INVITED (by Beat Neuenschwander)
An important challenge in the field of three-dimensional ultrafast laser processing is to achieve in the bulk structuring of silicon and narrow gap materials. Attempts by increasing the energy of infrared ultrashort pulses have simply failed. Our solution is inspired by solid-immersion microscopy to produce hyper-focused beams which are intrinsically free from aberrations and associated with an extreme energy confinement deep into the matter. Its validity is demonstrated by controlled refractive index modifications inside silicon. This opens a way to the direct writing of 3D monolithic devices for silicon photonics and provides perspectives for new strong-field physics and warm-dense-matter experiments.
In this paper we show an experimental procedure for fabrication of metal nanoparticle arrays on metal substrates. The nanostructures are fabricated by laser processing of thin metal films. The films are deposited on the metal substrates by classical PLD technology. The as deposited films are then annealed by nanosecond pulses delivered from a THG Nd:YAG laser system (λ = 355 nm). At certain conditions, the laser treatment leads to a formation of discrete nanoparticle structure on the substrate surface. The optical properties of samples fabricated at different conditions and having different characteristics of the nanostructures are examined by optical spectroscopy measurement. Such analysis shows that the optical spectra of the obtained nanostructures are characterized by plasmon excitation. Finite difference time domain (FDTD) model is used for theoretical description of the near field optical properties of the fabricated nanoparticle arrays. The simulation demonstrates high efficiency of the fabricated structures in enhancement of the near field intensity. The great enhancement observed in the Raman spectra of Rhodamine 6G deposited on the fabricated samples makes such structures very appropriate for applications in Surface Enhanced Raman Spectroscopy (SERS). The produced systems can be also applied in plasmonic solar cells (PSC).
3D laser microfabrication inside narrow band gap solids like semiconductors will require the use of long wavelength
intense pulses. We perform an experimental study of the multiphoton-avalanche absorption yields and thresholds with tightly focused femtosecond laser beams at wavelengths: 1.3μm and 2.2μm. For comparisons, we perform the experiments in two very different materials: silicon (semiconductor, ∼1.1 eV indirect bandgap) and fused silica (dielectric, ∼9 eV direct bandgap). For both materials, we find only moderate differences while the number of photons required to cross the band gap changes from 2 to 3 in silicon and from 10 to 16 in fused silica.
We investigate the non-linear absorption of 1.3 μm femtosecond laser pulses strongly focused inside silicon and fused
silica. Through transmission diagnostics, multiphoton initiated energy deposition is clearly observed inside these two
materials with nanojoules laser pulse energy when using 0.3 numerical aperture objective. For silicon, the non-linear
interaction is strongly dependent on the focusing depth due to the presence of spherical aberration contrarily to fused
silica. Below the surface, we find a difference of three orders of magnitude between the intensity thresholds for non-linear
absorption at 1.3 μm wavelength inside the two tested materials due to the difference of number of photons
required for non-linear absorption. By measuring the transmission of the ionizing pulses during multiple pulse
irradiation, irreversible modifications of the material are monitored inside fused silica in accordance with previous
studies at 800 nm. For similar laser energy deposition, the response of bulk silicon remains unchanged over more than
twenty thousands pulses suggesting no irreversible modifications are initiated.
Laser-matter interaction is a unique and simple approach to structure materials or locally modify their properties at the
micro and nanoscale level. Playing with the pulse duration and the laser wavelength, a broad range of materials and
applications can be addressed. Direct irradiation of surfaces with laser beam through a standard optical beam setup
allows an easy and fast structuring of these surfaces in the range of few micrometers. However, the irradiation of
materials through an array of dielectric nanospheres provides a unique opportunity to break the diffraction limit and to
realize structures in the range of hundred of nanometers. This simple, fast and low-cost near-field nanolithography
technique is presented and discussed, as well as its great potential.
The theoretical aspects of the near-field enhancement effects underneath the particles have been studied with a simple
model based on the Mie theory. A commercial FDTD software has also been used to study the influence of the substrate
and the surrounding media, on the energy profile of the photonic jet generated under the sphere. A specific study has
been dedicated to the influence of the dispersion of the sphere diameter on the morphology of the ablation craters. This
technique has been used for patterning bi-layer substrates. The process leads to the formation of a nanoporous membrane
which has been used to realize an array of gold nanodots on silicon. We have also associated the Laser-Induced Forward
Transfer (LIFT) process with the near-field nanolithography to print, in a single laser shot, an array of metallic
Laser near-field enhancements underneath transparent microspheres can be used to locally implement new functionalities in materials. Using this technique, we report micro- and nano-structuration on silicon (Si). The Langmuir-Blodgett (LB) technique is primarily used to realize monolayers of C18 functionalized silica (SiO<sub>2</sub>) microspheres on a large size area of the substrates. Afterwards, by irradiating the deposited monolayer with single shot UV nanosecond laser pulses in the ablation regime, we demonstrate the formation of dense arrays of craters in silicon substrate. In particular, we describe our works to obtain mono dispersed craters of sub micrometer size using LB technique and taking the fluence and sphere size as variable process parameters. Finite-difference time-domain (FDTD) simulations are presented to estimate the enhancement intensity factor and near-field distribution below the spheres in the experiments.
The unavoidable absorption of thin films used in antireflective coatings forms a permanent bottleneck in the
development of optics for high power laser applications. A valid alternative would be the micro-structuring of the optics
surface, realizing a diffraction grating which emulates the functioning of an Anti-Reflection thin film layer. Due to the
absence of film material, this diffractive structure would not contribute to the overall absorption of the optics. This paper
investigates the practical limits of this strategy, applied to zinc selenide as low absorption infrared substrate material.
Time- and space-resolved forward scattering detection was demonstrated as a suitable technique to characterize the dynamics of the ejected particles during dry laser cleaning. Silica particles with radii of 250 nanometers deposited on silicon substrates were irradiated by single nanosecond laser pulses with fluences above the particle removal threshold. The observation of different particle clouds propagating with different velocities was in support of the coexistence of at least two removal mechanisms. The ejection velocities were measured as a function of laser fluence in order to distinguish between the mechanisms involved in laser-assisted particle removal.
Femtosecond laser ablation of Ti, Zr and Hf has been investigated by means of in-situ plasma diagnostics. Fast plasma imaging with the aid of an intensified charged coupled device (ICCD) camera was used to characterize the plasma plume expansion on a nanosecond time scale. Time- and space-resolved optical emission spectroscopy was employed to perform time-of-flight measurements of ions and neutral atoms. It is shown that two plasma components with different expansion velocities are generated by the ultra-short laser ablation process. The expansion behavior of these two components has been analyzed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ultra-short laser ablation.