Method for improving the 355 nm laser damage threshold of calcium fluoride crystals

Abstract

The application relates to a method for improving the 355nm laser damage threshold of calcium fluoride crystals, in particular to a method for improving the 355nm laser damage threshold of calcium fluoride crystals by co-doping with inert rare earth ions and lanthanide ions. CaF2 powder is selected as raw material, YF3 and LaF3 are selected as doping sources, and the total amount of YF3 and LaF3 co-doped in the process of preparing calcium fluoride crystals by adopting a crucible lowering method or a temperature gradient method is not more than 2.0 at.%, so as to improve the damage threshold of the calcium fluoride crystals.

Description

TECHNICAL FIELD

[0001] The application relates to a method for improving the 355nm laser damage threshold of calcium fluoride crystals, in particular to a method for improving the damage threshold of calcium fluoride crystals by co-doping with inert rare earth ions and lanthanide ions, and belongs to the technical field of calcium fluoride crystals. BACKGROUND

[0002] The calcium fluoride crystal has the advantages of short ultraviolet cutoff wavelength, high transmittance, low nonlinear refractive index (n2=1.9x10 - 6 cm / W), small temperature coefficient of refractive index (dn / dT=-10.6x10 -6 ), large band gap (11.2eV) and high chemical resistance to halogen gas, so that the calcium fluoride crystal has small nonlinear self-focusing effect, thermal prism effect, high theoretical damage threshold and strong corrosion resistance, and becomes one of the preferred materials for window materials in ultraviolet laser equipment. However, research shows that the actual damage threshold of the calcium fluoride crystal is low, which limits its application.

[0003] As an optical material, the optical performance of the calcium fluoride crystal is closely related to the beam quality, and the thermal performance is related to the damage threshold. The inert rare earth ions and the lanthanide ions have the advantages of stable valence electron structure, almost no absorption in the ultraviolet band, and formation of a large number of clusters at a low doping concentration.

[0004] However, the single doping ion has limited cluster formation, and the cluster formation capacity is saturated when the doping concentration reaches a certain degree. SUMMARY

[0005] In order to solve the problem of low damage threshold in the application process of the existing calcium fluoride crystal, the application co-dopes specific contents of inert rare earth Y 3+ and lanthanide La 3+ to obtain a calcium fluoride crystal with a high damage threshold under the action of a 355nm laser.

[0006] On one hand, the application provides a method for improving the damage threshold of a calcium fluoride crystal by co-doping with inert rare earth (for example, Y 3+ ) and lanthanide (for example, La 3+ ). The method selects CaF2 powder as a raw material and selects YF3 and LaF3 as a doping source. In the process of preparing the calcium fluoride crystal by using a crucible lowering method or a temperature gradient method, YF3 and LaF3 are co-doped in a total amount of not more than 2.0at.% to improve the damage threshold of the calcium fluoride crystal. Preferably, the doping content of YF3 and the doping content of LaF3 are in a ratio of 1:6 to 6:1. Preferably, the doping content of YF3 is 0.1at.% to 0.6at.%, and the doping content of LaF3 is 0.1at.% to 0.6at.%.

[0007] In the present application, rare earth ions and lanthanide ions are doped into the calcium fluoride crystal lattice, and due to the dipole interaction, the doped ions and interstitial F ions are easily self-assembled to form clusters, and the formation of clusters will increase the number of chemical bonds between the calcium fluoride crystal surfaces and increase the interlayer binding force. The damage threshold can be further improved through the synergistic effect of co-doping.

[0008] In another aspect, the present application also provides an inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal, the chemical composition of the inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal is Ca 1-x-y Y x La y F2, wherein x+y≤2.0at.%; preferably x:y=1:6~6:1.

[0009] Preferably, the preferred range of x is 0.1~0.6at.%, or / and the preferred range of y is 0.1~0.6at.%; preferably, x:y=1:6~6:1.

[0010] Preferably, under nanosecond laser irradiation at a wavelength of 355nm, compared with pure calcium fluoride, the damage threshold of the optimized Y 3+ , La 3+ doped calcium fluoride is increased by nearly 1.5 times compared with pure calcium fluoride, and the damage threshold of the optimized Y 1-x-y Y x La y doped calcium fluoride is increased by nearly 1 times compared with Y 3+ doped calcium fluoride.

[0011] Preferably, compared with pure calcium fluoride, the damage area of the Y 3+ , La 3+ ion co-doped calcium fluoride crystal is observed to be smaller and the damage degree is lighter under the microscope at the same magnification under similar energy density.

[0012] Preferably, compared with pure calcium fluoride, the F / Ca ratio of the damage area of the rare earth ion doped calcium fluoride crystal is significantly reduced, indicating that the F loss in the damage area is small. The loss of F element during irradiation is closely related to the local vitrification of calcium fluoride crystal, because the rapid energy deposition during laser irradiation makes the damaged fragments non-adiabatic transition to the anti-bonding state, and in this process, complex material ionization and ejection can occur, forming neutral F 0 , Ca 0 , CaF 0 , thereby forming F loss. After optimizing the co-doping concentration, the F / Ca ratio of the Ca 1-x-y Y x La y F2 crystal is basically unchanged compared with pure calcium fluoride, indicating that the degree of local vitrification is weak.

[0013] In another aspect, the present application provides a method for preparing a calcium fluoride crystal co-doped with inert rare earth ions and lanthanide ions, according to the stoichiometric ratio Ca 1-x-y Y x La y F2The CaF2 powder and YF3, LaF3 powder are weighed as raw material powder, and the crystal is grown by a crucible lowering method. In the present application, the prepared doped calcium fluoride crystal has a high damage threshold and strong damage resistance.

[0014] Preferably, PbF2 powder is added as an oxygen scavenger in the raw material powder, and the amount of the PbF2 powder added is 0-3 wt.% of the CaF2 powder, preferably 0.2-2.0 wt.%, more preferably 0.5-1.0 wt.%.

[0015] Preferably, the crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99%; the crystal growth is carried out in a high-purity Ar atmosphere (purity of at least 99%) and / or a fluorine-containing atmosphere, or a high-vacuum atmosphere; and the fluorine-containing atmosphere is CF4 and / or HF gas, or a mixture of CF4 and / or HF gas and other inert gas Ar.

[0016] Preferably, the purity of the CaF2 powder, YF3 powder, LaF3 powder, and PbF2 powder is greater than 99.99%.

[0017] Beneficial effects:

[0018] The present application provides a method for co-doping Y 3+ and La 3+ to improve the damage threshold of calcium fluoride crystals. In addition, the damage threshold of the doped CaF2 crystal obtained by the method is high, which is 39.9% higher than the highest damage threshold reported for nanosecond laser action at a wavelength of 355 nm. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The damage transmittance diagrams of the calcium fluoride crystals doped with different concentrations of ions prepared in Examples 1-3 after three continuous laser shots at a wavelength of 355 nm and an energy density of 60 J / cm 2 ;

[0020] Figure 2 The damage morphology diagrams of the calcium fluoride crystals doped with different concentrations of ions prepared in Examples 1, 2, and 4 under the action of a single laser shot at a wavelength of 355 nm and similar energy density, wherein (d)-(f) are the damage morphology diagrams of crystals with different components after the action of pulsed laser, and the magnification of the optical microscope photographs of the samples after laser damage is the same;

[0021] Figures 3a-3cEDS results of the damage area of the low concentration inert rare earth ion doped calcium fluoride crystal prepared in Example 1, 2 and 4 after three continuous laser irradiations at a wavelength of 355 nm and an energy density of 60 J / cm 2 , in which the black line represents the initial surface and the red line represents the element content of F and Ca in the damage area. DETAILED DESCRIPTION

[0022] The present application is further illustrated by the following examples, which should not be construed as limiting the present application.

[0023] The inert rare earth Y 3+ ion and lanthanide ion La 3+ doped CaF2crystal was prepared by co-doping Y 3+ , La 3+ Doping can significantly improve the damage threshold of CaF2crystal.

[0024] The single-doped Y 3+ , single-doped La 3+ and Y 3+ , La 3+ co-doped calcium fluoride crystals were prepared under the same conditions as the preparation of pure calcium fluoride crystals. Specifically, the raw materials YF3, LaF3, CaF2were prepared according to the set molar ratio, and a certain mass fraction of PbF2was added as an oxygen scavenger, and then the crystal was grown by the crucible lowering method.

[0025] The crystal was oriented and processed to obtain a (111) surface calcium fluoride crystal with a size of 30 mm x 30 mm x 4 mm.

[0026] The crystal was ultra-precisely polished.

[0027] In the present application, the co-doping of yttrium ions and lanthanide ions significantly improves the zero-probability damage threshold of calcium fluoride crystals and enhances the damage resistance.

[0028] The present application is further illustrated by the following examples, which should not be construed as limiting the present application.

[0029] Example 1:

[0030] The growth method of CaF2 single crystal is a crucible descending method: the selected crucible material is a graphite crucible, a CaF2 single crystal rod with a normal direction of the oriented end face determined by an X-ray diffractometer being

[111] is placed at the bottom of the crucible, and the crystal growth is performed in a high vacuum atmosphere. -3 Pa, and then the temperature is increased to heat the material, the temperature is first increased to 300 DEG C, and is kept for 10 h to remove the moisture and air in the raw material; the temperature is continuously increased to 800 DEG C at a rate of 20-50 DEG C / h, and is kept for 15 h; then the temperature is continuously increased to 1400-1450 DEG C, and is kept for 10 h to heat the material, after the heating of the material is completed, the crucible is slowly descended to grow the crystal. The descending speed is 0.1-0.5 mm / h, and after the crystal growth is completed, the temperature is decreased to room temperature at a rate of 20-50 DEG C / h.

[0031] Example 2:

[0032] The growth method of 0.3at.% Y, 0.5at.% La-CaF2 single crystal is a crucible descending method: the selected crucible material is a graphite crucible, a CaF2 single crystal rod with a normal direction of the oriented end face determined by an X-ray diffractometer being

[111] is placed at the bottom of the crucible, the crystal growth is performed in a high vacuum atmosphere, and other parameters refer to example 1.

[0033] Example 3:

[0034] The growth method of 0.3at.% Y-CaF2 single crystal is a crucible descending method: the selected crucible material is a graphite crucible, a CaF2 single crystal rod with a normal direction of the oriented end face determined by an X-ray diffractometer being

[111] is placed at the bottom of the crucible, and the crystal growth is performed in a high vacuum atmosphere.

[0035] Example 4:

[0036] The growth method of 0.5at.% La-CaF2 single crystal is a crucible descending method: the selected crucible material is a graphite crucible, a CaF2 single crystal rod with a normal direction of the oriented end face determined by an X-ray diffractometer being

[111] is placed at the bottom of the crucible, the crystal growth is performed in a high vacuum atmosphere, and other parameters refer to example 1.

[0037] Example 5:

[0038] The growth method of 0.8at.% La-CaF2 single crystal is a crucible descending method: the selected crucible material is a graphite crucible, a CaF2 single crystal rod with a normal direction of the oriented end face determined by an X-ray diffractometer being

[111] is placed at the bottom of the crucible, the crystal growth is performed in a high vacuum atmosphere, and other parameters refer to example 1.

[0039] Example 6:

[0040] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0041] Example 7:

[0042] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0043] Example 8:

[0044] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0045] Example 9:

[0046] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0047] Example 10:

[0048] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0049] Example 11:

[0050] The growth method of 0.8 at.% Y-CaF2 single crystal is the crucible lowering method: the selected crucible material is graphite crucible, the CaF2 single crystal rod with the direction of the normal line of the oriented end face being

[111] by X-ray diffractometer is put into the bottom of the crucible, the crystal growth is carried out in a high vacuum atmosphere, and other parameters refer to example 1.

[0051] Example 12:

[0052] 0.3at. % Y, 0.3at. % La-CaF2 single crystal was grown by Bridgman method. The graphite crucible was selected as the crucible material, and the CaF2 single crystal rod with the normal direction of the oriented end face being

[111] was put into the bottom of the crucible. The crystal growth was carried out in a high vacuum atmosphere, and other parameters were referred to Example 1.

[0053] Example 13:

[0054] 0.3at. % Y, 0.3at. % La-CaF2 single crystal was grown by Bridgman method. The graphite crucible was selected as the crucible material, and the CaF2 single crystal rod with the normal direction of the oriented end face being

[111] was put into the bottom of the crucible. The crystal growth was carried out in a high vacuum atmosphere, and other parameters were referred to Example 1.

[0055] Table 1 is the test results of zero-probability damage of CaF2 crystals doped with different concentrations and components under the action of 355 nm laser. As can be seen from the table, in order to improve the zero-probability damage threshold, the optimal concentration of single-doped Y 3+ is about 0.3at. %, the optimal concentration of single-doped La 3+ is about 0.5at. %, and the damage threshold cannot be continuously improved by continuously increasing the concentration of Y 3+ or La 3+ . Under the condition that the total concentration of doping is equivalent, the zero-probability damage threshold of CaF2 crystal is obviously improved when Y 3+ and La 3+ are co-doped, reaching 29.8 J / cm 2 . Compared with pure CaF2, the zero-probability damage threshold is increased by 144%, which is 39.9% higher than the highest damage threshold of 21.3 J / cm 2 reported at present.

[0056] Figure 1 The damage transmittance diagram of the low-concentration inert rare earth ion doped calcium fluoride crystals prepared in Examples 1-3 after the action of three continuous 355 nm laser beams with an energy density of 60 J / cm 2 . The transmittance of the undamaged area of the crystals with different components is high and basically unchanged, indicating that a small amount of doping does not have a significant impact on the optical quality of the crystal, and no obvious absorption peak of other substances is found in the damage area transmittance spectrum. At a wavelength of 355 nm, the transmittance of the CaF2, 0.3at. % Y, 0.5at. % La-CaF2 and 0.3at. % Y-CaF2 crystals after damage is 17.16%, 75.54% and 27.93% respectively, corresponding to red, green and brown lines respectively. Obviously, the co-doped 0.3at. % Y, 0.5at. % La-CaF2 crystal can still maintain a high transmittance under the action of strong laser, indicating the strongest damage resistance.

[0057] Figure 2 The damage morphology of low concentration of inert rare earth ion doped CaF2 crystals prepared in Example 1, 2, 4 under the action of single shot laser with similar energy density and wavelength of 355 nm; CaF2 damage sample Figure 2 a) The surface has visible white damage points, the surface damage points of 0.3 at. % Y, 0.5 at. % La-CaF2 and 1.0 at. % La-CaF2 samples are small Figure 2 b and c) and cannot be observed by naked eye. It can be seen that under similar energy density, the damage area of pure CaF2 crystal is the largest and the damage is the most serious Figure 2 d), the damage area of 0.5 at. % La-CaF2 crystal is the second Figure 2 f), the damage area of 0.3 at. % Y, 0.5 at. % La-CaF2 crystal is the smallest Figure 2 e).

[0058] Figures 3a-3c EDS results of low concentration of inert rare earth ion doped CaF2 crystals prepared in Example 1, 2, 4 after three continuous shots of laser with wavelength of 355 nm and energy density of 60 J / cm 2 As shown in the figure, points I, III, V are the energy spectrum of CaF2, 0.3 at. % Y, 0.5 at. % La-CaF2 and 0.5 at. % La-CaF2 crystals in the undamaged area, and the F / Ca ratio is 2.2, 2.18 and 2.26 respectively. The F / Ca ratio of the damage area II, IV, VI is 1.50, 2.20 and 1.57 respectively. As can be seen, compared with the F / Ca ratio of the original surface, the F / Ca ratio in the glassification particles of the damage area of CaF2 and 0.5 at. % La-CaF2 crystals decreases by 31.8% and 30.5% respectively, indicating that F atoms are lost in the damage area during the laser irradiation process. The loss of F atoms is related to the photo-physical interaction occurring during the laser irradiation process. After rapid energy deposition during the laser irradiation process, the damaged fragments undergo non-adiabatic transition from non-adiabatic state to antibonding state, complex material ionization and ejection occur, neutral F 0 , Ca 0 , CaF 0 , and thus CaF 2-x is formed. The F / Ca of 0.3 at. % Y, 0.5 at. % La-CaF2 crystal is almost unchanged before and after damage (the F / Ca ratio before and after damage is 2.18 and 2.2 respectively), and there is no loss of F atoms. As can be seen, Y 3+ , La 3+ co-doping can reduce the loss of F in CaF2 crystal during the laser irradiation process, and thus improve the laser damage resistance of CaF2 crystal.

[0059] Table 1 is a table of zero-probability damage thresholds at 355 nm for different concentrations of ion-doped calcium fluoride crystals prepared in Examples 1-4:

[0060]

[0061]

Claims

1. A method for improving the 355 nm laser damage threshold of calcium fluoride crystals by co-doping with inert rare earth ions and lanthanide ions, characterized in that, CaF2 powder is selected as raw material, YF3 and LaF3 are selected as doping sources, YF3 and LaF3 are co-doped in a total amount of not more than 2.0 at.% to improve the damage threshold of the calcium fluoride crystal in the process of preparing the calcium fluoride crystal by a crucible lowering method or a temperature gradient method.

2. The method of claim 1, wherein, The ratio of the doping content of YF3 to the doping content of LaF3 is 1:6-6:

1. The doping content of YF3 is 0.1-0.6 at.% and the doping content of LaF3 is 0.1-0.6 at.%.

3. An inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal characterized in that, The chemically-composed calcium fluoride crystal co-doped with the inert rare earth ions and lanthanide ions is Ca 1-x-y Y x La y F2, wherein x+y≤2.0 at. %; x:y=1:6~6:

1.

4. The inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal of claim 3, wherein, x=0.1-0.6 at.% or / and y=0.1-0.6 at.%.

5. The inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal of claim 4, wherein, The calcium fluoride crystal co-doped with the inert rare earth ions and lanthanide ions has a damage threshold of 15.4-29.8 J / cm at 355 nm 2 .

6. A method of producing the inert rare earth ion and lanthanide ion co-doped calcium fluoride crystal as claimed in any one of claims 3 to 5, characterized by, According to the stoichiometric ratio Ca 1-x-y Y x La y F2Take CaF2 powder, YF3 powder, LaF3 powder as raw material powder, and grow calcium fluoride crystal co-doped with inert rare earth ions and lanthanide ions by crucible lowering method or temperature gradient method.

7. The production method according to claim 6, characterized by, PbF2 powder is added to the raw material powder as an oxygen scavenger, and the addition amount of the PbF2 powder is 0-3 wt.% of the CaF2 powder. PbF2 powder is added to the raw material powder as an oxygen scavenger, and the addition amount of the PbF2 powder is 0.2-2.0 wt.% of the CaF2 powder.

8. The preparation method according to claim 7, characterized in that, The crucible material used in the crucible lowering method is high-purity graphite with a purity of at least 99%; the crystal growth is carried out in a high-purity Ar atmosphere and / or a fluorine-containing atmosphere, or a high-vacuum atmosphere; the purity of the high-purity Ar atmosphere is at least 99%, and the fluorine-containing atmosphere is CF4 and / or HF gas, or a mixture of CF4 and / or HF gas and other inert gas Ar. The purity of the CaF2 powder, YF3 powder, LaF3 powder and PbF2 powder is greater than 99.99%.

9. The production method according to claim 6, characterized by, ​ 10. The production method according to claim 7 or 8, characterized by, ​

Images