In situ three-dimensional opto-acousto-optical imaging of materials transformations at nanoscale – I2T2M

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.

In essence, 6 months before the official end of the project, a significant progress in the accomplishment of all final goals of the initial proposal can be stated.
We have demonstrated the feasibility of the time-domain Brillouin scattering (TDBS) imaging of all the materials/processes chosen for the development and testing of the functionalities of this novel 3D imaging technique.
- Imaging of HEMA polymerization by application of high pressure.
- Imaging of mechanical metal/epoxy interface including: a) time-resolved imaging with sub-optical depth resolution of the epoxy curing process at ambient conditions and its transformations caused by heating, b) 3D-imaging of the inhomogeneous modifications of the epoxy caused either by laser-induced heating or by two-photon absorption, c) imaging of the bulk epoxy at earlier unattainable distances, i.e., shorter than ten micrometers distances from the interface, d) imaging of the industrial epoxy in the vicinity of rough interface. Through the upgrading of the NETA ultrafast optical microscope, we have developed and confirmed experimentally a protocol of non-invasive 3D-TDBS imaging of epoxy with IR pump/probe.
- 3D-imaging of texture of polycrystalline materials including individual grains of water-ice at high pressures, evaluation of inclinations in space of the interfaces between the grains and between different materials/phases. The first 3D-TDBS imaging at high pressure with coherent shear acoustics waves and 3D-monitoring of the growth of a polycrystal of the phase VII of H2O-ice inside a single crystal of the phase VI of H2O-ice.
- We revealed high sensitivity of the TDBS imaging in characterization of the single-crystal destruction/degradation caused by non-hydrostatic loading in a diamond anvil cell, the rupture of the optically anisotropic LiNbO3 single-crystal to several parts as a path to poly-crystallization and rotation of the parts caused by non-hydrostatic loading.

From the theoretical viewpoint, we suggested the extensions of the classical TDBS theory for the plane acoustic and light waves to the more general geometry, when the interacting acoustic and light fields are Gaussian beams. The developed theory explains the detection in some of our TDBS experiments either only the narrow-frequency-band Brillouin oscillations, or wide-frequency-band coherent acoustic pulses, or both simultaneously.
We solved the long standing problem of systematic deviation of the acoustic sound velocities measured using the classical technique of Brillouin light scattering (BLS) and those derived from the aggregate bulk and shear moduli obtained from the experimental Cij(P) using the Hill approximation.
We have presented a prototypical continuum model of the Bending-Stretching (BS) transition, interpreting it in the framework of the Ginzburg-Landau formalism. It explains the markedly no-affine behavior and rationalizes the observed heterogeneity of the mixed BS phase.

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.

Through the upgrading of the NETA ultrafast optical microscope for its operation using a pair of wavelengths picked in six optical wavelengths (instead of four previously), now including UV, visible (green) and IR, we have developed and confirmed experimentally a protocol of non-invasive 3D-TDBS imaging of epoxy with IR pump/probe, avoiding residual modifications of the epoxy in imaging process.
The research results, were already reported in 10 journal publications and 17 presentations at National and International conferences. We presented Invited Talks at METANANO (Saint Petersburg, Russia, 2019) and International Conference on Photoacoustic and Photothermal Phenomena (Blend, Slovenia, 2022). We have been invited to present our achievements in the TDBS imaging at International Congress on Ultrasonics (Beijing, 2023, Plenary Talk) and World Congress on Nondestructive Testing (Seoul, 2024, Keynote Talk).

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.