Photochemistry by femtosecond pulse-train

在本研究计画中，我们建构了一套由锁模飞秒雷射与脉冲整形器所组成的脉冲整形系统。脉冲形状的控制是使用基于傅里叶变换技术的可编程脉冲整形器来完成的，其中液晶空间光调制器（LCSLM）由偏振器和具有宽度为0.1 mm的矩形像素的液晶显示器（LCD）组成，在频域中调节激光脉冲的每个光谱分量的强度和相位。此外，该系统由遗传算法（GA）控制，这是一个强大的工具，找到一个适合于特定的物理/化学过程的脉冲形状，而不需要输入脉冲和样本的详细结构的知识。GA是一种多目标优化方法，模仿实际发生在生物进化过程中的过程，包括选择，交叉，变异的个体。在遗传算法控制的脉冲整形系统中，将SLM的像素看作基因的碱基对，将SLM中LCD上的调制图案看作基因中核苷酸序列的图案。然后，该系统演变的调制模式，在搜索的脉冲形状适合于特定的过程的基础上从实验的反馈。在模式进化过程中，实验输出值与生物体对环境的适应度相对应，我们尝试利用该系统控制苝晶体中的STE生成过程。结果表明，E型STE发射与Y型STE发射的强度比可以作为脉冲形状的函数而改变。

In this research project, we have constructed a pulse-shaping system consisting of a mode-locked femtosecond laser and pulse-shaper. The control of pulse shape is accomplished using a programmable pulse shaper based on a Fourier transform technique where a liquid crystal spatial light modulator (LCSLM), consisting of the polarizer and liquid crystal display (LCD) which has the rectangular pixels with the width of 0.1 mm, adjusts the intensity and the phase of each spectral component of the laser pulses in the frequency domain. Furthermore, the system was controlled by the a genetic algorithm (GA) which is a powerful tool to find a pulse shape suitable for a specific physical/chemical process without the knowledge of the input pulses and the detailed structure of samples.The GA is a multi-objective optimization method which mimics process actually taking place in the biological evolution process consisting of selection, crossover, and mutation of individuals. For the pulse shaping system controlled by the GA, the modulation pattern printed on the LCD in the SLM is regarded as the pattern of nucleotide sequence in gene by considering the pixel of the SLM as the base pair of gene. Then, the system evolves the modulation pattern in search of the pulse shapes suitable for a specific process on the basis of feedback from the experiment. In the progress of the pattern evolution, the value of the experimental output corresponds to the fitness for the environment of the organism.We tried to control the STE creation process in perylene crystal using the system. It was found that the intensity ratio of E-type STE emission to Y-type STE emission could be changed as a function of the pulse shape.