86 research outputs found
Nonlinear Time-Frequency Control Theory with Applications
Nonlinear control is an important subject drawing much attention. When a nonlinear system undergoes route-to-chaos, its response is naturally bounded in the time-domain while in the meantime becoming unstably broadband in the frequency-domain. Control scheme facilitated either in the time- or frequency-domain alone is insufficient in controlling route-to-chaos, where the corresponding response deteriorates in the time and frequency domains simultaneously. It is necessary to facilitate nonlinear control in both the time and frequency domains without obscuring or misinterpreting the true dynamics. The objective of the dissertation is to formulate a novel nonlinear control theory that addresses the fundamental characteristics inherent of all nonlinear systems undergoing route-to-chaos, one that requires no linearization or closed-form solution so that the genuine underlying features of the system being considered are preserved. The theory developed herein is able to identify the dynamic state of the system in real-time and restrain time-varying spectrum from becoming broadband. Applications of the theory are demonstrated using several engineering examples including the control of a non-stationary Duffing oscillator, a 1-DOF time-delayed milling model, a 2-DOF micro-milling system, unsynchronized chaotic circuits, and a friction-excited vibrating disk.
Not subject to all the mathematical constraint conditions and assumptions upon which common nonlinear control theories are based and derived, the novel theory has its philosophical basis established in the simultaneous time-frequency control, on-line system identification, and feedforward adaptive control. It adopts multi-rate control, hence enabling control over nonstationary, nonlinear response with increasing bandwidth ? a physical condition oftentimes fails the contemporary control theories. The applicability of the theory to complex multi-input-multi-output (MIMO) systems without resorting to mathematical manipulation and extensive computation is demonstrated through the multi-variable control of a micro-milling system. The research is of a broad impact on the control of a wide range of nonlinear and chaotic systems. The implications of the nonlinear time-frequency control theory in cutting, micro-machining, communication security, and the mitigation of friction-induced vibrations are both significant and immediate
Nonlinear Time-Frequency Control Theory with Applications
Nonlinear control is an important subject drawing much attention. When a nonlinear system undergoes route-to-chaos, its response is naturally bounded in the time-domain while in the meantime becoming unstably broadband in the frequency-domain. Control scheme facilitated either in the time- or frequency-domain alone is insufficient in controlling route-to-chaos, where the corresponding response deteriorates in the time and frequency domains simultaneously. It is necessary to facilitate nonlinear control in both the time and frequency domains without obscuring or misinterpreting the true dynamics. The objective of the dissertation is to formulate a novel nonlinear control theory that addresses the fundamental characteristics inherent of all nonlinear systems undergoing route-to-chaos, one that requires no linearization or closed-form solution so that the genuine underlying features of the system being considered are preserved. The theory developed herein is able to identify the dynamic state of the system in real-time and restrain time-varying spectrum from becoming broadband. Applications of the theory are demonstrated using several engineering examples including the control of a non-stationary Duffing oscillator, a 1-DOF time-delayed milling model, a 2-DOF micro-milling system, unsynchronized chaotic circuits, and a friction-excited vibrating disk.
Not subject to all the mathematical constraint conditions and assumptions upon which common nonlinear control theories are based and derived, the novel theory has its philosophical basis established in the simultaneous time-frequency control, on-line system identification, and feedforward adaptive control. It adopts multi-rate control, hence enabling control over nonstationary, nonlinear response with increasing bandwidth ? a physical condition oftentimes fails the contemporary control theories. The applicability of the theory to complex multi-input-multi-output (MIMO) systems without resorting to mathematical manipulation and extensive computation is demonstrated through the multi-variable control of a micro-milling system. The research is of a broad impact on the control of a wide range of nonlinear and chaotic systems. The implications of the nonlinear time-frequency control theory in cutting, micro-machining, communication security, and the mitigation of friction-induced vibrations are both significant and immediate
Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging
We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (similar to 20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations, but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain-and symmetry-dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches
Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs
We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at
terahertz frequencies. For SRRs on doped GaAs films, incident terahertz
radiation with peak fields of ~20 - 160 kV/cm drives intervalley scattering.
This reduces the carrier mobility and enhances the SRR LC response due to a
conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field
enhancement within the SRR gaps leads to efficient impact ionization,
increasing the carrier density and the conductivity which, in turn, suppresses
the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude
in the carrier density in the SRR gaps on semi-insulating GaAs substrate.
Furthermore, we show that the effective permittivity can be swept from negative
to positive values with increasing terahertz field strength in the impact
ionization regime, enabling new possibilities for nonlinear metamaterials.Comment: 5 pages, 4 figure
Machine Learning for Optical Scanning Probe Nanoscopy
The ability to perform nanometer-scale optical imaging and spectroscopy is
key to deciphering the low-energy effects in quantum materials, as well as
vibrational fingerprints in planetary and extraterrestrial particles, catalytic
substances, and aqueous biological samples. The scattering-type scanning
near-field optical microscopy (s-SNOM) technique has recently spread to many
research fields and enabled notable discoveries. In this brief perspective, we
show that the s-SNOM, together with scanning probe research in general, can
benefit in many ways from artificial intelligence (AI) and machine learning
(ML) algorithms. We show that, with the help of AI- and ML-enhanced data
acquisition and analysis, scanning probe optical nanoscopy is poised to become
more efficient, accurate, and intelligent
Harnessing excitons at the nanoscale -- photoelectrical platform for quantitative sensing and imaging
Excitons -- quasiparticles formed by the binding of an electron and a hole
through electrostatic attraction -- hold promise in the fields of quantum light
confinement and optoelectronic sensing. Atomically thin transition metal
dichalcogenides (TMDs) provide a versatile platform for hosting and
manipulating excitons, given their robust Coulomb interactions and exceptional
sensitivity to dielectric environments. In this study, we introduce a cryogenic
scanning probe photoelectrical sensing platform, termed exciton-resonant
microwave impedance microscopy (ER-MIM). ER-MIM enables ultra-sensitive probing
of exciton polarons and their Rydberg states at the nanoscale. Utilizing this
technique, we explore the interplay between excitons and material properties,
including carrier density, in-plane electric field, and dielectric screening.
Furthermore, we employ deep learning for automated data analysis and
quantitative extraction of electrical information, unveiling the potential of
exciton-assisted nano-electrometry. Our findings establish an invaluable
sensing platform and readout mechanism, advancing our understanding of exciton
excitations and their applications in the quantum realm
Cooperative photoinduced metastable phase control in strained manganite films
A major challenge in condensed matter physics is active control of quantum
phases. Dynamic control with pulsed electromagnetic fields can overcome
energetic barriers enabling access to transient or metastable states that are
not thermally accessible. Here we demonstrate strain-engineered tuning of
La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme
photo-susceptibility where even a single optical pulse can initiate a
transition to a long-lived metastable hidden metallic phase. Comprehensive
single-shot pulsed excitation measurements demonstrate that the transition is
cooperative and ultrafast, requiring a critical absorbed photon density to
activate local charge excitations that mediate magnetic-lattice coupling that,
in turn, stabilize the metallic phase. These results reveal that strain
engineering can tune emergent functionality towards proximal macroscopic states
to enable dynamic ultrafast optical phase switching and control
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