Browsing by Subject "Photonics"

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Embedded dielectric microstructures in molecular beam epitaxy : high-quality planar coalescence toward enhanced optoelectronic materials
(2018-10-29) Ironside, Daniel Joseph; Bank, Seth Robert; Wasserman, Daniel; Li, Xiaoqin (Elaine); Yu, Edward T; Wang, Zheng
Seamless integration of embedded dielectric microstructures in III-V crystal growth is a continued area of research due to its numerous high-impact applications. Historically, investigations into embedded dielectric microstructures within existing crystal growth techniques were focused on blocking dislocations at the III-V/dielectric interface in the production of low defect relaxed high mismatched heteroepitaxy. However, recent efforts have broadened the use of embedded dielectric microstructures for enhancement of optoelectronic device functionality and development of monolithic growth schemes toward integrated photonic circuits. The central challenge of embedding dielectric microstructures in III-V materials is achieving single-crystal high-quality planar coalescence within existing conventional III-V crystal growth techniques without defect. While prevalent in the field of III-V crystal growth, solid-source Molecular Beam Epitaxy (MBE) has a well-known "coalescence problem," historically lacking approaches that achieve planar coalescence over dielectric microstructures. Limited coalescence is in large part due to low diffusion of III-adatoms on dielectric surfaces, typically below 300nm, readily forming polycrystalline deposition on dielectric surfaces exceeding this diffusion length. Several solid-source MBE highly-selective growth and lateral epitaxial overgrowth (LEO) growth approaches have been reported; however, none demonstrating complete planar coalescence over dielectric microstructures. In this dissertation, to overcome the "coalescence problem," we demonstrate for the first time a general methodology for an all-MBE growth of high-quality planar coalescence over a variety of embedded dielectric microstructures. Underpinning the approach, we developed a two-stage all-MBE growth approach for GaAs and InAs on (001) substrates, producing highly selective LEO and planarization, returning the growth front to the (001) surface. Characterization of the growth approach demonstrates for the first time an all-MBE approach to planar coalescence. In application of the two-stage all-MBE growth approach towards photonics, we demonstrate enhancement of quantum emitters using buried silica gratings arrays and develop several methodologies for embedded high-contrast photonic materials through self-formed air voids and molded air channel processes. Lastly, in application to high-quality relaxed high mismatch heteroepitaxy, we demonstrate for the first time an all-MBE approach to III-V metamorphic heteroepitaxy, demonstrating threading dislocation reduction in InAs/GaAs metamorphics with high fill factor embedded silica gratings. Thus, from the material presented here, we provide several significant advances to the long-standing challenge of marrying high-quality semiconductor crystal growth with dielectric microstructures, unlocking several high-impact applications, including high-quality material pathways for enhanced quantum emitters and embedded metasurfaces as well as an all-MBE approach toward heterogeneous III-V integration on silicon.
Optical resonators and quantum dots: and excursion into quantum optics, quantum information and photonics
(2007-08) Bianucci, Pablo, 1975-; Shih, Chih-Kang
Modern communications technology has encouraged an intimate connection between Semiconductor Physics and Optics, and this connection shows best in the combination of electron-confining structures with light-confining structures. Semiconductor quantum dots are systems engineered to trap electrons in a mesoscopic scale (the are composed of [approximately] 10000 atoms), resulting in a behavior resembling that of atoms, but much richer. Optical microrseonators are engineered to confine light, increasing its intensity and enabling a much stronger interaction with matter. Their combination opens a myriad of new directions, both in fundamental Physics and in possible applications. This dissertation explores both semiconductor quantum dots and microresonators, through experimental work done with semiconductor quantum dots and microsphere resonators spanning the fields of Quantum Optics, Quantum Information and Photonics; from quantum algorithms to polarization converters. Quantum Optics leads the way, allowing us to understand how to manipulate and measure quantum dots with light and to elucidate the interactions between them and microresonators. In the Quantum Information area, we present a detailed study of the feasibility of excitons in quantum dots to perform quantum computations, including an experimental demonstration of the single-qubit Deutsch-Jozsa algorithm performed in a single semiconductor quantum dot. Our studies in Photonics involve applications of microsphere resonators, which we have learned to fabricate and characterize. We present an elaborate description of the experimental techniques needed to study microspheres, including studies and proof of concept experiments on both ultra-sensitive microsphere sensors and whispering gallery mode polarization converters.
Photonic crystal waveguides based active and passive devices for phased array antenna systems
(2006) Jiang, Yongqiang; Chen, Ray T.
Photonic crystals are a new class of artificial optical materials with periodic dielectric structures, which promise to miniaturize photonic devices. Optical true-time delay techniques are an emerging technology for the squint-free beam steering of phased array antennas with wide bandwidth, reduced system weight and size, and low electromagnetic interference. In this dissertation, highly dispersive photonic crystal fibers based optical true time delay modules were designed, integrated and characterized. An ultra-compact optical modulator based on silicon photonic crystal waveguides for phased array antenna systems was designed, fabricated and characterized. A true-time delay controlled X-band phased array antenna system was designed, integrated, and characterized. The continuously tunable optical true time delay module employ highly dispersive photonic crystal fibers connected with various lengths of nondispersive telecom fiber. A highly dispersive silica photonic crystal fiber using dual-core structure was developed to achieve high chromatic dispersion. By employing photonic crystal fibers to increase the dispersion, the true time delay module size can be proportionally reduced. A 4-element linear X-band phased array antenna system using photonic crystal fibers based true-time delay modules was developed and demonstrated. The beam steering angle of the phased array antenna system was scanned by tuning the optical wavelength. Squint-free operation is experimentally confirmed. An optical modulator based on silicon photonic crystal waveguides was developed, which could be implemented in phased array antenna systems to replace conventional optical modulators. Silicon photonic crystal waveguides were firstly developed and demonstrated. Photonic crystal line-defect waveguides showed high group velocity dispersion and slow photon effect near the transmission band edge. An ultra-compact silicon electro-optic modulator based on silicon photonic crystal waveguides was proposed, developed and demonstrated for the first time. Modulation operation was demonstrated by carrier injection into an 80 µm-long silicon photonic crystal waveguide of a Mach-Zehnder interferometer structure.
Polymer-based integrated photonic devices for interconnects
(2018-06-19) Pan, Zeyu; Chen, Ray T.; Pan, Zhigang; Ho, Paul S.; Wang, Yaguo; Tao, Hu
Integrated photonic devices based on optical waveguides have been extensively studied for various applications, especially the high-speed intra- and inter-chip interconnects. Usually, a waveguide contains a core with high refractive index and cladding with lower refractive index. Among various waveguides, silicon, polymer, and silicon-polymer hybrid devices are the most promising candidates for low cost, small size, light weight, and low power consumption (CSWaP) optical interconnect. Firstly, silicon-based optical devices can be fabricated using CMOS compatible nanofabrication technology, which is already widely used to manufacture integrated circuits. Silicon photonic devices can have very small footprint and enable high density photonic circuits, due to high refractive index contrast. However, one of the intrinsic obstacles is the absence of χ⁽²⁾-nonlinearity in unstrained silicon due to its centrosymmetric crystal structure, making modulating photons on silicon platform a great challenge. Secondly, polymer-based devices have been found very attractive, owing to the advantages of high thermo-optic (TO) or electro-optic (EO) coefficient, high transparency in the telecommunication wavelength windows, and fabrication feasibility over large areas on printed circuit board (PCB) or other kinds of substrates. The roll-to-roll (R2R) compatible imprinting and ink-jet printing for developing polymer-based devices on flexible or rigid substrates enable large-area, light-weight, low-cost optical interconnects. However, due to the low refractive index contrast, the polymer photonic devices always require large footprint. Finally, the silicon-organic hybrid (SOH) platform enables the marriage of the best of these two materials and thus has been receiving substantial attention. In this dissertation, integrated photonic devices based on silicon, polymer, or hybrid platform will be presented. First, high-efficiency quasi-vertical tapers for polymer waveguide based inter-board optical interconnects will be demonstrated. A triangular-shape tapered structure is adopted above the waveguide core to transform a fiber mode into a single mode polymer rib waveguide mode as an optical mode transformer. A coupling loss of 1.79±0.30 dB and 2.23±0.31 dB per coupler for the quasi-TM and quasi-TE mode respectively have been experimentally demonstrated, across the C and L bands (1535 nm – 1610 nm). Then, a reconfigurable thermo-optic polymer switch based true-time-delay network will be analyzed and demonstrated. Thirdly, I will show a novel subwavelength-grating waveguide ring resonator based high-speed modulators, which is the largest bandwidth and the most compact footprint that has been demonstrated for the ring resonators on the silicon-organic hybrid (SOH) platform. Finally, the on-chip time-division multiplexing and de-multiplexing system will be designed and analyzed.
Single-stage large-angle beam steering optical phased array on silicon nanomembrane
(2010-05) Kwong, David Nien; Chen, Ray T.; Bank, Seth R.
In this paper, we present the results of the design and fabrication of a 12 channel nano-membrane-based optical phased array that allows for large angle beam steering operating at wavelength=1.55µm. Our device is fabricated on silicon-on-insulator using standard CMOS process. By implementing unequally spaced waveguide array elements, we can relax the half-wavelength spacing requirement for large angle beam steering, thereby avoiding the optical coupling between adjacent waveguides and reducing the side-lobe-level of the array radiation pattern. 1D beam steering of tranverse-electric polarized single mode light is designed to be achieved thermo-optically through the use of thin film metal phase shifters.
Sub-wavelength optical phenomena and their applications in nano-fabrication
(2006) Shao, Dongbing; Chen, Shaochen
This dissertation presents the numerical study of sub-wavelength optical phenomena and experimental demonstration of their applications in nanoscale patterning. The optical near-field enhancement associated with sub-wavelength scale nanostructures, such as nano-ridges and nano-tips, were utilized to produce nanoscale patterns. Numerical simulation using finite difference time domain (FDTD) method were employed as a modeling tool to predict and optimize a scheme to achieve parallel patterning by near-field enhanced direct nano-molding. In order to pattern the whole area of the substrate, the laser light was chosen to be transparent to the substrate and was shine from the back with an incident angle. The near-field enhancement facilitates the local ablation to form line or dot patterns on the thin film coated on the substrate. Fabrication process to manufacture the nanostructure used as the mold was also developed. Further investigation into the near-field enhancement phenomena leads to the study of underlying physics: surface plasmons (SPs) and SP-light coupling. Previous research efforts have shown that transmission through sub-wavelength apertures can be orders of magnitude higher than predicted by aperture theory due to SP-light coupling. In this dissertation study, the SPs excited on the nano apertures were combined with the SPs on the substrate to overcome the two fundamental restraints of light at sub-wavelength scale, transmission and diffraction. The discovery was exploited for nanolithography, coined Surface Plasmon Assisted Nanolithography (SPAN), and one-to-one nanoscale pattern transfer has been achieved using both laser light and UV lamp without losing convenience and simplicity of traditional photolithography technique. High contrast optical near-field interference generated by SP-light coupling was also studied and combined with the photolithography technique to produce threedimensional (3D) nanostructures. Different multi-layer 2D/3D periodic polymeric nanostructures have been directly fabricated using such a mechanism in a typical photolithography setup. The nanostructures fabricated can be easily controlled in terms of size, layout, and defects by designing the mask. This so-called Surface Plasmon Assisted 3D Nanolithography (3D-SPAN) was demonstrated in experiments to offer flexible and convenient 3D nanofabrication capabilities.
Ultrafast all-optical switching via grating-based fabry-perot resonators and surface normal fiber-to-waveguide couplers
(2014-12) Covey, John Luther; Chen, Ray T.
Electronic microchips have now firmly plateaued in switching speed. A promising solution for increasing performance on unparallelizable tasks is to switch digital data purely in the optical domain on a photonic chip, as all-optical switching can reach up to terahertz speeds and beyond. Because ultrafast optical effects are weak phenomena, materials with extremely high nonlinear sensitivity must be developed, and very high optical intensities and coupling efficiencies are required to adequately switch data streams of light. In this thesis, unique all-optical platforms, waveguides, fiber-waveguide grating couplers, and an ultrafast optical switch were experimentally demonstrated as proofs-of-concept for the validity of densely integrated all-optical switches. Two horizontal slot waveguiding structures were designed and fabricated from scratch: a multiple horizontal slot waveguide with polycrystalline silicon sandwiching third-order nonlinear slots and a nonlinear cover-cladding with slot-like behavior over a thin crystalline silicon waveguide. Perfectly vertical grating couplers were then designed from a novel genetic algorithm, fabricated, and experimentally tested for both platforms with two promising nonlinear materials: silicon nanocrystals or a supra-molecular assembly, DDMEBT. Vertical grating couplers in the multiple horizontal slot waveguide achieved a theoretical coupling efficiency of 63% and an experimental coupling efficiency of 60%, which is the highest coupling efficiency into nonlinear slot waveguides to date. Vertical grating couplers for the cover-slot waveguide experimentally demonstrated a coupling efficiency of 38% and an extrapolated 1 dB bandwidth of 66 nm, the largest grating-coupled 1 dB bandwidth obtained for slot waveguides to date. A grating coupler was then designed to be included as one of two grating reflectors in a nonlinear resonator switch. Coupled mode theory and vectorial eigenmode propagation simulations were used to optimally design the grating coupler/resonator device, resulting in a record low footprint of 710 μm² per combined switch and fiber coupler device. The third-order nonlinear molecular material, DDMEBT, was, for the first time, successfully spun onto pre-patterned silicon-on-insulator chips with repeatable, defect-free results. Extremely sensitive experimental autocorrelation of the resonator's impulse response yielded output pulse durations as low as 600 femtoseconds. At high power and low pulse repetition rates, the switch's resonances redshifted by 4 nm with 4 dB of switching contrast, revealing an ultrafast Kerr effect which matched previous works. The resonator switch is therefore capable of modulating a single optical carrier frequency at 1 THz and switching an optical data stream at 500 GHz. These are the fastest switching speeds demonstrated by an integrated all-optical switch and validate the proof-of-concept needed for a future of densely integrated all-optical processing.
Wavelength-selective micro- and nano-photonic devices for wavelength division multiplexing networks
(2005) Jiang, Wei; Chen, Ray T.
On the road toward the information age, the enormous bandwidth demanded by the explosive growth of Internet traffic has been supplied by fiber-optic communications technology; in particular, the widespread use of Wavelength-Division Multiplexing (WDM) techniques. My research focused on Optical Add-Drop Multiplexers (OADMs) and wavelength demultiplexers, two key devices for WDM technology. Various approaches, including nanophotonic approaches based on photonic crystals, were employed in the research. First, a variety of ball lens-based OADMs were designed and implemented. The overall performance of the ball lens-based OADMs was competitive compared to that of commercial GRIN lens-based OADMs, while the former devices were more cost-effective and simpler in packaging. Optical Add-Drop Multiplexers are one of the promising applications of photonic crystals. Prior theories have been limited to devices with frequencyindependent coupling and simple mirror symmetry. Through my research, a general model was developed to understand the optical add-drop process in realistic photonic crystal-based OADMs, where the interactions between the waveguides and cavities are frequency dependent and cavity modes depart from the accidental degeneracy of frequency. Furthermore, a class of devices with more freedom in choice of symmetry were proposed. An original idea was proposed to utilize the inevitable optical loss in a way that improves device performance. The last part of my research was aimed at developing a wavelength demultiplexer based on the superprism effect in photonic crystals. In the course of this research, a general and rigorous refraction theory was developed for a photonic crystal of any lattice type, any surface orientation, and any surface termination. The theory solved some long-standing problems in grating diffraction as well. Refraction at naturally emerging quasi-periodic surfaces was treated in a unified way with the refraction on periodic surfaces. A new concept, surface-dependent mode degeneracy, was introduced and was shown to be crucial to understanding photonic crystal refraction. Arbitrary incident beam profiles were investigated. The first-ever practical demultiplexer design that has less than 3dB loss over a 25-nm spectrum is given based on this theory. The theory is anticipated to be instrumental in understanding some interesting research topics such as photonic crystal slab-based superlenses.