Articles written in Pramana – Journal of Physics

    • Plasmonic properties of gold-coated nanoporous anodic alumina with linearly organized pores

      Dheeraj Pratap P Mandal S Anantha Ramakrishna

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      Anodization of aluminium surfaces containing linearly oriented scratches leads to the formation of nanoporous anodic alumina (NAA) with the nanopores arranged preferentially along the scratch marks. NAA, when coated with a thin gold film, support plasmonic resonances. Dark-field spectroscopy revealed that gold-coated NAA with such linearly arranged pores shows a polarization-dependent scattering, that is larger when the incident light is polarized parallel to the scratch direction than when polarized perpendicular to the scratch direction. Fluorescence studies from rhodamine-6G (R6G) molecules dissolved in polymethylmethacrylate (PMMA) and deposited on these NAA templates showed that fluorescence can be strongly enhanced with the bare NAA due to multiple light scattering in the NAA, while fluorescence from the molecules deposited on gold-coated NAA is strongly quenched due to the strong plasmonic coupling.

    • Thin nanoporous anodic alumina film on aluminium for passive radiative cooling


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      We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98% within the solar spectrum (0.4–2.2 $\mu$m) and simultaneously exhibits a high average emissivity of 88% within the atmospheric infrared (IR) transmission window of 8–13 $\mu$m with the peak IR emission approaching 99% at a wavelength of 10 $\mu$m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $\mu$m. Further, we estimate the average cooling power of NAA/Al to be about 136 W m$^{−2}$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.

    • Nanoporous alumina microtubes for metamaterial and plasmonic applications


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      Double anodisation of aluminium microwires in an acid bath yields cylindrical nanoporous alumina with non-branching radially emanating pores. The obtained nanoporous alumina is a cylindrically anisotropic as well as a radially inhomogeneous optical medium. Detailed structural characterisation reveals that the nanopore diameter varies linearly with the radius of the aluminium microwire along the radial direction. Microcracks form on the alumina shell during the anodisation when sufficient thickness is formed due to volume expansion and stress accumulation. The formation of the microcracks can be monitored by the anodisation current which shows sudden jumps when the cracks are formed. After removing the remaining aluminium at the core of the anodised wire, the anisotropic and inhomogeneous alumina microtube is obtained. Such nanoporous alumina microtubes form unique optical waveguides and are useful for microscale heat transfer applications.

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