Nadine Götte

Nanofabrication of tailored surface structures in dielectrics using temporally shaped femtosecond-laser pulses

We have investigated the use of tightly focused, temporally shaped femtosecond (fs)-laser pulses for producing nanostructures in two dielectric materials (sapphire and phosphate glass) with different characteristics in their response to pulsed laser radiation. For this purpose, laser pulses shaped by third-order dispersion (TOD) were used to generate temporally asymmetric excitation pulses, leading to the single-step production of subwavelength ablative and subablative surface structures. When compared to previous works on the interaction of tightly focused TOD-shaped pulses with fused silica, we show here that this approach leads to very different nanostructure morphologies, namely, clean nanopits without debris surrounding the crater in sapphire and well-outlined nanobumps and nanovolcanoes in phosphate glass. Although in sapphire the debris-free processing is associated with the much lower viscosity of the melt compared to fused silica, nanobump formation in phosphate glass is caused by material network expansion (swelling) upon resolidification below the ablation threshold. The formation of nanovolcanoes is a consequence of the combined effect of material network expansion and ablation occurring in the periphery and central part of the irradiated region, respectively. It is shown that the induced morphologies can be efficiently controlled by modulating the TOD coefficient of the temporally shaped pulses.

Temporal Airy pulses for controlled high aspect ratio nanomachining of dielectrics

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.

Temporal Airy pulses control cell poration

We show that spectral phase shaping of fs-laser pulses can be used to optimize laser-cell membrane interactions in water environment. The energy and peak intensity thresholds required for cell poration with single pulse in the nJ range can be significantly reduced (25% reduction in energy and 88% reduction in peak intensity) by using temporal Airy pulses, controlled by positive third order dispersion, as compared to bandwidth limited pulses. Temporal Airy pulses are also effective to control the morphology of the induced pores, with prospective applications from cellular to tissue opto-surgery and transfection.

Material Processing of Dielectrics via Temporally Shaped Femtosecond Laser Pulses as Direct Patterning Method for Nanophotonic Applications

Dielectric materials are of great interest for optical applications since they are transparent in the UV, visible and IR spectral range. That makes them very suitable for optical filters, polarizer, waveguides or even reflectors. When dielectrics are processed with conventional techniques based on electron or ion bombardments, they suffer from severe charging effects. For this reason, we present temporally shaped femtosecond laser pulses as a novel direct patterning method of wide band gap materials with very high precision to create photonic crystal structures in dielectrics. Material processing with temporally shaped femtosecond laser pulses overcomes the charging problems. Fabrication of structures well below the diffraction limit is feasible with temporally shaped asymmetric pulse trains due to nonlinear ionisation effects like multiphoton ionisation and avalanche ionisation. For the implementation as optical filters, a thin-film waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. The wave guiding layer consists of a material with a higher refractive index than the surrounding materials, in our case SiO2. Although the refractive index contrast is low, numerical design results prove that light with normal incidence to the plane of periodicity couples to waveguide modes and Fano resonances are excited. This makes the device extremely interesting as a compact narrow-band optical filter.

Modelling, design and fabrication of dielectric photonic crystal structures using temporally asymmetric shaped femtosecond laser pulses

We present high aspect ratio Fano-resonance structures for optical filter application along with their fabrication technique, design and numerical simulation. The structuring of photonic crystal elements using temporally shaped femtosecond laser pulses is described. Performance considerations for the presented device due to angular spread of a real optical beam are discussed in addition.

Temporally shaped femtosecond laser pulses as direct patterning method for dielectric materials in nanophotonic applications

We present a direct patterning method of dielectric materials via temporally shaped femtosecond laser pulses. A thinfilm waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. We use dielectrics since they are transparent especially in the visible spectral range, but also in UV and near infrared range. Thus, they are very suitable as optical filters in the very same spectral region. Since structuring of non-conductive dielectric materials suffers from charging, the implementation of laser processing as patterning method instead of conventional processing techniques like electron beam lithography or focused ion beams is a very attractive alternative. Despite a low refractive index contrast, we show by numerical results that normal incident of light to the plane of periodicity couples to a waveguide mode and can excite Fano resonances. That makes the device extremely interesting as narrow-band optical filter. Applications of optical filters in the visible and UV range require fabrication of photonic crystal structures in the sub-100 nm range. Temporally shaped femtosecond laser pulses are applied as a novel method for very high precision laser processing of wide band gap materials to create photonic crystal structures in dielectrics. Shaping temporally asymmetric pulse trains enable the production of structures well below the diffraction limit.1 We combine this process with deposition of a high refractive index layer to achieve the targeted resonant waveguide structure. Additionally, we focus on the rim formation arising by laser processing since this is an important issue for fabrication of photonic crystal arrays with small lattice constants.