【作者】 陈奇；

【导师】 张春峰；

【作者基本信息】 南京大学 ， 光学工程（专业学位）， 2013， 硕士

【摘要】 利用多光子吸收过程实现频率上转换的激光发射在光电子学特别是生物光子学领域具有广泛的应用前景。不同于二次谐波,多光子激发产生的上转换激光不受相位匹配条件的限制,因而在很多应用场景下特别在显微环境下比较便利。在生物光子学的应用上,一方面由于泵浦激发光是生物组织光学窗口波长的近红外光,使得激发光可深入生物组织；另一方面多光子吸收非线性依赖于局域场强,空间限制性好,使得我们在做生物标定或治疗时对其他正常组织的伤害降到最低。目前,真正实现多光子激光在上述规模应用还面临一些基本问题和技术挑战。多光子吸收是高阶非线性光学过程,低功率下激发的效率低,且典型的多光子材料很难产生粒子数反转,激发阈值都很高,因而材料的稳定性变差,寿命变短,限制了其在实际应用中可行性。因此,开发新的材料体系,以降低激发阈值,提高使用寿命,成为目前多光子激光的研究重点。为了解决多光子激光器在实际应用前的技术障碍,本论文提出了一种新的多光子激发方案,从物理机制上寻求突破,开发新的复合材料体系,从而降低激发阈值,提高使用寿命。将具有大双光子吸收截面的材料作为给体,同时将良好的激光材料作为受体,利用超快能量转移的物理机制将两者耦合起来,从而兼容两者的优势,优化能量转移效率,实现低阈值的多光子激光。为实现上述设计概念,本论文选取聚合物激光材料聚[2-甲氧基-5-(2’-乙基己氧基)对苯乙炔](MEH-PPV)作为受体,并利用具有大双光子吸收截面的染料作为给体,通过能量转移过程实现上转换光学增益。具体来说,论文的工作主要包括：(1)利用z扫描方法评估了掺杂双光子染料的聚合物双光子吸收系数,证实了复合薄膜有效双光子吸收增强；(2)利用飞秒瞬态光谱学的方法在该复合材料体系中观测到了频率上转换,并确认了能量转移在其中的主导作用；(3)变条长的方法定量标定了最优掺杂下复合材料的上转换光学增益系数为25cm,与利用单光子激发时的增益系数相仿；(4)通过激发功率依赖的光谱测量确定了该复合材料体系的产生自发辐射放大的阈值(0.13mJ/cm2)较纯聚合物激光材料(2.5mJ/cm2)低了一个数量级,将这一增益介质耦合到分布反馈结构中,实现低阈值的激光发射。更多还原

【Abstract】 Frequency up-converted lasing pumped by the process of multi-photon absorption is promising for the myriad applications in biomedical optics. It has significant advantages over the conventional method of sum-frequency generation for the applications where the phase-matching condition cannot be fulfilled. The pump source in near infrared wavelength for multi-photon-pumped laser can penetrate deeply into tissues, making it feasible to do in vivo operation. The multi-photon excitation has a better spatial confinement, which is also instrumental for targeted treatment.Nevertheless, there are still critical challenges remaining towards practical applications of multi-photon-pumped lasers. Ideal gain medium for up-converted lasing should hold the ease of establishing population inversion with efficient multi-photon absorption, which makes most of the conventional laser materials unsuitable for the application. Therefore in order to solve the problem, it is necessary to search the gain material with the potential for low threshold multi-photon-pumped laser.In this thesis, I introduce our exploration to address this issue with a new material design scheme of energy transfer. We try to design a hybrid gain medium with efficient two-photon absorbing donors and high gain acceptors. The strong coupling ensures an efficient energy transfer from the donor to acceptor that can be used to realized multi-photon-pumped lasers with low excitation threshold.This scheme of energy-transfer pumping is verified in this thesis with polymer gain medium of MEH-PPV as acceptor and two-photon-absorbing dye as donor. More specifically, we obtained the results including:(1) effective two-photon absorption is enhanced in the hybrid polymer/dye medium as measured by the method of z-scan;(2) Optical gain is observed at the up-converted frequency domain by the femtosecond transient absorption spectroscopy. The process of energy transfer is confirmed to be responsible for the gain.(3) The optical gain coefficient is measured to25cm-1with the technique of variable stripe length test in the optimally-doped hybrid system, which is comparable to the value in the neat polymer under one-photon excitation.(4) The excitation threshold (0.13mJ/cm2) for amplified spontaneous emission in the optimally-doped hybrid system is obtained to be over one-order lower than the magnitude in neat films, which can be incorporated onto distributed feedback structures to demonstrate low threshold two-photon-pumped lasers.更多还原