By: Kazuyoshi Itoh, YasuyukiOzeki
Department of Material and Life Science, Osaka University
Focused ultrafast laser pulses can cause a variety of the structural modifications including void, damage, refractive index (RI) change, and crack, depending on materials and irradiation conditions such as pulse energy, pulse width and numerical aperture (NA) of the focusing lens. Among these modifications, the RI change is interesting especially for the application to micro-fabrication of 3D photonic devices inside glass. The RI change occurs as a result of localized melting and rapid re-solidification. Fused silica shows positive RI change, allowing us to fabricate waveguides in glass. Other devices such as diffractive optical elements (DOE’s) can be fabricated regardless of the sign of RI change. We review this technique focusing on the fabrication of photonic devices (see Fig. 1).
In order to integrate micro-photonic devices, it is important to assemble several micro-devices. Here we introduce the technique of ultrafast laser welding, which allows us to weld transparent glass materials. Figure 2 shows a schematic of welding procedure. Ultrafast laser pulses are focused to the interface of transparent materials to be joined to cause nonlinear absorption. The molten pool caused by the nonlinear absorption is subsequently cooled down after irradiation, forming a welded region. By translating the sample with respect to the focal spot, the welding can be done in a point-by-point manner. We review this technique of micro-welding of glass pieces and glass and metal pieces.
For the clarification of the thermal process in the micro-joining technique, we present the technique of transient temperature sensing with Raman scattering. This method is advantageous in that the temperature can be directly measured and signal can be enhanced by strong excitation with short pulses. The temperature at the focus was obtained from the ratio of anti-Stokes scattering to Stokes scattering. Figure 3 shows the time evolution of the temperature, measured by changing the displacement of the focus of the ultrafast laser pulse. The localized temperature increase is confirmed only near the focal point. The temperature decreases rapidly with a time constant of approximately 0.14 μs.
We will introduce the techniques of ultrafast laser micro-processing and micro-welding, which allow us to fabricate micro-devices in glass and to join glass pieces for assembling them. We also described the experimental analysis of the thermal process by time-resolved Raman measurement. The rich interactions between ultrafast laser pulses and glass have allowed us to develop new tools for micro-fabrication.