Microfabrication - Optical torque could power micromotors

Researchers have been interested in optical methods for driving micro machines for a few years, with options often focusing on light-induced rotation of absorbing microscopic particles using elliptically polarized laser beams or beams with helical phase structures. These experiments, however, involved trapping absorbing particles and thus limited power to avoid heating effects. This power restriction, in turn, limited rotation rates to a few hertz.

Now, Marlies Friese, a postdoctoral fellow at the University of Queensland (Brisbane, Australia), has found an alternative optical trapping method to exert optical torque on microscopic objects without the heating problem. She also adapted microfabrication techniques developed for electron-beam lithography to mass-produce the objects. Friese's studies revolved around the simple fact that the rotation of a particle in a fluid can induce the rotation of a nearby particle. Her goal was to extend this concept to a new optical method for driving micromachines.

The experiments first proved that a calcite particle trapped in a circularly polarized laser beam experiences a constant torque in a direction determined by the sense of the polarization. Testing involved three-dimensional (3-D) trapping of fragments of calcite dispersed in distilled water using between 100 mW and 1 W of 1064-nm laser light from a Nd:YAG device. The trapping beam was initially linearly polarized, and the plane of polarization could be rotated using a half-wave plate. A quarter-wave plate allowed the ellipticity of polarization to be varied.

Friese's results showed that calcite fragments rotate at constant frequency in circularly polarized light and that this frequency is proportional to the laser power. She also found that reversing the handedness of the polarization by rotation of a quarter-wave plate through 90° caused the calcite particles to spin at the same rate, but in the opposite direction. The fastest rotation frequency measured was 357 Hz for a particle approximately 1 µm thick trapped in a 1-W laser beam.

A followup experiment then illustrated that a birefringent particle can drive a microscopic cog within an optical-trapping arrangement. More specifically, a circularly polarized beam from a multiline argon-ion laser induced an optically trapped calcite fragment to rotate at tens of Hz (through the transfer of optical angular momentum). The moving particle then induced rotation of a transparent silicon-oxide (SiO2) cog-like structure via a fluid interface.

Unlike other methods of optically rotating microscopic particles, Friese believes her method is potentially very controllable and can easily achieve rotation rates of approximately 100 Hz, without appreciable heating of the calcite fragments. "When a trapped rotating calcite particle is brought near another transparent particle," she said, "both may become trapped in the same optical potential, causing the second particle to also rotate. The motion of a transparent particle nearby to a spinning birefringent particle can be better controlled, though, if each particle has a separate optical trap."

Fig 2. Double lift-off technique developed for electron-beam lithography was adapted to microfabricate the SiO2 cogs that would be driven by the calcite particle in the optical-trapping experiments. Sequence of deposition and removal of layers results in free structures. Microfabrication was done at the Swedish Nano Lab, Chalmers University, Gothenberg, Sweden with Julie Gold.

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Friese fabricated the microscopic objects, including the cogs for this experiment and other shapes, with a photolithographic process that allowed producing large numbers of identical structures. Structures were developed with a double liftoff technique similar to techniques used in electron-beam lithography.

The first step in the fabrication process produces a glass photolithography mask for the structures using electron-beam lithography. Next, two resist layers are spun onto a silicon wafer, and a contact exposure of the mask pattern is made using UV light. According to Friese, the process exposes and develops the pattern for the desired shape only in the upper resist layer. Next, the exposed pattern is dissolved to leave patterned-shaped depressions. A SiO2 layer is then deposited on the patterned resist layers through electron-beam evaporation under vacuum. The top resist layer and the SiO2 layer above it are then removed, leaving the shapes on the bottom resist. Slightly dissolving the bottom resist releases the structures into a liquid suspension (see figure on p. 48).

Friese reports that the double-liftoff photolithography technique is suitable for production of structures that are essentially two-dimensional and have features larger than 0.5 µm in size. "With further development of the method," she said, "it may be possible to include more than one type of material in the structures. The cogs could then be driven to rotate directly by light instead of a calcite fragment." This capability could extend the technique into applications such as an optically powered microscopic fluid pump or a rotor for micromachine production.

Paula M. Noaker