Enabling time resolved microscopy with random Raman lasing

Angular dependence of random laser emission by using ZnO-CuO heterostructure as scatterer and its applications in biocompatible imaging

The angular dependence of random laser (RL) generation in a commercially available rhodamine 6G (Rh6G) dye has been demonstrated using ZnO-CuO heterostructure as passive scatterers. The grass-like superstructure formed at a 1M:1M molar ratio of ZnO-CuO significantly enhances scattering, resulting in RL spikes with a full width at half maximum (FWHM) of just a few nanometer and a noticeable reduction in the RL threshold. RL emission spectra were collected over an angular spread of 0-180 degrees, revealing a remarkable shift in RL emission from 566 nm to 580 nm. Additionally, we explored RL using a biological extract from Caesalpinia sappan L (Biancaeasappan L) and demonstrated speckle-free imaging with an affordable handheld microscope called the Foldscope. The biocompatibility of the wood extract shows significant potential for applications in speckle-free imaging. We had assessed the Pearson correlation coefficient between images captured under LED, RL, and conventional laser illumination. It is observed that the value of the coefficient becomes the lowest under RL illumination which confirms the formation of high-resolution and artifact-free images.

Multiscale electronic and thermomechanical dynamics in ultrafast nanoscale laser structuring of bulk fused silica

We describe the evolution of ultrafast-laser-excited bulk fused silica over the entire relaxation range in one-dimensional geometries fixed by non-diffractive beams. Irradiation drives local embedded modifications of the refractive index in the form of index increase in densified glass or in the form of nanoscale voids. A dual spectroscopic and imaging investigation procedure is proposed, coupling electronic excitation and thermodynamic relaxation. Specific sub-ps and ns plasma decay times are respectively correlated to these index-related electronic and thermomechanical transformations. For the void formation stages, based on time-resolved spectral imaging, we first observe a dense transient plasma phase that departs from the case of a rarefied gas, and we indicate achievable temperatures in the excited matter in the 4,000–5,500 K range, extending for tens of ns. High-resolution speckle-free microscopy is then used to image optical signatures associated to structural transformations until the evolution stops. Multiscale imaging indicates characteristic timescales for plasma decay, heat diffusion, and void cavitation, pointing out key mechanisms of material transformation on the nanoscale in a range of processing conditions. If glass densification is driven by sub-ps electronic decay, for nanoscale structuring we advocate the passage through a long-living dense ionized phase that decomposes on tens of ns, triggering cavitation.

High fidelity visualization of multiscale dynamics of laser-induced bubbles in liquids containing gold nanoparticles

Cavitation in pure liquids and in liquids containing nanoparticles enables applications in mechanics, bio-medicine, and energy. Its evolution carries a significant interest. We describe the multiscale dynamic evolution of ultrafast-laser-induced cavitation in pure and gold-nanoparticles-doped liquids in one-dimensional geometries induced by non-diffractive ultrashort Bessel-Gauss laser beams. Covering the complete electronic and thermomechanical cycle, from the early plasma phase to bubble cavitation and collapse on ms timescales, we reconstitute, using time-resolved imaging with amplitude and phase sensitivity, the hydrodynamic phenomena concurring to bubble evolution. We indicate geometry-specific instabilities accompanying the collapse. The insertion of gold nanoparticles of 200 nm size has subtle effects in the process energetics. Albeit a moderate field enhancement minimizing the contribution to breakdown, the nanoparticles play a role in the overall relaxation dynamics of bubbles. The evolving bubble border in nanoparticles-containing liquids create a snow-plough effect that sweeps the nanoparticles at the gas liquid interface. This indicates that during the macroscopic cavity development, the nanoparticles were removed from the interaction region and dragged by the hydrodynamic movement. We thus shed light on the evolution of cavitation bubbles not triggered but perturbed by the presence of nanoparticles.