Cyclotriphosphazene-based organic frameworks as third-order nonlinear optical materials

Abstract

A cyclotriphosphazene (CTP) core as a new NLOphore has been introduced to construct imine (CTP-TAPB) and vinylene-linked (CTP-TCPB) frameworks. The open aperture (OA) femtosecond (fs) Z-scan measurements resulted in the NLO absorption coefficients (β) of −0.023 and −0.019 cm GW⁻¹, whereas the closed aperture (CA) Z-scan transmission measurements yielded a nonlinear refractive index (n₂) of −2.12 × 10⁻⁷ and −1.62 × 10⁻⁷ cm² GW⁻¹ for CTP-TAPB and CTP-TCPB, respectively. This resulted in third-order nonlinear susceptibility (χ⁽³⁾) of −9.92 × 10⁻⁸ and −7.58 × 10⁻⁸ esu for CTP-TAPB and CTP-TPCB. The measured values for β and n₂ (or χ⁽³⁾) are attributed to the pure electronic excitations owing to the ultrashort timescales (fs) and it is ensured that the contribution from thermo-optic nonlinearities is negligible.