Bifunctional Bi2ZnOB2O6:Nd3+ Single Crystal for Near Infrared Lasers

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Bi-functional Bi2ZnOB2O6:Nd3+single crystal for NIR lasers: luminescence and µ-Raman investigations Konrad Jaroszewski, Pawel Gluchowski, Mikhail G. Brik, Tomasz Pedzinski, Andrzej Majchrowski, Maciej Chrunik, Edward Michalskli, and Dobroslawa Kasprowicz Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Crystal Growth & Design

Bi-functional Bi2ZnOB2O6:Nd3+ single crystal for NIR lasers: luminescence and µ-Raman investigations Konrad Jaroszewski1, Pawel Gluchowski2, Mikhail G. Brik3,4,5, Tomasz Pedzinski6, Andrzej Majchrowski7, Maciej Chrunik7, Edward Michalski8, Dobroslawa Kasprowicz1* 1

Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznań, Poland 2 Institute of Low Temperature and Structure Research of Polish Academy of Sciences, Okólna 2, 50-422 Wrocław, Poland 3 College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, 2 Chongwen Road, Nan'an District, Chongqing 400065, P.R. China 4 Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia 5 Institute of Physics, Jan Długosz University, Armii Krajowej 13/15, 42-200 Częstochowa, Poland 6 Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland 7 Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warszawa, Poland 8 Institute of Optoelectronics, Military University of Technology, Kaliskiego 2, 00-908 Warszawa, Poland Abstract Bi2ZnOB2O6 single crystal doped with Nd3+ ions is characterized by high values of nonlinear optical coefficients as well as the effective luminescence of excited Nd3+ ions, which make this system a unique candidate for NIR to VIS laser converters. The investigated Bi2ZnOB2O6:Nd3+ single crystal was grown by means of Kyropoulos method. The vibrational properties of Bi2ZnOB2O6:Nd3+ were studied using µ-Raman spectroscopy. In particular, the Raman-active modes detected in parallel and cross polarizations were assigned to the vibrations of the characteristic molecular groups BO3, BO4, ZnO4 and BiO6. In the absorption spectra of Bi2ZnOB2O6:Nd3+ the bands related to the optical transitions from the 4I9/2 ground state to the excited states of Nd3+ ions were detected at 432, 515, 528, 533, 577, 586, 689, 750, 810, 874 and 1605 nm. Moreover, the strong emission of Bi2ZnOB2O6:Nd3+ with maximum at about 1062 nm (4F3/2→4I11/2 transition) was detected under the excitation at 514 nm. The decay kinetics profile monitored for the 4F3/2→4I11/2 transition of Nd3+ ions show a relatively long fluorescence lifetime equal to 109 µs, which allows efficient emission from the 4F3/2 level of Nd3+ ions. Because of the good spectroscopic properties of investigated system as well as nonlinear *Corresponding author, Tel.: +48 61 6653247; Fax: +48 61 6653164 E-mail adress: [email protected] ACS Paragon Plus Environment

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optical properties of the host, the Bi2ZnOB2O6:Nd3+ single crystal can be effectively used for the self-frequency doubling lasers. Keywords: Bi2ZnOB2O6; rare earth ions; Nd3+, luminescence; bi-functional materials

1. Introduction Crystals doped with Nd3+ ions are one of the most useful sources of near infrared (NIR) radiation, which can be used for many applications including solid-state lasers,1 optical amplifiers utilized in infrared optical communications2 or many integrated optical devices.3 This utility of the Nd3+ doped crystals arise from the specific structure of the electronic energy levels of Nd3+ ions. It is known that the 4f3 electron configuration is well shielded in Nd3+ ions and the term values are virtually the same in a crystal or free Nd3+ ions.4 The most efficient NIR emission of Nd3+ ions at wavelength around 946, 1064 and 1350 nm are attributed to the 4F3/2→4IJ (J=9/2, 11/2, 13/2) transitions.4 In particular Nd3+ doped systems have been extensively investigated for their important laser radiation at around 1064 nm (4F3/2→4I11/2). One of the required properties of good laser efficiency is a long fluorescence lifetime for the 4F3/2 level of Nd3+ ions, which is significantly influenced by non-radiative decay. It is mostly caused by multi-phonon relaxation and energy transfer process such as emission self-quenching due to Nd3+–Nd3+ interactions.5 All these processes can be optimized by suitable Nd3+ ions concentration as well as matrix properties (crystals or glasses). It is extremely important to use the accurate concentration and homogeneous distribution of Nd3+ ions in the matrix to obtain the laser materials with expected luminescence properties of

Nd3+ ions. Maximum

concentration of the Nd3+ ions depends on the host properties and is typically lower for crystals and higher for glasses. Usually, increasing of the Nd3+ ions concentration results in emission quenching of the 4F3/2→4I11/2 transition. This leads to the shortening of this luminescence lifetimes and finally to the emission efficiency decreasing. Most of the commercially available laser crystals activated with neodymium ions such as YAG, YVO4 or YAlO3 are doped with small amount of Nd3+ ions, usually at about 1%.6,7 Luminescence quenching observed at higher Nd3+ concentration can be caused by the multi-phonon relaxation as well as enhanced Nd3+–Nd3+ interactions leading to the cross-

ACS Paragon Plus Environment

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relaxation among active ions.5,8 These processes affect the fluorescence decay kinetics so that, for the intensive non-radiative transition shortening of luminescence lifetimes is observed. Laser materials characterized by longer lifetime are better for Q-switch laser performance (due to the higher energy storage), while crystals with a shorter lifetimes are more adapted for cw-lasers because of the laser beam quality.9 For mostly used Nd:YAG single crystals the fluorescence lifetime is relatively long and is about 230 µs, while for the Nd:YVO4 crystals decay time is equal 100 µs.7,10 Recently, some new crystals or glasses including heavy elements in the lattice structure (such as bismuth) were found to be good hosts for Nd3+ ions.11 Presence of the heavy metal ions leads to the efficient radiative transitions in the rare earth (RE) doped systems due to their higher refractive index and low phonons energy.12 Many bi-functional optical crystals showing the luminescence originated from RE ions and the nonlinear optical (NLO) effect of the host were reported as very attractive materials for new generation of self-optical conversion lasers.13 Bi2ZnOB2O6 single crystal (abbreviated as BZBO) is known as an effective NLO material.14 Recently RE-doped BZBO crystals were intensively studied because of their promising optical properties and consequently numerous potential applications. Moreover, BZBO materials in the form of single crystals,15 glasses16 as well as powders17-19 may be efficiently doped with RE ions resulting in effective luminescence in VIS and/or NIR spectral range. The structure and some of vibrational properties of pure and Pr3+-doped BZBO single crystal were previously reported by us.20 In this paper we present for the first time the results of spectroscopic investigations (absorption, luminescence and vibrational properties) of a new NLO borate: BZBO:Nd3+ single crystal. To our best knowledge Nd3+-doped BZBO single crystal, in particular its spectroscopic properties, have not been reported so far. The vibrational properties of this system have been investigated using µ-Raman spectroscopy method. To study the optical properties the room and low temperature absorption spectra as well as luminescence spectra and fluorescence decay profile were measured and analyzed.


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2. Experimental details BZBO:Nd3+ single crystal, due to their congruent melting near 690°C and lack of unwanted phase transitions were grown from stoichiometric melts by means of the Kyropoulos method similar to that described in Ref.21-23 The growth was carried out under conditions of low temperature gradients, therefore crystals grew in the volume of the melt and were confined with crystallographic planes. The seeds were oriented along [001] direction. As-grown [001] BZBO:Nd3+ single crystal is presented in Figure 1.

Figure 1.

The Raman spectra as well as the absorption and luminescence investigations of BZBO:Nd3+ single crystal were measured for the oriented samples of sizes 5×4×1mm3 with edges parallel to the crystallographic axes a, b and c of the orthorhombic system. The samples' orientation was performed using the Laue method and analytical use of the stereographic Wulff net.24 The initial concentration of Nd3+ ions in the melt as-grown crystal was close to 1 at.%. The Raman investigations were performed in back-scattering geometry using Renishaw InVia Raman microscope equipped with confocal DM 2500 Leica optical microscope and CCD detector. The polarized Raman spectra were excited with Ar laser emitting at 488 nm and recorded in a single scan with a 60 s exposure time. The applied power of the laser beam before focusing was less than 0.5 mW. The position of the Raman peaks was calibrated before collecting data using a Si reference sample as an internal standard with the peak position at 520.3 cm-1. The diffraction-limited optical spatial resolution depends on the excitation wavelength and for the used 50 x LWD objective was approximately equal to 0.8 to 1.3 µm. The instrumental resolution of spectrometer was better than 2 cm-1. The spectral analysis was performed with the software packages WiRE 3.1 /Renishaw. The room-temperature absorption spectra of BZBO:Nd3+ crystal were recorded in the 300–2500 nm spectral range using UV-VIS-NIR spectrophotometer (Carry 5000) with spectral resolution equal to 0.1 nm.


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Absorption spectra at 10 K were recorded with Varian 5E UV–VIS-NIR spectrophotometer. Instrument spectral bandwidth was set to 0.1 nm in UV–VIS (200– 845 nm) and 0.7 nm in IR (845–2700 nm) region. Emission spectrum was measured using FLS980 Fluorescence Spectrometer from Edinburgh Instruments equipped with 450 W Xenon lamp (set at 514 nm), holographic grating of 1800 lines/mm, blazed at 250nm and 300 mm focal length monochromators in Czerny Turner configuration. The emission spectrum was recorded with 0.1 nm resolution. For decay times measurements the 450 W excitation source was replaced with 100 W lamp (µF2) with external triggering.

3. Results and discussion 3.1. µ-Raman spectra 8 The BZBO:Nd3+ crystallizes in the orthorhombic structure of Pba2 ≡ C 2v space

group. The structure of BZBO is formed by chains of (B3O6)3− and Zn2+ ions along the aaxis. Along the c-axis the borate–zinc–oxygen layers alternate with cationic layers made of Bi3+ ions.21 Due to the size similarity of the ionic radii of the Nd3+ and Bi3+ ions in the doped BZBO crystal the Bi3+ ions can be efficiently substituted by Nd3+ ions. The ionic radii of the Bi3+ and Nd3+ are very close to each other (1.03 and 0.983 Å, correspondingly).25 There are some characteristic molecular groups in the structure of BZBO single crystal: BO3 (planar molecule), BO4 tetrahedra, ZnO4 tetrahedra and BiO6 octahedra, giving most of the bands in Raman spectra. The structure of BZBO and BZBO:Pr3+ crystals has been already precisely described in our last paper.20 The primitive unit cell of BZBO:Nd3+ crystal has 4 formula units and contains 48 atoms, giving 144 fundamental vibrations. From the group theory analysis for orthorhombic BZBO crystal (space group Pba2) all active vibrations can be classified according to the following formula: 35A1 + 35A2 + 37B1 + 37B2, where A1, A2, B1 and B2 describe the possible symmetry of all normal modes of the considered system. Except three of them (acoustic modes) the remaining 141 normal modes are Raman or IR active.26,27 The Raman active modes of BZBO:Nd3+ crystal were registered in the set of different back-scattering geometries for parallel z(xx)z, z(yy)z and x(zz)x and cross z(xy)z, y(zx)y and x(yz)x configurations. The A1 modes were detected with the highest intensity in


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parallel, while A2, B1 and B2 modes in cross configurations, respectively. Figure 2a-b show polarized Raman spectra of BZBO:Nd3+ single crystal detected in the 1600 to 100 cm-1 spectral range.

Figure 2.

In the spectra we observe the bands related with the translations of Bi (Nd) and Zn atoms and BO3 and BO4 groups and modes corresponding to the stretching, bending and librational vibrations of BO3, BO4, ZnO4 and BiO6 groups. The detailed assignment of all Raman-active bands/modes and their determined wavenumbers values were presented in the Table 1. As seen from the Figure 2a-b and Table 1, in the spectral range from 1450 to 690 cm-1 the bands attributed to stretching vibrations of the BO3, BO4 or BiO6 groups occur. In particular, the medium modes at 1341 cm-1 (x(zz)x and y(zx)y polarizations), mode at 1331 cm-1 in x(yz)x polarization and the weak bands at 962/967 cm-1 (x(zz)x and z(xy)z polarizations) can be assigned to vibrations of BO3 or BO4 groups. The weak band located at 1270 cm-1 in z(xx)z and medium bands at 870/866 cm-1 in x(yz)x and z(xy)z polarizations, respectively originate from stretching vibrations of BiO6 octahedra.28 Moreover, the bands at 1182 and 1010 cm-1 detected with highest intensity in x(yz)x polarization

correspond

to

anti-symmetric

stretching

29

vibrations

of

BO4

-1

groups/tetrahedra, while the relatively low intensity Raman band at 917 cm detected in z(xx)z polarization corresponds to stretching vibrations of BO3 group. The most intensive

modes in this range recorded at 699 cm-1 and the lower intensity modes at 741 cm-1 detected in all parallel polarizations and with low or very low intensity in the cross polarizations and the weak band at 713 cm-1 detected in z(xy)z scattering configuration arise from the symmetric stretching vibrations of BO4 tetrahedra. The bending modes of BO3, BO4, BiO6 and ZnO4 groups occur in the spectral range 660 – 375 cm-1. The highest intensity mode in this range recorded at 635 cm-1 in z(yy)z scattering geometry corresponds to the bending vibration of BO3 group. The modes

detected in the range 610 – 600 cm-1 and 430 – 410 cm-1 are assigned to the vibrations of BiO6 polyhedra/octahedra and ZnO4 tetrahedra, respectively. The double structure band


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with modes at 589 and 581 cm-1 recorded for the z(xx)z and z(yy)z scattering configurations are assigned to the stretching vibrations of BiO6 polyhedra and ZnO4 tetrahedra, respectively.28 Moreover, the corresponding mode at 591 cm-1 were recorded in x(yz)x scattering configuration while the modes centered at about 580 cm-1 were also detected in x(zz)x and crossed polarizations z(xy)z and y(zx)y. Several low intensity modes were observed in the spectral range 560 – 410 cm-1 (the positions of the peaks are collected in Table 1) for different scattering configurations. These modes arise mainly from the bending vibrations of BO4 tetrahedra and the stretching vibrations of BiO6 polyhedra/octahedra. A strong bands occurring in the Raman spectrum centered at 397 and 390 cm-1 in the z(xx)z and x(zz)x scattering geometry, respectively can be attributed to Bi–O–Bi vibrations of BiO6 polyhedra/octahedra.20,30 The external/lattice modes assigned to translational and librational motions of BO3 and BO4 groups and Bi and Zn atoms are detected in the spectral range below 370 cm-1. The translational motions of the BO3 and BO4 groups were detected in the higher wavenumber spectral range 370 – 330 cm-1 with the relatively low intensity modes at around 352 cm-1 detected in the z(yy)z and all cross scattering geometries. Other very low intensity modes located in the spectral range 330 – 300 cm-1 can be assigned to the vibrations of BiO6 units. The Raman modes recorded in the spectral range 290 – 140 cm-1 mainly correspond to the librational motions of BO3 and BO4 groups. In particular, the other lattice modes centered at about 249 and 231 cm-1 were recorded with the highest intensity in the z(yy)z and z(xx)z scattering configurations, respectively. A relatively strong modes observed at 221 cm-1 in the x(zz)x scattering geometry and at 198, 160 and 141 cm-1 in z(yy)z scattering configuration were also detected in other configurations. The low energy vibrations at 103 cm-1 were detected with higher intensity for parallel than cross polarizations and can be assigned to the translational motions of BiO6 or ZnO4 groups (are connected with the relatively large atomic masses of Bi and Zn atoms).20,28,31 3.2. Absorption of BZBO:Nd3+ single crystal Absorption spectra of BZBO:Nd3+ single crystal were recorded at 300 and 10K in the spectral range 340 – 2500 nm. The bands in the spectra were assigned on the basis of the Dieke's diagram of energy levels of the Nd3+ ion.32 The 4f3 electronic configuration of


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Nd3+ ions consists of: the ground state 4I9/2 level and the excited states 4I15/2, 4F11/2, 4F3/2, 2

H11/2, 4F5/2, 2S3/2, 4F7/2, 2F9/2, 2G7/2, 4G5/2, 2K13/2, 4G9/2, 4G7/2, 2D3/2 and 2P1/2 energy levels.

Energy level scheme of Nd3+ ion is shown in Figure 3.

Figure 3.

The room temperature (RT) absorption spectra were recorded for three orientations of the sample for the incident light parallel to the direction of the crystallographic axes a, b and c of the BZBO:Nd3+ crystal (Figure 4a-b).

Figure 4.

As seen from Figure 4a and b, the RT absorption spectra are isotropic, both from the point of view of the position of the maxima and intensities of the absorption bands. For BZBO:Nd3+ single crystal strong host absorption with cutoff edge at about 350 nm is observed. A weak bands at 432 nm and in the range 455 – 485 nm can be assigned to transition from the 4I9/2 ground state to the 2P1/2 and 2D3/2 (or 2K15/2) excited states, respectively. A group of relatively low-intensive bands in the spectral range 500-545 nm arise from 4I9/2→4G7/2 (515 nm), 4I9/2→4G9/2 (528 nm) and 4I9/2→2K13/2 (533 nm) transitions. In the range from 570 nm to 605 nm the double structure band with maxima at 577 and 586 nm occurs and is attributed to the 4I9/2→4G5/2 and 4I9/2→2G7/2 transitions, respectively. A weak band at about 689 nm is assigned to transition from the 4I9/2 ground state to 4F9/2 state. The relatively intensive absorption bands are located in the spectral range 720 – 780 nm. These multi-structure bands (with maxima at about 740/744/750/755 nm) arise from 4I9/2→4F7/2 and 4I9/2→4S3/2 transitions. Another intensive band at about 810 nm corresponds to the 4I9/2→4F5/2 and 4I9/2→2H9/2 transitions. The band centered at 874 nm is due to 4I9/2→4F3/2 transitions. A very weak band at 1605 nm is assigned to the 4

I9/2→4I15/2 transitions. Additional absorption spectrum of BZBO:Nd3+ single crystal,

recorded at 10K, is presented in Figure 5. Figure 5.


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As can be seen from Figure 5 characteristic changes in spectra of BZBO:Nd3+ single crystal detected at 10K occur. These changes are typically observed for lowtemperature absorption spectra, in particular: increase in absorbance bands, reduction of their half-band width, changes in the number of bands, and shifts in the position of peak maxima. In comparison with RT absorption spectra, some additional bands can be observed for low-temperature spectra. A group of weak bands located in the spectral range of 350 – 362 nm and 622 – 630 nm can be assigned to 4I9/2→4D3/2 and 4I9/2→2H11/2 transitions, respectively. Relatively broad bands centered at 1384 and 2209 nm can arise from Bi3+ and OH- ions respectively. Low-intensive bands in NIR range located at about 2373 nm correspond to transition from the 4I9/2 ground state to the 4I13/2 excited state. Moreover, the ultraviolet cutoff absorption edge of the BZBO:Nd3+ crystals is observed at about 350 nm and in the spectral region below 350 nm the Nd3+ absorption is overlapping with strong host absorption.15 3.3 Luminescence of BZBO:Nd3+ single crystal The RT emission spectrum (Figure 6) of BZBO:Nd3+ single crystal was recorded after excitation at 514 nm wavelength and consists of two broad and asymmetric bands centered nearly at 893 and 1062 nm as well as one very weak band around 825 nm. The strong emission band of BZBO:Nd3+ single crystal in the range 1036 – 1110 nm with maximum at 1062 nm can be very useful to build lasers emitting at 1062 nm. This emission arises from 4F3/2→4I11/2 transition and is characteristic for Nd3+-doped hosts. In the range from 858 to 925 nm the broad emission band assigned to the 4F3/2→4I9/2 transition occurs. This band is finger-like structure due to the existence of Stark levels. The weak band at 825 nm is located in the spectral range 800 – 850 nm and can be assigned to (4F5/2,2H9/2)→4I9/2 transitions. Figure 6.

3.4 Decay kinetics In general, the observed lifetime τlum of the emission of RE-doped single crystals, can be determined using the following equation:33


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= + +

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(1)

where Arad is the radiative transition rate of excited RE ions, Wmp is the non-radiative transition rate of excited RE ions related to multi-phonon relaxation and WET is the nonradiative transition rate between RE ions. The radiative transition rate Arad is given as the sum of the , term calculated over all terminal states (J'), which is related to the radiative lifetime τR: 1 = , =

where , is the spontaneous emission probability for the transition between two

manifolds (the initial |〉 and final | 〉 state) of any RE ion. The , can be given by Füchtbauer–Ladenburg equation:34 8 !" # 2′ + 1 , = & "$ 2 + 1 '( ′

where c is the speed of light, λp is the wavelength of the absorption peak, n(λp) is the refractive index at λp, Σabs is the integrated absorption cross-section expressed by: & '( = ) * '( "+" = )

-! 10 /0" +" 12

σabs is the absorption cross-section, C is the Nd3+ concentration (ion/cm3), L is the sample

thickness (cm) and OD(λ) is the optical density at a given wavelength.35 In particular, for the BZBO:Nd3+ single crystal for the calculations the following values were used: J = 3/2, λp = 874 nm (4I9/2→4F3/2 transition), n(λp) = 2.08,36 L = 0.12 cm and C = 1.37x1020 ions/cm3. Finally, the total spontaneous emission probability Arad for BZBO:Nd3+ single crystal given as the sum emission probability of the 4F3/2→4I9/2 and 4

F3/2→4I11/2 transitions was estimated to be 6457 s-1 and corresponds to the Arad values

(6823 and 3868 s-1) of Nd3+ doped BZBO glass and YAG crystal.16,37 The contribution to non-radiative decay comes from multi-phonon relaxation and energy transfer between active ions. Multi-phonon relaxation can rapidly depopulate the upper excited state of RE ions, which ultimately results in luminescence quenching. This process only occurs when a small number of phonons is required to bridge the energy gap


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between the upper and lower electronic states. The energy gap depends on the type of the RE ions, while the effective phonon energy depends from the host material. In the BZBO:Nd3+ crystal the highest phonon energy at 1341 cm-1 registered in Raman spectra is related to the vibrations of B–O bonds of BO3 and BO4 groups and does not significantly influence on the multi-phonon relaxation process. As it was mentioned, the Nd3+ ions occupy the Bi positions in the crystal lattice. The Bi3+ is a heavy metal ion and the vibrational energy of Bi–O bonds of BiO6 octahedra in BZBO:Nd3+ can reduce the local phonon energy in the vicinity of Nd3+ ions. Since the highest vibrational energy of Bi–O bonds of BiO6 octahedra in BZBO:Nd3+ is around 870 cm-1, we used this value as the effective phonon energy involved in the multi-phonon relaxation process. Moreover, in BZBO:Nd3+ single crystal the energy gap of the 4F3/2→4I11/2 transition of Nd3+ ions is 9416 cm–1 (1062 nm). In particular, the Nd3+ luminescence in BZBO single crystal at 9416 cm–1 is only weakly quenched at room temperature, what can be confirmed by means of calculations of the multi-phonon relaxation rate Wmp. In the considered system the Wmp was estimated using the modified exponential energy-gap equation of Van Dijk and Schuurmans.38 0 3 = 456 7 89∆8#;

(2)

where βel and α are constants depending on the host material, ∆E is the energy gap, and hvmax is the effective phonon energy. The α constant can be calculated from the following

equation:39

∆

? = ℎA B 8 × D-! E − 1G , H ≈ ;

(3)

where p is the effective phonon number and g is the electron-phonon coupling strength. The electron-phonon coupling strength g as well as the effective phonon energy of the host hvmax (cm-1) can be determined from the phonon sideband measurements. The electron-phonon coupling strength g is given as a ratio of integrated intensity of the phonon sideband (PSB) JKLM N to that of the pure electronic transition (PET) JK N:


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O=

P QRST U U P QRVW U U

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(4)

while the effective phonon energy hvmax can be calculated as the difference between the position (given in cm-1) of the PSB and the one of the PET: ℎX B = 10Y ⁄"KLM − 10Y ⁄"K

(5)

where λPSB and λPET are the wavelengths of the PSB and PET (given in nm).40 The phonon sideband spectrum of the BZBO:Nd3+ single crystal was recorded by monitoring emission of 4F3/2 → 4I11/2 transition in Nd3+ at 1062 nm at 300 K (Figure 7). In the spectrum the features corresponding to the splitting of lines of the 4F3/2← 4I9/2 transition were detected. The most intensity band at 882.4 nm (“zero energy shift” of 4F3/2← 4I9/2 transition) was used as the PET for further calculations.40

Figure 7.

Using the Eq. 4 and the data obtained from the spectrum in Figure 7 the value of the electron-phonon coupling strength g for BZBO:Nd3+ single crystal was estimated to be 0.02. Moreover, the effective phonon energy hvmax calculated from Eq. 5 was equal to 810 cm-1 for λPSB = 866.2 nm and λPET = 882.4 nm. The calculated value hvmax corresponds to the experimental value of the phonon energy equal 870 cm-1, determined from the Raman spectra of BZBO:Nd3+ single crystal assigned to the stretching vibrations of Bi–O bonds of BiO6. Finally, according to Eq.3, the calculated value of α was equal to 6.1×10-3 cm. This value is about two times higher in comparison to that for the Nd3+:YAG (α = 3.1×10-3 cm, for the electron-phonon coupling strength g = 0.25), which has considerably longer lifetime.41 With increasing temperature T, the stimulated emission of photons increased and Wmp (T) can be expressed by the following equation:42


12

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[ = 0 3 × \1 − 7

8

]^=> _W

8

`

∆V ]^=>

(6)

By combining the Eqs. 3 and 6 and using the values: ∆E = 9416 cm–1, the effective phonon energy hvmax = 870 cm-1, βel = 7.1×107 s-1,43 and α = 6.1×10-3 cm, the multiphonon relaxation rate at room temperature Wmp (T) was calculated to be equal to 3.5×1013

s-1. We can conclude that this value is negligible in respect to Arad and does not

influence on the 1064 nm emission of BZBO:Nd3+ single crystal. The process of the energy transfer from Nd3+ excited ions to neighboring unexcited Nd3+ ions depends on the distance RDA between excited (donor – D) and unexcited (acceptor – A) ions and hence depends on the concentration of the Nd3+ ions. For the homogenous distribution of active ions in the matrix, the energy transfer relaxation rate WET can be written as:44

∝ d ∝ e # bc

(7)

where N is the dopant concentration. The mean RDA distance between Nd3+ ions in BZBO lattice is given by equation:45 i

fgh = Dj/l∗nG

⁄o

(8)

where V is the unit-cell volume, n is the number of available sites for the dopant in the unit cell and Z is number of formula units in the unit cell. For the BZBO:Nd3+ single crystal, V = 582.03(6) Å3, Z = 4 and n = 2.21,46 The mean RDA distance between Nd3+ ions in BZBO crystal was calculated to be equal to 24.4 Å. When the mean distance between active ions is greater than 5 Å only a multipole-multipole interaction is significant and the exchange interaction becomes ineffective. While, for the distance shorter than 5 Å, the exchange interaction becomes more effective. Since RDA value in BZBO:Nd3+ is relatively large the effective mechanism of energy transfer process is due to the multipolar interactions. Finally, we conclude that due to low dopant concentration in


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BZBO:Nd3+ crystal, the energy transfer has nominal contribution for lowering the fluorescence lifetimes and intensity. The room temperature emission decay curve of the BZBO:Nd3+ single crystal was monitored at 1062 nm (4F3/2 → 4I11/2 transition) under 514 nm excitation (Figure 8).

Figure 8. Decay curve shows a mono-exponential decay of 4F3/2 → 4I11/2 transition that can be fitted according to the following equation: r

J = Jp 7qH−

(9)

Where Jp is the intensity of the luminescence process at initial time and τ is its lifetime. The radiative lifetime is equal to 109 µs, that in the kinetic curve first rise of the intensity is observed. This relatively long lifetime of 1062 nm emission (4F3/2 → 4I11/2 transition) of BZBO:Nd3+ single crystal in comparison to other well-known host materials suggests occurring very weak Nd3+–Nd3+ interaction and weak participation of phonons in nonradiative transitions. Due to the size similarity of the ionic radii of Nd3+ and Bi3+ ions in BZBO crystals the Bi3+ ions can be efficiently substituted by Nd3+ ions. Moreover, bismuth Bi atoms are located at the 4c positions (C1 symmetry), being four and sixcoordinated by the oxygen atoms/there are two nonequivalent positions of Bi atoms in unit cell. This allows efficient emission from the 4F3/2 level of Nd3+ions.

Conclusions The nonlinear optical BZBO single crystal doped with Nd3+ ions was grown by means of the Kyropoulos method. To our best knowledge the luminescence and other spectroscopic properties of this system are reported for the first time. In the BZBO: Nd3+, the Nd3+ ions effectively substitute Bi3+ ions. The polarized Raman spectra were measured in order to analyze vibrational modes of BZBO:Nd3+ single crystal. In particular, the results show the low-energy phonons of BZBO lattice, mainly due to the presence of heavy element (Bi) in the lattice structure. This suggests that a large number of phonons is required to bridge the energy gap of Nd3+ ions and the multi-phonon relaxation process is of low probability. Finally, we observe the effective and long


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emission lifetime of Nd3+ ions. The RT and low temperature (10K) UV-VIS-NIR absorption spectra were recorded and analyzed in terms of the optical transitions of the investigated system. Moreover, luminescence spectra have shown intensive emission in the NIR region from the 4F3/2 level of Nd3+ ions (4F3/2→4I9/2, 4F3/2→4I11/2 transitions) in BZBO host, under the excitation at 514 nm. The fluorescence decay measurements monitored 4F3/2→4I11/2 transition/efficient emission of Nd3+ ions at 1062 nm. The fluorescence lifetime of 4F3/2→4I11/2 transition, equal to 109 µs, is comparable with the lifetimes of other Nd3+-doped host materials such as Nd3+:YVO4 (100 µs) or Nd3+:YAG (230 µs). Because of the relatively long upper-state lifetime of 1062 nm emission of Nd3+ ions, BZBO:Nd3+ may be a good candidate as a new laser crystal, especially for continuous-wave

operation

and

mode-locked

lasers.

Moreover,

the

effective

luminescence of exciting Nd3+ ions in BZBO:Nd3+ single crystals as well as excellent nonlinear optical properties of the BZBO host suggest that the investigated system can be very useful for new integrated optical systems such as self-frequency doubling lasers.

Acknowledgement This work was supported by the Research Project of the Polish Ministry of Sciences and Higher Education: 06/65/DSPB/0517. M.G. Brik thanks the supports from the Recruitment Program of High-end Foreign Experts (Grant No. GDW20145200225), the Programme for the Foreign Experts offered by Chongqing University of Posts and Telecommunications, Ministry of Education and Research of Estonia, Project PUT430, and European Regional Development Fund (TK141). References (1) Wei, B.; Lin, Z.; Zhang, L.; Wang, G. Thermal and polarized spectral properties of Nd3+-doped Gd1−xLaxCa4O(BO3)3 (x = 0.16 and 0.33) crystals. Cryst. Growth Des. 2008, 8, 186–191. (2) Zhang, J.; Wu, Y.; Zhang, G.; Zu, Y.; Fu, P.; Wu, Y. Growth of high-usage pure and Nd3+-doped La2CaB10O19 crystals for optical applications. Cryst. Growth Des. 2010, 10, 1574–1577. (3) Huang, S.; Huang, Z.; Fang, M.; Liu, Y.; Huang, J.; Yang, J. Nd-sialon microcrystals with an orthogonal array. Cryst. Growth Des. 2010, 10, 2439–2442. (4) Snitzer, E. Optical maser action of Nd+3 in a barium crown glass. Phys. Rev. Lett. 1961, 7, 444–446. (5) Wiglusz, R. J.; Marciniak, L.; Pazik, R.; Strek, W. Structural and spectroscopic characterization of Nd3+-doped YVO4 yttrium orthovanadate nanocrystallites. Cryst. Growth Des. 2014, 14, 5512–5520.


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(6) Kuhn, K.J. Laser Engineering. Prentice Hall Pub., 1998. (7) Krennrich, D.; Knappe, R.; Henrich, B.; Wallenstein, R.; L’huillier, J.A. A comprehensive study of Nd:YAG, Nd:YAlO3, Nd:YVO4 and Nd:YGdVO4 lasers operating at wavelengths of 0.9 and 1.3 µm. Part 1: CW-operation. Appl. Phys. B 2008, 92, 165–174. (8) Sontakke, A. D.; Biswas, K.; Mandal, A. K.; Annapurna, K. Concentration quenched luminescence and energy transfer analysis of Nd3+ ion doped Ba-Al-metaphosphate laser glasses. Appl. Phys. B 2010, 101, 235–244. (9) Corcoran, V. J. High-repetition-rate eyesafe rangefinders. SPIE Proceedings 1991, 1419, 160–163. (10) Semwal, K.; Bhatt, S. C. Study of Nd3+ ion as a dopant in YAG and glass laser. Int. J. Phys. 2013, 1, 15–21. (11) Kassab, L. R. P.; Junior, N. D. R.; Oliveira, S. L. Laser spectroscopy of Nd3+-doped PbO–Bi2O3–Ga2O3–BaO glasses, J. Non-Cryst. Solids. 2006, 352, 3224–3229. (12) Balda, R.; Fernandez, J.; Sanz, M.; Pablos, A. D.; Fdez-Navarro, J. M.; Mugnier, J. Laser spectroscopy of Nd3+ ions in GeO2−PbO−Bi2O3 glasses. Phys. Rev. B 2000, 61, 3384–3390. (13) Brenier, A.; Tu, C.; Qiu, M.; Jiang, A.; Li, J.; Wu, B. Spectroscopic properties, selffrequency doubling, and self-sum frequency mixing in GdAl3(BO3)4:Nd3+. J. Opt. Soc. Am. B 2001, 18, 1104–1110. (14) Iliopoulus, K.; Kasprowicz, D.; Majchrowski, A.; Michalski, E.; Gindre, D.; Sahraoui, B. Multifunctional Bi2ZnOB2O6 single crystals for second and third order nonlinear optical applications. App. Phys. Lett. 2013, 103, 231103-1–231103-4. (15) Kasprowicz, D.; Brik, M. G.; Jaroszewski, K.; Pędziński, T.; Bursa, B.; Głuchowski, P.; Majchrowski, A.; Michalski, E. Spectroscopic properties of Bi2ZnOB2O6 single crystals doped with Pr3+ ions: absorption and luminescence investigations. Opt. Mater. 2015, 47, 428–434. (16) Shanmugavelu, B.; Venkatramu, V.; Kumar, R. K. V. V. Optical properties of Nd3+ doped bismuth zinc borate glasses, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 2014, 122, 422–427. (17) Qiuhong, Z.; Jing, W.; Mei, Z.; Weijia, D.; Qiang, S. Luminescence properties of Sm3+ doped Bi2ZnB2O7. J. Rare Earths 2006, 24, 392–395. (18) Qiuhong, Z.; Jing, W.; Haiyong, N.; Lingli, W. Synthesis and luminescent properties of Ln3+ (Ln3+ = Eu3+, Dy3+) -doped Bi2ZnB2O7 phosphors. Rare Metals 2012, 31, 35– 38. (19) Erdoğmuş, E.; Korkmaz, E. Photoluminescence properties and effects of dopant concentration in Bi2ZnB2O7:Tb3+ phosphor. Optik, 2014, 125, 4098–4101. (20) Kasprowicz, D.; Runka, T.; Jaroszewski, K.; Majchrowski, A.; Michalski, M. Vibrational properties of nonlinear optical Bi2ZnOB2O6 single crystals doped with Pr: µ-Raman investigations. J. Alloys Compd. 2014, 610, 600–605. (21) Li F.; Hou, X.; Pan, S.; Wang, X. Growth, structure, and optical properties of a congruent melting oxyborate, Bi2ZnOB2O6. Chem. Mater. 2009, 21, 2846–2850. (22) Su, X.; Wang, Y.; Yang, Z.; Huang, X. C.; Pan, S.; Li, F.; Lee, M. H. Experimental and theoretical studies on the linear and nonlinear optical properties of Bi2ZnOB2O6. J. Phys. Chem. C 2013, 117, 14149–14157.


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Figure Captions Figure 1. As-grown (001) BZBO:Nd3+ single crystal. Figure 2. Polarized Raman spectra of BZBO:Nd3+ crystal for (a) z(xx)z, z(yy)z and x(zz)x (b) z(xy)z, y(zx)y and x(yz)x scattering geometries. Figure 3. Energy level scheme of Nd3+ ions with possible energy transfer processes shown by the arrows. Figure 4. The absorption spectra of BZBO:Nd3+ crystal in VIS (a) and NIR (b) spectral range detected at RT. Figure 5. The absorption spectra of BZBO:Nd3+ crystal in VIS/NIR spectral range recorded at 10 K. Figure 6. The luminescence spectra of BZBO:Nd3+ crystal after excitation at 514 nm. Figure 7. The phonon sideband spectrum of the BZBO:Nd3+ single crystal, recorded by monitoring emission of 4F3/2→ 4I11/2 transition in Nd3+ at 1062 nm detected at 300 K. Figure 8. The luminescence kinetics of BZBO:Nd3+ crystal for

4

F3/2→4I11/2 transitions

measured at 300 K.

Table 1 Wavenumbers (cm-1) of Raman active modes for BZBO: Nd3+ crystal with modes assignment.


18

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z(xx)z

z(yy)z

x(zz)x

z(xy)z

y(zx)y

x(yz)x stretching vibrations of BO3, BO4 and BiO6 groups

1415 w 1403 w

1405 vw

1404 vw

1381 w 1348 sh/w 1341 m

1348 sh/w 1345 vw

1341 m 1331 m

1270 w 1247 w

962 w 917 m

1005 sh/vw 967 w

1010 w

870 m

875 vw

1182 vw 1010 w

918 vw 870 m 847 sh/w

830 sh/w 820 w 741 m

741 m 728 sh/w

802 vw 741 m

713 sh/m 699 s

699 m

699 s

636 w

635 s

636 w

585 w 580 w

589 m 581 m 541 w 519 vw 507 vw 493 vw

518 sh/w 506 w

397 s

581 m

740 w 728 w 713 w 699 w 635 w 601 w 578 w

740 vw

737 vw

699 w 659 vw 636 vw 607 vw

699 w

580 w 545 w

498 w

493 w

500 vw

487 w

424 vw

428 vw

353 w 322 vw

352 vw

389 w 359 w

289 sh/vw 277 w 266 vw 248 vw 229 sh/w 220 m

287 m 278 sh/vw 254 vw

309 vw 282 sh/vw 274 w 258 vw

222 w 200 w

230 sh/w 221 m 194 sh/w

184 m

185 w 170 sh/vw

186 m 174 m 157 m 140 m

416 vw 396 vw

397 w 390 m

308 sh/vw 279 m 249 s 231 s 221 s 198 sh/m

bending vibrations of BO3, BO4, BiO6 and ZnO4 groups

516 w

352 m

249 sh/m 230 s

610 w 591 m

stretching vibrations of BO3, BO4 and BiO6 groups

198 s

librations and translations of Bi and Zn atoms as well as BO3 and BO4 groups

190 m 168 sh/w 160 s

171 w 160 vs

141 s

142 vs

155 m 141 s

103 m

103 m

103 m

162 m 150 sh/w 142 m 123 m

156 m 141 m 127 m

104 vw

103 vw

118 m 102 vw

translations of BiO6 and ZnO4 groups

vs – very strong; s – strong; m – medium; w – weak; vw – very weak; w – weak; sh – shoulder.


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For Table of Contents Use Only: Bi-functional Bi2ZnOB2O6:Nd3+ single crystal for NIR lasers: luminescence and µ-Raman investigations

Konrad Jaroszewski, Pawel Gluchowski, Mikhail G. Brik, Tomasz Pedzinski, Andrzej Majchrowski, Maciej Chrunik, Edward Michalski, Dobroslawa Kasprowicz

Synopsis A new nonlinear optical Bi2ZnOB2O6:Nd3+ single crystal to be applied as material for NIR and self-frequency lasers were obtained and characterized using µ-Raman and optical spectroscopy techniques.


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Figure 1. As-grown [001] BZBO:Nd3+ single crystal. 993x660mm (72 x 72 DPI)



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Figure 2. Polarized Raman spectra of BZBO:Nd3+ crystal for (a) z(xx)z, z(yy)z and x(zz)x (b) z(xy)z, y(zx)y and x(yz)x scattering geometries. 201x140mm (300 x 300 DPI)


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Figure 2. Polarized Raman spectra of BZBO:Nd3+ crystal for (a) z(xx)z, z(yy)z and x(zz)x (b) z(xy)z, y(zx)y and x(yz)x scattering geometries. 201x141mm (300 x 300 DPI)



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Figure 3. Energy level scheme of Nd3+ ions with possible energy transfer processes shown by the arrows. 207x158mm (300 x 300 DPI)


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Figure 4. The absorption spectra of BZBO:Nd3+ crystal in VIS (a) and NIR (b) spectral range detected at RT. 203x146mm (300 x 300 DPI)



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Figure 4. The absorption spectra of BZBO:Nd3+ crystal in VIS (a) and NIR (b) spectral range detected at RT. 203x146mm (300 x 300 DPI)


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Figure 5. The absorption spectra of BZBO:Nd3+ crystal in VIS/NIR spectral range recorded at 10 K. 201x140mm (300 x 300 DPI)



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Figure 6. The luminescence spectra of BZBO:Nd3+ crystal after excitation at 514 nm. 208x159mm (300 x 300 DPI)


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Figure 7. The phonon sideband spectrum of the BZBO:Nd3+ single crystal, recorded by monitoring emission of 4F3/2→ 4I11/2 transition in Nd3+ at 1062 nm detected at 300 K.

203x142mm (300 x 300 DPI)



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Figure 8. The luminescence kinetics of BZBO:Nd3+ crystal for 4F3/2→4I11/2 transitions measured at 300 K. 201x140mm (300 x 300 DPI)


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Bifunctional Bi2ZnOB2O6:Nd3+ Single Crystal for Near Infrared Lasers

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