Mohammad Matin

Eigenvalue Solution of Thermoelastic Damping in Beam Resonators Using a Finite Element Analysis

A finite element formulation is developed for solving the problem related to thermoelastic damping in beam resonator systems. The perturbation analysis on the governing equations of heat conduction, thermoleasticity, and dynamic motion leads to a linear eigenvalue equation for the exponential growth rate of temperature, displacement, and velocity. The numerical solutions for a simply supported beam have been obtained and shown in agreement with the analytical solutions found in the literature. Parametric studies on a variety of geometrical and material properties demonstrate their effects on the frequency and the quality factor of resonance. The finite element formulation presented in this work has advantages over the existing analytical approaches in that the method can be easily extended to general geometries without extensive computations associated with the numerical iterations and the analytical expressions of the solution under various boundary conditions. DOI: 10.1115/1.2748472

On-die diffractive alignment structures for packaging of microlens arrays with 2-D optoelectronic device arrays

A novel technique for aligning a microlens array to an electrically packaged optoelectronic device array is presented: reflective Fresnel zone plates (FZPs) are fabricated on the device die to provide registration spots during alignment. A proof-of-concept experiment in which an MSM array was aligned to a microlens array with an accuracy of better than 9 microns is described.

Synthesis and characterization of titanium-alloyed hematite thin films for photoelectrochemical water splitting

We have synthesized pure and Ti-alloyed hematite thin films on F doped SnO{sub 2} coated glass substrates by radio frequency magnetron co-sputtering of iron oxide and titanium targets in mixed Ar/O{sub 2} and mixed N{sub 2}/O{sub 2} ambient. We found that the hematite films deposited in the N{sub 2}/O{sub 2} ambient exhibit much poorer crystallinity than the films deposited in the Ar/O{sub 2} ambient. We determined that Ti alloying leads to increased electron carrier concentration and crystallinity, and reduced bandgaps. Moreover, Ti-alloyed hematite thin films exhibited improved photoelectrochemical performance as compared with the pure hematite films: The photocurrents were enhanced and the photocurrent onset shifted to less positive potentials.

Influence of Strains and Defects on Ferroelectric and Dielectric Properties of Thin-Film Barium–Strontium Titanates

Pristine, W and Mn 1% doped Ba0.6Sr0.4TiO3 epitaxial thin films grown on the LaAlO3 substrate were deposited by pulsed laser deposition (PLD). Dielectric and ferroelectric properties were determined by the capacitance measurements and X-ray diffraction was used to determine both residual elastic strains and defect-related inhomogeneous strains by analyzing diffraction line shifts and line broadening, respectively. We found that both elastic and inhomogeneous strains are affected by doping. This strain correlates with the change in Curie-Weiss temperature and can qualitatively explain changes in dielectric loss. To explain the experimental findings, we model the dielectric and ferroelectric properties of interest in the framework of the Landau-Ginzburg-Devonshire thermodynamic theory. As expected, an elastic-strain contribution due to the epilayer-substrate misfit has an important influence on the free-energy. However, additional terms that correspond to the defect-related inhomogeneous strain had to be introduced to fully explain the measurements.

Enhancing the Stability of CuO Thin-Film Photoelectrodes by Ti Alloying

A major drawback for CuO as an efficient photocathode in photoelectrochemical (PEC) water splitting is its instability in aqueous solution. In this paper, we report that Ti alloying can enhance the stability of CuO in PEC water splitting but at the cost of reduced crystallinity and optical absorption, and therefore reduced photocurrent. We further report that a balance between the stability and photocurrent can be realized by a bilayer configuration—a thin Ti-alloyed CuO layer on a pure CuO thin film. Our results indicate that the thickness of the top Ti-alloyed CuO layer should be optimized to realize the best stability and photocurrent.

High performance InPInGaAs-BASED MSM photodetector operating at 1.3-1.5 μm

We report the fabrication and characteristics of high-speed, low-capacitance, high-responsivity metal-semiconductor-metal photodetectors (MSMPDs). These detectors are based on an InGaAs absorption layer incorporating an InP barrier enhancement layer grown on a semi-insulating InP substrate by gas-source molecular beam epitaxy (GSMBE). All epitaxial layers were not intentionally doped. The interdigitated metal electrodes, with 2 μm finger width and spacing, were formed using a Pt/Ni/Au contact film on a 50 μm by 50 μm active area. A very low dark current of 200–400 nA was observed below the bias voltage of 10 V. The devices have a capacitance of less than 2 pF. Photoresponsivities were measured under various illumination powers. Recorded typical responsivity is 0.5-0.6 A W−1. The highest responsivity of 0.78 A W−1 was observed at 10 V bias, which corresponds to an external quantum efficiency of 0.74. High-speed performance of the detectors was assessed using an electro-optic sampling (EOS) technique. The impulse response to a short optical pulse of 100 fs was recorded to assess the high speed performance. The output pulse has a rise time of 3.4–5 ps, a fall time of 8.5–11 ps, and an 8.6–11 ps full-width at half-maximum (FWHM). This corresponds to an 8–10 GHz 3 dB bandwidth, which is shown to be comparable with high-frequency measurements.

Predictive modeling of thermoelastic energy dissipation in tunable MEMS mirrors

The design of microstructures with a high quality factor Q value is of significant importance in many microelectromechanical sys- tem MEMS applications. Thermoelastic damping can cause an intrinsic energy loss that affects the Q value of high-frequency resonance in those devices such as MEMS mirrors. We deal with the simulation and analysis of thermoelastic damping of MEMS mirrors based on the finite element method. Four designs of MEMS mirrors with different geometric shapes are studied. In each model, the dynamic responses of the sys- tem subjected to thermoelastic damping are compared to those of the undamped modes. Then we present a systematic parametric study on both the resonant frequency and the Q value as functions of various representative parameters. These results are useful for early prediction of thermoelastic energy loss, not only restricted to the MEMS mirrors but also applicable in more general MEMS resonators and filters design.

Study of the BER performance in RoF-OFDM system modulated by QAM and PSK

Orthogonal frequency division multiplexing (OFDM) is considered as a one of the essential components in most of recent telecommunication systems. To maintain a high bit rate and provide a high bandwidth, using the OFDM as a modulation format in RoF system is preferred over other modulation formats. In this paper, up-converting a 20 Gb/s and a 30 Gb/s OFDM signal on a 20-GHz microwave carrier over 40 km SMF was applied under a different modulation methods of OFDM such as QAM and PSK in order to study the BER performance in all proposal cases.

Fiber Bragg sensor for smart bed sheet

This research proposes use of Fiber Bragg Grating (FBG) sensors to measure and monitor patient body temperature non-intrusively on a Smart Bed Sheet. The use of FBG sensors allows smart bed sheet to have the look and feel of an ordinary conventional bed sheet since FBG sensors have a very thin and light linear geometry. Additionally, they are dielectric in nature, and have a total immunity to electromagnetic and RF interferences. Recent developments in FBG research have made these sensors considerably inexpensive but very reliable. Simple signal processing techniques can extract very precise temperature from FBG sensors.

Results from an experiment that collected visible-light polarization data using unresolved imagery for classification of geosynchronous satellites

In order to protect critical military and commercial space assets, the United States Space Surveillance Network must have the ability to positively identify and characterize all space objects. Unfortunately, positive identification and characterization of space objects is a manual and labor intensive process today since even large telescopes cannot provide resolved images of most space objects. The objective of this study was to collect and analyze visible-spectrum polarization data from unresolved images of geosynchronous satellites taken over various solar phase angles. Different collection geometries were used to evaluate the polarization contribution of solar arrays, thermal control materials, antennas, and the satellite bus as the solar phase angle changed. Since materials on space objects age due to the space environment, their polarization signature may change enough to allow discrimination of identical satellites launched at different times. Preliminary data suggests this optical signature may lead to positive identification or classification of each satellite by an automated process on a shorter timeline. The instrumentation used in this experiment was a United States Air Force Academy (USAFA) Department of Physics system that consists of a 20-inch Ritchey-Chrétien telescope and a dual focal plane optical train fed with a polarizing beam splitter. Following a rigorous calibration, polarization data was collected during two nights on eight geosynchronous satellites built by various manufacturers and launched several years apart. When Stokes parameters were plotted against time and solar phase angle, the data indicates that a polarization signature from unresolved images may have promise in classifying specific satellites.