Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption

We report the fluorescence upconversion properties of a class of improved pyridinium toluene-p-sulfonates having donor–π–acceptor (D–π–A) structure under two-photon excitation at 1064 nm. The experimental results show that both the two-photon excited (TPE) fluorescence lifetime and the two-photon pumped (TPP) energy upconversion efficiency were increased with the enhancement of electron-donating capability of the donor in the molecule. It is also indicated that an overlong alkyl group tends to result in a weakened molecular conjugation, leading to a decreased two-photon absorption (TPA) cross section. By choosing the donor, we can obtain a longest fluorescence lifetime of 837 ps, a highest energy upconversion efficiency of 6.1%, and a maximum TPA cross-section of 8.74×10⁻⁴⁸ cm⁴ s/photon in these dyes.     

Two-photon absorption-induced optical properties of a new lasing dye in two solvents

Pumped by an infrared lasing and its frequency-doubling source, the fluorescence and superradiance emission properties of a new lasing dye trans-4-[4^′-(N,N-diethylamino)styryl]-N-methyl pyridinium methyl sulfate (abbreviated DEASPS), have been studied in benzyl alcohol and chloroform. The two-photon absorption (TPA) and emission properties of DEASPSare influenced by the solvents used. The emission wavelength of the dye in benzyl alcohol is redshifted relative to that in chloroform. The lifetime of two-photon absorption-induced fluorescence is about 529 ps in benzyl alcohol, whereas it is 340 ps in chloroform. Correspondingly, the one-photon-excited fluorescence lifetimes in the two solvents are also given. The upconversion efficiency of DEASPS in chloroform is higher than in benzyl alcohol. Finally, the effective molecular two-photon absorption cross-sections were measured by the nonlinear transmittance method. It is found that the optical limiting effect of the dye in benzyl alcohol is stronger than that in chloroform. Received: 3 May 2001 / Revised version: 6 July 2001 / Published online: 19 September 2001     

Energy transfer and infrared-to-visible upconversion luminescence of Er/Yb-codoped halide modified tellurite glasses

We report on the energy transfer and frequency upconversion spectroscopic properties of Er³⁺-doped and Er³⁺/Yb³⁺-codoped TeO₂-ZnO-Na₂O-PbCl₂ halide modified tellurite glasses upon excitation with 808 and 978 nm laser diode. Three intense emissions centered at around 529, 546 and 657 nm, alongwith a very weak blue emission at 410 nm have clearly been observed for the Er³⁺/Yb³⁺-codoped halide modified tellurite glasses upon excitation at 978 nm and the involved mechanisms are explained. The quadratic dependence of fluorescence on excitation laser power confirms the fact that the two-photon contribute to the infrared to green-red upconversion emissions. And the blue upconversion at 410 nm involved a sequential three-photon absorption process.     

Characterization of ultra-weak fluorescence using picosecond non-collinear optical parametric amplifier

We report a technique for characterization of ultra-weak fluorescence based on a 355-nm pumped picosecond non-collinear optical parametric amplifier (OPA). In the experiment, we effectively reduced the strong super-fluorescence background by using a series of methods. With the picosecond OPA as the pre-amplifier and the gating pulse, the decay of the fluorescence of Rhodamine 6G dye in ethanol was measured and the fluorescence lifetime was found to be about 941 ps. The gain factor of this parametric fluorescence amplifier was measured to be ∼4.2 × 10⁶, while the energy detection limit was ∼160 aJ per pulse within a 15-ps gating pulse.     

Properties of two-photon absorption-induced cavity lasing of an organic dye HEASPI pumped by picosecond infrared laser beam

The first operation of cavity lasing of a two-photon absorption (TPA) organic dye, trans-4-[p-(N-ethyl-N-hydroxyethylamino) styryl]-N-methylpyridinium iodide (HEASPI), by using picosecond infrared laser as pump source is reported in this paper. Temporal profile of cavity lasing shows obvious oscillations, and magnification of the feedback light can be obtained. By using the difference of re-absorption coefficient at different conditions, we explained the blue shift for cavity lasing and superradiance compared with TPA-induced fluorescence. The lifetime of TPA-induced fluorescence is measured to be 134 ps.     

Two-photon induced optical-power limiting and upconverted superradiance properties of a new organic dye HEASPI

A new organic dye trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently. The TPA cross section was measured to be σ₂′=7.0×10⁻⁴⁸ cm⁴ s/photon by using an open aperture Z-scan system. Linear absorption, single-photon induced fluorescence, two-photon induced fluorescence and two-photon pumped (TPP) upconverted superradiance properties were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted superradiance energy is as high as 19.6% at the pump energy of 2.07 mJ from a mode-locked Nd : YAG picosecond laser. The dye solution also shows a clear optical power limiting effect.     

Upconversion fluorescence property of Er/Yb-codoped novel bismuth-germanium glass

Infrared-to-visible upconversion fluorescence property of Er³⁺/Yb³⁺-codoped novel bismuth-germanium glass under 975 nm LD excitation has been studied. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. The structure of the bismuth-germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel bismuth-germanium glass with low maximum phonon energy (∼750 cm⁻¹) can be used as potential host material for upconversion lasers.     

Facile synthesis, optical properties and theoretical calculation of two novel two-photon absorption chromophores

Two heterocycle-based derivatives that can be used as two-photon absorption chromophore, 9-butyl-3-(2, 6-diphenylpyridin-4-yl)-9H-carbazole (BDPYC) and 9-butyl-3-(4-(2, 6-diphenylpyridin-4-yl)styryl)-9H-carbazole (BDPSC) have been successfully synthesized and fully characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR and MS. The molecules possess D-π-A structures, but have different π bridge. The 9-butylcarbazole is used as a donor (D), and the pyridine ring is used as an acceptor (A). One- and two-photon absorption and excited fluorescence properties in various solvents were experimentally investigated. Two-photon initiated optical data recording experiments have been carried out under 740 nm laser radiation, and the possible mechanism of optical data storage is discussed based on theoretical calculations.     

Broadband amplification and upconversion luminescence properties of Er/Yb co-doped fluorophosphate glass

Broadband and upconversion properties were studied in Er³⁺/Yb³⁺ co-doped fluorophosphate glasses. Large Ω₆ and S_ed/(S_ed+S_md) values and the flat gain profile over 1530-1585 nm indicate the good broadband properties of the glass system. And a premise of using Ω₆ as a parameter to estimate the broadband properties of the glasses is proposed for the first time to our knowledge. Results showed that fluorescence intensity, upconversion luminescence intensity, the intensity ratio of red/green light (656 nm/545 nm) are closely related to the Yb³⁺:Er³⁺ ratio and Er³⁺ concentration, and the corresponding calculated lifetime of ⁴F_9/2 and ⁴S_3/2 states for red and green upconversion samples proves this conclusion. The upconversion mechanism is also discussed.     

Absorption and Emission Properties of a New Two-photon Absorption Dye

The emission properties of a newly synthesized organic two-photon absorption dye, trans-4-[p-(N-ethyl-N-ethylamino)-styryl]-N-methyl-pyridinium tetraphenylborate, have been investigated. When pumped by infrared pulses from a picosecond Nd∶YAG laser, the dye exhibits intense upconverted fluorescence and strong superradiance properties. For comparison, the one-photon induced fluorescence and superradiance are also measured. The one- and two-photon excited fluorescent lifetimes are 86 and 64 ps, respectively. The maximum efficiency of the dye is measured to be 1.97% and the upconversion efficiencies at different pump wavelengths have also been investigated by the optical parametric amplifier.     

Absorption and Emission Properties of a New Two-photon Absorption Dye

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Excitation relaxation and structure of TPPS4 J-aggregates

The energy relaxation kinetics and the structure of the J-aggregates of water-soluble porphyrin 5,10,15,20-tetrasulphonatophenyl porphine (TPPS₄) were investigated in aqueous medium by means of time-resolved fluorescence spectroscopy and confocal laser-scanning fluorescence microscopy. The excitation of the J-aggregates, at excitation intensities higher than ∼10¹⁵ photons/cm² per pulse, results in a remarkable decrease of the fluorescence quantum yield and in the appearance of an additional, non-exponential energy relaxation channel with a decay constant that depends on the excitation intensity. This relaxation mechanism was attributed to the exciton single-singlet annihilation. The exciton lifetime in the absence of the annihilation was calculated to be ∼150 ps. Using exciton annihilation theory, the exciton migration within the J-aggregates could be characterized by determining the exciton diffusion constant (1.8±0.9)  10⁻³ cm²/s and the hopping time (1.2±0.6) ps. Using the experimental data, the size of the J-aggregate could be evaluated and was seen to yield at least 20 TPPS₄ molecules per aggregate. It was shown by means of confocal fluorescence laser scanning microscopy that TPPS₄ does self-associate in polyvinyl alcohol (PVA) at acidic pH forming molecular macro-assemblies on a scale of ∼1 μm in PVA matrices.     

Linear and nonlinear optical properties of two-photon absorption dye doped linear copolymer

2-hydroxyethyl methacrylate (abbreviated as HEMA) and acrylonitrile have been used as the monomers to form a trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HMASPS) doped linear copolymer. The spectra of one and two-photon excited fluorescence and two-photon pumped superradiance and two-photon pumped lasing of the polymer all shifted to short wavelengths compared with the solution sample of the dye. The copolymer shows much longer one- and two-photon excited fluorescence lifetimes of nanoseconds. When pumped by the picosecond Nd:YAG laser, the superradiance and lasing can be simultaneously obtained. The maximum two-photon pumped lasing conversion efficiency is 3.3%. The place of the maximum upconversion efficiency of the copolymer does not coincide with that of the maximum nonlinear absorption.     

Thermal stability and spectroscopic properties of Er-doped lead fluorogermanate glasses

The thermal characterization and spectroscopic properties of Er³⁺-doped 0.6GeO₂-(0.4-x)PbO-xPbF₂ glasses were investigated experimentally. With the replacement of PbO by PbF₂ the thermal stability of glasses is improved and the infrared fluorescence intensity at 1530 nm is increased. The Judd-Ofelt intensity parameters, radiative transition rates, and fluorescence lifetimes of the excited ⁴I_13/2 level of Er³⁺ ions were calculated from Judd-Ofelt theory. The asymmetric ligand field around Er³⁺ ions resulted from the incorporation of PbF₂ into germanate glasses, broadens the infrared emission spectra at 1530 nm. Upconversion luminescence in the investigated glasses was observed at room temperature under the excitation of 976 nm laser diode. The glass 0.6GeO₂-0.3PbO-0.1PbF₂ exhibits the maximum upconversion emission intensity, while no frequency upconversion luminescence was observed in the 0.6GeO₂-0.4PbO glass. The quadratic dependence of the green and red emissions on excitation power indicates that two-photon absorption contributes to the visible emission under the 976-nm excitation.     

Upconversion luminescence and optical power limiting effect based on two- and three-photon absorption processes of ZnO crystal

Near-infrared to UV and visible upconversion luminescence was observed in single-crystalline ZnO under an 800 nm infrared femtosecond laser irradiation. The optical properties of the crystal reveal that the UV and VIS emission band are due to the exciton transition (D₀X) bound to neutral donors and the deep luminescent centers in ZnO, respectively. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to three-photon sequential band-to-band excitation and the VIS emission belongs to two-photon simultaneous defect-absorption induced luminescence. A saturation phenomenon and polarization-dependent effect are also observed in the upconversion process of ZnO. A very good optical power limiting performance at 800 nm has been demonstrated. The two- and three-photon absorption coefficients of ZnO crystal were measured to be 0.2018 cm GW⁻¹ and 7.102 × 10⁻³ cm³ GW⁻², respectively. The two- and three-photon cross sections were calculated to be 1.189 × 10⁻⁵¹ cm⁴ s and 1.040 × 10⁻⁸⁰  cm⁶ s², respectively.     

Optical transitions of Dy in tellurite glass: observation of upconversion

Dy³⁺ doped tellurite glasses were prepared and its optical absorption, fluorescence and upconversion have been studied. The absorption data has been used to calculate the Judd-Ofelt parameters, oscillator strengths along with transition probability, branching ratio, etc. The lifetime of the level is found to be 0.66 ms for 1.0 mol% of Dy³⁺ doped in tellurite glass. NIR to visible upconversion has been observed at room temperature. The increase in upconversion luminescence at lower temperature indicates that upconversion is not due to a thermally activated process. The quantum efficiency of the transition is found to be ∼0.17 and the upconversion efficiency is ∼0.022. The strong dependence of the upconversion efficiency on the Dy³⁺ concentration reveals that the upconversion is due to energy transfer interaction.     

Optical properties of Yb-doped fibers and fiber lasers at high temperature

Recent advances in power scaling of Yb^+ 3-doped fiber lasers to the kilowatt level suggest a need to examine the performance of Yb^+ 3-doped silica at temperatures well above ambient. We report experimental results for the absorption coefficient, emission cross-section, fluorescence lifetime, and slope efficiency of a Yb³⁺-doped large mode area (LMA) silica fiber for temperatures spanning 23 °C-977 °C. To the best of our knowledge these are the highest temperatures to date for which these optical properties have been measured. We find a sharp reduction in the energy storing capability and lasing performance of Yb^+ 3:SiO₂ above 500 °C that coincides with the onset of non-radiative transitions in the excited state manifold (thermal quenching). As the temperature increases from room temperature to 977 °C, absorption in the 1020-1120 nm operating band increases monotonically, concurrent with a reduction in absorption at the 920-nm and 977-nm pumping bands. Conversely, the spectral weight of the emission cross-section shifts from transitions above 1010 nm to those below, with the exception of the 977-nm emission band.     

An efficient lasing action from pyrromethene 556 dye-doped organically modified silicates

Organically modified solid-state silicates (ORMOSILS) doped with a new laser dye 1,3,5,7,8-pentamethylpyrromethene-2,6-disulfonate-BF₂ complex (pyrromethene 556) have been synthesized by a sol-gel method and the compositional effects on pore characteristics, fluorescence and lasing properties have been investigated. It is found that the use of dimethylsulfoxide and γ-glycidoxypropyltrimethoxysilane could greatly change the structure properties of sol-gel derived ORMOSILS cage, and thus the fluorescence and lasing properties of the materials could improve significantly. A successful laser oscillation from this dye-doped ORMOSILS sample has been achieved upon pumping with a Q-switched frequency-doubled Nd:YAG laser at 532 nm. A slope efficiency of 54% with a useful lifetime greater than 10,000 shots has been demonstrated at a pump repetition rate of 1 Hz and a pump intensity of 1 J/cm² by using the new ORMOSILS cage on our newly designed laser system. Our results have shown that it is possible to obtain a high-efficiency with a long-lifetime for a compact new laser device by low cost dye-doped solid-state ORMOSILS.     

Multiphoton ultraviolet and visible upconversion luminescence of TmYb co-doped oxyfluoride glass

The ultraviolet upconversion luminescence of Tm³⁺ ions sensitized by Yb³⁺ ions in oxyfluoride glass when excited by a 975 nm diode laser was studied in this paper. One typical ultraviolet upconversion luminescence lines positioned at 362.3 nm was found. It can be attributed to the five-photon upconversion luminescence transition of ¹D₂ → ³H₆. Several visible upconversion luminescence lines at 451.1 nm, (477.9 nm, 462.5 nm), 648.7 nm, (680.5 nm, 699.5 nm) and (777.5 nm, 800.7 nm) were found also, which results from the fluorescence transitions of five-photon ¹D₂ → ³F₄, three-photon ¹G₄ → ³H₆, three-photon ¹G₄ → ³F₄, two-photon ³F₃ → ³H₆ and two-photon ³H₄ → ³H₆ of Tm³⁺ ion, respectively. The theoretical analysis suggests that the upconversion mechanism of the 362.3 nm ¹D₂ → ³H₆ upconversion luminescence is the cross energy transfer of {³H₄(Tm³⁺) → ³F₄(Tm³⁺), ¹G₄(Tm³⁺) → ¹D₂(Tm³⁺)} and {¹G₄(Tm³⁺) → ³F₄(Tm³⁺), ³H₄(Tm³⁺) → ¹D₂(Tm³⁺)} between Tm³⁺ ions. In addition, the upconversion luminescence of ¹G₄ and ³H₄ state results from the sequential energy transfer {²F_5/2(Yb³⁺) → ²F_7/2(Yb³⁺), ³H₄(Tm³⁺) → ¹G₄(Tm³⁺)} and {²F_5/2(Yb³⁺) → ²F_7/2(Yb³⁺), ³F₄(Tm³⁺) → ³F₂(Tm³⁺)} from Yb³⁺ ions to Tm³⁺ions, respectively.     

Structural and upconversion fluorescence properties of Er/Yb-codoped oxychloride lead-germanium-bismuth glass

Structural and infrared-to-visible upconversion fluorescence properties of Er³⁺/Yb³⁺-codoped oxychloride lead-germanium-bismuth glass have been studied. The Raman spectrum investigation indicates that PbCl₂ plays an important role in the formation of glass network, and has an important influence on the upconversion luminescence owing to lower phonon energy. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation.