Yu. V. Orlovskii

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.

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.

Kinetics of the direct energy transfer of optical excitation in crystalline nanoparticles: Theory and Monte Carlo computer simulation

A comparative analysis of direct energy transfer kinetics from donors to acceptors (obtained by Monte Carlo simulation in case of donor excitation by short (tpulse < τD) pulse) in a sample consisting of spherical nanoparticles with equal diameters and in a bulk crystal is performed. The influence of the finite size of nanoparticles on the process of direct (static) donor-acceptor quenching of optical excitation in nanoparticles with a crystalline structure is analytically studied. It is found that, in three-dimensional spherical nanoparticles, the quenching rates WAor and WBor found on an ordered stage of kinetics and quenching macroparameters γFA and γFB found on an disordered stage of kinetics for donors populating sites at the center (A) and in the surface layer (B) of nanoparticles differ by a factor of 2 (donors populating central sites A possess higher rates). An analytical expression describing disordered stage of kinetics of direct Nd3+ → OH− quenching in the sample consisting of spherical nanoparticles with equal diameters and the Y2O3 (Ia3, Th37) crystal structure using just two energy transfer macroparameters γFA and γFB is proposed.

Temperature dependencies of excited states lifetimes and relaxation rates of 3–5 phonon (4–6 μm) transitions in the YAG, LuAG and YLF crystals doped with trivalent holmium, thulium, and erbium

Abstract The temperature dependencies of the nanosecond multiphonon relaxation (MR) rates of the 3 F 3 state of Tm3+ in the YLF crystal and of the 5 F 5 state of Ho3+ ion in the YAG and LuAG crystals and of the microsecond MR rates of the 4 F 9/2 ( 2 H 9/2 ) state of Er3+ ions in YLF were measured in the wide temperature range using direct laser excitation and selective fluorescence kinetics decay registration. For YLF the observed relations are explained by 4-phonon process in the frame of a single-frequency model with hω eff =450±30 cm −1 for the 3 F 3 state of Tm3+ and by 5-phonon process with hω eff =445 cm −1 for the 4 F 9/2 ( 2 H 9/2 ) state of Er3+. For YAG and LuAG crystals these dependencies are explained by the 3-phonon process with hω eff =630 cm −1 . The decrease of the relaxation rate with the temperature in the range from 13 to 80 K was observed for the 4 F 9/2 ( 2 H 9/2 ) state of Er3+ in the YLF crystal. It is explained by the redistribution of excited electronic states population of erbium ions over the higher lying Stark levels with different MR probabilities. A good fit of experimental temperature dependence (including the dropping part of the experimental curve) was obtained for single-frequency model ( hω eff =450 cm −1 ) with W 01 =8.0×10 4 s −1 and W 02 =4.7×10 4 s −1 accounting Boltzmann distribution of population over two excited Stark levels of the excited state of erbium ions. Employment of this model improves the fit between the experiment and the theory for the 5 F 5 state of Ho3+ ion in YAG as well. Strong influence of the parameters of the non-linear theory of MR, i.e. the reduced matrix elements U(k) of electronic transitions and the phonon factor of crystal matrix η on the spontaneous MR rates was observed experimentally. The smaller these parameters the slower the spontaneous MR W0. This fact can be used for searching new active crystal laser media for the mid-IR generation.

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.

Luminescent properties of doped dielectric nanocrystals

Taking into account electron-phonon interaction effects, we have studied the radiative characteristics of doped nanoellipsoids in a dielectric medium with an arbitrary fraction of nanoparticles in a nanocomposite. The obtained analytical expressions for the radiative characteristics allow it to be concluded to what extent they are susceptible to changes in the phonon subsystem and electron-phonon interaction caused by the spatial confinement effect.

Synthesis and study of the properties of K2Y1 − x − yEuxTby(MoO4)(PO4) and K2Y1 − x − yEuxTby(MoO4)(PO4)1−δ(VO4)δ solid solutions

Al synthesized samples are isostructural and crystallize in the orthorhombic symmetry system, space group Ibca. Particles of the final product of ∼200 nm in size have been obtained. The introduction of the vanadate anion into the matrix composition leads to the lowering of the symmetry of the Eu3+ environment and to the rise of the defect luminescence at 450–550 nm because of the unit cell distortion. The luminescence of defects in terbium-europium-containing samples is determined by the sample surface area, which decreases on annealing. The τ, W0 and γ parameters of the luminescence kinetics of the samples have been determined.

Multiphonon relaxation in fluoride and ternary sulfide laser crystals with neodymium ions

It is shown that, in ternary sulfide crystal matrices, in contrast to fluoride matrices with similar phonon spectra, the multiphonon relaxation rate as a function of the number peff of “effective phonons” (the energy gap ΔEmin between the J′ and J levels of the Nd3+ ion) has a steep slope, which does not decrease with increasing peff. This indicates that, compared to fluoride crystals, the multiphonon relaxation rate of mid-IR transitions (in the range 4–5 μm) in ternary sulfide matrices sharply decreases, by three to four orders of magnitude.

Continuously tunable cw lasing near 2.75 {mu}m in diode-pumped Er{sup 3+} : SrF{sub 2} and Er{sup 3+} : CaF{sub 2} crystals

CW lasing is obtained in Er3+(5%) : CaF2 and Er3+(5%) : SrF2 crystals near 2.75 μm with 0.4 and 2 W of output powers, respectively, upon transverse diode laser pumping into the upper 4I11/2 laser level of erbium ions at 980 nm. Continuous tuning of the laser wavelength between 2720 and 2760 nm is realised in the Er3+ : SrF2 crystal.

Mid- IR laser transitions in Nd/sup 3+/ doped CaGa/sub 2/S/sub 4/, PbGa/sub 2/S/sub 4/, and PbCl/sub 2/ laser crystals

Multiphonon and radiative relaxation of mid-IR laser transitions is analyzed in neodymium doped calcium and lead thiogallates and lead chloride laser crystals. Higher radiative and much lower nonradiative rates are found comparing to fluorides.