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Title: Design and fabrication of low-loss plasmonic and upconversion lasers
Authors: Siu, Chun Kit
Degree: Ph.D.
Issue Date: 2019
Abstract: Theoretical analysis of plasmonic lasers is performed based on typical optical waveguide theory and laser physics. The study begins with the analysis of 1-dimensional planar waveguides, which has a highly symmetric structure with relatively simple mathematical derivation process from Maxwell equations. A two-layer planar waveguide is used to demonstrate the behavior of surface plasmon polariton propagating in a single metal-dielectric interface, in which light is confined within the sub-wavelength range along the transverse direction. Hence, this shows the significance of incorporating metal in laser designs to confine light within a small region. In addition, a three-layer planar waveguide (with two metal-dielectric interfaces) shows the interaction of surface plasmon between multiple adjacent metal-dielectric interfaces, resulting in a totally different behavior when compared to the two-layer system. It is noted that the degree of interaction highly depends on the distance between the two interfaces. Reflection Pole Method (RPM) is also used to analyze the optical behaviour of multi-layer planar waveguide structures. RPM is applied to study the longitudinal guided modes of common planar laser designs in recent researches. In particular, it is confirmed that by adding a thin dielectric protection layer between the gain medium and the metallic layer, the lasing threshold of Fabry-Perot modes in nano-cavities can be effectively reduced due to the suppression of metallic absorption. Similar analysis also applies to the study of the whispering-gallery modes (WGMs) of radially-layered cylindrical waveguides. The experimental part of this study involves the discussion of techniques in cavity excitation and spectral analysis, followed by design and fabrication of upconversion lasers with micro-and nano-scale features. There are two major works discussed in this area. First, single-crystalline microplates of the semiconductive perovskite CsPbCl₃ are fabricated. The microplate laser cavities, which have square shape of side lengths ranging from 2 to 10 μm, support single-or multi-mode WGM lasing emission with quality factor Q ~ 1400 at peak wavelength of about 425 nm. The low lasing threshold of the cavities enables lasing emission through excitation via 2-photon absorption (2PA) with 800 nm pump pulses, as well as 3-photon absorption (3PA) with 1280 nm pump pulses, at low temperature (83 K) and pressure (10⁻³ mbar). Second, plasmonic laser cavity is also designed using Yb³⁺-Er³⁺-Tm³⁺ tri-doped β-NaYF₄ hexagonal microrods as the gain material. White light emission, which is contributed by emissions from the red (654 nm), green (540 nm) and blue (450 nm) regimes, can be obtained from the gain medium through 980-nm excitation at room temperature. The laser is constructed by placing the NaYF4 microrods with 4-μm diameter onto an Ag-coated (50 nm) substrate, which alters the light distribution and enhances the emission intensity of WGM by more than 10 times due to plasmonic effect compared to a bare microrod without Ag coating. A micro-sized rod of the gain material ensures most of the light in the WGM is confined within it without a significant amount of metallic absorption in the Ag layer. This results in a much lower lasing threshold and compensates for the low quantum yield of the doped NaYF4 in order to achieve lasing, without requiring extremely high excitation power that often causes severe optical damage to the cavity.
Subjects: Hong Kong Polytechnic University -- Dissertations
Lasers -- Design and construction
Optical wave guides
Pages: xviii, 124 pages : color illustrations
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