JP2005070194A - Method for manufacturing periodical polarization reversal structure and optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form an excellent polarization reversal part even when a period of polarization reversal is lengthened. <P>SOLUTION: A periodical polarization reversal structure, which comprises a plurality of polarization reversal parts and a plurality of polarization non-reversal parts periodically aligned in a specified direction A, is manufactured by disposing comb shaped electrodes 14 on one principal surface of a ferroelectric single crystal substrate made to be the mono-domain, disposing a uniform electrode on the other principal surface of the substrate and applying a voltage between the comb shaped electrodes 14 and the uniform electrode. The comb shaped electrodes 14 include a plurality of electrode pieces 16a, 16b, 16c respectively aligned in the specified direction A corresponding to the respective polarization reversal parts 16. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a periodically poled structure and an optical device.

  By periodically forming a polarization inversion structure that forcibly inverts the polarization of a ferroelectric, an optical frequency modulator using surface acoustic waves, an optical wavelength conversion element using polarization inversion of nonlinear polarization, etc. Can be realized. In particular, if it is possible to periodically invert the nonlinear polarization of the nonlinear optical material, a highly efficient wavelength conversion element can be produced, and if this is used to convert light such as a solid-state laser, A compact and lightweight short wavelength light source that can be applied to fields such as printing, optical information processing, and optical applied measurement control can be configured.

A so-called voltage application method is known as a method of forming a periodic domain-inverted structure in a ferroelectric nonlinear optical material. In this method, a comb-shaped electrode is formed on one main surface of a ferroelectric single crystal substrate, a uniform electrode is formed on the other main surface, and a pulse voltage is applied between them. Such a method is described in Patent Document 1.
JP-A-8-220578

  In order to generate the second harmonic from a non-linear optical material such as a lithium niobate single crystal, it is necessary to form a periodic polarization inversion in the single crystal. In this case, for example, a comb-shaped electrode 20 as shown in FIG. 4 is formed on the upper surface of the substrate. The comb-shaped electrode 20 includes a large number of elongated electrode pieces 18 arranged in a periodic manner, and a power supply pad 15 that electrically connects the electrode pieces 18. A gap 17 is formed between the adjacent electrode pieces 18. P is the polarization inversion period, and W is the width of the electrode piece. When a voltage is supplied to the comb-shaped electrode 20 so as to be equal to or higher than the coercive electric field, the domain-inverted portion extends mainly from the tip portion of each electrode piece 18 to generate a periodic domain-inverted structure.

  The inventor attempted to form a periodically poled structure having a long period of, for example, a period P of 10 μm or more. Such a periodically poled structure is suitable for generating laser light for optical communication, for example. However, in this case, it was found that a good periodic polarization inversion structure was not generated. That is, in each electrode piece 18, each polarization inversion part is generated separately from each of the two corner points 18 a of the electrode piece, and each polarization inversion part is not connected. Therefore, no polarization inversion part corresponding to the width W is generated. It has been found. For this reason, for example, as shown in FIG. 5, the polarization inversion portions are generated only in the vicinity of each corner point 18a, and the non-polarization inversion portions remain between the adjacent polarization inversion portions. In such a periodically poled structure, the light conversion efficiency is low.

  It is an object of the present invention to form a favorable domain inversion part even when the domain inversion period is increased when a domain inversion part is manufactured on a single domain ferroelectric single crystal substrate by a voltage application method. Is to be able to do it.

  The present invention provides a comb-shaped electrode on one main surface of a single-domain ferroelectric single crystal substrate, a uniform electrode on the other main surface side of the ferroelectric single crystal substrate, A method of manufacturing a periodic polarization reversal structure in which a plurality of polarization reversal portions and non-polarization reversal portions are periodically arranged in a predetermined direction by applying a voltage between the electrodes and a uniform electrode The electrode includes a plurality of electrode pieces arranged in a predetermined direction corresponding to each polarization inversion portion.

  The present invention also relates to an optical device comprising a periodically poled structure manufactured by the above method.

The present invention will be described below with reference to the drawings as appropriate.
For example, as shown in FIG. 1, the inventor forms a plurality of electrode pieces 16a, 16b in a predetermined direction A corresponding to each polarization inversion portion when forming the comb-shaped electrode 14 on the substrate. It was conceived to arrange 16c. That is, in the comb-shaped electrode 14 of FIG. 1, gaps 17 are formed between adjacent domain-inverted regions 16, and the domain-inverted regions 16 and the gaps 17 are arranged in the direction of arrow A. Therefore, the generated periodically poled structure extends in the direction of arrow A. In such a structure, a plurality of electrode pieces 16 a, 16 b and 16 c are further arranged in the direction of arrow A in each polarization inversion region 16. t is the interval between adjacent electrode pieces.

  According to such a structure, the polarization inversion part extends from each corner point of each electrode piece 16a, 16b, 16c (see FIG. 2). Then, by adjusting the voltage application time and magnitude, it was found that the polarization inversion portions are continuous with each other and one wide polarization inversion portion is generated (see FIG. 3), and the present invention has been achieved.

  In the present invention, a plurality of electrode pieces 16a, 16b, 16c arranged in the predetermined direction A are provided corresponding to each polarization inversion portion. Here, the lower limit of the formation period P of the domain-inverted regions 16 is not particularly limited, but it is generally difficult to form a wider domain-inverted portion as P is larger, and thus the operational effects of the present invention are particularly large. From this viewpoint, the period P is preferably 5 μm or more, and more preferably 10 μm or more. The upper limit of the period P is not particularly limited, but may be 40 μm or less from a practical viewpoint.

  The number of electrode pieces in each polarization inversion region 16 is not particularly limited as long as it is two or more. From the viewpoint of satisfactorily forming a wide domain-inverted portion, the number of electrode pieces in each domain-inverted region is more preferably 3 or more. There is no particular upper limit on the number of electrode pieces in each domain-inverted region.

  The interval t between adjacent electrode pieces is not particularly limited. By setting t to 3.0 μm or less, the polarization inversion portions extending from the electrode pieces are easily connected to each other. From this viewpoint, it is more preferable that t is 2.5 μm or less. In addition, by setting t to 1.0 μm or more, short-circuiting between adjacent electrode pieces can be prevented, and the polarization inversion portion can be easily controlled to extend from each corner point of each electrode piece.

  The line width W of each electrode piece is preferably 1.0 μm or less, and preferably 0.5 μm or less so that the polarization inversion portions extending from the tip corners of each electrode piece can be easily connected to each other. Is more preferable.

The type of the ferroelectric single crystal constituting the ferroelectric single crystal substrate is not limited. However, single crystals of lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), lithium niobate-lithium tantalate solid solution, and K ₃ Li ₂ Nb ₅ O ₁₅ are particularly preferable.
The ferroelectric single crystal is selected from the group consisting of magnesium (Mg), zinc (Zn), scandium (Sc), and indium (In) in order to further improve the light damage resistance of the three-dimensional optical waveguide. More than one metal element can be contained, and magnesium is particularly preferred. From the viewpoint that the domain inversion characteristics (conditions) are clear, it is particularly preferable to add magnesium to a lithium niobate single crystal, a lithium niobate-lithium tantalate solid solution single crystal, or a lithium tantalate single crystal. Further, a rare earth element can be contained as a doping component in the ferroelectric single crystal. This rare earth element acts as an additive element for laser oscillation. As this rare earth element, Nd, Er, Tm, Ho, Dy, and Pr are particularly preferable.

  However, when these light damage resistance improving elements and rare earth elements are added, the conductivity of the ferroelectric single crystal is increased, and the periodic domain-inverted portion is hardly formed uniformly over the entire comb-shaped electrode. The following embodiments are particularly suitable in such a case.

  That is, in order to form a domain-inverted structure uniformly over the entire area where the comb-shaped electrode is patterned, for example, a separate base substrate 13 as shown in FIG. 6 is laminated. For example, a comb-shaped electrode 14 is formed on one main surface 1 a of the substrate 2 made of MgO-doped lithium niobate single crystal, and a uniform electrode 4 is formed on the other main surface 2 b of the substrate 2. A separate base substrate 13 is laminated under the substrate 2. A first conductive film 6 is formed on one main surface 5 a of the main body 5 of the base substrate 13, and a second conductive film 7 is formed on the other main surface 5 b of the main body 5. In this example, the first conductive film 6 and the uniform electrode 4 are brought into contact with each other to be electrically connected, but a conductive material (preferably between the first conductive film 6 and the uniform electrode 4 is preferable. Can be electrically connected to each other by interposing a conductive film.

  For example, as shown in FIGS. 7 and 8, the insulating oil 8 is accommodated in the container 9, and the laminate 1 is immersed in the insulating oil 8. At this time, the electric wire 11 is connected to the comb electrode 14, and the electric wire 10 is connected to the second conductive film 7. The electric wires 10 and 11 are connected to a high voltage source 12. When a pulse voltage having a predetermined voltage and a pulse width is applied in this state, a periodic polarization inversion portion is formed between the comb electrode 14 and the uniform electrode 4.

  Here, by laminating the base substrate 13 and applying a voltage through the conductive films 6 and 7 on the base substrate 13, the periodic polarization inversion portions are easily generated uniformly over the entire comb-shaped electrode 3. .

The material of the comb electrode and the uniform electrode is not limited, but Al, Au, Ag, Cr, Cu, Ni, Ni—Cr, Pd, and Ta are preferable.
The material of the first conductive film and the second conductive film is not limited, but Al, Au, Ag, Cr, Cu, Ni, Ni—Cr, Pd, and Ta are preferable.

  The material of the substrate body 5 of the base substrate is required to have a high insulation property, a uniform volume resistivity within the material, and a predetermined structural strength. This material includes silicon, sapphire, crystal, glass, lithium niobate, lithium tantalate, lithium niobate-lithium tantalate solid solution MgO-doped lithium niobate, MgO-doped lithium tantalate, ZnO-doped lithium niobate, ZnO-doped lithium tantalate. Can be illustrated.

  The substrate 2 is particularly preferably a so-called Z-cut substrate, off-cut X plate, or off-cut Y plate. When using an off-cut X plate and an off-cut Y plate, the off-cut angle is not particularly limited. Particularly preferably, the off-cut angle is 1 ° or more, or 20 ° or less.

  The magnitude of the applied voltage is preferably 3 kV to 8 kV, and the pulse frequency is preferably 1 Hz to 1000 Hz.

  The periodic domain-inverted part formed by the present invention can be applied to any optical device having such a domain-inverted part. Such an optical device includes, for example, a harmonic generation element such as a second harmonic generation element. When used as a second harmonic generation element, the wavelength of the harmonic is preferably 330-1600 nm.

Comparative Example 1

A laminate 1 as shown in FIG. 6 was prepared, and a periodic domain-inverted structure was formed by a voltage application method using an apparatus as shown in FIGS. However, the comb electrode 20 having a pattern as shown in FIG. 4 was formed.
Specifically, a 0.5 mm thick z-cut substrate 2 made of MgO-doped lithium niobate single crystal and a 5 mm off-y cut 0.5 mm thick substrate 5 were prepared, and each was z-cut. The comb electrode 20 was patterned on the + z surface 2a of the substrate 2, and the uniform electrode 4 was formed on the -z surface 2b. For the 5 degree off-y cut substrate 5, uniform electrodes 6 and 7 were formed on the upper and lower surfaces 5a and 5b. The polarization inversion period P was 18 μm. The line width W of each electrode piece was 7 μm. Ta was used as the material of each electrode. All electrode thicknesses were 1000 angstroms. Further, a SiO ₂ film having a thickness of 2000 Å was formed on the surface of the comb-shaped electrode 20 of the z-cut substrate 2. As shown in FIG. 6, the z-cut substrate 2 was laminated on the upper side, and the 5-degree off-cut substrate 5 was laminated on the lower side, whereby the laminate 1 was obtained. The laminate 1 was immersed in the insulating oil 8 as shown in FIG. 7, and a pulse voltage of 6 kV and a pulse width of 10 Hz was applied at a pulse interval of about 1 second.

In order to confirm whether the polarization inversion was formed, the + z plane of the wafer surface was wet etched with a hydrofluoric acid mixed solution (hydrofluoric acid: nitric acid = 1: 2). FIG. 5 is a photomicrograph of this. As shown in FIG. 5, it can be seen that the portion that looks whitish in the photograph is the reversal part, and the reversal structure is obtained only in the vicinity of the corner points of the comb-shaped electrode. Although the polarization inversion portions were generated in the vicinity of the corner points of the electrode pieces, the polarization inversion portions extending from the corner points were not connected to each other. Various attempts were made to increase the voltage or change the pulse width, but it was difficult to form a polarization inversion portion over the entire width of the electrode piece, although the size of the inversion portion slightly increased or decreased. It was.

In the same manner as in Comparative Example 1, a periodic domain-inverted structure was formed by a voltage application method. However, a comb electrode 14 having a pattern as shown in FIG. 1 was formed.
Specifically, a 0.5 mm thick z-cut substrate 2 made of MgO-doped lithium niobate single crystal and a 5 mm off-y cut 0.5 mm thick substrate 5 were prepared, and each was z-cut. The comb electrode 14 was patterned on the + z surface 2a of the substrate 2, and the uniform electrode 4 was formed on the -z surface 2b. For the 5 degree off-y cut substrate 5, uniform electrodes 6 and 7 were formed on the upper and lower surfaces 5a and 5b. The polarization inversion period P was 18 μm. The width of the gap between the polarization inversion portions 16 was 9 μm. The line width W of each electrode piece 6a, 6b, 6c was 0.3 μm, and the gap between adjacent electrode pieces was about 2.5 μm. Ta was used as the electrode material. All electrode thicknesses were 1000 angstroms. Further, a SiO ₂ film having a thickness of 2000 Å was formed on the surface of the comb-shaped electrode 14 of the z-cut substrate 2. As shown in FIG. 6, the z-cut substrate 2 was laminated on the upper side, and the 5-degree off-cut substrate 5 was laminated on the lower side, whereby the laminate 1 was obtained. The laminate 1 was immersed in the insulating oil 8 as shown in FIG. 7, and a pulse voltage of 6 kV and a pulse width of 10 Hz was applied at a pulse interval of about 1 second.

  FIG. 2 shows the result of observing the + z plane of the wafer after etching with hydrofluoric acid. It can be seen that a periodic polarization inversion is formed from the base of the electrode to the tip. However, there is a region that is not reversed in part. Therefore, as a voltage application condition, the number of applied pulses was increased and applied 2000 times. The results are shown in FIG. Although the magnification is different from that in FIG. 2, it can be confirmed that periodic polarization inversion is formed over the entire area of the comb-shaped electrode.

It is a top view which shows the comb-shaped electrode 14 which concerns on embodiment of this invention. 3 is a micrograph showing a pattern example of a domain-inverted part formed in Example 1. FIG. 3 is a micrograph showing a pattern example of a domain-inverted part formed in Example 1. FIG. It is a top view which shows the comb-shaped electrode 20 which concerns on a comparative example. It is a microscope picture which shows the example of a pattern of the polarization inversion part formed in the comparative example. 2 is a front view showing a laminate 1 of substrates 2 and 5. FIG. It is a schematic diagram which shows the apparatus for forming a polarization inversion part in the laminated body 1 by the voltage application method. FIG. 8 is a top view of the apparatus of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Laminated body 2 Ferroelectric single crystal substrate 2a One main surface of the substrate 2b The other main surface of the substrate 2 4 Uniform electrode 5 Substrate body of the base substrate 5a One main surface of the substrate body 5b 5b of the substrate body 5 The other main surface 6 First conductive film 7 Second conductive film 8 Insulating oil 9 Container 12 High voltage source 13 Base substrate 14 Comb electrode 15 according to the embodiment of the present invention 15 Feed pad 16 Polarization inversion region 16a of the comb electrode, 16b, 16c Electrode piece 17 Gap between polarization inversion portions 18 Electrode piece of comparative example 20 Comb electrode of comparative example A A predetermined direction P Polarization inversion period W Line width of electrode piece t Distance between electrode pieces

Claims (6)

A comb-shaped electrode is provided on one main surface of a single-domain ferroelectric single crystal substrate, and a uniform electrode is provided on the other main surface side of the ferroelectric single crystal substrate. A method of manufacturing a periodic polarization reversal structure in which a plurality of polarization reversal portions and non-polarization reversal portions are periodically arranged in a predetermined direction by applying a voltage between the electrodes,
The method of manufacturing a periodic polarization reversal structure, wherein the comb-shaped electrode includes a plurality of electrode pieces arranged in the predetermined direction corresponding to each polarization reversal portion.   The method according to claim 1, wherein the period of the periodically poled structure is 10 μm or more.   The ferroelectric single crystal substrate is made of a single crystal selected from the group consisting of a lithium niobate single crystal, a lithium tantalate single crystal, and a lithium niobate-lithium tantalate solid solution single crystal. Item 2. The method according to Item 1.   The method according to any one of claims 1 to 3, wherein the single crystal contains at least one of magnesium oxide and zinc oxide.   The method according to claim 1, wherein the ferroelectric single crystal substrate is a Z-cut substrate.   An optical device comprising a periodically poled structure manufactured by the method according to any one of claims 1 to 5.

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