Boutinguiza et al. Nanoscale Research Letters 2011, 6:255 http://www.nanoscalereslett.com/content/6/1/255 irradiance range between 2 × 105 and 106 W/cm2. The laser beam was coupled to an optical fiber of 50 μm dia- meter using the same focusing system and processing setup than in the case of the Nd:YAG laser. Precursor material was characterized by means of X-ray diffraction (XRD) using a Siemens D-500 equipment and by X-ray fluorescence (XRF) taken by a Siemens SRS 3000 unit. TEM, selected area electron diffraction (SAED), and HRTEM images were taken on a JEOL-JEM 210 FEG transmission electron microscope equipped with a slow digital camera scan, using an accelerating voltage of 200 kV, to reveal their crystalline. The morphology as well as the composition is described by the scanning elec- tron microscopy (SEM) using a JEOL-JSM-6700F Results The starting material used as target submerged in de- ionized water consisted in pellets obtained from calcined fish bones. The calcined fish bones exhibited an appear- ance of rod-like with micrometric size as shown in Figure 1. X-ray diffraction patterns of precursor materials compared with that of stoichiometric HA are shown in Figure 2. As can be seen, the biological material is com- posed of well crystallized HA. The composition detected by XRF revealed the presence of Ca and P as main elements in the samples with a Ca/P molar ratio of 1.68. Some minor elements were also detected, such as Mg, Na, Si, etc. The use of pulsed laser with 3 ms pulse width, 1.8 J, and 10 Hz of frequency (laser irradiance: 8 × 5 × 106 W/cm2) lead mainly to the formation of particles with rounded shape and nanometric size as can be seen from Figure 3. The HRTEM micrograph demonstrates that these particles are crystalline, showing the lattice fringes used to quantify the inter-planar spacing by means of the fast Fourier transform. The results of crystalline phases identified by inter-planar distances revealed that Figure 1 SEM photograph of fish bones used as target showing the size and appearance of crystals. Page 2 of 5 the obtained nanoparticles are mainly composed of HA and b-TCP, as listed in Table 1. The microanalysis per- formed on this kind of particles showed also the pre- sence of trace elements, such as Mg and Si. The use of continuous wave Yb:YAG fiber laser at irradiances around 6 × 105 W/cm2 led to the formation of particles with spherical shape ranging from nano- metric to micrometric size (Figures 4 and 5), but the predominance of the nanometric ones is evident from Figure 4. According to the results of the SAED per- formed on a group of this kind of particles they are amorphous. Nevertheless, the microanalysis performed on groups of these particles revealed the presence of the same elements of precursor material. Discussion 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 prop- erties, and the surrounding environment [13]. 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 inter- face, and for some materials thermal stress effects are important since they may cause the surface fracture of the solid [14]. 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 inter- action [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 ≪ ttc). Under this condition, pulse duration is shorter than the time needed for bubbles formation and diffu- sion in the process of heterogeneous boiling [17,18]; therefore, the material can be overheated over the boil- ing temperature leading to explosive vaporization at low fluences or phase explosion at higher fluences [19,20]. On the other hand, stress confinement condition is ful- filled when the energy is deposited in the irradiated volume more rapidly than it can be dissipated through collective molecular motion according to τp ≪ tsc,