Integrated Planar Optical Devices Based on Silicon Nitride Waveguides
Silicon nitride is a subject of growing interest with the potential of delivering planar integrated optical devices as a complementary part of silicon photonics. The material has a moderate refractive index, wide optical transparency window, lack of two-photon absorption, and low nonlinear susceptibility. Thanks to its CMOS fabrication compatibility, the freedom of integrating different materials into Si3N4 renders the platform omnipotent. This dissertation is dedicated to developing Si3N4 based optical devices integrated with Si nanowires, fishbone antennas, bowtie antennas and gold nanoparticles to achieve active and passive optical functionalities. To be specific, the thesis covers Si3N4 based optical leaky wave antennas that emit narrow beams towards desired directions in free space, bimetallic fishbone waveguide-based detectors that detect optical radiations plasmonically and thermo-mechanically, and trench waveguides that can be equipped with bowtie antennas for optical trapping or with gold nanoparticles for nonlinearity enhancement.
The purpose of the first part of the dissertation is to experimentally demonstrate emission from a leaky wave antenna and to investigate the possible modulation method in tuning the radiation beam. The optical leaky wave antenna is composed of a Si3N4 waveguide and periodic Si nanowires. The antenna has a single directive radiation peak at the angle of 85.1° in the measured range from 65° to 112° at the wavelength of 1550 nm. The side lobes are at least 7 dB lower than the main peak. The peak radiation angle moves to the broad side as the wavelength increases. The device can find promising applications in optical communications, especially for multi-wavelength space division multiplexing owing to its capability of beam scanning with frequency. The study on the optical leaky wave antenna proves the functionality of off-plane emission from a waveguide and explores the potential electronic modulation methods.
The second part of the dissertation presents a plasmo-thermomechanical radiation detector. The goal of the second work is to investigate if thermomechanical vibrations can be detected in an on-chip optical readout system. To study the problem, I designed a device that is composed of a Si3N4 waveguide and 13 fishbone nanowires suspended above the waveguide. Each wire is 12.54 µm long and consists of 16 nanoantennas with a period of 660 nm. Under the illumination of 660 nm light, the detector shows a responsivity of 3.954×10-3 µm2/µW. The noise equivalent power, dominated by the waveguide coupling instability, is 3.01 µW/√Hz. The 3dB bandwidth of the device is 9.6 Hz corresponding to a time constant of 16.6 ms. Besides the demonstrated radiation detector for visible wavelength, another device for mid-infrared wavelength has also been designed and optimized for fabrication. This study verifies the possibility of using on-chip waveguide-based readout system to detect the mechanical vibration that is induced by radiation.
The third part of the dissertation focuses on Si3N4 trench waveguides. The objective of this part is to thoroughly explore the optical properties of the trench waveguides and investigate their applications. The trench waveguide shape is determined by the silicon wet etching properties and thus can be controlled in either triangle or trapezoidal shape. Experimental results show that the propagation loss of the TM mode can be as low as 0.8 dB/cm. The nonlinear parameter of the waveguide is measured to be 0.3 W-1/m. Coating gold nanoparticles can enhance the waveguide nonlinearity. The trench waveguide is promising for liquid sensing thanks to its unique structure that can combine fluidic channel and waveguide together. Explorations on the trapezoidal trench waveguide and bowtie antennas also show that the platform is suitable for trapping nanoparticles. Switching between trapping and releasing the particles can be done by changing the mode polarization states from the TE mode to the TM mode. This work provides an in-depth study of the trench waveguides from optical properties, fabrication, to applications.
This study expands the knowledge and capabilities of conventional silicon photonics and paves way for novel devices pertinent to communications and sensing. In particular, this thesis shows that the use of Si3N4 based planar optical circuits, the operational bandwidth of silicon photonics can cover wavelengths shorter than 1.1μm for novel applications. The presented work on new optical planar emitters, waveguides, detectors, optical trapping, and microfluidics at wavelengths from visible to mid-wave infrared will be beneficial to both future research and industry applications.