JPH07261218A - Formation of domain inversion structure of ferroelectric substance - Google Patents

Abstract

PURPOSE:To make it possible to form domain inversion parts having high uniformity within a plane parallel with electrodes and to prohibit easy destruction of a ferroelectric substance by impressing an electric field while controlling the current flowing in this ferroelectric substance constant. CONSTITUTION:A corona power source device 5 has a current driver 21, a current detecting circuit 22 which detects the current flowing between a corona wire 4 and a grounding wire 3 and a differential amplifier 23 which receives the current detection signal S and variable reference signal R outputted by this current detecting circuit 22 and feeds the signal indicating the deviation therebetween back to the current driver 11. This power source device has a function to control the current flowing in an MgO-LiNbO3 substrate 1 to the specified value indicated by the variable reference signal R. Local domain inversion is hardly generated in the ferroelectric substance if the electric field is impressed to the ferroelectric substance while the constant current is controlled by this constitution and consequently, the local flow of the large current is averted and the uniformity of the domain inversion is improved. The easy destruction of the ferroelectric substance is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a domain inversion structure having a predetermined pattern in a ferroelectric material having a nonlinear optical effect, for example, for producing an optical wavelength conversion element having a periodic domain inversion structure. It is a thing.

[0002]

2. Description of the Related Art A method of converting a fundamental wave into a second harmonic using an optical wavelength conversion element provided with a region in which spontaneous polarization (domain) of a ferroelectric substance having a nonlinear optical effect is periodically inverted. Have already been proposed by Bleombergen et al. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)).
In this method, the period Λ of the domain inversion part is Λc = 2π / {β (2ω) -2β (ω)} (1) where β (2ω) is the propagation constant 2β (ω) of the second harmonic. Is set to be an integral multiple of the coherent length Λc given by the propagation constant of the fundamental wave, so that the fundamental wave and the second harmonic can be phase-matched. When wavelength conversion is performed using a bulk crystal of a non-linear optical material, the phase matching wavelength is limited to a specific wavelength unique to the crystal, but according to the above method, a period that satisfies (1) for any wavelength By selecting Λ, phase matching can be efficiently achieved.

As one of the methods for forming the above-mentioned periodic domain inversion structure, there is a conventional method, for example, Japanese Patent Laid-Open No. 4-33.
As disclosed in Japanese Patent No. 5620 and Japanese Patent Laid-Open No. 5-210132, an electrode having a predetermined pattern is formed on one surface of a ferroelectric substance having a monopolarized nonlinear optical effect.
A method of forming a counter electrode on the other surface facing the one surface, applying a charge to the ferroelectric substance via these two electrodes, and forming a domain inversion portion corresponding to the predetermined pattern thereat Are known.

Further, Japanese Patent Application No. 5-21152 filed by the present applicant
As shown in the specification, while forming an electrode of the above-mentioned predetermined pattern on one surface of the ferroelectric substance, a corona wire is arranged at a position facing the other surface of the ferroelectric substance,
A method has also been considered in which a ferroelectric substance is corona-charged by the corona wire and the electrode to form a domain inversion portion corresponding to the predetermined pattern thereon.

In the method of forming the domain inversion part by applying an electric field to the ferroelectric substance through the pair of electrodes as described above or using one electrode and the corona wire,
Conventionally, in order to stabilize the electric field application conditions, the electric field is applied while the voltage applied to the ferroelectric is controlled to be constant.

[0006]

However, in the conventional method of forming the domain inversion portion of a predetermined pattern in the ferroelectric substance by applying the electric field, the problem that the domain inversion is not uniform in the plane parallel to the electrodes. Is recognized. That is, originally, domain inversion should be performed in all the regions facing the predetermined pattern electrode in the plane, and domain inversion should be suppressed in the other regions. It may not happen, or conversely, domain inversion may occur in part of other areas, which may cause problems.

Further, in the above conventional method, it has been recognized that the ferroelectric substance is easily destroyed when a relatively high voltage is applied to the ferroelectric substance in order to obtain the optimum domain inversion shape.

Therefore, according to the present invention, it is possible to form a domain inversion portion having high uniformity in a plane parallel to the electrodes, and to form a domain inversion structure of a ferroelectric substance without easily destroying the ferroelectric substance. It is intended to provide a method.

[0009]

As described above, the method for forming a domain inversion structure of a ferroelectric substance according to the present invention comprises forming a predetermined pattern of electrodes on one surface of a ferroelectric substance having a non-linear optical effect monopolarized. Then, an electric field is applied between the one surface of the ferroelectric substance and the other surface facing the ferroelectric substance through the electrode to locally invert the domain of the ferroelectric substance facing the electrode. In the method of forming a domain inversion structure of a ferroelectric substance, an electric field is applied while controlling a current flowing through the ferroelectric substance to be constant.

[0010]

According to the research conducted by the present inventor, the uniformity of domain inversion is impaired when the conventional method is used.
Further, the ferroelectric substance is easily destroyed because local domain inversion easily occurs in the ferroelectric substance when an electric field is applied while constant voltage control is performed, and a large current flows as an inversion current in the region. It turned out that

On the other hand, when an electric field is applied while constant current control is performed as in the method of the present invention, local domain inversion is less likely to occur in the ferroelectric substance, so that a large current locally flows can be avoided. As a result, the uniformity of domain inversion is improved, and the ferroelectric is prevented from being easily destroyed.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 shows the formation of a domain inversion structure according to the first embodiment of the present invention. In FIG. 1, reference numeral 1 is a ferroelectric having a nonlinear optical effect, for example, Li doped with 5 mol% of MgO.
It is a substrate of NbO ₃ . As the MgO-LiNbO ₃ substrate 1, so that the largest nonlinear optical material constant d ₃₃ can be effectively utilized, z plate which is optically polished in z-plane it is used. Also, this MgO-LiNbO ₃ substrate 1
Has been monopolarized in advance by a known method, and is formed to have a thickness of 0.3 mm as an example.

Then, on the + z surface 1a of the MgO-LiNbO ₃ substrate 1, Ta periodic electrodes 2 having a thickness of 30 nm arranged at a predetermined pitch are attached. The periodic electrode 2 is formed, for example, by forming a periodic mask pattern on the + z surface 1a by photolithography, sputtering Ta to form a Ta thin film, and then removing the mask. The Ta periodic electrode 2 has a comb shape in which portions corresponding to comb teeth arranged in a predetermined cycle are connected to each other by one base portion, and is grounded via a ground wire 3.

[0014] slightly away from the other hand MgO-LiNbO ₃ -z surface 1b of the substrate 1, corona wires 4 are arranged in a state extending in parallel with the -z face 1b. The corona wire 4 is connected to the corona power supply device 5. This corona power supply device 5 includes a current driver 21 and
A current detection circuit 22 for detecting a current flowing between the corona wire 4 and the earth wire 3 and a signal indicating a deviation between the current detection signal S and the variable reference signal R output from the current detection circuit 22 are received. A differential amplifier 23 that feeds back to the current driver 11 is provided, and MgO-LiN
It has a function of controlling the current flowing through the bO ₃ substrate 1 to a constant value indicated by the variable reference signal R.

By using the corona power supply device 5 and the corona wire 4 having the above-mentioned structure, the MgO-LiNbO ₃ substrate 1 is -600.
A constant current of μA was applied for 2 seconds. In this way MgO
-When an electric field is applied to the LiNbO ₃ substrate 1, this substrate 1
The polarization of the portion facing the Ta periodic electrode 2 is inverted, and the domain inversion portion 6 having a pattern repeating in the predetermined period Λ is formed. Here, the period Λ is MgO-LiNbO.
Considering the wavelength dispersion of the refractive index of ₃ , the thickness was set to 4 μm along the x direction of the substrate 1 so as to have a first-order period near 946 nm.

Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 2, elements that are the same as the elements in FIG. 1 are given the same numbers, and duplicated description thereof is omitted.

[0017] In the second embodiment, -z face 1b of the same MgO-LiNbO ₃ substrate 1 with those of the first embodiment
The Cr electrode 7 is attached over the entire surface. The Cr whole surface electrode 7 is formed by depositing Cr as an example to a film thickness of 30 nm. The Cr whole surface electrode 7 is connected to a power supply device 9 via a lead wire 8. This power supply device 9 is also similar to the corona power supply device 5 of the first embodiment in that MgO-Li.
It has a function of controlling the current flowing through the NbO ₃ substrate 1 to a constant value indicated by the variable reference signal R.

In this embodiment, the power supply device 9 is used to
A constant current of −1200 μA was applied to the O—LiNbO ₃ substrate 1 for 5 seconds. Thus, the MgO-LiNbO ₃ substrate 1
When an electric field is applied to the electrode, the polarization of the portion of the substrate 1 facing the Ta periodic electrode 2 is also inverted in this case, and the domain inversion portion 6 having a pattern repeating with a predetermined period Λ is formed. Here, the period Λ is set to 4 μm as in the first embodiment.

Regarding the MgO-LiNbO ₃ substrate 1 on which the domain inversion portion 6 is formed in the above first and second embodiments, the domain inversion portion 6 in the plane parallel to the + z plane 1a.
When observed under a microscope, the domain inversion was observed over the entire region in the region facing the Ta periodic electrode 2 in this plane, and domain inversion was suppressed in all other regions. It was confirmed that the uniformity was extremely high.

In both the first and second examples, no breakage of the MgO-LiNbO ₃ substrate 1 was observed.

Furthermore, it was confirmed that the methods of the first and second embodiments described above also improved the reproducibility of the domain inversion portion formation under the same conditions. That is, in the methods of the first and second embodiments, various conditions were set as described above and 10 MgO—LiNbO ₃ substrates 1 each.
When the domain inversion portion 6 is formed on the
The domain inversion portions 6 in substantially the same state were formed on the individual substrates 1. On the other hand, when the electric field is applied while the voltage applied to the MgO-LiNbO ₃ substrate 1 is controlled to be constant, the periodic electrode 2 and the corona wire 4 are used as in the first embodiment.
In both the case of using the periodical electrode 2 and the case of using the periodic electrode 2 and the whole surface electrode 7 as in the second embodiment, the number of the substrates 1 on which the domain inversion portions 6 are formed in almost the same state is 2 out of 10.

Next, an optical wavelength conversion device manufactured from the MgO-LiNbO ₃ substrate 1 having the periodic domain inversion structure formed in the first embodiment will be described. By polishing the x-plane and the -x-plane of the substrate 1 to form the light transmitting surfaces 10a and 10b, respectively, the periodic domain reversing portion 6 is formed as shown in FIG.
A bulk crystal type optical wavelength conversion device 10 having the following is obtained.
This bulk crystal type optical wavelength conversion element 10 was placed in the resonator of the laser diode pumped YAG laser shown in FIG.

This laser diode pumped YAG laser has a laser diode 12 which emits a laser beam 11 as pumping light having a wavelength of 809 nm, a condenser lens 13 which converges the laser beam 11 in a divergent light state, and Nd.
A YAG crystal 14 which is a laser medium doped with (neodymium) and is arranged at the converging position of the laser beam 11, and a resonator mirror 15 arranged on the front side (right side in the figure) of the YAG crystal 14. Consists of. The light wavelength conversion element 10 has a crystal length of 1 mm and is arranged between the resonator mirror 15 and the YAG crystal 14.

The YAG crystal 14 is excited by the laser beam 11 having a wavelength of 809 nm and emits a laser beam 16 having a wavelength of 946 nm. The solid-state laser beam 16 resonates between the YAG crystal end face 14a provided with a predetermined coating and the mirror face 15a of the resonator mirror 15, and the optical wavelength conversion element 10
Second harmonic of wavelength ½ or 473 nm
It is converted to 17, and only the second harmonic 17 is emitted from the resonator mirror 15. The solid-state laser beam 16 as the fundamental wave and the second harmonic wave 17 are phase-matched (so-called pseudo-phase matching) in the periodic domain inversion region.

In this example, when the output of the laser diode 12 is 200 mW, the second harmonic 17 having a high output of 10 mW is generated.
was gotten. Similarly create the optical wavelength conversion element from the periodic domain inversion MgO-LiNbO ₃ substrate 1 the structure is formed in the second embodiment, is applied to the light wavelength converting element to the laser diode pumped YAG laser of FIG. 3 , The efficiency in that case was also the same as above.

On the other hand, as described above, an optical wavelength conversion device manufactured from a MgO-LiNbO ₃ substrate on which a periodic domain inversion structure was formed by applying an electric field while controlling a constant voltage was manufactured by using the same laser diode pumped YAG as described above. When applied to a laser, the laser diode 12 output is 20
At 0 mW, the output of the second harmonic 17 was 1 mW at the maximum.

As described above, since the optical wavelength conversion element formed by applying the method of the present invention realizes extremely high wavelength conversion efficiency, the domain inversion portion 6 is uniformly formed according to the method of the present invention. Proven to get.

The method of the above embodiment is applied for producing a bulk crystal type optical wavelength conversion element, but the method of the present invention is also applicable to the production of an optical waveguide type optical wavelength conversion element. It is also possible to apply for.

Further, the method of the present invention comprises LiNbO ₃ , MgO-L in addition to MgO-LiNbO ₃ in the above-mentioned embodiment.
The same can be applied when forming a domain inversion structure in a ferroelectric substance such as iTaO ₃ , LiTaO ₃ , KNbO ₃ , or KTP.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating a method of forming a domain inversion structure according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a method of forming a domain inversion structure according to a second embodiment of the present invention.

FIG. 3 is a side view of a solid-state laser including an optical wavelength conversion element in which a domain inversion structure is formed according to the present invention.

[Explanation of symbols]

1 MgO-LiNbO ₃ Substrate 1a MgO-LiNbO ₃ Substrate + z Surface 1b MgO-LiNbO ₃ Substrate -z Surface 2 Ta Periodic Electrode 3 Earth Wire 4 Corona Wire 5 Corona Power Supply 6 Domain Inversion Section 7 Cr Full Surface Electrode 8 Lead Wire 9 Power supply device 10 Optical wavelength conversion element 11 Laser beam (pumping light) 12 Laser diode 13 Condensing lens 14 YAG crystal 15 Resonator mirror 16 Solid-state laser beam (fundamental wave) 17 Second harmonic wave 21 Current driver 22 Current detection circuit 23 Differential amplifier

Claims (3)

[Claims] 1. An electrode having a predetermined pattern is formed on one surface of a ferroelectric substance having a non-linear optical effect that is monopolarized, and the one surface of the ferroelectric substance and the other surface facing it are formed through this electrode. In the method for forming a domain inversion structure of a ferroelectric in which an electric field is applied between and to locally invert the portion of the ferroelectric facing the electrode, a current flowing in the ferroelectric is controlled to be constant. A method for forming a domain inversion structure of a ferroelectric material, characterized by applying an electric field while being applied. 2. A corona wire is disposed at a position facing the other surface of the ferroelectric substance, and the ferroelectric substance is corona-charged by the corona wire and the electrode and an electric field is applied thereto. The method for forming a domain inversion structure of a ferroelectric according to claim 1. 3. The full-scale electrode is formed on the other surface of the ferroelectric material, and the electric field is applied through the electrode facing the electrode of the predetermined pattern and the full-scale electrode. Method of forming domain inversion structure of ferroelectric substance.