【作者】 赵阳；

【导师】 夏元钦；

【作者基本信息】 哈尔滨工业大学 ， 光学工程， 2015， 博士

【摘要】 随着超短脉冲激光技术的发展,飞秒科学(主要包括飞秒化学、飞秒物理、飞秒生物等)被广泛的研究。飞秒相干反斯托克斯拉曼光谱(Coherent Anti-Stokes Raman Spectroscopy,简称CARS)是飞秒科学研究中一种重要的非线性光谱技术,利用飞秒激光脉冲作为泵浦光和斯托克斯光共同作用激发分子的拉曼振动模并通过时间延迟探测光探测被激发的分子拉曼振动模的时间演化,探测得到的飞秒CARS信号不但能够反映物质微观的分子超快动力学过程,也可以反映分子的宏观温度信息,因此飞秒CARS是开展分子超快动力学研究和气体燃烧测温的一种重要手段。本论文利用飞秒时间分辨CARS,通过优化泵浦光和斯托克斯光的延迟时间、偏振特性和激光波长等实验参数,开展了BBO晶体(βphase barium metaborate crystals)、乙醇溶液、蒸馏水、PMMA(Poly methyl methacrylate)片材和若丹明B水溶液等样品的分子超快动力学以及气体燃烧测温的理论和实验研究工作。在飞秒时间分辨CARS理论研究中,理论分析了非共振背景对飞秒CARS信号的影响。对飞秒时间分辨CARS实验中常用的减小非共振背景噪声的方法进行了改进,并将这种改进后的方法应用到BBO晶体和PMMA片材,对BBO晶体中内环B-O键的伸缩振动和外环B-O’键的伸缩振动的相干弛豫过程以及PMMA片材的CH3基团中的对称和非对称的伸缩振动的相干弛豫过程进行了研究。通过控制泵浦光的波长对拉曼振动模进行选择性激发,进而研究了乙醇溶液3000 cm-1附近的三个拉曼振动模(CH3基团的对称与非对称振动和CH2基团的对称振动)的相干弛豫过程。另外,通过改变泵浦光的波长,利用飞秒时间分辨CARS研究了水中从3100 cm-1到3700 cm-1的OH键的四个伸缩振动的相干弛豫过程。由于飞秒激光脉冲宽度为飞秒量级(频域半高宽为几十纳米),这就导致了多个拉曼振动模会同时被激发,这种情况在大分子中更加严重。通过改变飞秒CARS实验中的延迟时间和偏振特性可以实现对拉曼振动模的选择性激发。通过改变泵浦光和斯托克斯光的延迟时间,利用激光脉冲的啁啾,对拉曼振动模进行选择性激发,进而使用探测光对被选择性激发的拉曼振动模进行探测。利用这个方法选择性激发了乙醇溶液中CH3基团、C-C键和C-O键等的伸缩振动。使用同样的方法选择性激发了若丹明B水溶液中从300 cm-1到1650 cm-1的多个拉曼振动模。飞秒CARS可以获得处于飞秒时域的超快过程的有关信息,捕捉瞬态变化的信息,用来测量大气压条件下火焰燃烧温度,在测温精度和测量范围方面较之以往利用纳秒激光测量有了大大的提高。与传统的纳秒CARS气体燃烧测温相比,飞秒CARS可以很好的消除非共振背景噪声,同时千赫兹以上的高重频飞秒CARS信号的获得可以实现燃烧场(湍流场)温度单点测量,进而实现毫秒量级时间尺度内燃烧场动态分析与监测,是一种燃烧场瞬时测温的新手段方法,本论文利用飞秒时间分辨CARS和飞秒单脉冲CARS对气体燃烧测温进行了研究。在飞秒时间分辨CARS气体燃烧测温的研究中,通过理论分析和实验验证的方法,分析了激光参数的测量和调节误差对测量结果的影响,以期望提高测量精度。通过飞秒时间分辨CARS对气体燃烧中的氮气温度进行了测量。理论计算了探测光的展宽情况对氮气飞秒单脉冲CARS信号的影响,为飞秒单脉冲CARS在气体燃烧测温中的应用提供参考。利用飞秒单脉冲CARS对火焰燃烧中氮气温度进行了测量,通过拟合得到了1000 K氮气的温度信息,测量速率达到了千赫兹。本论文改进的减少非共振背景噪声的方法,对乙醇溶液和水溶液相干弛豫过程的研究,提出的选择性激发并进行探测的方法以及利用飞秒CARS进行气体燃烧测温的研究丰富了飞秒CARS理论和实验研究,拓宽了飞秒CARS的研究范围,对于飞秒CARS的改进具有参考价值。更多还原

【Abstract】 With the development of ultrashort pulse laser technology, femtosecond science(including femtosecond chemistry, femtosecond physics, femtosecond biology, etc.) is widely studied. Femtosecond coherent anti-Stokes Raman spectroscopy(CARS) is an important nonlinear spectroscopy technology in femtosecond science, in which the Raman vibrational modes are excited by femtosecond laser pump and Stokes pulses, and then the time evolution of the Raman vibrational modes is detected by probe pulses. Femtosecond CARS not only can reflect the material microscopic molecular ultrafast dynamics process, but also can reflect the macroscopically molecular temperature information. Therefore the femtosecond CARS is an important tool for molecular dynamics research and gas combustion temperature measurement. In this paper, using the femtosecond time-resolved CARS, by optimizing the experimental parameters such as delay time bet ween pump and Stokes pulses, the polarization characteristics and the laser wavelength, the theoretical and experimental research about molecular ultrafast dynamics of the BBO crystals(β phase barium metaborate crystals), ethanol solution, distilled water, PMMA(Poly methyl methacrylate) sheets and rhodamine B water solutions and gas combustion temperature measurement was carried out.The properties of the quantum beats were analyzed with a function for quantum beats. The effects of nonresonant background on the performance of the quantum beats are discussed. If the quantum beats only are Fourier transformed in time domain with a little or little nonresonant background, the signal to noise ratio in the Fourier transform power spectra of the quantum beats signal will be enhanced. The best delay time is on the first peak of the time-domain signal. The time-domain CARS signals of BBO crystals and PMMA sheets were Fourier transformed after the best delay time. The difference frequency of the Raman modes in them was obtained.The vibrational dynamics in PMMA sheets and BBO crystals were investigated by femtosecond time-resolved CARS. The C-H stretch modes in PMMA sheets(at 2870 cm-1 and 3008 cm-1) are excited and detected. The Raman modes of B-O stretching vibrations at 1214 cm-1 and 1437 cm-1 in BBO crystals were obtained at room temperature. The dephasing times for the four vibrational modes are obtained by fitting function of quantum beats.The Raman vibrational modes can be changed by wavelengths of the laser pulses. The ultrafast dynamics process of ethanol solution was studied by using femtosecond time-resolved CARS. The three Raman modes of C-H stretching vibrations from 2800 cm-1 to 3000 cm-1 in ethanol were excited and successfully investigated at room temperature. The three Raman vibrational modes of ethanol are around 2973 cm-1, 2927 cm-1 and 2878 cm-1. The coherence relaxation times for the three Raman vibrational modes of C-H stretching vibrations in ethanol were measured.The femtosecond time-resolved CARS was performed to investigate the vibrational dynamics in distilled water. The ultrafast dynamics process of the OH-stretching modes between 3100 cm-1 and 3700 cm-1 in water was obtained and analyzed. The dephasing times of four Raman vibrational modes in water between 3100 cm-1 and 3700 cm-1 were detected and compared.Femtosecond time-resolved CARS has emerged as an attractive method for studying the vibrational dynamics of Raman vibrational modes in time-domain. Multiple Raman vibrational modes can be coherently excited simultaneously due to the spectrally broad femtosecond laser pulses. The restriction of the femtosecond CARS resolutions is especially severe in the case of biological macromolecule, the separation and recognition of individual Raman modes become rather challenging. As three laser pulses are used for the generation of the CARS signal, many degrees of freedom can be varied, such as the delay time between pump and Stokes pulses and the polarization characteristics.The ultrafast vibrational dynamics in ethanol and rhodamine B water solutions were investigated by spectrally dispersed femtosecond time-resolved CARS where we combined both wavenumber and time resolution. Many Raman vibrational modes of rhodamine B dye molecules which are from 300 cm-1 to 1800 cm-1 was selectively excited by changing the delay time between pump and Stokes pulses. The Raman modes of C-H stretching vibrations from 2700 cm-1 to 3500 cm-1 in ethanol were obtained at room temperature. The Raman vibrational modes of C-O and C-C stretching vibrations from 800 cm-1 to 1100 cm-1 in ethanol were also obtained. This technique allows one to track and determine the wavenumber of the excited Raman transitions. The ratio of intensity between Raman vibrational modes in BBO crystals can be changed by the polarization of pump pulses or crystal geometries.Femtosecond CARS can be obtained in the femtosecond time domain information about the ultrafast process, capture the transient change information. Especially in recent years, the developed femtosecond time-resolved CARS can be used to measure the atmospheric pressure nearly adiabatic flame temperature, which has greatly improved in the temperature measurement accuracy and range than nanosecond CARS measurement.Femtosecond CARS was utilized to overcome most of the problems associated with nanosecond CARS recently. Femtosecond CARS has many of advantages such as high spatial resolution, high temporal resolution and high sensitivity, etc, which is widely utilized to measure the methane/air flame temperat ure at atmospheric-pressure. The femtosecond time-resolved and single-shot CARS signals of the nitrogen molecule were measured and then simulated the theoretical results with a simple model.The effects of laser parameters on temperature measurements are discussed. Timing jitter is added to the pump/probe pulses and Stokes pulses. In 2000 K, the results indicate that timing jitter of 10% lead to less than 2% error for temperature measurements. In the higher temperature measurement, the impact of the error in laser parameters is greater.The methane/oxygen/nitrogen flame temperatures 300 K and 1325 K are extracted from the comparison of theoretical and experimental data(femtosecond time-resolved CARS) by least-square fit. The procedure for fitting theoretical spectra to experimental spectra is explained. The experimental results show good agreements with theoretical ones and present a good repeatability.The influence of the broadening of the probe beam on nitrogen femtosecond single-shot CARS was theoretically calculated. Femtosecond single-shot CARS was utilized to measure the flame temperatures 300 K and 1000 K at atmospheric-pressure. Femtosecond single-shot CARS temperature measurement speed reached a millisecond level.更多还原