PhD Defence Lisi Xia | Broadband transverse multimode nonlinear optics in strongly coupled waveguides

Broadband transverse multimode nonlinear optics in strongly coupled waveguides

Lisi Xia is a PhD student in the department Laser Physics & Nonlinear Optics. (Co)Promotors are prof.dr. K.J. Boller en dr. P.J.M. van der Slot from the Faculty of Sciences & Technology (TNW), University of Twente.

The thesis explores broadband transverse multimode nonlinear optical phenomena within integrated, multimode silicon nitride waveguides. It exploits spatially transverse degrees of freedom present as distinct supermodes for broadband nonlinear generation. The nonlinear conversion process is fundamentally governed by the universal third-order nonlinearity, employing supercontinuum generation as the undelying nonlinear dynamics. Our study focuses on a simplest prototype structure supporting two fundamental spatial supermodes, which is a dual-core waveguide. The fundamental, supported modes are a spatially symmetric mode (SM) and an anti-symmetric mode (AM). With controlled excitation of these supermodes via separate single-mode input and output waveguides,  we theoretically and experimentally explore new opportunities for dispersion management and nonlinear interactions to generate coherent light with broad spectral bandwidth. The thesis shows that a dual-core waveguide provides an additional degree of freedom in dispersion engineering for the supermodes, which is advantageous for controlling cascaded nonlinear optical processes or controlling the spectral location of dispersive waves. The dispersion of supermodes can also be engineered to be significantly different from each other, such that rather different output spectra are generated in parallel or separately. Furthermore, by arranging for measurement of the light from each core individually as well as collectively, a physical mode decomposition is facilitated to aid the analysis of multimode nonlinear and cascaded processes.

In Chapter 3, we proposed a novel strategy towards on-chip control of supercontinuum generation (SCG) that provides supermode selection through solely phase tuning, taking advantage of the additional spatial degree of freedom in strongly coupled dual-core waveguides. The significance of this work lies in its ability to achieve dynamic dispersion control between its extremes, spanning from all-normal to anomalous regimes. Such flexibility should allow the same waveguide circuit to generate markedly different supercontinuum spectra.

In Chapter 4, we presented the first demonstration of broadband transverse multimode frequency conversion of light from the near-infrared to the visible light using a fully integrated photonic platform, similar to what was presented in Chapter 3. Pumping with ultrashort pulses at 1.5-µm wavelength (195-THz light frequency), we observed simultaneous dual-supermode, near-infrared supercontinuum generation with different spectral widths, alongside third-harmonic generation near 515 nm (582 THz). Cascaded four-wave mixing with supercontinuum components upconverts the third-harmonic radiation toward four blue wavelengths in a range between 485-450 nm (617 to 661 THz). The approach used chip-integrated spatial multiplexing and demultiplexing for excitation and analysis of broadband transverse nonlinear conversion. Using ultrashort pump pulses, we simultaneously excited two supermodes, initiating nonlinear light generation across multiple transverse modes, including those contributing to blue-shifted spectral components. For analysis, we employed physical mode decomposition, which enables identification of both parallel and cascaded nonlinear processes, providing better insight into the underlying multimode dynamics.

In Chapter 5, we demonstrated blue shifting of dispersive waves located at the high-frequency side of supercontinuum spectra. The approach is based on using strong coupling to an additional waveguide core, as is present in dual-core waveguides when the separation gap is narrow. This work shows that supermode dispersion engineering in multi-core waveguides not only enables visible-light components through cascaded frequency conversion as described in Chapter 4, but it also enables a blue-shifting of the shortest zero-dispersion wavelength, beyond what a single core with the same cross section dimensions can achieve. More specifically, when pumping with ultrashort infrared pump pulses at 1-µm wavelength (285-THz frequency), solitons were formed that generate dispersive waves. When using a dual-core waveguide, the short-wavelength-side dispersive wave was found located at 540 nm (green, 555 THz). This corresponds to a blue-shift by 80 nm (70 THz) compared to a reference experiment where the ultrashort pulses are injected into a single-core waveguide having the same core cross section. Moreover, the investigated dual-core waveguide facilitates non-phase-matched third-harmonic generation, emitting broadband radiation extending into the UV below 350 nm (above 855 THz), though typically 25 dB weaker than the dispersive wave. Such radiation was not observed in the respective single-core waveguide.

Summarizing, the results and findings presented in this thesis, achieved through the use of integrated photonic circuits, provide progress in the field of multimode nonlinear optics, advancing capabilities in control, deepening understanding, and improving analysis of complex multimode interactions. As a first careful step, all these investigations were carried out using prototype dual-core waveguides with identical core dimensions— because this is the simplest spatially multimode structure that already introduces significant complexity. Despite the advances presented in the thesis, it also shows that multimode nonlinear optics remains a vast and largely unexplored space, offering considerable opportunities for future research.