Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses

Gold nanorods have been found to change their shape after excitation with i ntense pulsed laser irradiation. The final irradiation products strongly de pend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system produci ng pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorptio n spectroscopy by monitoring the changes in the plasmon absorption bands ch aracteristic for gold nanoparticles; (2) transmission electron microscopy ( TEM) in order to analyze the final shape and size distribution. While at hi gh laser fluences (similar to 1 J cm(-2)) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observ ed when the laser energy is lowered. Upon decreasing the energy of the exci tation pulse, only partial melting of the nanorods takes place. Shorter but wider nanorods are observed in the final distribution as well as a higher abundance of particles having odd shapes (bent, twisted, phi-shaped, etc.). The threshold for complete melting of the nanorods with femtosecond laser pulses is about 0.01 J cm(-2). Comparing the results obtained using the two different types of excitation sources (femtosecond vs nanosecond laser), i t is found that the energy threshold for a complete melting of the nanorods into nanodots is about 2 orders of magnitude higher when using nanosecond laser pulses than with femtosecond laser pulses. This is explained in terms of the successful competitive cooling process of the nanorods when the nan osecond laser pulses are used. For nanosecond pulse excitation, the absorpt ion of the nanorods decreases during the laser pulse because of the bleachi ng of the longitudinal plasmon band. In addition, the cooling of the lattic e occurring on the 100 ps time scale can effectively compete with the rate of absorption in the case of the nanosecond pulse excitation but not for th e femtosecond pulse excitation. When the excitation source is a femtosecond laser pulse, the involved precesses (absorption of the photons by the elec trons (100 fs), heat transfer between the hot electrons and the lattice (<1 0 ps), melting (30 ps), and heat loss to the surrounding solvent (>100 ps) are clearly separated in time.