Spring 2008, Issue 8

Manipulating Light

Fiorenzo OmenettoFiorenzo (Fio) Omenetto, PhD, joined Tufts in September 2005 as an associate professor in the departments of biomedical engineering and physics. While finishing his PhD in electrical engineering - applied physics from the University of Pavia, Italy, he worked as a department of physics visiting research specialist in the Laboratory of Atomic, Molecular and Radiation Spectroscopy at the University of Illinois. He did postdoctoral research in the Material Science and Technology Division of the Los Alamos National Laboratory and was a J. Robert Oppenheimer Fellow before being appointed staff scientist in the Physics Division in 2002.

Omenetto directs the Ultrafast Nonlinear Optics and Biophotonics Group in the Department of Biomedical Engineering. This group investigates the generation, shaping, control, and use of light, especially femtosecond [one quadrillionth (10-15) of a second] laser pulses. Ultrafast pulsed lasers produce enormous peak power (100 million to a billion watts) from a small amount of energy because the energy is divided by an extremely small amount of time (e.g., 100 watt-seconds/10-7 second = 109 watts). One well-known and successful use of femtosecond lasers is to open the flap of the cornea during Lasik eye surgery. The laser delivers precise cutting in such short bursts that heat doesn’t have time to dissipate and there is very little “collateral damage.”

Such laser pulses can be reshaped by varying their temporal length, their profile, and their delivery rate. “These shaped pulses deliver a designed excitation to the material under study,” says Omenetto. “The addition of feedback to a laser setup with a programmable pulse shaper allows the experiment to ‘learn by itself’ and search for an optimal operating point.” Omenetto’s research group is using adaptive feedback techniques to optimize and enhance light propagation and nonlinear imaging in biological samples. They are also looking into using lasers to interact with discrete biological targets as small as a single cell, or even a cell nucleus.

The materials that guide the pulses can also shape and transform them. Photonic crystal fibers (PCFs) are a new class of optical fibers that allow unprecedented control of light. Rather than the solid cores of regular optical fibers, PCFs have microscopic hollow tubes running the length of the fibers. The arrangement and diameters of these tubes determine the shape and properties of the laser pulses. For example, PCFs can be engineered to change a typical narrow-bandwidth one-color laser pulse into a broad-bandwidth white-light laser—a supercontinuum laser that covers the whole visible spectrum (and beyond). Supercontinua are currently being investigated for use in medical imaging, fluorescence microscopy, flow cytometry, and communications systems. Fluids and biological compounds can also be introduced into the PCF air spaces to further change the light properties and expand applications into sensing.

PCFs are usually made of glass or plastic. Omenetto is very excited about new classes of two-dimensional and three-dimensional photonic crystals that he is working on with David Kaplan, Stern Family Professor in Engineering and chair of biomedical engineering. “David had a project to do corneal replacement using silk as a scaffold to seed [corneal] cells,” Omenetto explained. “He wanted some porosity in the scaffold so it could interact very naturally with the environment, so he brought it to me to poke some holes in the scaffold with my femtosecond laser.” Omenetto realized the silk was a good optical material, and began working with Kaplan making optics out of silk.

Liquefied silk can be poured onto a template made of any nonhygroscopic material such as glass or plastic, allowed to crystallize, and peeled off with the template pattern replicated onto the silk. The silk films look like clear plastic. The pattern defines the silk optic’s use, such as for lenses, optical sensors, or projection optics encoding letters and other images. Silk optical sensors would have the advantage of the strong, flexible, benign, and biodegradable nature of silk, which is ideal for certain biological implants. “The other thing that strikes me is that it’s all processed in water at room temperature, which means you can include biological markers and biological indicators,” comments Omenetto. “So we’ve made optical elements that embed enzymes, and others that embed proteins such as hemoglobin. In the latter case, for instance, the hemoglobin stays active within the optic. You can store it on the shelf.” Omenetto’s research group, in collaboration with Kaplan's group, is currently doing pilot studies on the life times of peroxidase and other enzymes embedded in silk optics.

“As a sensing platform it becomes very interesting,” says Omenetto. “Imagine where you can entrain biologicals within a very sophisticated optical element in which the projected image depends on the activity of the biological, so you have projectable diagnostics. The silk optics and the silk photonic crystals are the most exciting projects for me right now. It’s entirely a new thing. You sense that there’s a lot of potential in the air.”

Omenetto is actively collaborating with many researchers in his own departments and several in other departments, while continuing collaborations outside Tufts. His lab embraces both basic and applied physical and biomedical science. Potential collaborators could be users of existing ultrafast laser pulses and biophotonic applications, or they could propose an entirely new application for these tools.

For more information, please go to http://ase.tufts.edu/biomedical/unolab/home.html.

 

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