Harnessing induced second- and intrinsic third-order nonlinearities for amorphous nonlinear integrated photonics

Abstract

Nonlinear integrated photonics has emerged as a powerful route to miniaturize optical systems, enabling compact and energy‑efficient devices for light generation, modulation and spectroscopy on a chip. By confining light within sub-micron‑scale waveguides, these platforms dramatically enhance light–matter interactions, allowing nonlinear processes that traditionally require centimeters of bulk crystal to be realized in sub‑millimeter lengths. Such functionality is particularly attractive for applications in broadband frequency generation and ultrafast modulation, where low‑power operation, small device footprints and compatibility with wafer‑scale fabrication are crucial.
This thesis investigates amorphous material platforms that support induced second‑order (χ⁽²⁾) and intrinsic third‑order (χ⁽³⁾) nonlinearities, with a particular emphasis on integrating electro‑optic modulation and broadband mid‑infrared supercontinuum generation within a unified photonic technology.
The first part of this work begins with numerical modeling to design and evaluate an electro‑optic phase modulator based on amorphous poled sodo‑niobate (Na₂O₅:Nb₂O₅) waveguides on Si/SiO₂ substrate. Single‑mode waveguide dimensions are selected to support the fundamental transverse electric (TE) and transverse magnetic (TM) polarizations while ensuring strong optical mode confinement. The traveling‑wave gold electrode is positioned sufficiently close to the waveguides that the additional loss from metallic absorption and substrate radiation remains below 1 dB/cm. An applied voltage across the waveguides drives the Pockels effect, modifying the refractive index of the active material. Using a strong induced χ⁽²⁾ of 29 pm/V converted into an effective r₁₁ of 19.8 pm/V, the simulations show that optimized engineered rib dimensions can achieve voltage–length products as low as 3.86 V.cm for TE‑polarized light at 1550 nm, highlighting the potential of poled amorphous niobate for integrated electro‑optic modulation. Sodo-niobate thin films were deposited on glass substrates using RF magnetron sputtering. The films were optically characterized using prism coupling and ellipsometry (λ=192-1690 nm). The propagation loss of the films was found to be around 1 dB/cm at 1550 nm. The films were structured into waveguides through UV lithography and dry etching via ion beam milling. The propagation loss of the waveguides was measured using two techniques, including, Fabry-perot interference and cut-back with the estimated losses of 4.54 and 4.33 dB/cm at 1550 nm, respectively. Thermal poling with patterned electrodes was then used to migrate the sodium ions within the films and create a depletion layer, thereby breaking inversion symmetry and inducing χ⁽²⁾. The underlying charge migration mechanism was confirmed by Raman spectroscopy, second-harmonic generation microscopy (μ-SHG) and current-voltage monitoring during the poling process. Poled waveguides were then fabricated in poled regions and induced reflected SHG signals were measured, revealing an optimal gap of 3 μm between the edge of the poling electrodes and the waveguides for achieving maximum induced nonlinearity within the waveguide. Photomask layouts were designed to define the poling electrodes, straight waveguides, and Mach–Zehnder interferometer structures, including the modulating electrodes. The fabrication and characterization pipeline for these devices was established and experimentally validated. This includes the realization of patterned poling electrodes on BF33 glass to pole sodo-niobate films, waveguide structured through UV lithography and argon ion beam milling, and the subsequent integration of lift‑off‑defined gold electrodes over poled regions. Polarization‑controlled μ-SHG and Raman spectroscopy confirm that the induced nonlinearity is spatially localized, robust to etching and accurately aligned to the guiding cores, thereby demonstrating precise control of the nonlinear response at the scale required for integrated photonics.
The final part of the thesis explores supercontinuum generation in dispersion engineered chalcogenide buried rib waveguides based on As₂Se₃ and As₂S₃. The high refractive index and strong third‑order nonlinearity of these glasses are exploited to design waveguide geometries that position the zero‑dispersion wavelength near 4.4 μm for the fundamental TE mode. Experiments using 200 fs pump pulses with 720 mW average power at 3.8 μm demonstrate coherent supercontinuum spectra extending from approximately 2.5 μm to 6.5 μm, covering a portion of the mid‑infrared molecular fingerprint region that is highly relevant for spectroscopic sensing. The measured spectra are in good agreement with simulations based on the generalized nonlinear Schrödinger equation, which incorporate linear dispersion and third-order nonlinear effects. These results confirm that the engineered chalcogenide platform can deliver broadband, stable supercontinuum generation at moderate pump powers.
This thesis demonstrates versatile amorphous material platforms for nonlinear integrated applications, supported by numerical studies, fabrication processes, and nonlinear optical measurements. The approaches presented herein offer a pathway toward compact, fully integrated nonlinear optical spectroscopic systems.

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Identifiers

ORCID for Sirawit Boonsit: orcid.org/0000-0002-1453-2855

ORCID for Senthil Murugan Ganapathy: orcid.org/0000-0002-2733-3273

ORCID for Milos Nedeljkovic: orcid.org/0000-0002-9170-7911

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