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14 Feb 2006
Microfluidics is a rapidly emerging field with researchers around the globe working on lab-on-a-chip systems for diverse and widespread applications. Rob van den Berg talks to three groups that are trying to shrink optical components to the nanoscale.
Microfluidic systems are already being touted for applications that include drug discovery, DNA analysis and aerosol detection. In order to enhance the device's functionality, lab-on-a-chip inventors are thinking optically and are developing nanoscale versions of techniques such as spectroscopy.
Fabricating radically scaled-down versions of successful optical techniques is easier said than done, but recently there have been considerable successes in integrating discrete components.
Ben Eggleton from the University of Sydney, Australia, is pioneering ways of shrinking optical techniques onto a lab-on-a-chip. "We are developing the devices to do spectroscopy and sensing on a chip," he told OLE. "Unique optical and physical properties allow us to impart photonic control in ways that are highly compact and tunable. We may also turn the technology around and use photonics to sense fluid properties, which is of increasing importance to medical diagnostics."
In 2002, while still at Bell Labs, US, Eggleton used multiple microfluidic plugs to tune the optical properties of a microstructured fibre. "By loading some fluid into the interior microchannels via capillary action, we were able to vary the transmission wavelength and attenuation of the fibre," he explained.
Using the same approach, he was also able to tune the transmission of a photonic crystal. "With a capillary heater, we managed to move a microfluidic plug back and forth in a photonic crystal and measure its response," he said. "We could clearly show that features in the optical spectrum became attenuated due to the lowered index contrast between the photonic bandgap (PBG) material and the surrounding silica. When the PBG structure was made transparent by the right index matching fluid, diffraction ceased completely."
Eggleton and his group have also developed a Mach-Zehnder interferometer based on a microfluidic channel. Here, the output from a singlemode fibre is sent through the channel and into another optical fibre. This creates a single-beam interferometer because the light is phase-shifted when it interacts with the fluid.
By moving the fluid back and forth into the interaction zone, Eggleton varies the degree of interference and attenuation. "This is a very efficient way to modulate the intensity," he explained. "If you wanted to do the same with, for instance, an electro-optic polymer, you would need an interaction length of several centimetres. Our technique may not be as fast as those based on MEMS, which can modulate at kilohertz rates, but I am confident we can reach that."
The latest development from the Eggleton lab, which will be published in Applied Physics Letters, is an optofluidic refractometer. The device is based on a singlemode fibre waveguide, which is split in two and sits either side of a microfluidic channel. Fibre Bragg gratings in each waveguide form a Fabry-Perot resonator, with a resonance wavelength that depends on the refractive index of the fluid in the channel. According to Eggleton, the device is capable of detecting 0.2% changes in refractive index of the fluid in real time.
The next goal for the team is to find optical methods to move the fluid in the microfluidic channels. "Our goal is to have full optical control of all components in a single optofluidic chip, without any electrical wires or electric power at the point where you manipulate," he said.
Eggleton's first idea was to use optical tweezers to move the fluid "particle" directly, but this required too much energy. Another approach is to optically trap and displace a silica microsphere, which refracts a laser beam and provides variable attenuation and beam steering.
Zhu Lin and colleagues from the California Institute of Technology in Pasadena, US, have also constructed a microfluidic variable attenuator and say that they have adopted a different approach from that of Eggleton.
"We aligned the microfluidic channel on an opening in the cladding layer in the waveguide," explained Lin. "The fluid in the channel acts as a segment of upper cladding and creates a hybrid fluid-solid state structure. This design keeps the waveguide core intact and confines the interaction between the fluid and the optical waveguide within the cladding layer."
Lin claims that, fabricated using a multistep (soft) photolithography process, it is possible to tune the optical confinement of the waveguide by passing different fluids through the microfluidic channel.
If the refractive index of the fluid is the same as that of the cladding, the light is confined, and the maximum output power is reached. However, fluids with a higher refractive index can attenuate this output up to 28 dB. "We are now trying to integrate a grating into the fluid in order to be able to change the wavelength by changing the fluid's refractive index," said Zhu "This should offer a really wide tuning range."
Another essential component for a lab-on-a-chip is a light source, and Anders Kristensen and his group at the Technical University of Denmark have come up with a solution: a liquid dye laser on a chip.
Manufactured in a single microlithographic step, the laser is confined to an 8 μm thick polymer layer on top of a glass substrate. The optical resonator consists of a Bragg grating formed by an array of channels interleaved by sections of polymer, each approximately 20 μm wide.
"It may sound like a complicated structure, but if you are making a lab-on-a-chip, the price of adding more optical devices is zero," Kristensen told OLE. "The laser structure is just another microfluidic channel."
The on-chip laser, which is pumped by a frequency-doubled Nd:YAG laser with an energy fluence of 0.02 mJ/mm2, has a singlemode output power of 1.2 mJ. Kristensen says that he can also tune the laser's output wavelength. "Diluting a solution of the laser dye Rhodamine 6G in ethanol enables concentration tuning and refractive index tuning within the laser resonator," he explained. "The output can be varied continuously between 573 and 583 nm. You could even mix in a different solvent, like ethylene glycol." This research will be published in the Journal of Applied Physics.
Kristensen sees two potential applications for the on-chip dye lasers. "One could couple the light from several dye lasers emitting at different wavelengths into several discrete waveguides," he said. "Bringing this light to various locations along a microfluidic channel allows you to make absorbance measurements."
The second application involves turning the laser cavity into an evanescent wave sensor by putting molecules on the surface of the waveguide. This changes the propagating field in the cavity and induces a phase or frequency shift, which is a very sensitive detection technique.
Kristensen has also set his sights on further miniaturization of these devices. "In order to have a polymer film with just a single propagating mode, the height of the film must be in the submicron range and we do this by thermal nano-imprint lithography," he explained. "Using this approach, we intend to make cavities with a higher Q that are less lossy and have a lower lasing threshold. This might ultimately allow us to replace the YAG pump laser with a flash lamp or a small diode laser and make a more compact hand-held cartridge."
This, however, is easier said than done, because fabricating polymer nanostructures with a high aspect ratio is a demanding task. The sidewalls also need to be optically smooth and absolutely vertical.
To overcome these difficulties, Kristensen is working with several research groups on a European project on emerging nanopatterning methods.
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