Femtosecond nearly resonant self-focusing in gold nanorod colloids

Influence of photothermal and plasma-mediated nano-processes on fluence thresholds for ultrafast laser-induced cavitation around gold nanoparticles

Laser fluence thresholds of ultrafast excitation of vapor bubbles around gold nanoparticles are determined experimentally. An optical scattering technique of limited minimum bubble size resolution is employed and analyzed for that purpose. Measurements were performed for spherical gold nanoparticles of varying sizes (40-200 nm) and for laser pulses of varying pulse width (55 fs to 4.3 ps) to estimate the limits where the evaluated thresholds are attributed to either plasma-mediated or photothermal cavitation. Furthermore, thresholds were obtained by double 55 fs pulsed excitation (varying delay 0.0-4.3 ps), providing insights into the dynamics of the excited plasma. A relationship is established between particle properties, (size, near-field amplification factor, and absorption efficiency) and the crossover pulse width of the transition from plasma-mediated to photothermal cavitation. Further, by comparing theory and experiments, we examine the approximative optical breakdown density of ∼10-21 cm-3 at a distance of 1-2 nm from the particle surface as a criterion of plasma-mediated cavitation around gold nanoparticles in analogy to the spinodal criterion for photothermal cavitation. For a given pulse width, the breakdown density appears to be nearly size-independent, establishing the aforesaid criterion applicable. However, a small pulse width dependence of the breakdown density is still observed. Based on these criteria, a comparison is further provided between theoretical thresholds of cavitation and the ones of detectable bubbles. An increasing discrepancy is observed between them with decreasing size for the case of photothermal cavitation. For plasma-mediated cavitation, the latter discrepancy is seemingly smaller, presumably due to the highly nonlinear nature of the process.

Phase-type diffractive micro-optics elements in sulfur-based polymeric glass by femtosecond laser direct writing

Sulfur-based polymeric glasses are promising alternative low-cost IR materials due to their profoundly high IR transparency. In this Letter, femtosecond-laser-induced refractive index change (RIC) was investigated in one typical sulfur-based polymeric glass material, poly(S-r-DIB), for the first time, to the best of our knowledge. The RIC in the laser-engineered region was quantitively characterized, which laid a foundation for phase-type optical element design. By the integration of RIC traces, embedded phase-type micro-optics elements, including Fresnel zone plates, and a Dammann grating were fabricated in bulk poly(S-r-DIB) polymeric glass substrate via the femtosecond laser direct writing technique. The imaging and beam shaping performance were demoed in the near-infrared (NIR) region.