Laser processing technology provides an effective means of controlling the colors of plasmonic metamaterials across large areas with micrometer resolution, allowing for the printing of images with structural colors that are visible to the naked eye. Despite the inhomogeneity in size and spatial distribution of metallic nano-objects inherent to the laser process, the colors generated are highly reproducible and can exhibit unique visual properties due to collective resonances that add to the localized surface plasmon resonances (LSPR). Here we investigate the effect of inhomogeneity in size and position on the surface lattice resonances that strongly contribute to the strong dichroism observed in laser-induced sself-organized metasurfaces. By exploiting the dichroic properties of these structures, we have successfully implemented two- and three-image multiplexing using direct laser printing techniques. These results demonstrate the potential of nanosecond laser processing for high-end anticounterfeiting applications. By combining our experimental findings with advanced computational approaches, we aim to advance the creation of even more complex and sophisticated images with enhanced optical properties.
Laser can be an effective tool to modify materials at the nanoscale in order to achieve desired optical properties. When dealing with metal-dielectric nanocomposite thin films, different mechanisms can be triggered by laser on large areas to control the statistical properties of these materials. Nanoparticles can be reshaped, resized and ordered according to self-organization mechanisms that set over micrometer wide areas. The dielectric crystal phase and film thickness can be changed upon laser-induced temperature rise. These mechanisms lead to changes in the optical properties of the films. Here, we investigate the structural changes that a Ag:TiO2 nanocomposite thin film undergoes under nanosecond laser scanning and their resulting optical properties. We especially focus on the color properties in different modes of observation such as reflection and diffraction. The colors originate from combination of absorption by the localized surface plasmon resonance of metallic nanoparticles, diffraction by the nanoparticles assemblies and interference between the incident, reflected and guided waves, the latter being excited by scattering on the nanoparticles. The morphological characterizations unveil the role of nanoparticle size, density and arrangement on the transition from a diffractive to a dichroic behavior. A full color image is also drawn to demonstrate the potential of the technique in industrial applications ranging from design, coloration to information storage and data security.
Understanding the time-dependent states of plasmonic metasurfaces under intense laser irradiation is crucial to optimize the laser processing parameters and reach specific optical properties. In this work, the physico-chemical mechanisms of laser-induced shape transformation of Ag nanoparticle ensemble embedded in thin TiO2 layers is investigated in details. The combination of high-repetition-rate pump-probe microscopy in transmission configuration with ex situ scanning electron microscopy reveals the variety of mechanisms that contribute to the reshaping of near-coalescence Ag nanoparticles with broad size and complex shape distributions into an ensemble of spherical particles. The description of the mechanisms on multiple time scales from the ultrafast electron dynamics to the much slower thermal processes triggered by the accumulation of 800 pulses at high repetition rate reveals different pulse number regimes. Owing to the high-repetition-rate data acquisition, the effect of the heat accumulation on the nanoparticle reshaping kinetics is demonstrated. The observed structural changes are discussed in the scope of the in situ transmission spectra. The presented results are intended to help improving applications of such laser-processed plasmonic metasurfaces in security printing, data storage and digital fabrication of nanoscale electronics.
Light scattering is well known to be dependent on the optical properties and surface roughness or bulk inhomogeneities of components. Angular scattering measurements and the development of electromagnetic theories at Institut Fresnel Marseilles permit to quantify the roughness behaviour. Measurements can be performed at different wavelengths from the UV to the near IR to access to different scales of characterization. Atomic Force Microscopy is used to complete these measurements at the microscopic scale, and predict the surface behaviour in the X rays domain. All these techniques rise to a multiscale characterization of all surfaces, which reveals in most cases fractal behaviours. The scatterometer has been extended and allows to perform ellipsometric measurements on scattered light in each direction of space. Results can be investigated by electromagnetic theories. They permit to directly separate bulk and surface effects in the case of bare substrates and reveal the high sensitivity of the polarimetric phase difference to the presence of contaminants on surfaces, even in the case of first order contaminants, that is to say whose size is in the same order as the substrate roughness.
Creating the conditions so that matter naturally self-arranges at the nanoscale under a homogeneous excitation is an exciting challenge for the development of efficient and cost-effective processes. Sub-micrometer periodic templates can be formed spontaneously on materials by low-energy ion sputtering or with lasers. In the latter case, the formation of self-organized grating-like structures requires a high temperature rise and generally results from interactions with ultrashort laser pulses. Recently, a few studies have dealt with self-formed periodic patterns of metal nanoparticle assemblies, but they only reported changes in the spatial and size distributions of metal nanoparticles deposited on surfaces prior to interaction with femtosecond lasers. Here, we show that metal nanoparticles can grow in a selforganized manner within a waveguide illuminated from free-space by a continuous wave visible laser. We report the conditions that give rise to the generation of such 1D nanoparticle gratings and describe the parameters that influence the grating characteristics. We explain the mechanisms involved in the formation of such nanostructures on the basis of interference phenomena between the incident wave and guided modes.
Laser-induced periodic surface structures, also known as ripples, have been investigated for a long time on various materials. They are self-organized grating-like structures that form spontaneously upon irradiation with a single laser beam; their formation requiring a high temperature rise, it generally results from interactions with ultrashort laser pulses. Recently, similar phenomena leading to periodic changes in the morphology of metallic nanoparticles embedded in polymer films have been reported under femtosecond laser exposure. Here, we demonstrate that chain-like self-organized silver nanoparticles can be grown, from an ionic silver precursor, within titania films under continuous wave visible laser beam. The nanoparticle chains are periodically spaced with a period that depends on the wavelength and are oriented parallel to the laser polarization. The samples present no significant surface modulation since all of the grown nanoparticles are located at the substrate-film interface, protected by a crystallized TiO2 film. SEM, TEM, HRTEM and EDX characterizations of the film nanostructuring are shown. Due to the coupling of incident light to a guided mode of the TiO2 film by the first diffraction order of the NP grating, such samples exhibit a strong dichroism whose characteristics depend on the laser exposure conditions. Color changes and spectral variations with polarization are measured and interpreted. The high stability of such colored films under high temperature rises or high intensity UV or visible exposures is demonstrated; it makes them good candidates for colored data storage.
Thanks to the localized surface plasmon resonance of silver nanoparticles, mesoporous titania films loaded with
silver salts manifest a photochromic behavior that can be used to perform updatable laser microinscriptions. Under UV
illumination, the silver salts are reduced into silver nanoparticles and the illuminated areas become grey-brown. This
coloration can be completely erased by oxidizing the silver nanoparticles with a polychromatic or monochromatic visible
light whose spectrum lies in the resonance band of silver nanoparticles. The paper investigates the usage of such
photochromic Ag/TiO2 films for creating an updatable random texturing. Random textures are produced on coated glass
samples, initially homogeneous, by exposing them to speckle patterns resulting from the scattering of a UV laser beam
from an optically rough surface. The stability of such textures under homogeneous UV post-exposures is investigated as
a function of the speckle exposure time. Under optimized exposure conditions, the textures remain stable enough for a
long time and the differences between textures are sufficiently discriminative to use the texturing process for goods
authentication. This is demonstrated by calculating the correlation coefficient of thousands of couples of texture images.
The numerical treatment of images has the advantage to be robust to changes in the sample repositioning between
different image records. The rewritability of the samples is characterized through the comparison of different textures
successively erased and written at the same place on multiple samples.
We investigate the effects of three reduction processes on the formation of silver nanoparticles in mesoporous titania
films. The later are impregnated with silver salt and then either exposed to UV laser light, chemically treated or
annealed. Depending on the reduction process, the NP are confined inside the mesopores or not, leading to different NP
size distributions and to various film colors. These TiO2/Ag nanocomposite films also exhibit different photochromic
behaviours when exposed to visible laser radiations. We characterize and interpret the color changes as well as the NP
deformation and oxidation under visible illuminations.
Due to their surface plasmon resonance silver nanoparticles are known to absorb visible light and give glasses
various colors. Grown in mesoporous titania films, they give the material a photochromic behaviour that can be used to
produce rewritable data carriers. On the one hand, UV light forms silver nanoparticles thanks to the photo-induced
generation of electrons by titania matrix. On the other hand, visible light oxidizes the silver nanoparticles via the
photoexcitation of electrons on Ag and their stabilization by oxygen molecules. The well controlled porosity of the
mesoporous films allows to tune the nanoparticles size and to obtain, under UV illumination, homogenous distributions
of small nanoparticles embedded within the titania matrix, which color the films. As all nanoparticles absorb light
similarly, the film can then be completely bleached under exposure to a visible laser beam whose wavelength falls in the
SPR band of the particles. Therefore, CW UV and visible focused-laser radiations, respectively, can repeatedly print and
completely erase colored micropatterns within TiO2/Ag films. The paper shows patterns printed under different
conditions, deals with the reproducibility of the process and the inscription stability, and explains the nanoscale
mechanisms, including silver migration during exposures, leading to the reversible color changes on the basis of TEM,
SEM, absorption spectroscopy and Raman micro-spectroscopy characterizations. This paper also evidences that CW
laser illuminations at higher intensity locally crystallize the titania matrix and investigates the influence of the
absorption-induced heating around nanoparticles.
In this paper we give an overview of the results obtained after benchmarking instrumentation tools for the characterization of micro-optics within the EC Network of Excellence on Micro-Optics NEMO. After a call within the NEMO network six different partners decided to organize a round robin. In this paper we will give an overview of all the experimental values obtained in the 6 different round robins, then we will comment on these results by explaining
the measurement differences and the uncertainties for all measurands.
The round robin "measurement of subwavelength diffractive elements" tackles the metrology problems related to the measurement of diffraction gratings by AFM. It aims at quantifying the absolute precision and the uncertainty of the measurement considering some features of such structures like the depth, the period, the fill factor and the shape of the profile. This round robin involved four partners within NEMO. Each partner has measured three different samples: one 2D small depth grating, one 1D small depth grating and one 1D high aspect ratio grating. In order to get rid of the samples inhomogeneity, the measurements were performed exactly at the same location on each sample by all partners. This was achieved by using a multiscale resist pattern transferred on the gratings which precisely defined a 5×5 μm2 area. The paper will sum up the experimental values obtained by all the partners, draw general conclusions and make suggestions to improve the accuracy of AFM measurements.
Top hat diffraction efficiency in an all-dielectric SiO2/HfO2 grating femtosecond pulse compression grating is demonstrated with a close to 100% flat top over more than 20 nm around 800 nm wavelength. New perspectives are open for high average power femtosecond laser machining.
The device presented in this paper is designed for coupling a free space optical wave under quasi-normal incidence in and out of a highly multimode waveguide with high efficiency. It uses two resonant diffraction gratings at the substrate-waveguide interface that are made of a shallow metal grating, covered with a high refractive index layer. It is shown that the resonant structure can theoretically diffract up to 90% of the incident energy in and out of the waveguiding layer. The geometrical parameters of the structure and the tolerances can easily be achieved by conventional technology means.
We present an innovative method Optical Diffraction Microscopy (ODM). for the simultaneous measurement of specular and non-specular diffraction patterns of sub-micron periodic structures. A sample is illuminated with broadband light and the diffraction pattern is collected by using a pair of ellipsoidal mirrors, optical fibers and a spectrometer. This method allows for rapid measurements and makes used of the Rigorous Coupled Wave algorithm for data analysis. In the present work the method has been applied to binary and multi-layer sub-micron gratings. A series of binary gratings with periods of 318 nm and 360 nm with different exposure levels of the photoresist were investigated. We succeded in characterize underexposed, ideally exposed and overexposed photoresist grating profiles. The measurements are well-suited to determine the delivered exposure energy density to photoresist gratings. The ODM technique may thus be applied to specify the exposure window and as a feedback in order to adjust the exposure energy density on-line. The homogeneity of a grating on multi-layered substrate has been investigated. Heights and duty cycles ranging from 50 nm to 55 nm and 0.25 to 0.97, respectively, have been found. AFM measurements of the gratings verify the ODM results and demonstrate that the ODM technique can be used to determine grating topology.
The presented interference detector comprises a standard pn junction in a silicon substrate and a corrugation grating engraved at its surface. Two beams with unknown phase difference impinge onto the detector under the Littrow condition for some diffraction order of the grating. The detected power exhibits a non-zero AC component as the relative phase between the incident beams changes. The present paper describes the operation principle and brings the evidence of non-zero interference contrast in the application case of a displacement sensor.
Energetic nanosecond UV sources could be advantageously used in laser material processing, biomedicine and to create laser- produced plasmas emitting soft X-ray radiation. SOPRA, in collaboration with IRPHE, is then developing an oscillator- regenerative amplifier XeCl laser system of short duration (1 - 3 ns), high energy and moderate divergence. Insertion in the amplification loop of the seed pulse and final extraction of the amplified laser pulse are realized by controlling the evolution of its polarization state by means of a HT driven Pockels cell and a half-wave plate. The experimental results are discussed and compared to numerical ones issued from a code describing the amplification of the seed pulse in the active medium. Finally, it is shown that the maximum output peak power is fairly low, PL approximately 1.4 MW (EL approximately 4.8 mJ, (tau) FWHM approximately equals 3.4 ns), due to important energetic loss as the highly divergent amplified beam is truncated by low-diameter aperture.
For a large number of specific applications, optical materials must be used in powder forms. In this context it has become highly necessary to characterize the optical and microstructural properties of such powders. Preliminary scattering experiments have shown that the scattering intensity from materials in powder forms could be quite different than that of the same homogeneous materials. In particular we have noted a strong increase in the level of the scattered light that could come from the bulk scattering. Here we show how to use the light scattering techniques in order to separate and determine the roughness and inhomogeneities of the samples. The same techniques are used to determine with accuracy the refractive index and absorption of the powders. In a second step, the same powders are evaporated in thin film forms, and we use classical spectrophotometric techniques to determine their refractive index and dispersion laws. The result are compared and discussed with those obtained with light scattering. In a general way, this study involves surface and bulk theories of light scattering, together with angle- resolved measurements, and atomic force microscopy.
It has been shown that measuring the polarimetric phase of the field scattered from a thin film multilayer gives information on the origin of scattering. Numerous numerical simulations have shown that the behavior of the polarimetric phase can be used to separate surface and bulk scattering. In the case of stacks with correlated interfaces the polarimetric phase depend only on the origin of scattering, whatever the microstructural parameters. Slight deviations from vertical correlation within the stack lead to ripples in the polarimetric phase, that can be observed experimentally. Moreover in certain cases second order effects, due for example to localized defects, can lead to depolarization. This phenomenon has strong influence on the polarimetric phase. This can be used to detect contamination of the surface. In our communication we will present some experimental results that show that the origin of scattering can be determined.
Total integrated scattering (TIS) and bi-directional reflectance distribution function measurements give information on the mean roughness of the surface and on the modulus of the Fourier Transform of the 2D surface profile. To obtain a more precise description of a rough surface and to get an estimation of the relative contribution of surface and volume scattering in multilayers stack, we have developed a new interferometric method which allows us to measure the variation of the phase of the Fourier Transform of the surface profile as a function of the scattering angle. This paper includes a detailed presentation of this method, a description of the experimental set-up used for its principle validation, as well as some preliminary results that we have obtained with it on high reflectance samples.
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