- IR technique enables optical trapping on silicon microchips
Gwynne D. Koch
Optical trapping is a well-established method that uses visible light to manipulate small objects and to measure forces at the single-molecule level. So when Matthew J. Lang and David C. Appleyard, researchers at MIT in Cambridge, Mass.,wanted to measure the adhesive force between a single biological molecule and a semiconductor microchip surface to investigate the mechanisms underlying such interactions, they looked to optical trapping.
However, semiconductor microchips are typically manufactured from silicon wafers, which are opaque to the visible spectrum. To enable optical trapping on silicon substrates, the researchers employed a laser in the near-infrared because silicon is transparent to those wavelengths. According to Appleyard, “Most biophysicists working with optical traps use 1064-nm lasers because they are powerful, inexpensive and aren’t horrendously damaging to biological samples. Rather than change equipment, we wanted to see if this could be done with equipment that many labs already use.”
With minimal modifications to a conventional setup, optical trapping can be used to manipulate objects and cells on the surface of silicon wafers. The schematic illustrates the optical layout for the dual-objective, dual-camera system, which includes a near-IR trapping and a 975-nm position detection laser; an acousto-optic deflector; various dichroic (D1, D2) and other (M1, M2, M3) mirrors; telescope systems for beam expansion and steering (T1, T2, T3); various lenses (L1, L2, L3); CCD cameras and a position-sensing device (PSD).
Unlike most optical trapping methods, which are performed on glass substrates mounted inside a microscope to enable observation, their IR technique required some minor modifications to a conventional imaging setup. As Appleyard explained, there are two geometries for trapping in a silicon environment: In the “through” system, the trap focus is formed after the laser penetrates the substrate, and in the “before” system, the trap focus is formed before the beam reaches the wafer.
Each of these geometries requires a reflective imaging arrangement to enable the visualization, necessitating a dual-objective, dual-camera system. The system comprised a 1064-nm trapping laser from IPG Photonics Corp. of Oxford, Mass., a 975-nm position detection laser from Avanex Inc. of Fremont, Calif., Nikon 100× 1.4-NA objectives, Dage CCD cameras, an acousto-optic deflector for beam steering as well as various other optical components.
Researchers used the IR optical trap to position 2.23-μm-diameter silica beads (left) and to arrange E. coli cells to form the letters “MIT” (right) on an undoped double side polished 190-μm-thick silicon wafer.
To determine the optimal substrate thickness and surface texture, the researchers collected and experimented with silicon wafers discarded from other labs at the university. Using a 190-μm-thick double side polished undoped wafer, they demonstrated the system’s object control and force- and position-sensing capabilities using a variety of cells, silica beads and nonspherical particles ranging from 560 nm to 20 μm. They also successfully collected and arranged Escherichia coli cells on a microchip to form the letters “MIT.”
The method could become an important tool for biological and materials research. Besides measuring single-molecule interactions at the semiconductor interface, the technique has several potential applications, including microchip design and manufacturing, MEMS device assembly on silicon and precise optical control of silicon-based microfluidic devices. It also could be used to position cells directly on fabricated sensors, providing a tool for disease diagnosis, cell-signaling investigation, and on-chip tissue scaffolding or seeding. One advantage of silicon is that its higher index of refraction compared with that of glass enables the use of samples up to 800 μm thick.
The more laser power transmitted, the more powerful the force the optical trap can provide. Because absorption by the silicon wafer changes as a function of the wavelength, scientists interested in specializing in this technique could optimize the setup by using a higher power (about 5 W), a longer-wavelength (above 1150 nm) laser or by using thinner wafers with different doping to reduce the absorption.
Photon-induced currents can restrict concurrent optical trapping and measurement applications. For unbiased results, the laser used to position the cell would have to be turned off prior to taking a measurement. However, laser modulation techniques such as those used to couple optical trapping and single-molecule fluorescence may help to overcome this limitation of the system.
Lab on a Chip, December 2007, pp. 1837-1840.
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