A photoconductive, miniature terahertz source

We discuss the performance of a micromachined, photoconductive terahertz emitter that is fabricated on low-temperature-grown GaAs. The device is mounted on a pair of single-mode optical fibers that allows the source to be freely positionable. A strong radiation burst is emitted due to the large magnetic moment created by the electrode. The emitter’s small feature size of 300 μm by 300 μm with a photoconductive switch area of 30 μm by 30 μm suggests its application for terahertz, time-domain, near-field spectroscopy and imaging. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70594/2/APPLAB-72-24-3100-1.pd

A field‐sensitive photoconductive probe for sampling through passivation layers

A field‐sensitive photoconductive sampling technique has been demonstrated in measurements performed through an insulating layer without the need for conductive contact. Sampled signals are sensed by a virtual‐ground, floating‐gate amplifier without draining charge from the device under test or the photoconductive switch. The minimum detectable signal is 2.5 μV/Hz1/2 with a spatial resolution of 7 μm, while the sampling bandwidth is essentially that observed using photoconductive sampling with a conductive contact to the device under test. The photovoltaic and shot current noise are negligible in comparison with the lock‐in amplifier noise since the current flowing in this high‐impedance, floating‐gate probe is negligible. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70307/2/APPLAB-69-15-2211-1.pd

A fiber‐mounted, micromachined photoconductive probe with 15 nV/Hz1/2 sensitivity

We report the performance of a micromachined, photoconductive‐sampling probe that is fabricated on low‐temperature‐grown GaAs and mounted on a single‐mode optical fiber. The epitaxial probe has a temporal resolution of 3.5 ps, a spatial resolution of 7 μm, and a sensitivity of 15 nV/(Hz)1/2 when integrated with a high impedance, junction field‐effect transistor source follower. The fiber, which couples short laser pulses to the interdigitated detector pattern on the probe, also provides flexible support and mobility. The probe’s compact cross section makes it ideal for applications as an internal‐node, picosecond‐response, photoconductive sampling probe or wave form launcher for test and characterization of integrated circuits. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70779/2/APPLAB-69-13-1843-1.pd

Field-sensitive photoconductive sampling and probes.

This dissertation has investigated thoroughly the sensitivity of photoconductive sampling, including the limitations of picosecond photoconductive material, Schottky metal/semiconductor contacts, miniaturized MSM interdigitated switches, and the parasitic effects of external readout circuits. The dominant noise source is found to be the photovoltaic noise induced by the 1/f gating laser amplitude fluctuation. By using an integrated JFET source follower with an extremely high input impedance and low parasitic capacitance, the sampling method is converted to a voltage measurement. The sensitivity of photoconductive sampling with a conductive contact is improved to a few tens of nV/Hz\sp{1/2}, a factor of 100 over the conventional method. The invasiveness to the device under-test and the required laser power are both greatly reduced. In addition, this voltage measurement method can avoid the intrinsic high electric field effect of photoconductive material. Overall, this sensitivity has already reached the limitation of the most sensitive readout electronics available. The dynamics of photoconductive sampling circuits using an equivalent-time method are modeled and verified using a new charge injection-dissipation model, which including the intrinsic photoconductive charge injection rate and the leakage current due to the external readout circuit. By measuring the modulation bandwidth of sampling switches at various gating laser power for various sizes of MSM interdigitated switches, this model is verified to be consistent with the experimental results. This high impedance source follower has also been incorporated with low-temperature-grown MBE GaAs based, epitaxial lift-off layer probes and measures picosecond waveforms through insulating layers. A sensitivity of 2.5 $\mu$V/Hz\sp{1/2} and a spatial resolution of 5 $\mu$m, consistent with the charge injection-dissipation model is verified. A conductive contact is no longer required and the invasiveness in term of current drainage from device-under-test is eliminated. The dominant noise source is reduced to a few nV/Hz\sp{1/2} of the readout circuit, instead of the 1/f photovoltaic noise since there has no complete current loop. The possibility of a photoconductive scanning probe with a submicron spatial resolution and a voltage sensitivity of a few $\mu$V/Hz\sp{1/2} is discussed by the charge injection-dissipation model.Ph.D.Applied SciencesCondensed matter physicsElectrical engineeringOpticsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130020/2/9711994.pd

Optimization of Vacuum Brazing Process Parameters in Ti-6Al-4V Alloy

In this study, the optimal parameters of the vacuum brazing titanium alloy Ti-6Al-4V with TiCuNi filler (30 μm-thick metal foil) were investigated by the Taguchi method. The microstructures, microhardness, and fractographs of the titanium brazed joints produced by these optimal parameters were also analyzed. The results of this study demonstrate that for the best tensile strength, the optimal combination of process parameters is: 890 °C soaking temperature, 60 min soaking time, 975 °C brazing temperature, and 45 min brazing time. The tensile strength obtained by welding with the optimal parameters was found to be 1265 MPa. A small error of 0.24% between experimental and predicted values confirmed the validity of the combined optimized parameters. Finally, from the means of variance analysis (ANOVA), out of the four factors, the highest contribution to the optimal parameters was found to be the brazing time, accounting for 47.3%. The base material of vacuum brazing (VB) weldment is mainly composed of white granular α titanium, slender β titanium, and layered structures that are interlaced by α and β. The weld bead, composed of Ti-15Cu-15Ni, contains many slender needle-shaped Widmanstätten structures. This structure is associated with higher strength and lower ductility. The weld bead hardness of the vacuum brazed parts is higher than that of laser beam weldment and gas tungsten arc weldment. This study demonstrates the feasibility of the Taguchi method for obtaining the optimal process parameters of titanium vacuum brazed joints

Prediction of Fatigue Crack Growth in Vacuum-Brazed Titanium Alloy

The assessment of fatigue is a crucial concern in welded components and structures. This study investigates the fatigue properties and models for predicting fatigue crack growth in Ti-6Al-4V titanium alloy when processed by vacuum brazing with TiCuNi filler. Fatigue properties and the impact of the stress ratio were determined through constant amplitude fatigue tests. By utilizing the results obtained from variable amplitude fatigue tests, various prediction models for fatigue crack growth were examined: modifications for load interaction, residual stress, and crack closure. The results indicate that the microstructures in the brazed zone consist of numerous fine, elongated needle-like Widmanstätten structures. In terms of cycle counting methods, the rainflow method outperforms the simple-range method. In the stable crack growth rate region, fatigue crack growth rate increases with the rise in stress ratio in a manner similar to high-strength steels. The Paris model without any modification obtains good predictions. For models modified with crack closure, the Elber model yields slightly better prediction results than the Schijve model. Among fatigue crack growth prediction models, the Willenborg model with residual stress modification produces the best results. Fracture surfaces within fatigued specimens’ brazed zones exhibit ductile failure characteristics, where fatigue striations and secondary cracks were observed

Optimization of Vacuum Brazing Process Parameters in Ti-6Al-4V Alloy

In this study, the optimal parameters of the vacuum brazing titanium alloy Ti-6Al-4V with TiCuNi filler (30 μm-thick metal foil) were investigated by the Taguchi method. The microstructures, microhardness, and fractographs of the titanium brazed joints produced by these optimal parameters were also analyzed. The results of this study demonstrate that for the best tensile strength, the optimal combination of process parameters is: 890 °C soaking temperature, 60 min soaking time, 975 °C brazing temperature, and 45 min brazing time. The tensile strength obtained by welding with the optimal parameters was found to be 1265 MPa. A small error of 0.24% between experimental and predicted values confirmed the validity of the combined optimized parameters. Finally, from the means of variance analysis (ANOVA), out of the four factors, the highest contribution to the optimal parameters was found to be the brazing time, accounting for 47.3%. The base material of vacuum brazing (VB) weldment is mainly composed of white granular α titanium, slender β titanium, and layered structures that are interlaced by α and β. The weld bead, composed of Ti-15Cu-15Ni, contains many slender needle-shaped Widmanstätten structures. This structure is associated with higher strength and lower ductility. The weld bead hardness of the vacuum brazed parts is higher than that of laser beam weldment and gas tungsten arc weldment. This study demonstrates the feasibility of the Taguchi method for obtaining the optimal process parameters of titanium vacuum brazed joints

Photoconductive sampling with an integrated source follower/amplifier

A hybrid, optoelectronic sampling circuit based on a photoconductive switch and a junction‐field‐effect‐transistor (JFET) source follower/amplifier has been demonstrated to have picosecond response, high‐sensitivity, absolute‐voltage capability, and a very high impedance. The distributed capacitance of the electrical measurement system is reduced to the gate input capacitance of the JFET, and the conventional photoconductive current measurement is transferred into an absolute voltage measurement. Gating measurements with an improvement of 150 times in output voltage over unamplified photoconductive gates have been made using only 10 μW of average optical power. The effective on‐state resistance of the photogate has also been increased by more than 150 times, indicating that a photoconductive probe with very low invasiveness may be produced. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69446/2/APPLAB-68-11-1464-1.pd

Macrophage Activation Syndrome as Initial Presentation of Systemic Lupus Erythematosus

Macrophage activation syndrome (MAS) is known to be a severe and potentially life-threatening complication of rheumatic disorder, especially systemic juvenile rheumatoid arthritis. It is very rare for MAS to be an initial presentation of systemic lupus erythematosus (SLE). Here, we report a 14-year-old girl in whom MAS developed as an initial presentation of SLE. With early diagnosis and administration of cyclosporine A, she had a fair outcome. Further testing showed positive anti-dsDNA about 8 months later

Typhoid Fever in Southern Taiwan: A Medical Center Experience

Typhoid fever is caused by Salmonella enterica serotype Typhi (S. typhi). Growing communication between Taiwan and neighboring regions in recent years could be an important source of typhoid fever. The aim of this study was to analyze the clinical characteristics of typhoid fever treated at a medical center in southern Taiwan. Methods: The study group consisted of 33 patients (aged 1–68 years) who had clinical symptoms and culture-confirmed typhoid fever. They were studied over a 20-year period from January 1987 to December 2006 at a tertiary referral hospital in southern Taiwan. Their medical records were reviewed and clinical data were collected. Results: The study group included 17 males and 16 females. They were divided into two groups: group I (≤ 17 years; n= 14) and group II (> 17 years; n= 19). Only one patient (7.1%) in group I had a history of travel, compared with 47.4% (n = 9) in group II (p= 0.045). Patients in group I also had less chills (group I vs. group II = 7.1% vs. 57.9%, p = 0.004). A higher proportion of group I patients had respiratory problems (cough, group I vs. group II = 57.1% vs. 26.3%) and a higher proportion of group II patients developed complications (group I vs. group II = 14.3% vs. 36.8%), but the differences were not significant. Conclusion: Pediatric patients with typhoid fever in southern Taiwan had less history of travel and developed fewer chills than adult patients. Careful examination and familiarity with the clinical manifestations are important factors in the early diagnosis of typhoid fever