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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 wavefunction 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., Comment: 5 pages, 4 figures

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 $\omega_2$ generates or amplifies two waves with frequencies $\omega_3>\omega_2$ and $\omega_1 = 2 \omega_2 - \omega_3$. 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 $\mu$m, to generate radiation with wavelengths longer than 2 $\mu$m, are discussed for silica and fluoride fibers., Comment: 13 pages, 10 figures

We introduce electrically-poled small molecule assemblies that can serve as the active electro-optic material in nano-scale 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 $\sim 80$ $^\circ$C, and an electro-optic coefficient of $20$~pm/V., Comment: 3 pages, 2 figures

Small organic molecules with a {\pi}-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., Comment: 11 pages, 5 figures

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., Comment: 5 pages, 2 figures

We observe quantum beats in the nanosecond-scale photoluminescence decay of rubrene single crystals after photoexcitation with short laser pulses in a magnetic field of 0.1 to 0.3 T. The relative amplitude of the quantum beats is of the order of 5\%. Their frequency is $1.3$ GHz when the magnetic field is oriented parallel to the two-fold rotation axis of the rubrene molecules and decreases to $0.6$ GHz when the magnetic field is rotated to the crystal's molecular stacking direction. The amplitude of the quantum beats decays alongside the non-oscillatory photoluminescence background, which at low excitation densities has an exponential decay time of $ 4.0 \pm 0.2$~ns. We interpret this as the effective lifetime of a multiexciton state that originates from singlet-fission and can undergo geminate recombination back to the singlet state., Comment: 4 pages, 4 figures

Chalcogenide glass-silver heterostructures are candidates for photoresist and diffractive optical applications. To optimize their processing, we report the kinetics of Ag photo-dissolution in As2S3 matrix using in-situ optical transmission/reflection measurements and real time atomic force microscopy (AFM) imaging under optical illumination. The results indicate that photodissolution occurs in three stages with the extent and kinetics of each stage depending strongly on Ag film thickness. By contrast, the photo-dissolution is found to be independent of As2S3 matrix thickness. The extent of three stages also depends strongly on the laser dose and can be reduced to two stages at higher laser fluence. A comparative study of two oppositely stacked sample configurations: As2S3/Ag/glass and Ag/As2S3/glass show that the heterostructures respond differently to light illumination. For the former, Ag dissolves completely into As2S3 matrix at a faster rate than for the latter case. The origin of this difference is established by energy dispersive X-ray spectroscopy and AFM measurements., Comment: 21 pages, 8 figures, 4 tables, Journal of Non-Crystalline Solids, 2018

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are only created through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime., Comment: 4 pages, 3 figures

This is a comment inspired by recently published results [Y. Zhang et al., Phys. Rev. Lett. 91, 157404 (2003)] that introduced the name "amphoteric refraction" for the fact that at the interface with an optically anisotropic material there can be a range of incidence angles for which the component of the Poynting vector parallel to the interface changes sign upon refraction. The latter effect is a well-known consequence of optical anisotropy, but it was described as a new negative refraction phenomenon that can be put in the same class as the negative refraction observed at an interface with a left-handed material with negative refractive index., Comment: LaTeX/RevTeX, 2 pages, 2 figures