David Veysset

The selection of light sources for photoacoustic imaging, a biomedical imaging technology, remains a critical yet under-informed topic. This review discusses the key trade-offs in light source parameters (e.g. wavelength, pulse energy, repetition frequency), safety, and practical considerations. We analyse these factors in the context of underlying imaging physics and the limits imposed by regulatory standards across various wavelength ranges. Attention is given to light source selection for photoacoustic tomography, optical and acoustic resolution microscopy, and endoscopy, which pose different requirements for pulse energies and repetition frequencies, to achieve real-time imaging at the desired depth. Current PA imaging light sources, including high-power lasers with optical parametric oscillators, fibre lasers, laser diodes, vertical-cavity surface-emitting lasers, and light-emitting diodes, are reviewed in terms of wavelength availability, energy, repetition frequency, beam quality, portability, and safety. Clinically viable systems require careful consideration of fibre coupling limitations, maximum permissible exposure at the tissue interface, imaging field of view, and signal-to-noise ratio. Further practical considerations include energy efficiency and thermal management in clinical settings. We also outline the present gaps in the available light source technologies, where the development of new sources may enable new PA imaging capabilities, both scientific and clinical. By offering an overview of these considerations, this review aims to guide researchers, industry, and clinicians in selecting optimal light sources and advancing the development of light sources that address current gaps in available technology.

We present a method for numerically determining the optimal wavelengths to perform spectral unmixing in photoacoustic experiments. The proposed method employs a rugged-landscape-model-inspired stochastic hill-climbing approach that can reliably and efficiently converge to near-optimal wavelength selections. Compared to current methods, the algorithm consistently achieves superior spectral unmixing accuracy, as is demonstrated on synthetic and experimental data, while maintaining equivalent or faster run times, particularly in broad spectral ranges. The algorithm’s adaptability with respect to computational efficiency makes it effective for both targeted offline wavelength optimization and opens up prospects for time-critical, real-time wavelength correction.

Adaptive-optics optical coherence tomography (AO-OCT) allows the visualization of cellular-scale retinal structures; however, its adoption both at research and clinical levels has been restricted by hardware and software complexity. Based on the observation that aberrations other than defocus are depth-independent, we propose an approach for wavefront sensorless AO-OCT that utilizes the interferometric fringe modulation in wavenumber ( k -) space to optimize the wavefront correction. This approach avoids the need for tomogram reconstruction at each optimization iteration and increases robustness against axial motion. The proposed routine combines k -space optimization with focal plane shifting (i.e., defocus optimization) and evaluates the objective function B-scan-wise, achieving 8 Zernike modes correction in ∼1.89 s. Experimental testing with a phantom model eye and computational complexity analysis show the proposed algorithm has a lower computational complexity and faster optimization time per mode while performing at least as well as depth-resolved optimization, using a LabVIEW implementation without the need for high-performance dedicated software or GPU acceleration. We demonstrate its performance in human retinal imaging in vivo .

S: Controlling the tissue temperature rise during retinal laser therapy is essential for predictable outcomes, especially at non-damaging settings. We demonstrate a method for determining the temperature rise in the retina using phase-sensitive optical coherence tomography (pOCT) in vivo. Measurements based on the thermally induced optical path length changes (ΔOPL) in the retina during a 10-ms laser pulse allow detection of the temperature rise with a precision less than 1 °C, which is sufficient for calibration of the laser power for patient-specific non-damaging therapy. We observed a significant difference in confinement of the retinal deformations between the normal and the degenerate retina: in wild-type rats, thermal deformations are localized between the retinal pigment epithelium (RPE) and the photoreceptors’ inner segments (IS), as opposed to a deep penetration of the deformations into the inner retinal layers in the degenerate retina. This implies the presence of a structural component within healthy photoreceptors that dampens the tissue expansion induced by the laser heating of the RPE and pigmented choroid. We hypothesize that the thin and soft cilium connecting the inner and outer segments (IS, OS) of photoreceptors may absorb the deformations of the OS and thereby preclude the tissue expansion further inward. Striking difference in the confinement of the retinal deformations induced by a laser pulse in healthy and degenerate retina may be used as a biomechanical diagnostic tool for the characterization of photoreceptors degeneration.

Solid-state bonding can form when metallic microparticles impact metallic substrates at supersonic velocities. While the conditions necessary for impact-induced metallic bonding are relatively well understood, the properties emerging at the bonded interfaces remain elusive. Here, we use in situ microparticle impact experiments followed by site-specific micromechanical measurements to study the interfacial strength across bonded interfaces. We reveal a gradient of bond strength starting with a weak bonding near the impact center, followed by a rapid twofold rise to a peak strength significantly higher than the yield strength of the bulk material, and eventually, a plateau covering a large portion of the interface towards the periphery. We show that the form of the native oxide at the bonded interface—whether layers, particles, or debris—dictates the level of bond strength. We formulate a predictive framework for impact-induced bond strength based on the evolution of the contact pressure and surface exposure.

Mixed-dimensional nanomaterials composed of one-dimensional (1D) and two-dimensional (2D) nanomaterials, such as graphene-silver nanowire (AgNW) composite sandwiched structures, are promising candidates as building blocks for multifunctional structures and materials. However, their mechanical behavior and failure mechanism have not yet been fully understood. In this work, we have performed integrated experimental, theoretical, and numerical studies to explore the performance and failure modes of graphene-AgNW composite under tensile and impact loading conditions. In situ tensile tests using a nanoindenter, implemented with a push-to-pull device and a laser-induced projectile impact test system, are used to shed light on load-bearing mechanisms in graphene-AgNW composites. Multiple failure modes have been observed in both experimental setups and analyzed with numerical and theoretical models. Results show that in the tensile loading the distribution of AgNW, as characterized by the effective free length, is the key parameter determining the failure mode. As for the impact failure scenarios, compared with failure modes observed in pure graphene cases, the mechanical reinforcing effect of AgNW will transform the failure mode from a scattered tensile fracture along radial directions to a shear failure that is constrained in a relatively local domain. Theoretical analysis using shear lag modeling, Timoshenko plate theory, molecular dynamics modeling, and finite element modeling approaches are adopted to further establish the failure modes.

Nanoarchitected materials represent a class of structural meta‐materials that utilze nanoscale features to achieve unconventional material properties such as ultralow density and high energy absorption. A dearth of fabrication methods capable of producing architected materials with sub‐micrometer resolution over large areas in a scalable manner exists. A fabrication technique is presented that employs holographic patterns generated by laser exposure of phase metasurface masks in negative‐tone photoresists to produce 30–40 µm‐thick nanoarchitected sheets with 2.1 × 2.4 cm² lateral dimensions and ≈500 nm‐wide struts organized in layered 3D brick‐and‐mortar‐like patterns to result in ≈50–70% porosity. Nanoindentation arrays over the entire sample area reveal the out‐of‐plane elastic modulus to vary between 300 MPa and 4 GPa, with irrecoverable post‐elastic material deformation commencing via individual nanostrut buckling, densification within layers, shearing along perturbation perimeter, and tensile cracking. Laser induced particle impact tests (LIPIT) indicate specific inelastic energy dissipation of 0.51–2.61 MJ kg⁻¹, which is comparable to other high impact energy absorbing composites and nanomaterials, such as Kevlar/poly(vinyl butyral) (PVB) composite, polystyrene, and pyrolized carbon nanolattices with 23% relative density. These results demonstrate that holographic lithography offers a promising platform for scalable manufacturing of nanoarchitected materials with impact resistant capabilities.

Controlling the tissue temperature rise during retinal laser therapy is highly desirable for predictable and reproducible outcomes of the procedure, especially with non-damaging settings. In this work, we demonstrate a method for determining the optical absorption, the thermal conductivity, and the thermal expansion coefficients of RPE and choroid using phase-resolved optical coherence tomography (pOCT). These parameters are extracted from the measured changes in the optical path length (ΔOPL) using an axisymmetric thermo-mechanical model. This allows the calculation of the temperature rise during hyperthermia, which was further validated by imaging the temperature-sensitive fluorescence at the same location. We demonstrate that, with a temperature uncertainty of ±0.9°C and a peak heating of about 17°C following a laser pulse of 20 ms, this methodology is expected to be safe and sufficiently precise for calibration of the non-damaging retinal laser therapy. The method is directly translatable to in-vivo studies, where we expect a similar precision.

Precise control of the temperature rise is a prerequisite for proper photothermal therapy. In retinal laser therapy, the heat deposition is primarily governed by the melanin concentration, which can significantly vary across the retina and from patient to patient. In this work, we present a method for determining the optical and thermal properties of layered materials, directly applicable to the retina, using low-energy laser heating and phase-resolved optical coherence tomography (pOCT). The method is demonstrated on a polymer-based tissue phantom heated with a laser pulse focused onto an absorbing layer buried below the phantom’s surface. Using a line-scan spectral-domain pOCT, optical path length changes induced by the thermal expansion were extracted from sequential B-scans. The material properties were then determined by matching the optical path length changes to a thermo-mechanical model developed for fast computation. This method determined the absorption coefficient with a precision of 2.5% and the temperature rise with a precision of about 0.2°C from a single laser exposure, while the peak did not exceed 8°C during 1 ms pulse, which is well within the tissue safety range and significantly more precise than other methods.