Fluoride crystals, method for producing same, faraday rotator, optical isolator, optical processing device, and optical component

Abstract

The present invention provides defect-controlled fluoride crystals, a method for producing the same, and the use thereof. These fluoride crystals contain at least a rare earth element RE (RE is at least one element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), fluorine (F), and hydrogen (H), and have the same crystal structure as crystals represented by CeF3. A method for producing fluoride crystals according to the present invention comprises growing crystals from a raw material containing a rare earth element RE and a raw material containing fluorine (F) by a solvothermal method in the presence of at least one mineralizer selected from the group consisting of mineralizers containing an alkali metal element and / or an alkaline earth metal element, acid mineralizers, and mineralizers containing a normal salt.

Description

Fluoride crystals, methods for producing them, Faraday rotors, optical isolators, optical processing machines, and optical components.

[0001] This invention relates to fluoride crystals, methods for producing them, Faraday rotors, optical isolators, optical processing machines, and optical components.

[0002] In recent years, with the widespread use of laser processing machines for optical communications and precision machining, there has been a growing demand for higher power output and multi-wavelength capability in various lasers used as light sources. To stabilize the light source and prevent damage, optical isolators (OIs) that prevent backlighting are crucial.

[0003] Optical isolators typically include a Faraday rotator (FR). A Faraday rotator is a component that can rotate the polarization plane of incident light by applying a magnetic field.

[0004] Fluoride crystals grown by the Czochralski (CZ) method are known (see, for example, Patent Documents 1 and 2). Patent Document 1 describes REF grown by the CZ method. ３－ｘ This invention discloses a Faraday rotator mainly composed of a fluoride represented by (RE being a rare earth element, and x being 0 ≤ x < 0.1). Patent Document 2 discloses NdF grown from a molten raw material, similar to the CZ method. ３ An inorganic optical filter made of a single crystal is disclosed.

[0005] The CZ method is a method for growing crystals without contact with a container and is known as a bulk crystal growth method. In the CZ method, the raw material must be heated and melted, and the crystals must be grown by pulling them up from the surface of the melt. Therefore, a relatively steep temperature gradient is unavoidable at the crystal growth interface during the crystal growth process, which increases the likelihood of defects occurring during crystal growth and may prevent the growth of the most stable composition. As a result, crystals grown under the high-temperature conditions applied in the CZ method are likely to have many defects and retain instability. In addition, containers such as platinum crucibles, which are essential in the CZ method, are prone to breakage when used at high temperatures for extended periods, shortening their lifespan and making them undesirable from a manufacturing cost perspective.

[0006] International Publication No. WO 2012 / 133200, Japanese Unexamined Patent Application Publication No. 2014-149538

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide a fluoride crystal with controlled defects, a method for producing the same, and uses thereof.

[0008] The fluoride crystal according to the present invention contains at least a rare earth element RE (RE is an element selected from at least one element of the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), a fluorine element (F), and a hydrogen element (H), and has the same crystal structure as the crystal represented by CeF ３ and thereby solves the above problems. In the fluoride crystal, the concentration of hydrogen is 1×10 １４ atoms / cm ３ or more and 1×10 ２３ atoms / cm ３ or less. The fluoride crystal may further contain an alkali metal element and / or an alkaline earth metal element. The concentration of the alkali metal element and / or the alkaline earth metal element is 1×10 １２ atoms / cm ３ or more and 1×10 ２３ atoms / cm ３ or less. The alkali metal element may be an element selected from at least one element of the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). The alkaline earth metal element may be an element selected from at least one element of the group consisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The CeF ３A crystal structure identical to the crystal shown is a trigonal crystal and may have the symmetry of space group P 3c1. The lattice constants a, b, and c of the fluoride crystal may be in the range of a = 0.7129 ± 0.05 nm, b = 0.7129 ± 0.05 nm, and c = 0.7286 ± 0.05 nm, respectively. The method for producing the above fluoride crystal according to the present invention includes growing a crystal by solvothermal method from a raw material containing a rare earth element RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)) and a raw material containing fluorine (F) in the presence of a mineralizer selected from the group consisting of a mineralizer containing an alkali metal element and / or an alkaline earth metal element, an acid mineralizer, and a mineralizer containing a normal salt, thereby solving the above problem. The concentration of the mineralizing agent may be in the range of 1 M to 50 M. In growing the crystal by the solvothermal method, the temperature may be in the range of 400°C to 800°C, and the maximum achievable pressure may be in the range of 25 MPa to 250 MPa. In growing the crystal by the solvothermal method, a seed crystal may be used. The Faraday rotator according to the present invention contains the above fluoride crystal, thereby solving the above problem. The optical isolator according to the present invention comprises the above Faraday rotator and an analyzer or polarizer, thereby solving the above problem. The optical processing machine according to the present invention comprises the above optical isolator and a laser light source, thereby solving the above problem. The optical component according to the present invention is made of the above fluoride crystal, thereby solving the above problem. The optical component may be selected from the group consisting of a prism, lens, window material, optical filter, scintillator, phosphor, and laser medium.

[0009] The fluoride crystal of the present invention contains at least a rare earth element RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), a fluorine element (F), and a hydrogen element (H), and CeF ３Since it has the same crystal structure as the crystal shown, defects can be controlled and crack formation can be suppressed. Depending on the selection of RE, such fluoride crystals can be used to provide optical components such as Faraday rotators, optical isolators, processing equipment using them, prisms, lenses, window materials, optical filters, laser media, scintillators, and phosphors.

[0010] The present invention's method for producing fluoride crystals allows for synthesis at lower temperatures compared to the conventional CZ method. As a result, the resulting crystals have controlled defects and a stable composition. Furthermore, since the present invention does not require the use of expensive platinum crucibles essential for the conventional CZ method, the production cost of the target crystals can be reduced. In addition, the solvothermal method is suitable for mass production, making it practically advantageous. Moreover, by using the solvothermal method, it is relatively easy to increase the size of the crystals compared to conventional methods, and it is also possible to use seed crystals.

[0011] Figure 1 shows a flowchart for manufacturing the fluoride crystal of the present invention. Figure 2 shows a schematic diagram of a photoisolator using the fluoride crystal of the present invention. Figure 3 shows a schematic diagram of a photoprocessing device using the fluoride crystal of the present invention. Figure 4 shows a schematic diagram of a wavelength conversion system using the fluoride crystal of the present invention. Figure 5 shows an optical microscope image of the crystal of Example 1. Figure 6 shows an optical microscope image of the crystal and seed crystal of Example 5. Figure 7 shows an optical microscope image of the crystal and seed crystal of Example 1. Figure 1 shows the hydrogen and cesium concentrations of the crystal. Figure 2 shows the transmission spectrum of the crystal. Figure 5 shows the transmission spectrum of the crystal. Figure 1 shows the excitation and emission spectra of the crystal. Figure 2 shows the excitation and emission spectra of the crystal.

[0012] Embodiments of the present invention will be described below with reference to the drawings. Similar elements will be given the same numbers, and their descriptions will be omitted. In this specification, numerical ranges represented by "~" mean a range that includes the numbers written before and after "~" as the lower limit and upper limit, respectively.

[0013] (Embodiment 1) Embodiment 1 describes in detail the fluoride crystal of the present invention and the method for producing the same.

[0014] The fluoride crystal of the present invention contains, as constituent elements, at least a rare earth element RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), fluorine (F), and hydrogen (H), and CeF ３ It has the same crystal structure as the crystal shown. The fluoride crystal of the present invention, by containing the element hydrogen, has controlled defects and can exhibit the inherent properties of the material. The fluoride crystal of the present invention can exhibit a large Verde constant, high transmittance to light of a specific wavelength, or laser oscillation depending on the selection of the rare earth element RE, and can therefore function as optical components such as Faraday rotors, optical isolators, optical processing machines using these, prisms, lenses, window materials, optical filters, scintillators, phosphors, and laser media. Hereafter, for simplicity, the elements fluorine and hydrogen will be simply referred to as fluorine (F) and hydrogen (H).

[0015] In the fluoride crystal of the present invention, preferably, the hydrogen concentration is 1 × 10⁻⁶ １４ atoms / cm ３ The above 1 x 10 ２３ atoms / cm ３ The range is as follows. This allows for control of defects in the fluoride crystal. The hydrogen concentration is more preferably 1 × 10⁻⁶. １６ atoms / cm ３ The above 1 x 10 ２２ atoms / cm ３ The range is as follows. This allows for further control of defects in the fluoride crystal. In one example, the hydrogen concentration is 1 × 10⁻⁶. １８ atoms / cm ３ The above 1 x 10２１ atoms / cm ３ The following ranges are acceptable. In this specification, the hydrogen concentration is the value at a depth of 3 μm, measured using secondary ion mass spectrometry (SIMS).

[0016] The fluoride crystals of the present invention may further contain alkali metal elements and / or alkaline earth metal elements. The inclusion of alkali metal elements and / or alkaline earth metal elements in the fluoride crystals can deactivate defects within the crystal. Hereafter, for simplicity, alkali metal elements and alkaline earth metal elements will simply be referred to as alkali metals and alkaline earth metals.

[0017] The alkali metal is selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), and is preferably selected from the group consisting of K, Rb, and Cs. This allows for the deactivation of defects within the crystal.

[0018] Alkaline earth metals are selected from the group consisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), with a preference being the group consisting of Ca, Sr, and Ba. This allows for the deactivation of defects within the crystal.

[0019] Alkali metals and alkaline earth metals may be used individually or in combination of two or more types.

[0020] The concentration of alkali metals and / or alkaline earth metals is preferably 1 × 10⁻⁶ １２ atoms / cm ３ The above 1 x 10 ２３ atoms / cm ３ The range is as follows, preferably 1 × 10 １４ atoms / cm ３ The above 1 x 10 ２０ atoms / cm ３ The following range is used. This allows for further deactivation of defects within the crystal. In one example, the concentration of alkali metals and / or alkaline earth metals is 1 × 10⁻⁶.１７ atoms / cm ３ The above 1 x 10 ２０ atoms / cm ３ The following ranges are acceptable. In this specification, the concentrations of alkali metals and alkaline earth metals are values ​​measured at a depth of 6 μm using secondary ion mass spectrometry (SIMS).

[0021] The fluoride crystal of the present invention is CeF ３ It has the same crystal structure as the crystal shown. Specifically, it is a trigonal crystal, possessing the symmetry of space group P 3c1 (space group 165 in the International Tables for Crystallography), and occupying the crystal parameters and atomic coordinates shown in Table 1. In this specification, " 3" represents an overbar of 3.

[0022]

[0023] In Table 1, lattice constants a, b, and c represent the lengths of the unit cell axes, and α, β, and γ represent the angles between the unit cell axes. Atomic coordinates indicate the position of each atom in the unit cell as a value between 0 and 1, with the unit cell as the unit. Analysis of this crystal revealed that Ce and F atoms are present, with Ce occupying one type of position (Ce1) and F occupying three types of positions (F1 to F3).

[0024] CeF ３ As a crystal that has the same crystal structure as, CeF ３ The crystal itself, CeF ３ Crystals can be categorized into those where constituent elements are replaced by other elements, altering the lattice constant and atomic positions; those with missing elements; and those with excess elements. Here, an example of a crystal where constituent elements are replaced by other elements is CeF. ３ Some crystals have some or all of the Ce substituted with other REs (where the RE is at least one element selected from the group consisting of Pr, La, and Nd).

[0025] CeF ３While the lattice constant of a crystal changes as its constituent elements are replaced by other elements, or as hydrogen and, if necessary, alkali metals and / or alkaline earth metals are dissolved in it, the atomic positions given by the crystal structure, the sites occupied by atoms, and their coordinates do not change so drastically that the chemical bonds between the skeletal atoms are broken.

[0026] In this invention, the length of the chemical bond calculated from the lattice constant and atomic coordinates obtained by Rietveld analysis of the X-ray diffraction results using space group P 3c1 is compared with the length of the chemical bond of the crystal shown in Table 1. If this difference is, for example, within ±5%, it is defined as the same crystal structure, and a determination is made as to whether it is the same crystal. This determination criterion is based on experimental results showing that if the length of the chemical bond changes by more than ±5%, the chemical bond breaks and a different crystal is formed.

[0027] In one aspect, CeF ３ A crystal having the same crystal structure as the crystal shown is CeF ３ , PrF ３ LaF ３ , and NdF ３ The fluoride crystals of the present invention are these crystals (REF ３ ) may contain hydrogen (H), and optionally alkali metals and / or alkaline earth metals. For simplicity, CeF ３ When written as "crystal," CeF ３ The crystal itself, CeF ３－ｘ This includes crystals with missing F, such as (0 ≤ x ≤ 0.5). PrF ３ Crystal, LaF ３ crystal, NdF ３ The same applies to crystals. The parameter x is preferably 0 ≤ x ≤ 0.25, and more preferably 0 ≤ x < 0.1. This stabilizes the crystal structure.

[0028] In one aspect, CeF ３ A crystal having the same crystal structure as the crystal shown is (Ce,Pr)F ３ , (Nd, La) F ３ (Ce, La) F ３The fluoride crystals of the present invention may contain hydrogen (H), and optionally alkali metals and / or alkaline earth metals. In this specification, descriptions such as (Ce, Pr), (Nd, La), (Ce, La), etc., indicate that each element is present in any ratio. The same applies to any combination of Ce, Pr, La, and Nd such as (Pr, La), (Ce, Nd), (Ce, Pr, Nd), (Ce, Pr, La, Nd), etc., although not all are listed.

[0029] The fluoride crystal of the present invention is CeF ３ A crystal having the same crystal structure as the crystal shown is a trigonal crystal, possessing the symmetry of space group P ≥ 3c¹, and its lattice constants a, b, and c may be in the range of a = 0.7129 ± 0.05 nm, b = 0.7129 ± 0.05 nm, and c = 0.7286 ± 0.05 nm. Such a fluoride crystal has controlled defects.

[0030] One embodiment of the present invention is a CeF ３ crystal, PrF ３ Crystal, (Ce,Pr)F ３ Fluoride crystals containing hydrogen and, optionally, alkali metals and / or alkaline earth metals are suitable for Faraday rotators because the control of defects within the crystal maintains the reproducibility of the Faraday rotation angle (Verde constant) and suppresses variations in the Faraday rotation angle (Verde constant). Furthermore, defect control reduces the number of light scattering centers in the crystal and improves transmittance, allowing for high-precision rotation of the polarization plane.

[0031] In detail, the aforementioned fluoride crystals have a transmittance in the infrared to ultraviolet wavelength range that is similar to that of terbium gallium garnet (TGG:Tb), the material used for Faraday rotators. ３ Ga ５ O １２ It is higher than that of [another laser]. Furthermore, in addition to the wavelength range of 400 nm or less in which TGG exhibits absorption, it has the characteristic of high transmittance in the blue to green wavelength range. For this reason, it can be used as a light source, especially for short-wavelength lasers with wavelengths of 400 nm or less, and visible light lasers such as blue and green.

[0032] Furthermore, the fluoride crystals mentioned above are Ce ３＋ and Pr ３＋ It functions as a phosphor because it emits fluorescence based on [a specific factor]. For example, CeF ３ Fluoride crystals containing hydrogen and, optionally, alkali metals and / or alkaline earth metals are excited by light with wavelengths between 200 nm and 250 nm and emit light with a peak in the wavelength range between 290 nm and 320 nm. For example, PrF ３ Fluoride crystals containing hydrogen and, optionally, alkali metals and / or alkaline earth metals are excited by light with wavelengths between 200 nm and 220 nm and emit light having multiple peaks in the wavelength range between 230 nm and 440 nm.

[0033] As one embodiment of the present invention, NdF is a rare earth element RE which is Nd. ３ Fluoride crystals containing hydrogen and, optionally, alkali metals and / or alkaline earth metals, have a wavelength range of 400 nm to 900 nm in which the transmittance is 1% or less, and a wavelength range in which the transmittance is 70% or more, due to the control of defects within the crystal.

[0034] More specifically, the fluoride crystals described above have a transmittance of 1% or less in the ranges of 500 nm to 520 nm, 570 nm to 590 nm, 725 nm to 750 nm, 775 nm to 810 nm, and 855 nm to 865 nm, and a transmittance of 70% or more in the ranges of 400 nm to 450 nm, 525 nm to 555 nm, and 600 nm to 650 nm. Therefore, they can function as optical components such as optical filters that transmit a predetermined wavelength and block a predetermined wavelength.

[0035] As one embodiment of the present invention, LaF is a rare earth element RE of which is La. ３ Fluoride crystals containing hydrogen and, optionally, alkali metals and / or alkaline earth metals, can be used in optical components such as prisms, lenses, window materials, and laser media, as defects within the crystal are controlled to achieve a transmittance of 70% or more in the wavelength range of 0.2 μm to 10.5 μm.

[0036] As one embodiment of the present invention, LaF is a rare earth element RE of which is La. ３ The crystal functions as a scintillator by adding elements such as Nd, Ce, Tb, Pr, Eu, etc., to a fluoride crystal containing hydrogen and, optionally, alkali metals and / or alkaline earth metals. Furthermore, by controlling defects, the number of light scattering centers in the crystal is reduced, which can be expected to increase the amount of light emitted.

[0037] When Ce is activated in the above fluoride crystal, it can be excited by an ArF or KrF excimer laser and emit near-ultraviolet (275 nm to 315 nm) light. Furthermore, when Nd, Tm, Er, etc. are activated, near-infrared light is emitted, and it can function particularly as a high-power laser. From this, the above fluoride crystal can function as a laser matrix crystal and can function as an optical component such as a laser medium that emits light of various wavelengths by activating various activating elements. Specifically, the above fluoride crystal (with rare earth element RE being LaF) ３ When the crystal (which contains hydrogen and, optionally, alkali metals and / or alkaline earth metals) is activated with Nd, laser oscillation in the 1 μm band is possible; when activated with Tm and / or Ho, laser oscillation in the 2 μm band is possible; when activated with Er, laser oscillation in the 3 μm band is possible; when activated with Dy, laser oscillation in the 4 μm band is possible; and when activated with Tb or Pr, laser oscillation in the visible light band is possible.

[0038] Therefore, the fluoride crystal of the present invention may further contain an activating element A that functions as a luminescence center. Such an activating element A is at least one element selected from the group consisting of manganese (Mn), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), terbium (Tb), dysprosium (Dy), ytterbium (Yb), thulium (Tm), holmium (Ho), and erbium (Er), which are different from the rare earth elements RE that constitute the fluoride crystal. The content of activating element A is in the range of 0.01% to 5% relative to the rare earth elements RE. Within this range, the fluoride crystal is CeF ３ It can maintain the same crystal structure as the crystal shown.

[0039] These fluoride crystals are all high-quality crystals with minimal cracking because defects within the crystal are controlled by the inclusion of hydrogen. Therefore, crack formation within the crystal is effectively suppressed when the crystal is cut according to its intended use, resulting in excellent yield.

[0040] In one embodiment, the presence of hydrogen in the fluoride crystal of the present invention can be confirmed by preparing a thin plate-like sample of any size from the target crystal and measuring its transmission spectrum. Specifically, in the transmission spectrum of the fluoride crystal of the present invention, an absorption band related to hydrogen may be observed in the wavelength range of approximately 2200 nm to approximately 2400 nm. This absorption band is due to crystal growth by the solvothermal method under predetermined conditions, as can be understood from the manufacturing method described later, and is unique to the fluoride crystal of the present invention and is not seen in crystals produced by conventional methods such as the CZ method. It is presumed that this absorption band is due to the bonding (H-F bond) between hydrogen and fluorine contained in the target crystal, but is not limited to this. The arrangement of hydrogen contained in the fluoride crystal of the present invention may be random, and the wavelength range of the hydrogen-related absorption band is not limited to the above range, but if an absorption band specific to the above wavelength range is observed when the transmission spectrum of the target crystal is measured, it can be presumed that the crystal contains hydrogen.

[0041] Next, a method for producing the fluoride crystals of the present invention will be described. Figure 1 is a flowchart showing the production of the fluoride crystals of the present invention.

[0042] Step S110: Crystals are grown by solvothermal method from a raw material containing a rare earth element RE (RE is an element selected from the group consisting of Ce, Pr, La, and Nd) and a raw material containing fluorine (F) in the presence of a mineralizer selected from the group consisting of a mineralizer containing an alkali metal element and / or an alkaline earth metal element, an acid mineralizer, and a mineralizer containing a normal salt. Hereafter, for simplicity, alkali metal elements, alkaline earth metal elements, and fluorine will be simply referred to as alkali metals, alkaline earth metals, and fluorine. Step S110 will be described in detail below.

[0043] In step S110, the raw material containing the rare earth element RE can be the elemental RE, oxides, hydroxides, halides (fluorides, chlorides, bromides, iodides, etc.), inorganic salts (sulfates, nitrates, carbonates, etc.), organic salts (acetates, etc.), etc. Each compound other than the elemental RE may be anhydrous or hydrated.

[0044] In step S110, the raw material containing fluorine (F) can be an inorganic acid, an organic acid, or a fluoride of RE containing fluorine. Note that RE fluoride can be used as the raw material containing RE and the raw material containing F.

[0045] For example, the fluoride crystal of the present invention is CeF ３ If the crystal contains at least hydrogen, then the raw materials containing RE and F are CeF. ３ It is advisable to adopt this. For example, the fluoride crystal of the present invention is PrF ３ If the crystal contains at least hydrogen, then the raw materials containing RE and F are PrF. ３ It is advisable to adopt this. For example, the fluoride crystal of the present invention is NdF ３ If the crystal contains at least hydrogen, then NdF is used as the raw material containing RE and the raw material containing F. ３ It is advisable to adopt the following. For example, the fluoride crystal of the present invention is (Ce,Pr)F ３ If the crystal contains at least hydrogen, then the raw materials containing RE and F are CeF.３ and PrF ３ may be employed. By using these raw materials, there is no need to control the composition ratio of the raw material mixture, so it can be produced with good yield.

[0046] In addition, when producing a fluoride crystal activated by an activating element A as the fluoride crystal of the present invention, as a raw material, manganese (Mn), cerium (Ce), praseodymium different from the rare earth element RE constituting the fluoride crystal, (Pr), neodymium (Nd), samarium (Sm), europium (Eu), terbium (Tb), dysprosium (Dy), ytterbium (Yb), thulium (Tm), holmium (Ho), and erbium (Er) You may further employ | adopt the raw material containing at least 1 sort (s) chosen from the group which consists of.

[0047] The mineralizer is not particularly limited as long as it is a mineralizer selected from at least one of the group consisting of a mineralizer containing an alkali metal, a mineralizer containing an alkaline earth metal, a mineralizer containing a normal salt, and a mineralizer of an acid. The mineralizer containing an alkali metal and / or an alkaline earth metal is a compound containing an alkali metal and / or an alkaline earth metal. For example, halides, hydroxides, inorganic salts (such as carbonates) of an alkali metal and / or an alkaline earth metal can be used.

[0048] Non-limiting examples of the mineralizer include, for example, KOH, K ２ CO ３ [[ID=十六]]、KF、RbOH、Rb ２ CO ３ 、RbF、CsOH、Cs ２ [[ID=二十二]]CO ３ 、CsF、Ca (OH) ２ 、CaCO ３ 、CaF ２ 、Sr (OH) ２ 、SrCO ３ 、SrF ２ 、Ba (OH) ２ 、BaCO ３ 、BaF ２ etc. are mentioned, and it is preferable to select at least 1 or more from the group which consists of these compounds. Especially, fluoride is preferable. Thereby, it can manufacture with a favorable yield.

[0049] The mineralizing agent for normal salts is a salt produced by the complete neutralization of the hydrogen atoms of the acid and the hydroxyl groups of the salt. Specifically, examples include, but are not limited to, sodium chloride, calcium sulfate, and sodium phosphate. Examples of mineralizing agents for acids include, but are not limited to, hydrochloric acid, nitric acid, sulfuric acid, and formic acid.

[0050] The method for preparing the solution (reaction solution) to be used in synthesis by the solvothermal method is not particularly limited. For example, if a raw material mixture is prepared, a solution of the mineralizer may be added to the raw material mixture, and may be further mixed as needed. Alternatively, the mineralizer (preferably in powder or tablet form) may be added to the raw material mixture, and may be further mixed as needed. Alternatively, the above raw materials may be added to an aqueous solution of the mineralizer and mixed as appropriate. Here, the concentration of the mineralizer in the final reaction solution is preferably 1 M or more and 50 M or less, more preferably 1 M or more and 20 M or less, and even more preferably 1 M or more and 10 M or less.

[0051] The solvent can be any protic polar solvent, including water or organic solvents such as alcohols. If water is used, tap water, distilled water, RO water, deionized water, or pure water (ultrapure water) can be used.

[0052] Here, the reaction conditions (specifically temperature conditions) of the solvothermal method can be adjusted by taking advantage of the fact that the pH of the reaction solution changes depending on the type of alkali metal and / or alkaline earth metal contained in the mineralizing agent used, and the concentration of the mineralizing agent. Although certain care is required when handling highly alkaline solutions, the desired reaction solution can be prepared by appropriately adjusting the concentration of the mineralizing agent. It is also preferable to select the type of mineralizing agent depending on the raw materials used and / or the type of constituent elements of the target fluoride crystal.

[0053] The reaction conditions for the solvothermal method are not particularly limited, and conditions used in conventional solvothermal methods can be applied. The temperature is preferably in the range of 400°C to 800°C. This allows for a high yield of the desired fluoride crystals. Preferably, the temperature is in the range of 450°C to 750°C, more preferably 500°C to 750°C, even more preferably 550°C to 750°C, and particularly preferably 550°C to 650°C. The pressure is preferably in the range of 25 MPa to 250 MPa. The pressure is preferably adjusted by the amount of protic polar solvent, such as water, contained in the reaction vessel.

[0054] The crystal growth time using the solvothermal method can be adjusted as appropriate to ensure the reaction is completed, depending on the type and amount of raw materials used. For example, the crystal growth time may be between 1 hour and 300 hours within the temperature range mentioned above.

[0055] Within the temperature range described above, two or more temperature conditions may be set to establish a predetermined temperature profile. Such a temperature profile may be designed with consideration to improving the homogeneity and stability of the reaction solution, and to more efficiently generating the target fluoride crystals.

[0056] In the manufacturing method of the present invention, the raw materials (or mixture of raw materials) are reacted by the solvothermal method at significantly lower temperature conditions compared to the conventional CZ method. In the CZ method, due to the melting point of the target crystal, a phenomenon called decomposition and / or evaporation (decomposition evaporation phenomenon) is likely to occur during the crystal growth process (crystal growth process), in which specific components decompose and / or evaporate from the melt (or its interface). Furthermore, such decomposition evaporation may also occur in the grown crystal, potentially leading to defects in the grown crystal. However, in the manufacturing method of the present invention, such decomposition evaporation is less likely to occur. Therefore, the crystals obtained by the manufacturing method of the present invention can significantly suppress the formation of defects in the crystal due to decomposition evaporation. In addition, in general, with the solvothermal method, the crystals obtained have the most stable composition. Although there is a possibility that this composition may deviate slightly from the target composition, even if such a deviation occurs, the difference from the target composition is considered to be very small and is adjustable.

[0057] Furthermore, in the manufacturing method of the present invention, a seed crystal may be used during crystal growth by the solvothermal method. Specifically, for example, a seed crystal having a specific crystal orientation can be prepared, and the seed crystal can be placed so as to be immersed in the reaction solution described above, and crystal growth can be carried out. This makes it possible to efficiently grow fluoride crystals having a desired crystal orientation.

[0058] Previously, the product contained rare earth elements RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), fluorine (F), and hydrogen (H), and CeF ３ We have described fluoride crystals having the same crystal structure as the crystal shown, but by employing the manufacturing method of the present invention, it is possible to provide fluoride crystals that use other rare earth elements in addition to the rare earth elements mentioned above.

[0059] For example, the fluoride crystal of the present invention contains the rare earth element RE and fluorine (F), and has the general formula REF ３－ｘThe inorganic crystal represented by (where x satisfies 0 ≤ x < 0.1) may contain at least hydrogen (H). In this case, the rare earth element RE is at least one element selected from the group consisting of scandium (Sc), yttrium (Y), cerium (Ce), praseodymium (Pr), lanthanum (La), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0060] Here again, the hydrogen concentration is as described above, and alkali metals and / or alkaline earth metals may be included as needed, with their concentrations also as described above.

[0061] For example, when RE is Ce, Pr, La, or Nd, it can exhibit the functionalities described above, and when RE is Gd, it can function as a magnetic refrigeration material because of its high Gd concentration. For example, if RE is Tb, it emits light well and can function as a phosphor.

[0062] In the above case, the method for producing fluoride crystals is as follows: in step S110 of Figure 1, a raw material containing a rare earth element RE is used, which contains at least one element selected from the group consisting of scandium (Sc), yttrium (Y), cerium (Ce), praseodymium (Pr), lanthanum (La), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Otherwise, the method is as described above.

[0063] Such modifications are within the scope of the present specification and will be understood by those skilled in the art.

[0064] (Embodiment 2) Embodiment 2 describes the applications of the fluoride crystal of the present invention.

[0065] Figure 2 is a schematic diagram showing a photoisolator using the fluoride crystal of the present invention.

[0066] The optical isolator 200 comprises a polarizer 210, an analyzer 220, and a Faraday rotator 230 positioned between the polarizer 210 and the analyzer 220. The Faraday rotator 230 is made of the fluoride crystal described in Embodiment 1, so its description is omitted.

[0067] Here, the polarizer 210 and the analyzer 220 are arranged such that their transmission axes are non-parallel to each other, for example, at an angle of 45°.

[0068] The Faraday rotor 230 is columnar, usually roughly cylindrical, but can be thin film, bulk, fiber, etc., and is not limited to any particular shape.

[0069] A magnetic flux density B is applied to the Faraday rotator 230, for example, in the direction from the polarizer 210 toward the analyzer 220, that is, along the incident direction of light L. By applying the magnetic flux density B, the Faraday rotator 230 rotates the polarization plane of the light L that has passed through the polarizer 210, so that it passes through the transmission axis of the analyzer 220.

[0070] The optical isolator 200 is not limited to the above configuration; any configuration having at least one polarizer or analyzer is acceptable. That is, the analyzer 220 may be used instead of the polarizer 210, so that both are analyzers, or the polarizer 210 may be used instead of the analyzer 220, so that both are polarizers. This configuration is called a polarization-dependent type, but the configuration of the optical isolator is not limited to this; for example, a polarization-independent type may also be used. The polarization-independent type is an optical isolator in which birefringent crystal wedges are placed instead of polarizers 210 and analyzer 220. The birefringent crystal wedge on the light incidence side separates the polarization into ordinary and extraordinary light, passes through a Faraday rotator, and then enters the birefringent crystal wedge on the light emission side, where it is combined into a single beam of light before being emitted. However, light in the opposite direction does not ultimately become a single beam of light. The polarization-independent configuration can be used regardless of the state of the incident light and can be used as a highly versatile optical isolator.

[0071] The Faraday rotator 230 is made of the fluoride crystal of the present invention. Because defects are suppressed in the fluoride crystal of the present invention, the reproducibility of the Faraday rotation angle (Verde constant) can be maintained, and the variation in the Faraday rotation angle is suppressed. By using the fluoride crystal of the present invention, it is possible to suppress cracks and the generation of a second phase when cutting single crystals.

[0072] In particular, when using fluoride crystals where RE is Ce, Pr, and Nd, transparency is maintained in the infrared to visible light wavelength region, as well as in the wavelength region below 530 nm, allowing the use of short-wavelength lasers of 400 nm or less in the optical processing equipment described later. In particular, when RE is Ce, excellent transparency is maintained in the blue to green region, allowing the use of blue lasers and green lasers. Among these, fluoride crystals where RE is Ce and / or Pr have a large Faraday rotation angle and high transparency, resulting in a Faraday rotator with a high figure of merit (V / α) (where V is Verde's constant and α is the absorption coefficient).

[0073] Figure 3 is a schematic diagram showing an optical processing machine using the fluoride crystal of the present invention.

[0074] The optical processing machine 300 comprises a laser light source 310 and an optical isolator 200 (Figure 2) positioned on the optical path P of the laser light L emitted from the laser light source 310. With this optical processing machine 300, the laser light L emitted from the laser light source 310 passes through the optical isolator 200 and is emitted, making it possible to process the workpiece Q with the emitted light. Such an optical processing machine can be used as a laser processing machine.

[0075] Here, since the fluoride crystal used in the Faraday rotor 230 (Figure 2) of the optical isolator 200 is transparent, light absorption by the fluoride crystal is reduced. Therefore, in the optical processing machine 300, the damage resistance of the laser light source 310 due to the light from the Faraday rotor can be increased.

[0076] Furthermore, the fluoride crystal used as the Faraday rotor 230 has very few defects within the crystal, thus suppressing crack formation. Therefore, in the optical processing machine 300 according to this embodiment, the lifespan of the optical isolator 200 can be extended. As a result, the frequency of replacement of the optical isolator 200 in the optical processing machine 300 can be reduced.

[0077] The laser light source 310 is not particularly limited, but since the fluoride crystal of the present invention is transparent in the infrared to visible light wavelength range and in the wavelength range of 400 nm or less, laser light sources of various wavelengths can be used.

[0078] For example, an Nd:YAG laser with an oscillation wavelength of 1064 nm or a Yb-doped fiber laser with an oscillation wavelength of 1080 nm can be used. As the laser light source 310, for example, a 405 nm GaN-based semiconductor laser or a 700 nm titanium-sapphire laser can be used as a laser light source with an oscillation wavelength of 400 to 700 nm.

[0079] The laser light source 310 can be, for example, an ArF or KrF excimer laser, an argon ion laser, or the like, as a laser light source with an oscillation wavelength of 400 nm or less. In particular, by using a laser light source 310 with a wavelength of 400 nm or less, high-precision processing is possible.

[0080] Figure 4 is a schematic diagram showing a wavelength conversion system using the fluoride crystal of the present invention.

[0081] The wavelength conversion system 400 comprises a wavelength conversion element 410 and an optical filter 420 arranged in the optical path of the light emitted from the wavelength conversion element 410. The optical filter 420 is made of a fluoride crystal as described in Embodiment 1, so its description is omitted.

[0082] The wavelength conversion element 410 may be a single wavelength conversion element or a combination of two or more wavelength conversion elements. The wavelength conversion element 410 may be a second harmonic generator (SHG), a third harmonic generator (THG), a fourth harmonic generator (FHG), a fifth harmonic generator (FIHG), a sum frequency generator (SFG), a parametric oscillator (OPO), or a difference frequency generator (DFG).

[0083] For simplicity, the wavelength conversion element 410 is assumed to be an SHG (Single Harmonic Generator). The wavelength conversion element 410 converts the wavelength of the incident light X, which is the fundamental wave, into the second harmonic Y using the SHG. The wavelength conversion element 410 emits both the incident light X, which is the fundamental wave, and the second harmonic Y produced by the SHG. The optical filter 420 does not transmit the incident light X, but transmits only the second harmonic Y, thus obtaining a second harmonic with less contamination of the fundamental wave.

[0084] The optical filter 420 is made of the fluoride crystal of the present invention. Because defects are suppressed in the fluoride crystal of the present invention, cracks and the generation of a second phase during single crystal cutting can be suppressed.

[0085] In particular, when a fluoride crystal with RE being Nd is used, it has a wavelength range of 400 nm to 900 nm in which the transmittance is 1% or less, and a wavelength range in which the transmittance is 70% or more. Therefore, it can function as an optical filter in the wavelength range in which the second harmonic Y has a transmittance of 70% or more.

[0086] For example, if the wavelength conversion element 410 is SHG and the optical filter 420 is a fluoride crystal with RE being Nd, then incident light X having a wavelength of 1064 nm is wavelength-converted by the wavelength conversion element 410 to the second harmonic Y of 532 nm. The optical filter 420 transmits only the second harmonic Y of the incident light X and second harmonic Y emitted by the wavelength conversion element 410.

[0087] Figure 4 illustrates the case where the wavelength conversion element 410 is an SHG, but those skilled in the art will understand that it is not limited to this.

[0088] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0089] [Crystal Production] <Examples 1 to 4> In Examples 1 to 4, fluoride crystals were grown under the conditions shown in Table 2-1.

[0090] In Examples 1 to 4, in the presence of cesium fluoride (CsF, Furuuchi Chemical Co., Ltd.) as a mineralizing agent, the raw materials containing RE and F were respectively used as raw materials containing CeF. ３ Powder, PrF ３ Powder, LaF ３ Powder and NdF ３ Crystals were grown from powder (Furuuchi Chemical Co., Ltd.) using the solvothermal method (step S110 in Figure 1).

[0091] For details, see CeF ３ Powder, PrF ３ Powder, LaF ３ Powder and NdF ３ The powder, CsF powder, and 0.6 mL of pure water were placed in a silver ampoule (5 mm in diameter x 10 cm in length) and sealed. The CsF powder was mixed with 0.6 mL of pure water to form a 5 M aqueous solution. Next, the sealed ampoule was placed in a reaction vessel containing pure water, and the solvothermal method was carried out.

[0092] The conditions for synthesis by the solvothermal method were as follows: the temperature was raised to 600°C over 12 hours, held at the maximum pressure shown in Table 2-1 for the time shown in Table 2-1, and then cooled to room temperature over 1 hour. In this way, crystals of Examples 1 to 4 were obtained.

[0093] <Examples 5 to 7> In Examples 5 to 7, fluoride crystals were grown under the conditions shown in Table 2-2.

[0094] In Example 5, in the presence of cesium fluoride (CsF, Furuuchi Chemical Co., Ltd.) as a mineralizing agent, the raw materials containing RE and the raw materials containing F are CeF ３ Crystals were grown from powder (Furuuchi Chemical Co., Ltd.) by the solvothermal method (step S110 in Figure 1). Here, a thin, roughly rectangular plate-like crystal (a c-axis oriented crystal grown by the Bridgman method) was prepared as a seed crystal, and this seed crystal was placed in the reaction solution in the ampoule using a silver wire so as to be immersed. The conditions for synthesis by the solvothermal method were to raise the temperature to 610°C over 12 hours, hold at a maximum pressure of 144.8 MPa for 200 hours, and then cool to room temperature over 12 hours. In this way, the crystal of Example 5 was obtained.

[0095] In Examples 6 and 7, respectively, in the presence of rubidium fluoride (RbF, Furuuchi Chemical Co., Ltd.) and potassium fluoride (KF, Furuuchi Chemical Co., Ltd.) as mineralizing agents, the raw materials containing RE and F were used as raw materials containing CeF. ３ Crystals were grown from powder (Furuuchi Chemical Co., Ltd.) by solvothermal method (step S110 in Figure 1). The conditions for synthesis by solvothermal method were to raise the temperature to 630°C over 12 hours, hold at the maximum achievable pressure shown in Table 2-2 for 100 hours, and then cool to room temperature over 12 hours. In this way, the crystals of Example 6 and Example 7 were obtained.

[0096]

[0097] [Crystal Analysis] <Microscopic Observation> The crystals of Examples 1 to 7 were observed with an optical microscope. The results are shown in Figures 5 to 8, 9-1 and 9-2. Figure 5 is an optical microscope image of the crystal of Example 1. Figure 6 is an optical microscope image of the crystal of Example 2. Figure 7 is an optical microscope image of the crystal of Example 3.

[0098] Figures 5 to 7 demonstrate that bulk crystals can be obtained by performing the solvothermal method described in Figure 1. All crystals exhibited high transparency and no cracks were observed under a microscope. Although not shown, the crystals in Examples 4, 6, and 7 were also bulk crystals similar to those in Examples 1 to 3.

[0099] Figure 8 shows an optical microscope image of the crystal of Example 5, showing the seed crystal and the crystal grown on its surface being held with tweezers. Figure 9-1 shows optical microscope images of the crystal and seed crystal of Example 5, with the left side showing an observation of the surface of the seed crystal (thin plate-like crystal) before the solvothermal method is performed, and the right side showing an observation of the surface of the crystal grown on the surface of the seed crystal (crystal of Example 5) after the solvothermal method is performed. Figure 9-2 shows optical microscope images of the crystal and seed crystal of Example 5, with the left side showing an observation of the cross-section of the seed crystal (thin plate-like crystal) before the solvothermal method is performed, and the right side showing an observation of the cross-section of the crystal grown on the surface of the seed crystal (crystal of Example 5) after the solvothermal method is performed.

[0100] Figures 8, 9-1, and 9-2 confirm that a seed crystal can be used during crystal growth in the solvothermal method described in Figure 1. The crystal obtained in Example 5 using a seed crystal exhibited high transparency under microscopic observation, and no cracks were observed other than unavoidable cracks resulting from the thin plate-like structure of the seed crystal. The approximately circular holes seen in Figures 8 and 9-1 were formed to pass a wire through the seed crystal and were not created during crystal growth. Furthermore, Figure 9-2 shows that CeF was applied to the surface of the seed crystal, more specifically, to the upper and lower surfaces of the thin plate-like crystal. ３ It was confirmed that the crystal in Example 5 was thicker than the seed crystal before crystal growth due to epitaxial growth.

[0101] <X-ray Diffraction Analysis> Powder samples were prepared from the crystals of Examples 1 to 7 and subjected to powder X-ray diffraction measurements for structural analysis. The results are shown in Figures 10 to 14, Tables 1, 3, and 4.

[0102] Figure 10 shows the XRD pattern of the crystal powder of Example 1. Figure 11 shows the XRD pattern of the crystal powder of Example 2. Figure 12 shows the XRD pattern of the crystal powder of Example 3.

[0103] Figures 10 to 12 show the powder XRD patterns, along with the simulation results. According to Figure 10, the peak position (angle 2θ) and intensity (normalized intensity) of the XRD pattern of the crystal in Example 1 are CeF ３ The XRD pattern obtained from the simulation closely matched the observed pattern, and no second phase was seen. Furthermore, the crystal structure analysis of the crystal of Example 1 yielded the crystal structure parameters shown in Table 1. From this, it can be concluded that the crystal of Example 1 contains Ce and F, and CeF ３ It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and belongs to the symmetry of space group P ≥ 3c1.

[0104] Furthermore, by comparing Figure 10 with a comparable substance, a simple determination can be made as to whether or not it is the fluoride crystal of the present invention. It is advisable to make this determination based on about 10 peaks with strong diffraction intensity as the main peaks. In this sense, Table 1 is important as it serves as a reference for identifying the fluoride crystal of the present invention. In addition, the crystal structure of the fluoride crystal of the present invention can be approximated using other crystal systems of the trigonal crystal, in which case it will be expressed using different space groups, lattice constants and plane indices, but the X-ray diffraction results (e.g., Figure 10) and crystal structure will not change, and the identification method and identification results using them will also be the same. For this reason, in the present invention, X-ray diffraction analysis will be performed as if it were a trigonal crystal.

[0105] According to Figure 11, the peak position (angle 2θ) and intensity (normalized intensity) of the XRD pattern of the crystal in Example 2 are PrF ３ The XRD pattern obtained from the simulation closely matched the observed pattern, and no second phase was seen. Furthermore, the crystal structure analysis of the crystal of Example 2 yielded the crystal structure parameters shown in Table 3. From this, it can be concluded that the crystal of Example 2 contains Pr and F, and CeF ３ It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and belongs to the symmetry of space group P ≥ 3c1.

[0106]

[0107] According to Figure 12, the peak position (angle 2θ) and intensity (normalized intensity) of the XRD pattern of the crystal in Example 3 are LaF ３ The XRD pattern obtained from the simulation closely matched the observed pattern, and no second phase was seen. Furthermore, the crystal structure analysis of the crystal of Example 3 yielded the crystal structure parameters shown in Table 4. From this, it can be concluded that the crystal of Example 3 contains La and F, and CeF ３ It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and belongs to the symmetry of space group P ≥ 3c1.

[0108]

[0109] Although not shown in the illustration, the crystal of Example 4 contains Nd and F, and CeF ３It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and belongs to the symmetry of space group P ≥ 3c1.

[0110] As shown in Tables 1, 3, and 4, the lattice constants a, b, and c of the crystals in Examples 1 to 4 satisfy the following conditions, respectively: a = 0.7129 ± 0.05 nm, b = 0.7129 ± 0.05 nm, and c = 0.7286 ± 0.05 nm.

[0111] Figure 13 shows the powder XRD pattern of the crystal from Example 6. Figure 14 shows the powder XRD pattern of the crystal from Example 7.

[0112] The powder XRD patterns in Figures 13 and 14 are shown along with the simulation results. According to Figure 13, the peak position (angle 2θ) and intensity (normalized intensity) of the XRD pattern of the crystal in Example 6 are CeF ３ The XRD pattern obtained from the simulation closely matched the observed pattern, and no second phase was seen. Furthermore, the crystal structure analysis of the crystal of Example 6 yielded the crystal structure parameters shown in Table 1. From this, it can be concluded that the crystal of Example 6 contains Ce and F, and CeF ３ It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and belongs to the symmetry of space group P ≥ 3c1.

[0113] According to Figure 14, the XRD pattern of the crystal in Example 7 shows CeF ３ XRD patterns obtained by simulation and KCeF ３ The peak positions (angle 2θ) and intensities (normalized intensities) of the XRD patterns obtained from the simulation were found to be consistent. From this, it was determined that the crystal of Example 7 contains Ce and F, and CeF ３ It was confirmed that the crystal has the same crystal structure as the crystal shown, is a trigonal crystal, and includes crystals belonging to the symmetry of space group P 3c1. For Example 7, by adjusting the conditions shown in Table 2-2, KCeF ３ It is thought that single crystals without secondary phases such as the above can be obtained.

[0114] Although not shown in the illustration, the crystal in Example 5 contains Ce and F, and CeF ３It was confirmed that the crystals have the same crystal structure as those shown, are trigonal crystals, and include those belonging to the symmetry of space group P 3c1.

[0115] <Secondary Ion Mass Spectrometry> For the crystal in Example 1, the concentrations of hydrogen (H) and cesium (Cs) were measured using a secondary ion mass spectrometer (Cameca IMS-6F) and a time-of-flight secondary ion mass spectrometer (TOF-SIMS, IonTOF), respectively. The results are shown in Figure 15.

[0116] Figure 15 shows the concentrations of hydrogen and cesium in the crystal of Example 1.

[0117] According to Figure 15, in the crystal of Example 1, the hydrogen concentration at a depth of 3 μm is approximately 2 × 10⁻¹⁶. ２０ atoms / cm ３ Therefore, the cesium concentration at a depth of 6 μm is approximately 1 × 10⁻⁶ １８ atoms / cm ３ It was found that, although not shown in the diagram, the same component analysis was performed on the crystals of Examples 2 to 7, and the results showed that the crystals of Examples 2 to 7 contain at least hydrogen, and the hydrogen concentration is 1 × 10⁻⁶. １４ atoms / cm ３ The above 1 x 10 ２３ atoms / cm ３ The following range applies, and the cesium concentration is 1 × 10⁻⁶ １２ atoms / cm ３ The above 1 x 10 ２３ atoms / cm ３ The following ranges were confirmed (however, for Examples 6 and 7, the cesium concentration should be read as the rubidium or potassium concentration).

[0118] Although not shown in the figures, under the same conditions as in Example 1 described in Patent Document 1, CeF ３ Single crystals were grown using the CZ method, and the cesium concentration was measured to be below the detection limit.

[0119] Based on the above, by performing the solvothermal method described in Figure 1, it is possible to obtain a substance containing at least rare earth elements RE, fluorine (F), and hydrogen (H), and CeF ３ It was shown that a fluoride crystal having the same crystal structure as the crystal shown can be obtained. Furthermore, the crystal of Example 1 is CeF ３ The crystal is a fluoride crystal containing hydrogen and cesium, and the crystal in Example 2 is PrF ３ The crystal is a fluoride crystal containing hydrogen and optionally cesium, and the crystal in Example 3 is LaF ３ The crystal is a fluoride crystal containing hydrogen and optionally cesium, and the crystal in Example 4 is NdF ３ The crystal is a fluoride crystal containing hydrogen and optionally cesium, and the crystal in Example 5 is CeF ３ The crystal is a fluoride crystal containing hydrogen and optionally cesium, and the crystal in Example 6 is CeF ３ The crystal is a fluoride crystal containing hydrogen and optionally rubidium, and the crystal in Example 7 is CeF ３ The crystals were fluoride crystals containing hydrogen and, optionally, potassium.

[0120] <Optical Properties> Thin plate-shaped samples of arbitrary size were prepared from the crystals of Examples 1 to 7, and the transmission and reflection spectra were measured to calculate the absorption coefficient α. In addition, the wavelength dependence of the Verde constant was measured for the crystals of Example 1 and Example 2. Furthermore, the excitation and emission spectra of the crystals of Examples 1 to 4 were measured using a fluorescence spectrophotometer.

[0121] The absorption coefficient α of the crystal in Example 1 was confirmed to be nearly zero in the wavelength region of 306 nm to 800 nm. The absorption coefficient α of the crystal in Example 2 was confirmed to be nearly zero in the wavelength region of 220 nm to 400 nm. The Verde constants of the crystals in Example 1 and Example 2 in the wavelength region of 300 nm to 400 nm were larger than those of the TGG, which is known as a Faraday rotator. From these findings, it was confirmed that the crystals in Example 1 and Example 2 function as Faraday rotators and can be used in combination with short-wavelength lasers of 400 nm or less as a light source. Furthermore, it was confirmed that the crystals in Example 5 and Example 6, which have a crystal structure similar to that of the crystal in Example 1, also function as Faraday rotators. The crystal in Example 7, a single crystal without the aforementioned second phase, can also function as a Faraday rotator.

[0122] The crystal in Example 3 showed high transmittance in the region of 0.2 μm to 10.5 μm, indicating that it can be applied to optical components such as prisms, lenses, and window materials.

[0123] The crystal in Example 4 exhibits a transmittance of 1% or less in the ranges of 500 nm to 525 nm, 570 nm to 590 nm, 725 nm to 750 nm, 775 nm to 810 nm, and 855 nm to 865 nm, and a transmittance of 70% or more in the ranges of 400 nm to 450 nm, 525 nm to 555 nm, and 600 nm to 650 nm. It was found that this crystal can be applied to optical components such as optical filters that transmit a predetermined wavelength and block a predetermined wavelength.

[0124] Figure 16 shows the transmission spectrum of the crystal in Example 2. Figure 17 shows the transmission spectrum of the crystal in Example 5.

[0125] Figure 16 shows PrF grown by the CZ method. ３ The transmission spectrum of the (comparative crystal) is also shown. The thickness of the thin plate-like samples analyzed was 163 μm for the crystal in Example 2 and 130 μm for the comparison crystal.

[0126] As shown in Figure 16, the transmission spectrum of the crystal in Example 2 showed a significant decrease in transmittance in the wavelength range of approximately 2200 nm to 2400 nm, whereas the transmission spectrum of the comparison crystal did not show such a decrease in transmittance. Analysis revealed that the decrease in transmittance was due to absorption related to hydrogen contained in the crystal in Example 2. Note that in Figure 16, the baselines of both spectra are somewhat low (in the range of approximately 60% to 90%), which is due to PrF ３ This is due to the crystal exhibiting material-specific absorption in the wavelength range of 2100 nm to 2500 nm shown in Figure 16, and does not mean that the crystal has low transparency.

[0127] Figure 17 shows a seed crystal (CeF grown by the Bridgman method). ３ The transmission spectrum is also shown. The thickness of the thin plate-like samples analyzed was 420 μm for the crystal in Example 5 and 360 μm for the seed crystal.

[0128] As shown in Figure 17, the transmission spectrum of the crystal in Example 5 showed a significant decrease in transmittance in the wavelength range of approximately 2200 nm to approximately 2400 nm, whereas the transmission spectrum of the seed crystal did not show such a decrease in transmittance. Analysis revealed that the decrease in transmittance was related to absorption by hydrogen contained in the crystal in Example 5. In contrast to Figure 16 above, CeF ３ The crystal exhibits virtually no material-specific absorption in the wavelength range of 2100 nm to 2500 nm, as shown in Figure 17, resulting in high baseline values ​​for both spectra (in the range of approximately 85% to 90%).

[0129] Although not shown in the figures, the transmission spectra of the crystals in Examples 1, 3, 4, 6, and 7 also showed absorption bands related to the hydrogen contained in each crystal in the wavelength range of approximately 2200 nm to 2400 nm, similar to the crystals in Examples 2 and 5.

[0130] Figure 18 shows the excitation and emission spectra of the crystal in Example 1. Figure 19 shows the excitation and emission spectra of the crystal in Example 2.

[0131] According to Figure 18, the crystal in Example 1 can be excited most efficiently at 227 nm, and the emission spectrum when excited at 227 nm is Ce ３＋ It was found to emit ultraviolet light with a peak at 307 nm based on this.

[0132] According to Figure 19, the crystal in Example 2 can be excited most efficiently at 214 nm, and the emission spectrum when excited at 214 nm is Pr ３＋ It was found to emit multiple types of light, including violet light with a peak at 399 nm, based on the following analysis.

[0133] As described above, the present invention can provide fluoride crystals with controlled defects. The fluoride crystals of the present invention are industrially advantageous because they can be manufactured under milder conditions than conventional methods, and in particular, at lower temperatures. The fluoride crystals of the present invention can be applied to various optical components such as Faraday rotors, prisms, lenses, window materials, optical filters, laser media, phosphors, and scintillators.

[0134] 200 Optical isolator 210 Polarizer 220 Detector 230 Faraday rotator (optical isolator material) 300 Optical processing machine (laser processing machine) 310 Laser light source 400 Wavelength conversion system 410 Wavelength conversion element 420 Optical filter L Light (laser light) B Magnetic flux density P Optical path Q Workpiece

Claims

1. Contains at least one rare earth element RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)), fluorine (F), and hydrogen (H), CeF ３ A fluoride crystal having the same crystal structure as the crystal shown.

2. The hydrogen concentration is 1 × 10⁻⁶. １４ atoms / cm ３ The above 1 x 10 ２３ atoms / cm ３ The fluoride crystal according to claim 1, wherein the crystal is within the following range.

3. The fluoride crystal according to claim 1 or 2, further comprising an alkali metal element and / or an alkaline earth metal element.

4. The concentration of the alkali metal element and / or alkaline earth metal element is 1 × 10 １２ atoms / cm ３ or more and 1 × 10 ２３ atoms / cm ３ or less, and the fluoride crystal according to claim 3.

5. The fluoride crystal according to claim 3 or 4, wherein the alkali metal element is an element selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

6. The fluoride crystal according to any one of claims 3 to 5, wherein the alkaline earth metal element is an element selected from the group consisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

7. The CeF ３ The fluoride crystal according to any one of claims 1 to 6, wherein the crystal structure identical to that shown is a trigonal crystal and has symmetry of space group P 3c1.

8. The fluoride crystal according to claim 7, wherein the lattice constants a, b, and c of the fluoride crystal are in the range of a = 0.7129 ± 0.05 nm, b = 0.7129 ± 0.05 nm, and c = 0.7286 ± 0.05 nm, respectively.

9. A method for producing fluoride crystals according to any one of claims 1 to 8, comprising growing crystals by solvothermal method from a raw material containing a rare earth element RE (RE is an element selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), and neodymium (Nd)) and a raw material containing a fluorine element (F) in the presence of a mineralizer selected from the group consisting of a mineralizer containing an alkali metal element and / or an alkaline earth metal element, an acid mineralizer, and a mineralizer containing a normal salt.

10. The method according to claim 9, wherein the concentration of the mineralizing agent is in the range of 1 M or more and 50 M or less.

11. The method according to claim 9 or 10, wherein, in growing crystals by the solvothermal method, the temperature is in the range of 400°C to 800°C, and the maximum achievable pressure is in the range of 25 MPa to 250 MPa.

12. The method according to any one of claims 9 to 11, wherein a seed crystal is used in growing a crystal by the solvothermal method.

13. A Faraday rotator containing fluoride crystals according to any one of claims 1 to 8.

14. An optical isolator comprising the Faraday rotator described in claim 13 and an analyzer or polarizer.

15. An optical processing apparatus comprising the optical isolator described in claim 14 and a laser light source.

16. An optical component made of a fluoride crystal according to any one of claims 1 to 8.

17. The optical component according to claim 16, wherein the optical component is selected from the group consisting of a prism, lens, window material, optical filter, scintillator, phosphor, and laser medium.

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