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.
Nanoscale laser processing of wide-bandgap materials with temporally shaped femtosecond laser pulses is investigated
experimentally. Femtosecond pulse shaping in frequency domain is introduced and applied to two classes
of shaped pulses relevant to laser nano structuring. The first class, characterized by a symmetric temporal pulse
envelope but asymmetric instantaneous frequency allows us to examine the influence of the sweep of the photon
energy. In contrast, asymmetric temporal pulse envelopes with a constant instantaneous frequency serve as a
prototype for pulses with time-dependent energy flow but constant photon energy. In our experiment, we use a
modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting
in ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy
(AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters.
We find different thresholds for surface material modification with respect to an asymmetric pulse and its
time reversed counterpart. However, we do not observe pronounced differences between up- and down-chirped
radiation in the measured structure diameters and thresholds.
Transparent solids may absorb energy from a laser beam of sufficient high intensity. Several models are under
consideration to describe the evolution of the free-electron density. Some of these models keep track of the energy
distribution of the electrons. In this work we compare different models and give rules to estimate which one
is applicable. We present the inclusion of a term in the multiple rate equation approach, recently introduced,
describing fast recombination processes to exciton states. Moreover, we present experimental results with temporally
asymmetric femtosecond laser pulses, impinging on a surface of fused silica. We found different thresholds for
surface material modification with respect to an asymetric pulse and its time reversed counterpart. This difference
is due to a different time-and-intensity dependence of the main ionization processes, which can be controlled with
help of femtosecond shaped laser pulses.