JPH04273224A - Polarization inversion control method - Google Patents

Abstract

PURPOSE:To securely generate the polarization inversion of a fine pattern with good controllability by irradiating a ferroelectric material with charged particles to plural areas at intervals which are large enough to exert no mutual influence on each other. CONSTITUTION:The areas on the singly polarized ferroelectric material 1 are irradiated locally with the charged particles to form a polarization inversion structure 3. The areas are irradiate with the charged particles at the intervals LB where no mutual influence is exerted. Thus, interference between adjacent irradiated areas is evaded and polarization inversion 3A can be generated independently. After the areas are irradiated with the charged particles at said intervals LB where no mutual influence is exerted, accumulated charges on the ferroelectric material 1 are removed and other areas nearby the irradiated areas where an electric field is removed are irradiated with charged particles while the intervals LB are maintained to generate each polarization inversion 3A independently.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization inversion control method suitable for application to the formation of optical devices such as optical second harmonic generating elements (hereinafter referred to as SHG elements).

0002

2. Description of the Related Art In recent years, it has been proposed to improve characteristics such as optical output by forming a periodic domain inversion structure on the surface of optical devices such as SHG elements.

For example, an SHG element generates second harmonic light of a frequency of 2ω when light of a frequency ω is introduced, and this SHG element can expand the wavelength range of single wavelength light. Accordingly, it is possible to expand the scope of use of lasers and optimize the use of laser light in each technical field. For example, by shortening the wavelength of laser light, it is possible to improve the recording density in optical recording and reproduction using laser light, magneto-optical recording and reproduction, and the like.

[0004] As such an SHG element, for example, K
So-called bulk-type SHG elements using TP, waveguide-type SHG elements that perform phase matching using a larger nonlinear optical constant, and nonlinear optical materials such as ferroelectric crystals such as LiNbO3 (LN). Cerenkov radiation type SHG that forms a linear waveguide on a single crystal substrate, inputs the fundamental wave of near-infrared light, and extracts second harmonics, such as green and blue light, from the substrate side as a radiation mode. There are elements etc.

However, the bulk type SHG element has relatively low SHG conversion efficiency due to its characteristics, and cannot use LN, which is inexpensive and provides high quality. Further, in the Cerenkov radiation type SHG element, the radiation direction of the SHG beam is in the direction into the substrate, and the beam spot has a unique shape, for example, a crescent-shaped spot, which poses problems in actual use.

In order to realize a device with high conversion efficiency,
The phase propagation speeds of the fundamental wave and the second harmonic must be made equal. As a method to do this in a pseudo manner, a method has been proposed in which the nonlinear optical constants + and - are arranged periodically (J
．． A. Armstrong, N. Bloemberge
n, et al., Phys. Rev. , 127, 1918 (19
62)). One way to achieve this is to periodically reverse the direction of the crystal (for example, crystal axis). Specific methods include, for example, cutting crystals thinly and pasting them together (Okada, Takizawa, Ieiri, NHK Technical Research, 29 (1)
, 24 (1977)), and a method of forming a periodic domain inversion structure by controlling the polarity of the applied current during crystal pulling to form periodic domains (domains).
D. Feng, N. B. Ming, J. F. Hong,
et al., Appl. Phys. Lett. 37,607 (1
980), K. Nassau, H. J. Levinst
ein, G. H. Loiacano Appl. Phy
s. Lett. 6, 228 (1965), A. Feis
st,P. Koidl Appl. Phys. Lett
．． 47, 1125 (1985)). These methods aim to form a periodic structure throughout the crystalline material. However, the method described above not only requires a large-scale apparatus, but also has the problem that it is difficult to control domain formation.

On the other hand, as a method for forming the above-mentioned periodic domain inversion structure near the surface of a crystal material, for example, T
A method for diffusing i from the crystal surface (Hiromasa Ito, Yinghai Zhang, Fumio Inaba, Proceedings of the 49th Japan Society of Applied Physics Conference 919)
(1988)) and the method of externally diffusing LiO2 (Jo
nas Webjoern, et al, IEEE P
HOTONICS TECHNOL. LETT. 1,
1989, PP316-318) has been proposed. However, when using these methods, the refractive index of the polarization inversion portion changes, and the controllability of the polarization inversion shape deteriorates (F. Laurel et al, Integra
ted Photonics Research, Tu
12, 1989), etc. In addition, when SHG elements are formed using these methods, leakage of input light, leakage of second harmonic light, and a decrease in coupling efficiency between input light and second harmonic light may occur, resulting in so-called optical This may lead to a decrease in conversion efficiency.

[0008]

[Problems to be Solved by the Invention] Furthermore, the present applicant has previously filed Japanese Patent Application No. 1-8271 and Japanese Patent Application No. 1-184.
In the '362 patent application, we proposed a domain control method for nonlinear optical materials of ferroelectric materials. This method involves placing opposing electrodes on both opposing principal surfaces of a single-domain ferroelectric material, or placing them facing each other with an insulator interposed between them, and applying a DC voltage between the two electrodes. In this method, a periodic domain inversion structure is obtained by forming a polarization inversion part.

However, in the periodic domain inversion structure formed by such a method, as shown in FIGS. 7A and 7B, the ratio t/w of the width w to the thickness t of the domain inversion 3A was less than 1. Therefore, if the periodic domain inversion structure 13 is made fine, the absolute value of t becomes small, which is smaller than the thickness of the optical waveguide 2. That is, for example, if the width w of the polarization inversion 3A is made to be about 1.5 μm due to a small pitch, the thickness t will be about 0.5 μm, and the thickness of the optical waveguide 2 will be about 1.0 μm. In this case, the periodic domain inversion structure 13 is sufficiently formed in the optical waveguide portion and the evanescent region.
is not formed, the effect of equalizing the phase propagation speed of the fundamental wave and the second harmonic by periodically arranging the + and - of the nonlinear optical constants as described above cannot be obtained, and the SHG efficiency decreases. This is a factor that hinders the improvement of

In contrast to such methods, a method has been proposed in which a nonlinear optical material is irradiated with an electron beam to obtain a periodic domain inversion structure with a desired pattern (R.W. Keys
,A. Loni, B. J. Luff, P. D. Town
send, others, Electronics Lette
rs 1st February 1990 Vol.
．． 26 No. 3). In this method, as shown in a schematic cross-sectional view in FIG.
1, a NiCr layer 6 with a thickness of 50 nm is formed on the −c plane 61C of
2, and then an Au layer with a thickness of 400 nm, which is then patterned into a desired pattern, and an electron beam is irradiated onto the patterned Au layer 63. In this method, the substrate is heated to approximately 580°C, an electric field of 10 V/cm is applied to the substrate itself in the direction of its c-axis, and
At an energy of keV, electron beam irradiation is performed with a total dose of 1017, ie, approximately 1016 mm-2, over 9 mm2.

However, when using such a method,
There is a risk that the surface of the nonlinear optical material may become contaminated by performing high-temperature heat treatment or applying a voltage at high temperature after a substance such as an insulator or electrode material is coated and patterned on the surface of the nonlinear optical material. . Furthermore, in this case, since polarization inversion is formed by out-diffusion of oxygen molecules from the LN substrate, there is a risk that the refractive index will change due to a change in composition, similar to the Li out-diffusion method, which may cause a change in characteristics.

In order to solve such problems, the present applicant previously applied for Japanese Patent Application No. 3-26358 by accelerating charged particles onto a ferroelectric crystal such as a nonlinear optical material at an accelerating voltage of 15 kV or more. Alternatively, we have proposed a method for manufacturing an optical device having a periodic domain inversion structure in which polarization is inverted in the irradiated area by irradiation at a current density of 1 μA/mm 2 . According to this manufacturing method, as shown in a schematic enlarged perspective view of an example thereof, charged particles, for example, electron beams b, are applied from above one principal surface 1C of a ferroelectric material 1 made of LN single crystal or the like. For example, a parallel strip pattern is irradiated to form a parallel strip pattern of polarization inversion 3A on the main surface 1C of the ferroelectric material 1, thereby forming a periodic domain inversion structure 13.
I am getting .

[0013] In the case of using such a method, since no voltage is applied and high-temperature heating is not required, deterioration and fluctuation of characteristics due to surface contamination of the ferroelectric material 1 can be avoided, and the above-mentioned problems can be avoided. Unlike the Li out-diffusion method, etc., this method does not change the composition of the ferroelectric material 1, so it has the advantage that the polarization inversion 3A can be formed without changing the refractive index.

However, by such a method,
When forming a periodic domain inversion structure with a fine pattern,
Adjacent polarization inversions 3A overlap. For example, width 10
When forming polarization inversions 3A of μm, the interval between each polarization inversion 3A is about 7 to 8 μm, and there is a problem in that the interval between the polarization inversions 3A, and thus the pitch thereof, cannot be made sufficiently small.

The problem to be solved by the present invention is to obtain a polarization inversion control method that reliably forms such a fine pattern of polarization inversion 3A with good controllability.

[0016]

[Means for Solving the Problems] A process diagram of an example of the polarization inversion control method according to the present invention is shown in the schematic enlarged perspective view of FIG. The present invention provides polarization in which a plurality of regions on the ferroelectric material 1 are locally irradiated with charged particles to form a polarization inversion structure 3 in a single domain ferroelectric material 1. In the inversion control method, charged particles are irradiated onto a plurality of regions while maintaining an interval LB that does not affect each other.

Manufacturing process diagrams of another example of the polarization inversion control method according to the present invention are shown in FIGS. 3A to 3C. In the present invention, charged particles are locally irradiated onto a plurality of regions on the single-domain ferroelectric material 1 to form a polarization-inverted structure 3.
In the polarization inversion control method that forms the
As shown in A, the distance LB that does not affect the charged particles
The process of irradiating multiple areas while holding the
As shown in FIG. 3C, the process of removing the charge on the ferroelectric material 1, and the process of removing the charges on the ferroelectric material 1 while maintaining the distance LB between the other regions close to the plurality of irradiation regions so as not to affect each other, as shown in FIG. 3C. and a step of irradiating charged particles.

Manufacturing process diagrams of another example of the polarization inversion control method according to the present invention are shown in FIGS. 4A to 4C. The present invention provides a polarization inversion method in which charged particles are locally irradiated onto a plurality of regions on the ferroelectric material 1 to form a polarization inversion structure 3 in a single domain ferroelectric material 1. In the control method, after the charged particles are once irradiated as shown in FIG. 4A, the charge on the ferroelectric material 1 is removed as shown in FIG. 4B, and then the adjacent adjacent particles are A polarization-inverted structure 3 is obtained by irradiating charged particles onto the region.

[0019]

[Operation] As described above, according to the polarization inversion control method of the present invention, when forming the polarization inversion structure 3 by locally irradiating charged particles onto a plurality of regions on the ferroelectric material 1, Particles are irradiated onto a plurality of regions while maintaining a distance LB that does not affect each other, and by doing so, it was possible to form adjacent polarization inversions 3A separated from each other. This seems to be due to the following reasons.

First, the polarization inversion mode when the ferroelectric material 1 is irradiated with charged particles will be considered. FIGS. 5A and 5B are cross-sectional views showing a schematic polarization mode of the ferroelectric material 1. FIG. In FIG. 5A, 9 indicates each polarization within the ferroelectric material 1, and the direction of these polarizations is indicated by an arrow d. When the ferroelectric material 1 is an LN single crystal, each polarization direction is aligned in the c-axis (Z-axis) direction, and the ferroelectric material 1
This shows the case where the polarization is aligned in the thickness direction toward the inside. Reference numeral 5 denotes a conductor made of Au or the like which is entirely deposited on the back surface 1D.

[0021] For such a ferroelectric material 1, the main surface 1
When a charged particle, for example, an electron beam b, is irradiated from above C, some of the electrons remain in the irradiated area, and a negative electric field E is generated by this accumulated charge on the surface of the ferroelectric material 1. portion enters the ferroelectric material 1 and an electron flow is generated as indicated by arrow i. Therefore, in this portion, the + side of the polarization 9 is reversed so as to face the direction in which the electric field E or the electron current i passes, and as a result, polarization reversal 3A appears as shown in FIG. 5B.

At this time, as shown in the cross-sectional view of the polarization mode of the ferroelectric material in FIG.
is not erased, the electric field E is increased by the second irradiation.
2 is generated, these electric fields E1 and E2 interfere, and as a result, the regions where polarization inversion occurs substantially overlap, resulting in the formation of polarization inversion 3A with a larger shape than the intended pattern. .

Therefore, according to the polarization inversion control method of the present invention,
Since charged particles are irradiated onto multiple regions while maintaining a distance LB that does not affect each other, it is possible to avoid interference between the electric fields of adjacent irradiation regions as described above, and ensure polarization inversion 3A. can be formed independently of each other.

In another polarization inversion control method according to the present invention,
As shown in FIG. 3A, after irradiating charged particles to multiple regions while maintaining a distance LB that does not affect each other, as shown in FIG.
As shown in , after the accumulated charge on the ferroelectric material 1 is removed, that is, the electric field due to the charged particles is removed, other regions adjacent to the plurality of irradiation regions from which this electric field has been removed are exposed to each other. Since charged particles are irradiated while maintaining an unaffected interval LB, it is possible to avoid interference between electric fields due to accumulated charges in adjacent irradiation areas, and each polarization inversion 3A can be formed independently of each other. .

In the case of another polarization inversion control method according to the present invention, the ferroelectric material 1 is once irradiated with charged particles as shown in FIG. 4A, and then the ferroelectric material 1 is irradiated with charged particles as shown in FIG. 4B. In order to remove the accumulated charges above and sequentially irradiate charged particles onto adjacent regions as shown in FIG. 4C, even if the interval LS between the irradiation regions is relatively small,
It is possible to avoid interference between the electric fields of adjacent irradiation areas, and each polarization inversion structure 3A can be formed independently of each other, and the polarization inversion structures 3 are arranged at fine intervals.
can be obtained.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each example of the polarization inversion control method of the present invention will be described in detail below with reference to FIGS. 1 to 4. In each example, an LN single crystal of a nonlinear optical material is used as the ferroelectric material 1, and has a c-axis (Z-axis) in the thickness direction, and the main surface 1C side is the -c plane. This ferroelectric material 1 is heated to, for example, about 1200° C., just below its Curie temperature, and an external DC voltage is applied to the entire surface in the thickness direction, thereby aligning the entire surface in the thickness direction of the c-axis. This is an example in which charged particles are irradiated from above the -c plane 1C. Further, on the back surface 1D of the ferroelectric material 1, a conductor 5 made of Au or the like is entirely deposited by vapor deposition, sputtering, or the like.

Example 1 As shown in FIG. 1, charged particles, such as electron beams b, are scanned and irradiated onto the main surface 1C of the ferroelectric material 1 so as to draw loci in, for example, a linear pattern in the form of parallel bands. This electron beam b
is irradiated from above the principal surface 1C at an accelerating voltage of, for example, 15 kV, and hence an energy of 15 keV, and a current density of, for example, 1 μA/mm 2 . At this time, the width w of the irradiation pattern was 10 μm, and the interval between the irradiation areas was about 8 μm. By making the interval LB between the irradiation regions relatively large in this way, as shown in the schematic enlarged cross-sectional view of FIG. By the current flowing toward the body 5, polarization inversions 3A having polarization in the direction indicated by the arrow h, which is opposite to the polarization direction indicated by the arrow d, can be formed independently of each other. Therefore, it is possible to obtain a polarization inversion structure 3 in which these polarization inversions 3A are periodically arranged with a pitch (L+w).

Example 2 In this example, a polarization inversion structure 3 is obtained by another polarization inversion control method of the present invention. First, as shown in FIG. 3A, charged particles are For example, a plurality of regions are irradiated with the electron beam b. The irradiation conditions at this time are selected in the same manner as in Example 1 described above. The first polarization inversion 3B is formed by selecting the interval LB between these regions so that they do not influence each other, that is, so that the electric fields in each irradiation region do not interfere.

Next, the ferroelectric material 1 is heated, for example, in a vacuum.
Natural discharge due to being left for a long time or being left in the atmosphere for a short time,
Alternatively, as shown by arrow a in FIG. 3B, the accumulated charge on the ferroelectric material 1 is removed by, for example, irradiating the entire surface with ion wind, and the electric field caused by this is removed.

Then, as shown in FIG. 3C, the electron beam b is applied to a plurality of other regions close to the plurality of irradiation regions at intervals LB that do not affect each other, that is, the electric fields in each irradiation region do not interfere with each other, in accordance with the above-mentioned pattern. Same and spacing LB
Then, in the step shown in FIG. 3A, that is, between each polarization inversion 3B of the strip pattern formed by the first irradiation, irradiation is performed in parallel to these to form a second polarization inversion 3C. At this time, the first and second polarization inversions 3B and 3C are arranged at equal intervals. Further, the irradiation conditions are selected in the same manner as in Example 1 described above. In this way, it is possible to obtain a periodic domain-inverted structure 3 with a fine pitch, in which the interval LS between adjacent domain-inverted structures 3A is approximately 8 μm or less.

Example 3 This example is a case using another polarization inversion control method of the present invention.
First, as shown in FIG. 4A, the ferroelectric material 1 is once scanned and irradiated with charged particles, such as an electron beam b, in a parallel strip pattern, for example. The irradiation conditions at this time were as described in Example 1 above.
Select in the same way.

Next, the ferroelectric material 1 is heated, for example, in a vacuum.
The accumulated charge on the ferroelectric material 1 is removed by leaving it for about an hour, or by natural discharge by leaving it in the atmosphere for a short time, or by forced removal, such as by irradiating it with ion wind a, as shown in FIG. 4B. .

Thereafter, as shown in FIG. 4C, charged particles are sequentially applied to adjacent areas, for example, by forming strip-shaped straight lines parallel to the previously irradiated pattern at a desired small interval to finally obtain the interval LS between the irradiated areas. Scan and irradiate as a pattern. Even if the interval between the irradiation areas is small enough in this way,
Since the electric charge on the main surface 1C of the previously irradiated area is removed, it is possible to avoid interference between the electric field of the subsequently irradiated area and the electric field on the adjacent previously irradiated area. The polarization inversions 3A can be formed independently of each other. This makes it possible to obtain polarization inversion structures 3 arranged periodically with fine intervals LS.

In the above-described embodiments 2 and 3, even if the width w of the polarization inversions 3A is about 10 μm, the interval LS between each polarization inversion can be formed to have any desired small interval of 8 μm or less. . Further, for example, the polarization inversion structure 3 can be formed with a finer width and pitch by setting the width of the polarization inversion to 4 μm and the interval LS to about 4 μm.

In each of the above-mentioned examples, it is desirable to ground the conductor 5 in order to form each polarization inversion with better shape controllability.

In each of the above examples, the polarization direction is in the thickness direction of the ferroelectric material 1 made of LN single crystal,
Although we have explained the case where charged particles are irradiated in a direction along this polarization direction, in other cases, for example, for a material whose polarization is aligned in the in-plane direction along the main surface of a ferroelectric material, it is possible to irradiate a charged particle in a direction perpendicular to this polarization direction. The present invention can also be applied to the case where charged particles are irradiated to the surface.

Furthermore, in each of the above examples, a case has been described in which LN single crystal is used as the material of the ferroelectric material 1, but various other materials such as the above-mentioned KTP can be used.

Further, in each of the above examples, the electron beam b was used as the charged particle, but in other cases, for example, L was used as the ferroelectric material 1.
When using an N single crystal, the + direction of polarization inversion, that is, +
Various charged particles can be used, such as irradiating Li+ from the c-plane side.

[0039]

As described above, according to the polarization inversion control method of the present invention, when forming a plurality of polarization inversions 3A by irradiating the ferroelectric material 1 with charged particles, each adjacent polarization inversion is 3A can be formed reliably and at fine intervals, thereby making it possible to obtain a periodic polarization inversion structure 3 with a fine pitch.

Therefore, for example, a nonlinear optical material is used as the ferroelectric material 1, a fine pattern of polarization inversions 3A is formed at a fine pitch to form a periodic polarization inversion structure 3, and the well-known proton exchange method is used. By forming an optical waveguide using the above method, an SHG element with high light conversion efficiency can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a process diagram of an example of the polarization inversion control method of the present invention.

FIG. 2 is a schematic cross-sectional view showing a polarization inversion mode of a ferroelectric material.

FIG. 3 is a manufacturing process diagram of another example of the polarization inversion control method of the present invention.

FIG. 4 is a manufacturing process diagram of another example of the polarization inversion control method of the present invention.

FIG. 5 is a schematic cross-sectional view showing a polarization reversal mode of a ferroelectric material by charged particle irradiation.

FIG. 6 is a schematic cross-sectional view showing a mode of polarization reversal of a ferroelectric material by charged particle irradiation.

FIG. 7 is a schematic enlarged cross-sectional view showing a conventional periodic domain inversion structure.

FIG. 8 is a schematic enlarged cross-sectional view showing a method for manufacturing an optical device having a conventional periodic domain inversion structure.

FIG. 9 is a schematic enlarged perspective view showing a method for manufacturing an optical device having a periodic domain inversion structure.

[Explanation of symbols]

1 Ferroelectric material 1C Main surface 1D back side 3 Polarization inversion structure 3A Polarization inversion 3B First polarization inversion 3C Second polarization inversion 5 Conductor 9 Polarization

Claims (3)

[Claims] 1. Polarization inversion control in which a plurality of regions on the ferroelectric material are locally irradiated with charged particles to form a polarization inversion structure in a single domain ferroelectric material. A polarization inversion control method, characterized in that the ferroelectric material is irradiated with the charged particles in a plurality of regions while maintaining intervals that do not affect each other. 2. Polarization inversion control in which charged particles are locally irradiated onto a plurality of regions on a single domain ferroelectric material to form a polarization inversion structure. The method includes the steps of irradiating charged particles onto a plurality of regions of the ferroelectric material while maintaining intervals that do not affect each other, removing charges on the ferroelectric material, and the plurality of irradiation regions. A polarization inversion control method comprising the step of irradiating charged particles onto a plurality of other regions adjacent to the region while maintaining intervals that do not affect each other. 3. Polarization inversion control in which a plurality of regions on the ferroelectric material are locally irradiated with charged particles to form a polarization inversion structure in a single domain ferroelectric material. The method is characterized in that after once charged particles are irradiated, charges on the ferroelectric material are removed, and then charged particles are sequentially irradiated onto adjacent regions to obtain a polarization-inverted structure. A polarization reversal control method.