Femtosecond laser nanofabrication in transparent materials - Shane Eaton

Focused femtosecond laser pulses drive nonlinear absorption in a wide variety of transparent materials including glasses, crystals and polymers. This nonlinear interaction enables a micrometer-sized modification confined to the focus, which allows one to pattern micro-photonic devices with novel three-dimensional geometries.In glasses and crystals, the ultrashort laser pulse interaction leads to a permanent alteration of the refractive index, enabling the formation of 3D photonic circuits. Among the many parameters used to tailor the modification in transparent materials, the pulse delivery rate is perhaps the most significant. At suitably high repetition rates, typically greater than 100 kHz, the time between pulses is shorter than time for heat from the absorbed laser pulses to diffuse out of the focal volume. This leads to an accumulation of heat at the focus and if the pulse energy is sufficiently high, the material at the focus is melted. As further pulses arrive, the size of the melted zone expands outwards. After the final incident pulse, the melt rapidly cools into a permanent modification with refractive index greater than the bulk. In this way, the dwell time in static exposures or scan speed in scanned exposures can be used to tailor the final waveguide size for a specific application. Here we will describe how high repetition rate fabrication can be exploited to form high performance and novel photonic devices in transparent materials for the burgeoning fields of quantum information, sensing and lab on a chip.