Studies Of Spectrum Split, Amplitude Distributions And Phase Singularities In Femtosecond Laser Pulses Transmitting Through Optical Fiber Probes

It is well known that studies on the propagations of femtosecond laser pulses through the near-field optical microscopy have become a hot subject in recent years. If a ultroshort laser pulse is taken as the incident wave illuminating the near-field optical microscopy, the light wave probe can reach an ultra-high spatial and ultrafast temporal resolution,which provides novel means to study the transmission properties of light field within the interface and near-field .The optical fiber probe is one of the most important parts in near-field optical microscopy, and it determines the characteristics of the microscopy such as resolution, sensitivity and transmission efficiency. The study of an femtosecond laser pulse propagating in the optical fiber probes may be beneficial for understanding both the propagation dynamics and applications of the femtosecond lasers. The result can also help us to find optimized tip design.In this paper ,using the three-dimensional finite difference time domain (3D-FDTD) method, the near-field distribution of a light wave in metal-coated optical fiber probes under illumination of femtosecond laser pulses are numerically simulated. Firstly, The temporal signal and its frequency spectrum of an ultrashort light pulse propagating in the metal-coated optical fiber probes is studied, and the calculation results show that the structure of probe leads to the spread of the optical pulse. Then by choosing the maximum of the calculated light field data and the time when the light field reach the maximum data in the time period of one light wave cycle at each spatial point, we obtain the amplitude and phase distributions of the light field near the probe, and obtain the phase singularities in metal-coated optical fiber probes under illumination of femtosecond laser pulses. The whole paper includes six chapters.In chapter 1, we introduces the research background of the subject, and give a brief overview over the development of the femtosecond laser, near-field optical microscopy, femtosecond near-field optical spectroscopy theories and applicationsIn chapter 2, we give a present the fundamental theories of the finite-difference time-domain (FDTD) method. Based on the Maxwell's equations, we obtain the finite-difference equations, and discuss the stability of the FDTD numerical solution and its dependence on the time and space steps. We finally discuss the numerical dispersion problems. The fundamental theories of Perfectly Matched Layer (PML) absorbing boundary conditions in FDTD method are presented in detail. By setting particular medium layers on the boundary of the computational space, we give the numerical method for setting the light field components at the boundaries of problem space, and in limited space, and we modulate the light field propagation in boundless space.In chapter 3, using the three-dimensional finite difference time domain (3D-FDTD) method, we calculate light field in metal-coated optical fiber probes with different taper angles and different lengths under illumination of femtosecond laser pulses. The transmission of temporal fields of the laser pulses propagating in the probes and the properties of their frequency spectrum and phases are studied. The influences of the taper angle and the length of the probe on these properties are analyzed. The calculation results show that the structure of probe leads to the broadening, the amplitude oscillation and the spectrum split of the laser pulses. With the analysis of the spectrum and the phase variation, we give the initial explanations to the phenomena of the pulse broadening and the periodic variation of the pulse amplitude with time.In chapter 4, with three-dimensional finite-difference time-domain (3D-FDTD) method, we calculate light field in metal-coated optical fiber probes under illumination of femtosecond laser pulses. By choosing the maximum of the calculated light field data in the time period of one light wave cycle at each spatial point, we obtain the amplitude distributions instead of the conventional light field distribution of a time instant. We find that with an incident plane wave pulse of y-polarization, the output amplitude distributions of y-polarization has roughly a circled-cross-like pattern with two arc-like zero-amplitude zones in it. By analyzing the y-polarized waveforms versus time at the points near the arc-like zero-amplitude zones, we find that waveform distortions appear at points in the neighborhood of the zero amplitude lines and in the time intervals when two wave-packets overlap.In chapter 5, the near-field distribution of a light wave in metal-coated optical fiber probes under illumination of femtosecond laser pulses are numerically simulated. By choosing the maximum of the calculated light field data and the time when the light field reach the maximum data in the time period of one light wave cycle at each spatial point, we obtain the amplitude and phase distributions of the light field near the probe. We find that the output amplitude distributions of y-polarization has roughly a circled-cross-like pattern with two arc-like zero-amplitude zones in it. The zeros of light intensity are the phase singularities at which the phase of is undefined. By analyzing the output phase distributions of y-polarization , we obtain the temporal evolution of the phase singularities and their character.In chapter 6, we give a summary of this paper, and put forward the following work.