Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections.

We have studied plasma formation and relaxation dynamics along with the corresponding topography modifications in fused silica and sapphire induced by single femtosecond laser pulses (800 nm and 120 fs). These materials, representative of high bandgap amorphous and crystalline dielectrics, respectively, require nonlinear mechanisms to absorb the laser light. The study employed a femtosecond time-resolved microscopy technique that allows obtaining reflectivity and transmission images of the material surface at well-defined temporal delays after the arrival of the pump pulse which excites the dielectric material. The transient evolution of the free-electron plasma formed can be followed by combining the time-resolved optical data with a Drude model to estimate transient electron densities and skin depths. The temporal evolution of the optical properties is very similar in both materials within the first few hundred picoseconds, including the formation of a high reflectivity ring at about 7 ps. In contrast, at longer delays (100 ps–20 ns) the behavior of both materials differs significantly, revealing a longer lasting ablation process in sapphire. Moreover, transient images of sapphire show a concentric ring pattern surrounding the ablation crater, which is not observed in fused silica. We attribute this phenomenon to optical diffraction at a transient elevation of the ejected molten material at the crater border. On the other hand, the final topography of the ablation crater is radically different for each material. While in fused silica a relatively smooth crater with two distinct regimes is observed, sapphire shows much steeper crater walls, surrounded by a weak depression along with cracks in the material surface. These differences are explained in terms of the most relevant thermal and mechanical properties of the material. Despite these differences the maximum crater depth is comparable in both material at the highest fluences used $\left(16\text{\hspace{0.17em} J}/{\text{cm}}^2\right)$. The evolution of the crater depth as a function of fluence can be described taking into account the individual bandgap of each material.

Phase transformations of crystalline ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$ films upon pulsed laser irradiation have been studied using in situ reflectivity measurements with temporal resolution. Two different configurations allowed point probing with nanosecond temporal resolution and imaging with subpicosecond temporal and micrometer spatial resolution. The role of the pulse duration and laser fluence on the dynamics of the phase change and the degree of amorphization is discussed. Several advantageous features of femtosecond compared to nanosecond laser-induced amorphization are identified. Moreover, a high-resolution study of the amorphization dynamics reveals the onset of amorphization at moderate fluences to occur within $\sim 100\text{ps}$ after arrival of the laser pulse. At high fluences, amorphization occurs after $\sim 430\text{ps}$ and the molten phase is characterized by an anomalously low reflectivity value, indicative of a state of extreme supercooling.

We have investigated the temporal and spatial evolution of the ablation process induced in fused silica upon irradiation with single $120\mathrm{fs}$ laser pulses at $800\mathrm{nm}$ . Time-resolved microscopy images of the surface reflectivity at $400\mathrm{nm}$ reveal the existence of a transient plasma distribution with annular shape surrounding the visible ablation crater. The material in this annular zone shows an increased reflectivity after irradiation, consistent with a local refractive index increase of approximately 0.01. White light interferometry measurements indicate a shallow surface depression in this outer region, most likely due to material densification.