Two-Photon Absorption Enables Microfabrication

Kevin Robinson

Two-photon absorption tech-niques that have proved beneficial to biological imaging are spilling over into the industrial realm. Researchers at Osaka University in Suita, Japan, are using two-photon absorption to create polymer structures with details smaller than the diffraction limit.

The two-photon effect is well-known. When two low-energy, long-wavelength photons strike a molecule at almost the same time, they have approximately the same effect as one photon of half the wavelength. This has been used in microscopy to excite fluorescent markers. To make it work, however, you need a very fast laser -- in the femtosecond range -- focused to a very tight spot.

Researchers at Osaka University have combined radical quenching and two-photon infrared excitation to fabricate structures with details smaller than the diffraction limit, such as this linked chain and gear. Courtesy of Hong-Bo Sun, with permission of Applied Physics Letters.

University researchers Hong-Bo Sun, Tomokazu Tanaka and Satoshi Kawata are using infrared two-photon absorption to harden SCR 500 resin into tiny three-dimensional structures. Infrared radiation is particularly useful in such microfabrication applications because most materials are transparent to it, enabling it to penetrate deeply into the medium. By using two-photon absorption, the infrared radiation polymerizes the material only in the vicinity of the focal spot.

Ultraviolet, in contrast, will harden a polymer, but it experiences a large power loss to penetrate to the same depth and also hardens the material in a much larger volume.

A catch for infrared two-photon microfabrication is that many applications require details that are smaller than the diffraction limit. For instance, creating photonic crystals for 1550-nm telecommunications applications requires making a rod about 300 nm wide.

Two-step technique

To beat the diffraction limit, the researchers use a 1.4-NA lens to focus 0.13-nJ pulses from a Ti:sapphire laser operating at 780 nm to a diffraction-limited spot.

Crucially, they employ a material with a threshold -- in other words, one that requires a particular energy density to harden. By adjusting the power of the laser and by using photo-produced radical quenching in the polymer, they have achieved two-photon polymerization spots as small as 120 nm, less than half the system's diffraction limit.

In practice, the microfabrication technique involves two steps. The researchers first use the two-photon process to scan a 3-D outline, curing a shell of the polymer into the shape they desire.

After removing the uncured polymer surrounding this shell, they harden the structure throughout with UV radiation. Using two steps speeds processing by 90 percent by reducing the use of the slower two-photon scanning.

The technique is mature in terms of its optics, but there still is room for refinement. "Chemically, we can do a lot," Sun said. "For example, we can use photo-initiators with large absorption cross sections to ease two-photon absorption and particularly chosen radical quenchers [to better control] the threshold."

The group plans to focus on using the technique to make functional micro- and nanoscale devices and to build novel microelectromechanical systems.