We show that an array of semiconductor quantum dots in an organic host leads to a large two-photon absorption. The optical nonlinearity depends on the semiconductor, the dot size, and the dot-to-dot spacing. Using numerical simulations, we demonstrate that a large optical limiting is possible using thin films of this hybrid material.
We calculate the imaginary partofthethird order optical non-linearity for an array of semiconductor quantum dots in an organic host and show that it leads to large two-photon absorption. The calculated two-photon absorption is greater than currently measured materials. The large non-linearity results from a hybrid exciton formed in the inorganic-organic medium. The band gap of the semiconductor dot determines the spectral region of the resonances that vary from the visible to the near, mid and far infrared regions. We show that relatively small changes in the ratio ofthe quantum dot size to the quantum dot-to-dot spacing result in significant changes in the non-linearity. We briefly describe applications in communications, optical filters, and bio photonics for thin films comprising these hybrid excitons.
Dissipation of heat efficiently from a hot object via radiation while minimizing the inward heat conduction is the key requirement of radiation thermal protection. In this study, a Ca‐Cr co‐doped Y3NbO7 coating with lamellar porous structure is fabricated, which shows an ultra‐low thermal conductivity (<0.7 W m−1 K−1) and near‐unity emissivity (>0.9) across a broad wavelength range of ≈1–24 µm. This record high emissivity to thermal conductivity ratio (≈1.3) is experimentally and theoretically revealed from a multi‐scale perspective. The diffusoin‐mediated thermal conduction feature of niobates combined with lamellar porous structure of the coating reduces its thermal conductivity to an impressive 0.5 W m−1 K−1 at 25 °C, surpassing the theoretical amorphous limitation of 0.72 W m−1 K−1. Experiments and FDTD calculation results demonstrate that the intrinsic emissivity dips at shallow extinction wavelengths (1 and 8 µm) and strong phonon‐polariton resonances wavelengths (>13 µm) can be effectively compensated by the multiple scattering/absorption and gradual modulation of conical shape/effective refractive index induced by surface micro‐protrusion structures, respectively. Furthermore, the coating exhibits robust mechanical and thermal stability with a high bonding strength (18.3 MPa) and thermal expansion coefficient (≈11 × 10−6 K−1 at 1200 °C) comparable to YSZ, showing great potential in the radiation thermal protection field.
Many chromophores with a large two-photon absorptive cross section are hybrid materials where the two-photon absorption (TPA) is coupled to an excited-state absorption (ESA). We develop a numerical technique to investigate hybrid two-photon processes in nonlinear absorbers. Our numerical method compares well with published results. In addition to customary calculation of the transmission curve, we demonstrate the importance of the ESA following the TPA, which may cause significant temporal and radial distortion. We also show that improvements in the transmission can result in significant radial and temporal pulse distortion, which may actually reduce the material effect.
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