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

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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.