By combining a vertical-cavity surface-emitting laser (VCSEL) with diffractive optics, researchers at Sandia National Laboratories (Albuquerque, NM, and Livermore, CA) have developed an instrument that, aided by its dime-sized optical system, analyzes the chemical makeup of liquids using electrokinetically driven capillary separation. The instrument is part of a package that the re searchers call a "chem-lab on a chip," a hand-held device that will detect liquid- and gas-borne substances on the battlefield, such as explosive residues and sarin nerve gas. Sandia is also pursuing commercial and medical applications for its technology.
The electro phoresis instrument is built en tirely around a small plane-parallel slab of silicon dioxide (SiO2) that serves as a base for mounting a gallium arsenide/gallium aluminum ar senide VCSEL and a silicon PIN photodiode, as well as a substrate in which to etch diffractive optics and a liquid-carrying capillary channel (see figure). The VCSEL is flip-chip bonded to a smaller piece of SiO2, which is in turn aligned and mounted to the slab to a tolerance of a few microns. Folded by a pair of gold reflective patches, the optical path of the instrument lies entirely within the slab. The first diffractive optical element focuses light from the VCSEL to a point on the 10 p 100-µm liquid-carrying channel, while the second collects the resulting fluorescence and directs it to the photodiode.
Electrokinetically driven separation resolves the components of chemical mixtures based on their physical properties, such as charge-to-mass ratio or chemical affinity. By applying a voltage between the ends of a capillary, a sample fluid containing a number of chemical components is driven into the capillary, which in some cases is filled with a gel or other impeding substance. The various components move at different rates, eventually separating out. "The miniaturization of the optics is important [in the Sandia device]," says Mial Warren, manager of photonics research. He notes that the surface-mounting compatibility of the VCSEL is also a factor in the device`s small size.
A laser emitting a short wavelength such as blue or ultraviolet--which causes direct fluorescence--would be ideal for the Sandia instrument, says Warren. But because the VCSEL emits at 750 nm, the device must rely on indirect fluorescence, in which a sample is mixed with a dye before the analysis begins. As the separated chemical components move along the channel and past the VSCEL focus, they briefly quench the fluorescence. "You know how long it takes specific substances to move through the channel, so you can identify them," says Warren. "Things go by one at a time."
Because the fluorescence occurs at a wavelength only 15 nm longer than that of the laser, the researchers had to place a narrowband filter in front of the photo diode to reject the laser light. One challenge, notes Warren, was to come up with a design in which the diffractive optics--fabricated by electron-beam lithography in a multistep process--were all on the same side of the SiO2 slab. Reducing stray light was another design problem, one on which they are still working. "Most of the background light is coming right out of the side of the VCSEL," says Warren.
Even so, when the Sandia researchers ran a sensitivity test using a 10-4 molar dye solution, the result was a 100:1 signal-to-noise ratio (S/N). "The system has [successfully] separated components of various laser dyes in preliminary ex perimental tests," says Warren. In practical use, he notes, solutions will tend to be about 10-5 molar, resulting in a 10:1 S/N.
Possible commercial applications of the device include testing of waste water and blood samples, process control in manufacturing, and drug research, says Warren. "We`ve already had negotiations with a couple of companies in the instrumentation business that are pretty excited about this," he adds.
Further development will include the integration of blue-emitting semiconductor lasers into the device, according to Warren. Gallium nitride-based lasers are one possibility, he says. Another is a frequency-doubled VCSEL being developed at Sandia that uses a thin potassium niobate crystal to generate 490-nm light.
