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Peer Reviewed

Editorial for the Special Issue on Silicon Photonics Bloom

UC Irvine Previously Published Works (2020)

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Cover page: Editorial for the Special Issue on Silicon Photonics Bloom

Thesis
Peer Reviewed

Integrated Planar Optical Devices Based on Silicon Nitride Waveguides

Zhao, Qiancheng
Advisor(s): Boyraz, Ozdal

UC Irvine Electronic Theses and Dissertations (2017)

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.

Cover page: Integrated Planar Optical Devices Based on Silicon Nitride Waveguides

Creative Commons 'BY' version 4.0 license

Article
Peer Reviewed

Electronic control of optical tweezers using space-time-wavelength mapping

UC Irvine Previously Published Works (2016)

We present a new approach for electronic control of optical tweezers by using space-Time-wavelength mapping (STWM), a technique that uses time-domain modulation to control local intensity values and hence the resulting optical force in space. The proposed technique enables direct control of magnitude, location, and polarity of force hot spots created by Lorentz force (gradient force). In this paper we develop an analytical formulation of the proposed STWM technique for optical tweezing. In the case study presented here we show that 150 fs optical pulses are dispersed in time and space to achieve a focused elliptical beam that is ∼20 μm long and ∼2 μm wide. By choosing the appropriate RF waveform and electro-optic modulator we can generate multiple hot spots with >200 pN force per pulse.

Cover page: Electronic control of optical tweezers using space-time-wavelength mapping

Article
Peer Reviewed

Silicon Nitride Waveguides for Plasmon Optical Trapping and Sensing Applications

UC Irvine Previously Published Works (2015)

We demonstrate a silicon nitride trench waveguide deposited with bowtie antennas for plasmonic enhanced optical trapping. The sub-micron silicon nitride trench waveguides were fabricated with conventional optical lithography in a low cost manner. The waveguides embrace not only low propagation loss and high nonlinearity, but also the inborn merits of combining micro-fluidic channel and waveguide together. Analyte contained in the trapezoidal trench channel can interact with the evanescent field from the waveguide beneath. The evanescent field can be further enhanced by plasmonic nanostructures. With the help of gold nano bowtie antennas, the studied waveguide shows outstanding trapping capability on 10 nm polystyrene nanoparticles. We show that the bowtie antennas can lead to 60-fold enhancement of electric field in the antenna gap. The optical trapping force on a nanoparticle is boosted by three orders of magnitude. A strong tendency shows the nanoparticle is likely to move to the high field strength region, exhibiting the trapping capability of the antenna. Gradient force in vertical direction is calculation by using a point-like dipole assumption, and the analytical solution matches the full-wave simulation well. The investigation indicates that nanostructure patterned silicon nitride trench waveguide is suitable for optical trapping and nanoparticle sensing applications.

Cover page: Silicon Nitride Waveguides for Plasmon Optical Trapping and Sensing Applications

Article
Peer Reviewed

Field enhancement and optical trapping with plasmonic nano antennas on silicon-based waveguides

UC Irvine Previously Published Works (2015)

Plasmonic nano antennas like dimers, bowties, and others have been investigated for their capability to provide a strong near-field enhancement when illuminated by external light. Traditionally these nano antennas, isolated or arrayed, are placed on a substrate and used in spectroscopy techniques. Surfaces made of such plasmonic nano antennas have been very useful, for example for surface enhanced Raman scattering, providing various orders of magnitude of enhanced sensitivity. These instruments however are not economic and are often not mobile since surfaces require an external beam illumination and the Raman scattering is detected by a large aperture microscope. We propose a technique where optical plasmonic nano antennas are located in proximity of silicon based waveguides that provide both for illumination and detection channels. The evanescent field surrounding the waveguide is capable to excite the proximity resonant nano antenna and generate the very strong field enhancement in the antenna gaps. Intensity enhancement reaches three or four orders of magnitude. Such nano antennas are used also for detection schemes and they provide very high sensitivity to molecular scattering, for example, when molecules are placed in the antenna 'hot spot'. A couple of different silicon based waveguide will be investigated with coupling to various possible plasmonic nano antennas. First we investigate the field enhancement and then the coupling to waveguide modes.

Cover page: Field enhancement and optical trapping with plasmonic nano antennas on silicon-based waveguides

Article
Peer Reviewed

Plasmon optical trapping using silicon nitride trench waveguides

UC Irvine Previously Published Works (2016)

We theoretically demonstrate optical trapping using a silicon nitride (Si3 N4 ) trench waveguide on which bow-tie plasmonic nanoantennas are employed for enhancing optical forces. The electric field tailing away from the waveguide is transformed and then enhanced by the plasmonic nanoantennas deposited on the waveguide surface. We show that, with gold bow-tie nanoantennas, the waveguide system exhibits outstanding trapping capability on a 10 nm radius polystyrene nanoparticle, due to a 60-fold electric field enhancement in the proximity of the nanoantenna gap. This enhancement causes a boost of the optical trapping force by 3 orders of magnitude. The gradient force in the vertical direction is also calculated semi-analytically by using a dipole approximation of a scattering polystyrene nanosphere, and the analytical solution well matches the full-wave simulations. Mode polarization effects are discussed in this paper as a way to switch trapping. These investigations indicate that the patterned Si3 N4 trench waveguide is suitable for optical trapping and nanoparticle sensing applications.

Cover page: Plasmon optical trapping using silicon nitride trench waveguides

Article
Peer Reviewed

Fully symmetric controllable integrated three-resonator photonic molecule

UC Santa Barbara Previously Published Works (2021)

Photonic molecules can be used to realize complex optical energy states and modes, analogous to those found in molecules, with properties useful for applications like spectral engineering and quantum optics. It is desirable to implement photonic molecules using high quality factor photonic integrated ring resonators due to their narrow atom-like spectral resonance, tunability, and the ability to scale the number of resonators on a photonic circuit. However, to take full advantage of molecule spectral complexity and tuning degree of freedom, resonator structures should have full symmetry in terms of inter-resonator coupling and resonator-waveguide coupling as well as independent resonance tuning, and low power dissipation operation, in a scalable integration platform. To date, photonic molecule symmetry has been limited to dual- and triple-cavity geometries coupled to single- or dual-busses, and resonance tuning limited to dual resonator molecules. In this paper, we demonstrate a three-resonator photonic molecule, consisting of symmetrically coupled 8.11 million intrinsic Q silicon nitride rings, where each ring is coupled to the other two rings. The resonance of each ring, and that of the collective molecule, is controlled using low power dissipation, monolithically integrated thin-film lead zirconate titanate (PZT) actuators that are integrated with the ultra-low loss silicon nitride resonators. This performance is achieved without undercut waveguides, yielding the highest Q to date for a PZT controlled resonator. This advance leads to full control of complex photonic molecule resonance spectra and splitting in a wafer-scale integration platform. The resulting six tunable supermodes can be fully controlled, including degeneracy, location and splitting as well as designed by a model that can accurately predict the energy modes and transmission spectrum and tunable resonance splitting.

Cover page: Fully symmetric controllable integrated three-resonator photonic molecule

Article
Peer Reviewed

Experimental demonstration of directive Si3N4 optical leaky wave antennas with semiconductor perturbations at near infrared frequencies

UC Irvine Previously Published Works (2015)

Directive optical leaky wave antennas (OLWAs) with tunable radiation pattern are promising integrated optical modulation and scanning devices. OLWAs fabricated using CMOS-compatible semiconductor planar waveguide technology have the potential of providing high directivity with electrical tunability for modulation and switching capabilities. We experimentally demonstrate directive radiation from a silicon nitride (Si3N4) waveguide-based OLWA. The OLWA design comprises 50 crystalline Si perturbations buried inside the waveguide, with a period of 1 μm, each with a length of 260 nm and a height of 150 nm, leading to a directive radiation pattern at telecom wavelengths. The measured far-field radiation pattern at the wavelength of 1540 nm is very directive, with the maximum intensity at the angle of 84.4° relative to the waveguide axis and a half-power beam width around 6.2°, which is consistent with our theoretical predictions. The use of semiconductor perturbations facilitates electronic radiation control thanks to the refractive index variation induced by a carrier density change in the perturbations. To assess the electrical modulation capability, we study carrier injection and depletion in Si perturbations, and investigate the Franz-Keldysh effect in germanium as an alternative way. We theoretically show that the silicon wire modulator has a -3 dB modulation bandwidth of 75 GHz with refractive index change of 3×10-4 in depletion mode, and 350 MHz bandwidth with refractive index change of 1.5×10-2 in injection mode. The Franz-Keldysh effect has the potential to generate very fast modulation in radiation control at telecom wavelengths.

Cover page: Experimental demonstration of directive Si3N4 optical leaky wave antennas with semiconductor perturbations at near infrared frequencies

Article
Peer Reviewed

Highly nonlinear sub-micron silicon nitride trench waveguide coated with gold nanoparticles

UC Irvine Previously Published Works (2015)

We demonstrate the fabrication of a highly nonlinear sub-micron silicon nitride trench waveguide coated with gold nanoparticles for plasmonic enhancement. The average enhancement effect is evaluated by measuring the spectral broadening effect caused by self-phase-modulation. The nonlinear refractive index n2 was measured to be 7.0917×10-19 m2/W for a waveguide whose Wopen is 5 μm. Several waveguides at different locations on one wafer were measured in order to take the randomness of the nanoparticle distribution into consideration. The largest enhancement is measured to be as high as 10 times. Fabrication of this waveguide started with a MEMS grade photomask. By using conventional optical lithography, the wide linewidth was transferred to a 100 wafer. Then the wafer was etched anisotropically by potassium hydroxide (KOH) to engrave trapezoidal trenches with an angle of 54.7°. Side wall roughness was mitigated by KOH etching and thermal oxidation that was used to generate a buffer layer for silicon nitride waveguide. The guiding material silicon nitride was then deposited by low pressure chemical vapor deposition. The waveguide was then patterned with a chemical template, with 20 nm gold particles being chemically attached to the functionalized poly (methyl methacrylate) domains. Since the particles attached only to the PMMA domains, they were confined to localized regions, therefore forcing the nanoparticles into clusters of various numbers and geometries. Experiments reveal that the waveguide has negligible nonlinear absorption loss, and its nonlinear refractive index can be greatly enhanced by gold nano clusters. The silicon nitride trench waveguide has large nonlinear refractive index, rendering itself promising for nonlinear applications.

Cover page: Highly nonlinear sub-micron silicon nitride trench waveguide coated with gold nanoparticles

Article
Peer Reviewed

422 Million intrinsic quality factor planar integrated all-waveguide resonator with sub-MHz linewidth

UC Santa Barbara Previously Published Works (2021)

High quality-factor (Q) optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration in a photonic waveguide platform is key to reducing cost, size, power and sensitivity to environmental disturbances. However, to date, the Q of all-waveguide resonators has been relegated to below 260 Million. Here, we report a Si3N4 resonator with 422 Million intrinsic and 3.4 Billion absorption-limited Qs. The resonator has 453 kHz intrinsic, 906 kHz loaded, and 57 kHz absorption-limited linewidths and the corresponding 0.060 dB m−1 loss is the lowest reported to date for waveguides with deposited oxide upper cladding. These results are achieved through a careful reduction of scattering and absorption losses that we simulate, quantify and correlate to measurements. This advancement in waveguide resonator technology paves the way to all-waveguide Billion Q cavities for applications including nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications.

Cover page: 422 Million intrinsic quality factor planar integrated all-waveguide resonator with sub-MHz linewidth