Mark H. Bergen

Characterization of Image Receivers for Optical Wireless Location Technology

In this letter, image receivers are characterized to address the challenges of indoor positioning with optical wireless location systems. Image receivers are studied according to their field-of-view (FOV) characteristics: 1) the first image receiver uses a wide-FOV (95°) microlens and 2) the second image receiver uses an ultrawide-FOV (130°) microlens. An angle of arrival characterization of the image receiver is used to quantify azimuthal, φ, and polar, θ, angles of incident light from optical beacons. A dilution of position (DOP) characterization is used to quantify geometrical effects of the optical beacon distribution. It is found that the ultrawide-FOV microlens (with a mean positioning error of 1.5 cm) can better image widely separated optical beacons, and thus operate at a lower DOP, compared with the wide-FOV microlens (with a mean positioning error of 3.2 cm).

Optimization processes for pulsed terahertz systems

In this work, a pulsed terahertz (THz) system is designed and implemented. The work introduces a methodology for implementing such THz systems through three design processes: collineation, autocorrelation, and electro-optic. The collineation process establishes spatial alignment between the overlapped pump and probe beams, to ensure that there is similar spatial alignment between the subsequent THz and probe beams. The autocorrelation process characterizes the optical path difference between pulses in the THz and probe beams to define the precise temporal zero-time of the THz system. The electro-optic process optimizes the polarization-sensitive optics in the THz system to maximize the THz-induced modulation on the probe polarization. The processes are applied to design and implement a successful THz system.

Characteristics of Angular Precision and Dilution of Precision for Optical Wireless Positioning

The challenges of optical wireless positioning are addressed in this paper by a thorough investigation of angle of arrival (AOA) positioning characteristics. The overall positioning precision for AOA positioning is studied in terms of two contributing factors-being angular precision and geometric dilution of precision (DOP). Angular precision is characterized for an optical wireless receiver having an especially wide angular field-of-view (FOV). Geometric DOP is characterized for optical beacons deployed in the form of triangle, square, and hexagon cell geometries. The mean and standard deviation of the positioning errors are extracted from the positioning error distributions for each of the three cell geometries. It is found that the overarching goal to establish low and uniform positioning error distributions can be met by implementing an optical wireless receiver with a wide angular FOV and by implementing the optical beacon geometry with a correspondingly small height-to-side-length ratio. The prospects of these findings are discussed for future optical wireless positioning systems.

Retroreflective imaging system for optical labeling and detection of microorganisms

A retroreflective imaging system for imaging microscopic targets over macroscopic sampling areas is introduced. Detection of microorganism-bound retroreflector (RR) targets across millimeter-scale samples is implemented according to retroreflection directionality, collimation, and contrast design characteristics. Retroreflection directionality is considered for corner-cube (CC) and spherical geometries. Spherical-RRs improve directionality and reliability. Retroreflection collimation is considered for spherical-RRs. Retroreflective images for micro-CC-RRs and micro-spherical-RRs with varying refractive indices show optimal results for high refractive index BaTiO3 micro-spherical-RRs. A differential imaging technique improves retroreflection contrast by 35 dB. High refractive index micro-spherical-RRs and differential imaging, together, can detect microscopic RR targets across macroscopic areas.

Design and Implementation of an Optical Receiver for Angle-of-Arrival-Based Positioning

Optical wireless (OW) technology has attracted significant interest for indoor positioning in the past decade. An emerging form of this technology makes use of angle-of-arrival (AOA) measurements to carry out positioning via triangulation off of an optical beacon grid. Such AOA-based OW positioning systems can yield accurate position estimates—but only given sufficient attention to the optical receiver. The design, operation, and implementation of such a receiver are presented in this work. The optical receiver is designed to have a sufficiently small AOA error, being σAOA = 1°, over a wide angular field-of-view (FOV), being 100°. The design allows the optical receiver to carry out positioning based off a 3 × 3 grid of optical beacons, where each optical beacon is uniquely identified using multiple frequency and color channels. The optical beacons are widely spaced to fully utilize the optical receivers wide angular FOV. The overall AOA-based OW positioning system exhibits a position error of 1.7 cm, which is comparable to those obtained by more complex positioning systems. Thus, the presented AOA-based technologies can play a role in emerging indoor positioning systems.

Retroreflective imaging systems for enhanced optical biosensing

Biosensing is important for detection and characterization of microorganisms. When the detection and characterization of targeted microorganisms require micron-scale resolutions, optical biosensing techniques are especially beneficial. Optical biosensing can be applied through direct or indirect optical sensing techniques. The latter have demonstrated especially high sensitivities for the detection of targeted microorganisms with labeling. Unfortunately, such systems rely on high-resolution microscopy with microscopic sampling areas to image the labeled target microorganisms. This leads to long characterization times for applications such as pathogen detection in water quality monitoring where users must scan the micron-scale sampling areas across millimeter- or even centimeter-scale samples. This work introduces retroreflector labels for the detection and characterization of microorganisms for macroscopic sample sizes. The demonstrated retroreflective imaging system uses a laser source to illuminate the sample, in lieu of the fluorescent excitation source, and micron-scale retroreflector labels, in lieu of fluorescent stains/proteins. Antibodies are used to bind retroreflectors to targeted microorganisms. The presence of these microscopic retroreflector-microorganism pairs is monitored in a retroreflected image that is captured by a distant image sensor which shows a well-localized retroreflected beamspot for each pair. Characteristics of an appropriately-designed retroreflective imaging system which provide a quantifiable record of microorganism-coupled retroreflectors across macroscopic sample sizes are presented. Retroreflection directionality, collimation, and contrast are investigated for both corner-cube retroreflectors and spherical retroreflectors (of varying refractive indices). It is ultimately found that such a system is an effective tool for the detection and characterization of microorganism targets, down to a single-target detection limit.

A smart solar energy collecting device

Flat silicon solar cells are the standard for solar technology implementations, due to the simplicity of its form and the low cost of its material. However, the bare material of such a technology is known to suffer from high reflectivity and low solar conversion efficiency. Engineered surfaces (e.g., textured) and bulk structures (e.g., quantum dots) are often used to diminish the reflection and enhance the efficiency, but such processes come at the expense of complexity and cost. The proposed work responds to these challenges by introducing a new architecture for traditional silicon solar technology. The architecture takes the form of a Smart Solar Sensing (S-Cubed) Array. It consists of a macroscopic close-packed array of corner-cube- (CC-) shaped solar cells. Each CC-cell has three silicon solar cells lining its interior corners. The three silicon solar cells establish multiple internal reflections for enhanced overall absorption. At the same time, the impedances of the three silicon solar cells in each CC-cell of the S-Cubed Array can be sensed and independently matched to a common load. This allows for maximized electric power transfer to the load over a broad range of illumination conditions. It is shown in this work that the collected energy density of the CC-cell array, over the course of a day, can be increased by 33.02% when compared to an array of conventional (flat) silicon solar cells. Such findings can lay the groundwork for future implementations of high-efficiency solar technology.

Characterization and Integration of Terahertz Technology within Microfluidic Platforms

In this work, the prospects of integrating terahertz (THz) time-domain spectroscopy (TDS) within polymer-based microfluidic platforms are investigated. The work considers platforms based upon the polar polymers polyethylene terephthalate (PET), polycarbonate (PC), polymethyl-methacrylate (PMMA), polydimethylsiloxane (PDMS), and the nonpolar polymers fluorinated ethylene propylene (FEP), polystyrene (PS), high-density polyethylene (HDPE), and ultra-high-molecular-weight polyethylene (UHMWPE). The THz absorption coefficients for these polymers are measured. Two microfluidic platforms are then designed, fabricated, and tested, with one being based upon PET, as a representative high-loss polar polymer, and one being based upon UHMWPE, as a representative low-loss nonpolar polymer. It is shown that the UHMWPE microfluidic platform yields reliable measurements of THz absorption coefficients up to a frequency of 1.75 THz, in contrast to the PET microfluidic platform, which functions only up to 1.38 THz. The distinction seen here is attributed to the differing levels of THz absorption and the manifestation of differing f for the systems. Such findings can play an important role in the future integration of THz technology and polymer-based microfluidic systems.

Terahertz microjets and graphene: Technologies towards ultrafast all-optical modulation

Key technologies for ultrafast all-optical THz modulation are introduced. A graphene monolayer is applied for modulation, on a picosecond timescale, and a dielectric sphere is applied to form a high-intensity THz microjet within the graphene monolayer.

Textured semiconductors for enhanced photoconductive terahertz emission

There are severe limitations that photoconductive (PC) terahertz (THz) antennas experience due to Joule heating and ohmic losses, which cause premature device breakdown through thermal runaway. In response, this work introduces PC THz antennas utilizing textured InP semiconductors. These textured InP semiconductors exhibit high surface recombination properties and have shortened carrier lifetimes which limit residual photocurrents in the picoseconds following THz pulse emission—ultimately reducing Joule heating and ohmic losses. Fine- and coarse-textured InP semiconductors are studied and compared to a smooth-textured InP semiconductor, which provides a baseline. The surface area ratio (measuring roughness) of the smooth-, fine-, and coarse-textured InP semiconductors is resolved through a computational analysis of SEM images and found as 1.0 ± 0.1, 2.9 ± 0.4, and 4.3 ± 0.6, respectively. The carrier lifetimes of the smooth-, fine-, and coarse-textured InP semiconductors are found as respective values of 200 ± 6, 100 ± 10, and 20 ± 3 ps when measured with a pump-probe experimental system. The emitted THz electric fields and corresponding consumption of photocurrent are measured with a THz experimental setup. The temporal and spectral responses of PC THz antennas made with each of the textured InP semiconductors are found to be similar; however, the consumption of photocurrent (relating to Joule heating and ohmic losses) is greatly diminished for the semiconductors that are textured. The findings of this work can assist in engineering of small-scale PC THz antennas for high-power operation, where they are extremely vulnerable to premature device breakdown through thermal runaway.