Ivan Biaggio

Rubrene single crystals are seen as a “fruit-fly” benchmark material for organic semiconductors. The article reviews several investigations and discoveries that focused on rubrene and on the development of a physical understanding of its properties.

This corrects the article DOI: 10.1103/PhysRevLett.133.056901.

We analyze quantum interference in the triplet-exciton pair generated by singlet exciton fission in a molecular crystal and introduce transport-induced dephasing (TID) as a key effect that can suppress the expected fluorescence quantum beats when the triplet-exciton wave function can localize on inequivalent sites. TID depends on the triplet-exciton hopping rate between inequivalent sites and on the energy shifts among the stationary states of the entangled triplet pair in different spatial configurations. The theoretical model is confirmed by experiments in rubrene single crystals, where triplet pairs remain entangled for more than 50 ns but quantum beats are suppressed by TID within a few nanoseconds when the magnetic field is misaligned by just a few degrees from specific symmetric directions. Our experiments deliver the zero-field parameters for the rubrene molecule in its orthorhombic lattice and information on triplet-exciton transport, in particular, the triplet-exciton hopping rate between inequivalent sites, which we evaluate to be of the order of 150 ps in rubrene.

Small organic molecules with a π-conjugated system that consists of only a few double or triple bonds can have significantly smaller optical excitation energies when equipped with donor- and acceptor groups, which raises the quantum limits to the molecular polarizabilities. As a consequence, third-order nonlinear optical polarizabilities become orders of magnitude larger than those of molecules of similar size without donor-acceptor substitution. This enables strong third-order nonlinear optical effects (as high as 1000 times those of silica glass) in dense, amorphous monolithic assemblies. These properties, accompanied by the possibility of deposition from the vapor phase and of electric-field poling at higher temperatures, make the resulting materials competitive towards adding an active nonlinear optical or electro-optic functionality to state-of-the-art integrated photonics platforms.

We introduce electrically poled small molecule assemblies that can serve as the active electro-optic material in nanoscale guided-wave circuits such as those of the silicon photonics platform. These monolithic organic materials can be vacuum-deposited to homogeneously fill nanometer-size integrated-optics structures, and electrically poled at higher temperatures to impart an orientational non-centrosymmetric order that remains stable at room temperature. An initial demonstration using the DDMEBT molecule and corona poling delivered a material with the required high optical quality, an effective glass transition temperature of the order of similar to 80 degrees C, and an electro-optic coefficient of 20 pm/V. (C) 2022 Optica Publishing Group

Appropriately designed dual-core fibers using coupling-length phase matching (CLPM) allow for phase-matched frequency downconversion over wide frequency intervals using the third-order optical nonlinearity of glass. By tuning the distance between the two cores, CLPM allows continuously tunable phase matching for widely different wavelengths for the process in which a pump wave at a frequency ω 2 generates or amplifies two waves with frequencies ω 3 > ω 2 and ω 1 = 2 ω 2 − ω 3 . The intensity-dependent correction that accounts for nonlinear phase modulation is derived in general. In addition, a specific CLPM configuration is found to be insensitive to phase modulation, and can achieve 100% theoretical quantum yield for pump wave injection in one core. Fiber-based frequency converters can thus be designed for large differences between pump wavelengths and generated wavelengths, with the phase-matched interaction enabling the use of meter-long fibers to compensate for low third-order susceptibilities. Examples of fiber designs for pump wavelengths at 1.3 and 1.55 µ m , to generate radiation with wavelengths longer than 2 µ m , are discussed for silica and fluoride fibers.

The transport mechanisms leading to spatial separation of triplet pairs after singlet exciton fission can be investigated using the photons emitted if the triplet excitons meet again. Dimensionality of transport can strongly affect fission efficiency.

Small organic molecules with a π‐conjugated system that consists of only a few double or triple bonds can have significantly smaller optical excitation energies when equipped with donor and acceptor groups, which raises the quantum limits to the molecular polarizabilities. As a consequence, third‐order nonlinear optical polarizabilities become orders of magnitude larger than those of molecules of similar size without donor‐acceptor substitution. This enables strong third‐order nonlinear optical effects (as high as 1000 times those of silica glass) in dense, amorphous monolithic assemblies. These properties, accompanied by the possibility of deposition from the vapor phase and of electric‐field poling at higher temperatures, make the resulting materials competitive towards adding an active nonlinear optical or electro‐optic functionality to state‐of‐the‐art integrated photonics platforms.

The geminate annihilation of two triplet excitons created by singlet exciton fission is affected by the dimensionality of transport as determined by typically anisotropic triplet exciton mobilities in organic molecular crystals. We analyze this process using a random-walk model where the time dynamics of the geminate annihilation probability is determined by the average exciton hopping times along the crystallographic directions. The model is then applied to the geminate fluorescence dynamics in rubrene, where the main channel for triplet-triplet annihilation is via triplet fusion and subsequent photon emission, and we identify the transitions between transport in one, two, and three dimensions.