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Crystal Growth of LiNa5Mo9O30 Crystals of High Optical Quality
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17 August 2024
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19 August 2024
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18 December 2024
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Abstract
The bulk LiNa5Mo9O30 (LNM) crystals were successfully grown in the [010] and [001] directions without internal inclusions and cracks by the Czochralskii method with low temperature gradient. The crystal grown in the [010] direction showed a tendency to twinning. The crystal grown in the [001] direction demonstrated high structural perfection (FWHM=13'') for the (001) plane and high optical quality Δn≈210-5. The laser induced damage threshold was measured along a, b and c axes and was 15.7, 27.0 and 27.5 J/cm2 respectively. The thermo-optical coefficient dn/dT was measured for the main crystallographic axes, which was -5.7510-6, -2.0210-5 and 3.6510-6 K-1 along the a, b and c axes, respectively. The second harmonic generation (SHG) is conducted in crystalline LNM sample. The maximum efficiency value of 3.5% at pump power of 12 W had been achieved.
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Subject: Chemistry and Materials Science - Materials Science and Technology
1. Introduction
The development of the laser industry is impossible without development of new optical materials to improve the laser characteristics. Molybdate family crystals are widely known in laser technology.
LiNa5Mo9O30 (LNM) crystal was first presented in 2012 [1]. The crystal has a wide transparency window of 430-5500 μm [2], high birefringence, nonlinear optical coefficients within the values 1.4, 4.3 и 1.1 pm/V for d31, d32 and d33 respectively [3], and a rather high radiation resistance of 2.64 GW/cm2 [4]. Previously [5], we demonstrated the possibility for frequency conversion by a LNM crystal for almost the entire transparency range from UV to mid-IR.
The possibility of producing Glan polarizing prisms with a higher laser damage threshold than widely used materials such as CaCO3, YVO4 and α-BBO was demonstrated in [4].
LNM crystals presented in the literature were obtained by both the Czochralski method [7,8] and the TSSG method [2,3,4,6]. Crystals grown by the Czochralski method have an elongated shape along the [100] direction with pronounced planes (040). The LNM material is potentially applicable in laser optics, but no data on the measurement of thermo-optical coefficients have been reported in the literature.
In the current research a high quality LNM crystal of 30×30×90 mm3 obtained by the Czochralski method at low temperature gradient with a mass up to 190 g is presented. Its structural perfection and optical quality are investigated. Thermo-optical coefficients dn/dT and the laser induced damage threshold (LIDT) along three axes are measured. First results on second harmonic generation (SHG) obtained.
2. Experimantal Part
2.1. Materials and Preliminary Preparation for Growth
Stoichiometric amounts of Na2CO3 (99.99%, Lenreactiv GSC, St.Peterburg, Russia ), Li2CO3 (99.99%, Fox-Chemicals GmbH, Pfinztal-Germany), MoO3 (99.9997%, ARMOLED Ltd, Moscow, Russia) were placed in a platinum crucible d=70 mm h=200 mm after careful mechanical homogenization. The crucible was kept in a muffle furnace at 580°C for 24 h to homogenize the melt. After extracting the crucible from the furnace and cooling it down, phase compositions of the preparations were determined by PXRD method. To determine the synthesis regimes for full homogenization TG-DTA data were used.
2.2. Crystal Growth
LNM crystal growth was carried out by the Czochralski method. The setup specific feature was the 4-zone resistant furnace, which allowed creating a low temperature vertical gradient at the level of 5 °C/cm above the melt. We used a platinum crucible D = 70 mm h = 200 mm with a conical lid for the growth process. Before the growth start we superheated the melt to 650 °C and exposed for 24 hours to homogenize the melt. The growths were realized in two crystallographic directions [010] and [001] on an oriented seeds with 5×5 mm cross-section. The pulling speed was varied from 0.3 to 1 mm/hour. The crystal rotation speed was 3-10 rpm. To the end of the growth process, the growing speed was increased to 2 mm/hour and the melt was slightly heated until the crystal broke away from the melt spontaneously. The grown crystal was cooled to room temperature at a rate of 7 °C/hour.
2.2. Crystal Characterization
Crystalline perfection was estimated by high-resolution X-ray diffraction (HRXRD) using a Malvern PANalytical Empyrean instrument (Malvern Panalytical Ltd., Worcestershire, GB) with a Cu Kα radiation (λ=1.541874 Å). The scanning range was ω=0.6 in steps of 0.0003° at room temperature. (001) oriented plate LNM crystal was mechanically polished on both sides and used for the HRXRD measurements.
Optical quality of the grown crystals was estimated both qualitatively and quantitatively. For the qualitative study we used the standard Schlieren method [9]. The quantitative study of optical homogeneity was carried out by interferometry on a Mach-Zehnder interferometer [7,9].
To measure dn/dT thermo-optic coefficient we used the method proposed in Ref. [10].
To measure the laser induced damage threshold (LIDT), the R-on-1 method in the single pulse mode was used [11]. A fiber laser with a wavelength of 1.06 μm was used for measurement, the pulse time was 15 ns, the energy in the pulse was up to 1 mJ, and the beam diameter in the waist was 25 μm. The method allows collecting a sufficient amount of statistical data from a small size area. The fact of tested area destruction was registered by a sharp distortion of the profile of the transmitted radiation.
The experimental setup for second harmonic generation in crystalline LNM sample is shown in Figure 1a. Linear-polarized laser radiation at a wavelength of 1030 nm is focused via lens system into the sample installed in a heater with PID controller. The pulse duration is 15 ps with the repetition frequency 2 MHz, the output power is up to 25 W, beam diameter is varied from 450 to 900 μm, beam parameter product M2 = 1.1. After nonlinear conversion the radiation propagates through a system of dichroic dielectric mirrors, which separate radiation by wavelength. The sample (5×10×20 mm3) was cut in direction =42,9º, =90,0º (fsf–type) (Figure 1b).
3. Results and Discussion
3.1. LiNa5Mo9O30 Synthesis
X-ray measurements was carried out using a Malvern PANalytical Empyrean instrument (Malvern Panalytical Ltd., Worcestershire, GB) with Cu Kα radiation (λ=1.541874 Å), in the 2θ range from 5° to 115° at room temperature. The X-ray powder diffraction patterns of the polycrystalline LiNa5Mo9O30 are shown in Figure 2. The experimental XRD pattern of the polycrystalline LiNa5Mo9O30 is in good agreement with the literature data, which indicates that the pure single phase was obtained.
To determine the melt homogenization temperature, it is necessary to know the temperature at which mass loss begins. This is important to consider when working with melts containing MoO3 due to its high volatility. According to TG-DTA analysis data obtained by a Netzsch Jupiter STA 449 F1 instrument (NETZSCH-Gerätebau GmbH, Selb, Germany), the onset of mass loss occurs at 783 °C. This indicates that the melt homogenization occurs without changing the composition stoichiometry. The melting temperature was determined as 550±1.5 °C (Figure 3), which corresponds to the literature data [3].
3.2. LNM Crystal Growth
The first experiments on LNM crystal growth were carried out on the seed oriented in the [010] direction. The plane (040) is the most stable and is pronounced on the crystals obtained in [3,7]. The shape of the crystals had a lamellar appearance and this is not optimal for growth technology. At the same time, the crystals grown in the [010] direction presented in the literature have defects and cracks [12].
A series of crystal growth experiments using the low temperature gradient technique of the Czochralski method in the [010] direction resulted in several crystals weighing up to 185 g (Figure 4). We did not observed any cracks and defects in the grown LNM crystals.
To investigate the optical quality of the grown crystals, 5 mm thick plates were cut parallel to (010) crystallographic plane. During the polishing process in a slightly acidic environment (pH = 5), areas of inhomogeneity were noticed on the sample (Figure 5). An optical microscope Olympus GX53 (Olympus Corporation, Hamburg, Germany) was used to make crystal photos. Investigations of inhomogeneity areas were carried out using the Quasi-Static Piezo d33/d31 Meter ZJ-6B (Beijing Jkzc Echnology Development Co., Ltd, Beijing, China). The values of the piezoelectric constant in regions 1 and 2 had the same value with the opposite signs. At the border of regions 1 and 2, the value of the constant turned to zero. Similar defects were observed along the length of the entire boule and were specific for all crystals grown in this direction.
Due to the presence of inhomogeneous areas in the [010] as-grown LNM crystals, we decided to change the growth direction to [001]. We grew a number of crystals weighing up to 190 g without visible defects in the volume and cracks were obtained (Figure 6). The crystals had a volumetric shape with a hexagon at the base and a pronounced plane (040) along the growth direction, the ratio between the [010] and [100] directions is almost 1:1. The typical diameter of a grown crystal was 30 mm. As a consequence, the bulk shape of the crystal allowed the fabrication of optical elements designed for frequency conversion tasks. The crystals shown in Figure 5 and Figure 6 were grown under the same thermal conditions, with the same growth and rotation rates.
To study the optical quality of the crystal, a 4.8 mm thick plate was cut from the central part of the boule parallel to the (001) plane. Then, a plate parallel to the (010) plane with a thickness of 3 mm was cut from the lower part of the boule. No inhomogeneity similar to Figure 5 were detected during grinding and polishing of the samples (Figure 7).
The crystalline quality of the as-grown LNM crystal was checked by HRXRD. FWHM for the rocking curve of the (001) diffraction plane (Figure 7A) was 13'' which is the best result presented in the literature [2,3,7]. This indicates a high structural perfection of the obtained crystal.
The optical homogeneity of the crystal was qualitatively assessed in two directions along the growth axis on the (040) plane (Figure 7B) and perpendicular to the growth axis on the (001) plane (Figure 7A).
Quantitative assessment of optical homogeneity was performed on a plate cut perpendicular to the growth axis. The figure 8 shows interferograms obtained in the infinite-width strip mode (A) and the result of processing the corresponding fragments of interferograms taken in the finite-width strip mode in the 5×5 mm region. The magnitude of distortions introduced into the wave front was 0.1λ. The obtained value Δn≈2×10-5, indicates the high optical quality of the obtained crystal.
3.3. Measurement of dn/dT
The LNM material is potentially applicable in laser optics, but no data on the measurement of thermo-optical coefficients have been reported in the literature. To measure the thermo-optic coefficient dn/dT and LIDT three elements of size 3×3×10 mm along 3 crystallographic axes were fabricated. The measured dn/dT values along three crystallographic axes are presented in Table 1.
3.4. Laser Induced Damage Threshold
After measuring the thermo-optical coefficient dn/dT, destructive measurements of the laser damage threshold were performed on the same elements. The obtained measurement values are presented in Figure 9.
The graphs show the probabilistic values of the destruction of the LNM crystal when exposed to laser radiation. The graphs highlight the values obtained on the samples cut in the direction [100]. The output facet withstands lower energy exposure to radiation. Probably, such results are associated with not enough polishing quality and need additional verification.
The obtained values indicate that the grown LNM crystal can withstand a sufficiently high power density and be competitive in this characteristic with widely known non-linear optic KDP, KTP, LBO, BBO, CLBO, LiNbO3, and LiTaO3 commercial crystals [13].
3.5. Second Harmonic Generation
Phase synchronism (wave synchronism) is a condition for the most effective implementation of the ability of a nonlinear medium to convert frequency. For the grown LNM crystals we determined the optimum temperature to fulfill the phase matching condition was determined to be 24 ºС. The dependence of the output second harmonic power P515 on the input pump power Pin was measured, and then the conversion efficiency was calculated.
One can see (Figure 10) that the second harmonic power is the same regardless of the beam diameter in the initial part of the dependence, and then reaches different constant values. The maximum power value is achieved at a beam diameter of 900 μm with the value of 0.5 W, and the maximum efficiency value is 3.5% at 12 W pump power.
4. Conclusions
LNM crystals were grown by the Czochralski method in two directions [010] and [001]. Crystals grown in the [010] direction are prone to growth heterogeneities. When grown in the [001] direction, the crystal has a sufficiently high optical uniformity Δn≈2*10-5. 6 elements were made from this crystal along 3 crystallographic axes to measure the thermo-optical coefficients dn/dT and the LIDT. The values of dn/dT were -5.75×10-6, -2.02×10-5 and 3.65×10-6 K-1 along the a, b and c axes, respectively. LIDT values were measured for three crystallographic directions. The second harmonic generation is conducted in crystalline LNM sample. The maximum efficiency value is 3.5% at pump power of 12 W
Thus, the paper shows the possibility of obtaining a volumetric crystal with a size of 30×30×90 mm and a mass of up to 190 g of high optical quality for birefringence and frequency conversion problems.
Author Contributions
Conceptualization, N.K and I.A.; methodology, N.K., I.T. and E.S.; software, D.B., E.S. and O.R.; validation, D.D, D.Dm. and E.B.; formal analysis, A.K.; investigation, N.K., I.G., D.B., D.D., I.T. E.S. and E.Y.; resources, E.S.; data curation, D.B. and D.D.; writing—original draft preparation, E.B., D.Dm and O.R.; writing—review and editing, R.A., A.K. and I.A.; visualization, I.G. and E.B.; supervision, I.A.; project administration, E.Y.; funding acquisition, R.A. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the Ministry of Science and Higher Education of Russia through the project FSSM-2020-0005.
Data Availability Statement
Not applicable.
Acknowledgments
The authors are grateful to the Mendeleev Center for the Collective Use of Scientific Equipment for computational resources.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Experimental setup for second harmonic generation in a crystalline LNM sample (a); crystalline LNM sample cutting and placement for the study (b).
Figure 1.
Experimental setup for second harmonic generation in a crystalline LNM sample (a); crystalline LNM sample cutting and placement for the study (b).
Figure 2.
PXRD pattern of the synthesized LiNa5Mo9O30 compound.
Figure 3.
TG-DTA data for the nominal composition of LiNa5Mo9O30.
Figure 4.
The LNM crystal grown in the [010] direction.
Figure 5.
Inhomogeneities in the [010] as-grown LNM crystal.
Figure 6.
The LNM crystal grown in the [001] direction.
Figure 7.
Plates cut from boules grown in the [001] direction A-slice perpendicular to the growth direction; B-slice parallel to the growth direction.
Figure 7.
Plates cut from boules grown in the [001] direction A-slice perpendicular to the growth direction; B-slice parallel to the growth direction.
Figure 8.
Interferograms of the LNM crystal obtained on a Mach-Zander interferometer.
Figure 9.
The laser induced damage threshold for LNM elements cut in the direction a – [100], b - [010], c - [001] (facet 1 – input, facet 2 – output).
Figure 9.
The laser induced damage threshold for LNM elements cut in the direction a – [100], b - [010], c - [001] (facet 1 – input, facet 2 – output).
Figure 10.
The second harmonic power (left) and the efficiency at different pump power (right).
Table 1.
dn/dT values obtained along the main crystallographic axes.
| Direction | dn/dT, K-1 |
|---|---|
| along the a axis | -5.75∙10-6 |
| along the b axis | -2.02∙10-5 |
| along the c axis | 3.65∙10-6 |
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