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

Abstract

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 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 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.

Anomalous carrier transport model for broadband infrared absorption in metals

“…The second, less obvious regime occurs when μ ∈ (0, 1]. In this regime, the anomalous conductivity component (18) vanishes in the low-frequency limit. In this case, the model is appropriately treated with the KKRs for dielectric media (as is necessary when assessing, e.g., the Lorentz oscillator).…”

“…Its popularity stems from the * Corresponding author: ccoimbra@ucsd.edu remarkable accuracy with which it predicts the experimental observations for many common metals despite its simple two-parameter structure. Due to its straightforward classical interpretation, the Drude model is often employed for the analysis of experimental data, as a starting point for the development of models that seek to extend its range of use, and as a baseline metric for the assessment of model performance [12][13][14][15][16][17][18][19][20]. Here we undertake an extension of the Drude model to account for anomalous intraband absorption by metals to broadband infrared radiative forcing.…”

“…To address some of these limitations in the intraband absorption by metals, we develop an anomalous model for the broadband infrared response. Engineering design applications requiring accurate models of the infrared optical response of metals are numerous and far reaching, with use cases ranging from the development of nanoscale infrared antennas [18] to the design of structural IR signal suppression and mimicry technologies [23].…”

Altering infrared metamaterial performance through metal resonance damping

“…Of the two, metal-film relaxation time provides the largest range of variation through the alteration of impurity scattering. 20 It should also be noted that this type of design optimization is limited to frequencies close to the relaxation frequency, or ͑2͒ −1 . For most metals, the relaxation frequency falls in the infrared and terahertz frequency bands.…”

Temperature-dependent carrier transport: Low-complexity model for the infrared optical and radiative properties of nickel

“…This is an important regime for a wide range of applications, ranging from the design of specialized metamaterials for heat signature privacy 2 to the modeling of infrared antennas at nanoscale proportions. 3 The need for thermophysical models is also prevalent, with use cases including heat shielding design, 4 noncontact thermometry, 5 and engine component fabrication. 6 In all but the most idealized scenarios, the Drude model fails to completely account for experimental observations.…”