Oscillator-only micromachining of transparent materials

By focusing femtosecond laser pulses with high numerical-aperture microscope objectives, we achieve the intensities required for bulk damage in transparent materials with energies in the range of modern laser oscillators. When a femtosecond laser pulse is tightly focused inside a transparent material, energy deposition occurs only at the focus, where the laser intensity is high enough to cause absorption through nonlinear processes. When enough energy is deposited, a localized change in the index of refraction is produced, i.e. the material is damaged. By scanning the focus through the sample, very precise, three-dimensional microstructuring can be achieved. We demonstrate for the first time, to our knowledge, bulk damage to a transparent material produced with un-amplified femtosecond pulses from a Ti:Sapphire oscillator. Because a complicated, expensive amplifier chain is not required, our technique greatly simplifies the laser technology required to reap the benefits of micromachining with femtosecond laser pulses. We will present single- and multiple-shot studies of damage threshold and damage morphology produced with tightly-focused femtosecond pulses. The dependence on the numerical aperture of the microscope objective, the wavelength of the laser, the bandgap of the material, and the energy of the laser pulse have all been investigated. In addition we will discuss the effects of megahertz repetition rates on the morphology of multiple-shot damage. The potential of our technique for producing industrially-relevant structures such as waveguides and Bragg gratings will also be discussed.