A micromachined thermo-optical light modulator based on semiconductor-to-metal phase transition

In this research, a micromachined thermo-optical light modulator was realized based on the semiconductor-to-metal phase transition of vanadium dioxide (VO2) thin film. VO2 undergoes a reversible phase transition at approximately 68 0C, which is accompanied with drastic changes in its electrical and optical properties. The sharp electrical resistivity change can be as great as five orders. Optically, VO2 film will switch from a transparent semiconductor phase to a reflective metal phase upon the phase transition. The light modulator in this research exploits this phase transition related optical switching by using surface micromachined low-thermal-mass pixels to achieve good thermal isolations, which ensures that each pixel can be individually switched without cross talking. In operation, the pixel temperature was controlled by integrated resistor on each pixel or spatially addressed thermal radiation sources. Active VO2 thin film was synthesized by thermal oxidation of e-beam evaporated vanadium metal film. The oxidized film exhibits a phase transition at ~65°C with a hysteresis of about 15°C. A transmittance switching from ~90% to ~30% in the near infrared and a reflectance switching from ~50% to 15% in the visible have been achieved. The surface microstructure was studied and correlated to its optical properties. A study on the hysteresis loop reveals that the VO2 can be repetitively switched between the on and off\u27 states. The micromachined thermal isolation pixel was a bridge-like silicon dioxide platform suspended with narrow supporting legs. The pixel design was optimized with both thermal and optical simulations. The VO2 light modulator was fabricated by surface micromachining based on dry processing. Silicon dioxide was deposited on a polyimide sacrificial layer by PECVD and patterned to form the structural pixel. Vanadium film was e-beam evaporated and patterned with lift-off process. It was thermally oxidized into VO2 at 390°C. The thermal isolation pixel was anchored on substrate by aluminum pedestals. Finally, the structure was released in an oxygen plasma barrel asher. The VO2 array was experimentally tested and its light switching and modulation ability were demonstrated. Further study shows that the surface micromachining process has no degrading effect on the optical property of VO2 thin film

Algorithmic Regularization in Model-free Overparametrized Asymmetric Matrix Factorization

We study the asymmetric matrix factorization problem under a natural nonconvex formulation with arbitrary overparametrization. The model-free setting is considered, with minimal assumption on the rank or singular values of the observed matrix, where the global optima provably overfit. We show that vanilla gradient descent with small random initialization sequentially recovers the principal components of the observed matrix. Consequently, when equipped with proper early stopping, gradient descent produces the best low-rank approximation of the observed matrix without explicit regularization. We provide a sharp characterization of the relationship between the approximation error, iteration complexity, initialization size and stepsize. Our complexity bound is almost dimension-free and depends logarithmically on the approximation error, with significantly more lenient requirements on the stepsize and initialization compared to prior work. Our theoretical results provide accurate prediction for the behavior gradient descent, showing good agreement with numerical experiments.Comment: 30 pages, 7 figure

A Normalization Model for Analyzing Multi-Tier Millimeter Wave Cellular Networks

Based on the distinguishing features of multi-tier millimeter wave (mmWave) networks such as different transmit powers, different directivity gains from directional beamforming alignment and path loss laws for line-of-sight (LOS) and non-line-of-sight (NLOS) links, we introduce a normalization model to simplify the analysis of multi-tier mmWave cellular networks. The highlight of the model is that we convert a multi-tier mmWave cellular network into a single-tier mmWave network, where all the base stations (BSs) have the same normalized transmit power 1 and the densities of BSs scaled by LOS or NLOS scaling factors respectively follow piecewise constant function which has multiple demarcation points. On this basis, expressions for computing the coverage probability are obtained in general case with beamforming alignment errors and the special case with perfect beamforming alignment in the communication. According to corresponding numerical exploration, we conclude that the normalization model for multi-tier mmWave cellular networks fully meets requirements of network performance analysis, and it is simpler and clearer than the untransformed model. Besides, an unexpected but sensible finding is that there is an optimal beam width that maximizes coverage probability in the case with beamforming alignment errors.Comment: 7 pages, 4 figure

Adaptive Control Strategy for Active Power Sharing in Hybrid Fuel Cell/Battery Power Sources

Hybrid systems composed of fuel cells and batteries combine the high energy density of fuel cells with the high power density of batteries. A dc/dc power converter is placed between the fuel cell and the battery to balance the power flow between them and greatly increase the peak output power of the hybrid. This paper presents an adaptive control strategy for active power sharing in the hybrid power source. This control strategy can adjust the output current setpoint of the fuel cell according to the state-of-charge (or voltage) of the battery, and is applicable in two topologies of active fuel cell/battery hybrids. The control strategy is implemented in Simulink and then tested under arbitrary load conditions through simulation and experiments. Simulation and experimental results show that the adaptive control strategy is able to adjust the fuel cell output current to adapt to the charge state of the battery, and appropriately distribute the electrical power between the fuel cell and the battery. Experiments demonstrate the generality of the adaptive control strategy

Flexible Multiobjective Control of Power Converter in Active Hybrid Fuel Cell/Battery Power Sources

Hybrid power sources composed of fuel cells and secondary batteries can combine the high energy density of fuel cells with the high power density of batteries. A dc/dc power converter can be placed between the fuel cell and the battery to balance the power flow and greatly augment the peak output power. This paper presents a novel, flexible strategy for multiobjective control of the power converter in the hybrid power source. The control strategy is able to regulate the output current of the fuel cell and the charging current or voltage of the battery while limiting the discharging current of the battery. It can be used in two different configurations without any change. The control strategy is implemented in MATLAB/Simulink and tested by simulation and experiments. Simulation and experimental results show that the multiobjective control strategy is able to select the regulation mode correctly and the fuel cell current, battery current and battery voltage are regulated appropriately. Experiment results demonstrate the great flexibility and generality of the control strategy and validate that the peak power capacity of the active hybrid power source is increased significantly. Simulation and experiment results also show that power converter can be appropriately regulated to meet the multiple objectives required by hybrid power sources