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

在这个项目中，我们提出研究气体和分子样品与微结构、超薄光纤的倏逝光场之间的线性和非线性相互作用，光纤的直径约为<s:1> /2，其中，<s:1>是光的波长，通常在红外或可见光谱范围内。这种相互作用异常强烈，因为大约一半的传播光功率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，光沿着超薄光纤传播了几厘米，强烈地限制在一个尺寸为ײ的区域内，对应于一个极长的（“无限”）焦点。相比之下，在均匀材料中，聚焦到<s:1>²的光束的聚焦深度被限制在一个非常短的范围内。因此，我们可以预期，通过超薄光纤引导光束，光-物质相互作用将提高约4个数量级。对于非线性光-物质相互作用，我们计划在第一步将超薄纤维置于密集的铯蒸气中，使强倏逝场与铯原子相互作用。在约1000 nm附近，近共振铯- D2跃迁会增强三阶非线性磁化率。通过仔细选择光纤直径，可以通过模态色散实现相位匹配，通过调整铯蒸气压可以实现微调。我们计划用皮秒脉冲和连续波（cw）钛蓝宝石激光辐射研究三阶非线性。一旦该方法建立，我们计划研究三阶非线性，例如，分子表面吸附。此外，发现由非中心对称发色团组成的适当定向的吸附膜的二阶非线性是否会导致二次谐波的增强，似乎很有吸引力。在项目的第二部分，我们计划通过干涉沿单个锥形光纤传播的两种不同的径向模式来实现双峰马赫-曾德尔干涉仪。分束器将通过短的非绝热锥形截面实现，即超薄光纤直径的小变化。因此，我们期望构建一种本征干涉仪，它对周围介质或表面吸附物的折射率变化高度敏感，因为两种模式的倏逝场强非常不同。作为第一个演示，我们计划用这种色散方法来检测吸附在介电表面的分离的3,4,9,10-苝-四羧酸二酐（PTCDA）分子的性质，而不是在第一个阶段以非常高的灵敏度成功证明的吸收技术。

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.