Seminar: “Femtosecond Laser Processing of Glass: Optofluidics Fabrication and Glass Welding”, Koji Sugioka

Monday, August 13, 2012 1:00 PM to 2:00 PM
CREOL 102

Koji Sugioka
RIKEN
Wako, Saitama 351-0198, Japan
ksugioka@riken.jp

Abstract:
Femtosecond laser direct writing is a promising technique for fabricating optofluidic devices since it can modify the interior of glass in a spatially selective manner through multiphoton absorption. The chemical properties of laser-irradiated regions in glass are modified allowing them to be selectively etched by subsequent wet etching using aqueous solutions of etchants such as hydrofluoric (HF) acid. This technique can be used to directly form three-dimensional microfluidic systems. In addition, femtosecond laser direct writing can alter the optical properties of a substrate to write optical waveguides. The unique ability of femtosecond laser direct writing to simultaneously alter the chemical and optical properties of glass opens up a new avenue for fabricating a variety of optofluidic microchips for biological analysis. The most typical optofluidics fabricated by femtosecond laser consists of microfluidic channel integrated with optical waveguides which intersect the microfludic channel. Such optofluidics can be used for measuring the concentrations of liquid samples. The sensitivity is, however, limited by the interaction length of the probe beam with the samples, which corresponds to the width of the microfluidic channels. To greatly enhance the sensitivity, we propose a new type of optofluidics in which the probe light can be propagated inside the microfluidic channel.
  Meanwhile, glass microwelding has been recently attracting great interest due to its potential applications in optics, microelectromechanical systems, optofluidics, etc. Then, a few groups have successfully demonstrated glass welding using femtoscond lasers. Glass welding by femtosecond laser irradiation is considered to occur due to melting induced at the interface between two glass substrates by irradiation of a focused laser beam. In this scenario, electrons are first excited from the valence band to the conduction band by multiphoton absorption of the ultrafast laser light (multiphoton ionization). The excited electrons are accelerated by the intense electric field of the ultrafast laser beam and collide with surrounding atoms, generating secondary electrons (avalanche ionization). Generated free electrons eventually relax to the valence band, generating heat that causes the glass to melt. These successive processes that generate free electrons (i.e., multiphoton ionization and avalanche ionization) occur within a single pulse in a conventional laser irradiation scheme (i.e., single-pulse train irradiation). If each process could be individually controlled, more electrons could be generated and the glass substrates could be heated more efficiently. Our proposal of using irradiation by a double-pulse train permits individual control of each process since the first pulse causes multiphoton ionization and the second pulse causes avalanche ionization. In fact, the bonding strength of glasses welded by the double-pulse train irradiation is significantly higher than that for irradiation by a conventional single-pulse train.

For More Information:
Martin Richardson
mcr@creol.ucf.edu

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