CN1717620A - Polarization reversal structure constructing method and optical device having polarization reversal structure - Google Patents

Polarization reversal structure constructing method and optical device having polarization reversal structure Download PDF

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CN1717620A
CN1717620A CN 200380104116 CN200380104116A CN1717620A CN 1717620 A CN1717620 A CN 1717620A CN 200380104116 CN200380104116 CN 200380104116 CN 200380104116 A CN200380104116 A CN 200380104116A CN 1717620 A CN1717620 A CN 1717620A
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polarization
electrode
polarization reversal
strong dielectric
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CN100390651C (en
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水内公典
森川显洋
杉田知也
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Panasonic Holdings Corp
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Matsushita Electric Industrial Co Ltd
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Abstract

A method for forming a domain-inverted structure includes the following: using a ferroelectric substrate (1) having a principal surface substantially perpendicular to the Z axis of crystals; providing a first electrode (3) on the principal surface of the ferroelectric substrate, the first electrode having a pattern of a plurality of electrode fingers (5) that are arranged periodically; providing a counter electrode (6) on the other side of the ferroelectric substrate so as to be opposite from the first electrode; and applying an electric field to the ferroelectric substrate with the first electrode and the counter electrode, thereby forming domain-inverted regions corresponding to the pattern of the first electrode in the ferroelectric substrate. Each of the electrode fingers of the first electrode is located so that a direction from a base to a tip (5a) of the electrode finger is aligned with the Y-axis direction of the crystals of the ferroelectric substrate. This method can provide a short-period uniform domain-inverted structure.

Description

The formation method of polarization reversal structure and optical element with polarization reversal structure
Technical field
The present invention relates to a kind of utilization and apply the method that electric field forms polarization reversal structure, and have polarization reversal structure and can be applied to the optical element that optical wavelength changes element, polarization element, photoswitch, phase-modulator etc.
Background technology
Utilization makes the polarization reversal phenomenon of the mandatory counter-rotating of polarization of strong dielectric, just can form the polarization reversal structure that periodically is arranged with polarization-reversed region in the inside of strong dielectric.Like this polarization reversal structure of Xing Chenging be used to utilize the light frequency modulator of surface acoustic wave and utilize the polarization reversal of nonlinear polarization the optical wavelength conversion element, utilize the polarizer etc. of the inversion structures of prism shape and lens shape.Particularly, if the nonlinear polarization of nonlinear optics material is periodically reversed, then can make the very high optical wavelength conversion element of conversion efficiency.If use such element that the light of semiconductor laser etc. is carried out wavelength conversion, the small-sized short wavelength light source of so just can obtain being applied to printing, instrumentation control field being used in optical information processing, light etc.
Strong dielectric has in crystal because the electric charge skew that spontaneous polarization causes.The direction of spontaneous polarization can change by the electric field that applies with spontaneous polarization resists mutually.The direction of spontaneous polarization is different because the kind of crystallization (material) is different.LiTaO 3, LiNbO 3, or as their LiTa of mixed crystallization (1-X)Nb XO 3The crystallization of (0≤x≤1) substrate owing to only have spontaneous polarization at the C direction of principal axis, therefore for this crystallization, polarization only along the C axle+both direction of direction or its reciprocal-direction exists.By applying electric field, the polarization Rotate 180 degree of this crystallization is up to becoming in the other direction.This kind phenomenon is called polarization reversal.Be called the counter-rotating electric field in order to produce the necessary electric field of polarization reversal, it is for LiNbO 3, LiTaO 3Deng crystallization, when room temperature, be about 20kV/mm, for MgO: LiNbO 3The value that then is about 5kV/mm.
In strong dielectric, the phenomenon that becomes the crystallization with single polarised direction is called " poling of polarization ".In order to make the polarization poling, after forming, crystallization carries out in high temperature, applying the method for electric field usually.
As the existing method that forms periodic polarization-reversed region, for example in the flat 4-19719 communique of Japanese Patent Application Laid-Open, put down in writing at LiNbO 3Form comb electrode on the substrate of (lithium columbate crystal), this comb electrode is applied the method for the electric field of pulse type.Thus, at LiNbO 3Substrate+formed comb electrode on the C face, on-C face, form plane electrode.Make+C face ground connection, give-plane electrode of C face applies the pulse voltage that pulse width is 100 μ s, thus because the impulse electric field that is applied on the substrate causes polarization reversal.Be used to make the necessary electric field of polarization reversal to be about more than the 20kV/mm.If apply the electric field of this value, then under the thick situation of substrate, because the applying of electric field, the crystallization of substrate may be destroyed.But, be the degree of 200 μ m by the thickness that makes substrate, can avoid the crystalline fracture that causes that applies owing to electric field, can form polarization-reversed region in room temperature.Thus, obtain connecting the dark polarization-reversed region of substrate.
In order to make optical wavelength conversion element high efficiency, need short-period polarization reversal structure of the scope of 3～4 μ m.If form polarization-reversed region, so after the polarization reversal under the electrode, broaden in the zone of the polarization reversal in a lateral direction of substrate by applying electric field.For this reason, the short periodization of polarization reversal structure becomes difficult.In order to overcome this problem, in existing method, be about 100 μ s by making pulse width, apply the pulse voltage of short time to electrode, can form short-period polarization reversal structure.
As at the LiNbO of Mg that mixed 3Substrate (below be expressed as MgLN) is gone up the method that forms short-period polarization reversal structure, for example discloses on the MgLN of Z plate the periodically method of formation polarization reversal structure in the flat 6-242478 communique of Japanese Patent Application Laid-Open.So, by MgLN+form comb electrode on the Z face, from back side illuminaton corona (corona), the cycle of obtaining is 4 μ m and to connect thickness be the polarization reversal structure of the substrate of 0.5mm.
Also have, be documented in the flat 9-218431 communique of Japanese Patent Application Laid-Open in the last method that forms polarization reversal structure of (the オ Off カ Star ト) MgLN that downcuts.MgLN substrate by the cutting-out of tilting a little from substrate surface in polarised direction forms electrode, and this electrode application voltage is formed the polarization reversal structure of needle-like.Polarization-reversed region increases in the polarised direction of crystallization, and the formation cycle is the polarization reversal structure of 5 μ m.
Yet, in the doping of Z plate the LiTa of Mg (1-X)Nb XO 3It is difficult forming fine polarization reversal structure on (0≤x≤1) substrate.Forming polarization reversal structure by existing method with respect to the substrate that downcuts can be undertaken by applying electric field.Yet as the method that forms uniform and fine inversion structures with respect to the Z base board, known only have complicated approach such as corona polarizing.Thereby corona polarizing is to pile up charged particle to produce electric field on substrate, and makes the method for polarization reversal thus.But because the electric field that is produced by charged particle is big or small limited, so the thickness that can form the substrate of polarization reversal structure is limited in the degree of 0.5mm, and the polarization reversal structure of wanting to form above the substrate of the thickness of 1mm all is difficult.On the other hand, be effectively though the method by electrode application voltage forms on the substrate that downcuts aspect the polarization reversal structure, on the Z plate, form wide and uniform polarization reversal structure is difficult by this method.
Also have, in the flat 2001-66652 communique of Japanese Patent Application Laid-Open, disclose by on the MgLN of Z plate, forming comb electrode, and this electrode application voltage is formed periodic polarization reversal structure.The speciality of this method is to be formed uniformly periodic polarization reversal structure.Yet the polarization reversal that there is formation is limited in the vast scope interior degree of depth ground under the part of eletrode tip, the electrode and is formed uniformly the problem of polarization reversal structure difficulty.
Summary of the invention
The purpose of this invention is to provide a kind of in the strong dielectric substrate degree of depth ground and be formed uniformly the short period and the method for the polarization reversal structure that width is big.
In the formation method of polarization reversal structure of the present invention, use has the strong dielectric substrate with the first type surface of the Z axle approximate vertical of crystallization, first electrode is set on the first type surface of above-mentioned strong dielectric substrate, this first electrode has and is formed with periodically the pattern that a plurality of electrodes of arranging refer to, on another surface of above-mentioned strong dielectric substrate, be provided with and the relative comparative electrode of above-mentioned first electrode, apply electric field by above-mentioned first electrode and above-mentioned comparative electrode to above-mentioned strong dielectric substrate, and on above-mentioned strong dielectric substrate, form the corresponding polarization-reversed region of pattern with above-mentioned first electrode.Method of the present invention is characterised in that: each electrode that above-mentioned first electrode is set refers to, makes the base portion that refers to from the electrode of above-mentioned first electrode Y direction towards the direction at tip along the crystallization of above-mentioned strong dielectric substrate.
Optical element of the present invention comprises: have the crystallization of being approximately perpendicular to the Z axle the plane the strong dielectric substrate and periodically be formed on a plurality of polarization-reversed regions on the above-mentioned strong dielectric substrate, above-mentioned polarization-reversed region has the rotational symmetry flat shape respectively, and is arranged to its axis of symmetry and is parallel to each other.Optical element of the present invention is characterised in that: form above-mentioned polarization-reversed region and make the direction of above-mentioned axis of symmetry along the Y-axis of the crystallization of above-mentioned strong dielectric substrate, above-mentioned polarization-reversed region has from+Z towards-shape that the Z face extends, and with respect to the whole area of above-mentioned polarization-reversed region, the ratio of area of above-mentioned polarization-reversed region that penetrates into the back side from the surface of above-mentioned strong dielectric substrate is below 50%, and perhaps the mean depth of above-mentioned polarization-reversed region is in 40～95% scope of the thickness of above-mentioned strong dielectric substrate.
Description of drawings
Figure 1A is the planimetric map that is illustrated in the electrode structure that uses in the formation method of the polarization reversal structure in the embodiments of the present invention 1, and Figure 1B is a cut-open view.
Fig. 2 A is the planimetric map of expression by the situation of the polarization-reversed region of the formation method formation of this polarization reversal structure, and Fig. 2 B is an outboard profile.
Fig. 3 A is the oblique view that is used to illustrate the advantage of the electrode with small tip, and Fig. 3 B is a cut-open view, and Fig. 3 C is the curve of situation of the characteristic variations of the expression expansion strong dielectric substrate of following polarization-reversed region.
Fig. 4 A and 4B are planimetric map and the cut-open views of representing to be used to enlarge the method for polarization-reversed region respectively.
Fig. 5 A and 5B are planimetric map and the cut-open view of representing to be used to enlarge the method for polarization-reversed region respectively equally.
The characteristic diagram of the length L r of Fig. 6 polarization-reversed region that to be expression make by this polarization reversal structure formation method and the relation of substrate crystal orientation.
Fig. 7 A is a planimetric map of representing other electrode structure, and Fig. 7 B is a cut-open view.
Fig. 8 A is the planimetric map of the formation method of expression polarization reversal structure of the present invention, and Fig. 8 B is a cut-open view.
Fig. 9 A is the planimetric map of the formation method of expression polarization reversal structure of the present invention, and Fig. 9 B is a cut-open view.
Figure 10 A is the figure of temperature curve of annealing in process of the characteirstic element of the expression stability that is used to illustrate the expression polarization-reversed region, and Figure 10 B is the figure of relation of the attenuation rate of expression programming rate and polarization-reversed region.
Figure 11 A is the planimetric map of the formation method of polarization reversal structure in the expression embodiment 3, and Figure 11 B is a cut-open view.
Figure 12 is the cut-open view of the state that polarization-reversed region forms in first electrode and second electrode in this embodiment of expression.
Figure 13 is the figure of the relation of the length L r of distance L and polarization-reversed region between first electrode and second electrode in this embodiment of expression.
Figure 14 is the figure of the relation of the length L r of the temperature of insulating solution and polarization-reversed region in this embodiment of expression.
Figure 15 is the figure of substrate thickness and the relation in polarization reversal cycle in this embodiment of expression.
Figure 16 A is the planimetric map of the formation method of polarization reversal structure in the expression embodiment 4, and Figure 16 B is a cut-open view.
Figure 17 is the figure of the relation of the expression length L r that applies the pulse width of voltage and polarization-reversed region in the embodiment 3,4.
Figure 18 is the oblique view of optical element in the expression embodiment 6.
Figure 19 A is the planimetric map of expression as the polarizer of an example of this optical element, and Figure 19 B is a cut-open view.
Embodiment
Formation method according to polarization reversal structure of the present invention, the electrode assignment of first electrode by will being used for applying electric field on the strong dielectric substrate is set to from its base portion to the direction at the tip Y direction along the crystallization of strong dielectric substrate, can form fine polarization-reversed region.The width of polarization reversal that this effect is based on Y direction is than X-axis big several times and uniform phenomenon and produce.Also have because voltage is concentrated on each tip that a plurality of electrodes of periodic arrangement refer to, therefore by such electrode when the substrate of Z plate causes polarization reversal, can efficient form polarization-reversed region well.
In the formation method of polarization reversal structure of the present invention preferably, on above-mentioned strong dielectric substrate, apply electric field, make whole area with respect to above-mentioned polarization-reversed region, the ratio of area that penetrates into the above-mentioned polarization-reversed region at the back side from the surface of above-mentioned strong dielectric substrate is suppressed in below 50%.Thus, can be formed uniformly fine polarization reversal structure.In the roughly vertical strong dielectric substrate of crystallization Z axle with base plan, if partly be formed with polarization-reversed region, and this part connects substrate and makes short circuit between the electrode, and the width of polarization-reversed region concentrates on the counter-rotating zone of perforation so, hinders the formation of uniform polarization reversal.Therefore, the area that suppresses the split pole counter-rotating zone of this perforation is effective for the homogeneity of guaranteeing polarization reversal structure.
In this structure, the thickness T of above-mentioned strong dielectric substrate is preferably more than 1mm.
Also have, by on above-mentioned strong dielectric substrate, apply electric field make the mean value of depth D of above-mentioned polarization-reversed region be above-mentioned strong dielectric substrate thickness 40%～95%, can obtain and above-mentioned same effect.
The LiTa that it is doped with Mg that above-mentioned method is particularly suitable for above-mentioned strong dielectric substrate (1-X)Nb XO 3The situation of (0≤x≤1).
Above-mentioned first electrode is a comb electrode, and above-mentioned electrode refers to it can is the striated structure.Perhaps, the electrode of above-mentioned first electrode refers to it can is triangle, can be the structure that tip that electrode refers to is formed by vertex of a triangle.Perhaps, above-mentioned electrode refers to have with respect to from the axisymmetric shape of its base portion to its most advanced and sophisticated direction, also above-mentioned electrode assignment can be set to the Y direction of its axis of symmetry along the crystallization of above-mentioned strong dielectric substrate.
Preferably, make the width at the tip that above-mentioned electrode refers to below 5 μ m.
Also have, preferably, the operation that applies electric field at above-mentioned strong dielectric substrate comprises the operation of the pulse voltage that applies electric field strength E 1 and applies the operation of the DC voltage of electric field strength E 2, makes that satisfying E1＞E2 carries out.Thus, under the electrode of design, can control the pulse waveform that applies voltage, make in wide region, forming counter-rotating zone uniformly along electrode.If have most advanced and sophisticated electrode to produce polarization reversal on the substrate of Z plate in use, voltage concentrates on the tip of electrode so, and efficient forms the polarization-reversed region of this part well.For this polarization-reversed region is broadened on entire electrode easily, share pulse voltage and DC voltage is effective as applying electric field.That is, can form polarization reversal nuclear by pulse voltage, be middle auxocardia polarization-reversed region by DC voltage with polarization reversal nuclear.
Preferably make above-mentioned electric field E1 greater than 6kV/mm, above-mentioned electric field E2 is less than 5kV/mm.Also have, preferred above-mentioned pulse voltage is made of plural a plurality of train of impulses.
Also have, in the formation method of polarization reversal structure of the present invention preferably, after forming above-mentioned polarization-reversed region, more than 200 ℃ above-mentioned strong dielectric substrate is being implemented thermal treatment, in above-mentioned thermal treatment, suppress the generation of the thermoelectric electric charge of above-mentioned strong dielectric substrate.Thus, make stability raising, and reduce the scattering (at random) that causes by polarization reversal by the polarization-reversed region that applies electric field formation.
Preferably, make the surface and the back side electrical short of above-mentioned strong dielectric substrate in the above-mentioned thermal treatment.Also have, programming rate is below 10 ℃/minute in the preferred above-mentioned thermal treatment.
The formation method of polarization reversal structure of the present invention is fit to the situation of polarization reversal electric field below 5kV/mm of above-mentioned strong dielectric substrate.Also having, can be that to make the crystallization of above-mentioned strong dielectric substrate roughly be the composition of stechiometry.
Also have, preferably at above-mentioned first type surface second electrode makes and a plurality of electrodes of above-mentioned first electrode refer to tip being set in the formation method of polarization reversal structure of the present invention, to have a compartment of terrain relative.Second electrode is realized the auxiliary purpose that electric field is concentrated on the tip of first electrode.Form polarization reversal nuclear by the tip that electric field is concentrated on first electrode, begin the growth of polarization reversal easily apace.
Preferably, the relation of the thickness T of bee-line L between most advanced and sophisticated and above-mentioned second electrode that refers to of above-mentioned electrode and above-mentioned strong dielectric substrate is set for and is satisfied L＜T/2.Thus, can fully obtain the effect of second electrode.The relation of the thickness T of interelectrode distance L and substrate exerts an influence to the Electric Field Distribution at the tip that electrode refers to, if interelectrode distance L becomes this more than value then too small to the influence of second electrode.
Preferably, under above-mentioned first and second electrodes, form polarization-reversed region by between above-mentioned first electrode and above-mentioned comparative electrode, applying voltage.By in same plane, applying electrode respectively, under each adjacent electrode, form polarization-reversed region.Thus, very effective for the polarization-reversed region that forms wide region.
Also have, preferably, have first electric field that between above-mentioned first electrode and above-mentioned comparative electrode, applies voltage and apply operation, and second electric field that applies voltage between above-mentioned second electrode and above-mentioned comparative electrode applies operation.Also have, preferably, apply operation and above-mentioned second electric field by above-mentioned first electric field and apply operation and under first and second electrodes, form polarization-reversed region.Also have, preferably, carry out separately respectively that above-mentioned first electric field applies operation and above-mentioned second electric field applies operation.
Above-mentioned second electrode has most advanced and sophisticated relative a plurality of electrodes most advanced and sophisticated and that the electrode of above-mentioned first electrode refers to and refers to, the electrode of above-mentioned second electrode refers to also can be configured to direction from its base portion towards the tip along the Y direction of the crystallization of above-mentioned strong dielectric substrate.
The distance L that preferably makes above-mentioned first electrode and above-mentioned second electrode is 50 μ m≤L≤200 μ m.
When the polarization-reversed region cartographic represenation of area that also has preferably spontaneous polarization when above-mentioned strong dielectric substrate to be expressed as Ps, expectation is A, apply operation or second electric field by above-mentioned first electric field and apply in the operation any one, apply the quantity of electric charge more than 100 times of 2Ps A.Also have, preferably apply operation, apply electric field intensity and be the pulse voltage of E1, pulse width τ≤10 millisecond, apply operation, apply electric field intensity and be the DC voltage of E2, pulse width τ 〉=1 second, E1＞E2 for above-mentioned second electric field for above-mentioned first electric field.
Also have, the operation that preferably applies electric field on above-mentioned strong dielectric substrate is carried out in the insulating solution more than 100 ℃.Also have, the angle θ that preferred above-mentioned first type surface and above-mentioned Z axle form is in the scope of 80 °≤θ≤100 °.Also have, the thickness T of preferably making above-mentioned strong dielectric substrate more than 1mm, the polarization-reversed region of periods lambda below 2 μ m.Also have the depth D of preferred above-mentioned polarization-reversed region to manufacture the relation that satisfies D＜T with respect to substrate thickness T.
Also have, the thickness T that preferably makes above-mentioned strong dielectric substrate is T 〉=1mm, between above-mentioned comparative electrode and above-mentioned strong dielectric substrate, form insulation film, between above-mentioned first electrode and above-mentioned comparative electrode, apply pulse width and be 1 millisecond～50 milliseconds pulse voltage.Above-mentioned insulation film can be SiO 2Film, TiO 2Film or Ta 2O 5Film.The thickness T that perhaps also can make above-mentioned strong dielectric substrate is T 〉=1mm, between above-mentioned comparative electrode and above-mentioned strong dielectric substrate, form semiconductor film, between above-mentioned first electrode and above-mentioned comparative electrode, apply pulse width and be 1 millisecond～50 milliseconds pulse voltage.Above-mentioned semiconductor film is Si film, ZnSe film or GaP film.
According to optical element of the present invention, by whole area with respect to polarization-reversed region, the ratio of area of polarization-reversed region that penetrates into the back side from the surface of strong dielectric substrate is below 50%, or the mean depth of polarization-reversed region can be formed uniformly fine polarization reversal structure in 40%～95% scope of the thickness of strong dielectric substrate.In the strong dielectric substrate of the Z of crystallization axle and base plan approximate vertical, if make polarization-reversed region perforation substrate and make short circuit between the electrode, the width of polarization-reversed region concentrates on the counter-rotating zone of perforation so, hinders to form uniform polarization reversal.Therefore, the area that suppresses the polarization-reversed region of this perforation is effective for the homogeneity of guaranteeing polarization reversal structure.
The LiTa that preferred above-mentioned strong dielectric substrate is a doped with Mg (1-X)Nb XO 3(0≤x≤1).Also have, the cycle of preferred above-mentioned polarization-reversed region is below 4 μ m.Also have, the thickness of preferred above-mentioned strong dielectric substrate is more than 1mm.Also have, the substrate thickness T of preferred above-mentioned strong dielectric substrate is 1mm, and the periods lambda of above-mentioned polarization-reversed region is below 2 μ m.Also have the depth D of preferred above-mentioned polarization-reversed region to satisfy the relation of D＜T with respect to substrate thickness.Also have, the angle θ that preferred above-mentioned first type surface and above-mentioned Z axle form is in the scope of 80 °≤θ≤100 °.
Below, specifically describe at embodiments of the present invention with reference to accompanying drawing.
(embodiment 1)
Figure 1A is the planimetric map of the electrode structure of the expression formation method that is used for implementing embodiment of the present invention 1 polarization reversal structure, and Figure 1B is a cut-open view.
First type surface 2 at MgLN substrate 1 forms first electrode 3 with pectination pattern.The a plurality of electrodes that constitute first electrode 3 refer to that 5 have elongated striated, and periodically arrange.Thus, electrode refers to that 5 small most advanced and sophisticated 5a periodically arranges.Form second electrode 4 at the first type surface that is provided with predetermined space 2 from the most advanced and sophisticated 5a of first electrode 3.First electrode 3 and second electrode, 4 electrical isolations.Comparative electrode 6 is set at the back side of MgLN substrate makes it relative with second electrode 4 with first electrode 3.Comparative electrode 6 has for example flat shape of rectangle, thereby comprises and first electrode 3 and second electrode, 4 corresponding zones, does not need to have special pattern.
The a plurality of electrodes that form first electrode 3 refer to that 5 are configured to make the Y direction of the axis of symmetry of striped along the crystallization of MgLN substrate 1 respectively.In other words, most advanced and sophisticated 5a refers to that from electrode 5 base portion extends towards the direction of Y direction.
Between first electrode 3 and comparative electrode 6, controlled voltage is imposed on MgLN substrate 1, in interelectrode strong dielectric, form polarization-reversed region thus by pulse producer 7.Though in the back controlled voltage is specifically described, controlled voltage has predetermined voltage level or duration, is pulse voltage or DC voltage.
Discharge when applying voltage, substrate 1 is arranged in insulating solution or the vacuum (10 -6Holder is following), and apply DC voltage.If the generation polarization reversal, size and the proportional electric current of electrode area (being called " reverse current ") with the spontaneous polarization of strong dielectric are so flowing between first electrode 3 and second electrode 4.
For existing electrode structure, though only apply pulse, only apply DC voltage, be applied to the voltage of superimposed pulse in the DC voltage in addition, it also all is difficult forming the good polarization-reversed region of repeatability on the MgLN of Z plate.Relative therewith, according to present embodiment,, can form short-period uniform polarization reversal structure by the condition that employing describes below.
Here, the homogeneity of the periodic structure of polarization reversal means the stability of cycle or dutycycle.Homogeneity influences conversion efficiency under the situation that polarization reversal structure is used for wavelength conversion.For example, periodic polarization reversal structure is being crossed under the situation about forming about 10mm length, partly be formed with the part of periodic structure confusion.Uneven main cause is the horizontal direction that partly enlarges polarization-reversed region, and forms the part of the big confusion of dutycycle change partly.This uneven part is formed with tens places for existing method in the scope of the average 10mm of inversion structures, for the situation below the cycles 3 μ m, occupied by uneven part on whole front almost.For this reason, conversion efficiency is had to a few percent～50 percent of theoretical value.Relative therewith, the good uniformity of present embodiment for example means on 10mm length, and uneven part is below several.Also have, thus, mean that conversion efficiency under the situation that is used for wavelength conversion at more than 90% of theoretical value, obtains approaching the very high efficient of theoretical value.
The condition of present embodiment relates generally to:
(a) electrode shape
(b) relation of electrode direction and crystallographic axis
(c) apply pulse waveform
Under this situation that fully satisfies specified conditions, be formed uniformly small polarization reversal structure.
At first, be described at (a) electrode shape.With reference to Fig. 2 A, 2B, the shape of polarization-reversed region when applying voltage by first electrode 3 with small tip, that form in strong dielectric describes.If apply voltage, the tip portion at first electrode 3 forms polarization-reversed region 8 so.At this moment, concentrate on the small most advanced and sophisticated 5a of electrode, at first form polarization reversal nuclear, enlarge polarization-reversed region in this part in order to make electric field.
Desirable polarization reversal structure is that the width W of polarization-reversed region is narrower, the structure that the length L r in counter-rotating zone is long.Width W is more little, easy more polarization-reversed region is controlled to small.If for example width W is little, then can form short-period polarization reversal structure.Also have length L r long more, may form big polarization-reversed region more.
For the tip is not the electrode of the bigger pattern of small width, can not be formed uniformly polarization-reversed region.This is because owing to similarly produce electric field under electrode, therefore nuclear produces polarization reversal in the position that is arrived, and polarization-reversed region is that the center becomes big cause with this nuclear.According to the electrode structure of present embodiment,,, form uniform polarization reversal structure so can improve controlled that polarization-reversed region forms owing to specifically form the formation zone of polarization reversal nuclear at the tip of electrode.This method is for the LiNbO of the Mg that mixed 3, the Mg that mixed LiTaO 3Perhaps as their LiTa of doped with Mg of potpourri (1-X)NNb XO 3The substrate of (0≤x≤1) is effective especially.
The LiTa of doped with Mg particularly (1-X)Nb XO 3The crystallization of (0≤x≤1) is well known that the polarization-reversed region of formation has rectification characteristic.Thus, form polarization-reversed region and flow in this part electric current is arranged.Therefore, if once just form polarization-reversed region, be that the polarization-reversed region at center is big so with the inversion section.On the other hand, in the part that does not form polarization reversal, apply voltage, therefore form very difficult that opposite counter-rotating becomes owing to the rectified action by polarization-reversed region has reduced.Thus, increase the unevenness of polarization-reversed region, formed the difficulty that uniform polarization reversal structure becomes.This tendency is remarkable especially in small shape.
Relative therewith, describe at the superiority of electrode with small tip.As shown in Figure 3, if MgLN substrate 10+form comb electrode 11 on the Z surface, on-Z surface, form plane electrode 12, between electrode, apply voltage, so because electric field 13 concentrates on the tip of comb electrode 11, so most advanced and sophisticated electric field ratio other parts are big.Producing polarization reversal nuclear thus, as triggering, is that center polarization-reversed region 14 become big with counter-rotating nuclear with this polarization reversal nuclear.But for the smooth electrode structure in tip, because the unevenness of crystallization and the existence of little magnetic domain, polarization reversal nuclear produces at random site, so be difficult to control.On the other hand,, have the electrode structure at small tip, electric field can be concentrated on eletrode tip by use as present embodiment.Because the electric field intensity of this part is grow partly, so can control the generation position of polarization reversal nuclear.Shown in Fig. 3 B, if polarization reversal nuclear is produced at the tip of comb electrode 11, so with this nuclear be the center polarization-reversed region along electrode growth, the length L r of polarization-reversed region increases.Thus, have the electrode at small tip, can control the generation zone of polarization reversal nuclear, form uniform polarization reversal structure by use.
Under the situation of the electrode of the electrode shape that does not have small tip, for example ladder shape and plane electrode, owing to can not cause electric field convergence to the tip, so the random site of polarization reversal nuclear under electrode produces.Therefore, can not control polarization reversal, it is difficult forming necessary uniform shape.Therefore, the tip that refers to for electrode is that small situation means that can to make the electric field that applies by electrode fully concentrated, thus the state that the width at the tip diminishes.Electric field is enough concentrated to be meaned in order to be formed uniformly the necessary degree of polarization reversal structure.Usually, most advanced and sophisticated width is preferably below 5 μ m, if below the 2 μ m, owing to the homogeneity that has improved the polarization reversal structure that forms, with better.If the tip becomes below the 1 μ m,, will be preferred owing to can form small polarization reversal structure.
In MgLN, as previously mentioned, reduced the electrical impedance of polarization-reversed region widely.Thus, impedance reduces when enlarging polarization-reversed region.Therefore, if make the magnitude of current that applies pulse certain, when enlarging polarization-reversed region, shown in Fig. 3 C, apply voltage and reduce so.Reduce if apply voltage, become below the polarization reversal voltage Vc, will automatically reduce the growth of polarization-reversed region so.
Need consider the variation of the electrical specification of above-mentioned polarization-reversed region for the bigger expansion that realizes polarization-reversed region.With reference to Fig. 4 A, 4B, 5A, 5B, further describe at the polarization reversal method that is used to enlarge polarization-reversed region.As previously mentioned, polarization-reversed region 14 stops along with its expansion.If set current value significantly for fear of this phenomenon, under the high state of impedance so in the early stage, the big electric current that flows in polarization-reversed region produces the insulation breakdown that causes and the rapid horizontal expansion of polarization-reversed region because temperature rises.Carry out for fear of this phenomenon is following.At first, the lower electric current that flows, for example the maximum current flow of 0.1mA degree forms polarization-reversed region shown in Fig. 4 A, 4B.Thereafter, the growth of polarization reversal stops, and further improves the maximal value of electric current, shown in Fig. 5 A, 5B, promotes the growth of polarization-reversed region.By this process repeatedly, can realize the expansion of length L r.
As the method for length L r of lengthening polarization-reversed region, it is effective that second electrode 4 is set.Second electrode 4 is formed on the position of only leaving distance L from the tip of first electrode 3.Second electrode 4 is realized the auxiliary purpose that electric field is concentrated on the most advanced and sophisticated 5a of first electrode 3.When forming polarization-reversed region, form counter-rotating nuclear as previously mentioned, the growth of beginning polarization reversal by the tip that electric field is concentrated on first electrode 3.Electric Field Distribution in the tip of first electrode 3 is subjected to the influence of the comparative electrode 6 and second electrode 4.Distance L and substrate thickness T refer to that to electrode the Electric Field Distribution of 5 most advanced and sophisticated 5a exerts an influence between the electrode of first electrode 3 and second electrode 4, and therefore the length L r to the polarization-reversed region that forms produces bigger influence.
According to experimental result, distance L between the electrode than the short situation of substrate thickness T under, form uniform polarization reversal easily.If distance more than it then that the influence of second electrode 4 becomes is too small, can not obtain the effect that length L r that second electrode 4 causes increases.Distance between electrodes L is undue to be shortened then generation discharge between first and second electrodes 103,104 owing to making if also have, so the size of the distance L between the preferred electrode is more than 5 μ m.Preferably the two makes the length L r of polarization-reversed region elongated under the situation of L＜T/2.
Next, be described at the relation between (b) electrode and the crystallographic axis.The research electrode refers to the relation of the length L r of 5 direction and polarization-reversed region.MgLN is the crystallization of uniaxiality, considers perpendicular to crystallization on the plane of Z axle it is symmetrical structure.Particularly about the characteristic of polarization reversal, consideration and X, Y direction do not have dependence.But the X of polarization reversal characteristic and crystallization, Y-axis have dependence very much in the substrate of known Z plate.Fig. 6 is the figure of crystallographic axis mutual relationship of the length L r of the polarization-reversed region that forms of expression.Make electrode refer to that 5 direction rotates in Y direction, the length L r of the polarization reversal that forms under the situation of all directions is by representing with the distance of initial point.If make electrode refer to that 5 tip towards Y direction, makes electrode refer to that 5 direction of principal axis is consistent with Y direction, so the length L r of polarization-reversed region become very long.Relative therewith, if make electrode refer to 5 along X-direction, length L r is reduced to below half so.
Also have, electrode refers to that 5 situations about forming in X-direction compare with situation about forming in Y direction, and it is big that the unevenness of the size of polarization reversal becomes.Under the situation that Y direction forms, the deviation of the size of the polarization-reversed region of formation obtains practical uniform polarization reversal structure below a few percent.Refer to 5 direction as electrode, if tilt below ± 10 ° with respect to Y-axis, length L r is long so, obtains satisfying practical uniform polarization reversal structure.If below ± 5 °, then homogeneity is better.If surpass ± 10 ° with respect to Y-axis as can be known, length L r will reduce significantly so, and unevenness increases simultaneously.
As above-mentioned, make the Y direction of axial direction that electrode refers to and crystallization as one man form electrode and refer to small tip, be the essential condition that is used to form uniform polarization reversal structure.The forming process of polarization-reversed region is that electric field is concentrated on the tip that electrode refers to, makes the part height of the surface field of this part than other, thereby at first forms polarization reversal nuclear.Thereafter, thus be that the center refers to that at electrode enlarging polarization-reversed region down forms polarization reversal with nuclear.At this moment, by making axial method that electrode refers to towards Y direction, the characteristic that the width that activates polarization reversal enlarges easily in the Y direction of crystallization, thus form uniform polarization reversal.Under the situation that does not have small tip,,, be difficult to be formed uniformly the following inversion structures of small polarization reversal shape, particularly 10 μ m so polarization-reversed region becomes greatly brokenly owing to be irregularly formed polarization reversal nuclear.Also have,, be difficult to guarantee enough length L r, and be formed uniformly micro-structure making the tip under the situation of X-direction.
As the periodic pattern of first electrode, beyond the shape of stripes of comb electrode, also can use the triangle shown in Fig. 7 A, 7B.Can periodically form leg-of-mutton polarization-reversed region 9 by this first electrode 3a.Leg-of-mutton periodic polarized counter-rotating zone can be applied to prism, deflector etc.Even under leg-of-mutton situation,, also can make polarization-reversed region become big by making the Y direction of axis of symmetry along the substrate crystallization.In this case, vertex of a triangle becomes the tip, is that the center produces and the growth polarization reversal with this summit.
Next, applying waveform at (c) electric field is described the influence of polarization reversal.If apply DC voltage between electrode, so the polarization-reversed region that forms is under the situation of small polarization-reversed region that is a few μ m, and the polarization-reversed region of formation is inhomogeneous in the extreme.That is, form polarization-reversed region, and, will contact with the polarization-reversed region that becomes by approaching electrode finger-type because each polarization reversal nuclear polarization enlarges significantly at each electrode place.Thus, polarization-reversed region can not be controlled to small.Next, be 0.1ms～100ms if apply pulse width, apply the pulse-like voltage that voltage is about 8kV/mm, then can be formed uniformly small polarization reversal structure.Apply the preferably impulse electric field of pulse width τ≤10 millisecond of electric field.And, by applying a plurality of train of impulses, can form uniform polarization reversal structure.If apply voltage below 6kV/mm, then can not form polarization-reversed region.
Though can form polarization reversal structure by applying train of impulses, the polarization-reversed region that forms is confined to can not reach formation and refer to that along electrode the length of extending is the big polarization-reversed region of Lr near the tip that electrode refers to.Even change pulse waveform, umber of pulse also can only obtain identical result.Optimum umber of pulse can decide at the voltage waveform shown in the oscillograph by observation.At first, the voltage amplitude when monitoring voltage applies beginning replenishes and applies pulse.Be accompanied by the increase of umber of pulse, voltage amplitude reduces, if reach a certain quantity then the reduction of voltage amplitude stops.Voltage amplitude saturated and minimum applies umber of pulse and has dependence, and the reduction amount by the monitoring voltage amplitude decides and applies umber of pulse.Can not enlarge the counter-rotating zone even apply this above umber of pulse.The minimum umber of pulse that applies has dependence with the setting electric current, and current value is big more, and its umber of pulse reduces.That is, under the situation of formation with the polarization reversal structure of one-period, current value is high more, stops the growth of polarization reversal by few more umber of pulse, can not enlarge polarization-reversed region even apply its above pulse.
Therefore, the additional DC voltage that applies on the pulse voltage that applies.Application time is 1～100 second a degree.Be difficult to form uniform polarization reversal structure though only apply direct current, apply DC voltage if follow applying of train of impulses, polarization-reversed region enlarges along electrode so, and comparing length L r with the situation that only applies pulse increases several times.That is, after applying train of impulses, apply DC voltage, can form the small polarization reversal structure in the wide zone of uniform leap by replenishing.If apply electric field pulse is that for example pulse width 0.5ms, umber of pulse are about 200～5000, and applying voltage is 5～6kV at substrate thickness during for 2mm, then can obtain good result.The maximal value that makes electric current is about 0.2～1mA.The size of DC voltage is compared quite little with pulse voltage, obtain good result under the situation below 0.2～4kV/mm.The process that forms polarization reversal by low-down voltage is to form polarization reversal nuclear by applying train of impulses, and it is the effect that the center enlarges polarization-reversed region that the applying of DC voltage considered to have with polarization reversal nuclear.If in the DC voltage that applies more than the after-applied 5kV of pulse, polarization-reversed region too enlarges so, be difficult to form small polarization-reversed region.
Electric current when next, applying voltage at restriction in the practicality, the peaked necessity of voltage describe.When making polarization reversal such as MgLN, because the electrical impedance of grade slab reduces significantly as described above, big electric current flows in the part of to the utmostization counter-rotating.In common strong dielectric, the quantity of electric charge that flows is the very little amount that is subjected to the area constraints of polarization-reversed region, but in MgLN, because the continuous electric current that flowing needs the special potential circuit of considering to apply.That is, need have the performance that is controlled at the maximum current that flows in the circuit that applies electric field.Under the situation of MgLN, if Control current value not, so because the big electric current that flowing will produce crystalline fracture.In order to prevent this phenomenon, need control gear to make current value be no more than the maximal value of setting, and apply voltage and descend automatically.Though the area with electrode under the situation of the MgLN of reality has dependence, the maximum magnitude of current is preferably below 10mA.Cycle is that the situation of the following short period structure of 3 μ m need be controlled to below the 5mA.
Also have, when applying continuous impulse, the different pulse voltage of maximum current flow that applies each pulse is effective.Thereby when applying a plurality of impulse electric fields and forming polarization reversal, because therefore the impedance height of initial stage polarization inversion unit can apply high voltage by a spot of magnitude of current.If also have inhomogeneous that polarization inversion unit becomes because electric current quantitative change in the early stage is big, therefore maximum current need be set for low current below the 1mA at the initial stage that forms polarization reversal.Yet when polarization-reversed region enlarged, because the impedance of polarization inversion unit reduces significantly, if the therefore maximal value of current limit amount, polarization reversal can not reach necessary voltage.Thus, when polarization-reversed region enlarged, it was effective that the maximal value that applies electric current is increased.
In the formation method of the polarization reversal structure of present embodiment, the substrate thickness of preferred MgLN is more than the 1mm.Substrate thickness is under the situation more than the 1mm, and the length L r under the electrode of the homogeneity of polarization reversal structure and polarization-reversed region obtains good effect.Its reason is owing to use thick substrate, can prevent that polarization-reversed region from connecting substrate.As described later, if polarization-reversed region connects substrate, the increase of the unevenness of polarization-reversed region is difficult to form small polarization reversal structure so.By increasing the thickness of substrate, suppress the perforation of polarization-reversed region, can form uniform polarization reversal structure.Existing being thinned to by the thickness that makes substrate forms polarization-reversed region easily below the 0.5mm, and can form small polarization reversal structure.Can make the LiTa of the phenomenon of regional homogenising of counter-rotating and microminiaturization by adding thick substrate for the Mg that mixed (1-X)Nb XO 3The substrate of (0≤x≤1) is remarkable especially.Also has the LiTa of the Mg that mixed (1-X)Nb XO 3Reversal voltage is common below 1/4 of LN in the substrate of (0≤x≤1).Though for common LN etc. under the situation that makes the substrate thickening because reversal voltage produces insulation breakdown, can not cause the low part of reversal voltage of insulation breakdown, also can apply polarization reversal voltage.
Above description of the present embodiment is the example of the formation method of polarization reversal structure under the situation of the MgLN that uses the Z plate.Because the C axle of Z base board crystallization is present in the direction perpendicular to substrate, therefore can efficient utilize the electric field of electrooptics effect to apply well.Also have, deepen the advantage that waits, so be desirable substrate as large-scale optical element owing to have the degree of depth of polarization-reversed region.Yet, even near the substrate of the cutting-out the Z plate also can observe same effect.As the angle of downcutting, the vertical line of research base plan and the C axle angulation of crystallization are the situation more than 0 °, can confirm can form the uniform polarization reversal structure same with the Z plate in the cutting-out angle in the situation below ± 10 °.Surpass ± 10 ° if downcut angle, it is difficult using the same method so and forming uniform and small polarization reversal structure.
Also have, the formation method of the polarization reversal structure of present embodiment is formed beyond the MgLN of (コ Application グ Le エ ン ト Group one-tenth) in congruent melting, even for the LiTa of the Mg that mixed (1-X)Nb XO 3The substrate of (0≤x≤1), the doping of stoichiometric composition the LiTa of Mg (1-X)Nb XO 3The substrate of (0≤x≤1) also can be suitable for.
In the MgLN that congruent melting is formed, estimate doping and the polarization reversal characteristic of Mg.The thickness that makes substrate is 1mm.The doping of Mg influences the polarization reversal characteristic widely.Follow the variation of the electrical impedance of polarization reversal to depend on the doping of Mg and increase, form short-period polarization reversal structure and also have dependence with the doping of Mg.Cycle is that the following periodic structure of 3 μ m only forms in the scope of 4～5.5 μ m in the doping of Mg.Even being the material of 2～7mol%, the doping of the big periodic structure Mg that the cycle, to be 10 μ m above also can form.Also have,, then be difficult to form polarization reversal owing to crystallinity degenerates if doping surpasses 7mol%.Also have for less than 2mol%, it is big that the expansion of the transverse direction of polarization reversal becomes, and is difficult to form periodic structure.Therefore, preferred mol concentration is 2～7mol% when forming periodic structure.Realize that the short period structure is more preferably 4～5.5mol%.
Also have, about the composition of substrate, relatively congruent melting composition and stoichiometric composition do not have big difference for the doping of Mg and the relation of polarization reversal characteristic.Even MgLN, the MgLT of stoichiometric composition and as they potpourri doping the LiTa of Mg (1-X)Nb XO 3In (0≤x≤1), the relation of the doping of Mg and polarization reversal characteristic also is same.
Also have, judge of the homogeneity generation big influence of the degree of depth of the polarization-reversed region that forms polarization-reversed region.Under the situation of the formation method of polarization reversal structure, polarization-reversed region penetrates into the back side from the surface and forms in the MgLN of existing Z plate substrate.But, when forming polarization-reversed region, have the short period in formation by same structure, particularly under the situation of the polarization reversal structure of the periodic structure below the 4 μ m, unevenness increases significantly.In MgLN, the polairzed area of formation has rectification characteristic and since the voltage that polarization reversal produces following apply voltage and streaming current.Therefore, applying voltage between the electrode and forming under the situation of polarization reversal,, thereby will between electrode, flow by the polarization-reversed region electric current so if a part of polarization reversal also connects between substrate.Consequently, with this partial polarization significantly growth phase right be, in other parts, the part that the polarization reversal of going ahead of the rest connects makes current vanishes, the growth of polarization reversal will stop.Consequently form polarization-reversed region very unevenly.
To form small polarization-reversed region difficulty under the situation of polarization reversal also be same reason by applying direct current.Formation method for the polarization reversal structure of present embodiment causes polarization reversal owing to apply pulse, does not make the polarization reversal depth D reach substrate thickness T so may be controlled to.That is, control applies umber of pulse, may be controlled to the polarization reversal depth D and does not reach substrate thickness T, can limit the ratio that polarization-reversed region penetrates into the back side thus, and improves the homogeneity of polarization reversal.In experiment, in the whole area by the polarization-reversed region that will form, the ratio of area that penetrates into the polarization-reversed region at the back side is suppressed at more than 1% below 50%, can form uniform polarization reversal structure.If this ratio is suppressed at below 20%, forms the following small structure of 4 μ m so and also become easy.By making applying the after-applied DC voltage of pulse is low-down voltage, to examine by the polarization reversal that applies pulse shaping is that the center enlarges along electrode, institute finally can keep the polarization reversal depth D less than substrate thickness T so that the polarization reversal degree of depth can not increase.As above-mentioned, form polarization-reversed region by the relation that keeps T＞D, can form even and small polarization reversal structure.
Be formed uniformly the method for small polarization-reversed region as growth, the mean value of polarization reversal depth D be controlled to 40～95% of substrate thickness T, effectively by the polarization-reversed region that suppress to connect.If the mean value of polarization reversal depth D surpasses 95%, the perforation ratio of polarization-reversed region will be above 50% so, and the unevenness of counter-rotating increases considerably.If be lower than 40% on the other hand, the part that does not form polarization-reversed region so becomes many, and the result also becomes uneven polarization reversal structure.If the mean value of polarization reversal depth D is suppressed at 50～80% of substrate thickness T, will further improve homogeneity so.
Also have, it is effective implementing ion-exchange and crystallinity is changed for controlling polarization reversal minutely on the surface of MgLN substrate.When applying electric field by pattern electrode on MgLN, the surface state of substrate produces big influence to reverse speed characterisstic.Under the situation by electrode application voltage, below electrode, make the polarization-reversed region growth, but laterally also enlarging simultaneously.The expansion of horizontal polarization-reversed region is difficult to form minutely polarization-reversed region.For example, under the situation that forms periodic polarization reversal structure, the horizontal expansion of polarization-reversed region is difficult to form short-period inversion structures.For anti-phenomenon here, the generation that suppresses polarization reversal nuclear is effective.Polarization reversal nuclear forms below the electrode of crystal surface and on every side, and polarization inversion unit is the center growth with counter-rotating nuclear.The generation of this counter-rotating nuclear can make the strong dielectric deterioration of crystallization reduce by the ion-exchange of carrying out crystal surface.For example, by implementing a kind of proton exchange, can suppress the horizontal expansion of polarization-reversed region, and form short-period polarization reversal structure as ion-exchange.But, if form the difficulty of polarization-reversed region change because the ion-exchange degree of depth became dark, so the ion-exchange degree of depth is preferably below 0.5 μ m.
And, shown in Fig. 8 A, 8B, refer to 15 if on second electrode 4, form comb electrode, then improve the qualification rate that polarization-reversed region forms.When applying voltage between first electrode 3 and comparative electrode 6, produced the situation that produces discharge and can not cause polarization reversal between first electrode 3 and second electrode 4, this is the reason that makes the qualification rate decline of polarization-reversed region formation.On the other hand,, can prevent interelectrode discharge by on second electrode 4, forming the comb electrode identical with first electrode 3, and the raising qualification rate.
Also have, cause polarization reversal by between second electrode 4 and comparative electrode 6, applying voltage in the bottom of first electrode 3 as can be known.By reducing the interval between second electrode 4 and first electrode 3, and apply pulse voltage, form polarization-reversed region in the bottom of first electrode 2 at second electrode 4.Because like this polarization-reversed region that forms is even, polarization-reversed region can not connect substrate and become inhomogeneous, so be formed uniformly small polarization-reversed region.Also have by on first electrode 3 and second electrode 4, applying voltage respectively repeatedly, can form more even and long polarization-reversed region.
And, shown in Fig. 9 A, 9B,, can increase the homogeneity of polarization-reversed region and enlarge the polarization-reversed region that under electrode, forms by at least one of first electrode 3 and second electrode 4 is configured to the sandwich construction of metal 16 and dielectric 17.This is because when applying pulse voltage between electrode, the reason that the increase of the capacity by electrode changes the transient characteristic of pulse waveform.As the method that increases capacity, make electrode become metal and dielectric multilayer film is effective.As the dielectric big SiO of specific inductive capacity preferably 2, Ta 2O 5, Nb 2O 5, other high dielectric constant materials.
(embodiment 2)
The formation method of the polarization reversal structure in the embodiment 2 relates to the improvement that is used to make the polarization reversal stabilization.At first, the instable experimental result at the polarization reversal of having confirmed MgLN describes.
The Z plate LiNbO of the 5mol Mg that mixed is used in experiment 3Substrate.The thick substrate of 1mm ± form electrode on the Z face, apply the pulse voltage about 10kV, under electrode, form polarization-reversed region.By by HF solution etching substrates, because ± the etched pattern difference of Z face, thus the state that can observe polarization-reversed region become.
Next, 100 ℃ of left and right sides thermal treatments after 30 minutes, if carry out the HF etch processes once more and observe the polarization reversal part, the area that will observe the polarization-reversed region that at first forms so reduces near half at the substrate that will be formed with polarization-reversed region.Other observed phenomenon is as follows:
(1) even carry out Low Temperature Heat Treatment about 80 ℃, also reduce in the counter-rotating zone.
(2) the counter-rotating zone depends on heat treated temperature, time and reduces.
(3) even apply the electric field of low-voltage, polarization-reversed region also reduces.
(4) minimizing in counter-rotating zone produces unevenly.
(5), also can observe same phenomenon on the substrate of the cutting-out that the C axle of crystallization tilts a little even at normal with respect to substrate surface.
As above-mentioned,, judge owing to the polarization reversal structure that applies electric field formation is very unstable for the Z plate of MgLN.Problem below this produces.
At first, because therefore the minimizing that also can produce polarization-reversed region under the situation of unusual low temperature on the substrate that has formed polarization reversal, can not be accompanied by the processing of the processing of heating.Also have, owing to polarization reversal changes with timeliness, so element characteristic changes in time.
The formation method of the polarization reversal structure in the present embodiment has solved the problems referred to above.The feature of this method for example is to use substrate and the electrode structure identical with embodiment 1, by after applying voltage and forming polarization-reversed region, implements annealing in process.The condition of the annealing in process after forming by suitable setting polarization-reversed region can suppress the minimizing of polarization-reversed region.
The result that suitable annealing conditions is studied is that the minimizing of judging polarization-reversed region depends on the programming rate of annealing in process very much.Figure 10 A represents the temperature curve of annealing in process.After reaching annealing temperature with certain programming rate, annealed 1 hour at 100 ℃, and with certain cooling rate cool to room temperature.Figure 10 B represents result that the relation of the slip in the programming rate of annealing in process and counter-rotating zone is measured.Can know from Figure 10 B: programming rate is fast more, and the minimizing of polarization reversal is big more, if programming rate surpasses 20 ℃/minute, the decay of counter-rotating zone is more than 50% so.Relative therewith, if becoming 10 ℃/minute, programming rate becomes below 10%, if programming rate is reduced to a few percent at 5 ℃/minute with next attenuation rate with next attenuation rate.Therefore, in order to suppress the regional decay of reversing, preferably programming rate is set in below 10 ℃/minute.And be more preferably below 5 ℃/minute.Carry out the same experiment relevant with cooling rate, the influence of judging cooling rate does not almost have.Consider that this is because the caused electric field of thermoelectric electric charge that produces when heating up, the stability of polarization reversal is exerted an influence.
Owing to the instable reason that has proved polarization-reversed region is because thermoelectric electric charge causes the reversal development again of polarization reversal, so study at anti-other method of phenomenon here.Under the situation of Z base board, thermoelectric electric charge appears at the surface and the back side of substrate, forms the electric field of Z-direction.For anti-phenomenon here, the surface and the back side electrical short of substrate got final product.Therefore, stick with paste at the surface and the back side metallizing of the substrate that has formed polarization-reversed region, and make surface and back side electrical short.Under this state, carry out annealing in process.Annealing temperature is 400,600,800 ℃.The situation of MgLN is although produce the minimizing of polarization-reversed region in the time of 800 ℃, below 600 ℃, all to have guaranteed the stability of polarization reversal for any high speed thermal treatment.Like this, annealing in process at a high speed can be carried out to eliminate the electric field by thermoelectric charge generation in the surface by the short circuit substrate and the back side.
Also have, judge by carrying out annealing in process more than 200 ℃, the stability of polarization reversal structure is improved significantly.Even heating up, lowering the temperature and test with the high speed of carrying out repeatedly after carrying out annealing in process more than 200 ℃ more than 100 ℃, the counter-rotating shape can not change fully yet.
Also have, for the thermal treatment more than 400 ℃, the scattering loss that exists in substrate reduces significantly, can form the high polarization reversal structure of transparency.For this reason, for example be applicable to that the optical wavelength of utilizing the nonlinear optics effect changes under the situation of element, changes efficient and increases significantly.Also have, owing to the transmission loss in the crystallization under the situation that is applicable to polarization element is reduced to below 1/2, so can realize losing few polarizer.
The instable reason of polarization reversal structure be the polarization reversal electric field of MgLN below 5kV/mm, LiNbO normally 3, LiTaO 3Deng the very little degree below 1/4.Because polarization reversal voltage is low, so the inversion section instability after the polarization reversal, will cause counter-rotating again by very little thermoelectric effect.Because reversal voltage is also low under the situation of stoichiometry crystallization, so need carry out same thermal treatment.Also have, the upper limit of heat treatment temperature is about the Curie temperature of substrate.Because the Curie temperature of the situation of MgLN is about 1200 ℃, so heat treatment temperature need be limited in below 800 ℃.If surpass 800 ℃, then polarization-reversed region diminishes.Also have, because LiTaO 3Curie temperature be about 600 ℃, so the heat treated upper limit is 500 ℃.
The thermal treatment of present embodiment though the polarization reversal structure that forms for the method by embodiment 1 has special effect, also goes for making the polarization reversal structure stabilization that forms by other method.
(embodiment 3)
The formation method of the polarization reversal structure in the present embodiment 3 has the feature of the voltage application method under the situation of the use electrode structure shown in Figure 11 A and 11B.In the present embodiment, use have perpendicular to the MgLN substrate 1 of the first type surface 2 of Z axle+first electrode 3 and second electrode 4 that form on the Z face and apply voltage.That is,, utilize and under another electrode, also form polarization-reversed region, can form the polarization-reversed region of wide region by to any one electrode application voltage.In the following description, describe as an example with the situation that on the thick Z plate MgLN substrate of 1mm, forms polarization-reversed region.
In Figure 11 A and 11B, the key element identical with embodiment 1 used identical Reference numeral and omitted explanation to its repetition.In the present embodiment, a plurality of electrodes of first electrode 3 of formation pectination refer to the axis of symmetry of 5 its elongated shapes separately with the Y direction arrangement of predetermined cycle along the crystallization of MgLN substrate, yet most advanced and sophisticated 5a refers to that from electrode 5 base portion extends towards Y direction.The electrode that second electrode 4 also has pectination refers to 15, and its most advanced and sophisticated 15a extends towards Y direction from base portion.
By at first electrode 3 be formed on the voltage that applies between another surperficial comparative electrode 6 by pulse producer 7 control, between electrode, form polarization-reversed region.Can on MgLN substrate 1, apply and have pulse voltage predetermined voltage level, as required or DC voltage.Produce discharge when applying voltage, MgLN substrate 1 is configured in dielectric or the vacuum (10 -6Holder is following) and apply voltage.
Describe at the distinctive voltage application method of present embodiment.At first, apply DC voltage after between second electrode 4 and comparative electrode 6, applying pulse voltage.Next, after applying pulse voltage equally between first electrode 3 and the comparative electrode 6, apply DC voltage.Thus, generation polarization reversal nuclear forms polarization reversal below most advanced and sophisticated 5a, the 15a of first electrode 3 and second electrode 4.
Here, at applying voltage by one in first electrode 3 or second electrode 4, describe thereby under electrode, form polarization reversal as second electrode 4 of another electrode or first electrode 3.
In order to study the influence below first electrode 3 when second electrode 4 applies voltage, apply voltage for first electrode 3, the situation of research polarization reversal after applying pulse voltage between second electrode 4 and the comparative electrode 6, in the strong dielectric substrate.Make the most advanced and sophisticated 5a of first electrode 3 and second electrode 4 most advanced and sophisticated 15a be spaced apart 400 μ m.Apply the etching of fluorine nitric acid (the Off Star nitric acid) solution that carries out heat behind the voltage, carry out the observation of the following polarization reversal of first electrode 3.Its result confirms to have formed polarization-reversed region under the electrode of first electrode 3 that does not apply voltage.Similarly, applying voltage at first electrode 3 applies for second electrode 4 to confirm also to form polarization-reversed region under the voltage condition under the electrode of second electrode 4.
With reference to Figure 12, give the account in greater detail.Figure 12 is the cut-open view of the formation situation of expression polarization-reversed region.At first, apply under the voltage condition for second electrode 4, forming polarization-reversed region R2 under the electrode of second electrode 4 and under the electrode of first electrode 3.Next, if apply voltage for first electrode 3, the then polarization-reversed region further growth that forms under two electrodes forms polarization-reversed region R1.Thus, prove that also be effective in order to enlarge polarization-reversed region by another electrode application voltage that forms on same plane.
Next, at being purpose to enlarge the polarization-reversed region that forms for 3 times at first electrode, the result that the variation following conditions is studied describes.
(a) voltage application method
(b) electrode gap
(c) electrode direction and crystallographic axis
(d) voltage waveform and the quantity of electric charge
(e) shape of second electrode 4
(f) temperature of insulating solution
At first, describe at (a) voltage application method.As the voltage application method at simultaneously to first electrode 3 with second electrode 4 applies and respectively applying individually of applying separately of each electrode studied.For applying simultaneously, because+electrorheological that the Z near surface flows is many, and big electric current flows through and becomes easily in the same plane of first electrode 3 and second electrode 4, so that the discharge generation rate becomes is very high.Thereby, preferably to apply individually as the voltage application method.This is elaborated below.
If first and second electrodes 3,4 are applied electric field simultaneously, then reduce the growth of obstruction polarization-reversed region owing to concentrate on the electric field of each eletrode tip.Apply in the early stage electric field thus, it is effective applying electric field respectively.Further, by applying electric field by adjacent electrode, under the electrode that does not apply voltage, also produce the effect of polarization reversal,, obtain enlarging the effect of the polarization-reversed region that under mutual electrode, forms biglyyer by being used alternatingly adjacent electrode and applying electric field.Also have,, compare, obtain the elongated effect of polarization-reversed region with the situation that applies electric field by single electrode by alternately applying electric field.In experiment, be under the situation of 200 μ m for example in electrode gap, with simultaneously first and second electrodes 3,4 are applied voltage condition and compare, only apply under the situation of electric field at an electrode, the length of polarization-reversed region increases about 2 times, and by alternately applying the length L r that electric field obtains 3 times polarization-reversed region about 1.5 times, when applying simultaneously.
Like this first and second electrodes being applied the electric field ratio alternately, to apply impulse electric field more effective.Also have, with respect to the electrode that has applied electric field at first, the length L r that has been applied the polarization-reversed region that the electrode of electric field forms by the back tends to increase.Therefore, it is effective main electrode being applied electric field in the back.
There is not to find alternately to apply the effect of electric field on the other hand in the situation that applies the after-applied DC electric field of impulse electric field.By applying electric field simultaneously for adjacent electrode, process is shortened.Also have, also have the effect of uniform of the polarization reversal structure change of whole formation.For applying the after-applied DC electric field of impulse electric field, the method that a plurality of electrodes are applied is effective simultaneously.
Therefore,, apply operation, and second electric field that applies voltage between second electrode 4 and comparative electrode 6 applies operation and applies electric field by first electric field that between first electrode 3 and comparative electrode 6, applies voltage as a preferred example.Therefore, apply in the operation, apply the pulse voltage of electric field strength E 1, pulse width τ≤10 millisecond, apply in the operation, apply the DC voltage of electric field strength E 2, pulse width τ 〉=1 second, set E1＞E2 at second electric field at first electric field.
Next, describe at (b) electrode gap.Figure 13 is illustrated under the identical applying condition, the figure of the relation of the length L r of the polarization-reversed region that the interval L of the most advanced and sophisticated 5a of first electrode 3 and the most advanced and sophisticated 15a of second electrode 4 and first electrode form for 3 times.As from this figure, judging, be accompanied by electrode gap L and diminish, length L r increases.Also have, begin near electrode gap L length L r 200 μ m saturated, so preferably below 200 μ m.On the other hand, if electrode gap L too near (L≤50 μ m), the discharge generation rate uprises so.In the present embodiment, the electrode gap L of first electrode 3 and second electrode 4 is set at L=200 μ m, can obtains good effect.
For (c) electrode direction and crystallographic axis explanation in embodiment 1.
Next, describe at (d) voltage waveform and the quantity of electric charge.Identical for voltage waveform with the content of explanation in embodiment 1.Result of study at the quantity of electric charge that applies to electrode is as follows.In order to enlarge the polarization-reversed region below first electrode 3, the quantity of electric charge of supplying with second electrode, 4 surpluses is effective.In second electrode 4, be Ps if make spontaneous polarization, the polarization reversal area is A, so Shi Yi quantity of electric charge C is: C=2Ps * A.By applying the quantity of electric charge more than 100 times of suitable quantity of electric charge C, the polarization-reversed region under first electrode 3 enlarges, and length L r increases greatly.At this moment, owing on second electrode 4, applied the excess charges amount, thus all form polarization reversal 4 times at second electrode, because the periodic polarization reversal that the shape of pectination produces disappears.
Next, the shape at (e) second electrode 4 describes.As the shape of second electrode 4, most advanced and sophisticated 15a refers to that from electrode 15 base portion is effective in the shape of Y direction extension.But, owing to use the dummy electrode of second electrode 4, enlarging polarization-reversed region under first electrode 3 so need only by applying electric field as the expansion that is used to enlarge the polarization-reversed region first electrode under, other electrode shape also can.In fact, as second electrode 4, under the situation of using rectangular electrode, by second electrode 4 is applied electric field, the polarization-reversed region of first electrode 3 also enlarges.
Next, the temperature at (f) insulating solution describes.Insulation breakdown in order to prevent to apply electric field preferably carries out applying of electric field in insulating solution.Figure 14 is the figure of relation of the length L r of expression temperature of insulating solution and polarization-reversed region.Near affirmation polarization-reversed region 80 ℃ increases, and the length L r in the counter-rotating of the temperature more than 100 ℃ zone is saturated as can be known.Consider that this is because the temperature of MgLN substrate rises, the counter-rotating electric field reduces, and the easy reason of the growth of polarization reversal change.Also have, for more than 150 ℃, it is remarkable that the growth of the polarization reversal of cycle direction becomes, the difficulty that the uniform polarization reversal structure of formation short period (5 μ m are following) becomes.Thereby, form the short period polarization-reversed region and preferably make the temperature of insulating solution below 150 ℃.This condition is equally applicable to the method for embodiment 1.
The formation method of the polarization reversal structure of the condition by having considered above explanation in the thick Z plate MgLN substrate of 1mm, obtains the following even and wide counter-rotating area of short period polarization reversal structure of cycles 10 μ m.In the formation method of the polarization reversal structure of present embodiment, the substrate thickness of MgLN obtains good effect under the situation more than the 1mm.That is, the width Lr under the electrode of the homogeneity of polarization-reversed region, polarization inversion unit is to be good under the situation more than the 1mm at substrate thickness.Its reason is can prevent that by the used thickness substrate polarization-reversed region from connecting substrate.
Figure 15 is the figure of the relation of expression substrate thickness T and the polarization reversal periods lambda that can form polarization reversal.For the thick substrate of 0.5mm, the polarization reversal of the cycle shape that 7 μ m are following is very difficult.Can form small polarization reversal by slabization.As described later, this is because if polarization-reversed region connects substrate, then the unevenness of polarization-reversed region increases and form the reason of the difficulty of small polarization reversal structure change.By making the thickness thickening of substrate, can suppress the perforation of polarization-reversed region and form uniform polarization-reversed region.Existingly polarization-reversed region can be formed, and small inversion structures can be formed by substrate thickness is thinned to below the 0.5mm.Make the LiTa of the easy homogenising microminiaturization in counter-rotating zone by adding thick substrate for the Mg that mixed (1-X)Nb XO 3The situation of (0≤x≤1) is effective especially.
(embodiment 4)
Describe at the formation method of the polarization reversal structure in the embodiment 4 with reference to Figure 16 A and 16B.Electrode structure in the present embodiment is general identical with the situation of embodiment 3.Difference sandwiches SiO between MgLN substrate 1-Z face and comparative electrode 6 2Film is as dielectric film 18.Also have by between electrode, applying low-frequency pulse voltage, on+Z face, form the polarization-reversed region of wide region under the formed electrode.
As describing in embodiment 3, MgLN has distinctive rectification characteristic, if a part of polarization reversal and connecting between MgLN substrate 1, so at this part streaming current, the polarization of this part is compared with other parts and grown greatlyyer.Consequently, not only extend, and the counter-rotating change is inhomogeneous owing to the voltage that does not apply expectation on whole M gLN substrate 1 stops the counter-rotating zone.Have in formation particularly that unevenness increases significantly under the situation of polarization reversal structure in the cycle below the 4 μ m.
For the perforation of the polarization of the top and bottom that prevent MgLN substrate 1 and obtain the homogenising of short-period polarization-reversed region or the expansion of polarization-reversed region, in the present embodiment, between-Z face and comparative electrode 6, sandwich SiO 2Film is as dielectric film 18.By between electrode, sandwiching the structure of insulator, the capacity of electrode is increased, the polarization-reversed region that the homogeneity of polarization reversal increases and forms under electrode enlarges.For the structure that between electrode, sandwiches insulator, on the books in the flat 7-281224 communique of Japanese Patent Application Laid-Open.In the document, put down in writing the polarization-reversed region that makes cycle 5 μ m for substrate and connected, application time has been set at 3 seconds at the substrate surface and the back side at thickness 0.3mm.
On the other hand, in order on the substrate of the thickness more than the 1mm, to form short-period polarization reversal structure, prevent that the polarization perforation between the substrate from being very important.The perforation of polarization is very big with the dependence of the pulse width of the pulse voltage that applies.Therefore, study at the pulse waveform that applies.At first, apply pulse width τ and be 10～100 seconds pulse waveform,, and find that discharge or whole surface produce the phenomenon of polarization reversal even very the polarization reversal that current value can not obtain the cycle shape is set in the lowland.Consider that this is owing to applying the long influence that produces of pulse width.On the other hand, apply the pulse waveform that has with existing 1 millisecond pulse width equally, even increase umber of pulse and electric current, the counter-rotating zone does not enlarge yet.
Therefore, carrying out the optimumization of pulse width, as shown in figure 17, is 1 millisecond～50 milliseconds scope at pulse width τ as can be known, and the counter-rotating zone enlarges.Particularly, find that the expansion in counter-rotating zone is remarkable for 10 milliseconds～50 milliseconds.And, the polarization reversal width W when general 0.5 Λ (Λ is the polarization reversal cycle) dutycycle near 50%, most effective.More than 1 second, polarization reversal is in the Width overgrowth for pulse width, and up to W=Λ, it is bigger than the cycle that the width of polarization-reversed region becomes, and can not obtain periodic structure.
Also have, the expansion of the polarization-reversed region of the situation of using the thick MgLN substrate of 2mm is studied, can confirm the dependence of reverse speed characterisstic equally for pulse width.That is, in pulse width is 10 milliseconds～2 seconds zone, the expansion in the counter-rotating zone of affirmation cycles 4 μ m.
Also have, for present embodiment, as dielectric film, except SiO 2Beyond the film, also can use TiO 2Film, Ta 2O 5Film, Nb 2O 5Film etc.
(embodiment 5)
The formation method of the polarization reversal structure in the embodiment 5, the SiO that uses as dielectric film 18 in the electrode structure of replacement embodiment 4 2Film uses the Si film as semiconductor film.By between-Z face and comparative electrode 6, sandwiching structure as the Si film of semiconductor film, the capacity of electrode is increased, prevent that the polarization between substrate from connecting, improve the homogeneity of polarization reversal and enlarge the polarization-reversed region that under electrode, forms.
Using under the situation of semiconductor film, study at the pulse waveform that applies according to present embodiment.At first, apply pulse width τ and be 10～100 seconds pulse waveform,, and find the phenomenon of discharge or whole surperficial polarization reversal even very the polarization reversal that current value can not obtain the cycle shape is set in the lowland.This is owing to apply the long influence of pulse width.On the other hand, apply the pulse waveform that has with existing 1 millisecond pulse width equally, even increase umber of pulse and electric current, the counter-rotating zone does not enlarge yet.Therefore, carry out the optimumization of pulse width, in pulse width τ was 10 milliseconds～1 second scope, the counter-rotating zone enlarged as can be known.Particularly, find that the expansion in counter-rotating zone is remarkable for 20 milliseconds～50 milliseconds.
Also have, the expansion of the polarization-reversed region of the situation of using the thick MgLN substrate of 2mm is studied, can confirm the dependence of reverse speed characterisstic equally for pulse width.That is, in pulse width is 10 milliseconds～2 seconds zone, the expansion in the counter-rotating zone of affirmation cycles 4 μ m.
For present embodiment,, except the Si film, also can use ZnSe film, GaP film etc. as semiconductor film.
(embodiment 6)
Optical element in the embodiment 6 can utilize the formation method of the polarization reversal structure in the above-mentioned embodiment to make.Describe at Wavelength conversion element with reference to Figure 18 as an example of the optical element of present embodiment.Figure 18 is the oblique view of Wavelength conversion element.On the MgLN substrate 20 of Z plate, form rhythmic polarization-reversed region 21.The first-harmonic of wavelength X carries out wavelength conversion by periodic polarization reversal structure, can be transformed to the higher hamonic wave of λ/2.The polarization reversal cycle can for example be 4 μ m, can be the light of wavelength 450nm with the optical wavelength conversion of wavelength 900nm.The thickness of substrate 20 for example is 1mm, and the degree of depth of polarization-reversed region 21 is about 0.8mm.Polarization-reversed region 21 extends along the Y-axis of substrate crystallization.Polarization-reversed region 21 also from substrate 20+Z forms towards-Z face side.It is more shallow than the thickness of substrate 20 that the degree of depth of polarization-reversed region 21 forms the major part in a plurality of polarization methods counter-rotating zone 21.Though part polarization-reversed region 21 connects substrate 20 and forms, the area of the polarization-reversed region 21 that connects becomes below 50% of whole polarization-reversed region area.
The length that makes polarization-reversed region 21 cross over 10mm in X-direction forms, and the light of scioptics incident 900nm carries out wavelength variations with conversion efficiency 5%/W, obtains the higher hamonic wave of 450nm.Judge and form uniform polarization-reversed region and carry out high efficiency wavelength conversion.Also have, by making substrate 20 thickness more than 1mm, it is big that the beam waist of first-harmonic, higher hamonic wave becomes.Thus, can reduce the power density of light, obtain high output.Compare the situation that forms polarization-reversed region at the thick substrate of 0.5mm, utilize the situation of the thick substrate of 1mm, can make output improve 4 times.
Also have, form in Y direction, can form even and short-period polarization reversal structure by making polarization-reversed region 21.The following polarization reversal structure of cycles 2 μ m can be formed, the following ultraviolet light of wavelength 400nm can be produced thus.By forming polarization-reversed region 21, can produce short-wavelength light in Y direction.Relative therewith, form in X-direction under the situation of polarization-reversed region 21, form the difficulty that short-period polarization reversal structure becomes, can only obtain the above light of wavelength 500nm.
Also have, be suppressed at below 50%, can form uniform polarization reversal structure by making polarization-reversed region form depth ratio substrate thickness the area shallow and polarization-reversed region that connects.Be under the situation of 1%～50% scope in the ratio of the polarization-reversed region that connects, obtain uniform polarization-reversed region.If polarization-reversed region is discontented with 1%, observe the instability that causes polarization reversal structure so and increase, and the time dependent phenomenon of the polarization-reversed region that produces.If polarization-reversed region, forms the difficulty that short-period polarization-reversed region becomes so more than 50%.Thus, by the second harmonic difficulty below the Wavelength conversion element generation wavelength 500nm that produces.By as the ratio of the polarization-reversed region that connects of above-mentioned restriction, can obtain the following and polarization-reversed region uniformly of polarization reversal cycles 3 μ m, produce the following ultraviolet light of wavelength 400nm.
As the optical element that utilizes polarization reversal structure, except above-mentioned optical wavelength conversion element, for example, can constitute polarizer by making polarization reversal structure form prism shape and grille-like.In addition, can be applied to phase shifter, photomodulator, lens etc.Also have by applying voltage,, can constitute switch, polarizer, modulator, phase shifter, beam shaping etc. as the optical element that utilizes this phenomenon owing to can control variations in refractive index by electro-optic effect at polarization-reversed region.The method of present embodiment can make these optical element high performances owing to can form small polarization reversal structure.
The polarizer of the polarization reversal that utilizes prism shape has been shown in Figure 19 A, 19B.On strong dielectric substrate 22, form the polarization-reversed region 23 of periodic prism shape.The electrode of formation up and down 24,25 at polarization-reversed region 23.By electrode 24,25 is applied electric field, refractive index is changed, direction (for example, angle θ) that can control bundle 26.Owing to make the electro-optic effect of variations in refractive index and polarised direction have dependence by applying electric field, if therefore apply electric field as shown in the figure, then can the symbol of refractive index be reversed, control the refractive direction of the light of prismatic portion by polarization-reversed region 23 and non-counter-rotating zone.
In the explanation of above embodiment, though with the mixed LiNbO of MgO of use 3Substrate as the situation of strong dielectric substrate as an example, but other doping the LiTaO of MgO 3The LiNbO of substrate, doping Nd 3Substrate, KTP substrate, KNbO 3The LiNbO of substrate, mixed Nd and MgO 3The LiTaO of substrate or mixed Nd and MgO 3The situation of the same substrate of substrate, stoichiometric composition etc. is applicable to present embodiment too.Wherein, owing to the substrate that the crystallization by the Nd that mixed constitutes can be realized laser generation, so can carry out simultaneously producing first-harmonic and producing second harmonic by its wavelength conversion by laser generation.Therefore, can constitute short wavelength light source with high-level efficiency and stable acting characteristic.
The present invention's possibility of utilizing industrially is, according to the present invention, and not only can be by force Degree of depth ground and be formed uniformly short period and the big polarization reversal of width on the dielectric base plate Structure can also be made the optical elements such as optical wavelength conversion element with good characteristic.

Claims (42)

1, a kind of formation method of polarization reversal structure, this method is used the strong dielectric substrate that has with the first type surface of the Z axle approximate vertical of crystallization, first electrode is set on the first type surface of above-mentioned strong dielectric substrate, this first electrode has and is formed with the pattern that a plurality of electrodes of periodically being provided with refer to, on another surface of above-mentioned strong dielectric substrate, be provided with and the relative comparative electrode of above-mentioned first electrode, apply electric field by above-mentioned first electrode and above-mentioned comparative electrode to above-mentioned strong dielectric substrate, the corresponding polarization-reversed region of pattern of formation and above-mentioned first electrode on above-mentioned strong dielectric substrate is characterized in that:
Each electrode that above-mentioned first electrode is set refers to, makes the base portion that refers to from the electrode of above-mentioned first electrode Y direction towards the direction at tip along the crystallization of above-mentioned strong dielectric substrate.
2, the formation method of polarization reversal structure according to claim 1, it is characterized in that: above-mentioned strong dielectric substrate is applied electric field, make whole area with respect to above-mentioned polarization-reversed region, the ratio of area that penetrates into the above-mentioned polarization-reversed region at the back side from the surface of above-mentioned strong dielectric substrate is controlled in below 50%.
3, the formation method of polarization reversal structure according to claim 2, it is characterized in that: the thickness T of above-mentioned strong dielectric substrate is more than 1mm.
4, the formation method of polarization reversal structure according to claim 1 is characterized in that: above-mentioned strong dielectric substrate is applied electric field, make the mean value of depth D of above-mentioned polarization-reversed region be above-mentioned strong dielectric substrate thickness 40%～95%.
5, the formation method of polarization-reversed region according to claim 1 is characterized in that: above-mentioned strong dielectric substrate is the LiTa of Mg of having mixed (1-X)Nb XO 3(0≤x≤1).
6, the formation method of polarization reversal structure according to claim 1 is characterized in that: above-mentioned first electrode is a comb electrode, and above-mentioned electrode refers to be striated.
7, the formation method of polarization reversal structure according to claim 1 is characterized in that: the electrode of above-mentioned first electrode refers to it is triangle, and the tip that electrode refers to is formed by vertex of a triangle.
8, the formation method of polarization reversal structure according to claim 1, it is characterized in that: above-mentioned electrode refers to have the shape with respect to the symmetry of the axle of the direction from its base portion towards the tip, and above-mentioned electrode is referred to be arranged so that the Y direction of its axis of symmetry along the crystallization of above-mentioned strong dielectric substrate.
9, according to the formation method of each described polarization reversal structure in the claim 1～8, it is characterized in that: the width that makes the tip that above-mentioned electrode refers to is below 5 μ m.
10, according to the formation method of each described polarization reversal structure in the claim 1～9, it is characterized in that: the operation that above-mentioned strong dielectric substrate is applied electric field comprises and applies the operation that electric field intensity is the pulse voltage of E1, with apply the operation that electric field intensity is the DC voltage of E2, and satisfy E1＞E2.
11, the formation method of polarization reversal structure according to claim 10 is characterized in that: make above-mentioned electric field E1 greater than 6kV/mm, above-mentioned electric field E2 is less than 5kV/mm.
12, the formation method of polarization reversal structure according to claim 10 is characterized in that: above-mentioned pulse voltage is made of plural a plurality of train of impulses.
13, according to the formation method of each described polarization reversal structure in the claim 1～12, it is characterized in that: after forming above-mentioned polarization-reversed region, more than 200 ℃ above-mentioned strong dielectric substrate is being implemented thermal treatment, in above-mentioned thermal treatment, suppress the generation of the thermoelectric electric charge of above-mentioned strong dielectric substrate.
14, the formation method of polarization reversal structure according to claim 13 is characterized in that: in above-mentioned thermal treatment, make the surface and the back side electrical short of above-mentioned strong dielectric substrate.
15, the formation method of polarization reversal structure according to claim 13 is characterized in that: the programming rate in the above-mentioned thermal treatment is below 10 ℃/minute.
16, according to the formation method of claim 1 or 13 described polarization reversal structures, it is characterized in that: the polarization reversal electric field of above-mentioned strong dielectric substrate is below 5kV/mm.
17, the formation method of polarization reversal structure according to claim 1 is characterized in that: the crystallization of the substrate of above-mentioned strong dielectric roughly is stoichiometric composition.
18, the formation method of polarization reversal structure according to claim 1 is characterized in that: on above-mentioned first type surface, to have at interval with tip that a plurality of electrodes of above-mentioned first electrode refer to and relative mode is provided with second electrode.
19, the formation method of polarization reversal structure according to claim 18 is characterized in that: the relation of the bee-line L between most advanced and sophisticated and above-mentioned second electrode that above-mentioned electrode refers to and the thickness T of above-mentioned strong dielectric substrate is configured to satisfy L＜T/2.
20, according to the formation method of claim 18 or 19 described polarization reversal structures, it is characterized in that:, under above-mentioned first and second electrodes, form polarization-reversed region by between above-mentioned first electrode and above-mentioned comparative electrode, applying voltage.
21. the formation method of polarization reversal structure according to claim 20, it is characterized in that: also have first electric field that between above-mentioned first electrode and above-mentioned comparative electrode, applies voltage and apply operation, and second electric field that applies voltage between above-mentioned second electrode and above-mentioned comparative electrode applies operation.
22. the formation method of polarization reversal structure according to claim 21 is characterized in that: apply operation and above-mentioned second electric field applies operation by above-mentioned first electric field, under above-mentioned first and second electrodes, form polarization-reversed region.
23. the formation method of polarization reversal structure according to claim 22 is characterized in that: carry out respectively that above-mentioned first electric field applies operation and above-mentioned second electric field applies operation.
24. the formation method of polarization reversal structure according to claim 18, it is characterized in that: above-mentioned second electrode has most advanced and sophisticated relative a plurality of electrodes most advanced and sophisticated and that the electrode of above-mentioned first electrode refers to and refers to that the electrode of above-mentioned second electrode refers to be configured to from its base portion to the Y direction of its most advanced and sophisticated direction along the crystallization of above-mentioned strong dielectric substrate.
25. the formation method of polarization reversal structure according to claim 18 is characterized in that: the distance L of above-mentioned first electrode and above-mentioned second electrode is 50 μ m≤L≤200 μ m.
26. the formation method of polarization reversal structure according to claim 21, it is characterized in that: when representing the area of the polarization-reversed region expected in the spontaneous polarization of representing above-mentioned strong dielectric substrate with Ps, with A, any one that applies by above-mentioned first electric field that operation or second electric field apply in the operation applies the quantity of electric charge more than 100 times of 2PsA.
27. the formation method of polarization reversal structure according to claim 21, it is characterized in that: apply in the operation at above-mentioned first electric field, apply the pulse voltage of electric field strength E 1, pulse width τ≤10 millisecond, apply in the operation at above-mentioned second electric field, apply the DC voltage of electric field strength E 2, pulse width τ 〉=1 second, wherein E1＞E2.
28. the formation method according to claim 1 or 18 described polarization reversal structures is characterized in that: the operation that applies electric field on above-mentioned strong dielectric substrate is carried out in the insulating solution more than 100 ℃.
29. the formation method of polarization reversal structure according to claim 1 is characterized in that: above-mentioned first type surface and the formed angle θ of above-mentioned Z axle are in the scope of 80 °≤θ≤100 °.
30. the formation method of polarization reversal structure according to claim 1 is characterized in that: the substrate thickness T of above-mentioned strong dielectric is more than 1mm, and manufacturing cycle A is the following polarization-reversed regions of 2 μ m.
31. the formation method of polarization reversal structure according to claim 30 is characterized in that: the depth D that manufactures above-mentioned polarization-reversed region satisfies the relation of D＜T with respect to substrate thickness T.
32. the formation method of polarization reversal structure according to claim 1, it is characterized in that: the thickness T of above-mentioned strong dielectric substrate is T 〉=1mm, between above-mentioned comparative electrode and above-mentioned strong dielectric substrate, form dielectric film, between above-mentioned first electrode and above-mentioned comparative electrode, apply pulse width and be 1 millisecond～50 milliseconds pulse voltage.
33. the formation method of polarization reversal structure according to claim 32 is characterized in that: above-mentioned dielectric film is SiO 2Film, TiO 2Film, or Ta 2O 5Film.
34. the formation method of polarization reversal structure according to claim 1, it is characterized in that: the thickness T of above-mentioned strong dielectric substrate is T 〉=1mm, between above-mentioned comparative electrode and above-mentioned strong dielectric substrate, form semiconductor film, between above-mentioned first electrode and above-mentioned comparative electrode, apply pulse width and be 1 millisecond～50 milliseconds pulse voltage.
35. the formation method of polarization reversal structure according to claim 34 is characterized in that: above-mentioned semiconductor film is Si film, ZnSe film or GaP film.
36. optical element, comprise: have the crystallization of being approximately perpendicular to the Z axle the plane the strong dielectric substrate and periodically be formed on a plurality of polarization-reversed regions on the above-mentioned strong dielectric substrate, above-mentioned polarization-reversed region has the rotational symmetry flat shape respectively, and be configured to its axis of symmetry and be parallel to each other, it is characterized in that:
Form above-mentioned polarization-reversed region, make the direction of above-mentioned axis of symmetry along the Y-axis of the crystallization of above-mentioned strong dielectric substrate;
Above-mentioned polarization-reversed region has from+Z towards-shape that the Z face extends, and with respect to the whole area of above-mentioned polarization-reversed region, the ratio of area of above-mentioned polarization-reversed region that penetrates into the back side from the surface of above-mentioned strong dielectric substrate is below 50%, and perhaps the mean depth of above-mentioned polarization-reversed region is in 40～95% scope of the thickness of above-mentioned strong dielectric substrate.
37. optical element according to claim 36 is characterized in that: above-mentioned strong dielectric substrate is the LiTa of Mg of having mixed (1-X)Nb XO 3(0≤x≤1).
38. optical element according to claim 36 is characterized in that: the cycle of above-mentioned polarization-reversed region is below 4 μ m.
39. optical element according to claim 36 is characterized in that: the thickness of above-mentioned strong dielectric substrate is more than the 1mm.
40. according to the described optical element of claim 38, it is characterized in that: the thickness T of above-mentioned strong dielectric substrate is 1mm, the periods lambda of above-mentioned polarization-reversed region is below the 2 μ m.
41. according to the described optical element of claim 40, it is characterized in that: the depth D of above-mentioned polarization-reversed region satisfies the relation of D＜T with respect to substrate thickness T.
42. optical element according to claim 36 is characterized in that: the angle θ that above-mentioned first type surface and above-mentioned Z axle form is in the scope of 80 °≤θ≤100 °.
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CN112987447A (en) * 2019-12-02 2021-06-18 济南量子技术研究院 Electrode structure for periodic polarization of ferroelectric crystal material and polarization method

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