Tupak García-Fernández

Nanosecond laser fragmentation in liquids of micrometer-scale feedstocks is poorly understood, particularly regarding how wavelength steers photothermal versus photomechanical pathways. We investigate the fragmentation of Au microparticles (10–20 μm) in water using 5.9 ± 0.7 ns Nd:YAG pulses at 1064, 532, and 355 nm (up to 30 000 pulses). Colloid evolution was tracked in situ by UV–Vis–NIR extinction and two pulsed photoacoustic (PA) regimes: (i) focused, high-fluence fragmentation pulses and (ii) unfocused, low-fluence thermoelastic pulses mainly sensitive to absorption. With Mie-based absorption/scattering efficiencies and electron microscopy, the results indicate wavelength-dependent regimes. At 1064 nm, fragmentation appears progressive and tends to self-limit as optical coupling decreases with shrinking size, yielding a colloid dominated by primary nanoparticles (mean ∼10 nm, mostly <50 nm) with limited aggregation. At 355 nm, strong interband absorption in a thin near-surface layer can promote transient thermal gradients and thermomechanical stresses; together with cavitation transients in the liquid, these effects may contribute to abundant irregular submicrometer fragments (hundreds of nm) alongside a smaller nanoparticle population (mean ∼5 nm). At 532 nm, plasmon-resonant coupling may initially generate both submicrometer fragments and nanoparticles, while continued irradiation can further break down larger species and, at longer exposure, aggregation/coalescence and partial sedimentation may also become relevant, producing nonmonotonic extinction and the lowest primary particle index. Overall, combined extinction–PA monitoring helps distinguish absorption-driven evolution from scattering-dominated contributions.

This study investigates the fluence-dependent evolution of gold nanoparticles formed through single nanosecond pulsed laser dewetting of a gold thin film on a fused silica substrate. By employing a well-defined Airy-like laser spatial profile and reconstructing scanning electron microscope images across the irradiation spot into a panoramic view, we achieve a detailed continuous analysis of the nanoparticle formation process. Our morphological analysis, combined with finite element thermal simulations directly correlated with the applied fluence, identifies two distinct thresholds. The first threshold corresponds to the dewetting of the gold film at its melting point, resulting in large, sparse nanoparticles. The second threshold, where the substrate temperature reaches values near its melting point, leads to the formation of numerous small nanoparticles and a significant increase in coverage area. Notably, the formation of these small nanoparticles is attributed to substrate heating, which alters the interaction between the molten gold film and the substrate, increasing adhesion. Contact angle measurements of the nanoparticles confirm this change, revealing a shift in wettability, and highlighting the crucial role of substrate heating in modulating the interactions leading to nanoparticle formation. Our findings underscore the intricate interplay between laser fluence, material properties, and substrate interactions in pulsed laser dewetting, with the well-defined laser profile offering valuable insights into these dynamics.

This study presents an experimental and analytical investigation of the interactions between laser pulses and air within an electric field generated by a parallel-plate capacitor, which is part of a circuit. This research offers a physical interpretation previously unexplored in the works on this topic. It was found a direct correlation between the measured voltage and the temporal changes in electric flux induced by plasma generated by the laser pulses, and consequently, that the variation of the electric flux is proportional to the time integral of the voltage measured across a resistor of the circuit. The behavior of this last parameter mirrors that of the laser pulse, particularly up to its peak value, which facilitates the determination of its duration. Furthermore, within a specific range, it is feasible to ascertain the incident energy of the laser pulses knowing the maximum value of the electric flux variation. These findings hold significant potential for practical applications in laser pulse diagnostics. Unlike common commercial devices, after a proper calibration, the proposed system can simultaneously determine the energy and duration of laser pulses.

We report the fabrication of metal alloy Au–Pd nanoparticles on semiconductor thin film substrates (ZnO) by laser-induced dewetting. Employing a UV excimer laser, a single pulse was directed onto a three-layer film stack on a glass substrate: glass/ZnO/Au/Pd and glass/ZnO/Pd/Au. We simulated the temperature attained by the thin films enabling the prediction of energy thresholds required for melting the metal films but avoiding modifying the ZnO film. A specific range is reported of the pulse energy conducive to nanoparticle formation and the energy threshold required to modify the ZnO film beneath them. Depending on the pulse energy applied, the mean diameter of the nanoparticles varied from approximately 150 to around 70 nm. Notably, higher fluences resulted in smaller particles but also induced surface cracks in the ZnO film. Additionally, we observed a reduction in nanoparticle size with increased Pd content. Our results show that laser-induced dewetting can produce bimetallic alloy nanoparticles and, at the same time, ensure the preservation of the optical properties of the ZnO film. This approach opens avenues for tailoring material characteristics and expanding the range of applications of metal nanoparticles on semiconductor-based systems.

Optical microresonators based on Whispering Gallery Modes have been extensively investigated as unique tools for fundamental studies and as core components in several applications. Specifically, microbubbles resonators (MBR) made from glass capillaries combine high optical and mechanical Q-factors together with intrinsic microfluidics, and in this work we demonstrate them as unique platforms for photoacoustic (PA) detection and photothermal (PT) spectroscopy. In a first configuration, we employed a MBR as an optical transducer to detect the PA signal generated by a suspension of gold nanorods (GNRs) placed within the MBR, demonstrating all-optical PA detection and high sensitivity towards nanoparticles optical absorption. The MBR acts both as the ultrasound sensor and as the microvial, enabling the inspection of submicroliter samples without the need for impedance-matching media. We then present a flow cytometry experiment where the GNRs run through the MBR, and we estimate PA intensity by a Fourier analysis of the read-out signal. The sharp MBR mechanical eigenmodes allowed to easily decouple the environmental noise from the PA contributions. In a second proof-of-concept experiment using a similar setup, we implemented the MBR transducer to reconstruct the GNRs absorption spectrum through the photothermal conversion. The high sensitivity of the MBR towards temperature variations allowed for the interrogation of small samples while using a low-intensity beam for their excitation. This thermal detection produces spectra that are insensitive towards light scattering in the sample, as proved experimentally by comparing the spectra of aqueous GNR suspensions in the presence or absence of milk powder.

In this paper, we implement a Whispering Gallery mode microbubble resonator (MBR) as an optical transducer to detect the photoacoustic (PA) signal generated by plasmonic nanoparticles. We simulate a flow cytometry experiment by letting the nanoparticles run through the MBR during measurements and we estimate PA intensity by a Fourier analysis of the read-out signal. This method exploits the peaks associated with the MBR mechanical eigenmodes, allowing the PA response of the nanoparticles to be decoupled from the noise associated with the particle flow whilst also increasing the signal-to-noise ratio. The photostability curve of a known contrast agent is correctly reconstructed, validating the proposed analysis and proving quantitative PA detection. The experiment was run to demonstrate the feasible implementation of the MBR system in a flow cytometry application (e.g., the detection of venous thrombi or circulating tumor cells), particularly regarding wearable appliances. Indeed, these devices could also benefit from other MBR features, such as the extreme compactness, the direct implementation in a microfluidic circuit, and the absence of impedance-matching material.

We implement a whispering-gallery-mode microbubble resonator (MBR) as a multifunctional component for all-optical photoacoustic inspection of materials in a platform combining microfluidic design and high sensitivity to spectral absorbance. In our setup, the MBR acts both as the acoustic transducer and as the vial for a sample causing the photoacoustic event, thus dictating its operational frequency and removing any need for impedance-matching media. We challenge our setup in the assessment of the photostability of gold nanorods to demonstrate its validity. We develop a numerical model to qualitatively interpret the experimental signal as a transient shift of the resonance condition of the MBR. Possible implementations of the MBR in photoacoustic sensors are discussed.

We report on the experimental and theoretical analysis of parametrical optomechanical oscillations in hollow spherical phoxonic whispering gallery mode resonators due to radiation pressure. The optically excited acoustic eigenmodes of the phoxonic cavity oscillate regeneratively leading to parametric oscillation instabilities.