Efficiency Of Nonradiative Transfer Research Articles

Extended emission wavelength of random dye lasers by exploiting radiative and non-radiative energy transfer

We demonstrate long-wavelength operation (>700 nm) of random dye lasers (using a methylene blue dye) with the addition of rhodamine 6G and titania, enabled by radiative and non-radiative energy transfer. The pump energy is efficiently absorbed and transferred to the acceptors, to support lasing in random dye lasers in the near infrared. The optimum random laser performance with the highest emission intensity and the lowest lasing threshold was achieved for a concentration of methylene blue as the acceptor equal to 6× the concentration of rhodamine 6G (donor). Excessive levels of methylene blue increased the lasing threshold and broadened the methylene blue emission linewidth due to dye quenching from re-absorption. This is due to competition between the donor emission and energy transfer and between absorption loss and fluorescence quenching. The radiative and non-radiative energy transfer is analyzed as a function of the acceptor concentration and pump energy density, with consideration of the spectral overlap. The dependence of the radiative and non-radiative transfer efficiency on the acceptor concentration is obtained, and the energy transfer parameters, including the radiative and non-radiative energy transfer rate constants (K R and K NR), are investigated using Stern–Volmer analysis. The analysis indicates that radiative energy transfer is the dominant energy transfer mechanism in this system.

Investigation of energy transfer from pyrromethene dye to cresyl violet 670 in ethanol

In this paper, radiative and nonradiative energy transfer from laser dye pyrromethene 567 (PM567) and pyrromethene 580 (PM580) as donors to cresyl violet 670 (CV670) as acceptor in ethanol are investigated by using the steady-state emission measurement and the second harmonic generation (532 nm, ∼ 13 ns) of a Q-switched Nd:YAG laser as the pumping source. The fluorescence intensity of the acceptor is improved due to the introduction of the donors, and the largest enhancement is obtained to be 128% in the PM567:CV670 dye mixture system. Energy transfer parameters, including the radiative and nonradiative energy transfer rate constants (KR and KNR), critical distance (R0), and half quenching concentration ([A]1/2) are investigated using the Stern—Volmer plots, and the acceptor concentration dependencies of radiative and nonradiative transfer efficiencies are also obtained. The values of KR for PM567:CV670 and PM580:CV670 systems are 2032.0×109 L·mol−1·s−1 and 2790.4×109 L·mol−1·s−1, respectively, and the values of corresponding KNR are 3.3 × 109 L·mol−1·s−1 and 4.2×109 L·mol−1·s−1, respectively. The results indicate that the dominant mechanism responsible for the energy transfer in the dye mixture systems is of the radiative type.

Investigation of energy transfer between PM567:Rh610 dye mixture in modified poly (methyl methacrylate)

In this paper, solid dye samples were prepared by codoping laser dyes Pyrromethene 567 (PM567) as the energy donor and Rhodamine 610 (Rh610) as the energy acceptor into the ethanol modified poly (methyl methacrylate) matrix (MPMMA) to enhance the properties of the solid dye lasers. The fluorescence intensity of the acceptor was enhanced by up to 9 fold with the introduction of the donor molecules. The laser efficiency of the dye mixture doped samples was improved by up to 8 times relative to that of the samples solely doped with the acceptor, and the highest slope efficiency was obtained as 70.4%. The radiative and nonradiative energy transfer rate constants (KR and KNR) were calculated using the Stern–Volmer plots and the acceptor concentration dependence of the radiative and nonradiative transfer efficiencies were also obtained. The KR was three orders of magnitude higher than the KNR, indicating the dominance of the radiative energy transfer mechanism in the present system. The deviation of the Stern–Volmer plot from the linearity demonstrated that both the dynamic and transient quenching mechanism exist in the present energy transfer system.

Energy transfer studies in binary dye solution mixtures: Acriflavine + Rhodamine 6G and Acriflavine + Rhodamine B

The effect of acceptor concentration on the energy transfer in Acriflavine (donar) plus Rhodamine 6G (Rh 6G) and Acriflavine plus Rhodamine B (Rh B) binary solution mixtures has been studied. The theoretical calculations are done to determine their lifetimes. Effect of these values on the total transfer efficiency at various acceptor concentrations have been studied to identify the appropriate energy transfer mechanism responsible for photon emissions, enhancement in lasing efficiency and dreading of the tenability of such mixed solutions. The energy transfer rate constants and critical transfer radius ( R 0) are calculated using Stern–Volmer plots and concentration dependence of radiative and non-radiative transfer efficiencies have also been determined. The experimental results indicate that dominant mechanism responsible for the efficient energy transfer in the binary mixtures is of non-radiative kind and is due to long-range dipole–dipole interaction.

Photophysical processes that determine the photoisomerization selectivity of Z,Z-bilirubin IXα in complexes with albumins

This paper discusses the photophysical processes that determine the regularities of Z,Z bilirubin IXα photoisomerization - the mechanism that forms the basis of the widely used optical technology for treating hyperbilirubinemia (jaundice) of newborns. Based on a study of the spectrofluorescence and polarization characteristics of complexes of Z,Z bilirubin IXα with human serum albumin (HSA) and bovine serum albumin (BSA), it is established that one cause of the predominant formation of Z,E bilirubin IXα among the cis-trans photoisomerization products of Z,Z bilirubin IXα is the bichromophore character of the structure of the pigment, the nonidentity of the chromophores that form it, and the presence of interchromophore intramolecular energy transfer. The highest efficiency of nonradiative transfer is found for the bilirubin-HSA complex, and this correlates with the literature data concerning the higher selectivity of the bilirubin photoisomerization process in the indicated complex by comparison with BSA. The factor that promotes the process of intermolecular energy transport when the fluorescence lifetime of bilirubin is anomalously short (the picosecond or subpicosecond range) is the presence of excitonic interactions between the chromophores.

Energy transfer and optical gain studies of FDS: Rh B dye mixture investigated under cw laser excitation

Radiative and non-radiative (Forster type) energy transfer processes in a dye mixture of FDS and Rh B in methanol under cw Ar ion laser excitation were investigated for fixed donor concentration and varying acceptor concentration. It is found that most of the pump power absorbed by FDS is transferred to Rh B as a useful pump power. Transfer probability (PDA), transfer efficiency both radiative (ηR) and non-radiative (ηNR) and optical gain (G) of the system were studied for various pump powers. The gain characteristics of Rh B are found to alter due to the change in the effective fluorescence lifetime caused by energy transfer reaction. Theoretical calculations were also done to find the total transfer efficiency (ηT) at various acceptor concentrations to identify the appropriate energy transfer mechanism responsible for gain enhancement in Rh B. Both radiative and non-radiative transfer processes are taken into consideration in all the calculations. Various energy transfer parameters viz. radiative rate constant (KR), non-radiative rate constant (KNR), critical concentration (C0), critical radius (R0) and half quenching concentration ([A]1/2) are calculated by using the Stern–Volmer plots and concentration dependence of radiative and non-radiative transfer efficiencies. Concentration and pump power dependence of the peak gain and lasing wavelengths of the energy transfer dye lasers (ETDL) have also been studied. The experimental results show that the dominant mechanism responsible for the efficient excitation transfer in this mixture is of radiative nature, whereas the long range dipole–dipole (d–d) interaction (Forster type) is comparatively smaller.

Study of laser behaviour in C-440 and C-485 dye mixture

The laser behaviour in the C-440 (donor) and C-485 (acceptor) dye mixture in ethanol is studied and discussed by investigating the gain characteristics at various acceptor concentrations (1 X 10 -4 M/L to 1 X 10 -2 M/L) and pump powers of the N 2- laser. It is found that most of the pump power absorbed by C-440 is transferred to C-485 as a useful pump power. The gain characteristics of C-485 are found changed due to the change in the effective fluorescence lifetime caused by the energy transfer reaction. The theoretical calculations were also done to find the total transfer efficiency at various acceptor concentrations to identify the appropriate energy transfer mechanism responsible for gain enhancement in C-485. These calculations take into account the contributions due to both radiative and nonradiative energy transfer. The energy transfer rate constants and critical transfer radius ( R 0) are calculated by using Stern-Volmer plots and concentration dependence of radiative and nonradiative transfer efficiencies. The concentration dependence of laser peak gain and the lasing wavelengths of the ETDL have also been studied. The experimental results indicate that the dominant mechanism responsible for the efficient excitation transfer in this mixture is of nonradiative nature due to long range dipole-dipole interaction. It is also found that the TICT state formation possibility in excited states is either reduced for the acceptor dye in the presence of donor or these TICT states become nonradiative.

New possibilities for Cr3+ions as activators of the active media of solid-state lasers

An analysis is made of the relationship between the excited-state lifetime of chromium and the ratio of the luminescence intensities in the 4T2–4A2 and 2E–4A2 channels, on one hand, and the Cr3+→acceptor energy transfer efficiency, on the other. A method is proposed for determination of the energy gap ΔE between the 2E and 4T2 levels of Cr3+ from the temperature dependence of the Cr3+→acceptor nonradiative transfer efficiency. Two independent methods were used to determine ΔE for gadolinium gallium garnet (GGG) crystals doped with chromium and with chromium and neodymium. The probabilities of Cr3+ intracenter relaxation and typical parameters of the Cr3+–Nd3+ energy transfer process were also determined. It was found that the values ΔE in gadolinium scandium gallium (GdScGaG) and lanthanum lutetium gallium garnet crystals are comparable with the values of kT at liquid helium temperature. It is shown that the high efficiency of the Cr3+→Nd3+ nonradiative transfer in GGG and GdScGaG crystals doped with chromium and neodymium at room temperature is due to the relatively low value of ΔE. It is noted that there are many crystals having the garnet structure and low values of ΔE that are potentially suitable for developing lasers utilizing the 4T2–4A2 electronic-vibrational transition in Cr3+ and having an emission frequency continuously tunable over a wide spectral range.

Intramembrane positions of membrane-bound chromophores determined by excitation energy transfer.

A detailed theory has been derived to evaluate the efficiency of nonradiative transfer of electronic excitation energy between nonassociated membrane-bound chromophores. Two different approaches are presented and shown to lead to identical numerical results. In the first of these the efficiency of transfer is computed from the decay with time of the donor excited state. In the second approach, the efficiency is calculated directly, demonstrating that to a high degree of accuracy the array of acceptors can be represented as consisting of a single nearest acceptor plus a continuum of secondary acceptors. A general expression is derived for the dipole-dipole orientation factor as a function of the position of an acceptor. It is shown that, by invoking the range of orientations that must be present at the very least in a particular case, the expected values of transfer efficiency may be limited to a relatively narrow band of uncertainty about those predicted for total randomization. In the limit of total randomization, the theory reduces to functions of but two dimensionless parameters: an effective number of acceptors and a normalized distance of closest approach, a parameter which in turn is a function of an excluded surface area and the depth in the membrane of a donor relative to that of an acceptor. Finally, data analysis procedures are presented whereby one can determine the surface density of acceptors for a known geometry or, alternatively, determine the distance of closest approach for known surface densities.

Multiple Sensitization Effects in CaF2:Yb+Nd

Transfer of energy between three different ions has been observed in CaF2 doped with Yb and Nd. In CaF2, Nd occurs in only the trivalent state, while Yb forms both divalent and trivalent luminescent centers. Evidence for both Yb2+→Nd3+ and Nd3+→Yb3+ energy transfer is presented. Measurement and analysis of Yb2+ fluorescence decay times in crystals containing 0.1 mole % Yb and 1.0 mole % Nd, gave a figure of 0.62 for nonradiative transfer efficiency at 77°K. Sensitization of Yb3+ by Nd3+ produces strong ir emission, principally at 0.9752μ. An approximate estimate of sensitized increase of Yb3+ emission achieved approaches two orders of magnitude.