GB2245263A

GB2245263A - Inorganic nonlinear optical material

Info

Publication number: GB2245263A
Application number: GB9111951A
Authority: GB
Inventors: Hiroshi Daimon; Itsuhiro Fujii; Shinichi Sakata
Original assignee: Ube Industries Ltd
Current assignee: Ube Corp
Priority date: 1990-06-12
Filing date: 1991-06-04
Publication date: 1992-01-02
Anticipated expiration: 2011-06-04
Also published as: FR2663131B1; DE4118460A1; DE4118460C2; FR2663131A1; GB9111951D0; GB2245263B

Description

1 :22;2 l. E5:2. L= NONLINEAR OPTICAL MATERIAL

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a nonlinear optical material and more particularly to an inorganic nonlinear optical material containing a specific metal-oxygen four coordination structure. DESCRIPTION OF THE PRIOR ART

The nonlinear optical materials are useful for the purposes of varying wavelength of laser beam, switching, modulating, and memorizing laser beams and, therefore, provide the most important elements in all the optoelectronic materials. Particularly, the second harmonic generation (SHG) which-is one form of wave variation constitutes the most important field in which the nonlinear optical materials find utility.

The nonlinear optical materials are broadly divided into two groups, one group made of organic substances-and the other group of inorganic substances. Organic nonlinear optical materials possessing a prominent nonlinear optical effect have been proposed and active studies on these materials are still under way. The organic substances, however, are vulnerable to heat.

In contrast, inorganic nonlinear optical materials are resistant to heat as compared with organic nonlinear optical materials. Among the known inorganic nonlinear optical materials, the nonlinear optical material Ca3(VO4)2 which contains a vanadiumoxygen tetrahedron is particularly popular.

This Ca 3 (VOI 4) 2' however, possesses a feeble nonlinear optical effect barely comparable with potassium acid phosphate (KDP) As one reason for the poor nonlinear optical effect manifested by Ca 3 (VO 4)2' the absence of an orienting property from the vanadium-oxygen tetrahedron in the crystalline structure may be cited.

SUMMARY OF THE INVENTION

An object of this invention is to provide a nonlinear optical material exhibiting notably high nonlinear optical effect without entailing the problem of the conventional inorganic nonlinear optical material described above.

Another object of this invention is to provide a novel inorganic nonlinear optical material e:kccelling in durability and avoiding appreciable generation of heat on exposure to a laser beam.

The nonlinear optical material of this invention is characterized by comprising inorganic oxide crystals of a structure containing M-oxygen tetrahedra having the element M coordinated with four oxygen atoms, having these M-oxygen tetrahedra oriented in one direction, and lacking a center of symmetry and having the element M composed of an element Ma and an element Mb, the element Ma being at least one member selected from the group consisting of the elements of Group Ia, Group Ib, Group IIa, and Group IIb and the element Mb being at least one member selected from the group consisting of the elements of Group Va, Group Vb, and Group IVb, both in the Periodic Table of Elements.

W The M-tetrahedron having a specific element M coordinated with four oxygen atoms in the structure of the nonlinear optical material of this invention exhibits a notably high orienting property. This explains why the nonlinear optical material of this invention manifests a highly desirable nonlinear optical effect.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a type diagram for the illustration of elementary configuration of the structure of an element-oxygen tetrahedron in the nonlinear optical material of this invention.

Fig. 2 is a diagram showing an X-ray diffraction chart of lithium vanadate obtained in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail below with reference to the accompanying drawings.

Fig. 1 is a type diagram for the illustration of an elementary configuration of the structure of an element-oxygen tetrahedron in the nonlinear optical material of this invention. In the diagram, 1 stands for an oxygen atom, 2 an element Ma, and 3 for an element Mb. The nonlinear optical material of this invention thus contains the structure of a (Ma, Mb)-ox ygen tetrahedron of a regular tetrahedral shape having the element Ma 2 and the element Mb 3 each coordinated with four oxygen atoms 1.

Preferably, this material is composed of inorganic oxide crystals of a structure solely comprising the tetrahedra, having these tetrahedra oriented in one direction, and lacking a center of symmetry. In the nonlinear optical materi al of this invention, 1 the element Ma is at least one member selected from the group consisting of the elements of Group Ia, Group Ib, Group-IIa, and Group IIb and the element Mb is at least one member selected from the group consisting of the elements of Group Va, Group Vb, and Group IVb.

Preferably, the element Ma is at least one member selected from the group consisting of Li, Na, Zn, and Mg and the element Mb is at least one member selected from the group consisting of V, P, As, Si, and Ge.

As examples of the crystalline compound to be used in the construction of the nonlinear optical material of the present invention, the compounds indicated in (1) to (3) below may be cited.

(1) Compounds having Li or Na as Ma and V as Mb, specifically lithium vanadates represented by the general formula, Li k V m 0 n (wherein k, m, and n each stand for an integer of an arbitrary value).

These lithium vanadates contain a plurality of phases. Their typical compositions are 2Li 2 0-5V 2 05r Li 2 O-V 2 0 5, and 3Li 2 O-V 2 0 51 for example.' They are only required to form crystals having no center of symmetry.

(2) Compounds represented by the general formula, Ma3 MbO 4 (wherein the element Ma is at least one member selected from the group consisting of the elements of Group Ia and Group Ib and the element Mb is at least one member selected from the group consisting of the elements of Group Va and Group Vb, and preferably Ma is Li or Na and Mb is P or As). Their typical compositions are Li 3 PO 4 and Li 3 AsO 41 for example.

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(3) Compounds represented by the general formula, Mc 2 MdMbO 41 providing that Ma is composed of Mc and Md (wherein the'element Mc is at least one member selected from the group consisting of the elements of Group Ia and Group 1b, the element Md is at least one member selected from the group consisting of the elements of Group IIa and Group IIb, and the element Mb is at least one member selected from the group consisting of the elements of Group IVb, and preferably Mc is Li or Na, Md is Zn or Mg, and Mb is Si or Ge). Their typical compositions are Li 2 (Zn or Mg)SiO 4 and Li 2 (Zn or Mg)GeO 41 for example.

The drystals of the compounds of (2) and (3) likewise contain a plurality of phases. They also form solid solutions. These crystals are only required to be devoid of a center of symmetry.

The method to be used for the synthesis of the compounds of (1) to (3) mentioned above is not particularly restricted. Any of the known methods such as the solid-phase reaction method and the solution method may be employed for the synthesis. By the solid-phase reaction method, for example, such a compound can be easily produced by mixing powdered raw materials of the component elements of the compound in proportions calculated to give a desired composition and firing the resultant mixture. In this case, the firing temperature must be selected to as to give rise to crystals devoid of a center of symmetry. Generally, the firing is obtained sufficiently at a temperature in the range of from 400 to 1,200C for a period in the range of from 4 to 20 hours.

The raw materials to be used are not particularly restricted.

1 Various compounds containing the relevant elements can be used. For example, oxides, carbonates, halogenides, hydroxide;, and ammonium salts are usable.

The nonlinear-optical material according with this invention manifests a literally prominent nonlinear optical effect and, on exposure to a laser beam, generates SHG efficiently, emits heat very slightly under the impact of the laser beam, and enjoys outstanding durability.

The nonlinear optical material of this invention is extremely useful economically as various electrooptical materials such as, for example, piezoelectric materials and dielectric materials.

Now, the present invention will be described more specifically below with reference to working examplf--s and comparative experiments. Example 1:

A white powder was obtained by thoroughly mixing vanadium pentoxide (V 2 0 5' purity 99.99%) and lithium carbonate (L'2CO3' purity 99.99%) in a molar ratio of 1: 3 and firing the resultant mixture in the open air at 600C for five-hours.

The produced powder, on analysis by X-ray diffraction, was found to be crystals of lithium vanadate (Li 3 VO 4) devoid of a center of symmetry. The X-ray diffraction chart of this powder is shown in Fig. 2. This lithium vanadate lacking a center of symmetry possessed a structure having the position of zinc atom of a wurtzite type crystal substituted with lithium and vanadium atoms at 3: I and the position of sulfur atom thereof substituted with an oxygen atom. In the crystal structure, vanadium-oxygen 1 i i 1--- 11.

tetrahedra and lithium-oxygen tetrahedra are oriented in the direction of the C axis of crystal toward the apex theieof.

When the powder was interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd-YAG laser (1,064 nm), it was observed to emit SHG in a green color. The pulse produced absolutely no change on the surface of the sample exposed thereto. Example 2:

A white powder was obtained by thoroughly mixing arsenic oxide (As 2 0 5' purity 99.9%) and lithium carbonate (L'2CO3' purity 99.9%) in a molar ratio of 1: 3 and firing the resultant mixture in the open air at 600C for five hours. The produted powder, on analysis by X-ray diffraction, was found to be crystals of Li 3 AsO 4 having no center of symmetry. The crystals, similarly to those of Example, had a structure in which arsenicoxygen tetrahedra and lithium-oxygen tetrahedra were oriented in the direction of the C axis of crystal toward the apex.

When the produced powder was interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd-YAG laser (1,064 nm), it was observed to emit SHG in a green color. The pulse produced absolutely no change on the surface of the sample exposed thereto. Example 3:

Crystals of lithium magnesium germanate (Li 2 MgGeO 4) were obtained by thoroughly mixing germanium oxide (Geo 2' purity 99.99%), magnesium oxide (MgO, purity 99.9%), and lithium carbonate (Li 2 CO 3' purity 99.99%) in a molar ratio of 1: 1 7 - z firing the resultant mixture in the open air at 900C, and further firing it at 6000C. These crystals, similarly to those of Example 1, had a structure having germanium-oxygen tetrahedrons, magnes3-um-oxygen tetrahedra, and lithium-oxygen tetrahedra oriented in the direction of the C axis of crystal toward the apex thereof.

When the crystals were interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd:YAG laser (1,064 nm), it was observed to emit SHG in a green color. The pulse produced absolutely no change on the surface of the sample exposed thereto. Example 4:

Lithium zinc germanate (Li 2 ZnGeO 4 crystals were obtained by thoroughly mixing germanium oxide (GeO 2' purity 99.99%), zinc oxide (ZnO, purity 99.9%), and lithium carbonate (Li 2 CO 3' purity 99.99%) in a molar ratio of 1: 1: 1, firing the resultant mixture in the open air at 9000C, and further firing it at 600'C.

The crystals, similarly to those of Example 1, had a structure having germanium-oxygen tetrahedra, zinc-oxygen tetrahedra, and lithium-oxygen tetrahedra oriented in the direction of the C axis of crystal toward the apex thereof.

When the crystals were interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd:YAG laser (1,064 nm), it was observed to emit SHG in a green color. The pulse produced absolutely no change on the surface exposed thereto. Example 5:

Lithium zinc silicate (Li 2 ZnSiO 4) crystals were obtained by 1 il' ' thoroughly mixing silicon oxide (SiO 2' purity 99.9%), zinc oxide (ZnO, purity 99.9%), and lithium carbonate (Li 2 CO 3' pur ity 99.99%) in a molar ratio of 1: 1: 1, firing the resultant mixture in the open air at 1,0001C, and further firing it at 6000C. The crystals, similarly to those of Example 1, had a structure having silicon oxygen tetrahedra, zinc-oxygen tetrahedra, and lithium-oxygen tetrahedra oriented in the direction of the C axis of crystal toward the apex thereof.

When the crystals were interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd:YAG laser (1,064 nm), it was obserbed to emi-t SHG in a green color. The pulse produced absolutely no change on the surface exposed thereto. Example 6:

Sodium zinc silicate (Na2ZnSiO,) crystals were obtained by thoroughly mixing silicon oxide (SiO 2' purity 99.9%), zinc oxide (ZnO, purity 99.9%), and sodium carbonate (Na2C0 3' purity 99.99%) in a molar ratio of 1: 1: 1, firing the resultant mixture in the open air at 1,OOOOC, and further firing it at 6000C. The crystals, similarly to those of Example 1, had a structure having silicon-oxygen tetrahedra, zinc-oxygen tetrahedra, and sodium-oxygen tetrahedra of crystal toward the apex thereof.

oriented in the direction of the C axis When the crystals were interposed between opposed celophane sheets and exposed to a pulse of 10 nsec from a Nd:YAG laser (1,064 nm), it was observed to emit SHG in a green color. The pulse produced absolutely no change on the surface exposed thereto.

Comparative Experiment 1:

When lithium niobate (LiNbO 3) was exposed to the lser beam in the same manner as in Example 1, it was observed to emit SHG but sustain a burn at the site exposed to the pulse. This fact indicates that lithium vanadate generates less heat than lithium niobate. Comparative Experiment 2:

Ca 3 (VO 4)2 crystals were obtained by thoroughly mixing vanadium pentoxide (V 2 0 5' purity 99.99%) and calcium carbonate (CaC03. purity 99.99%) in a molar ratio of 1: 3 and firing the resultant mixture in the open air at 1,2500C. The Ca 3 (VO 4)2 crystals were found to have a structure in which vanadium-oxygen tetrahedra were not oriented.

When the crystals were exposed to a pulse of 10 nsec from a Nd:YAG laser in the same manner as in Example 1, they were found to emit SHG in a green color. The intensity of the SHG was about 50% of that obtained in example 1.

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Claims

WHAT IS CLAIMED IS:

1. A nonlinear optical material formed of inorganic oxide crystals possessing a structure containing M-oxygen tetrahedra having said element M coordinated with four oxygen atoms, having said M-oxygen tetrahedra oriented in one direction, and lacking a center of symmetry, which nonlinear optical material is characterized by the fact that said element M is composed of an element Ma and an element Mb and said element Ma is at least one member selected from the group consisting of the elements of Group Ia, Group Ib, Group IIa, and Group IIb, and said element Mb is at least one member selected from the group consisting of the elements of Group Va, Group Vb, and Group IVb.

2. A nonlinear optical material according to claim 1, wherein saidinorganic oxide crystals are formed solely of a Moxygen four coordination structure.

3. A nonlinear optical material according to claim 1, wherein said element Ma is at least one member selected from the group consisting of Li, Na, Zn, and Mg and said element Mb is at least one member selected from the group consisting of V, P, As, Si, and Ge.

4. A nonlinear optical material according to claim 3, wherein said element Ma is Li or Na and said element Mb is V.

5. A nonlinear optical material according to claim 1, wherein said inorganic oxide is a compound represented by the general formula, Ma 3 Mb04 (wherein Ma stands for at least one member selected from the group consisting of the elements of Group Ia and Group Ib and Mb for at least one member selected from the group consisting of the elements of Group Va and Group Vb).

6. A nonlinear optical material according to claim 5, wherein the element Ma is Li or Na and the element Mb is P or As.

7. A nonlinear optical material according to claim 1, wherein said element Ma is composed of an element Mc and an element Md and said inorganic oxide is a compound represented by the general formula, Mc 2 MdMbO 4 (wherein Mc stands for at least one member selected from the group consisting of the elements of Group Ia and Group Ib, Md for at least one member selected from the group consisting of the elements of Group IIa and Group IIb, and Mb for at least one member selected from the group consisting of the elements of Group IVb).

8. A nonlinear optical material according to claim 7, wherein said element Mc is Li or Na, sLid element Md is Zn or Mg, and said element Mb is Si or Ge.

9. A nonlinear optical material according to claim 4, wherein said inorganic oxide is Li 3 vo 4'

10. A nonlinear optical material according to claim 6, wherein said inorganic oxide is Li 3 AsO 4

11. A nonlinear optical material according to claim 8, wherein said inorganic oxide is Li 2 MgGeO 4 or Li 2 ZnGeO 4

12. A nonlinear optical material according to claim 8, wherein said inorganic oxide is Li 2 ZnSiO 4 or Na 2 ZnSiO 4'

13. A nonlinear optical material as defined in any of the preceeding claims, substantially as herein described.

14. A nonlinear optical material substantially as described in any of Examples 1-6.

Published 1991 at The Patent Office. Concept House. Cardiff Road, Newport. Gwent NP9 I RH. Further copies may be obtained from Sales Branch, Unit 6. Nine Mile Point. Cwnifeliffach. Cross Keys. Newport. NPI 71-17- Printed by Multiplex techniques lid, St Mary Cray. Kent.

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