JP2008065140A - Lithium niobate single crystal or lithium tantalate single crystal with improved optical damage characteristic, and optical element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium niobate single crystal and a lithium tantalite single crystal, having high OPTICAL DAMAGE resistance, and to provide an optical element using the single crystal. <P>SOLUTION: A lithium niobate single crystal or a lithium tantalite single crystal having a substantially stoichiometric composition contains at least one element selected from a group consisting of Er, Tm, Yb and Lu by 0.3-3.0 mol%. Otherwise, the single crystal may contain the above element by 0.4-1.0 mol%. An optical element is manufactured by using the above single crystal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a lithium niobate single crystal and a lithium tantalate single crystal, and an optical element using the same. More specifically, the present invention relates to a lithium niobate single crystal and a lithium tantalate single crystal having high light damage resistance, and an optical element using the same.

  In recent years, a wavelength conversion element using an optical single crystal having a nonlinear optical effect has attracted attention. Among such optical single crystals, a lithium niobate single crystal having a congruent composition (hereinafter simply referred to as “CLN”), a lithium tantalate single crystal having a congruent composition (hereinafter simply referred to as “CLT”), and a substantially constant crystal. A lithium niobate single crystal having a specific composition (hereinafter simply referred to as “SLN”) and a lithium tantalate single crystal having a substantially stoichiometric composition (hereinafter simply referred to as “SLT”) have a large nonlinear optical constant and are industrially high. It is useful because of the establishment of a method for producing single crystals with high quality and large diameter.

  In particular, SLN and SLT are attracting attention because they have an extremely small coercive electric field compared to CLN and CLT, and thus it is possible to manufacture an optical element having a high aspect ratio periodically poled structure.

  However, when these SLNs and SLTs are irradiated with laser light, optical damage (or light-induced refractive index change) occurs in the SLNs and SLTs, resulting in a decrease in wavelength conversion efficiency, a decrease in light transmittance, generation of beam fanning, etc. It is known that the optical characteristics deteriorate.

  In order to reduce the occurrence of such optical damage, there are methods using various dopants. For example, there is a technique for improving light damage resistance by doping Mg, Zn, In, or Sc into SLN or SLT (see, for example, Patent Document 1). There is a technique for improving light damage resistance by doping Sc with CLN or CLT (see, for example, Patent Document 2).

  When the dopants shown in Patent Document 1 and Patent Document 2 are used, the Curie temperature may increase. In that case, since the Curie temperature and the decomposition start temperature (1215 ° C.) are close, the polling process may be difficult. It is known that the Curie temperature of non-doped SLN is 1202 ° C to 1203 ° C. When an additive that improves the light damage resistance of Mg, Zn, or Sc is added to this non-doped SLN, the Curie temperature rises and becomes very close to or exceeds the decomposition start temperature, making poling almost impossible. . Moreover, it is desirable that an appropriate dopant can be applied according to various applications.

On the other hand, there is a technique for manufacturing a large-diameter polycrystal by doping Nd, Eu, Yb, Ce, and Tb into SLN or SLT (see, for example, Patent Document 3).
Although the said patent document 3 is not disclosing the effect of a specific dopant, these dopants are mainly known as a dopant aiming at light emission. It is desirable to clarify the effects of these dopants and the appropriate doping amount.

JP 2003-267798 A Registration No. 1956727 JP 2003-171199 A

  Accordingly, an object of the present invention is to provide a lithium niobate single crystal and a lithium tantalate single crystal having high light damage resistance.

  The substantially stoichiometric lithium niobate single crystal or lithium tantalate single crystal according to the present invention contains at least one element selected from the group consisting of Er, Tm, Yb and Lu in an amount of 0.3 mol% to 3.0 mol. %, Thereby achieving the above objective.

  The single crystal may contain 0.4 to 1.0 mol% of the element.

  The single crystal may be for an optical element.

  The optical element according to the present invention uses the above single crystal, thereby achieving the above object.

  The substantially constant ratio lithium niobate single crystal and lithium tantalate single crystal according to the present invention contain 0.3 mol% to 3 mol% of an element selected from the group consisting of Er, Tm, Yb and Lu. . The light damage resistance can be improved by doping these specified elements. Since the number of dopant species for improving the light damage resistance is increased, a wide range of material designs for various applications is possible. In addition, since the light damage resistance can be improved even with a smaller doping amount than in the past, cracks, inclusions and the like based on the dopant during single crystal growth can be less likely to occur. As a result, the yield of single crystal production can be improved.

(Outline of the present invention)
The inventors of the present invention include a group consisting of Er, Tm, Yb, and Lu in a substantially stoichiometric lithium niobate single crystal (SLN) and a substantially stoichiometric lithium tantalate single crystal (SLT). It has been found that the light damage resistance is improved by doping at least one element selected from the group consisting of 0.3 mol% to 3 mol%.
In the present specification, the phrase “having a stoichiometric composition” substantially means Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) or Li ₂ O / (Ta ₂ O ₅ + Li ₂ O). Although the molar fraction is not completely 0.50, the composition is close to the stoichiometric ratio (Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) or Li ₂ O / (Ta ₂ O ₅ + Li ₂ O). Having a molar fraction of 0.495 to 0.5).

  The inventors have found (A) rare earth elements, (B) specific ionic radii, and (C) doping amounts as requirements for dopants that contribute to the improvement of light damage resistance. Each requirement will be described.

(A) About rare earth elements The inventors have found that rare earth elements are effective in improving the light damage resistance. The rare earth elements are substituted as monovalent ions Li and trivalent ions in SLN and SLT. At this time, two excessive charges are generated per molecule of SLN and SLT. This excessive charge contributes to an increase in photoconductivity σ. As the photoconductivity σ increases, the rise time τ of the photoinduced refractive index effect decreases. This means that the rise time of the light-induced refractive index effect is reduced, so that the difference in refractive index between ordinary light and extraordinary light is reduced. As a result, the light damage resistance is improved.

In the rare earth element, it is considered that such an effect is caused by the electron arrangement. Among the rare earth elements, lanthanoids are filled with electrons before starting 5s, 5p, and 6d outside the 4f electron orbit, and then enter the 4f electron orbit (that is, the inner transition element). As a result, the electronic configuration of the trivalent ions of the lanthanoid is 5s ² 5p ⁶ (La), 4f ¹ 5s ² 5p ⁶ (Ce), 4f ² 5s ² 5p ⁶ (Pr), 4f ³ 5s ² 5p ⁶ ( Nd), 4f ⁴ 5s ² 5p ⁶ (Pm), 4f ⁵ 5s ² 5p ⁶ (Sm), 4f ⁶ 5s ² 5p ⁶ (Eu), 4f ⁷ 5s ² 5p ⁶ (Gd), 4f ⁸ 5s ² 5p ⁶ ( Tb), 4f ⁹ 5s ² 5p ⁶ (Dy), 4f ¹⁰ 5s ² 5p ⁶ (Ho), 4f ¹¹ 5s ² 5p ⁶ (Er), 4f ¹² 5s ² 5p ⁶ (Tm), 4f ¹³ 5s ² 5p ⁶ (Tm) Yb) and 4f ¹⁴ 5s ² 5p ⁶ (Lu). For this reason, lanthanoids have characteristics common to each other.

(B) Ion radius In order to improve the light damage resistance by the rare earth element, it is necessary to dope a predetermined amount described in (C) below. The inventors have determined that the necessary amount of dopable (6-coordinated) ionic radius is 0.86-0.89 Å (RD Shannon and CT Prewitt, Acta Cryst., B29 (1996) 925). It was found to be in the range.

  In the case of a dopant having an ionic radius larger than 0.89Å, for example, in the case of Sm in rare earth elements (ionic radius of 0.96Å), a sufficient amount of Sm cannot be substituted with Li, although substitution is possible. Therefore, the improvement of light damage resistance is not seen. Further, in the case of a dopant having an ionic radius smaller than 0.86 Å, particularly in the case of a dopant having an ionic radius smaller than that of Li (0.65 Å), the dopant is likely to be located between lattices, and is resistant to light damage. There may be no improvement in sex.

(C) About doping amount About SLN and SLT, doping amount is the range of 0.3-3.0 mol%, More preferably, it is the range of 0.4-1.0 mol%. This is because when the doping amount is less than 0.3 mol%, no improvement in light damage resistance is observed. In addition, when the doping amount exceeds 3.0 mol%, the improvement in light damage resistance becomes saturated, and the doping amount is too large, which may make it difficult to grow crystals. When the doping amount is in the range of 0.3 to 3.0 mol%, particularly 0.4 to 1.0 mol%, the light damage resistance is surely improved and the crystal growth is not adversely affected.

  Therefore, the inventors have specified that the dopants that satisfy all the conditions (A) to (C) are Er, Tm, Yb, and Lu among the rare earth elements described in (A) above. The SLN and SLT of the present invention contain at least one element selected from the group consisting of Er, Tm, Yb and Lu. Needless to say, even when two or more elements are selected from these groups as dopants, the optical damage resistance can be improved if doping is performed so as to satisfy the condition (C). Further, as the dopant, an element selected from the group consisting of the above-mentioned Er, Tm, Yb and Lu may be used in combination with an element selected from the group consisting of Mg, Zn, In and Sc according to the prior art. .

  SLN and SLT containing the above-mentioned dopants are suitable for optical elements. Specifically, the SLN and SLT of the present invention are suitable for optical elements such as wavelength conversion elements, optical modulators, and optical switches that are sensitive to optical damage. Those skilled in the art can easily analogize application to a specific optical element. Further, in addition to the conventional dopant for improving the light damage resistance, the fact that there are further dopant options can be advantageous in manufacturing since the dopant can be appropriately selected according to the device design.

  In addition, since all of the above-described dopants are trivalent, it is possible to expect an improvement in light damage resistance with a small doping amount as compared with the above-described divalent dopants Mg and Zn. Further, since the light damage resistance can be improved with a small doping amount, cracks, inclusions and the like based on the dopant during single crystal growth are less likely to occur. As a result, the yield of single crystal production is improved.

  The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples.

  In this example, using a double crucible method equipped with a raw material continuous supply system, Er was selected as a dopant, and SLN single crystals with various dopant concentrations were produced.

Using commercially available raw material powders of high purity Li ₂ O, Nb ₂ O ₅ , Li component excess raw material of Li ₂ O: Nb ₂ O ₅ = 0.58: 0.42, and Li ₂ O: Nb ₂ O ₅ = A raw material with a constant ratio composition of 0.50: 0.50 was prepared. Next, the Li component excess raw material was subjected to rubber press molding at a hydrostatic pressure of 1 ton / cm ² and sintered at about 1050 ° C. in the atmosphere. The stoichiometric composition raw material was sintered in the atmosphere at about 1150 ° C. and pulverized to a particle size of 50 μm to 500 μm.

Next, the outer crucible and inner crucible of the double crucible were filled with the sintered Li component excess raw material and commercially available high purity Er ₂ O ₃ powder. The Er ₂ O ₃ powder is filled so that the Er concentration with respect to Nb in the melt in the double crucible is 0, 0.2, 0.5, 1.0 and 2.0 mol%, respectively. It was. The high purity used in the examples is 99.999% or more.

  SLN single crystals of each Er concentration were produced by the following procedure.

  A seed crystal was attached to the Li component excess raw material melt in the double crucible, and the single crystal was pulled at a pulling speed of 0.5 mm / h and a crystal rotation speed of 20 rpm. At this time, an amount of the stoichiometric composition raw material corresponding to the crystallized SLN single crystal was automatically supplied to the outer crucible. As a result, lithium niobate single crystals (SLN) having substantially stoichiometric compositions doped with various concentrations of Er were obtained.

  About the obtained SLN doped with various concentrations of Er, the composition and the like were measured using inductively coupled high-frequency plasma emission analysis (ICP emission analysis) (Iris advantage, Japan Jarrell Ash, Japan). Moreover, Curie temperature was measured using the differential thermal analysis method (Thermo Plus DSC8270, Rigaku Corporation, Japan). The results are shown in Table 1 and will be described later.

Next, the absorption spectrum of the obtained SLN doped with various concentrations of Er was measured using Fourier transform infrared spectroscopy (FTIR) (Bio-Rad Win-IR, Varian, Inc., USA). Then, the peak shift of the band edge (3450 cm ^{−1 to} 3550 cm ⁻¹ ) of the OH stretching vibration was examined. The measurement results are shown in FIGS. 1 and 2 and will be described later.

  With respect to the obtained SLN doped with various concentrations of Er, a He-Ne laser and a green laser (532 nm) are incident on the same axis, and a refractive index dispersion (ordinary and extraordinary light) is measured using a device that measures birefringence change. The time dependence of the difference in refractive index between the two was measured. The measurement results are shown in FIG. 3 and will be described later. Next, the relationship between the rise time of the photoinduced refractive index effect obtained from refractive index dispersion and the photoconductivity was determined. The relationship is shown in FIG. 4 and will be described later.

  Photodamage was measured for SLN having an Er charge concentration of 0.2 and 1.0 mol% using a system 500 (to be described later) for measuring photodamage. The results are shown in FIG. 6 and will be described later.

A SLN single crystal doped with 0.5 mol% of Lu was produced. Specifically, Lu ₂ O ₃ powder is used instead of Er ₂ O ₃ powder, and the Lu concentration with respect to Nb in the melt in the double crucible is filled so as to be 1.0 mol%. The procedure was the same as in Example 1.

  For the obtained SLN single crystal doped with Lu, optical damage was measured in the same manner as in Example 1. The results are shown in FIG. 6 and will be described later.

Comparative Example 1

SLN single crystals doped with 1.0 mol% and 2.75 mol% of Mg were manufactured. Specifically, MgO powder is used instead of Er ₂ O ₃ powder, and the Mg concentration with respect to Nb in the melt in the double crucible is 1.0 mol% and 2.75 mol%, respectively. Otherwise, the procedure was the same as in Example 1.

About the obtained Mg-doped SLN single crystal, the absorption spectrum was measured using Fourier transform infrared spectroscopy (FTIR) in the same manner as in Example 1, and the band edge (3450 cm ^{−1 to} 3550 cm) of OH stretching vibration was measured. The peak shift of ^-1 ) was examined. The measurement results are shown in FIG. 2 and will be described later. Moreover, the Curie temperature of the obtained SLN single crystal doped with Mg was measured. The results will be described later.

Comparative Example 2

A SLN single crystal doped with 0.2 mol% of Nd was manufactured. Specifically, Nd ₂ O ₃ powder was used instead of Er ₂ O ₃ powder, and the Nd concentration with respect to Nb in the melt in the double crucible was filled so as to be 2.0 mol%. The procedure was the same as in Example 1. The obtained single crystal was composed of three planes and had side surfaces, and many inclusions were observed inside. This suggests that doping of Nd at a higher concentration makes it difficult to grow crystals and lowers the quality of the crystals obtained.

  For the obtained Nd-doped SLN single crystal, optical damage was measured in the same manner as in Example 1. The results are shown in FIG. 6 and will be described later.

  Table 1 shows the composition, segregation coefficient, and Curie temperature of SLN single crystals doped with various concentrations of Er prepared in Example 1. All of the single crystals were found to be lithium niobate single crystals having Li / Nb in the range of 0.96 to 1.0 and having a substantially stoichiometric composition. When the Er concentration in the melt (that is, the charged concentration of Er) is 0.2 mol%, 0.5 mol%, 1.0 mol% and 2.0 mol%, the Er concentration actually doped in the single crystal Were found to be 0.08 mol%, 0.26 mol%, 0.46 mol% and 0.83 mol%, respectively. Hereinafter, description will be made using the actually doped concentration. For example, in the case of SLN doped with 0.26 mol% of Er, for simplicity, it is represented as Er 0.26 mol% SLN.

  The Curie temperature of SLN doped with Er is lower than the Curie temperature of SLN doped with Mg (Mg 1.0 mol% SLN: 1212 ° C., Mg 2.0 mol% SLN: 1216 ° C.). It was found to be about 20 ° C. lower than (1215 ° C.). This suggests that the Curie temperature of the lithium niobate single crystal is lowered by Er doping, so that poling becomes easy, and as a result, the yield during processing can be improved.

  FIG. 1 is a diagram showing the results of FTIR according to Example 1.

It is known that the degree of resistance of the SLN single crystal to optical damage can be determined by the absorption position of the —OH group contained in the SLN single crystal (that is, OH stretching vibration). Specifically, it is known that the light damage resistance is higher as the band end of the OH stretching vibration appears on the higher energy side. Therefore, the band edge of OH stretching vibration was examined for Er0 mol% to 0.83 mol% SLN obtained in Example 1. As a result, the band edge of the OH stretching vibration in the spectrum of Er0 mol% to 0.26 mol% SLN appears at about 3465 cm ^−1, and the band edge of the OH stretching vibration in the spectrum of Er 0.46 mol to 0.84 mol% SLN is about It was found to appear at 3520 cm ⁻¹ . From this, it is suggested that the band end of the OH stretching vibration is shifted to the high energy side (the direction indicated by the arrow in FIG. 1) and the light damage resistance can be improved by increasing the doping amount. Next, the band end shift amount of OH stretching vibration was examined in more detail.

  FIG. 2 is a graph showing the relationship between the band edge shift amount of OH stretching vibration and the dopant concentration according to Example 1 and Comparative Example 1. FIG.

  According to Example 1, when the doping amount was between 0.26 mol% and 0.46 mol%, a remarkable shift of the band edge of the OH stretching vibration was observed. On the other hand, according to Comparative Example 1, a remarkable shift of the band edge of the OH stretching vibration was observed when the doping amount was between 0 mol% and 1.0 mol%. From this, it was found that the band edge of the OH stretching vibration shifts faster with Er with a smaller doping amount than with Mg. FIG. 1 and FIG. 2 show that Er can improve the photodamage with a smaller doping amount than Mg.

  FIG. 3 is a diagram illustrating the time dependence of the refractive index difference according to the first embodiment.

  From each plot, it can be seen that the refractive index difference between ordinary light and extraordinary light is the smallest at Er 0.46 mol% SLN and the largest at Er 0 to 0.08 mol% SLN. A small difference in refractive index suggests that beam fanning due to optical damage is unlikely to occur.

The obtained plots were fitted with the relational expression δΔn = δΔn _s [1-e (t / τ)] to determine the rise time τ of the photo-induced refractive index effect. Here, [Delta] n is the refractive index difference between the ordinary light and extraordinary light, [Delta] n _s is the saturated refractive index difference after t time. Next, the photoconductivity σ was calculated from the relational expression σ = ε ₀ ε _r τ using the determined rise time τ. Here, ε ₀ is the permittivity of free space (= 8.85 × e ⁻¹² F / m), and ε _r is the permittivity of lithium niobate single crystal (= 28).

  FIG. 4 is a graph showing the relationship between the rise time τ, the photoconductivity σ, and the Er doping amount according to the first embodiment.

  It is known that the greater the photoconductivity, the less photodamage. FIG. 4 shows that the photoconductivity increases as the amount of dopant increases. In particular, it was found that the maximum was obtained in the vicinity of the doping amount of 0.4 mol%. This suggests that the light damage resistance is particularly improved when the doping amount is 0.4 mol% or more.

  FIG. 5 is a diagram illustrating a system 500 for measuring optical damage.

  System 500 includes a light source 510, a condenser lens 520, and a screen 530. Here, an Nd: YAG SHG laser (wavelength 532 nm) was used as the light source 510. In the measurement, the laser light generated from the light source 510 is condensed on the sample 540 via the condenser lens 520, and the shape 550 of the laser light transmitted through the sample 540 is displayed on the screen 530. The projected shape 550 was photographed using a beam profiler (not shown), and optical damage was observed. The laser beam was irradiated in the Y-axis direction of SLN.

  FIG. 6 is a diagram showing a beam shape photographed by the system 500.

  6A shows the beam shape of Er0.26 mol% SLN, FIG. 6B shows the beam shape of Er 0.45 mol% SLN, and FIG. 6C shows the beam of Lu 0.5 mol% SLN. FIG. 6D shows a beam shape of Nd 0.24 mol% SLN.

  The beam shape in FIG. 6A shows an ellipse extending in the Z-axis direction, indicating that beam fanning has occurred. From this, it was found that even when Er is selected as the dopant, the optical damage resistance is not improved unless 0.3 mol% or more is doped. The beam shape in FIG. 6D also shows an elliptical shape extending in the Z-axis direction, and beam fanning occurs. This suggests that when Nd is selected as the dopant, a predetermined amount of doping cannot be achieved, and thus it is not suitable for improving light damage resistance.

  On the other hand, the beam shapes in FIGS. 6B and 6C remain circular, indicating that beam fanning, that is, no optical damage has occurred. From this, it was confirmed that the lithium niobate single crystal (SLN) doped with predetermined amounts of Er and Lu has light damage resistance.

  In the examples, only SLN is shown, but it can be easily understood by those skilled in the art that similar results can be obtained for SLT.

  As described above, the substantially constant ratio lithium niobate or lithium tantalate single crystal according to the present invention has light damage resistance. Such a single crystal can be applied to optical elements such as a wavelength conversion element, an optical modulator, and an optical switch.

The figure which shows the result of FTIR by Example 1 The figure which shows the relationship between the shift amount of the band edge of OH expansion-contraction vibration by Example 1 and Comparative Example 1, and dopant concentration. The figure which shows the time dependence of the refractive index difference by Example 1. FIG. The figure which shows the relationship between the rise time (tau) by Example 1, the calculated | required photoconductivity (sigma), and the doping amount of Er. Diagram showing a system 500 for measuring optical damage The figure which shows the beam shape image | photographed by the system 500

Explanation of symbols

500 System 510 Light source 520 Condensing lens 530 Screen 540 Sample 550 Shape

Claims (4)

A lithium niobate single crystal or a lithium tantalate single crystal having a substantially stoichiometric composition,
A single crystal containing 0.3 mol% to 3.0 mol% of an element selected from at least one selected from the group consisting of Er, Tm, Yb and Lu.   The single crystal according to claim 1, comprising 0.4 mol% to 1.0 mol% of the element.   The single crystal according to claim 1, wherein the single crystal is for an optical element.   The optical element using the single crystal of Claims 1-3.