Luminescence Of Color Centers Research Articles

Nonlinear Excitation of Color Centers in a LiF Crystal by Femtosecond Laser Radiation

Experiments were carried out in which the luminescence of color centers in lithium fluoride crystals was excited by two different femtosecond lasers with significantly different energies, durations and pulse repetition rates. It was established that in all experiments the main center, the luminescence of which was excited nonlinearly, was the F3+ color center. Unusual experimental data were obtained; a laser with low pulse energy (4 nJ) excited triplet luminescence of these centers (570 nm) but did not excite singlet luminescence (540 nm). Another laser with a higher pulse energy (0.3 mJ), on the other hand, excited singlet luminescence and did not excite triplet luminescence. A proposed diagram of energy levels and quantum transitions is presented, explaining the possible mechanisms of nonlinear excitation of luminescence in these experiments.

Transformation of Radiation-Induced Molecular Point Defects and Color Centers in LiF Crystals under Influence of Light

The influences of UV light and integral light pulses from lamps on γ-irradiated LiF crystals with impurities of hydroxyl and magnesium ions have been investigated. The optical absorption and luminescence of color centers in different bleaching stages are studied depending on the crystal exposure time under UV light or on the number of light pulses. IR spectra of molecular hydrogen-bonded complexes and hydroxyl ions in different bleaching stages are presented. The transformation of strong hydrogen-bonded complexes under illumination into complexes with weak hydrogen bond (and vice versa) is studied. It is shown how molecular hydrogen-bonded complexes involving hydroxyl affect the transformation of color centers during the crystal bleaching.

Преобразования радиационно-наведенных молекулярных точечных дефектов и центров окраски в кристаллах LiF под воздействием света

The effect of UV light and light pulses of the integral light of lamps on γ-irradiated LiF crystals with impurities of hydroxyl ions and magnesium were investigated. The optical absorption and luminescence of color centers at different stages bleaching depending on the exposure time of the crystal under UV light or from the number of light pulses were studied. IR spectra of molecular complexes with hydrogen bond and hydroxyl ions at different stages of bleaching are present. The transformation of complexes with a strong hydrogen bond in complexes with a weak hydrogen bond, and vice versa under the action of light are shown. It is presented as molecular complexes with hydrogen bond affect transformations color centers in the process of crystal bleaching.

Size‐dependent luminescence of color centers in composite nanodiamonds

Luminescence properties of nitrogen‐vacancy (NV) and silicon‐vacancy (SiV) centers were investigated for the series of O‐terminated composite nanodiamonds consisting of a high‐pressure–high‐temperature (HPHT) diamond core and a chemical‐vapor‐deposition (CVD) diamond outer layer of different thickness. It was found that emission of NV and SiV centers cease to “feel” the diamond surface at a distance of 12 and 4 nm, respectively, from it. This finding determines minimum sizes of O‐terminated nanodiamonds in which stable single photon emitters could be formed based on NV and SiV centers. Suggested composite diamond nanostructure are optimal for design of two‐color luminescent markers.The studied diamond nanoparticles present composite structure of “core‐outer layer” type. The core is 20 nm HPHT diamond containing NV centers, whereas outer diamond layer containing SiV centers is formed by CVD synthesis.

Radioluminescence of color centers in LiF crystals

Abstract Luminescence of color centers in the near-infrared region under cathode beam excitation is revealed for the first time in as-grown pure LiF and LiF:Mg,O crystals. Emission efficiency of F 2 + , F 3 − and F 2 − centers is higher for the samples enriched by oxygen and magnesium than for ultrapure crystals. The intensity of the NIR luminescence remains practically constant with the increase of the electron beam density. In contrast, visible emission typical for F 3 + and F2 centers is suppressed due to the induced absorption bands. It is assumed that the high-density electronic irradiation leads to the strongly pronounced thermal- and radiation-stimulated diffusion in a relatively thin layer. It causes the association of point radiation defects with the formation of various complex centers and aggregates. Results obtained suggest that LiF crystals can be used for in-situ detection of ionizing radiation.

Lithium fluoride crystal as a novel high dynamic neutron imaging detector with microns scale spatial resolution

AbstractRecently, a new field of application of optically stimulated luminescence of color centers (CCs) in lithium fluoride (LiF) crystals was proposed – using them for high‐performance neutron imaging – and promising results were obtained (Matsubayashi et al., Nucl. Instrum. Methods A 622, 637 (2010) and Matsubayashi et al., Nucl. Instrum. Methods A 651, 90 (2011)). Here we present the overview of main findings, which clearly demonstrated that the LiF crystal performs efficiently as neutron imaging detector in areas, where a high spatial resolution with a high image gradation resolution is needed. It was shown that the obtained neutron images are almost free from granular noises, have spatial resolution of ∼ 6 μm, and have practically linear response with the dynamic range of at least 103. It was also found that the LiF crystal detector offers a fairly good sensitivity. Moreover, detailed evaluation using a standard sensitivity indicator for neutron radiography showed that two holes with less than 2% transmittance differences could be distinguished. Additionally, we recently demonstrated that the high resolution neutron imaging with LiF crystals could be useful for quantitative characterizations of neutron sources and electric devices, comprising of low‐Z elements, for example, such as fuel cells. All of this gives new opportunity for microns scale spatial resolution imaging by neutrons (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diamond processing by focused ion beam—surface damage and recovery

The nitrogen vacancy color center (NV−) in diamond is of great interest for photonic applications. Diamond nano-photonic structures are often implemented using focused-ion-beam (FIB) processing, leaving a damaged surface which has a detrimental effect on the color center luminescence. The FIB processing effect on single crystal diamond surfaces and their photonic properties is studied by time of flight secondary ion mass spectrometry and photoluminescence. Exposing the processed surface to hydrogen plasma, followed by chemical etching, drastically decreases implanted Ga concentration, resulting in a recovery of the NV− photo-emission and in a significant increase of the NV−/NV0 ratio.

Modulation frequency doubling in the axially periodic dependence of the luminescence of F 3 + centers in LiF crystals

A spatially periodic distribution of the luminescence intensity of F2 and F3+ color centers is observed in LiF crystals with induced anisotropy. The period of spatially periodic modulation of the luminescence intensity generally coincides with the period of the change in the polarization state of excitation light. For F2 centers, the experimental axial spatially periodic dependence of the luminescence intensity is close to the calculated dependence obtained in the description of the absorption and radiative transitions in the centers by linear oscillators oriented along six C2 axes of the cubic crystal. However, for F3+ centers, the same experiment shows a spatially periodic distribution of luminescence intensity with a modulation period shorter by a factor of 2. It is shown that this effect of modulation frequency doubling in the spatially periodic pattern of the luminescence of color centers arises due to the nonlinear dependence of the luminescence intensity on the excitation light intensity. For F3+ centers, upon luminescence excitation in the absorption band with λmax = 452 nm, the modulation frequency doubling is due to the saturation of the metastable triplet state of such a center.

Luminescence of color centers formed in alkali-earth-doped yttrium orthoaluminate crystals

Alkali-earth-doped yttrium orthoaluminate crystals grown in a reducing atmosphere are found to show bright photoluminescence (PL) in visible wavelength regions under the excitation by UV light source. From the results of transmission, PL, PL excitation and time-resolved PL spectra for the samples with different types and concentrations of dopants and the comparison to the results for the samples grown under different conditions, the origin of principal emission is determined to be color centers stabilized by heterovalent ions. The observed fast lifetime and high quantum yield of the luminescence can be explained by dipole-allowed transition between the levels localized in a vacancy. Comparing the effects on the optical properties from several types of dopant ions and taking influence from the UV irradiation into account, a model for the structure of emission centers is proposed.

Luminescence depolarization mechanisms for color centers with the luminescence peak at 670 nm in gamma-irradiated LiF:Mg crystals

Processes of tunneling and thermal depolarization of luminescence of color centers with two absorption bands at 385 and 560 nm and the emission band peaked at 670 nm were studied in gamma-irradiated LiF:Mg crystals. As was previously concluded, relaxed excited states of the center have trigonal symmetry and are characterized by four equivalent trigonal Jahn-Teller minima corresponding to the C 3 crystallographic axes. The luminescence depolarization observed at 110–120 K results both from the thermally stimulated transition and the tunnel transition of the system between the Jahn-Teller minima. Temperature dependence of the polarization degree was calculated and compared with experimental data.

Color center luminescence decay times in optical microcavities based on LiF films

Controlling and modifying spontaneous emission in optical microcavities is of great interest for both their basic properties and for their potential applications in emitting solid-state devices. Spatial and temporal spontaneous emission modifications of F 2 color centers in LiF films placed inside planar optical microcavities are reported. Angularly resolved photoluminescence measurements show a narrowing and an enhancement of the emission spectrum along the cavity axis; time resolved luminescence measurements in different geometrical configurations show a shortening of luminescence decay time along the microcavity axis.

Color center luminescence in pure and Mg-doped LiBaF 3 crystals

Radiation-induced absorption as well as thermoluminescence of irradiated pure and Mg-doped LiBaF 3 crystals were studied. Color center properties were found to depend on the Mg-content and heat treatment. The possibility of Mg + and Mg 0 centers formation is shown. Intense thermally stable color center luminescence and thermostimulated emission parameters indicate that LiBaF 3(Mg) crystals are prospective laser active and dosimetric media.

Two-photon polarization spectroscopy of F3+ and F2 color centers in LiF crystals

We identified energy levels, and their corresponding symmetries, of F3+ and F2 color centers in LiF crystals, using the Feofilov method of polarized luminescence as applied to two-photon excitation. For this purpose, we measured the azimuthal dependencies of the polarization degree, P(ϕ), and the intensity, I(ϕ), of the F3+ and F2 color center luminescence in LiF crystals under two-photon excitation. To increase the reliability of the results we measured the polarization dependencies of three LiF crystal plates cut parallel to the three main crystallographic planes (100), (110) and (111), respectively. The measured polarization dependencies were seen to be in good agreement with the theoretical polarization dependencies derived for P(ϕ) and I(ϕ). We established that the new energy level of the F2 center has the same parity as the ground state. Consequently, the one-photon electric-dipole transitions are forbidden between the states. The symmetry of the new electronic state of the F3+center was found to be 1A1.

Study of color centers in hot-filament CVD diamond films by cathodoluminescence and photoluminescence and their correlations with film quality

The color centers on the front and back of five free-standing diamond films grown by hot-filament CVD were characterized by cathodoluminescence and photoluminescence. The film quality was assessed by measuring the full width at half-maximum (FWHM) of the characteristic Raman diamond peak at 1332 cm −1. A correlation between the color center luminescence and the film quality was derived. The cathodoluminescence study clearly showed that the intensities of band A and the 600 nm emission band increased with increasing quality. Conversely, the intensity of the 1.68 eV photoluminescence decreased as the film quality improved. The lower intensity of the 1.68 eV luminescence in higher quality films, observed in the samples grown in a silicon-free reactor, is different from the usual 1.681 eV luminescence from silicon-related centers, which shows a greater intensity in higher quality films.

Luminescence of color centers in α-Al2O3 single crystals

Luminescence of color centers in α-Al2O3 single crystals

Enhancement of color center luminescence in neutron irradiated Al 2O 3

Enhancement of color center luminescence in neutron irradiated Al 2O 3

Tunable laser radiation, nonlinear absorption, and “anti-stokes” luminescence of color centers in α-Al2O3

This work investigates the generation of laser radiation by color centers of leucosapphire, nonlinear absorption, and the excitation mechanism of UV-luminescence. It is shown that UV-luminescence from color centers, excited in the 0.68 μm absorption band, is caused by a two-staged excitation mechanism. The optical characteristics of the color centers are measured and calculation is made of the variation of the absorption coefficient with laser pulse's duration.

Luminescence of color centers in KI:Li +

New luminescence bands have been observed in KI:Li + excited in the F band region. As suggested by the dependence of the bands upon the Lithium content of the samples, these bands are due to perturbed F and M centers (F A, F B and M A centers). The excitation spectrum, the wavelength and the polarization of the emission indicate that the 1.75 μm band is due to the M A center and the 2.08 μm band to the F A (I) center. The band at 2.88 μm is tentatively assigned to the F B (II) centers.

An instrument for the automatic measurement of time-variant decay-time constants

An instrument for the automatic measurement, as a function of a varying parameter, of the decay-time constant of low level statistically fluctuating exponential waveforms, is presented. Developed to investigate the temperature dependence of excited color center luminescence in alkali halide crystals, it allows a continuous recording of the decay time constant from some μs to several ms, even in the case of rather fast temperature variations. Use of the instrument is however not limited to this case. The block diagram of the instrument is described, a discussion is given of possible applications and the limits of its performance are evaluated.

Luminescence of Color Centers in Alkali Halides

Luminescence of Color Centers in Alkali Halides