Nonlinear Frequency Conversion and Interferometric Sensing with Ultra-Thin Optical Fibers

在这个项目中，我们建议研究气体和分子样品的线性和非线性相互作用与微结构的倏逝光场，直径约为λ/2的光纤，其中λ是通常在红外或可见光谱范围内的光的波长。这种相互作用非常强，因为传播光功率的大约一半包含在与蒸汽或表面吸附物强烈相互作用的倏逝光场中。此外，光沿着光纤传播沿着几厘米，强烈地限制在大约1/2的区域内，对应于极度拉长（“无限”）的焦点。相比之下，在均质材料中，聚焦到λ 2的光束的焦深被限制在λ 2量级的非常短的范围内。对于非线性光-物质相互作用，我们计划在第一步中将双折射光纤置于稠密的铯蒸气中，使强倏逝场与铯原子相互作用。在λ = 1000 nm附近，近共振铯D2跃迁将强烈增强三阶非线性极化率。相位匹配可以通过仔细选择光纤直径的模式色散来实现，并且可以通过调节铯蒸气压来进行微调。我们计划研究皮秒脉冲和连续波Ti：Sapphire激光辐射的三阶非线性。一旦建立了方法，我们计划研究三阶非线性，例如，分子表面吸附物。此外，它似乎很有吸引力，以找出是否二阶非线性的适当取向的吸附膜组成的非中心对称的生色团可以导致增强的二次谐波generation.In第二部分的项目，我们计划实现一个双模态马赫-曾德尔干涉仪通过干涉两个不同的径向模式传播沿着一个单一的锥形光纤。分束器将通过短的非绝热锥形部分来实现，即超薄光纤直径的微小变化。因此，我们期望构建一个内在的干涉仪，这是高度敏感的周围介质或表面吸附物的折射率的变化，由于两种模式的倏逝场强度非常不同。作为第一个演示，我们计划检测孤立的3，4，9，10-二萘嵌苯四甲酸二酐（PTCDA）分子吸附到电介质表面的这种分散的方法，而不是吸收技术，成功地证明了非常高的灵敏度在第一阶段的属性。

In this project we propose to study linear and nonlinear interactions of gaseous and molecular samples with the evanescent light field of micro-structured, ultrathin optical fibers with diameters on the order of /2, where , is the wavelength of the light which is typically in the infrared or visible spectral range. The interaction is unusually strong because about half of the propagating light power is contained in the evanescent light field which interacts strongly with vapors or surface adsorbates. Furthermore, the light travels up to several centimeters along the ultrathin fiber, strongly confined to an area on the order of ², corresponding to an extremely elongated ("infinite") focus. For comparison, in a homogeneous material the depth of focus of a light beam focused to ² is limited to a very short range of order . Thus we can expect about 4 orders of magnitude enhancement of the light-matter interaction by guiding a light beam with an ultrathin fiber.For nonlinear light-matter interaction we plan in the first step to place the ultrathin fiber into a dense caesium vapor, such that the strong evanescent field interacts with the caesium atoms. Near  = 1000 nm the third-order nonlinear susceptibility will be strongly enhanced by the near-resonant caesium D2 transition. Phase matching can be achieved via the modal dispersion by carefully choosing the fiber diameter, and fine tuning is possible by adjusting the caesium vapor pressure. We plan to study third-order nonlinearities with both picosecondpulsed and continuous wave (cw) Ti:Sapphire laser radiation. Once the method is established we plan to investigate the third-order nonlinearities of, e.g., molecular surface adsorbates. In addition it seems attractive to find out whether second-order nonlinearities of suitably oriented adsorbate films consisting of non-centro symmetric chromophores can lead to enhanced second harmonic generation.In the second part of the project we plan to realize a bi-modal Mach-Zehnder interferometer by interfering two different radial modes propagating along a single tapered fiber. The beam splitters will be realized by short non-adiabatic taper sections, i.e. small variations of the diameter of the ultrathin fiber. We thus expect to construct an intrinsic interferometer, which is highly sensitive to changes in refractive index of the surrounding medium or surface adsorbates, due to the very different evanescent field strengths of the two modes. As a first demonstration we plan to detect the properties of isolated 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules adsorbed to the dielectric surface with this dispersive method rather than the absorptive technique that was successfully demonstrated with very high sensitivity in the first period.