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Investigations of EuGa2S4 have been done on the photoluminescence (PL) related to the transition between 4f65d and 4f7 configuration of the Eu2+ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa2S4 surely has phonon-terminated character.

The Ce emission appearing in the blue–green region was investigated for CaGa2S4 and SrGa2S4. The room temperature quantum efficiency as high as 73% was observed for a 0.4 wt% Ce-doped CaGa2S4 single crystal grown by the melt method. The quantum efficiency decreased for a 0.8 wt% Ce-doped CaGa2S4 crystal and clear photoacoustic signal was detected in this sample, indicating an increase of non-radiative transition. For each of these compounds, ultraviolet (UV) excitation produced a hitherto unreported new UV emission which is thought to originate from 5d(Eg) level of the Ce ion. Temperature dependent intensity changes observed in this UV emission and in the blue–green emission suggest temperature dependent population transfer from the upper level of Eg to the lower level of T2g in the excited state of 5d electrons.

Photoluminescence properties of the stoichiometric compound of EuGa 2 S 4 has been investigated for the first time. The emission appearing in the region of 510–620 nm has a high quantum efficiency value reaching 100% at low temperatures. The characteristics of the emission are similar to those of CaGa 2 S 4 :Eu, and can be considered to be due to phonon-terminated transition of the Eu ion. Based on the measured luminescence data, construction feasibility of unique high gain tunable lasers is discussed.

Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of ∼2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ∼0.2 eV and ∼0.94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.

Significant acceleration of the photoluminescence (PL) decay rate was observed in water solutions of two rare earth ions (REIs), Tb and Eu. We propose that the time-resolved PL spectroscopy data are explained by a fluorescence resonance energy transfer (FRET) between the REIs. FRET was directly confirmed by detecting the induced PL of the energy acceptor, Eu ion, under the PL excitation of the donor ion, Tb, with FRET efficiency reaching 7% in the most saturated solution, where the distance between the unlike REIs is the shortest. Using this as a calibration experiment, a comparable FRET was measured in the mixed solution of REIs with single-wall nanotubes (SWNTs) wrapped with DNA. From the FRET efficiency of 10% and 7% for Tb and Eu, respectively, the characteristic distance between the REI and SWNT/DNA was obtained as 15.9 ± 1.3 A, suggesting that the complexes are formed because of Coulomb attraction between the REI and the ionized phosphate groups of the DNA.

The rubrene molecular crystal has the unique property of showing a strong photoconductivity for light wavelengths that are close to the absorption edge. We studied the microsecond dynamics of the photoconductivity induced by short light pulses to characterize the way in which photoinduced excitons efficiently ionize to produce free charge carriers. We found that the photoconductivity is produced by carriers released after an average time of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ from a ``reservoir state'' that originates from the photoexcited molecular excitons. The conversion of photoexcited excitons into this reservoir state happens only at the surface of the crystal within a depth of the order of a few micrometers, but in this region close to the surface, a photoexcited molecular exciton has a probability of the order of unity to ultimately lead to a mobile charge carrier. This high carrier photoexcitation efficiency leads to a pronounced shortening of the photocurrent rise time for decreasing wavelength or increasing energy of the excitation pulses because of the effect of quadratic recombination of the photocarriers.

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

Excitons in rubrene single crystals dissociate into free charge carriers via two mechanisms whose relative importance depends on the illumination wavelength through the optical penetration depth into the crystal. The first mechanism is defect-induced dissociation in less than 10 ns after photoexcitation. For low photoexcitation densities, about 10% of the excitons that survive radiative recombination dissociate through this channel. The second mechanism, affecting the remaining 90% of the excitons, involves a previously reported state localized close to the surface of the crystal that leads to a delayed release of photocarriers a fraction of a millisecond after photoexcitation.