In opto-acousto-optic imaging technique called time-domain Brillouin scattering (TDBS), thermal phonons interacting with light in classic (frequency-domain) Brillouin scattering are substituted by laser-generated coherent acoustic pulses. As a consequence, information on material parameters could be gathered not from a volume where the light is focused but from a much smaller volume occupied by propagating laser-generated acoustic pulse, opening opportunities for fast nanoscale imaging.
In optically transparent materials, time-domain Brillouin scattering (TDBS) [1,2] provides imaging of material inhomogeneities traversed by acoustic pulse [Fig. (a)] with nano-scale spatial resolution along its path. The first experimental applications of this technique for depth profiling (with 30-40 nm spatial resolution) of nanoporous material in France  and ion-irradiated semiconductor in the USA  were reported in 2009. Following the original research [3,5], the TDBS technique has been already applied for 2D imaging of the inhomogeneous media in stationary state [Fig. (b)] [6,7], of the front motion in a laser-induced phase transformation [Fig. (c)] , and for the determination of single crystal elastic moduli of highly compressed polycrystalline cubic water ice . <br />In the present project, we propose to apply for the first time the TDBS technique for 3D imaging and analysis of transient processes in materials. This requires multiple methodological developments providing in particular a drastic improvement of the data acquisition rate. The measurements should indeed be recorded in multiple spatial positions for multiple different parameters of their initiation (such as temperature, pressure or UV radiation flux change) and should be resolved in time. <br /> H. T. Graham et al., IEEE J. Quantum Electron. 25, 2562 (1989).  V. E. Gusev and P. Ruello, Appl. Phys. Rev. 5, 031101 (2018).  C. Mechri et al., Appl. Phys. Lett. 95, 091907 (2009).  A. Steigerwald et al., Appl. Phys. Lett. 94, 111910 (2009).  A. M. Lomonosov et al., ACS Nano 6, 1410 (2012).  S. M. Nikitin et al., Sci. Rep. 5, 9352 (2015); Faits marquants CNRS (2015).  M. Kuriakose et al., Ultrasonics 69, 201 (2016).  M. Kuriakose et al., New J. Phys. 19, 053206 (2017).  M. Kuriakose et al., Phys. Rev. B 96, 134122 (2017).
To accelerate data acquisition, we will apply in the pump-probe TDBS 3D imaging of the transient phenomena, for the first time, the asynchronous optical sampling (ASOPS) technique. In ASOPS, off-setting the repetition rate frequency of two lasers gradually increases the time delay between the pump and the probe pulses and makes the use of a slow mechanical delay lines, usually used for this purpose, unnecessary. Within the main goal to get fast data acquisition rate, we will apply also for the first time in TDBS imaging, ultrafast optical interferometry. Indeed, an interferometric technique for the detection should provide the separation of the amplitude and of the phase of the TDBS signals and thus, the access, from a single acquisition, to quantitative and independent measurements of a larger number of material parameters than when detected by the reflectometry technique, which is conventionally used in TDBS imaging. We can therefore consider that the acquisition will be faster because we can reduce the number.
Currently (t0+3 of the project), we do not have any published results to communicate yet. We will be updating this section all along the run of the project, after publications of the results.
The realization of our proposal will push further the use of the TDBS technique, thanks to achieved improvements of the experimental realization allowing the 3D imaging of transient phenomena at the sub-µm to nm scale and thanks to its commercialization. Our success will also provide unprecedented insights to academic and industrial issues that could only be addressed by its use. In general, our researches and their results will accelerate the tendency of the BS replacement by TDBS in most of its applications where nanometers resolution is either advantageous or necessary.
We envision publishing few papers in open access peer-reviewed international scientific journals to share our results with the international scientific community. Based on the expertise of patenting of the industrial partners in the project, we might deposit technical patents following our developments leading to acceleration of the data acquisition rate for the 3D TDBS imaging technique.
The project goal is to develop, for the first time, quantitative fast three-dimensional (3D) in-situ imaging of complex spatiotemporal material transformations at nanometer scale by upgrading existing technique of time-domain Brillouin scattering (TDBS). TDBS uses ultrafast high repetition rate lasers for the generation and detection of nanometers in length coherent acoustic pulses (CAPs). Monitoring the CAPs in transparent materials reveals information on material properties in their current spatial position. This imaging with nanometers resolution along CAPs path drastically outperforms the classic frequency-domain Brillouin scattering (FDBS) microscopy which spatial resolution along the probed direction in the material is not better than ten micrometers. Lateral spatial resolution of both TDBS and FDBS, is controlled by the laser beam focusing. TDBS was already successfully applied for imaging of stationary distributions of acoustic, optic and acousto-optic parameters in inhomogeneous nanoporous films, semiconductors/dielectrics subjected to ion implantation/damage, polycrystalline textured aggregates and vegetal/animal cells.
In this project the 3D TDBS will be applied for the first time to imaging of the transient spatially inhomogeneous phenomena. Achievement of this goal will require the application for the TDBS of asynchronous optical sampling (ASOPS). ASOPS provides opportunity to achieve a 10 ns duration scan 1 million times faster than when the time delay between the pump and probe laser pulses is provided by a mechanical delay line. ASOPS is a way to 3D imaging of transient processes at multiple magnitudes of a variety of external actions, such as mechanical loading, temperature or radiation flux, in reasonable time. The ASOPS system from NETA company, originally designed for the lateral 2D measurement of the lumped parameters of opaque layers (metal layers in microelectronics industry), will be used for 3D imaging of transient phenomena in transparent media. It will be upgraded by application of optical interferometry/polarimetry (to obtain independently a larger number of material properties than with common reflectometry) and through the development of advanced methods of signal processing (to avoid the related limitation in spatial resolution).
The primary scientific objectives are in-situ real-time imaging and quantitative characterization of (1) a stress-induced single crystal fracture, polycrystallization and creep, (2) of the radiation/pressure-induced formation of a biocompatible polymer from monomer, and (3) of the adhesive interface formation between metals and epoxy resins in the process of curing.
The proposed research will have impact on several domains of science. Advancing the knowledge on single crystal destruction processes under high mechanical loading is of extreme importance for condensed-matter physics, planetology and prevision of the earthquakes and nuclear weapons tests consequences. Understanding of pressure-induced polymerization will promote its application for commercial production of bio-compatible objects. Evaluation of the parameters of metal/epoxy interfaces at nanoscale, especially of the industrial samples from SAFRAN company, would open the new avenues for the improvement of technical performance of adhesive joints/paints in multiple applications, in particular in aerospace/automotive domains. In general, our research will accelerate progress in already existing applications of TDBS to imaging nano- micro-scale objects and the tendency of the FDBS replacement by TDBS in many others.
Our innovative methods could after industrialization increase the capacities of NETA’s technology in imaging of extended number of materials/structures (polymers/monomers, epoxy resins, composites, 3D stacks for microelectronics, layered organic light-emitting diode (OLED) screens) and open for turnkey NETA’s products the additional industrial markets, in particular the aerospace and OLED ones.
Monsieur Vitali Goussev (LABORATOIRE D'ACOUSTIQUE DE L'UNIVERSITE DU MAINE)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LAUM LABORATOIRE D'ACOUSTIQUE DE L'UNIVERSITE DU MAINE
IMMM INSTITUT DES MOLÉCULES ET MATÉRIAUX DU MANS
LSPM Laboratoire des Sciences des Procédés et des Matériaux
Help of the ANR 390,708 euros
Beginning and duration of the scientific project: February 2019 - 48 Months