Quantum technologies

This thesis presents an examination of ultrathin glass fibers as a novel tool for coupling light and matter. As a basic concept, matter, i.e. atoms, molecules, etc., will be coupled to the evanescent field in the vicinity of the fiber surface, which contains a large portion of the power of the guided light. Here, the effects of forming and microstructuring of these fibers on the field strength at the surface relative to the field strength in the center of the fiber are studied. Chapter 1 reports on the construction and optimization of a pulling system for glass fibers, which can be used to reproducibly manufacture subwavelength diameter fibers of centimeters length from standard glass fibers with high accuracy. The transmission through the whole structure is measured to be up to 97 %. The properties of the evanescent field and its potential for coupling of light and matter are analyzed in chapter 2 by spectroscopy of thin surface adsorbed films of organic molecules (PTCDA). It is theoretically and experimentally shown that the spectroscopic sensitivity can be increased by several orders of magnitude with respect to conventional techniques. This method allows for the first time the observation of sub-monolayer dynamics of structural changes of PTCDA on glass at ambient conditions. In chapter 3 two types of resonators, which can be built from glass fibers, are investigated. Firstly, a Bragg-mirror is integrated into an ultrathin glass fiber and the reflectivity is measured qualitatively. The second resonator type can be formed from a 16 µm thick fiber. Selective coupling of light from an ultrathin fiber into a whispering-gallery mode of the resonator is realized with an efficiency of 99,3% and tuning of the resonance frequency by more than one free spectral range of order 100 GHz is demonstrated. The quality factor of the excited modes has been measured to be of order 105 and limitations are discussed. Finally, a method is developed to determine the quantum numbers of a resonator mode. Thereby, this thesis makes a contribution towards the utilization of ultrathin and structured glass fibers to couple light and matter and opens the route to fiber-based quantum optics experiments.

The guided modes of sub-wavelength diameter air-clad optical fibers exhibit a pronounced evanescent field. The absorption of particles on the fiber surface is therefore readily detected via the fiber transmission. We show that the resulting absorption for a given surface coverage can be orders of magnitude higher than for conventional surface spectroscopy. As a demonstration, we present measurements on sub-monolayers of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) molecules at ambient conditions, revealing the agglomeration dynamics on a second to minutes timescale.