Dispersion Engineering In Micro-and Nano-Optical Devices

As the building blocks of micro-and nano-optical devices,integrated optical waveguides have high index contrasts and high mode confinement,which will induce strong waveguide dispersion.The chromatic dispersion will not only broaden the transmitting pulses,which leads to the signal distortion,but also act together with the nonlinearity to transform the spectra of pulses.Therefore,dispersion engineering is critical for many applications including nonlinear optics,high-speed optical communication,and optical imaging.This thesis further develops the dispersion engineering technology in micro-and nano-devices,then explores new physical mechanisms and phenomena based on dispersion engineering.The novelties and main works of this thesis are listed below.In dispersion engineering technology,firstly,the principles of dispersion engineering based on a bilayer waveguide and a single-material arrayed waveguide are demonstrated to obtain low and flat dispersion profiles with four zero-dispersion wavelengths(ZDWs).Secondly,the fabrication tolerance is improved and the steps of fabrication processes are greatly reduced for dispersion-tailored waveguides.The generation of broadband highly anomalous dispersion is also obtained in the single-material arrayed waveguide.The peak value of the generated dispersion is 9316 ps/nm/km with an anomalous bandwidth of about one octave.Thirdly,the performance limits of dispersion engineering in integrated waveguides are investigated.The obtained dispersion flatness can be as high as 8000 nm2·km/ps in a novel kind of hybrid waveguides,which is about 20 times larger than other works,and the generations of five and six ZDWs are also demonstrated.Lastly,the mode characteristics and dispersion are optimized simultaneously and a dispersion-flattened waveguide with the single-mode and single-polarization feature is proposed.Then,the dispersion engineering technology is applied to both nonlinear optics and optical imaging,facilitating the generations of on-chip octave-spanning frequency comb and achromatic metalens,respectively.For the generation of frequency comb,a highly nonlinear germanium-based microresonator is proposed.By dispersion flattening and dispersion hybridization,a two-octave mode-locked mid-IR frequency comb is generated over 2.3-10.2 μm wavelengths with the low pump power of 180 mW.For the generation of achromatic metalens,the dispersion-flattened micro-and nano-units are proposed to form an achromatic metalens with a diameter of 26 μm.By building a target function which consists of multiple goals for optimization,nearly diffraction-limited achromatic focusing over 1.1-2.2 μm wavelengths is obtained with an averaged efficiency of 42%.