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Thermally induced diffusion of F2+ color centers in lithium fluoride crystals
Journal of Luminescence 192 (2017) 283–287
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermally induced diffusion of F2+ color centers in lithium fluoride crystals a,⁎
b
a
a
N.V. Shipitsin , A.I. Krivoshein , N.A. Ivanov , I.K. Petrushenko , A.E. Rzhechitskii a b
MARK
a
Irkutsk National Research Technical University, 664074, 83 Lermontov st., Irkutsk, Russia Bauman Moscow State Technical University, 105005, 5 Vtoraya Baumanskaya st., Moscow, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: LiF Color centers Diffusion Dichroism Reorientation
The existence of thermally induced diffusion of F2+ centers was shown by measuring the coefficient of optical absorption dichroism of gamma-irradiated LiF crystals. An elementary act of diffusion is a rotation of an F2+ center with the shift of one of the vacancies in the neighbor lattice site, thus, there is a displacement of the center as a whole. The measured activation energy of rotation is 1.0 eV, which is in a good agreement with the energies of the activation process of F2+ centers thermal destruction obtained before.
1. Introduction The crystals of lithium fluoride (LiF) with radiation-induced color centers are widely used in dosimetry [1] and as the laser media with broadband rearrangement of the radiation frequency in the visible and near infrared regions of the spectrum [2]. Generation of the radiation at laser pumping in gamma-irradiated LiF crystals is available in a few types of color centers, such as F2 , F3+, F2+, F2−, and in the color centers associated with an impurity of Mg [3–5]. There is a growing interest in color centers in thin films of LiF at present [6,7]. F2+ centers are unstable at room temperature in all alkali halide crystals (AHC), which are not activated by impurities, and they are destructed in a few hours. A small concentration of stable at room temperature F2+ centers can be formed only in LiF and NaF by doping the crystals with a hydroxyl impurities [8]. A number of radiation-induced defects are mobile, therefore they have a significant impact on the formation of the aggregation of color centers and the optical properties of crystals. In particular, the mobility of anion vacancies and F2+ color centers is well known [9]. However, the direct observations of the diffusion of F2+ centers are lacking in literature, although F2+ centers, owing to their charge states, can interact with neutral and negatively charged defects during the migration over the crystal. The work [10] studied the reorientation of the F2 centers in LiF crystals using the fluorescence microscopy. The process of the destruction of F2+ centers was investigated in papers [9,11,12] mainly by measuring the kinetics, which reflects the overall process of centers destruction, comprising several reactions of interaction with radiation defects of the crystal as a result of their mutual thermal diffusion. However, the issue on the contribution of thermal diffusion of F2+ color centers on the interaction of defects
⁎
Corresponding author. E-mail address: [email protected] (N.V. Shipitsin).
http://dx.doi.org/10.1016/j.jlumin.2017.06.065 Received 5 September 2016; Received in revised form 16 June 2017; Accepted 28 June 2017 Available online 30 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
remains open. This paper studies the process of thermally induced reorientation and, as a consequence, the diffusion of F2+ color centers in LiF crystals. We measured the kinetics of relaxation of dichroism nearly the maximum (620 nm) of the F2+ centers absorption band induced by intense laser radiation of a colored LiF crystal with a wavelength of 532 nm. The aims of this work are the direct observation of the thermal reorientation of F2+ centers as an elementary act of diffusion by the method of measurement of polarization absorption dichroism, determination of the activation energy of this process, and comparison of the values obtained from the kinetic measurements. 2. Materials and methods In this work, we used LiF crystals grown in the air by Kyropoulos method. They were irradiated with gamma radiation by using 60Co source at room temperature, exposure dose of about 4.4 × 105 Gy. The absorption coefficient at the F2 centers band maximum (446 nm) is 30.8 cm−1. The thickness of the studied samples is 1.5 mm. In order to create dichroism in the crystals, we employ photoionization of F2 centers using intense linearly polarized radiation from a neodymium (Nd: YAG) laser operating in a pulsed mode with a pulse repetition rate of 1 Hz, a pulse duration of 20–25 ns, and the pulse energy is 2 mJ with frequency doubling (λ = 532 nm); the centers with larger projection of the polarization on the axis of the center were mainly ionized. As a result, the formed F2+ centers are dominantly oriented in parallel to the polarization vector. A He-Ne laser LH-75 (λ = 632.8 nm) with a linearly polarized radiation was used as the source of probe radiation. Photodiodes PD-24K (in the photoconductive mode) were used as photodetectors in the experimental setup. Tempering of the sample was carried out in a cryostat SHI-4-1 (JanisResearch, USA). The
Journal of Luminescence 192 (2017) 283–287
N.V. Shipitsin et al.
d = α∥ − α⊥, where α|| is the absorption of light with the direction of the polarization vector parallel to the direction of the polarization vector of the photoionizing beam, α⊥ is the light absorption with the direction of the polarization vector perpendicular to the direction of the polarization vector of the photoionizing beam. The polarization vector of the photoionizing beam was parallel to the [110] axis of the crystal. The angle between the direction of the polarization vector of the probe beam and the direction of the polarization vector of the photoionizing beam was 45°. The probe beam that has passed through the crystal is split into two beams by a Glan prism (Fig. 2). The first measured beam had the direction of the polarization vector coinciding with that of the photoionizing beam; the second beam had the perpendicular direction of the polarization vector. Thus, we simultaneously measure the absorption of the probe beam with the polarization vector both parallel to the direction of that of the photoionizing beam and perpendicular to it. It should be noted that upon photoionization, the formation of F2+ color centers occurs not only in the (001) plane, but also in the (100) and (010) planes. However, the possibility of formation of F2+ color centers oriented in the [110] direction (in parallel to the polarization vector of photoionized radiation) is much more essential. Thus, the larger quantity of F2+ color centers formed upon photoionization and oriented in the [110] direction leads to observed dichroism.
Fig. 1. The scheme of vacancy shifts upon the reorientation of an F2+ center. The arrows depict the possible directions of vacancy shifts.
measurements of kinetics of relaxation of dichroism were performed in the temperature range from 300 to 320 K. Optical absorption spectra of the samples were measured by a spectrophotometer Shimadzu UV3600. As is well known, an F2+ center is two anionic vacancies in neighbor lattice sites and an electron localized on them. F2+ centers in LiF crystals, not perturbed by other defect, are destructed for about 10 h at room temperature. At present, it is assumed that they are destructed as a result of diffusion and the following entry into reaction with other defects in the crystal lattice [12]. The proposed mechanism of diffusion of F2+ centers is a sequence of reorientations of an F2+ center with the shift of one of the vacancies in the neighbor lattice site, thus, there is a displacement of the center as a whole (Fig. 1). The sequence of such shifts leads to diffusion of the center over long distances and its destruction as a result of interaction with various lattice defects. In this paper, we experimentally verify this mechanism of diffusion. It is also known, F2+ color centers in AHC which oriented, for example, by means of photoionization of F2 defects with polarized radiation have pronounced absorption dichroism [13]. The degree of orientation of the centers is associated with absorption dichroism (d ), which can be expressed in terms of the absorption of polarized light:
3. Result and discussion Fig. 3 shows optical absorption spectra (400–850 nm) of irradiated spot of studied LiF samples before the experiment (curve 1), after photoionization (curve 2), and after the experiment (curve 3). F2+ centers are characterized by the band in the region of 620 nm. There may be exist centers different from F2+ in used crystals; they can absorb the probe beam at λ of 632.8 nm (for example, N1,2 -centers), as well as the stable F2+ centers can also exist. However, they do not give any input into the measured degree of dichroism owing to their absorbance being equal for the both directions of polarization. Also, we should note that the concentration of induced F2+ centers is prevailing. In this work, we measure the light intensity of the probe beam (λ = 632.8 nm) transmitted through the LiF crystal with induced F2+ centers. We record the probe radiation split by a Glan prism into two beams having different directions of the polarization vector. The light intensity in the split beams is proportional to the voltage U at the recording
Fig. 2. The direction of polarization of ionizing and probe beams relatively orientations of color centers: a – photoionization of - centers, b – probe beam immediately after photoionization of - centers, c – probe beam after second relaxation of centers, d – polarization direction of the probe beam separated by a Glan prism.
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Journal of Luminescence 192 (2017) 283–287
N.V. Shipitsin et al.
Fig. 3. Optical absorption spectra (400–850 nm) of irradiated spot on the studied LiF sample: before the experiment (curve 1), after photoionization (curve 2), and after the experiment (curve 3).
Fig. 4. a – Photodetector voltage vs. time dependency (directly proportional to light intensity transmitted through the sample). Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation I∥. Curve 2 denotes voltage U⊥ at the channel recording intensity I⊥ of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. b – Dichroism vs. time dependency. Solid line is experimental data. Line with filled circles is an approximation using exponent with the decay constant of 3.63 × 10−2 s−1.
parallel to the direction of that of the photoionizing beam, and increase of population F2+ color centers oriented perpendicular to the direction of that of the photoionizing beam. It confirms the reorientation of F2+ color centers. Further, the populations of F2+ color centers in both orientation directions tend to the same value. The substitution of the light intensities, expressed from the Bouguer-Baer law into the expression for d , yields a formula for the determination of dichroism in terms of the intensity of the polarization components of the probe beam:
photodiodes. Fig. 4a shows the experimental data of U as a function of time at temperature of 305 K. Fig. 5a shows the experimental data of U as a function of time at temperature of 320 K. Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation (I∥). Curve 2 denotes voltage U⊥ at the channel recording intensity (I⊥) of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. The sharp spikes observed in Figs. 4a and 5a are the pulses of photoionizing radiation (2 (Fig. 4a), 3 pulses (Fig. 5a)). After these pulses, the abrupt decrease in U∥ at the channel recording I∥ is observed. It witnesses that the absorption of the probe beam owing to the creation of oriented F2+ color centers occurs. We then observe the increase in U∥ (and corresponding increase in I∥) and simultaneous decrease in U⊥ (and corresponding decrease in I⊥). It speaks for the decline of population of F2+ color centers oriented
d = ln
I I⊥ − ln . I0 I0 ⊥
The time dependences of dichroism are shown in Fig. 4b (T = 305 K) and Fig. 5b (T = 320 K), solid line is experimental data, line with filled circles is an exponential approximation. The decline of dichroism from 0.1 to 0 within the limits of experimental accuracy was also observed. The time dependence of d within 285
Journal of Luminescence 192 (2017) 283–287
N.V. Shipitsin et al.
Fig. 5. a – Photodetector voltage vs. time dependency (directly proportional to light intensity transmitted through the sample). Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation I∥. Curve 2 denotes voltage U⊥ at the channel recording intensity I⊥ of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. b – Dichroism vs. time dependency. Solid line is experimental data. Line with filled circles is an approximation using exponent with the decay constant of 2.01 × 10−1 s−1.
Fig. 6. Logarithm of the decrease rate of dichroism vs. temperature−1. Circles – experimental data, solid line – linear approximation.
using the temperature dependence of the exponent index. The obtained experimental value of the activation energy (Ea) is 1.00 ± 0.02 eV (herein, mean square error is indicated). The data on the kinetics of the degree of F2+ centers orientation were shown in Fig. 6. Ea is obtained by mean of a linear best fit procedure. The reduction of d witnesses that the reorientation of F2+ centers occurs.
the experimental error is exponential. Error of dichroism measurements is about 0.005. The Fig. 4b data are approximated using the decay constant of 3.63 × 10−2 s−1, whereas the Fig. 5b data are approximated with the decay constant of 2.01 × 10−1 s−1. In addition to absorption dichroism, during the formation of F2+ centers in LiF crystals birefringence arises [14], however, it does not lead to changes in the measured intensities as the observation is carried out for polarizations corresponding to the ordinary and extraordinary rays, which propagate without changes in the intensities due to birefringence. Besides this, in our measurements, dichroism is no more than 0.1, which leads to a phase difference only, and it allows one to ignore the errors associated with small inaccuracies in the alignment of the experimental setup. Fresnel losses do not affect measured dichroism, because the probing beam falls normally to the surface of the sample, while the Fresnel losses are the same for any direction of polarization. The activation energy of the F2+ centers rotation was determined
4. Conclusions In works [10,11], it was founded that the activation energies of the destruction of F2+ color centers are 1.07 ± 0.01 eV and 1.1 ± 0.1 eV, respectively. The difference between the activation energy obtained in the present study and that of the destruction of F2+ centers obtained in [9] is 0.07 eV. This is slightly higher than the statistical evaluation of measurement uncertainty, although keeping in mind the possible systematic error one should assume that the results are in a good agreement. In addition, the destruction of F2+ color centers is a complex and 286
Journal of Luminescence 192 (2017) 283–287
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laser tunable in 820–1210 nm range, Opt. Commun. 147 (1–3) (1998) 107–111. [5] A.G.V. Spivey, V.V. Fedorov, M.M. McKerns, C.M. Lawson, S.B. Mirov, Amplification of narrow line LiF: F2 + ** color center laser oscillation, Opt. Commun. 254 (4–6) (2005) 290–298. [6] G. Baldacchini, R.M. Montereali, New perspectives of coloured LiF for optoelectronic devices, Opt. Mater. 16 (1–2) (2001) 53–61. [7] R.M. Montereali, F. Bonfigli, M. Piccinini, E. Nichelatti, M.A. Vincenti, Photoluminescence of colour centres in lithium fluoride thin films: from solid-state miniaturised light sources to novel radiation imaging detectors, J. Lumin. 170 (2016) 761–769. [8] N.A. Ivanov, E.D. Isyanova, F.V. Karpushko, B.D. Lobanov, N.T. Maksimova, A.M. Provorov, N.A. Saskevich, G.V. Sinitsyn, V.M. Khulugurov, A.G. Shneider, A.S. Yasyukevich, Flash lamp-pumped LiF crystal laser with stable F2+ color centers, Sov. J. Quantum Electron. 16 (11) (1986) 1536–1538. [9] A.P. Voitovich, V.S. Kalinov, L.P. Runets, A.P. Stupak, E.F. Martynovich, R.M. Montereali, G. Baldacchini, Color centers aggregation kinetics in lithium fluoride after gamma irradiation, J. Lumin. 143 (2013) 207–214. [10] S.A. Zilov, A.P. Voitovich, S.V. Bojchenko, A.V. Kuznetsov, V.P. Dresvyanskiy, A.L. Rakevich, A.V. Bartul, K. Koenig, E.F. Martynovich, Quantum trajectories of photoluminescence of F2 centers in a LiF crystal, in: Proceedings of the XIV International Conference Luminescence and Laser Physics, Bulletin of the Russian Academy of Sciences: Physics, vol. 80(1), 2016, pp. 81–84. [11] L.A. Lisitsyna, Kinetics of the relaxation of the absorption by F2 centers during irradiation pulses in lithium fluoride crystals, Sov. Phys. Solid State 34 (9) (1992) 1441–1447. [12] A.P. Voitovich, V.S. Kalinov, E.F. Martynovich, A.N. Novikov, A.P. Stupak, Systematic features of diffusion and aggregation of intrinsic defects in dielectric crystals, Phys. Solid State 54 (9) (2012) 1768. [13] P.P. Feofilov, The Physical Basis of Polarized Emission, Consultants Bureau, New York, 1961. [14] V.A. Astapenko, V.M. Buimistrov, V.G. Dmitriev, B.G. Lysoi, Photo-induced birefringence in LiF crystals with F2 colour centres, Quantum Semiclass. Opt. 10 (1998) 233–237.
multi-stage process, and some other processes, complementary to the center rotation, can moderately affect the values of the measured activation energies. It can also leads to some differences in the measured activation energies. Thus, activation energy of the elementary process of thermally induced F2+ centers rotation is consistent with that of their destruction, which confirms the hypothesis of a prevailing contribution of F2+ centers diffusion to the mechanism of their destruction in LiF crystals. Acknowledgements This work was partially supported by the base part of the Government Assignment for Scientific Research from the Ministry of Education and Science of Russia (Project codes: 13.7232.2017/8.9 and 3.7913.2017/7.8). The equipment of the joint center ‘Baikal Center of Nanotechnologies’ was used. References [1] C. Furetta, P.-S. Weng, Operational Thermoluminescence Dosimetry, World Scientific Publ. Co., Singapore, 1998. [2] T.T. Basiev, P.G. Zverev, S.B. Mirov, in: Handbook of Laser Technology and Applications, Taylor & Francis Group, CRC Press, Boca Raton, USA, 2003, pp. 499–522. [3] T.T. Basiev, S.B. Mirov, V.V. Osiko, Room-temperature color center lasers, IEEE J. Quantum Electron. 24 (1988) 1052–1069. [4] A.Y. Dergachev, S.B. Mirov, Efficient room temperature LiF: F2 + ** color center
287
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermally induced diffusion of F2+ color centers in lithium fluoride crystals a,⁎
b
a
a
N.V. Shipitsin , A.I. Krivoshein , N.A. Ivanov , I.K. Petrushenko , A.E. Rzhechitskii a b
MARK
a
Irkutsk National Research Technical University, 664074, 83 Lermontov st., Irkutsk, Russia Bauman Moscow State Technical University, 105005, 5 Vtoraya Baumanskaya st., Moscow, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: LiF Color centers Diffusion Dichroism Reorientation
The existence of thermally induced diffusion of F2+ centers was shown by measuring the coefficient of optical absorption dichroism of gamma-irradiated LiF crystals. An elementary act of diffusion is a rotation of an F2+ center with the shift of one of the vacancies in the neighbor lattice site, thus, there is a displacement of the center as a whole. The measured activation energy of rotation is 1.0 eV, which is in a good agreement with the energies of the activation process of F2+ centers thermal destruction obtained before.
1. Introduction The crystals of lithium fluoride (LiF) with radiation-induced color centers are widely used in dosimetry [1] and as the laser media with broadband rearrangement of the radiation frequency in the visible and near infrared regions of the spectrum [2]. Generation of the radiation at laser pumping in gamma-irradiated LiF crystals is available in a few types of color centers, such as F2 , F3+, F2+, F2−, and in the color centers associated with an impurity of Mg [3–5]. There is a growing interest in color centers in thin films of LiF at present [6,7]. F2+ centers are unstable at room temperature in all alkali halide crystals (AHC), which are not activated by impurities, and they are destructed in a few hours. A small concentration of stable at room temperature F2+ centers can be formed only in LiF and NaF by doping the crystals with a hydroxyl impurities [8]. A number of radiation-induced defects are mobile, therefore they have a significant impact on the formation of the aggregation of color centers and the optical properties of crystals. In particular, the mobility of anion vacancies and F2+ color centers is well known [9]. However, the direct observations of the diffusion of F2+ centers are lacking in literature, although F2+ centers, owing to their charge states, can interact with neutral and negatively charged defects during the migration over the crystal. The work [10] studied the reorientation of the F2 centers in LiF crystals using the fluorescence microscopy. The process of the destruction of F2+ centers was investigated in papers [9,11,12] mainly by measuring the kinetics, which reflects the overall process of centers destruction, comprising several reactions of interaction with radiation defects of the crystal as a result of their mutual thermal diffusion. However, the issue on the contribution of thermal diffusion of F2+ color centers on the interaction of defects
⁎
Corresponding author. E-mail address: [email protected] (N.V. Shipitsin).
http://dx.doi.org/10.1016/j.jlumin.2017.06.065 Received 5 September 2016; Received in revised form 16 June 2017; Accepted 28 June 2017 Available online 30 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
remains open. This paper studies the process of thermally induced reorientation and, as a consequence, the diffusion of F2+ color centers in LiF crystals. We measured the kinetics of relaxation of dichroism nearly the maximum (620 nm) of the F2+ centers absorption band induced by intense laser radiation of a colored LiF crystal with a wavelength of 532 nm. The aims of this work are the direct observation of the thermal reorientation of F2+ centers as an elementary act of diffusion by the method of measurement of polarization absorption dichroism, determination of the activation energy of this process, and comparison of the values obtained from the kinetic measurements. 2. Materials and methods In this work, we used LiF crystals grown in the air by Kyropoulos method. They were irradiated with gamma radiation by using 60Co source at room temperature, exposure dose of about 4.4 × 105 Gy. The absorption coefficient at the F2 centers band maximum (446 nm) is 30.8 cm−1. The thickness of the studied samples is 1.5 mm. In order to create dichroism in the crystals, we employ photoionization of F2 centers using intense linearly polarized radiation from a neodymium (Nd: YAG) laser operating in a pulsed mode with a pulse repetition rate of 1 Hz, a pulse duration of 20–25 ns, and the pulse energy is 2 mJ with frequency doubling (λ = 532 nm); the centers with larger projection of the polarization on the axis of the center were mainly ionized. As a result, the formed F2+ centers are dominantly oriented in parallel to the polarization vector. A He-Ne laser LH-75 (λ = 632.8 nm) with a linearly polarized radiation was used as the source of probe radiation. Photodiodes PD-24K (in the photoconductive mode) were used as photodetectors in the experimental setup. Tempering of the sample was carried out in a cryostat SHI-4-1 (JanisResearch, USA). The
Journal of Luminescence 192 (2017) 283–287
N.V. Shipitsin et al.
d = α∥ − α⊥, where α|| is the absorption of light with the direction of the polarization vector parallel to the direction of the polarization vector of the photoionizing beam, α⊥ is the light absorption with the direction of the polarization vector perpendicular to the direction of the polarization vector of the photoionizing beam. The polarization vector of the photoionizing beam was parallel to the [110] axis of the crystal. The angle between the direction of the polarization vector of the probe beam and the direction of the polarization vector of the photoionizing beam was 45°. The probe beam that has passed through the crystal is split into two beams by a Glan prism (Fig. 2). The first measured beam had the direction of the polarization vector coinciding with that of the photoionizing beam; the second beam had the perpendicular direction of the polarization vector. Thus, we simultaneously measure the absorption of the probe beam with the polarization vector both parallel to the direction of that of the photoionizing beam and perpendicular to it. It should be noted that upon photoionization, the formation of F2+ color centers occurs not only in the (001) plane, but also in the (100) and (010) planes. However, the possibility of formation of F2+ color centers oriented in the [110] direction (in parallel to the polarization vector of photoionized radiation) is much more essential. Thus, the larger quantity of F2+ color centers formed upon photoionization and oriented in the [110] direction leads to observed dichroism.
Fig. 1. The scheme of vacancy shifts upon the reorientation of an F2+ center. The arrows depict the possible directions of vacancy shifts.
measurements of kinetics of relaxation of dichroism were performed in the temperature range from 300 to 320 K. Optical absorption spectra of the samples were measured by a spectrophotometer Shimadzu UV3600. As is well known, an F2+ center is two anionic vacancies in neighbor lattice sites and an electron localized on them. F2+ centers in LiF crystals, not perturbed by other defect, are destructed for about 10 h at room temperature. At present, it is assumed that they are destructed as a result of diffusion and the following entry into reaction with other defects in the crystal lattice [12]. The proposed mechanism of diffusion of F2+ centers is a sequence of reorientations of an F2+ center with the shift of one of the vacancies in the neighbor lattice site, thus, there is a displacement of the center as a whole (Fig. 1). The sequence of such shifts leads to diffusion of the center over long distances and its destruction as a result of interaction with various lattice defects. In this paper, we experimentally verify this mechanism of diffusion. It is also known, F2+ color centers in AHC which oriented, for example, by means of photoionization of F2 defects with polarized radiation have pronounced absorption dichroism [13]. The degree of orientation of the centers is associated with absorption dichroism (d ), which can be expressed in terms of the absorption of polarized light:
3. Result and discussion Fig. 3 shows optical absorption spectra (400–850 nm) of irradiated spot of studied LiF samples before the experiment (curve 1), after photoionization (curve 2), and after the experiment (curve 3). F2+ centers are characterized by the band in the region of 620 nm. There may be exist centers different from F2+ in used crystals; they can absorb the probe beam at λ of 632.8 nm (for example, N1,2 -centers), as well as the stable F2+ centers can also exist. However, they do not give any input into the measured degree of dichroism owing to their absorbance being equal for the both directions of polarization. Also, we should note that the concentration of induced F2+ centers is prevailing. In this work, we measure the light intensity of the probe beam (λ = 632.8 nm) transmitted through the LiF crystal with induced F2+ centers. We record the probe radiation split by a Glan prism into two beams having different directions of the polarization vector. The light intensity in the split beams is proportional to the voltage U at the recording
Fig. 2. The direction of polarization of ionizing and probe beams relatively orientations of color centers: a – photoionization of - centers, b – probe beam immediately after photoionization of - centers, c – probe beam after second relaxation of centers, d – polarization direction of the probe beam separated by a Glan prism.
284
Journal of Luminescence 192 (2017) 283–287
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Fig. 3. Optical absorption spectra (400–850 nm) of irradiated spot on the studied LiF sample: before the experiment (curve 1), after photoionization (curve 2), and after the experiment (curve 3).
Fig. 4. a – Photodetector voltage vs. time dependency (directly proportional to light intensity transmitted through the sample). Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation I∥. Curve 2 denotes voltage U⊥ at the channel recording intensity I⊥ of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. b – Dichroism vs. time dependency. Solid line is experimental data. Line with filled circles is an approximation using exponent with the decay constant of 3.63 × 10−2 s−1.
parallel to the direction of that of the photoionizing beam, and increase of population F2+ color centers oriented perpendicular to the direction of that of the photoionizing beam. It confirms the reorientation of F2+ color centers. Further, the populations of F2+ color centers in both orientation directions tend to the same value. The substitution of the light intensities, expressed from the Bouguer-Baer law into the expression for d , yields a formula for the determination of dichroism in terms of the intensity of the polarization components of the probe beam:
photodiodes. Fig. 4a shows the experimental data of U as a function of time at temperature of 305 K. Fig. 5a shows the experimental data of U as a function of time at temperature of 320 K. Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation (I∥). Curve 2 denotes voltage U⊥ at the channel recording intensity (I⊥) of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. The sharp spikes observed in Figs. 4a and 5a are the pulses of photoionizing radiation (2 (Fig. 4a), 3 pulses (Fig. 5a)). After these pulses, the abrupt decrease in U∥ at the channel recording I∥ is observed. It witnesses that the absorption of the probe beam owing to the creation of oriented F2+ color centers occurs. We then observe the increase in U∥ (and corresponding increase in I∥) and simultaneous decrease in U⊥ (and corresponding decrease in I⊥). It speaks for the decline of population of F2+ color centers oriented
d = ln
I I⊥ − ln . I0 I0 ⊥
The time dependences of dichroism are shown in Fig. 4b (T = 305 K) and Fig. 5b (T = 320 K), solid line is experimental data, line with filled circles is an exponential approximation. The decline of dichroism from 0.1 to 0 within the limits of experimental accuracy was also observed. The time dependence of d within 285
Journal of Luminescence 192 (2017) 283–287
N.V. Shipitsin et al.
Fig. 5. a – Photodetector voltage vs. time dependency (directly proportional to light intensity transmitted through the sample). Curve 1 denotes voltage U∥ at the channel recording intensity of the beam with the direction of polarization vector coinciding with that of photoionizing radiation I∥. Curve 2 denotes voltage U⊥ at the channel recording intensity I⊥ of the beam with the direction of polarization vector perpendicular with that of photoionizing radiation. b – Dichroism vs. time dependency. Solid line is experimental data. Line with filled circles is an approximation using exponent with the decay constant of 2.01 × 10−1 s−1.
Fig. 6. Logarithm of the decrease rate of dichroism vs. temperature−1. Circles – experimental data, solid line – linear approximation.
using the temperature dependence of the exponent index. The obtained experimental value of the activation energy (Ea) is 1.00 ± 0.02 eV (herein, mean square error is indicated). The data on the kinetics of the degree of F2+ centers orientation were shown in Fig. 6. Ea is obtained by mean of a linear best fit procedure. The reduction of d witnesses that the reorientation of F2+ centers occurs.
the experimental error is exponential. Error of dichroism measurements is about 0.005. The Fig. 4b data are approximated using the decay constant of 3.63 × 10−2 s−1, whereas the Fig. 5b data are approximated with the decay constant of 2.01 × 10−1 s−1. In addition to absorption dichroism, during the formation of F2+ centers in LiF crystals birefringence arises [14], however, it does not lead to changes in the measured intensities as the observation is carried out for polarizations corresponding to the ordinary and extraordinary rays, which propagate without changes in the intensities due to birefringence. Besides this, in our measurements, dichroism is no more than 0.1, which leads to a phase difference only, and it allows one to ignore the errors associated with small inaccuracies in the alignment of the experimental setup. Fresnel losses do not affect measured dichroism, because the probing beam falls normally to the surface of the sample, while the Fresnel losses are the same for any direction of polarization. The activation energy of the F2+ centers rotation was determined
4. Conclusions In works [10,11], it was founded that the activation energies of the destruction of F2+ color centers are 1.07 ± 0.01 eV and 1.1 ± 0.1 eV, respectively. The difference between the activation energy obtained in the present study and that of the destruction of F2+ centers obtained in [9] is 0.07 eV. This is slightly higher than the statistical evaluation of measurement uncertainty, although keeping in mind the possible systematic error one should assume that the results are in a good agreement. In addition, the destruction of F2+ color centers is a complex and 286
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laser tunable in 820–1210 nm range, Opt. Commun. 147 (1–3) (1998) 107–111. [5] A.G.V. Spivey, V.V. Fedorov, M.M. McKerns, C.M. Lawson, S.B. Mirov, Amplification of narrow line LiF: F2 + ** color center laser oscillation, Opt. Commun. 254 (4–6) (2005) 290–298. [6] G. Baldacchini, R.M. Montereali, New perspectives of coloured LiF for optoelectronic devices, Opt. Mater. 16 (1–2) (2001) 53–61. [7] R.M. Montereali, F. Bonfigli, M. Piccinini, E. Nichelatti, M.A. Vincenti, Photoluminescence of colour centres in lithium fluoride thin films: from solid-state miniaturised light sources to novel radiation imaging detectors, J. Lumin. 170 (2016) 761–769. [8] N.A. Ivanov, E.D. Isyanova, F.V. Karpushko, B.D. Lobanov, N.T. Maksimova, A.M. Provorov, N.A. Saskevich, G.V. Sinitsyn, V.M. Khulugurov, A.G. Shneider, A.S. Yasyukevich, Flash lamp-pumped LiF crystal laser with stable F2+ color centers, Sov. J. Quantum Electron. 16 (11) (1986) 1536–1538. [9] A.P. Voitovich, V.S. Kalinov, L.P. Runets, A.P. Stupak, E.F. Martynovich, R.M. Montereali, G. Baldacchini, Color centers aggregation kinetics in lithium fluoride after gamma irradiation, J. Lumin. 143 (2013) 207–214. [10] S.A. Zilov, A.P. Voitovich, S.V. Bojchenko, A.V. Kuznetsov, V.P. Dresvyanskiy, A.L. Rakevich, A.V. Bartul, K. Koenig, E.F. Martynovich, Quantum trajectories of photoluminescence of F2 centers in a LiF crystal, in: Proceedings of the XIV International Conference Luminescence and Laser Physics, Bulletin of the Russian Academy of Sciences: Physics, vol. 80(1), 2016, pp. 81–84. [11] L.A. Lisitsyna, Kinetics of the relaxation of the absorption by F2 centers during irradiation pulses in lithium fluoride crystals, Sov. Phys. Solid State 34 (9) (1992) 1441–1447. [12] A.P. Voitovich, V.S. Kalinov, E.F. Martynovich, A.N. Novikov, A.P. Stupak, Systematic features of diffusion and aggregation of intrinsic defects in dielectric crystals, Phys. Solid State 54 (9) (2012) 1768. [13] P.P. Feofilov, The Physical Basis of Polarized Emission, Consultants Bureau, New York, 1961. [14] V.A. Astapenko, V.M. Buimistrov, V.G. Dmitriev, B.G. Lysoi, Photo-induced birefringence in LiF crystals with F2 colour centres, Quantum Semiclass. Opt. 10 (1998) 233–237.
multi-stage process, and some other processes, complementary to the center rotation, can moderately affect the values of the measured activation energies. It can also leads to some differences in the measured activation energies. Thus, activation energy of the elementary process of thermally induced F2+ centers rotation is consistent with that of their destruction, which confirms the hypothesis of a prevailing contribution of F2+ centers diffusion to the mechanism of their destruction in LiF crystals. Acknowledgements This work was partially supported by the base part of the Government Assignment for Scientific Research from the Ministry of Education and Science of Russia (Project codes: 13.7232.2017/8.9 and 3.7913.2017/7.8). The equipment of the joint center ‘Baikal Center of Nanotechnologies’ was used. References [1] C. Furetta, P.-S. Weng, Operational Thermoluminescence Dosimetry, World Scientific Publ. Co., Singapore, 1998. [2] T.T. Basiev, P.G. Zverev, S.B. Mirov, in: Handbook of Laser Technology and Applications, Taylor & Francis Group, CRC Press, Boca Raton, USA, 2003, pp. 499–522. [3] T.T. Basiev, S.B. Mirov, V.V. Osiko, Room-temperature color center lasers, IEEE J. Quantum Electron. 24 (1988) 1052–1069. [4] A.Y. Dergachev, S.B. Mirov, Efficient room temperature LiF: F2 + ** color center
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