Tel Aviv scientists put matter in its place

Marie Freebody, marie.freebody@photonics.com

The ability to manipulate matter using lasers is enabling scientists at Tel Aviv University to develop innovative photonic devices. Dr. Yael Roichman and colleagues at the university’s school of chemistry have come up with a novel way of assembling photonic crystals, which they hope will aid the development of anything from optical microscopes to light-fueled computer technology.

The ultimate goal is to build lab-on-a-chip devices that will be better able to control light and guide it on paths on the scale of microns. Using these chips, high-throughput, small-scale experiments could be built, which will enable biological studies of bacteria and yeast, for example.

“We hope to have our first prototype done within the coming year,” Roichman said. “I hope the devices we are trying to make will benefit the entire photonics community by paving the way to new designs. What’s more, our fabrication process lends itself to integration in larger all-optical devices based on soft lithography.”

Dr. Yael Roichman is pictured in her laboratory at Tel Aviv University with her students. Courtesy of Tel Aviv University.

Roichman employs a tool known as holographic optical tweezers (HOTs) to painstakingly move photonic crystals into place to form larger structures. HOTs operate by trapping and moving tiny particles within a highly focused laser beam. If light from the laser is sufficiently focused, it provides an attractive or repulsive force on the particle, enabling matter to be manipulated on the nanoscale.

Using HOTs to assemble photonic devices was first done by Roichman during her time at New York University as a postdoctoral student. Now, she is improving the process so that the quality of the final product is better in two key ways: The positioning of the building blocks is more accurate, and it is coupled to input and output fibers in an optimal way.

“What we have achieved is to design a method to fabricate these materials within the coupling box, putting the coupling optical fibers in place before we assemble the colloidal particles,” Roichman explained. “This allows us to assemble the device and position it directly around the input/output optical fibers.”

This means that the new material can be fabricated around light sources such as lasers as well as measuring devices such as power meters and spectrometers. Until now, this step in the construction involved coupling a device measuring 50 x 50 µm with an accuracy of ±1 µm – a complicated process that must be done skillfully for the device to work.

In the approach, 2- and 3-D holograms are used: 2-D for first assembly, and then a sequence of 3-D holograms is projected to guide the particles into their 3-D positions. Once the 3-D structure is assembled, Roichman shines UV light onto the fluid to fix the particles within the gel.

To aid construction, the Tel Aviv team has also built a unique microscope combining HOTs and confocal microscopy. This enables the group to monitor the fabrication process in real time, ensuring accurate positioning of the particles and also playing an essential role in optimizing the performance of the devices.

The group is currently working on the design and fabrication of its first prototype, which it hopes to characterize in the coming year. “I believe we could use our soft material to make tunable and durable devices,” Roichman commented. “Ultimately, we plan to apply our technique to make lab-on-a-chip devices as well as all-optical devices.”