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Lasers trap particles on silicon wafers

12 Nov 2007

Microfluidic devices and molecule manipulation could benefit from research into optical trapping on silicon wafers.

Researchers at the Massachusetts Institute of Technology (MIT) have found a way to optically trap particles through silicon wafers with comparable results to trapping through glass. The integration of optical trapping into silicon environments could pave the way to optical control on small, portable silicon chips. (Lab on a Chip 2007 DOI: 10.1039/b711507e)

"There is a great need to couple biophysics methods of optical trapping to the silicon microchip community," Matthew Lang and David Appleyard, researchers from MIT, told optics.org. "Part of why we developed the method was to deliver particles to features such as waveguides and other sensors."

The device enables scientists to manipulate, track and position cells or particles over silicon substrates. "There are many medical applications of our system such as disease diagnostics, drug screening, delivering cells or other molecules to sensor locations, building biological structures, active assembly and studying cell interactions," commented Lang and Appleyard. "One advantage of our device is that it can operate on through/ above silicon surfaces and can move particles around in 3D."

The team investigated two geometries for trapping; 'before' and 'through'. In the 'before' system, the trap focus is formed before the beam path reaches the silicon substrate. In the 'through' system, the laser penetrates the silicon substrate and then forms the trap focus. A crucial hurdle in the design of the instrument was overcoming silicon's lack of transmission at visible wavelengths and limited transmission in the near-infrared.

"Currently, the community uses conventional glass slides or other transparent flow chambers for optical trapping because silicon wafers and microchips are non-transparent to visible light," explained Lang and Appleyard. "The key insight is that silicon wafers transmit in the infrared, which means that it is possible to trap through silicon."

The team used a 1064nm trapping laser and a reflective imaging arrangement. The setup involved building a trapping microscope that could operate in tandem with a conventional imaging microscope, where both microscopes looked at the same point on the sample. This enabled object control and measurement comparable to trapping through a classical glass substrate.

"Our device works largely like a conventional optical trap where a tight focus can exert optical forces on micro-nano sized objects," explained Lang and Appleyard. "The double microscope is needed so that you can form the trap, track its position and visualize what is going on."

The team intends to increase transmission by using a trapping wavelength further into the infrared as well as improving the imaging arrangement and optical components.

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