Electrical & Computer Engineering
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Item Microwave Nonlinearities in Photodiodes(1994) Williams, Keith Jake; Dagenais, Mario; Electrical & Computer Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)The nonlinearities in p-i-n photodiodes have been measured and numerically modeled. Harmonic distortion, response reduction, and sinusoidal output distortion measurements were made with two singlefrequency offset-phased-locked Nd: YAG lasers, which provided a source dynamic range greater than 130 dB, a 1 MHz to 50 GHz frequency range, and optical powers up to 10 mW. A semi-classical approach was used to solve the carrier transport in a one-dimensional p-i-n photodiode structure. This required the simultaneous solution of three coupled nonlinear differential equations: Poisson's equation and the hole and electron continuity equations. Space-charge electric fields, loading in the external circuit, and absorption in undepleted regions next to the intrinsic region all contributed to the nonlinear behavior described by these equations. Numerical simulations were performed to investigate and isolate the various nonlinear mechanisms. It was found that for intrinsic region electric fields below 50 kV/cm, the nonlinearities were influenced primarily by the space-charge electric-field-induced change in hole and electron velocities. Between 50 and 100kV/cm, the nonlinearities were found to be influenced primarily by changes in electron velocity for frequencies above 5 GHz and by p-region absorption below 1 GHz. Above 100 kV/cm, only p-region absorption could explain the observed nonlinear behavior, where only 8 to 14 nm of undepleted absorbing material next to the intrinsic region was necessary to model the observed second harmonic distortions of -60 dBc at 1 mA. Simulations were performed at high power densities to explain the observed response reductions and time distortions. A radially inward component of electron velocity was discovered, and under certain conditions, was estimated to have the same magnitude as the axial velocity. The model was extended to predict that maximum photodiode currents of 50 mA should be possible before a sharp increase in nonlinear output occurs. For capacitively-limited devices, the space-charge-induced nonlinearities were found to be independent of the intrinsic region length, while external circuit loading was determined to cause higher nonlinearities in shorter devices. Simulations indicate that second harmonic improvements of 40 to 60 dB may be possible if the photodiode can be fabricated without undepleted absorbing regions next to the intrinsic region.Item Metamaterial Model of Tachyonic Dark Energy(MDPI, 2014-02-17) Smolyaninov, Igor I.Dark energy with negative pressure and positive energy density is believed to be responsible for the accelerated expansion of the universe. Quite a few theoretical models of dark energy are based on tachyonic fields interacting with itself and normal (bradyonic) matter. Here, we propose an experimental model of tachyonic dark energy based on hyperbolic metamaterials. Wave equation describing propagation of extraordinary light inside hyperbolic metamaterials exhibits 2 + 1 dimensional Lorentz symmetry. The role of time in the corresponding effective 3D Minkowski spacetime is played by the spatial coordinate aligned with the optical axis of the metamaterial. Nonlinear optical Kerr effect bends this spacetime resulting in effective gravitational force between extraordinary photons. We demonstrate that this model has a self-interacting tachyonic sector having negative effective pressure and positive effective energy density. Moreover, a composite multilayer SiC-Si hyperbolic metamaterial exhibits closely separated tachyonic and bradyonic sectors in the long wavelength infrared range. This system may be used as a laboratory model of inflation and late time acceleration of the universe.Item Fractional Effective Charges and Misner-Wheeler Charge without Charge Effect in Metamaterials(MDPI, 2016-07-08) Smolyaninov, IgorTransformation optics enables engineering of the effective topology and dimensionality of the optical space in metamaterials. Nonlinear optics of such metamaterials may mimic Kaluza-Klein theories having one or more kinds of effective charges. As a result, novel photon blockade devices may be realized. Here we demonstrate that an electromagnetic wormhole may be designed, which connects two points of such an optical space and changes its effective topological connectivity. Electromagnetic field configurations, which exhibit fractional effective charges, appear as a result of such topology change. Moreover, such effects as Misner-Wheeler “charge without charge” may be replicated.Item Thermally Induced Effective Spacetimes in Self-Assembled Hyperbolic Metamaterials(MDPI, 2017-03-08) Smolyaninov, Igor I.Recent developments in gravitation theory indicate that the classic general relativity is an effective macroscopic theory which will be eventually replaced with a more fundamental theory based on thermodynamics of yet unknown microscopic degrees of freedom. Here we consider thermodynamics of an effective spacetime which may be formed under the influence of an external magnetic field in a cobalt ferrofluid. It appears that the extraordinary photons propagating inside the ferrofluid perceive thermal gradients in the ferrofluid as an effective gravitational field, which obeys the Newton law. Moreover, the effective de Sitter spacetime behaviour near the metric signature transition may mimic various cosmological inflation scenarios, which may be visualized directly using an optical microscope. Thus, some features of the hypothetic microscopic theory of gravity are illustrated in the ferrofluid-based analogue models of inflation.Item Extra-Dimensional “Metamaterials”: A Model of Inflation Due to a Metric Signature Transition(MDPI, 2017-09-20) Smolyaninov, Igor I.Lattices of topological defects, such as Abrikosov lattices and domain wall lattices, often arise as metastable ground states in higher-dimensional field theoretical models. We demonstrate that such lattice states may be described as extra-dimensional “metamaterials” via higher-dimensional effective medium theory. A 4 + 1 dimensional extension of Maxwell electrodynamics with a compactified time-like dimension is considered as an example. It is demonstrated that from the point of view of macroscopic electrodynamics an Abrikosov lattice state in such a 4 + 1 dimensional spacetime may be described as a uniaxial hyperbolic medium. Extraordinary photons perceive this medium as a 3 + 1 dimensional Minkowski spacetime in which one of the original spatial dimensions plays the role of a new time-like coordinate. Since the metric signature of this effective spacetime depends on the Abrikosov lattice periodicity, the described model may be useful in studying metric signature transitions.Item Secure Degrees of Freedom in Networks with User Misbehavior(MDPI, 2019-09-26) Banawan, Karim; Ulukus, SennurWe investigate the secure degrees of freedom (s.d.o.f.) of three new channel models: broadcast channel with combating helpers, interference channel with selfish users, and multiple access wiretap channel with deviating users. The goal of introducing these channel models is to investigate various malicious interactions that arise in networks, including active adversaries. That is in contrast with the common assumption in the literature that the users follow a certain protocol altruistically and transmit both message-carrying and cooperative jamming signals in an optimum manner. In the first model, over a classical broadcast channel with confidential messages (BCCM), there are two helpers, each associated with one of the receivers. In the second model, over a classical interference channel with confidential messages (ICCM), there is a helper and users are selfish. By casting each problem as an extensive-form game and applying recursive real interference alignment, we show that, for the first model, the combating intentions of the helpers are neutralized and the full s.d.o.f. is retained; for the second model, selfishness precludes secure communication and no s.d.o.f. is achieved. In the third model, we consider the multiple access wiretap channel (MAC-WTC), where multiple legitimate users wish to have secure communication with a legitimate receiver in the presence of an eavesdropper. We consider the case when a subset of users deviate from the optimum protocol that attains the exact s.d.o.f. of this channel. We consider two kinds of deviation: when some of the users stop transmitting cooperative jamming signals, and when a user starts sending intentional jamming signals. For the first scenario, we investigate possible responses of the remaining users to counteract such deviation. For the second scenario, we use an extensive-form game formulation for the interactions of the deviating and well-behaving users. We prove that a deviating user can drive the s.d.o.f. to zero; however, the remaining users can exploit its intentional jamming signals as cooperative jamming signals against the eavesdropper and achieve an optimum s.d.o.f.Item The Capacity of Private Information Retrieval from Decentralized Uncoded Caching Databases(MDPI, 2019-11-28) Wei, Yi-Peng; Arasli, Batuhan; Banawan, Karim; Ulukus, SennurWe consider the private information retrieval (PIR) problem from decentralized uncoded caching databases. There are two phases in our problem setting, a caching phase, and a retrieval phase. In the caching phase, a data center containing all the K files, where each file is of size L bits, and several databases with storage size constraint 𝜇𝐾𝐿 bits exist in the system. Each database independently chooses 𝜇𝐾𝐿 bits out of the total 𝐾𝐿 bits from the data center to cache through the same probability distribution in a decentralized manner. In the retrieval phase, a user (retriever) accesses N databases in addition to the data center, and wishes to retrieve a desired file privately. We characterize the optimal normalized download cost to be 𝐷∗=∑𝑁+1𝑛=1(𝑁𝑛−1)𝜇𝑛−1(1−𝜇)𝑁+1−𝑛(1+1𝑛+⋯+1𝑛𝐾−1). We show that uniform and random caching scheme which is originally proposed for decentralized coded caching by Maddah-Ali and Niesen, along with Sun and Jafar retrieval scheme which is originally proposed for PIR from replicated databases surprisingly results in the lowest normalized download cost. This is the decentralized counterpart of the recent result of Attia, Kumar, and Tandon for the centralized case. The converse proof contains several ingredients such as interference lower bound, induction lemma, replacing queries and answering string random variables with the content of distributed databases, the nature of decentralized uncoded caching databases, and bit marginalization of joint caching distributions.Item Data Acquisition Interface of a VLSI Cochlea Model(1993) Edwards, Thomas G.; Shamma, Shihab; Electrical Engineering; Digital Repository at the University; University of Maryland (College Park, Md)Computer models of cochlear processing take exceedingly long times to run, even for short data sets. A data acquisition system was developed for a new switched-capacitor VLSI cochlea model chip, in order to rapidly perform cochleaI processing on digitzed speech samples. The system is capable of processing very long speech samples. Processing is in near-real-time, though it, takes about 2 minutes per second of speech to write the large amount of data to a hard drive. Software has also been developed to convert the output data into a form readable by the ESPS digital signal processing package from Entropic Speech, Inc.Item The crosstalk between EGF, IGF, and Insulin cell signaling pathways - computational and experimental analysis(Springer Nature, 2009-09-04) Zielinski, Rafal; Przytycki, Pawel F; Zheng, Jie; Zhang, David; Przytycka, Teresa M; Capala, JacekCellular response to external stimuli requires propagation of corresponding signals through molecular signaling pathways. However, signaling pathways are not isolated information highways, but rather interact in a number of ways forming sophisticated signaling networks. Since defects in signaling pathways are associated with many serious diseases, understanding of the crosstalk between them is fundamental for designing molecularly targeted therapy. Unfortunately, we still lack technology that would allow high throughput detailed measurement of activity of individual signaling molecules and their interactions. This necessitates developing methods to prioritize selection of the molecules such that measuring their activity would be most informative for understanding the crosstalk. Furthermore, absence of the reaction coefficients necessary for detailed modeling of signal propagation raises the question whether simple parameter-free models could provide useful information about such pathways. We study the combined signaling network of three major pro-survival signaling pathways: E pidermal G rowth F actor R eceptor (EGFR), I nsulin-like G rowth F actor-1 R eceptor (IGF-1R), and I nsulin R eceptor (IR). Our study involves static analysis and dynamic modeling of this network, as well as an experimental verification of the model by measuring the response of selected signaling molecules to differential stimulation of EGF, IGF and insulin receptors. We introduced two novel measures of the importance of a node in the context of such crosstalk. Based on these measures several molecules, namely Erk1/2, Akt1, Jnk, p70S6K, were selected for monitoring in the network simulation and for experimental studies. Our simulation method relies on the Boolean network model combined with stochastic propagation of the signal. Most (although not all) trends suggested by the simulations have been confirmed by experiments. The simple model implemented in this paper provides a valuable first step in modeling signaling networks. However, to obtain a fully predictive model, a more detailed knowledge regarding parameters of individual interactions might be necessary.Item Networks for Fast and Efficient Unicast and Multicast Communications(1992) Lee, Ching-Yi; Oruç, A. Yavuz; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)This dissertation presents new results on networks for high-speed unicast and multicast communications which play key roles in communication networks and parallel computer systems. Specifically, (1) we present past parallel algorithms for routing any one-to-one assignment over Beneš network, we propose new multicasting networks that can efficiently realize any one-to-many assignments, and we give an explicit construction of linear-size expanders with very large expansion coefficients. Our parallel routing algorithms for Beneš networks are realized on two different topologies. Using these algorisms, we show that any unicast assignment that involves )(k) pairs of inputs and outputs can be routed through and n-input Beneš network in O(log2 k+lg n) time without pipelining and O(lg k) time with pipelining if the topology is complete, and in O(lg4k+lg2k lg n) time without pipelining and O(lg3 k+lg k lg n) time with pipelining if the topology is extended perfect shuffle. These improve the best-known routing time complexities of parallel algorithms of Lev et al. and Nassimi and Sahni by a factor of O(lg n). Our multicasting networks uses a very simple self-routing scheme which requires no separate computer model for routing. Including the routing cost, it can be constructed with O(n lg2 n) bit-level constant fanin logic gates, O(lg2 n) bit-level depth, and can realize any multicast assignment in O(lg3 n) bit-level time. These complexities match or are better than those of multicasting networks with the same cost that were reported in the literature. In addition to its attractive routing scheme, our multicasting network is input-initiated and can pipeline multicast assignments through itself. With pipelining, the average routing time for O(lg2 n) multicast assignments can be reduced to O(lg n) which is the best among those of the multicasting networks previously reported in the literature. Our linear-size expanders are explicitly constructed by following a traditional design and analysis technique. We construct a family of linear-size with density 33 and expansion coefficient 0.868. This expansion coefficient is the larges among the linear-size expanders that were similarly constructed. Using these expanders, we also report a family of explicitly constructed superconcentrators with density 208.