When a material surface is excited by laser irradiation, the photon energy is converted to heat due to photon-atom interaction, leading to a rapid temperature rise. As a result, a plume formed by high energetic species can be generated, where the amount of the mass removed and the energy of the laser are involved in a complex process, which depends on the laser parameters (pulse duration, energy, wavelength, etc.), the solid target properties, and the surrounding environment . Due to the high energy density reached at the target surface, several changes may occur, such as vaporization, surface melting into a liquid with a moving solid-liquid interface, and for some materials thermal stress effects are important since they may cause the surface fracture of the solid . All these mechanisms can contribute to the formation of particles, which can be obtained from condensation of evaporated material, from solidification of liquid droplets ejected by the recoil pressure induced by vaporization, as well as fragmented material from the target. According to TEM and SEM observations, there are differences among the particles obtained with the pulsed and the CW laser. However, the majority of obtained particles in both cases are spherical, which means they are probably formed by explosive ejection due to the high temperature reached at the zone interaction [15, 16] or melting and rapid solidification. In thermal confinement regime, pulse duration is shorter than the time needed for heat dissipation in target (τp ≪ t
tc). Under this condition, pulse duration is shorter than the time needed for bubbles formation and diffusion in the process of heterogeneous boiling [17, 18]; therefore, the material can be overheated over the boiling temperature leading to explosive vaporization at low fluences or phase explosion at higher fluences [19, 20]. On the other hand, stress confinement condition is fulfilled when the energy is deposited in the irradiated volume more rapidly than it can be dissipated through collective molecular motion according to τp ≪ t
sc, which can lead to material fracture into more or less chunks . As the use of CW laser is dominated by thermal regime, both conditions can be estimated in our work in the case of pulsed laser when thermal diffusivity (
) and the speed of sound (v
s = 1801 m/s) for spongy bone [22, 23] are assumed valid for fish bones. Calculations for the used laser beam diameter ( φ = 0.14 mm) confirm that thermal confinement condition is fulfilled for laser pulse durations in our experiments. The characteristic time t
ch for heat dissipation in fish bones can be estimated according to
where d is the smallest dimension of the heated volume (beam diameter) and α is the thermal diffusivity, resulting in t
= 18 ms, which is considerably longer than the used laser pulses. On the other hand, stress confinement characteristic can be estimated as
= 83 ns, which is orders of magnitude shorter than the used pulse durations. This corroborates that thermal confinement is the only mechanism responsible for material explosive ejection and subsequent particles formation, which is consistent with the size as well as the spherical shape of the obtained particles. As the irradiance when using the pulsed laser is higher than when using the CW laser, the particles obtained in the latter conditions are amorphous, while the obtained in the former case are crystalline; other authors have obtained crystalline hydroxylapatite particles using pulsed laser ablation at higher irradiance [24, 25].
Concerning the composition, crystalline particles obtained by the use of pulsed laser still preserve the composition of precursor material, although some of them undergo transformation phase from precursor HA to β-TCP promoted by longer pulse and high temperature. The effect of laser irradiation is expected to induce structural changes in material precursor constituted by HA due to the elevated temperature. Investigations in enamel irradiated with laser reported the formation of traces of α-TCP phase  when CO2 laser is used and the presence of traces of α-TCP and β-TCP when the source is Nd:YAG laser , which are in accordance with the obtained results. The amorphous particles obtained when using CW laser are calcium phosphate compounds, probably formed by melting and rapid solidification due the low irradiance delivered by the CW laser.