• S ANANTHA RAMAKRISHNA

      Articles written in Pramana – Journal of Physics

    • Estimation of light transport parameters in biological media using coherent backscattering

      S Anantha Ramakrishna K Divakara Rao

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      The suitability of using the angular peak shape of the coherent backscattered light for estimating the light transport parameters of biological media has been investigated. Milk and methylene blue doped milk were used as tissue phantoms for the measurements carried out with a He-Ne laser (632.8 nm). Results indicate that while the technique accurately estimates the transport length, it can determine the absorption coefficient only when the absorption is moderately high (α>1 cm−1) for the long transport lengths typical of tissues. Further, the possibility of determining the anisotropy factor by estimating the single scattering contribution to the diffuse background is examined.

    • Photon transport in thin disordered slabs

      Venkatesh Gopal S Anantha Ramakrishna AK Sood N Kumar

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      We examine using Monte Carlo simulations, photon transport in optically ‘thin’ slabs whose thickness L is only a few times the transport mean free path l*, with particles of different scattering anisotropies. The confined geometry causes an auto-selection of only photons with looping paths to remain within the slab. The results of the Monte Carlo simulations are borne out by our analytical treatment that incorporates directional persistence by the use of the Ornstein-Uhlenbeck process, which interpolates between the short time ballistic and long time diffusive regimes.

    • Cloaks for suppression or enhancement of scattering of diffuse photon density waves

      LALRUATFELA RENTHLEI S ANANTHA RAMAKRISHNA HARSHAWARDHAN WANARE

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      Enhancement of wave-like characteristics of heavily damped diffuse photon density waves in a random medium by amplification can induce strongly localised resonances. These resonances can be used to either suppress or enhance scattering from an inhomogeneity in the random medium by cloaking the inhomogeneous region by a shell of random medium with the correct levels of absorption or amplification. A spherical core–shell structure consisting of a shell of a random amplifying medium is shown to enhance or suppress specific resonant modes. A shell with an absorbing random medium is also shown to suppress scattering which can also be used for cloaking the core region.

    • Alcohol sensor based on gold-coated nanoporous anodic alumina membrane

      SALEEM SHAIK ANJANI KUMAR TIWARI S ANANTHA RAMAKRISHNA

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      We report a refractive index sensor consisting of a gold-coated nanoporous anodic alumina membrane on aluminium substrate that can distinguish between different kinds of alcohols such as methanol and ethanol due totheir different refractive indices. The nanoporous volume allows the loading of liquids with low surface energy into its nanopores. Upon dipping one end of the membrane into the alcohol, the entire nanoporous surface experiences wetting. The wavelength shift of the Fabry–Perot resonating modes formed between the gold-coated nanoporous alumina surface and aluminium on the other side due to the changed effective refractive index form the basis of the sensor. The sensitivity of the nanosensor to the refractive index of the loaded liquid is sufficient to distinguish between different alcohols such as methanol, ethanol and isopropanol, and to detect about 5–10% of methanol in a methanol–ethanol mixture.

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

      JITENDRA K PRADHAN DHEERAJ PRATAP S ANANTHA RAMAKRISHNA

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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.

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