Intensity-driven, laser induced transformation of Ag nanospheres to anisotropic shapes

Tailoring the surface plasmon resonance of embedded silver nanoparticles by combining nano- and femtosecond laser pulses

“…This behavior is consistent with a purely thermal mechanism, involving melting up to temperatures close to the boiling point, without the initial elongation due to electron ejection/Ag-ion emission/ recombination as induced by fs laser pulses. 10,12 The larger inter-particle separation implies reduced inter-particle interactions, consistent with the experimentally observed blue-shift of the SPR. 16 Exploiting the small size of the white light probe spot (4 lm), we have performed transmission measurements across the laser-irradiated area with a step size of 2 lm.…”

“…This process is accompanied by emission of Ag ions, which recombine with the trapped electrons to form elongated NPs. 10 It is worth noting that the estimated temperature increase after the initial shaping process is well above the melting point, leading to melting of the NPs, 11 which likely contributes to their smooth shape.…”

“…This confirms that there is no shape anisotropy, as expected due to the relatively long pulse duration and opposed to the case of irradiation with ultrashort laser pulses. 7,8,10,12 In order to precisely determine the SPR of the sample, we have calculated the absorption spectrum from the reflectivity and transmission spectra as We neglect scattering, which seems a reasonable assumption due to the measurement conditions used, based on microscope objective lenses with relatively high numerical apertures for high collection efficiency of scattered light. For calculating the absorbance spectrum of the embedded NP layer, the contribution of the two reflections at the two substrate interfaces had to be taken into account.…”

“…12 However, a clear advantage of fs laser pulses is their ability to generate anisotropy in the NP shape (and thus in the optical response) due to the high peak intensities, which trigger the electron emission at the poles of the NP along the laser polarization axis. 10 We have combined both techniques, ns laser irradiation to optimize the SPR in terms of amplitude and shape and subsequent fs laser irradiation to induce polarization anisotropy in the SPR. First, we have written an array of spots with single ns laser pulses at 0.74 J/cm 2 , separated sufficiently to avoid pulse overlap.…”

“…In the second step, we have exposed each spot to multiple fs laser pulses, since it has been reported that irradiation with tens or hundreds of low energy pulses has a stronger effect on the NP shape than a single pulse of high energy. 7,8,10,12 The fs laser fluence/pulse number was increased between different spots and the whole study was performed for fs laser pulses at 400 nm and 800 nm. The optimum fluence and pulse number was identified by evaluation of the SPR in terms of amplitude and polarization anisotropy in each case.…”