In: Optica. - Washington, DC : Optical Society of America. - Vol. 3, No. 4, April 2016, S. 389-395
Understanding the interplay between optical pulse parameters and ultrafast material response is critical in achieving efficient and controlled laser nanomachining. In general, the key to initiate material processing is the deposition of a sufficient energy density within the electronic system. In dielectrics this critical energy density corresponds typically to a plasma frequency in the near-IR spectral region. Creating this density instantaneously with ultrashort laser pulses of a few tens of femtoseconds pulse duration in the same spectral region, the penetration depth into the material will strongly decrease with increasing electron density. Consequently, staying below this critical density will allow deep penetration depths. This calls for delayed ionization processes to deposit the energy for processing, thus introducing the temporal structure of the laser pulses as a control parameter. In this contribution we demonstrate this concept experimentally and substantiate the physical picture with numerical calculations. Bandwidth-limited pulses of 30 fs pulse duration are stretched up to 1.5 ps either temporally symmetrically or temporally asymmetrically. The interplay between pulse structure and material response is optimally exploited by the asymmetrically structured temporal Airy pulses leading to the inherently efficient creation of high aspect ratio nanochannels. Depths in the range of several micrometers and diameters around 250 nm are created within a single laser shot and without making use of self-focusing and filamentation processes. In addition to the machining of nanophotonic devices in dielectrics, the technique has the potential to enhance laser-based nanocell surgery and cell poration techniques.