V. Yu. Yakovlev

Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH4H2PO4) and KDP (KH2PO4) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H0 centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H0 centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects.

Transient optical absorption and luminescence of lithium triborate LiB3O5

The transient optical absorption and luminescence of LiB3O5 (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li0 electronic centers, which represent neutral lithium atoms. At 290 K, the Li0 centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B2+ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.

Transient optical absorption and luminescence in Li2B4O7 lithium tetraborate

This paper reports on a study of the transient optical absorption exhibited by Li2B4O7 (LTB) in the visible and UV spectral regions. Using absorption optical spectroscopy with nanosecond time resolution, it is established that the transient optical absorption (TOA) in these crystals originates from optical transitions in hole centers and that the kinetics of the optical-density relaxation is controlled by interdefect tunneling recombination, which involves these hole centers and electronic Li0 centers representing neutral lithium atoms. At 290 K, the Li0 centers migrate in a thermally stimulated, one-dimensional manner, without carrier ejection into the conduction or valence band. The kinetics of the pulsed LTB cathodoluminescence is shown to be controlled by a relaxation process connected with tunneling electron transfer from a deep center to a small hole polaron migrating nearby, a process followed by the formation of a self-trapped exciton (STE) in an excited state. Radiative annihilation of the STE accounts for the characteristic σ-polarized LTB luminescence at 3.6 eV, whose kinetics is rate-limited by the tunneling electron transfer.

Transient hole-polaron optical absorption in Li6Gd(BO3)3 crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li6Gd(BO3)3 (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li0 centers. At 290 K, the Li0 centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.

Electronic excitations and defects in nanostructural Al2O3

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al2O3 (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al2O3 nanopowders probed before and after Fe+ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ∼17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al2O3 nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ1 = 0.5 ns, τ2 = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P− centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F+ center (FS+-type centers). In addition to the fast luminescence, nano-Al2O3 was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al2O3 suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.

Recombination kinetics in nonlinear defective LiB3O5 crystals

A study of recombination kinetics in LiB3O5 (LBO) crystals by time-resolved luminescence and absorption spectroscopy is reported. An investigation of the kinetics of transient optical absorption (TOA) and luminescence under ns-scale electron-beam excitation performed within a broad temperature range of 77–500 K and a 1.2–5-eV spectral interval has established that the specific features in the recombination kinetics observed in LBO involve electronic, B2+, and hole, O−, trapping centers. The TOA and luminescence kinetics, as well as their temperature dependence, are interpreted by a model of competing hole centers. Relations connecting the kinetics parameters and the temperature dependence to the parameters of the main LBO point defects are presented.

Dynamics of electronic excitations and localized states in LiB3O5

Abstract The paper presents the results of study of dynamics of electronic excitations and their interaction with the localized states for non-linear crystals LiB3O5 (LBO). The results were gained mainly by the use of spectroscopy of transient optical absorption under excitation with an electron beam. Origin of the absorption centers, and role of the localized states in the electronic excitation dynamics of LBO are discussed.

Transient optical transmission changes induced by pulsed electron radiation in commercial crown silicate glasses

Abstract We report on results of time-resolved induced optical absorption measurements in commercial crown silicate glass K8 (similar to Schott BK7 glass) and its radiation-resistant counterpart K108 under 0.25-MeV pulsed electron radiation. The spectra have been obtained in a wavelength range 280–1100 nm on a time interval 10 ns–1 s after the end of a 20-ns pulse. In contrast to behavior of stable defects, the efficiency of non-stationary color centers’ generation in the long-wavelength spectrum range is similar for both standard and radiation-resistant glasses. The characteristic time for transmission recovery in the visible range at room temperature was found to be about 100 μs. Based on the Kramers–Kronig relations we have estimated transient refractive index changes. For the same radiation dose such changes can be two orders of magnitude higher than those observed in stationary conditions.

Short-lived color and luminescence centers in LiNbO3 crystals irradiated by pulsed electron beams

Data on spectra of short-lived optical absorption (SLOA) and luminescence induced in congruent crystals of lithium niobate by a pulsed electron beam (0.25 MeV, 20 ns, 15–160 mJ/cm2) in a temperature range of 80 to 350 K are presented. Anisotropic bands with maxima at 1.6 and 4.0 eV, originating from the capture of one and two conduction electrons by an (NbNb-NbLi) complex, respectively, and weakly polarized bands at 2.5 and 3.3 eV due to holes located at Li and Nb vacancies are identified in the structure of SLOA spectra. Cathodoluminescence (CL) of lithium niobate crystals is characterized by fast (τ<4 ns) decay and a broad spectrum, which contains the same bands as the SLOA spectra. It is shown that the change in the initial amount of defects in reduced crystals at 830 K results in an identical change in both the CL and SLOA spectra. A model which treats the luminescence as the result of radiative nonphonon transitions that accompany the thermalization of charge carriers captured into the ground state of a polaron is discussed.

Creation of excitons and defects in a CsI crystal during pulsed electron irradiation

Data presented on the influence of the temperature in the range 80–650 K on the spectral kinetics of the luminescence and transient absorption of unactivated CsI crystals under irradiation by pulsed electron beams (〈E〉=0.25 MeV, t1/2=15 ns, j=20 A/cm2). The structure of the short-wavelength part of the transient absorption spectra at T=80–350 K exhibits features, suggesting that the nuclear subsystem of self-trapped excitons (STE’s) transforms repeatedly during their lifetime until their radiative annihilation at T⩾80 K, alternately occupying di-and trihalide ionic configurations. It is established that a temperature-induced increase in the yield of radiation defects, as well as F and H color centers, and quenching of the UV luminescence in CsI occur in the same temperature region (above 350 K) and are characterized by identical thermal activation energies (∼0.22 eV). It is postulated that the STE’s in a CsI crystal can have a trihalide ionic core with either an on-center or off-center configuration; the high-temperature luminescence of CsI crystals is associated with the radiative annihilation of an off-center STE with the structure (I−(I0I−e−))*.