Nanostructured optical fibre tapers and related applications

Ding, Ming (2013) Nanostructured optical fibre tapers and related applications. University of Southampton, Faculty of Physical Sciences and Engineering, Doctoral Thesis, 190pp.

Record type: Thesis (Doctoral)

Abstract

In the last decade, optical fibre tapers have attracted considerable interest because they offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These distinctive features have been exploited in a wealth of applications ranging from telecommunication devices to sensors, from optical manipulation to high-Q resonators. Nanostructures on the optical fibre tapers are very promising since the size of the device can be extremely small. With the development of nanostructuring methods, sub-wavelength feature sizes have been achieved. In this thesis, nanostructured optical fibre tapers and some related applications are discussed.

Light confinement is limited by diffraction and the minimum spot size is related to the light wavelength. In this thesis, light confinement in two and three dimensions is proposed and achieved with two typologies of nanostructured optical fibre tapers.

The first group of devices exploits plasmons excited at the optical fibre tips to obtain high transmissivity, and confine light to a sub-wavelength dimension. Optical fibre tips were designed according to numerical simulations and coated by a layer of gold; an extremely small aperture was then opened at the tip apex. The experimental characterization and simulation results showed their improved transmission efficiency (higher than 10^-2) and thermal expansion measurements showed no shape changes could be detected within the accuracy of the system (~2 nm) for 9 mW injected powers. Effective confinements to 10 nm or smaller can be envisaged by decreasing the aperture size and slope angle. Application of this small spot size source can include scanning near-field optical microscope, optical recording, photolithography and bio-sensing.

The second group achieves three dimensional light confinement exploiting a Fabry-Perot microcavity formed by a microfibre grating similar to those used in distributed feedback lasers. Microfibres were patterned using a Focused Ion Beam (FIB) system. In this structure, the microcavity provides longitudinal light confinement, whereas air dielectric guiding by the microfibre provides diffraction limited confinement in the other two dimensions. Due to the high refractive index contrast between silica and air, strong reflection can be obtained by only dozens of notches. This device can be used for a wide range of applications, e.g. sensing and triggered single-photon sources.

Light confinement in nanostructured optical fibre tapers was exploited in a micrometric thermometer. A compact thermometer based on a broadband microfibre coupler tip showed a dynamic range spanning from room temperature to 1511ºC with a response time of tens of microseconds. This is the highest temperature measured with a silica optical fibre device. An average sensitivity of 11.96 pm/ºC was achieved for a coupler tip with ~2.5 µm diameter. A resolution of 0.66ºC was achieved for a coupler tip diameter of ~12.6 µm. Better resolution can be achieved with smaller size microfibre coupler tips.

Optical fibre tapers are commonly used to couple light to selected resonator modes. Here FIB was used to inscribe microgrooves on optical Bottleneck Microresonator (BMR) surfaces to excite selected whispering gallery modes. By monitoring the transmission spectrum of the optical fibre taper, substantial spectral clean-up was obtained in appropriately scarred BMRs. Single high-Q mode operation can be achieved by either using two asymmetrical perpendicular scars or placing the grooves closer to the BMR centre, providing the potential for high performance sensors and other optical devices.

Finally, strong three dimensional localization has been achieved in Plasmonic Slot Nano-Resonators (PSNRs) embedded in a gold-coated optical fibre tapers. Different shapes PSNRs, embedded in thin gold metal film coated plasmonic microfibre, were numerically investigated. The intensity enhancement (in excess of 10⁶) and the resonance wavelength depend on both the PSNR and microfibre dimensions. Theoretically and experimentally, the transversal excitation of a rectangular PSNR embedded in a thin gold film coated plasmonic fibre tip was discussed for the first time, and showed high localization and strong enhancement (7.24×10³). This device can find a wide range of applications such as surface-enhanced Raman scattering, optical filtering, spectroscopy and bio-sensing.