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  • The thesis reports on research in the general field of light interaction with matter. According to the topics addressed, it can be naturally divided into two parts: Part I, many-body aspects of the Rabi oscillations which a two-level systems undergoes under a strong resonant drive; and Part II, absorption of the ac field between the spectrum branches of two-dimensional fermions that are split by the combined action of Zeeman and spin-orbit (SO) fields. The focus of Part I is the following many-body effects that modify the conventional Rabi oscillations: Chapter 1, coupling of a two-level system to a single vibrational mode of the environment. Chapter 2, correlated Rabi oscillations in two electron-hole systems coupled by tunneling with strong electron-hole attraction. In Chapter 1, a new effect of Rabi-vibronic resonance is uncovered. If the frequency of the Rabi oscillations, R, is close to the frequency, !0, of the vibrational mode, the oscillations acquire a collective character. It is demonstrated that the actual frequency of the collective oscillations exhibits a bistable behavior as a function of ΩR ω0. The main finding in Chapter 2 is, that the Fourier spectrum of the Rabi oscillations in two coupled electron-hole systems undergoes a strong transformation with increasing ΩR. For ΩR smaller than the tunneling frequency, the spectrum is dominated by a low-frequency (<
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  • rabi oscillations
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  • Microelectromechanical gyroscopes are readily used in cars and cell phones. Tactical gyroscopes are available inexpensively and they offer 0.01 to 0.1 % scale factor inaccuracy. On the other hand, strategic gyroscopes with much better performance levels are 100,000 times more expensive. The main objective of this work is to explore the possibility of developing inexpensive strategic grade gyroscopes using microelectromechanical systems technology. Most of the available gyroscopes are surface micromachined due to fabrication issues and misalignment problems involved in multistep fabrication processes necessary to use the bulk of the wafer as the proofmass in MEMS gyroscopes. It can be shown that the sensitivity of the gyroscope is inversely proportional to the natural frequency; so if bulk micromachining technique is used it is possible to decrease the natural frequency further than current limits of surface micromachining in order to increase sensitivity. This thesis is focused on proposing a way to use bulk of the silicon wafer in the gyroscope to decrease the natural frequency to very low levels, such as sub-KHz regime, that cannot be achieved by single mask surface micromachining processes. It then proposes a solution for solving the misalignment problems caused by using multiple fabrication steps and masks instead of using only one mask in surface micromachined gyroscopes. In our design discrete proofmasses are linked together around a circle by compliant structures to ensure the highest effective mass and lowest effective spring constant. By using a proposed double sided fabrication technology the effect of misalignments on frequency mismatch can be reduced. ANSYS software simulations show that 20 µm misalignment between the masks causes a frequency shift equal to 0.3% of the natural frequency that can be compensated using electrostatic frequency tuning. Acceleration parasitic effects can also be a major problem in a low natural frequency gyroscope. In our design a multiple sensing electrode configuration is used that cancels the acceleration effects completely. The sensitivity of the gyroscope with 3126 Hz natural frequency is simulated to be 574 mV/(deg/sec) , or about four times higher than 132 mV/(deg/sec) , which was used as a benchmark for a sensitive gyroscope.
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  • Rabi Oscillations
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  • This work focuses on the study of low-magnetic field (<10mT) magnetoresistance effects of organic polymer diodes based on the n-conjugated polymer MEH-PPV in presence of oscillating magnetic fields in the radio frequency range. In these conditions, the combination of static and ac fields can magnetic resonantly influence the electronspin degree of freedom of localized charge-carrier states. As long as bipolar injection conditions influence the net current of the polymer diode, magnetic-resonance changes of the charge carrier spin state can affect spin-dependent charge carrier recombination rates and therefore the material's conductivity. Since the observed spin-dependent recombination currents are governed by the charge carrier pair's spin-permutation symmetry, magnetoresistance measurements under ac drive allow for the electrical detection of magnetic resonance under very low magnetic field conditions where inductive magnetic resonance detection schemes fail due to a lack of spin polarization. In this thesis, this effect was utilized for two effects. Firstly, for the exploration of a magnetic resonance regime where the driving field B 1 approaches the same magnitude as the static magnetic field B0. When Bi approaches B0, a regime where magnetic resonance effects become nonlinear emerges and interesting collective spin-phenomena occur. This includes spin-cooperativity, where the resonantlydriven spin ensemble assumes a macroscopically collective state. Experiments are presented that tested and confirmed previous theoretical predictions. When B}~B0, the emerging spin-cooperativity of recombining polaron pairs in organic semiconductors can be observed through magnetoresistance measurements. The experiments confirmed the theory in all aspects and demonstrated the emergence of the spin-Dicke effect. Secondly, for the exploration of whether magnetic resonantly-controlled spindependent currents can be used for magnetometry of inhomogeneous magnetic fields. This work is a continuation of the previously introduced idea to utilize spin-dependent charge carrier recombination in organic semiconductors for an absolute low-magnetic field magnetometry that is robust against fluctuating environmental conditions. The work focuses on the measurement of magnetic field distributions in gradient magnetic fields. It is shown that organic semiconductor-based magnetic resonance magnetometers can reveal magnetic field distributions. However, this measurement approach can be compromised by inductive resonance artifacts introduced by the large-bandwidth RF stripline resonators needed to operate the magnetometer.
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  • Oscillations
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  • The main objectives of this study were to identify the leading propagating patterns of atmospheric variability over the Midwest, and to determine the relationships of these patterns with Midwest precipitation. Complex Hilbert empirical orthogonal function (HEOF) analysis was performed on daily mean 850-hPa horizontal moisture transport, 850-hPa temperature advection, jet relative frequency, and the difference between 850-hPa and 250-hPa vorticity advection. Atmospheric fields were derived from the 6-hourly NCEP-NCAR reanalysis on a year-round and within-season basis. Additionally, the HEOFs were phase-shifted to maximize the correlation between the real part of the score series and area-weighted power-transformed Midwest precipitation. In the year-round analysis, the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport captured the seasonal migration of the jet and attendant low-level circulation features. The second HEOF showed high jet relative frequency over the Midwest on the upstream side of a trough, and moisture transport from the Gulf of Mexico into the Midwest. The leading within-season HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport showed a similar pattern in winter, spring, and fall. In all seasons, the monthly mean scores of the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport were better estimates of Midwest precipitation than the Pacific-North American pattern, North Atlantic Oscillation, and El Ni˜no-Southern Oscillation teleconnection indices. In addition, this study examined the relationship between the leading winter propagating patterns of variability and lake effect precipitation over the Great Lakes region. Here, the leading HEOF of combined jet relative relative frequency and 850-hPa horizontal moisture transport was phase-shifted to maximize the correlation between the real part and a lake effect precipitation fraction time series. The phase-shifted HEOF did not resolve the mesoscale features of lake effect snow, but did position the synoptic-scale circulation so that flow developed the expected northerly component over the Great Lakes.
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  • This thesis presents the design, fabrication and characterization of a microelectromechanical system (MEMS) based complete wireless microsystem for brain interfacing, with very high quality factor and low power consumption. Components of the neuron sensing system include TiW fixed-fixed bridge resonator, MEMS oscillator based action-potential-to-RF module, and high-efficiency RF coil link for power and data transmissions. First, TiW fixed-fixed bridge resonator on glass substrate was fabricated and characterized, with resonance frequency of 100 - 500 kHz, and a quality factor up to 2,000 inside 10 mT vacuum. The effect of surface conditions on resonator's quality factor was studied with 10s of nm Al2O3 layer deposition with ALD (atomic layer deposition). It was found that MEMS resonator's quality factor decreased with increasing surface roughness. Second, action-potential-to-RF module was realized with MEMS oscillator based on TiW bridge resonator. Oscillation signal with frequency of 442 kHz and phase noise of -84.75 dBc/Hz at 1 kHz offset was obtained. DC biasing of the MEMS oscillator was modulated with neural signal so that the output RF waveform carries the neural signal information. Third, high-efficiency RF coil link for power and data communications was designed and realized. Based on the coupled mode theory (CMT), intermediate resonance coil was introduced and increased voltage transfer efficiency by up to 5 times. Finally, a complete neural interfacing system was demonstrated with board-level integration. The system consists of both internal and external systems, with wireless powering, wireless data transfer, artificial neuron signal generation, neural signal modulation and demodulation, and computer interface displaying restored neuron signal.
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  • Low-cost wireless embedded systems can make radio channel measurements for the purposes of radio localization, synchronization, and breathing monitoring. Most of those systems measure the radio channel via the received signal strength indicator (RSSI), which is widely available on inexpensive radio transceivers. However, the use of standard RSSI imposes multiple limitations on the accuracy and reliability of such systems; moreover, higher accuracy is only accessible with very high-cost systems, both in bandwidth and device costs. On the other hand, wireless devices also rely on synchronized notion of time to coordinate tasks (transmit, receive, sleep, etc.), especially in time-based localization systems. Existing solutions use multiple message exchanges to estimate time offset and clock skew, which further increases channel utilization. In this dissertation, the design of the systems that use RSSI for device-free localization, device-based localization, and breathing monitoring applications are evaluated. Next, the design and evaluation of novel wireless embedded systems are introduced to enable more fine-grained radio signal measurements to the application. I design and study the effect of increasing the resolution of RSSI beyond the typical 1 dB step size, which is the current standard, with a couple of example applications: breathing monitoring and gesture recognition. Lastly, the Stitch architecture is then proposed to allow the frequency and time synchronization of multiple nodes' clocks. The prototype platform, Chronos, implements radio frequency synchronization (RFS), which accesses complex baseband samples from a low-power low-cost narrowband radio, estimates the carrier frequency offset, and iteratively drives the difference between two nodes' main local oscillators (LO) to less than 3 parts per billion (ppb). An optimized time synchronization and ranging protocols (EffToF) is designed and implemented to achieve the same timing accuracy as the state-of-the-art but with 59% less utilization of the UWB channel. Based on this dissertation, I could foresee Stitch and RFS further improving the robustness of communications infrastructure to GPS jamming, allow exploration of applications such as distributed beamforming and MIMO, and enable new highly-synchronous wireless sensing and actuation systems.
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  • Hippocampal network oscillations are important for learning and memory. Theta rhythms are involved in attention, navigation, and memory encoding, whereas sharp wave-ripple complexes (ripples) are involved in memory consolidation. Cholinergic neurons in the medial septum-diagonal band of Broca (MS-DB) influence both types of hippocampal oscillations, promoting theta rhythms and suppressing ripples. They also receive frequency-dependent hyperpolarizing feedback from hippocamposeptal connections, potentially affecting their role as neuromodulators in the septohippocampal circuit. However, little is known about how the integration properties of cholinergic MS-DB neurons change with hyperpolarization. By potentially altering firing behavior in cholinergic neurons, hyperpolarizing feedback from the hippocampal neurons may, in turn, change hippocampal network activity. To study how hyperpolarizing inputs change in membrane integration properties, we used whole-cell patch-clamp recordings targeting genetically labeled, choline acetyltransferase-positive neurons in mouse medial septal brain slices. Hyperpolarization of cholinergic MS-DB neurons resulted in a long-lasting decrease in spike firing rate and input-output gain. Additionally, voltage-clamp measures implicated a slowly inactivating, 4-AP-insensitive, outward K+ conductance. Using a conductance-based model of cholinergic MS-DB neurons, we show that the ability of this conductance to modulate firing rate and gain depends on the expression of an experimentally verified shallow intrinsic spike frequency-voltage relationship. Finally, we show that cholinergic suppression of hippocampal ripples can be achieved through an imbalance in drive, caused by cholinergic modulation, to hippocampal excitatory and inhibitory neurons. Together, these findings show possible mechanisms through which cholinergic MS-DB neurons may both influence and be influenced by hippocampal rhythms.
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