Konstantin K. Pukhov

Spontaneous emission in dielectric nanoparticles

An analytical expression is obtained for the radiative-decay rate of an excited optical center in an ellipsoidal dielectric nanoparticle (with sizes much less than the wavelength) surrounded by a dielectric medium. It is found that the ratio of the decay rate Anano of an excited optical center in the nanoparticle to the decay rate Abulk of an excited optical center in the bulk sample is independent of the local-field correction and, therefore, of the adopted local-field model. Moreover, the expression implies that the ratio Anano/Abulk for oblate and prolate ellipsoids depends strongly on the orientation of the dipole moment of the transition with respect to the ellipsoid axes. In the case of spherical nanoparticles, a formula relating the decay rate Anano and the dielectric parameters of the nanocomposite and the volumetric content c of these particles in the nanocomposite is derived. This formula reduces to a known expression for spherical nanoparticles in the limit c ≪ 1, while the ratio Anano/Abulk approaches unity as c tends to unity. The analysis shows that the approach used in a number of papers {H. P. Christensen, D. R. Gabbe, and H. P. Jenssen, Phys. Rev. B 25, 1467 (1982); R. S. Meltzer, S. P. Feofilov, B. Tissue, and H. B. Yuan, Phys. Rev. B 60, R14012 (1999); R. I. Zakharchenya, A. A. Kaplyanskii, A. B. Kulinkin, et al., Fiz. Tverd. Tela 45, 2104 (2003) [Phys. Solid State 45, 2209 (2003)]; G. Manoj Kumar, D. Narayana Rao, and G. S. Agarwal, Phys. Rev. Lett. 91, 203903 (2003); Chang-Kui Duan, Michael F. Reid, and Zhongqing Wang, Phys. Lett. A 343, 474 (2005); K. Dolgaleva, R. W. Boyd, and P. W. Milonni, J. Opt. Soc. Am. B 24, 516 (2007)}, for which the formula for Anano is derived merely by substituting the bulk refractive index by the effective refractive index of the nanocomposite must be revised, because the resulting ratio Anano/Abulk turns out to depend on the local-field model. The formulas for the emission and absorption cross sections σnano for nanoparticles are derived. It is shown that the ratios σnano/σbulk and Anano/Abulk are not equal in general, which can be used to improve the lasing parameters. The experimentally determined and theoretically evaluated decay times of metastable states of dopant rare-earth ions in crystalline YAG and Y2O3 nanoparticles are compared with the corresponding values for bulk crystals of the same structure.

Multiphonon relaxation rates measurements and theoretical calculations in the frame of non-linear and non-Coulomb model of a rare-earth ion-ligand interaction

The fluorescence kinetics decay from 3F3, 3H5, 3H4, 3F4 multiplets of Tm3+ and 5F5, 5I5, 5S2 + 5F4, 5I6, 5I7 multiplets of Ho3+ ions in Y3Al5O12, Lu3Al5O12 and LiYF4 crystals was directly measured. On the basis of these measurements the multiphonon relaxation (MPR) rates of the transitions were determined. The theoretical dependence of multiphonon relaxation rates on the rare-earth ionic radii was derived using the non-linear (NL) theory of multiphonon relaxation. A good qualitative agreement of this dependence with new and earlier measured MPR rates in rare-earth ions in the row from Tm3+ to Pr3+ in YAG, YLF, and LaF3 crystals was obtained. The dependence of multiphonon relaxation rates on the crystal type is also considered.

Nonlinear mechanism of multiphonon relaxation of the energy of electronic excitation in optical crystals doped with rare-earth ions

Abstract A review of the nonlinear theory of multiphonon relaxation (MR) in the crystal-field approximation and the super-weak electron-phonon coupling limit is presented. The analysis of the measured multiphonon relaxation rate dependence on the number of phonons involved in the nonradiative transition in a LiYF 4 : Er 3+ crystal was done considering both the Coulomb interaction between the rare-earth ions and the ligands and the corrections to the Coulomb interaction resulting from the spatial distribution of the ligand electronic charge, the energy of exchange interactions and the contributions from the charge transfer states.

Cooperative quenching: experiment, theory and Monte-Carlo computer simulation

Abstract An analytical expression and results of Monte-Carlo computer simulations are proposed for the kinetics of cooperative static quenching decay in crystals with arbitrary acceptor concentration and arbitrary multipolarity of interaction process S =6, 8, 10, which at a long decay time can be approximated by the nonexponential law I ( t )exp[−( wt ) k ], where k =0.41, 0.3, 0.25 for multipolarities S =6, 8, 10, respectively. The phenomenon of cooperative energy transfer from single Nd 3+ , Tm 3+ or Ho 3+ ion simultaneously to the pair of Ce 3+ ions and from single Er 3+ ion to three Ce 3+ ions with, respectively, two and three times smaller transition energy each in highly concentrated La 1− x Ce x F 3 crystals is presented. The influence of overlap integral (between donor ion and cooperative pair and triple Ce 3+ acceptor) and electronic matrix elements of donor transitions on the cooperative energy transfer rate for a set of donor ions is discussed.

Spontaneous and induced emission in dielectric nanoparticles

An analytical expression is proposed for the rate of radiative decay of excitations of optical centers in an ellipsoidal nanoparticle that is considerably smaller than the light wavelength (a, b, c ≪ λ) and is embedded in the surrounding medium. This expression is compared with the corresponding expression for a bulk crystal. The time of radiative decay in nanoparticles spherical in shape and identical in size (2R ≪ λ) is derived as a function of the dielectric characteristics of a nanocomposite and the nanoparticle volume fraction C in suspensions and aerosols. The expressions describing the emission and absorption cross sections in spherical nanoparticles are obtained. The experimentally measured and theoretically calculated decay times of metastable levels of rare-earth impurity ions in YAG and Y2O3 crystalline nanoparticles are compared with those for bulk crystals with the same structure.

Peculiarities of initial flux line structure in fine-grained YBaCuO ceramics

Abstract It was shown that grain size R to penetration depth λ ratio had an essential effect on magnetization curves M(H) in the case of fine- grained YBaCuO at high temperatures. Peculiarities of initial flux line structure in fine- grained HTSC samples were studied to interpret the experimental data. The fields of penetration of the first and subsequent flux lines were obtained.

Multiphonon relaxation of the electronic excitation energy of rare-earth ions in laser crystals

The paper discusses both the linear and nonlinear mechanisms of multiphonon relaxation (MR) transitions in trivalent rare-earth (RE) ions in crystals. In the first well-known case, the linear terms of electron-phonon interaction (EPI) with respect to vibronic coordinates are responsible for MR transitions. In the last case, the nonlinear part of EPI induces the multiphonon relaxation. The expression for total rate of the MR transition due to both mechanisms taken into account was obtained.

Multiphonon relaxation in the rare-earth ions doped laser crystals

The nanosecond and microsecond fluorescence kinetics decay from 3F3, 3H4, 3H5, 3F4 multiplets of Tm3+ and 5S2, 5F5, 5I5, 5I6, 5I7, multiplets of Ho3+ ions in Y3Al5O12, Lu3AL5O12, and LiYF4 crystals with low dopant concentration was directly measured. On the basis of these measurements the multiphonon relaxation (MPR) rates for three, four, and five phonon transitions were determined. The theoretical depedence of multiphonon relaxation rates versus rare-earth ionic radius was derived using the non-linear (NL) theory of multiphonon relaxation. A good qualitative agreement of this dependence with new and earlier measured MPR rates in rare-earth ions in the row from Tm3+ to Pr3+ in YAG, YLF, and LaF3 crystals was obtained. The regularities of niultiphonon relaxation rates on the number of phonons n, 4f-4f electronic transitions selection rules, and crystal type are also considered.

Radiative properties of lanthanide and transition metal ions in nanocrystals

We examine the physical factors that influence the radiative transitions of small-radius optical centers in ellipsoidal nanoparticles. The main objective of this work is to reveal to the extent to which changes in the phonon subsystem and electron-phonon interaction caused by spatial confinement effects affect the radiative characteristics of optical centers in nanocrystals.

Laser heating of the Y 1-x Dy x PO 4 nanocrystals

We propose a novel material prepared by microwave-hydrothermal treatment, the tetragonal xenotime-type yttrium orthophosphate YPO4 nanocrystals doped by different concentrations of Dy3+. It may be suitable for laser-induced local heating of cancer tumors for hyperthermia. We heated a powder consisted of the nanoparticles by focused quasi-CW laser irradiation at different wavelengths in the near IR spectral range fitting the transparency window of biological tissues. The local temperature on the surface of the powder in the place of irradiation increases linearly with increasing laser power and increasing the Dy3+ concentration. At the same time the efficiency of local heating Φ = ΔT / (P f) (ΔT is a local temperature increase, f is an oscillator strength of absorption transition, and P is the quantity of laser power) is proportional to the energy of the initially excited electronic level. The proposed method allows for high rates of heating and cooling. The laser power used for heating was rather low, tens of milliwatts that together with short heating time to required temperature may result in extremely low doses of laser irradiation for heating.