Norman Nan Shi

Correlated Perovskites as a New Platform for Super-Broadband-Tunable Photonics

We report strong and non-volatile optical modulation utilizing electron-doping induced phase change of a perovskite, SmNiO<inf>3</inf>. Broadband modulation (λ=400nm–17μm) is demonstrated using thin-film SmNiO<inf>3</inf>, and narrowband modulation is realized with metasurfaces integrated with SmNiO<inf>3</inf>.

Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling.

Painting on the cool Passive radiative cooling materials emit heat. They can reduce the need for air conditioning by providing daytime cooling but are often challenging to apply to rooftops and other building surfaces. Mandal et al. fabricated porous poly(vinylidene fluoride-co-hexafluoropropene) to create an excellent radiative cooling material. Better yet, the polymer is easy to paint or spray onto a wide range of surfaces, has good durability, and can even be dyed. This makes it a promising candidate for widespread use as a high-performance passive radiative cooling material. Science, this issue p. 315 Tuning the air-filled void distribution in a polymer can produce a film with excellent radiative cooling properties. Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space. Current PDRC designs are promising alternatives to electrical cooling but are either inefficient or have limited applicability. We present a simple, inexpensive, and scalable phase inversion–based method for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)HP] coatings with excellent PDRC capability. High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave infrared emittances (0.97 ± 0.02) allow for subambient temperature drops of ~6°C and cooling powers of ~96 watts per square meter (W m−2) under solar intensities of 890 and 750 W m−2, respectively. The performance equals or surpasses those of state-of-the-art PDRC designs, and the technique offers a paint-like simplicity.

Nanostructured fibers as a versatile photonic platform: radiative cooling and waveguiding through transverse Anderson localization

Broadband high reflectance in nature is often the result of randomly, three-dimensionally structured materials. This study explores unique optical properties associated with one-dimensional nanostructures discovered in silk cocoon fibers of the comet moth, Argema mittrei. The fibers are populated with a high density of air voids randomly distributed across the fiber cross-section but are invariant along the fiber. These filamentary air voids strongly scatter light in the solar spectrum. A single silk fiber measuring ~50 μm thick can reflect 66% of incoming solar radiation, and this, together with the fibers’ high emissivity of 0.88 in the mid-infrared range, allows the cocoon to act as an efficient radiative-cooling device. Drawing inspiration from these natural radiative-cooling fibers, biomimetic nanostructured fibers based on both regenerated silk fibroin and polyvinylidene difluoride are fabricated through wet spinning. Optical characterization shows that these fibers exhibit exceptional optical properties for radiative-cooling applications: nanostructured regenerated silk fibers provide a solar reflectivity of 0.73 and a thermal emissivity of 0.90, and nanostructured polyvinylidene difluoride fibers provide a solar reflectivity of 0.93 and a thermal emissivity of 0.91. The filamentary air voids lead to highly directional scattering, giving the fibers a highly reflective sheen, but more interestingly, they enable guided optical modes to propagate along the fibers through transverse Anderson localization. This discovery opens up the possibility of using wild silkmoth fibers as a biocompatible and bioresorbable material for optical signal and image transport.Optical fibres: Unique properties woven in silkSilk cocoon fibres of the comet moth Argema mittrei could be used to create new materials for transporting optical signals and images, possibly serving as biocompatible and resorbable light guides for medical applications. Researchers led by Nanfang Yu at Columbia University in New York, explored the unique optical properties of the silk. The fibres have many filamentary air voids arranged in a dense array that effectively scatters sunlight. This allows them to act as natural cooling devices, protecting the moth pupae from temperature fluctuations. The researchers were inspired to make synthetic fibres mimicking the natural ones. In addition to the useful radiative cooling effect, the fibres guide light in a manner that may allow efficient transmission of signals and images. Applications in optical therapies, medical diagnostics and tissue engineering should be explored.

Active metasurface devices based on correlated perovskites

There has been persistent exploration of new active materials and novel device architectures to dynamically control light with larger modulation depth and increased spectral range, at faster speed, and using less power. In this presentation, I will present experimental results showing that samarium nickelate (SmNiO3), a prototypical phase-change perovskite nickelate, exhibits reversible large refractive index changes over an ultra-broad spectral range, from the visible to the long-wavelength mid-infrared (λ = 400 nm-17 μm). The super broadband performance is due to strong electron correlation effects that allow extraordinarily large bandgap tuning of the order of 3 eV, and this new mechanism can be exploited to create active photonic devices.

Correlated perovskites as a new platform for super broadband tunable photonics

We report strong and non-volatile optical modulation utilizing electron-doping induced phase change of a perovskite, SmNiO 3 . Broadband modulation (λ=400nm–17μm) is demonstrated using thin-film SmNiO 3 , and narrowband modulation is realized with metasurfaces integrated with SmNiO 3 .