KLN Crystals Research Articles

The role of alkali additives in the crystallization of ferroelectric potassium lithium niobate crystals

Ferroelectric K3Li2Nb5O15 (KLN-1) crystals have been grown by the top-seeded solution growth method from pure, Na+, Rb+ and Cs+ doped melt. The impact of alkali additives was assessed all over the entire pulling range by investigating the variation of the structural and physical properties by using spectroscopic and dielectric methods. The incorporation of alkali homologs has been correlated with their ionic radii: Na+ ions were found to enter both Li and K sites with high segregation coefficient (k∼1.6), Rb+ ions were detected only at K sites (k∼0.34) and Cs+ ions practically did not incorporate into the lattice (k∼0.12). Alkali additives have been found to play a dual role in the growth process by affecting the crystallization temperature and promoting the K and Li site occupancy. By decreasing the crystallization temperature the Cs2O additive reduced the concentration both of the antisite Nb ions at Li site and that of alkali vacancies; therefore it can be considered as a promising fluxing agent in the growth of KLN crystals.

Laser heated pedestal growth of potassium lithium niobate for UV generation

Potassium lithium niobate (KLN) is a nonlinear optical material with a high nonlinearity. It has the potential to improve the performance and reduce the cost of blue and UV lasers. KLN crystals are not commercially viable because growth by traditional techniques is not possible. In an effort to develop commercially viable KLN, single crystals of the material were grown by the laser heated pedestal growth method (LHPG) with compositions of x=0.02, 0.06 and 0.2 following K3Li2−xNb5+xO15+2x. Noncritical phase matching at 20°C for previously unreported compositions of x=0.02 and 0.06 was measured at 795nm and 805nm, respectively. Overall, the results suggest that single crystal KLN can be used for SHG into the UV region of the spectrum and can be developed into a commercially viable nonlinear optical material.

Photorefractive Properties of Potassium Lithium Niobate Crystals with CeO 2 and Nd 2O 3

KLN and Ce:Nd:KLN crystals were grown by Czochralski method and polarized into single ferroelectrics domain along c-axis. The properties of KLN and Ce:Nd:KLN crystals, such as Curie temperature, Raman spectra, exponential gain coefficient ( T) and thin crystal sheet effect, were measured. The results showed that the two spectra resembling Ce:Nd:KLN crystal were of tetragonal tungsten bronze structure, the exponential gain coefficient of Ce:Nd:KLN crystal was higher than that of KLN crystals and Ce:Nd:KLN crystal had thin crystal sheet effect, for its exponential gain coefficient increasing with crystal sheet thinning. The thin crystal sheet effect of Ce:Nd:KLN crystal was also discussed.

The influence of composition on optical properties of ferroelectric potassium lithium niobate single crystals

Homogeneous crack-free Potassium Lithium Niobate (KLN) single crystals with different Li content were grown up by the TSSG technique. Lattice vibration spectra of these samples were investigated using Raman spectroscopy. The results showed that characteristic Raman spectra of the [NbO6]7− octahedral ions were strikingly influenced by the Li ions. Thus, the symmetric bend vibration mode ν5 of the KLN sample with higher Li content was split into three Raman peaks, while the sample with lower Li content displayed a single peak, supporting that the bend vibration modes of the [NbO6]7− octahedrons were obviously perturbed by Li ions occupying C-sites. Enhanced Raman peak intensities of the KLN sample post-annealed at 900°C for 24h evidenced that content of defects in KLN crystal might be reduced. The transmittance spectra showed that the crystal with higher Li content had better transmittance intensity.

Growth of Na modified potassium lithium niobate crystal by the Czochralski method

Transparent Na modified potassium lithium niobate (Na0.23K2.60Li1.82Nb5.35O15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na2O:K2O : Li2O:Nb2O5=2:30:25:43mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.5446±0.0010Å and c=4.0129±0.0005Å at room temperature. The dielectric constant along the c-axis ε33 showed a sharp maximum around 480°C. Optical transmission edge was 370nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal.

Photorefractive Effect of Fe:KLN Crystal

We have grown KLN and Fe:KLN crystals using the TSSG method. The crystal lattice constants are measured by the X-ray diffraction technique and the crystal optical absorption edges are characterized by an ultraviolet-visible absorption spectrum measurement. The results show that the absorption edge of the Fe:KLN crystal shifts to the longer wavelengths compared with that of the KLN crystal. The exponential gain coefficient and erasure time are examined by two-beam coupling experiment. The results indicate that the exponential gain coefficient of Fe:KLN crystal is twice as large as that of the KLN crystal, and the photoconductivity of the Fe:KLN is three orders of magnitude higher than that of KLN. The enhancement of photorefractive properties of Fe:KLN crystal is also discussed.

Photorefractive properties of Mn doped K 3Li 2Nb 5O 15 crystal

KLN(K 3Li 2Nb 5O 15, KLN) and Mn:KLN crystal was grown by Czochralski method. Compared with KLN, the absorption edge of Mn:KLN crystal shifted to infrared band. No correlative property data of KLN was obtained for its weaker photorefractive effect. However, the photorefractive effect of Mn:KLN was enhanced because Mn 2+ or Mn 4+ existed in the crystal, which applied more donor and acceptor centers. The two beam coupling coefficient Γ of Mn:KLN crystal with the thickness of 2 mm was up to its maximum value 0.7 cm −1 at the external angle of 40°, whereas that of the crystal with the thickness of 0.8 mm was up to its maximum value 1.4 cm −1 at the external angle of 34°. The response time decreased with the increasing of pump intensity I P. The response time of Mn:KLN was just several seconds, which was one order of magnitude faster than that of Fe doped LiNbO 3 crystal.

Growth and optical properties of Zn:KLN single crystal

KLN and Zn:KLN crystal were grown by the Czochralski method along the [100] orientation. All crystals were transparent from 380 to 760nm by the ultraviolet-visible transmittance spectra of KLN and Zn:KLN crystals. Compared with KLN crystals, the transmittance of Zn:KLN increased and its absorption edge shifted to the ultraviolet band. The infrared transmittance spectra of KLN and Zn:KLN crystals were measured. The replacement of Zn2+ in the KLN crystals was analyzed. Laser lights of 420 and 405nm were obtained through noncritical phase-matching second harmonic generation for KLN and Zn:KLN crystal, respectively. A shorter wavelength output was obtained for Zn:KLN crystal. The frequency efficiency for all crystals reached 6–7%.

Photorefractive properties of potassium lithium niobate doped with copper

Summary Potassium lithium niobate doped with copper (Cu:KLN) were grown by the Czochralski method for the first time. The structure of Cu:KLN was measured by the x-ray powder diffraction method, and its lattice constants were obtained. The position of copper ions in KLN crystal was determined. The exponential gain coefficient, response time and erasure time were measured. It was found that the exponential gain coefficient of Cu:KLN is 10.5 cm−1, as two times high as that of KLN, and its response time of 1.53 s is one order of magnitude shorter than that of Cu:LiNbO3. The type of light exciting carriers in Cu:KLN has been investigated. The result showed that the electron acts the main role in Cu:KLN.

Second-Harmonic Generation of Blue Light in Epitaxial K3Li2-xNb5+xO15+2x Waveguides on K3Li2-x(Nb0.98Ta0.02)5+xO15+2x Substrates by Metalorganic Chemical Vapor Deposition

We report a new second-harmonic generation blue laser using a K3Li2-xNb5+xO15+2x single crystal film waveguide on a K3Li2-x(Nb0.98Ta0.02)5+xO15+2x crystal substrate by the metalorganic chemical vapor deposition method. Second-harmonic blue light with output power of 2 mW at 487 nm was successfully generated. The phase-matched wavelength bandwidth of the device was 1.3 nm, and the temperature coefficient of the phase-matched wavelength was 0.36 nm/°C. Crystallographic evaluation was also performed for the KLN thin film crystal. The (001) plane of KLN crystal film was well orientated to the (001) plane of the KLNT substrate, and the surface morphology of the film was very smooth. Moreover, it was revealed that an as-deposited KLN film crystal has a single domain structure.

RAMAN AND INFRARED TRANSMITTANCE SPECTRA OF DOPED KLN CRYSTAL

Raman and Infrared Transmittance Spectra of KLN and Doped KLN Crystal have been carried out. The Raman spectra of KLN and doped KLN crystal were basically alike, which justified the fact that the ions of impurities entered the lattice by replacement of original ions of KLN crystal. The result of Infrared Transmittance Spectra showed that, compared with the KLN crystal, the number and site of absorption peaks of OH−1 vibration in doped KLN changed, only one peak existed in Fe:KLN and Cu:KLN, which located at higher wave numbers 3520 cm−1 and lower wave numbers 3488 cm−1 respectively, while, there existed two peaks in KLN at 3519 cm−1 and 3504 cm−1 respectively. It suggested that Fe cation replaced Nb cation, and Cu cation replaced K cation when they entered the lattice of crystal.

Optical properties of nonlinear potassium lithium niobate crystals

Crack-free potassium lithium niobate (KLN) single crystals of different compositions have been grown by the resistance-heating Czochralski technique. The crystal structure was studied using synchrotron radiation X-ray powder diffraction, and the crystal compositions were determined by X-ray fluorescent spectroscopy. Homogeneous and crack-free samples of about 6×6×7 mm 3 were used to study the lattice vibration spectra by Raman and infrared reflection spectroscopy. The results show a strong influence of Li ions in the C sites of the KLN crystal on the characteristic Raman and infrared reflectivity spectra of the Nb–O octahedral ions. In particular, the symmetric bend vibrational mode, ν 5, is split into three Raman peaks; the antisymmetric bend vibrational mode, ν 4, in the infrared spectrum is split into three peaks as well. The antisymmetric stretch vibrational mode, ν 3, is broadened greatly and slightly split. The crystals obtained have good quality and improved second harmonic generation properties.

Second-Harmonic Generation of Ferroelectric Potassium Lithium Niobate Crystals

The second-harmonic generation (SHG) properties of a potassium lithium niobate (KLN) crystal with the size of 8×20×43 mm3 were studied in detail. The KLN crystal was grown from a melt consisting of 32 mol% K2O, 24 mol% Li2O and 44 mol% Nb2O5 (denoted as the melt 32:24:44) by the top-seeded solution growth (TSSG) method. The ferroelectricity of the KLN crystal was revealed by the marked dielectric anomaly at the Curie temperature. The full-widths at half maximum (FWHMs) of the wavelength- and temperature-tuning curves of a 4-mm-long KLN crystal sample were found to be 0.6 nm and 2°C, respectively. The SHG conversion efficiency of this crystal was studied and estimated to be 0.03%/W for an input power of 748 mW. The CW tunable blue laser covering the range of 431.1–437.6 nm was achieved by tuning the crystal temperature in the range of 24–80°C. The crystal studied in this paper is more inhomogeneous and has lower SHG efficiency than the KLN crystal grown from the melt 31:26:43, which has higher Li2O content.

Ferroelectric potassium lithium niobate crystals grown by the vertical Bridgman method

Ferroelectric potassium lithium niobate (KLN) single crystals are very promising candidates for direct second harmonic generation of semiconductor diode lasers. In this paper, crack-free KLN crystals up to 10 mm in diameter by 50 mm in length were grown by the modified vertical Bridgman method from potassium and lithium enriched solutions. Prior to growth, KLN powder was prepared by sintering the mixture of K 2CO 3, Li 2CO 3 and Nb 2O 5 at 850°C for 8 h. Two temperature zones were designed in the Bridgman furnace to prevent crystals cracking. The growth rate is less than 0.25 mm h −1 and the temperature gradient in the solid–liquid interface is about 20°C cm −1. The transparent KLN crystal has a very good transmittance in the 420–800 nm region with an absorption edge at 378 nm. Dielectric constant of ε 33 and ε 11 at room temperature are 127 and 376, respectively. Curie temperature of the crystal is about 380°C.

Blue SHG characteristics and homogeneity of the TSSG grown potassium lithium niobate (KLN) crystal with high Li2O content

The high quality and crack-free KLN crystal with high Li2O content has been grown along the 〈110〉 axis by the TSSG method from the melt of 31mol% K2O, 26mol% Li2O and 43mol% Nb2O5. The size of the as-grown KLN crystal is about 6×9×29mm3. The XRD analysis gives the lattice constant a=12.583Å, and c=4.041Å near the c value of the stoichiometric KLN. The Curie temperature of this crystal is determined to be 540°C by the dielectric constant measurement. The blue SHG characteristics of the KLN crystal are investigated by using a 3900S Ti: sapphire CW tunable laser. The FWHMs of the wavelength- and temperature-tuning curves for the NCPM SHG are determined to be 0.4nm and 1.2°C, respectively. The blue laser light with 1.45mW output at 416.7nm is achieved at room temperature by the single-pass SHG for 903mW input power. The blue SHG conversion efficiency is estimated to be 0.25%/W that is the highest value obtained up to now. The temperature-tuned blue laser light with continuous output from 416.7 to 423.5nm is obtained by increasing the crystal temperature from 23.5°C to 80°C. The temperature coefficient of the NCPM wavelength is 0.295nm/°C. The investigated KLN crystal, especially its middle part, has a good homogeneity of the room temperature NCPM wavelength along the growth axis (<2nm).

Influence of [K]/[Li] and [Li]/[Nb] ratios in melts on the TSSG growth and SHG characteristics of potassium lithium niobate crystals

KLN crystals were grown along the 〈110〉 axis by the TSSG method from different melts with 30–32mol% K2O, 23–27mol% Li2O and 43–46mol% Nb2O5. Lattice constants were measured by the single crystal XRD analysis. Curie temperatures were determined with an LCR meter HP 4284A at 1kHz. The blue SHG characteristics of the KLN crystals were investigated by using a 3900S Ti: sapphire CW tunable laser with the wavelength range of 675–1100nm. It is found that the non-critical phase matching (NCPM) wavelength at room temperature decreases significantly from 974 to 827.4nm with the increase of [K]/[Li] and [Li]/[Nb] ratios in the melts. On the basis of this survey, the melt compositions suitable for the TSSG growth of high quality KLN crystals with high lithium content are outlined, as indicated by the large c-axis lattice constant, high Curie temperature and short NCPM wavelength.

Cracking mechanism of the potassium lithium niobate crystal related to its internal structure

The potassium lithium niobate (KLN) crystal is attractive due to its excellent second harmonic generation (SHG) properties. It is difficult to get large sized samples because of cracking. In this paper, the cracking mechanism of the KLN crystal in relation to its structure is based on the cracking morphology, the thermal mechanical analysis, and crystallochemistry. It is found that the cracking always occurs parallel to the cleavage plane (001) and that the surface cracking lines are perpendicular to the [001] direction. The cracking is due to the thermal stress generated by the big difference in the thermal expansion coefficients along different directions during the cooling process after the crystal growth. From 600°C to 20°C, the KLN crystal exhibits a linear expansion along the [110] direction, but a large contraction along the [001] direction. The contraction along the [001] direction increases with decreasing content of Nb. The formation mechanism of the observed cracking is also discussed by the crystallochemistry approach in which the generation of cracking is related to the anisotropy of the inter-atom bond strength.

Growth and properties of ferroelectric potassium lithium niobate (KLN) crystal grown by the Czochralski method

The ferroelectric potassium lithium niobate (KLN) crystals with tungsten–bronze-type structure were grown by the Czochralski method. Usually, the growth of ferroelectric KLN crystal was very difficult and as-grown KLN crystals have cracks. These problems were investigated in relation with growth axis and different solid solution. For the growth of high quality and crack-free KLN crystals, the growth conditions and properties of ferroelectric KLN crystal were investigated. The grown KLN crystals are single-crystalline and belong to the tetragonal system with the lattice constants a= b= 1.2500–1.2551 nm and c=0.3996–0.4009 nm. The diffuse dielectric anomaly was observed for these KLN crystals. The width of phase-transition region and phase-transition temperature depends both on the compositional fluctuations and the compositional variations.

Photorefractive property of Ce:KLN crystal

Ce:KLN crystal was grown by Czochraski method. The two coupling coefficient Γ as a function of the external crossing angle was measured. The two coupling coefficient Γ of the sample with the thickness of 2 mm was up to its maximum value 0.9 cm−1 at the external angle of 45°, whereas that of the sample with the thickness of 0.4 mm was up to its maximum value 4.3 cm−1 at the external angle of 55°. The erasing time decreased with the increasing of pump intensity I P. Photoconductivity σph have a relationship with the pump intensity as I P 0.5. The exponential coefficient was smaller than unity, which might indicated two photorefractive centers or one center with three valence states in as Ce:KLN crystal.

Growth and property of Er:KLN crystal

KLN and Er:KLN crystal was grown by Czochraslki method. The growth technology was discussed. The growth of Er:KLN was easier than that of KLN. Pure KLN and Er:KLN crystal exhibited the TB structure by X-ray analysis. Compared with pure KLN, there was no new peaks in X-ray diffraction and Raman spectra results for Er:KLN, which showed that Er entered the lattice of crystal. But, dopant resulted in the change of peak intensity for both spectra. The absorption edge of Er:KLN shifted to ultraviolet band because the capacity of attracting electron of Er3+ was larger than that of K+, Li+ and Nb5+.