Optical "Tweezers” for Microscopy

Using a white-light laser, researchers have produced a new type of optical "tweezers” that can trap, hold, and move microscopic objects and also perform characterization of the object via spectroscopy at the same time.

Optical spectroscopy enables researchers to probe the size, shape, refractive index, and chemical composition of a particle. Engineers at Penn State (University Park, PA, USA) have demonstrated the capabilities of the tweezers with three kinds of polymer microspheres of different sizes. The researchers have also incorporated a white light laser into a confocal microscope system to speed image production while retaining the image clarity and the ability to observe an object in layers, which are available in conventional instruments. Images that require a second or more to be produced with a confocal microscope need only tens of milliseconds in the white-light instrument.

The project was led by Dr. Zhiwen Liu, assistant professor of engineering at Penn State. "Our team is among the first to demonstrate the 3-dimensional trapping and manipulation of microscopic objects using white laser light,” he noted. "Our novel tweezers, thanks to the broadband nature of white light, also have the potential to perform optical scattering spectroscopy of the trapped object over a broad wavelength range.”

The new confocal microscope has the potential to film biologic processes as they happen, in milliseconds or less. Dr. Liu expects that both the tweezers and the new microscope will have applications in the biologic and medical sciences, as well as in the microcircuit chip industry. Propagating short laser pulses of infrared light, for example, in a photonic crystal fiber broadens its spectrum dramatically and generates supercontinuum white light. The white light produced this way can be focused to a tiny spot, like a normal laser.





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