Interaction Between Femtosecond Laser Pulses And Nanostructures And Ultrafast Spectroscopic Properties

Ultrashort laser pulses exhibit ultra-intensive peak powers and enable ultrashort interaction time duration,which are of important meanings for photophysics,nonlinear optics,time-resolved spectroscopy,and material machining techniques.The high peak powers of femtosecond laser pulses can be utilized to achieve high-quality and high-precision machining and manufacturing,or produce strong nonlinear processes having potential applications.Based on the performance of ultrashort interaction time and high time resolution,femtosecond lasers can be used to investigate the ultrafast phenomena or processes with high precision,which may include ultrafast electronic processes,ultrafast dynamics in physical or chemical reaction processes in atoms and molecules,optoelectronic processes in semiconductors.Plasmonic collective electron oscillation in metallic nanostructures is a typical ultrafast photophysical process.Ultrafast laser spectroscopy may not only give deep insights into the plasmonic resonance modes and physical mechanisms of local field enhancement,but also facilitate the development of novel ultrafast photonic devices.This thesis utilizes ultrafast spectroscopy as the main tool and has its research work centered around the interactions between ultrafast pulses and metallic and semiconductor nanostructures,which mainly includes:(1)Studies on the transient phase transition induced by femtosecond laser pulses in polymer films.Making use of the ultrahigh peak power and ultrashort interaction time,transient annealing process was carried out on the F8 BT polymer films and new physics involved in the laser-induced “solid-liquid-solid” phase-transition processes was discovered.Photoluminescence and Raman spectroscopic performance was investigated on the product of the transient annealing processing.Thus,modifications on the physical properties of the molecules are discovered,which were induced during the melting of the polymeric molecular chains with large flexibility and their rearrangements during the solidification.These discoveries supply new techniques and physical basis for the organic semiconductor physics and optoelectronics.(2)Investigation on the ultrafast optical switching effects in a self-supported gold nanowire grating film.Using the flexible transfer technique,the gold nanowire grating structure prepared on an ITO glass substrate was lift off to produce a self-supported thin film.Ultrafast spectroscopic response of the transferred structures was investigated and flexible ultrafast optical switch is thus achieved.It is revealed that the transient modulation on band structure of gold atoms and its effects on the plasmonic resonance spectra by strong laser excitation are the main physical mechanisms.Meanwhile,by resonance excitation,the working efficiency of the optical switch is largely enhanced.Thus,ultrafast optical switching functions and the corresponding devices are achieved with high signal contrast,high signal-to-noise ratio,and low excitation threshold.(3)Fiber end integrated gold nanowire gratings and all-optical switching performance.Gold nanowire grating structures were integrated onto the end facet of an optical fiber,realizing the miniaturization and integration of the plasmonic ultrafast optical switching devices and enabling long-range transmission of the ultrafast optical modulation signals.Combination of the optical switch with flexible optical fibers implies enhancement and extension of the functions and applications of the ultrafast optical switching devices.The localized surface plasmons that support the ultrafast optical switching effects are studied systematically,leading to the discovery of the quadrupolar and dipolar plasmon resonance modes.Thus,the main physical mechanisms for the ultrafast optical switch are determined as the interaction between the multiple localized surface plasmons and the bandegde modulation in gold under excitation by femtosecond laser pulses.An optical switching signal with a speed higher than 3.2 ps and a modulation depth greater than 7.6% is achieved.