The explosive growth of data centers driven by the enormous search engine libraries of companies like Google is creating a demand for faster and more compact optical components that are preferably directly integrated with CMOS silicon circuitry. Among the key optical functions that are needed and are not directly available in silicon, optical modulation is one of the most significant. An attractive solution is to directly integrate electro-optic (EO) polymers with silicon nanostructures, resulting in low voltage EO modulators with small footprints and low power consumption4.
However, to date it has proven difficult to efficiently pole EO polymers in silicon nanostructures, resulting in performance that is far from optimal. In the EO polymer poling process, a high voltage is applied to the EO polymer while it is held at a temperature near its glass transition. We have previously shown the importance of treating the cladding materials and EO polymers as a complete system with respect to the poling process, notably in the case of sol-gel cladding materials5-6. We propose to have an IOU project to develop screening methods for assisting in the development of optimal poling procedures for EO polymers in silicon nanostructures. Our primary approach will be for the IOU student to fabricate EO polymer/silicon "nanosandwiches," simple silicon/EO polymer/silicon samples that simulate the behavior of an EO polymer silicon nanoslot; parameters that will be varied include the conductivity of the silicon, the poling temperature and time, the poling voltage, and the EO polymer material composition. Fabrication of these samples will involve learning semiconductor manufacturing techniques, such as: spin-coating, vacuum deposition systems, and dicing. The IOU student will then work together with a graduate student mentor to learn the EO poling process.
Finally, the IOU student will measure the EO coefficient of the resulting samples using several different existing EO polymer characterization systems; note that in the course of this work the student will also use our well-equipped materials testing laboratory, which includes instruments for measuring film thickness (profilometer), refractive index (prism coupler and ellipsometer), and spectral characteristics (UV-VIS and FTIR). The IOU student will then have the task of interpreting the data and determining whether optimal poling conditions have been achieved by comparison with previous data on samples poled on other substrates.
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Challenges facing the access and core networks stem from the rapidly growing number of users and applications that demand dynamic reconfigurability in a highly aggregated network environment. At the Columbia Lightwave Systems and Networks Research Laboratory (LRL), our mission as part of the networking Thrust is to create a seamless high-bandwidth optical equivalent of the access network with cross-layer capabilities. The full endeavor includes developing a programmable and flexible platform for cross-layer information exchange and optimization based on an optical network test-bed. Our research drives the development of networking functionalities within CIAN, including such functions as programmable high-bandwidth multicasting. The network test-bed architecture has been uniquely adapted to support future cross-layer communication capabilities.
A key aspect of access networking is the seamless interface to wireless applications, and this forms a major area for undergraduate research opportunities. Future broadband access networks will leverage both fiber and wireless networks giving rise to fiber-wireless (FiWi) converged access networks. In this project, the student will design and implement a new interface that will efficiently connect wireless networks to high speed fiber networks operating at the edge of the Internet. This interface will later be used in order to study and demonstrate the various design tradeoffs related to transmitting data originating from wireless networks over fiber networks. The Lightwave Research Lab operates a state-of-the-art optical testbed providing data rates of up to 40Gb/sec.
We plan to connect this testbed to a number of wireless networks (e.g., WiMax, Cellular, and WiFi) by creating a number of dedicated interfaces. The projects will focus on designing both the software components as well as the implementation (in collaboration with senior graduate students) of the hardware design. These components will deal with functionalities such as link layer control, routing, and Quality of Service provisioning, and will enable a seamless integration of both network types. The project will introduce students to this critically important and emerging field including various concepts in optical and wireless networking.
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Within the framework of various research projects our group is working on photonic subsystems for synchronization, regeneration, and other types of signal conditioning. Several Optical Sciences and Engineering undergraduates, as well as IOU students, have already successfully participated in these activities. In the project proposed here IOU students will be assigned the task to characterize and utilize photonic components. Following an introductory literature research, the students will learn how to characterize photonic components experimentally and then integrate them into an existing, or to be built, experimental set-up.
The components to be characterized include optical fibers (various types), passive and active fiber-based devices (e.g. grating filters and optical amplifiers), and other photonic components (e.g. arrayed waveguide grating de-/multiplexers, and optical semiconductors). Quantities to be measured include attenuation, amplification, insertion loss, gain, power transfer, filter function, and chromatic dispersion. The students will learn how to: handle delicate optical components appropriately, operate basic as well as complex instruments efficiently, analyze collected data and compare those to given specifications, draw conclusions regarding the characterized component’s actual condition, and communicate and present their results and accomplishments professionally.
Depending on the student’s progress, and on the status of the research laboratories needs, students will be asked to select specific components to be integrated into running experiments, making their choice based on the measurement results they have obtained before and by comparing it to requirements dictated by the given set-up. In case requirements are not matched by measured component characteristics, students will get the chance to propose a modified/alternative component or change the experimental set-up in order to accommodate the actual component. The project is designed to allow inexperienced undergraduates to connect with the research environment into which they will be immersed; gain a sense of achievement; leave enough room for challenging extensions; and, last but not least, benefit the hosting group’s ongoing research.
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Demand for high-speed optical communication systems, which require multi-level modulation for high spectral efficiency, has increased the need for high bandwidth and resolution analog-to-digital converters (ADC). Additionally, these ADCs are required in advanced laboratory instruments, such as RF vector spectrum analyzers and high speed real-time digitizers. Photonic Time-Stretch Analog-to-Digital Converter (TSADC) is a promising candidate for such applications.
It can provide continuous digitization of ultrahigh bandwidth signals by exploiting multiple parallel wavelength channels. New operation mode of the TSADC is called Time Stretch Enhanced Recording (TiSER) which uses a single wavelength channel and reconstructs the ultrahigh bandwidth signal in equivalent time mode. Using the new sampling technique, Real-time Burst Sampling (RBS), offered by TiSER, fast non-repetitive dynamics and rare events can be captured that cannot be captured with sampling oscilloscopes because they lack real-time capability or with real-time digitizers because they lack sufficient bandwidth. Polarization multiplexing (PolMux) has been recently developed to efficiently use the optical bandwidth in current generation and next generation of optical communication. Since TSADC is analogous to a broadband optical link, PolMux can be proposed in TiSER to effectively increase the time aperture of the TiSER, resulting in a faster test-time and more samples in less time intervals. This idea can be explored by undergraduate students and requires them to become familiar with PolMux transmission and impairment compensations, which are practical challenges in optical communication systems.
Post signal-processing using MATLAB and/or C++ is performed to correct all impairments due to the optical system and reconstruct the high speed signal. In order to monitor the performance of an optical communication channel continuously, post-processing should be performed automatically using FPGAs or DSP units. The FPGA/DSP programming would be a suitable introduction to research for undergraduate students as they can become familiar with MATLAB and FPGA programming, which are widely used in research projects and industrial applications.
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The proposed project is on the development of efficient Computational Electromagnetic methods for the study of optical fields. Specifically, the IOU student will be involved in two activities: optimizing our Fortran codes and using these codes for the study of optical fields in photonic waveguiding systems. The development of codes will involve working on our code for computing periodic Green’s functions for free space and layered medium environments.
Some of these codes currently exist only for scalar potential fields and the IOU student will extend them to full dyadic forms, including the code optimization for cases in which many source-observer locations are needed. The investigation of optical phenomena will include the study of open optical waveguides comprised of arrays of metallic and dielectric particles, as well as, nanowires and dielectric waveguides. This project will serve two purposes. First, it is well suited as an introduction into the field of computational optics for an undergraduate student. The project evolves from already existing codes, which facilitates the learning process. In addition, our graduate students have experience in collaborating with undergraduates on various aspects of these codes. Second, the IOU students will contribute to CIAN’s project on Optical Simulation Programs for Integrated Circuits Emphasis (SPICE) with the goal of advancing Computational Electromagnetic methods for the design and analysis of complex photonic systems
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While integrated optics has demonstrated virtually all of the functions required in optical networks, integrated optical isolation has remained elusive, owing to the difficulty of forming waveguide structures in the conventional garnet crystals that are used in conventional bulk optical isolators. In order to realize a compact, high performance integrated optical isolator, materials are required with exceptionally large Verdet constants (~ 1 106 /T-m), good processability, and adequate optical transparency. Recently3 we have demonstrated the development of magnetite nanoparticle polymer composites with exceptional Verdet constants and transparency. Figure 3 is a transmission electron micrograph of a composite using 15nm magnetite nanoparticles; the upper right inset illustrates that polymer shells that have been formed around the magnetite nanoparticles in order to improve their dispersion in an acrylate polymer host. We have recently a developed a process whereby the polymer nanocomposite samples are raised to a temperature above the glass transition temperature of the polymer and subjected to a poling magnetic field. The poling process both increases Verdet constant and can effect the refractive index. For the IOU project, the student will learn how to make nanocomposite films and to photolithographically deposit thermo-optic electrodes onto the films. The primary goal of the project is to experiment with localized heating of polymer composites using Ohm heating of the overlying electrodes, thereby allowing regions of the nanocomposite to be selectively poled, which will result in refractive index changes that can be used for waveguide formation or post fabrication birefringence trimming. The IOU student will then have the task of interpreting the data and using these observations to assess the feasibility of the localized poling process. Throughout this work, the student will have a designated graduate student or post-doc mentor and will have access to state-of-the-art facilities.
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We are developing an on-chip wavelength division multiplexer (WDM) using coupled vertical gratings. Our proposed WDM device will build upon the 1X2 wavelength selective coupler (WSC) which was developed earlier. The approach is advantageous over state-of-the-art technologies, such as ring-resonator add/drop filters since they are not limited by the free spectral range within the telecommunications bandwidth. The investigation of each device involves finite difference time domain simulations, device fabrication and experimental characterization. A schematic of the proposed device is shown in Figure 4. By cascading several coupled waveguides, we can obtain a multi-port add/drop filter which essentially behaves as a WDM. We also wish to study the behavior of the device and other issues such as athermalization and tuning.
To achieve athermalization, we propose to clad the device using a polymer with a thermo-optic coefficient opposite to that of silicon to eliminate changes in operation wavelength through environmental fluctuations. We propose to have the IOU student investigate the feasibility of different polymers to athermalize the WDM device. The student will start off by measuring the refractive indices of the various polymers as a function of temperature. The polymer cladding will then be deposited on samples with the WDM device on it without the cladding. Testing of the device as a function of temperature fluctuations can be performed to verify the effectiveness of the polymers for athermalization. This project enables the student to get involved quickly and learn the theory and function of the device as they make progress experimenting with the materials.
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This project involves lots of hands-on work in an experimental physics laboratory, and will provide the undergraduate student with exposure both to ideas from fundamental physics as well as advanced semiconductor device technologies. The student will learn to characterize surfaces using atomic force microscopy (AFM); to operate spectroscopic instrumentation, including lasers, spectrometers, and various detector technologies; and to collect, analyze and interpret spectroscopic data from a variety of low-dimensional semiconductor systems. Using the data collected from these various techniques, the student will be challenged to help determine what improvements in the samples or devices need to be made in order to produce systems that will facilitate the fundamental experiments.
Most of these experiments are performed in cryogenic environments, and will thus also provide the student with valuable experience in the methods of low-temperature physics. More specifically, the student will be actively involved in our ongoing work in experimental cavity quantum electrodynamics of the solid state. This involves studying fundamental quantum mechanical interactions between single semiconductor quantum dots and the radiation field confined in a photonic crystal slab nanocavity. We will use various spectroscopic techniques ranging from traditional grating spectroscopy to advanced single-photon counting experiments for performing measurements of the statistical properties of the radiation field.
We will especially be focusing on resonant interactions of laser fields with the quantum dots in nanocavities, which present fascinating opportunities to study fundamental physical processes such as lasing with single quantum dots and lasing without inversion. These experiments are on the cutting edge of fundamental research in the field of semiconductor optics, and will provide the undergraduate student a great opportunity to participate in important experiments, learn some of the most advanced experimental techniques in the field of laser spectroscopy, and observe first hand what work is like in a real-life laboratory environment.
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Growth and Characterization of Carbon Nanotubes
The scope of this project is to grow and characterize carbon nanotubes (CNTs) on two types of substrates - quartz and silicon for electronic device applications. CNTs are allotropes of carbon that are a few nanometers in diameter and several micrometers in length. Numerous studies report various methods to grow CNTs including electric arc discharge, laser vaporization, and chemical vapor deposition (CVD). There are strong correlations between the synthesis, structure, and properties of the CNTs. Among these methods, CVD becomes commonly used due to its ability to produce high quantities of CNTs at relatively low temperatures. In CVD process, the CNTs are formed by the dissociation of hydrocarbon gas molecules into carbon atoms, dissolution and saturation of the carbon in the catalyst particles and further precipitation into tubular carbon solid structure.
In our study, key variables controlling the growth process include the flow rate of the hydrocarbon source, the catalysts and the growth time and temperature. The structures of CNTs are usually characterized using electron microscopy, which provides information of the distribution, morphology, crystal versus amorphous structure. On the other hand, optical and electronic properties of the nanotubes are often studied using Raman spectroscopy which can provide more details about types of nanotubes formed, stress/strain state, crystal symmetry, quality of crystal as well as metallic/semiconducting state of the nanotubes. These two analyses are usually done separately causing some difficulties to directly correlate the microstructure and optical properties of various forms of CNTs on the substrate. In the present study, we present for the first time the characterization of the nanotubes using a Raman spectrometer that is attached to a scanning electron microscope (SEM) for a simultaneous analysis to correlate the optical responses and the microstructure of the nanotubes. Moreover, internal structures of the tubes can also be obtained using transmission electron microscopy (TEM). The investigated nanotubes will be synthesized by systematically changing different growth parameters (flow rate, temperature, time) on two different types of substrates (silicon and quartz).
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