Relaxation time effects on dynamic conductivity of alloyed metallic thin films in the infrared band

The behavior of nanoscale infrared antenna elements depends upon the dynamic conductivity of thin metallic films. Spectroscopic ellipsometer measurements of noble metal films show that when the product of the incident radiation frequency and the relaxation time is greater than unity, anomalous dynamic electron transport effects occur. In this regime electron scattering increases the conductivity of alloyed metallic films as demonstrated by ellipsometry measurements of films from the Au-Cu system. A binary alloy thin film was fabricated with equal parts of Au and Cu, and the dynamic conductivity was measured to be 300% larger than the high frequency conductivity of pure Au or pure Cu films at wavelengths in the 3-5 mu m band. When electronic scattering is reduced, ellipsometer measurements of Au and Cu films taken near 4 K demonstrate that the IR conductivity decreases to 20% of the value measured at 300 K at wavelengths in the 3-5 mu m band. Using measured dc relaxation times, a model to explain deviations from Drude behavior was developed using the theory of the anomalous skin effect and frequency dependent relaxation time. This model was in quantitative agreement with the measured data. The ability to design an alloyed metallic thin film using a calculated ideal dc relaxation time to produce the greatest possible dynamic conductivity for infrared antennas and metamaterials was demonstrated

Altering infrared metamaterial performance through metal resonance damping

Infrared metamaterial design is a rapidly developing field and there are increasing demands for effective optimization and tuning techniques. One approach to tuning is to alter the material properties of the metals making up the resonant metamaterial to purposefully introduce resonance frequency and bandwidth damping. Damping in the infrared portion of the spectrum is unique for metamaterials because the frequency is on the order of the inverse of the relaxation time for most noble metals. Metals with small relaxation times exhibit less resonance frequency damping over a greater portion of the infrared than metals with a longer relaxation time and, subsequently, larger dc conductivity. This leads to the unexpected condition where it is possible to select a metal that simultaneously increases a metamaterial\u27s bandwidth and resonance frequency without altering the geometry of the structure. Starting with the classical microwave equation for thin-film resistors, a practical equivalent-circuit model is developed predicting the sensitivity of infrared metamaterials to complex film impedance. Several full-wave electromagnetic models are developed to validate the resonant-circuit model, and excellent agreement is demonstrated between modeled and measured results

Statistical Methods for Detecting Ichthyoplankton Density Patterns that Influence Entrainment Mortality

Samples of drifting American shad eggs were collected at two transects in the Savannah River near industrial water intakes. At each transect the river was divided into four sectors that were sampled at two hour intervals over a 24 hour period. The actual risk of entrainment was approximately 35-50% lower that if the shad eggs were uniformly distributed, and the risk of entrainment was lower at one intake than the other

Fabrication of periodic microstructures on flexible polyimide membranes

Periodic metallic microstructures were fabricated on polyimide membranes. Techniques were developed to maintain flatness of the membrane during processing while still allowing for flexibility in the final structure. For proper functionality of the structures, it was necessary to first fabricate a continuous metallic film and a continuous dielectric layer on top of the flexible substrate, which underlaid the periodic microstructure. Flexibility of the overall structure was maintained by using a polymer as the dielectric layer, which was constrained to have high optical transmission over the infrared wavelength range of 6-14 mu m. Three candidate polymers were evaluated, and their measured optical properties are presented. Benzocyclobutene was found to be the best choice for this application. The final structure fully populated a 10 cm (4 in.) diameter flexible membrane with microstructures of excellent uniformity. (C) 2007 American Vacuum Society

Releasable infrared metamaterials

Infrared metamaterial arrays containing Au elements immersed in a medium of benzocyclobutene (BCB) were fabricated and selectively etched to produce small square flakes with edge dimensions of approximately 20 mu m. Two unit-cell designs were fabricated successfully: one employed crossed-dipole elements while the other utilized square-loop elements (the latter design was symmetric about a Cr ground plane). Infrared spectral reflectivity measurements from collected flakes were compared to infinite-surface simulations in Ansoft HFSS and spectral reflectance measurements of full array samples. Good agreement was found between modeled and experimental resonant behavior when taking into account the proportionality of flakes filling the input aperture of the measurement apparatus

Use of a Natural Isotopic Signature in Otoliths to Evaluate Scale-Based Age Determination for American Shad

We used delta O-18 signatures in otoliths as a natural tag for hatch year to evaluate the scale-based age determination method used for adult American shad Alosa sapidissima in the York River, Virginia. Juveniles of the 2002 year-class exhibited high delta O-18 values in otolith cores that identified adult members of the cohort as they returned to spawn. Recruitment of the 2002 cohort was monitored for three consecutive years, identifying age-4, age-5, and age-6 individuals of the York River stock. The scale-based age determination method was not suitable for aging age-4, age-5, or age-6 American shad in the York River. On average, 50% of the individuals from the 2002 year-class were aged incorrectly using the scale-based method. These results suggest that the standard age determination method used for American shad is not applicable to the York River stock. Scientists and managers should use caution when applying scale-based age estimates to stock assessments for American shad in the York River and throughout their range, as the applicability of the scale-based method likely varies for each stock. This study highlights a promising new direction for otolith geochemistry to provide cohort-specific markers, and it identifies several factors that should be considered when applying the technique in the future