Photon upconversion constitutes an exceptionally rich area of research in photonics and electronics, where low-energy light is converted to high-energy light through nonlinear processes represented by two-photon absorption (TPA) and triplet−triplet annihilation (TTA). Here, we report a cascade process of TPA in inorganic perovskite quantum dots (PQDs) of CsPbBr 3 and TTA in an organic molecule (9,10-diphenylanthracene) mediated by an octaethylporphyrinatoplatinum(II) (PtOEP) sensitizer. This sequential energy transfer enables upconversion from four photons from a near-infrared femtosecond laser at 800 nm to one photon at 430 nm with a large anti-Stokes shift of ∼1.3 eV. We characterize the energy transfer from PQDs to PtOEP by picosecond lifetime spectroscopy and a Stern−Volmer plot of the steady-state photoluminescence while considering dynamic and static quenching as well as trivial absorption and Forster (fluorescence) resonance energy transfer. The serial connection of TPA and TTA achieved in a simple system opens up an attractive avenue in nonlinear photonics and harvesting of low-energy photons.

Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration.

Powder electroluminescent (EL) devices with an electric field type excitation are surface light sources that are expected to have a wide range of practical applications, owing to their high environmental resistance; however, their low luminance has hindered their use. A clarification of the relationship between the properties of the film substrates and the electroluminescence is important to drastically improve light extraction efficiency. In this study, powder EL devices with different substrates of various levels of surface roughness and different optical transmittances were fabricated to quantitatively evaluate the relationships between the substrate properties and the device characteristics. A decrease in the surface roughness of the substrate caused a clear increase in both the current density and the luminance. The luminance was found to have a direct relationship with the optical transmittance of the substrates. The powder EL device, which was based on a cellulose nanofiber film and was the smoothest and most transparent substrate investigated, showed the highest luminance (641 cd/cm2) when 300 V was applied at 1 kHz.

This study reports that the nanoscale interfacial elastodynamics between cellulose nanofibers dynamically modulate the macroscopic thermal diffusivity.

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