【作者】 尹君；

【导师】 牛憨笨；

【作者基本信息】 华中科技大学 ， 光电信息工程， 2010， 博士

【摘要】 作为一种基于物质分子固有的分子振动特性的光谱研究手段,相干反斯托克斯拉曼散射(Coherent Anti-Stokes Raman Scattering, CARS)光谱探测和显微成像技术是一种具有高的时间和空间分辨率,高的灵敏度、高化学特异性和具有三维层析能力的非侵入光谱分析和显微成像技术。为了基于分子振动谱快速有效地识别含有多种物质成份或未知成份的样品中各种成份,实时监测物质分子所处环境的变化和物质分子之间的相互作用,要求CARS光谱探测技术具有快速获取具有高光谱分辨率的完整的分子振动谱的能力。在本论文中,我们提出了一种基于超连续谱激光输出的超宽带时间分辨CARS (time-resolved CARS, T-CARS)光谱探测技术。在这一技术中,利用飞秒激光脉冲泵浦光子晶体光纤(Photonic Crystal Fiber, PCF)产生的超连续谱激光输出同时作为泵浦光和探测光,利用窄线宽激光脉冲作为探测光。通过改变探测光和超连续谱激光脉冲之间的时间延迟能够有效抑制非共振背景噪声,提高系统的探测灵敏度和光谱分辨率。同时,通过时间分辨测量方法实现同时对多个分子振动模式的振动退相时间进行测量。本论文完成的主要研究工作包括以下几个方面：(1)在广泛调研的基础上,介绍CARS光谱探测和显微成像技术的发展历程和现状,分析其特性和优势,以及现存的问题。(2)利用半经典理论分析方法分析具有不同中心频率的三束激光脉冲与物质相互作用的CARS过程。对抑制非共振背景噪声的方法进行了详细的分析和讨论。在此基础上,通过数值模拟的方法模拟了T-CARS过程,并对利用时间分辨方法抑制非共振背景噪声给出了物理解释。理论分析了实现宽带T-CARS光谱探测技术的可行性。(3)为实现同时获取完整的分子振动谱,要求超连续谱激光输出在一定的光谱范围内的各光谱成份具有光谱连续性、时间一致性,强度分布均匀且稳定。为获得优化超连续谱激光输出的实验条件,对超连续谱产生机制进行了理论分析和模拟工作。利用有限元法获得现有PCF的色散曲线、零色散点等主要光学参数。利用分步傅立叶法分析了不同条件下超连续谱的频域和时域分布情况。利用互相关频率分辨光学快门方法分析了不同条件下超连续谱中各光谱成份的时间分布情况。通过模拟计算,获得了优化超连续谱激光输出的实验条件,并最终实现了满足实验要求的超连续谱光源。(4)在理论研究和数值模拟工作的基础上,实现了超宽带T-CARS光谱探测系统。通过对已知拉曼谱的有机溶液样品进行实验,对系统的主要参数进行了标定,如光谱分辨率、可同时探测的光谱范围等。实验得到了多种有机溶液样品波数在387-4092cm-1内的分子振动谱。对CARS信号的强度与激发光的强度之间的关系进行了实验研究。开展了以时间分辨方法同时测量多个分子振动模式的振动退相时间的实验研究工作。本论文研究工作的主要创新点如下：(1)通过理论研究得到了优化超连续谱激光光源的实验条件,实现了满足超宽带T-CARS光谱探测技术要求的,在较宽光谱范围内具有较好的光谱连续性、时间一致性,输出强度分布均匀且稳定的超连续谱光源；(2)实现了基于超连续谱的超宽带T-CARS光谱探测系统,能够同时获得波数在387-4092cm-‘内的具有高光谱分辨率有机物分子的分子振动谱,具有快速有效地区分不同成份的能力;(3)利用超宽带T-CARS光谱探测系统能够同时测量光谱范围内的多个分子振动模式的振动弛豫过程,同时获得振动退相时间,具有实时观察分子间相互作用和监测分子所处环境变化的能力。更多还原

【Abstract】 As a spectroscopic analyzing method based on the inherent molecular vibrations, the coherent anti-Stokes Raman scattering (CARS) spectroscopic and microscopic imaging technique is a noninvasive spectroscopic and microscopic imaging method with high temporal and spatial resolution, high sensitivity, high chemical specificity and 3D sectioning imaging capability.In order to quickly and effectively distinguish the various components of samples with different kinds of components and unknown components, and real-time monitor the change of environment and the interaction courses between different molecules based on the molecular vibrational spectrum, the capability of quickly obtaining the whole molecular vibrational spectrum with high spectral resolution is necessary with the CARS spectrography. In this dissertation, we present an ultra-broadband time-resolved CARS (T-CARS) spectrography based on the supercontinuum. In the ultra-broadband T-CARS, the output of one locked-mode femtosecond laser is split into two beams. One beam of femtosecond laser pulses is introduced into a photonic crystal fiber (PCF) to generate the supercontinuum used as the pump and Stokes simultaneously. The other is used as the probe after passing through a narrow-band-pass filter. The whole molecular vibrational spectrum can be simultaneously obtained. The nonresonant background (NRB) noise accompanying with resonant CARS signals can be effectively suppressed by changing the delay time between pulses of the probe and supercontinuum. The sensitivity and spectral resolution can be improved. Otherwise, the relaxation courses of all molecular vibrational modes in the simultaneously detectable spectral coverage can be recorded by the time-resolved method. The dephasing time of each molecular vibrational mode can be obtained by numerical fitting.In this dissertation, the main completed research work has been shown below.(1) Based on the extensive research, the developments and present state of the CARS spectroscopic and microscopic imaging technique are introduced.(2) We studied the mechanism of CARS process. The CARS course, in which three beams of laser pulse with different frequencies are used, is analyzed by using half-classic theoretical analysis. The methods of suppressing the NRB noise are presented and analyzed in detail. The T-CARS is studied by the numerical simulation method. The physical explanation is given for the suppressing NRB noise by the time-resolved method. The feasibilities are theoretically analyzed for achieving an ultra-broadband T-CARS spectrography.(3) In order to simultaneously obtain the whole molecular vibrational spectrum, besides an enough broad bandwidth, the spectral continuity, simultaneity, uniformity and stability of various spectral components in superconyinuum are all necessary. The theoretical analysis and numerical simulation are carried out for obtaining the experimental conditions to improve the supercontinuum. The main optical parameters of existing PCF, such as the dispersion curve, zero dispersion wavelength, are obtained by the finite element method. The distributions of the supercontinuum in the frequency and time domain are explored under various conditions with the split step Fourier method. And the temporal distributions of various spectral components in supercontinuum are analyzed with the cross-correlation frequency-resolved optical gating method. The experimental conditions for improving supercontinuum are obtained with numerical simulation. It is the guidance for achieving the required supercontinuum source.(4) Based on the theoretical analysis and numerical simulation, an ultra-broadband T-C ARS spectrometer system is successfully achieved. The main parameters of system are calibrated by measuring some samples with the known Raman spectrum, such as the spectral resolution, simultaneously detectable spectral range. The molecular vibrational spectrums in 387-4092cm-1 of a variety of organic samples are obtained by experiments. The relationships between the intensity of CARS signals and pump laser are studied. The simultaneous measurement of the dephasing time of various molecular vibrational modes is carried out.The main innovations are:(1) The experimental conditions for improving supercontinuum are obtained with numerical simulation. The supercontinuum laser source is achieved with the good spectral continuity, simultaneity, uniformity and stability. The requirements on the supercontinuum laser source of the ultra-broadband T-C ARS spectrography are met.(2) The ultra-broadband T-CARS spectrography based on the supercontinuum is achieved. The organic molecular vibrational spectrums with high spectral resolution can be simultaneously obtained in 387-4092cm-1. It has the capabilities of quickly and effectively distinguishing different components.(3) The relaxation courses of various molecular vibrational modes in the spectral coverage can be simultaneously measured to obtain the dephasing time. It has the capabilities of real-time observing the molecular interactions and monitoring the change of surrounding environment.更多还原