arXiv subjects

Nithin Jo Varghese

Publications and source records attributed to Nithin Jo Varghese.

5 recordsLinked to original sources

arXiv2026

A Disordered Photonic Medium Enabling Ultrabroadband Light Scattering and Selective Longwave Infrared Emission

A surface that selectively emits heat in the long-wave infrared (LWIR) can enable passive cooling in hot environments while retaining partial radiative insulation in cold conditions. However, its cost-effectiveness, practical deployment, and fundamentally, the optical design, remain limited by the reliance on metal reflectors. To overcome this limitation, here we use an absorption-scattering competition factor to establish design guidelines for enhancing reflection or absorption in disordered media across the ultrabroadband ultraviolet-to-far-infrared range. Based on electromagnetic simulations and the optical constants of real materials, we then propose disordered photonic media with a layered, multiscale scattering architecture which, unlike typical scattering designs, simultaneously attains ultraviolet-to-far-infrared reflection and selective LWIR emission. We validate this approach by developing a metal-free selective emitter that exhibits high LWIR emittance (0.88), strong solar reflectance (0.97), and low thermal emittance outside the LWIR (0.49), independent of substrates. Field tests, supported by theoretical modelling, show both enhanced radiative cooling and seasonal thermoregulation performance relative to a state-of-the-art broadband radiative cooler. By expanding the spectral functionality of disordered scattering media as a scalable and low-cost optical materials platform across the solar-to-thermal infrared waveband, this work provides a pathway towards improved energy savings and thermal comfort through passive radiative thermal management.

physics.optics↗

arXiv2025

When Energy-Efficiency May Not Yield Positive Climate Impact -- The Case of Adaptive Radiative Coolers

The climate impact of building envelopes is often quantified using their energy savings and CO2 emission reduction benefits. However, building envelopes also trap solar and thermal infrared heat, which is dissipated as a direct heating penalty into our warming planet. For static or adaptive envelopes that passively heat buildings by radiatively retaining heat, these two effects are antagonistic. Yet, their net effect remains unexplored. In this study, we compare the emission reductions benefit, and direct heating penalty of two classes of roof envelopes, traditional and adaptive radiative coolers (TRCs and ARCs). Calculations for buildings in different urban climates show that relative to TRCs like cool roofs, ARCs like smart roofs may have a net heating impact on earth well past this century. Thus, despite their relative energy savings and CO2 emissions reductions benefits, adaptive envelopes on roofs have a negative climate impact relative to traditional cooling designs. Our findings are generalizable across climates and a range of building envelopes and call for a rethinking of how sustainability is quantified for building envelopes, and of material and architectural design for buildings.

physics.app-ph↗

arXiv2025

Radiative Cooling and Thermoregulation of Vertical Facades with Micropatterned Directional Emitters

We demonstrate a micropatterned directional emitter (μDE) with an ultrabroadband, azimuthally selective and tailorable emittance across the thermal wavelengths and over wide angles. The μDE can enable a novel and passive seasonal thermoregulation of buildings by reducing summertime terrestrial radiative heat gain, and wintertime loss. We show several types of μDE, such as metallic, white and transparent variants, made using low-cost materials and scalable manufacturing techniques that are already in large-scale use. Furthermore, we show that its directional emittance can be geometrically tailored to sky-view factors in different urban scenarios. Outdoor experiments show that μDEs stay 1.53-3.26°C cooler than traditional omnidirectional building envelopes in warm weather, including when they are sunlit. In cold weather, μDEs can be up to 0.46°C warmer. Additionally, μDEs demonstrate significant cooling powers of up to 40 Wm-2 in warm conditions and heating powers of up to 30 Wm-2 in cool conditions, relative to typical building envelopes. Building energy models show that μDEs can achieve all-season energy savings similar to or higher than those of cool roofs. Collectively, our findings show μDEs as highly promising for thermoregulating buildings.

physics.app-ph↗

arXiv2023

Semi-analytical technique for the design of disordered coatings with tailored optical properties

Disordered media coatings are finding increasing use in applications such as day-time radiative cooling paints and solar thermal absorber plate coatings which require tailored optical properties over a broad spectrum ranging from visible to far-IR wavelengths. Both monodisperse and polydisperse configurations with thickness of coatings up to 500 micrometer are currently being explored for use in these applications. In such cases it becomes increasingly important to explore utility of analytical and semi-analytical methods for design of such coatings to help reduce the computational cost and time for design. While well-known analytical methods such as Kubelka-Munk and four-flux theory have previously been used for analysis of disordered coatings, analysis of their utility has so far in literature been restricted to either solar spectrum or IR but not simultaneously over the combined spectrum as required for the above applications. In this work, we have analysed the applicability of these two analytical methods for such coatings over the entire wavelength range from visible to IR, and based on observed deviation from exact numerical simulation we propose a semi-analytical technique to aid in the design of these coatings with significant computational cost savings.

physics.optics↗

arXiv2022

Achieving passive daytime radiative cooling via TiO$_2$/PDMS coating

The exponential growth in population and the increasing global temperature trickles down to an explosive demand in cooling and refrigeration. The vicious cycle of carbon footprint generation by these cooling devices can be broken by a mechanism of passive cooling. This study outlines research undertaken with the aim to design such materials - exhibiting high reflectance in the solar spectrum and high emission in the atmospheric transparency window of 8-13 $μ$m. The Monte Carlo (MC) method is used to simulate light propagation in a composite material aiding the design of metamaterials with these specific thermo-optical properties. A TiO$_2$/PDMS coating is fabricated to obtain > 91 \% solar reflectivity and > 75 \% emissivity in the atmospheric transparency window. This translates to cooling the coated body by 4-9 $^\circ$C below the ambient under peak solar irradiation in Mumbai, India. The facile fabrication process supplemented with the potential versatility of this coating shows promise to attain passive daytime radiative cooling on a commercial scale.

physics.app-ph↗