RESUMO
The gate-controllable complex conductivity of graphene offers unprecedented opportunities for reconfigurable plasmonics at terahertz and mid-infrared frequencies. However, the requirement of a gating electrode close to graphene and the single 'control knob' that this approach offers limits the practical implementation and performance of these devices. Here we report on graphene stacks composed of two or more graphene monolayers separated by electrically thin dielectrics and present a simple and rigorous theoretical framework for their characterization. In a first implementation, two graphene layers gate each other, thereby behaving as a controllable single equivalent layer but without any additional gating structure. Second, we show that adding an additional gate allows independent control of the complex conductivity of each layer within the stack and provides enhanced control on the stack equivalent complex conductivity. These results are very promising for the development of THz and mid-infrared plasmonic devices with enhanced performance and reconfiguration capabilities.
RESUMO
OBJECTIVE: To provide a cost minimization analysis to support a paradigm shift in the use of comprehensive ultrasound and echocardiography in the prenatal diagnosis of congenital heart disease (CHD) in fetuses of women with diabetes mellitus (DM). METHODS: In this retrospective cohort study, the diabetic clinic service database of the University of California San Diego was searched from January 2001 to June 2004 for pregnant women with Type I or II DM and HbA1c >6.3%. Subjects underwent comprehensive ultrasound examination (with four-chamber views and outflow tracts) and fetal echocardiography according to a standard protocol. Newborns were examined for cardiac defects and underwent postnatal echocardiography as indicated. The cost of screening was evaluated. RESULTS: Of 115 neonates and two terminations of pregnancy there were 20 (17%) cases of CHD. Six of these CHD were major and all six were detected prenatally by both ultrasound and echocardiography. Three additional clinically insignificant cases of CHD were identified by fetal echocardiography. Eleven cases of CHD were identified by postnatal echocardiography only, all of which were clinically insignificant lesions. The prenatal detection rate of major CHD was 100% (6/6) for both ultrasound and echocardiography. The sensitivites of ultrasound (30% (95% CI, 13-54%)) and echocardiography (45% (95% CI, 24-68%)) were similar (P = 0.32). A cost minimization analysis was done using the published Medicaid (California) system's relative value unit and conversion factors. Accordingly, our current protocol costs $6503.43 per case of major CHD detected. If echocardiography had been performed only as indicated and postnatal echocardiography had been performed on all neonates, the cost would have been $7056.83 per case of major CHD detected. Alternatively, combined targeted ultrasound with indicated prenatal and postnatal echocardiography would have been associated with a cost of $4996.05 per case of major CHD detected. CONCLUSION: Detection of major CHD was excellent with both comprehensive prenatal ultrasound and echocardiography. Echocardiography added little to the prenatal diagnosis of CHD if the comprehensive ultrasound examination was normal. The highest cost-benefit ratio and most efficacious protocol for screening based on our data would be comprehensive ultrasound with prenatal and postnatal echocardiography only as indicated. Further prospective studies are warranted.
