Alumni (2009)

Abstract:
The purpose of this research is to characterize the quality of semiconductor nanostructure devices. The two structures that we characterize are quantum dots and nanocavities. Quantum dots are semiconductor structures that trap electrons and holes. One reason why quantum dots are interesting for fundamental physics research is that they have discrete energy levels. For this reason quantum dots are sometimes referred to as “artificial atoms.� Nanocavities are structures that confine light. Our nanocavities are slab photonic crystal devices that confine light in the plane of a semiconductor slab. The focus of our investigations is the interaction between the quantum dots and the nanocavities. The characterization parameters include the structural quality of the samples, the density, wavelength, and efficiency of the quantum dots, the linewidth of the nanocavity modes, the Purcell enhancement factor of the dots in the cavities, and whether a single quantum dot is strongly enhanced or coupled to a nanocavity mode. These characteristics are attractive to basic physicists because such devices would enable ultralow threshold lasers, single photon sources, and quantum information processing, which are potential building blocks for the next generation of telecom.

Abstract:
A fiber optic transmission line is a non-linear system due to the Index of Refraction vs. Power relationship. On the one hand, as light travels a fiber, the chromatic dispersion causes a broadening of the pulses due to the different velocities of the different spectral components. In high-speed transmission, this broadening can result in overlap of signal pulses and corruption of bit information. On the other hand, the differences in Index of Refraction along the temporal shape of the pulses has the potential to create pulse compression. The soliton effect occurs when the non-linearity of a transmission line exactly compensates for the chromatic dispersion of that same line creating an undistorted signal for high-speed transmission over very long distances. This work has been part of a project where the soliton effect will be used to synchronize high speed data streams in the time division multiplexing systems.

Absract: Magneto-optic (MO) materials employing novel conjugated polymers and commercial rare-earth garnets like bismuth iron garnet (BIG) with in-plane magnetization have opened the door to compact, low-power telecom modulators as well as high performance 6 magnetic field sensors and optical isolators. The Faraday effect is the rotation of the polarization vector of linearly polarized light when the light travels through a MO material having a length L with a uniform magnetic field B applied parallel to the direction of the light propagation. The motivation in this study was to improve performance of the existing MO setup used at the University. The current setup which uses a small solenoid has a narrow frequency response and spatial properties of the existing solenoid make alignment of the optics and placement of the MO material both very tedious. Our new approach is to create a wideband Helmholtz coil (HC)-based magnetic field delivery system along with driver electronics integrated into a differential polarimetric MO measurement setup. Due to the frequency limitations of existing lab gaussmeters, the spatial and frequency response of the magnetic field is to be characterized using an optical approach. The sensitivity limitations of the resulting MO sensor setup are to be determined using an experimental approach based upon previous work

Abstract:
This article presents the synthesis and characterization of carbon nanotubes (CNTs) on silicon substrates by chemical vapor deposition (CVD) at 900ºC using methane and hydrogen flow rates. The variation of H2 gas concentration and a set growth time of 15 minutes, have a significant effect on distribution, morphology, internal structure and electronic properties of the nanotubes. The transmission electron microscope (TEM) and state-of-the-art scanning electron microscope (SEM), equipped with Raman spectrometer allowed us to obtain critical information on the morphology and chemical and electronic structures of the CNTs. The results revealed substantial quantity trends as hydrogen flow rate increased from 100 to 700 standard cubic centimeter per minute (sccm). At a constant CH4 flow rate of 700 sccm and varied H2 of 100 and 200 sccm, we observed that few CNTs were produced. Between H2 flow rates of 300 and 400 sccm, the highest density of CNTs were grown; therefore, suggesting optimum growth conditions within that range. Increasing H2 to 700 sccm, the amount of CNTs decreased. The results from this study will guide a production process to obtain high quantity and quality CNTs with desired properties.

Abstract:
The goal of this research project is the design and implementation of a method for generation of multilevel optical signals operating at a symbol rate of 10Gs/s (gigasymbols per second). By using an FPGA (Field Programmable Gate Array) processor and time division multiplexers, different composite signals corresponding to different signal constellations can be generated. The electrical signals are then used to drive optical modulators which, when properly combined, generate multilevel optical signals for high spectral efficiency data transmission. The 7 main objective of this summer research was to test a method that could increase spectral efficiency. Maximizing the spectral efficiency involves the use of multilevel modulation formats for optical transmission where multiple orthogonal optical signals are modulated separately and mixed together in a proper fashion, then transmitted over the optical fiber channel. The graphical representation of the composite signal is a space where a constellation of possible values (points) exist for every combination of the number of bits carried by the optical pulse. Upon reception, the signal is decomposed and several detectors working in parallel determine the signal parameters of interest. This results in locating a point in the constellation space that corresponds to the transmitted pulse and subsequently, in detecting the sequence of bits carried by this pulse.

Abstract:
Development of a high powered, narrow linewidth, tunable VECSEL (Vertical External Cavity Surface Emitting Laser) holds promise for stimulating advances in a great many other fields of research. A specific application is the creation of a sodium guide star laser capable of creating an artificial star in the earth’s mesosphere which would aid adaptive optical telescopes in the creation of clearer pictures. This 589.159 nm yellow laser with over 10 Watts of output power and a linewidth of less than 1 GHz is the predominant goal of the research mentioned within this paper. A VECSEL was chosen for this task because of the potential for high power, high brightness, and access to the cavity allows for frequency doubling, and linewidth narrowing. Additionally, creation of a 589 nm output by a laser is traditionally very difficult with any other method due to the lack of gain materials that produce this wavelength. Thermally induced wavelength shift and wide linewidth are among the drawbacks of VECSELs. Thus, being able to tune and narrow the linewidth of the VECSEL output is an important feature. This narrowing and tuning is possible with the use of a birefringent filter (BF) within the VECSEL cavity. The specific goals of this research have been to determine a relationship between the thickness of the BF placed within a VECSEL cavity and the linewidth and output power that that VECSEL produces. The main goal is to reduce the VECSEL’s linewidth to 1GHz and increase the output power to the 10W range, so understanding how the BF’s thickness affects these characteristics will enable us to maximize our output power and minimize our linewidth

Abstract:
The "nano-texturing" of chloro-indium phthalocyanine (ClInPc) donor layers for use in "Type II Heterojunction" organic photovoltaics (OPV) has been investigated. A central problem in OPV optimization is that exciton diffusion lengths in donor and acceptor layers are on the order of 20nm, requiring the use of extremely thin, nearly transparent films of these materials. 8 Our goal is to create "textured" features, 20-40nm in size on the surface of a promising donor material, ClInPc using solvent vapor annealing. This improves OPV device efficiency by increasing the effective interaction volume for a ClInPc/C60 heterojunction, as well as allowing us to utilize thicker, more light absorbing donor layers, leading to higher short circuit currents, JSC, than from a planar heterojunction. The aim of this research has been to quantitatively compare the Phase I to Phase II surface morphology and absorption spectrum transformations of solvent vapor annealed thin films of ClInPc.
See Brittany talk about her research here

Abstract:
There are two digitizers/oscilloscopes that have been widely in use: equivalent-time digitizers, also known as sampling oscilloscopes, and real-time digitizers. Equivalent time oscilloscopes rely on continuous repetitive signals in order to be sampled. This is because equivalent time sampling collects samples at rates that are above the Nyquist sampling frequency (half of the sample rate); the oscilloscope constructs the signal by collecting more and more samples at each repetition and then “stitching� together the resulting repetitive waveform. Non-repetitve signals and rare events cannot be captured using the sampling oscilloscope. Also, it takes a long time to construct the repetitive signal. The electrical signal that is captured by real-time digitizers on the other hand have small bandwidths in comparison to equivalent-time digitizers, but differ in that they collect samples continuously. TiSER is a new sampling oscilloscope that combines the real time capabilities of the real time oscilloscope and the high bandwidth of the equivalent time oscilloscope to achieve the high sample rate needed to record transient events, which are impossible to record with the aforementioned methods.

Abstract:
Fabricating specific 3D nanostructures is a formidable challenge. Here we introduce a novel method for fabricating specific 3D nanophotonic structures. To do this, we take advantage of a previously practiced holographic lithography technique for fabricating relatively large arrays of periodic nanohole structures on polymer film. We demonstrate that these nanohole arrays can be integrated in a microfluidic platform, where we can effectively use the nanoholes as nanochannels—collectively, a nanochip. Nanoholes have previously been used as nanochannels for flow-through surface-plasmon resonance sensing. As we can control flow through these nanochannels, by creating pressure gradients, potential gradients, or concentration gradients, we are working to use them to align individual nanostructures to create functional 9 nanophotonic devices. In particular, we are working to produce specifically patterned nanosphere arrays that could be used as wave guides or for sensing purposes.

Abstract:
Several Gold nanohole arrays for a microfluidic sensor application were simulated, fabricated, and characterized to determine the effects of varying hole size, and periodic hole spacing on device performance. The samples were illuminated with 780nm laser light then rotated. At certain angles of incidence, a Surface Plasmon Polarition (SPP) is excited along the surface, which greatly enhances light transmission through the sub-wavelength sized holes. Changes in the refractive index surrounding the structure cause a measurable shift in the resonant incidence angle. The devices were fabricated by exposing SU-8 Negative Photoresist to the interference of two UV laser beams. A single exposure forms a grating. Rotating the sample 90 degrees then exposing again produces a grid of holes. Gold is sputtered on the developed SU-8 to a form a mushroom-like structure with holes at regular intervals. Hole size was calibrated by varying both exposure dosage, and gold deposition thickness. A hollow fluid chamber was cast is PDMS and bonded to the surface. Index of refraction was varied by injecting different fluids into the chamber. Device performance was then compared to simulated models.

Abstract:
This project deals with the design, fabrication and characterization of a laser at nanoscale, where laser’s physical dimensions are smaller than the vacuum wavelength of the light it emits. Characterization of optical gain of semiconductor materials is performed using variable-stripe method. In the experiment we use optical pump for the gain, and precise characterization of the pump beam profile is required. To achieve that we will use a razor blade mounted on programmed stages to incrementally cover portions of the beam, and use a power meter to measure the power in unblocked part of beam. This will be done in both horizontal and vertical direction. Using simple calculus one can calculate the beam profile from collected data. The experiment will be fully automated and controlled using MATLAB software

Abstract:
A major goal in the study of optoelectronics is the integration of high-quality III-V compounds onto inexpensive Silicon (Si) platforms supported by modern CMOS processes. A major advantage of the nanoneedle is its ability to grow on Si and serve as an optically active material. GaAs nanoneedles possess a single crystalline wurtzite structure and form without the assistance of catalysts. The sharp tips of nanoneedles may possess the advantage of enhanced electric fields at the tip. This strong electric field can then be used for field-enhanced applications such as surface enhanced Raman spectroscopy (SERS) and Terahertz generation. The focus of this particular research project is to characterize the optical properties of nanoneedles as a necessary step towards the ultimate goal of realizing nanoneedle nanolasers. The project consists of three primary objectives:
1. Optical characterization of an Aluminum Gallium Arsenide (AlGaAs) cladding on the surface of the nanoneedle to suppress non-radiative combination of electron hole pairs.
2. Characterization of nanoneedles grown on quartz and other new nanoneedle templates.
3. Characterization of effects of dopants added to nanoneedles to change the electrical properties. Tellurium (Te) will be investigated as the dopant because it is a good dopant at room temperature.