Our study introduces a new method for label-free super-resolved polarimetry on nanomaterials, compatible with in-situ analysis. Integrating Image Scanning Microscopy (ISM) with polarimetry techniques, we achieve remarkable resolutions down to 90 nm while acquiring polarization information. Overcoming limitations associated with fluorophores in challenging materials, our approach facilitates quantitative measurements of optical properties. Applied successfully to nanostructured surfaces created by femtosecond lasers and boron nitride nanotubes, our work showcases the versatility of this methodology.
Laser processing of material surfaces has been very known for the last five decades. Femtosecond LIPSS, are created generally on the surface, they could be classified into two groups depending on the periodicity of the structures: LSFL showing a periodicity lower than the incident wavelength (λ_l), and HSFL with a periodicity well below λ_l that could sit below the optical diffraction limit. In this paper, we show an unprecedented resolution of a noninvasive label-free optical method to observe such structures, that does not require a priori knowledge of the surface. We demonstrate that using a modified reflectance confocal microscope reflection (CMR), the characterization of HSFL(̴Λ_HSFL∽120 nm @ λ_l=257 nm) is possible and efficient. These results, pave the way toward a new, better, and more resolved optical technique to observe nanostructures below the diffraction limit.
Femtosecond lasers emit short pulses whose temporal width is in the range of less than a picosecond to a few femtoseconds (fs), thereby enabling extremely high peak-power machining with minimum thermal damages. Herein we employed femtosecond laser pulses as a versatile tool for surface processing of textiles made of 2 polymers commonly used in textile industry, Polyethylene terephthalate (PET), and Polyamide66 (PA66). This work focuses on a comparison of ultraviolet (UV, 257 nm) and infrared (IR, 800 and 1030 nm) femtosecond laser irradiation at the surface of the polymers PET and PA66, possible hybridization with chemical grafting, as well as the resulting liquid repellency from different process scenarios. The study highlights the different responses of the polymers to the laser irradiations and possible routines for surface functionalization of the textiles.
Spatial beam shaping is becoming an essential technique for optimized surface and bulk laser processing with ultrafast laser pulses. In this contribution, we discuss the interest of non-diffractive intensity distributions for transparent material processing but also for surface drilling where specific physical removal mechanisms are in play. Parallel irradiation with arrays of focused spots is also evoked especially in the context of ophthalmology where the drastic speed increase of the laser treatment have led to a change of paradigm of the most widespread surgery worldwide, the cataract surgery.
Ultrafast laser processing considerably gains in efficiency when using liquid crystal based spatial light modulators (LCSLM) to tailor the laser beam shape for upgradedsurface or bulk structuring.The fidelity of the experimental beam shape when compared to the target intensity distribution is of great importance for precise and controlled machining. Due to the physical characteristics of LCSLM, their non-perfect optical response has to be taken into account when designing phase masks. In this contribution, we'll discuss phase mask optimization for LCSLM for several beam shapes and present some applications in surface, bulk processing as well as in ophthalmology.
Spatial beam shaping can be achieved using wavefront modulators to increase ultrafast laser processing efficiency. These modulators can display a pre-calculated phase mask on the beam path in order to shape the laser intensity distribution following a user defined target in the processing plane or volume. Due to the non-perfect optical response of wavefront modulators, the experimental distribution may differ from the target. We investigate the use of electrically addressed and optically addressed liquid crystal spatial light modulators with ultrafast laser pulses. Applications in parallelized surface and bulk processing are achieved with both modulators showing the advantages and drawbacks of these technologies. In particular, we focus on the limitations of these devices in terms of spatial and phase delay resolution, showing the consequences on the shaped beam distribution. Calculation strategies to overcome these limitations are discussed.
Ultrashort laser irradiation of metal targets results in a variety of coupled processes, such as energy deposition
on surface, electron-ion heating and diffusion, as well as thermal ablation and plasma expansion, mechanical
rupture below the surface, and melt flow, modifying the initial surface morphology on micro/nanometric scales.
Multidimensional simulations capable to predict the consequences of inhomogeneous absorption on hydrodynamic
processes are performed in order to elucidate the mechanisms of surface micro/nanostructure formation and
material removal during multipulse laser ablation in regimes below, near and above laser ablation threshold. On
one hand, the numerical results suggest new ways of control over the properties of periodic and aperiodic surface
structures. On the other hand, the strategies to reduce the surface roughness and to improve the quality and the effciency of ultrashort laser ablation are discussed.
Ultrafast laser pulses can be used to achieve structuring of surfaces at the micro-nano scale. Under certain irradiation conditions, Laser Induced Periodic Surface Structures (LIPSS) are formed. The LIPSS dimensions range from 100 nm to 2-3 micrometers. The characterization is generally conducted after the laser irradiation by systems such as SEM and/or AFM with a resolution beyond the diffraction limit. In this paper, we use a super resolution microscopy technique based on structured illumination for in-situ observation of the irradiated surface. The LIPSS formation on steel and Si is observed IN-SITU and discussed for a multipulse sequence.
Femtosecond laser micro machining of surfaces knows a gain of interest as it demonstrates efficient and precise processing with reduced side effects around the irradiated zone, and also because of the remarkable costs reduction and reliability improvements of nowadays commercially available sources. Controlling the intensity distribution spatially can offer a supplementary degree of flexibility and precision in achieving user-defined ablation spatial profile, drilling, cutting of materials or in-volume laser-induced modifications. In this scope, the possibility to generate a top-hat intensity distribution by spatially shaping the beam wavefront is studied in this work. An optimization of Zernike polynomials coefficients is conducted to numerically determine an adequate phase mask that shapes the laser intensity distribution following a targeted top hat distribution in the processing plane, usually at the focal length of a converging lens. The efficiency of the method is numerically investigated in the optimization by evaluation of the root mean square error (RMS) between the top-hat target and the calculated laser distribution in the far field. We numerically verify that acceptable top-hat beam shaping of various size can be achieved with a sufficient number of Zernike polynomials, opening the way to phase mask calculations adapted to the wavefront modulator ability to reproduce Zernike polynomials.
Corneal therapeutic molecules delivery represents a promising solution to maintain human corneal endothelial cells (HCECs) viability, but the difficulty is transport across cell membrane. A new delivery method published recently consists in ephemerally permeabilizing cell membranes using a photo-acoustic reaction produced by carbon nanoparticles (CNPs) and femtosecond laser (FsL). The aim of this work is to investigate the size of pores formed at cell membrane by this technique. To induce cell permeabilization, HCECs were put in contact with CNPs and irradiated with a 500 μm diameter Ti:Sa FsL focalized spot. Four sizes of marker molecules were delivered into HCECs to investigate pore sizes: calcein (1.2 nm), FITC-Dextran 4kDa (2.8 nm) and FITC-Dextran 70kDa (12 nm) and FITC-Dextran 2MDa (50 nm). Delivery of each molecule was assessed by flow cytometry, a technique able to measure their presence into cells. We showed that the delivery rate was dependent of their size. Calcein was delivered in 56.1±8.2% of HCECs, FITC-Dextran 4kDa in 42.2±3.5%, FITC-Dextran 70 kDa in 21.5±2.7% and finally FITC-Dextran 2MDa in 12.9±2.0%. It means that a large number of pores in the size ranging from 1.2 to 2.8 nm were formed. However, 12 nm and larger pores were almost half more infrequent. Pore sizes formed at cell membrane by the technique of cell permeabilization by FsL activated CNPs was investigated. The results indicated that the pore sizes are large enough for the efficient delivery of small, medium and big therapeutics molecules on HCECs by this technique.
Ultrafast Bessel beams are ideal sources for high aspect ratio submicron structuring applications because of their nondiffracting nature and higher stability in nonlinear propagation. We report here on the interaction of ultrafast Bessel beams at various laser energies and pulse durations with transparent materials (fused silica) and define their impact on photoinscription regimes, i.e., formation of positive and negative refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the relevant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses, leading to soft positive index alterations via presumably nonthermally induced structural transitions involving defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to hydrodynamic expansion, resulting in high aspect ratio submicron-size wide voids. Delayed ionization, carrier defocusing, and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis.
An increase of industrial needs for micro-ablation and surface structuration using sub-picosecond laser working at high
repetition rate is required. In this context, new industrial lasers were recently commercialized for such a type of purpose.
The potential of a new industrial femtosecond laser source (Tangerine model from Amplitude Système) is investigated in
this work for different etching purposes. Our experimental results will be also compared to those obtained when using
Ti:Sa laser source, with the help of numerical simulations.
Nonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy for 3D laser-material processing. Due to their ability to maintain near-constant fluence profiles over an appreciable distance along the propagation direction in linear and nonlinear media, ultrafast Bessel beams are ideal sources for high aspect ratio sub-micron structuring applications. We report here on the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate cone angle at various laser energies and pulse durations and define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic (positive and negative) refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the significant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses leading to soft positive index alterations via presumably non-thermally induced structural transitions via defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to a hydrodynamic expansion resulting in high aspect ratio sub-micron size wide voids. Delayed ionization, carrier defocusing and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis.
The nonlinear absorption character determines a high potential of ultrafast laser pulses for 3D processing of transparent
materials, particularly for optical functions. This is based on refractive index engineering involving thermo-mechanical,
and structural rearrangements of the dielectric matrix. Challenges are related to the time-effectiveness of irradiation,
correct beam delivery, and the influence of material properties on the exposure results. Particularly for light-guiding
applications it is suitable to master positive refractive index changes in a time-efficient manner, considering that the
result depends on the deposited energy and its relaxation paths. To address these challenges several irradiation concepts
based on adaptive optics in spatial and temporal domains were developed. We review here some of the applications from
various perspectives. A physical aspect is related to temporal pulse shaping and time-synchronized energy delivery tuned
to material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal
results. Examples will be given concerning refractive index flip in thermally expansive glasses by thermo-mechanical
regulation and energy confinement by nonlinear control. A second engineering aspect is related to processing efficiency.
We give insights into beam-delivery corrections and 3D parallel complex photoinscription techniques utilizing dynamic
wavefront engineering. Additionally, in energetic regimes, ultrafast laser radiation can generate an intriguing nanoscale
spontaneous arrangement, leading to form birefringence and modulated index patterns. Using the birefringence
properties and the deriving anisotropic optical character, polarization sensitive devices were designed and fabricated. The
polarization sensitivity allows particular light propagation and confinement properties in 3D structures.
Ultrashort pulses lasers are tools of choice for functionalizing the bulk of transparent materials. In particular,
direct photoinscription of simple photonic functions have been demonstrated. Those elementary functions rely
on the local refractive index change induced when focusing an ultrashort pulse in the volume of a transparent
material. The range of possibilities offered by direct photoinscription is still under investigation. To help
understanding, optimizing and assessing the full potential of this method, we developed a time-resolved phase
contrast microscopy setup. The imaginary part (absorption) and the real part of the laser-induced complex
refractive index can be visualized in the irradiated region. The setup is based on a commercially available phase
contrast microscope extended into a pump-probe scheme. The originality of our approach is that the illumination
is performed by using a pulsed laser source (i.e. a probe beam). Speckle-related issues are solved by employing
adequate sets of diffusers. This laser-microscopy technique has a spatial resolution of 650 nm, and the impulse
response is about 300 fs. The laser-induced refractive index changes can be tracked up to milliseconds after the
energy deposition. The excitation beam (the pump) is focused with a microscope objective (numerical aperture
of 0.45) into the bulk of an a-SiO2 sample. The pump beam can be temporally shaped with a SLM-based pulse
shaping unit. This additional degree of flexibility allows for observing different interaction regimes. For instance,
bulk material processing with femtosecond and picosecond duration pulses will be studied.
Femtosecond laser processing of bulk transparent materials can generate localized increase of the refractive
index. Thus, translation of the laser spot give potential access to three dimensionnal photowriting of waveguiding
structures. Increasing the number of machining foci can considerably reduce the processing efforts when complex
photonic structures are envisaged such as waveguide arrays. The present report presents a technique of dynamic
ultrafast laser beam spatial tailoring for parallel writing of photonic devices. The wavefront of the beam is
modulated by a periodical binary (0-π) phase mask of variable pattern to achieve dynamic multispot operation.
The parallel photoinscription of multiple waveguides is demonstrated in fused silica. Using this method, light
dividers in three dimensions relying on evanescent coupling are reported and wavelength-division demultiplexing
(WDD) devices were achieved in single sample scan.
Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in
waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in
glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually
negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal
expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source
design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse
envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction
leads to a positive refractive index change and guiding structures may thus be created. From an application perspective,
the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric
interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the
structuring accuracy. The corrective process is becoming important when laser energy has to be transported without
losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.
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