can occur. “Either we create new uses for bio-based waste through government regulations or research brings about such cool demos and breakthroughs that it drives demand for renewable alternatives for optical applications. We believe that we need both political direction and solid research.”

One of the main obstacles is cost, but that is dropping enough to make industrial use possible. Another other is in lignocellulose’s affinity for water. “Cellulose loves water. To use it in optical applications, we need to find a way make it stable in humid conditions,” said Vapaavuori.