Supercontinuum and frequency comb generation in tantalum pentoxide waveguides

Abstract

In this thesis, the design, simulation, fabrication, and characterisation of tantalum pentoxide (Ta₂O₅) waveguides and micro-resonators on silicon substrates is presented as a potential technology for the generation of supercontinuum and Kerr frequency combs. Vertical external-cavity surface-emitting lasers and membrane external-cavity surface emitting lasers that operate on two frequencies simultaneously are shown for the purposes of either acting as pump sources for the waveguide systems or as sources for generating THz radiation through the beat between the two frequencies. The fabrication process of the Ta₂O₅ waveguides is discussed in detail, including the relevant fabrication parameters. The Ta₂O₅ layer is deposited using radio-frequency (RF) sputtering onto a 4 ” diameter silicon wafer, with a 5 µm thick layer of thermal silica on the top surface, using a Ta₂O₅ powder target. The waveguide and micro-ring structures are realised using photolithography and ion beam milling, and the samples are cladded with a layer of silica using either RF sputtering or plasma-enhanced chemical vapour deposition with a liquid precursor. Linear waveguides in Ta₂O₅ are presented for the purpose of generating supercontinuum; Experimental outputs from linear Ta₂O₅ waveguides for a range of pump wavelengths from 900 to 1500 nm are reported with hundreds of nm of spectral broadening being seen for each pump wavelength. The expected spectral broadening emission of the waveguides is shown, which is calculated using a generalised nonlinear Schrodinger equation (GNSLE) solver that is capable of solving for multiple modes. The GNLSE solver is used to estimate which mode is propagating through the waveguide. The propagation losses of linear waveguides, including an investigation into the impact of waveguide width and cladding deposition technique, are presented, with the lowest propagation loss reported to be on the order of 2.5 dB/cm. The dispersion engineering for new Ta₂O₅ linear waveguide samples is shown with a focus on the waveguide geometry, leading to the finalisation of a waveguide design that corresponds to the optimised dispersion relation for nonlinear broadening. Micro-ring and racetrack resonators are presented for the purpose of generating Kerr frequency combs. The dispersion relations for micro-resonators are calculated and compared to that of linear waveguides, this followed by an investigation into the critical coupling lengths of micro-ring resonators. Racetrack resonator systems are presented as a suitable alternative to micro-ring resonators due to the fact that the critical coupling lengths are larger than the micro-ring geometries. A Lugiato-Lefever equation solver, based on the GNSLE solver, is described and confirmed against the literature, and is used to simulate the expected frequency comb emission from the Ta₂O₅ micro-resonators. A brief investigation into the effect of propagation loss on the comb generation is presented using the LLE solver. Experimental Q-factor measurements are presented for Ta₂O₅ micro-ring and racetrack resonators with the highest Q-factor being 1.91×10^5. Frequency comb generation was not observed, this is expected to be due to the propagation losses being too high. Finally, vertical external-cavity surface-emitting lasers and membrane external-cavity surface-emitting lasers operating simultaneously on two distinct spatial modes, and therefore two distinct frequencies, are reported. The laser systems are forced to operate on two modes simultaneously through the inclusion of laser ablated masks on an intracavity high reflectivity mirror that introduces spatially dependent losses on the cavity modes in order to suppress the lasing threshold of the fundamental Hermite Gaussian, HG00, mode and allow the higher order, HG11, mode to operate. Simulated mode losses are calculated, and the lasers are characterised for a range of spatially dependent loss values and pump powers. The spectral emissions of the lasers are shown, with wavelength separations on the order of 5 nm, which corresponds to a beat frequency of 1.41 THz. These laser technologies also represent an exciting potential pump laser system for the micro-resonators with the aim of generating a Kerr frequency comb.

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Identifiers

ORCID for Jake Daykin: orcid.org/0000-0002-4513-6425

ORCID for Vasileios Apostolopoulos: orcid.org/0000-0003-3733-2191

ORCID for James Wilkinson: orcid.org/0000-0003-4712-1697

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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