Search

Wide-bandgap dielectrics such as glasses or water are transparent at visible and infrared wavelengths. This changes when they are exposed to ultrashort and highly intense laser pulses. Different interaction mechanisms lead to the appearance of various transient nonlinear optical phenomena. Using these, the optical properties of dielectrics can be controlled from the transparent to the metal-like state. Here we expand this range by a yet unexplored mechanism in excited dielectrics: amplification. In a two-colour pump–probe experiment, we show that a 400 nm femtosecond laser pulse is coherently amplified inside an excited sapphire sample on a scale of a few micrometres. Simulations strongly support the proposed two-photon stimulated emission process, which is temporally and spatially controllable. Consequently, we expect applications in all fields that demand strongly localized amplification. Ultrashort high-intensity laser pulses change the properties of dielectrics in different ways. One unexpected outcome is light amplification in an excited dielectric, observed in a two-colour pump–probe experiment.

In this work, laser excitation of water under ambient conditions is investigated by radially resolved common-path spectral interferometry. Water, as a sample system for dielectric materials, is excited by ultrashort bandwidth-limited and temporally asymmetric shaped femtosecond laser pulses, where the latter start with an intense main pulse followed by a decaying pulse sequence, i.e. a temporal Airy pulse. Spectral interference in an imaging geometry allows measurements of the transient optical properties integrated along the propagation through the sample but radially resolved with respect to the transverse beam profile. Since the optical properties reflect the dynamics of the free-electron plasma, such measurements reveal the spatial characteristics of the laser excitation. We conclude that temporally asymmetric shaped laser pulses are a promising tool for high-precision laser material processing, as they reduce the transverse area of excitation, but increase the excitation inside the material along the beam propagation. (C) 2015 Elsevier B.V. All rights reserved.

7 pags.; 4 figs.; 1 tab., © 2015 American Chemical Society. 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., This work has been partially supported by the Spanish MINECO projects TEC2011-22422 and MAT2013-33517. J.H.-R. acknowledges the FPI grant and the FPI-EEBB travel grant awarded by the Spanish Ministry of Science and Innovation. N.G., B.Z., C.S., M.W., and T.B. acknowledge support from the DFG priority program no. 1327.

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.

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.

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.

Thin films of the biopolymers gelatine and chitosan were treated using femtosecond pulse shaping techniques combined with a microscope-based setup for material processing. The polymer films were irradiated with laser pulses of 35 fs and a central wavelength of 790 nm provided by an amplified Ti:Sapphire system. The effect of temporal pulse shaping, with quadratic and cubic spectral phases, on the induced morphology was analyzed by characterization of the created surface structures via scanning electron microscopy. We observed different material modification thresholds and different structure sizes for temporally asymmetric pulse shapes. The results indicate the possibility of control of the generated microstructures and are discussed in relation to the formation of free electrons and the different contributions of multi-photon and avalanche ionization processes.

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.