No description available.

No description available.

No description available.

The detection for phase locking in a forced oscillator with dual attractors and ill-defined phase structure is hard until a quantitative approach was constructed for detecting phase locking via stroboscopic method. We study the route to weak phase locking in a chaotic system ¡§Chua oscillator¡¨ with complex attractor structure by analyzing the stroboscopic points. The onset of weak phase locking detected by using this statistical approach and the critical coupling strength calculated by conditional Lyapunov exponent are matched well. Detailed structure of phase locking intensity is described by the Arnold tongue diagram. Moreover, we apply this approach on three hyper-chaotic systems with multi-scroll attractor, including hyper-chaotic Rössler system, hyper-chaotic Lorenz system, and modified MCK oscillator. The weak phase locking between hyper-chaotic system and a periodic or a chaotic driving force is observable following the condition of stroboscopic point concentration.

The manufacturing of nanocontact-based spin-torque oscillators (NC-STOs)has opened the door for spintronic devices to play a part as active microwaveelements. The NC-STO has the capability of converting a direct current intoa microwave signal, and vice versa, by utilizing the spin transfer torque (STT)in ferromagnetic multilayer systems. However, the high-frequency operation ofNC-STOs typically requires high magnetic fields and the microwave power theygenerate is rather limited. As a result, NC-STOs are not yet commercially used,and they require improvements in both material systems and device geometriesbefore they can find actual use in microwave applications. In order to improve and advance this technology, NC-STOs are requiredwith both different nanocontact (NC) sizes and geometries, and using differ- ent stacks of magnetic materials. This dissertation presents experimental in- vestigations into the manufacturing of such devices using different fabrication techniques and a number of different magnetic material stacks. Currently, the fabrication of NC-STOs is limited to advanced laboratories, because NC fabri- cation requires high-resolution lithography tools. In the present work, we have developed an alternative method of fabrication, which does not require such tools and has the capability of fabricating NC-STOs having one to hundreds of NCs in a variety of sizes, possibly  down to 20 nm. Devices fabricated with this method have shown mutual synchronization of three parallel-connected NCs, and pairwise synchronization in devices with four and five NCs. Furthermore, the present work demonstrates low-field operation (down to0.02 Tesla) of NC-STOs at a record high frequency of 12 GHz. This wasachieved by implementing multilayers with a perpendicular magnetic anisotropy(PMA) material in the free layer of the NC-STO. In addition, the fabricateddevices revealed an unexpected dynamic regime under large external appliedfield (above 0.4 Tesla). The new dynamic regime was found to be due to anentirely novel nanomagnetic dynamic object â a so-called magnetic droplet soliton,predicted theoretically in 1977 but not experimentally observed until now.Detailed experiments and micromagnetic simulations show that the droplet hasvery rich dynamics. Finally,  spin-torque-induced  transverse spin wave instabilities have beenstudied.  A NC-STO with  a material stack consisting of a single ferromag- netic metal sandwiched between two non-ferromagnetic metals was fabricated. Prior to this work, evidence of spin wave instabilities was reported as resis- tance switching in nanopillar- and mechanical point contact based STOs. In the present  work, the fabricated NC-STOs showed actual microwave  signals up to 3 GHz under zero applied field with strong current hysteresis. All  the fabricated NC-STOs open up new means of studying STT in different environ- ments, in order to resolve their current drawbacks for industrial applications. / <p>QC 20130527</p>

Local gamma-band (~30-100Hz) oscillations in the brain, produced by feedback inhibition on a characteristic timescale, appear in multiple areas of the brain and are associated with a wide range of cognitive functions. Some regions producing gamma also receive gamma-rhythmic input, and the interaction and coordination of these rhythms has been hypothesized to serve various functional roles. This thesis consists of three stand-alone chapters, each of which considers the response of a gamma-rhythmic neuronal circuit to input in an analytical framework. In the first, we demonstrate that several related models of a gamma-generating circuit under periodic forcing are asymptotically drawn onto an attracting invariant torus due to the convergence of inhibition trajectories at spikes and the convergence of voltage trajectories during sustained inhibition, and therefore display a restricted range of dynamics. In the second, we show that a model of a gamma-generating circuit under forcing by square pulses cannot maintain multiple stably phase-locked solutions. In the third, we show that a separation of time scales of membrane potential dynamics and synaptic decay causes the gamma model to phase align its spiking such that periodic forcing pulses arrive under minimal inhibition. When two of these models are mutually coupled, the same effect causes excitatory pulses from the faster oscillator to arrive at the slower under minimal inhibition, while pulses from the slower to the faster arrive under maximal inhibition. We also show that such a time scale separation allows the model to respond sensitively to input pulse coherence to an extent that is not possible for a simple one-dimensional oscillator. We draw on a wide range of mathematical tools and structures including return maps, saltation matrices, contraction methods, phase response formalism, and singular perturbation theory in order to show that the neuronal mechanism of gamma oscillations is uniquely suited to reliably phase lock across brain regions and facilitate the selective transmission of information.

The primary purpose of this investigation was to determine whether temporal coding in the auditory system was the same for younger and older listeners. Temporal coding was assessed by amplitude-modulated auditory steady-state responses (AM ASSRs) as a physiologic measure of phase-locking capability. The secondary purpose of this study was to determine whether AM ASSRs were related to behavioral speech understanding ability. AM ASSRs showed that the ability of the auditory system to phase lock to a temporally altered signal is dependent on modulation rate, carrier frequency, and age of the listener. Specifically, the interaction of frequency and age showed that younger listeners had more phase locking than old listeners at 500 Hz. The number of phase-locked responses for the 500 Hz carrier frequency was significantly correlated to word-recognition performance. In conclusion, the effect of aging on temporal processing, as measured by phase locking with AM ASSRs, was found for low-frequency stimuli where phase locking in the auditory system should be optimal. The exploration, and use, of electrophysiologic responses to measure auditory timing analysis in humans has the potential to facilitate the understanding of speech perception difficulties in older listeners.

The main objective of this thesis is to explore the use of volume holographic elements recorded in photo-thermo-refractive (PTR) glass for power scaling of narrow linewidth diffraction-limited fiber lasers to harness high average power and high brightness beams. Single fiber lasers enable kW level output powers limited by optical damage, thermal effects and non-linear effects. Output powers can be further scaled using large mode area fibers, however, at the cost of beam quality and instabilities due to the presence of higher order modes. The mechanisms limiting the performance of narrow-linewidth large mode area fiber lasers are investigated and solutions using intra-cavity volume Bragg gratings (VBG) proposed. Selfpulsations-free, completely continuous-wave operation of a VBG-stabilized unidirectional fiber ring laser is demonstrated with quasi single-frequency ( < 7.5 MHz) output. A method for transverse mode selection in multimode fiber lasers to reduce higher order mode content and stabilize the output beam profile is developed using angular selectivity of reflecting VBGs. By placing the VBG output coupler in a convergent beam, stabilization of the far-field beam profile of a 20 μm core large mode area fiber laser is demonstrated. Beam combining techniques are essential to power scale beyond the limitations of single laser sources. Several beam combining techniques relevant to fiber lasers were compared in this study and found to be lacking in one or more of the following aspects: the coherence of the individual sources is compromised, the far-field beam quality is highly degraded with significant power in iv side lobes, spectrally broad and unstable, and uncertainty over scaling to larger arrays and higher power. Keeping in mind the key requirements of coherence, good far-field beam quality, narrow and stable spectra, and scalability in both array size and power, a new passive coherent beam combining technique using multiplexed volume Bragg gratings (M-VBGs) is proposed. In order to understand the mechanism of radiation exchange between multiple beams via these complex holographic optical elements, the spectral and beam splitting properties a 2nd order reflecting M-VBG recorded in PTR glass is experimentally investigated using a tunable single frequency seed laser. Two single-mode Yb-doped fiber lasers are then coherently combined using reflecting M-VBGs in both linear and unidirectional-ring resonators with > 90% combining efficiency and diffraction-limited beam quality. It is demonstrated that the combining bandwidth can be controlled in the range of 100s of pm to a few pm by angular detuning of the M-VBG. Very narrow-linewidth ( < 210 MHz) operation in a linear cavity and possibility of singlefrequency operation in a unidirectional ring cavity of the coherently combined system is demonstrated using this technique. It is theoretically derived and experimentally demonstrated that high combining efficiency can be achieved even by multiplexing low-efficiency VBGs, with the required diffraction efficiency of individual VBGs decreasing as array size increases. Scaling of passive coherent beam combining to four fiber lasers is demonstrated using a 4th order transmitting M-VBG. Power scaling of this technique to 10 W level combined powers with 88% combining efficiency is demonstrated by passively combining two large mode area fiber lasers using a 2nd order reflecting M-VBG in a unidirectional ring resonator. High energy compact single-frequency sources are highly desired for several applications – one of which is as a seed for high power fiber amplifiers. Towards achieving the goal of a monolithic solid-state laser, a new gain medium having both photosensitive and luminescence properties is investigated – rare-earth doped PTR glass. First lasing is demonstrated in this new gain element in a VBG-stabilized external cavity.

In the field of second harmonic (SH) generation most studies have been concerned with maximizing conversion efficiencies, generally achievable at the phase matching (PM) condition. Outside of the PM the conversion efficiency drastically decreases. This has caused that the possible working conditions out of PM to remain largely unexplored. In this thesis work we initiated a systematic study of the SH behavior in under conditions of large phase mismatch. When a pump pulse crosses an interface between a linear and a nonlinear medium there are always two generated SH components. These components may be understood on the basis of the mathematical solution of the inhomogeneous wave equations at the SH frequency. The homogeneous (HOM) solution is a component with wave-vector k(2¿) as expected from the dispersion relation and exchanges energy with the pump until the inevitable walk-off. The inhomogeneous (INH) solution is a component with a wave-vector 2k(¿), twice the pump wave-vector, and travels locked to the pump pulse. We divide our work in two parts, one for each generated component. Inhomogeneous component. We start a systematic study of the behavior of the generated INH component, phase locked to the pump. The consequences of phase locking (PL) can guide us towards new scenarios by allowing working conditions hitherto assumed inaccessible for absorbing materials. We show that while the HOM component travels with the group velocity given by material dispersion, the IHN component is captured by the pump pulse and experiences the same effective dispersion of the pump. It does not follow the PM condition. It naturally follows that the suppression of absorption at the SH wavelengths will occur if the pump is tuned to a region of transparency. We extended the same theory for the generated third harmonic (TH). We then studied the surprising behavior of SH and TH INH components with frequencies above the absorption edge when the material is placed inside a cavity resonant only at the fundamental frequency. We have shown that the PL mechanism not only inhibits absorption but also fosters the enhancement of harmonic generation by several orders of magnitude compared to the no-cavity case. Finally, we tested the INH SH and TH behaviors in metallic frequency regime of material. Homogeneous component. The techniques used to PM the nonlinear interaction enable efficient nonlinear interactions but drastically limit the spectral bandwidth of the nonlinear optical process, making the designed frequency converter only suitable for a fixed input wavelength and single interaction only. It has been shown that the use of disordered nonlinear media relaxes the PM condition thus allowing one to achieve relatively efficient broad bandwidth regime of the frequency conversion. An example of a quadratic nonlinear medium with a disordered domain structure is an un-poled Strontium Barium Niobate (SBN) crystal. It is composed of a system of random size anti parallel ferroelectric domains that allow to phase-match any second-order parametric process over a broad range of wavelengths without any poling. We have initiated an experimental and theoretical investigation of the properties of the SH waves generated in SBN crystals, with an extension to the generated TH. This study covers the coherence and polarization properties of the generated signal, as well as its spatial distribution. In addition, we have made an experimental study of the noncollinear interaction of short optical pulses in a SBN crystal by using two fundamental waves intersecting inside the crystal. We have shown that this effect may be employed as a simple tool for monitoring both the pulse duration and initial chirp. This method offers a simple and economic alternative to the existing methods for pulse characterization.

Phase control in microwave circuits is an impotant process. Especially, in certain applications such as phase array antennas, it is the main principle of opeation. In antenna arrays, each array element is fed by an individual oscillator. By controlling the phase of each oscillator, the radiation pattern and the RF power can be combined in space in certain directions. For such applications, phase shifters have been utilized extensively. However, their high costs, difficulties in design and efficiency are impotant disadvantages. More recently, another technique, &quot / Injection Locking&quot / or &quot / Phase Locking&quot / suggests to use a single reference signal injected into each oscilator element. Through this signal, the phase of the individual oscillators can be controlled and set to a desired value. Therefore, power combining in space or known as &quot / Spaial Power Combining&quot / is possible by using &quot / Phase Locking&quot / of individual oscillator elements. In this thesis, this new phase control technique is examined in theory and in application of a 1GHz oscillator system. A reference signal is injected into a voltage controlled oscillator, and the phase progression is obtained by tuning the oscilator&#039 / s free running frequency.