Method and system for polling ferroelectric medium

Abstract

To provide a method for polling a ferroelectric medium.SOLUTION: The method includes the steps of: preparing a ferroelectric medium; abutting a first contact surface of a first electrical contact to a first side section, the first contact surface being smaller than the first side section; abutting a second contact surface of a second electrical contact to a second side, the second contact surface covering at least a second side section; applying a voltage between the first electrical contact and the second electrical contact; performing a relative movement between the first contact surface and the ferroelectric medium in two mutually perpendicular directions such that the first contact surface sweeps the first side section; sensing a measurement value of a current flowing between the first electrical contact and the second electrical contact through the ferroelectric medium; and controlling the voltage such that a deviation of the current from a nominal polling current is minimized. The second electrical contact is an ionic conductor.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method of poling a ferroelectric medium having a first side and a second side located on the opposite side of the first side, such that the ferroelectric medium includes a first volume segment and a second volume segment, wherein the spontaneous polarization of the ferroelectric medium includes a first orientation in the first volume segment and a second orientation in the second volume segment, and the first orientation and the second orientation are different from each other. The first volume segment is defined by a first side section of the first side and a second side section of the second side, and the second volume segment is defined by a third side section of the first side and a fourth side section of the second side. The method comprises the steps of preparing the ferroelectric medium; contacting a first contact surface of a first electrical contact with the first side section, wherein the first contact surface is smaller than the first side section; contacting a second contact surface of a second electrical contact with the second side, wherein the second contact surface covers at least the second side section; applying a voltage between the first electrical contact and the second electrical contact; performing relative movement between the first contact surface and the ferroelectric medium in two mutually perpendicular directions such that the first contact surface sweeps the first side section; detecting a measured value of a current flowing between the first electrical contact and the second electrical contact through the ferroelectric medium; and controlling the voltage such that a deviation of the current from a nominal poling current is minimized.

[0002] The present invention also relates to a system for poling a ferroelectric medium having a first side and a second side located opposite the first side such that the ferroelectric medium includes a first volume segment and a second volume segment.The system includes a mount for holding a ferroelectric medium during operation of the system; a first electrical contact having a first contact surface, the first contact surface being arranged and positioned such that it can abut a first side surface section of the first side surface of the ferroelectric medium held on the mount during operation of the system; a second electrical contact having a second contact surface, the second contact surface being arranged and positioned such that it can abut a second side surface section of the second side surface of the ferroelectric medium received within the mount during operation of the system; a voltage source connected to the first and second electrical contacts such that a voltage can be applied between the first and second contact surfaces during operation of the system; a current measuring device arranged and positioned to detect a measured value of the current flowing between the first electrical contact and the second electrical contact through the ferroelectric medium during operation of the system; an actuator arranged and positioned to cause relative movement between the first contact surface and the mount during operation of the system; and a control and adjustment device operably connected to the actuator such that the actuator receives an actuator control signal from the control and adjustment device during operation of the system, operably connected to the current measuring device such that the current measuring device receives the measured value of the current from the current measuring device during operation of the system, operably connected to the voltage source such that the voltage source receives a voltage control signal from the control and adjustment device during operation of the system, and operably connected to the voltage source such that the voltage source is operably connected to the control and adjustment device during operation of the system, generating and outputting the actuator control signal such that the first contact surface sweeps the first side surface section of the ferroelectric medium received on the mount during operation of the system, and generating and outputting the voltage control signal such that the deviation between the value of the current and a nominal polling current is minimized during operation of the system.

Background Art

[0003] Ferroelectric media, especially ferroelectric crystals, often exhibit high non-linearity when interacting with electromagnetic fields. Ferroelectric media also have the advantage that they can be poled or repoled for each section based on a certain uniform polarity of each medium.

[0004] This possibility of forming domains or volume segments for each section with different, preferably opposite, orientations of spontaneous polarization makes the ferroelectric medium suitable for quasi-phase matching. In a non-linear optical process, when one or more electromagnetic fields pass through a propagation medium having a non-linear susceptibility, one or more new electromagnetic fields having different frequencies are generated. In order to successfully achieve this efficiently, the generated fundamental waves need to be in phase, that is, have a certain relationship between their vibrations. Only then will destructive interference not occur and the conversion efficiency be maintained high. Even in a medium that does not satisfy this phase matching condition, it is possible to effectively avoid destructive interference by aligning or reversing the orientation of spontaneous polarization in different sections of the non-linear medium. The phase of the fundamental wave generated from the repoled volume segment experiences a phase jump compared to the fundamental wave generated from the non-repoled volume segment. This enables the constructive addition of electromagnetic fields over a long propagation distance. Many methods for poling or repoling ferroelectric media are known from the prior art. All of them are based on the premise that a ferroelectric medium with the same orientation of spontaneous polarization is provided over the entire length of the medium. The individual volume segments will be selectively poled or repoled by applying an electric field so as to have an orientation of spontaneous polarization deviating from the initially specified orientation.

[0005] The aim is to increasingly realize more complex poling structures in ferroelectric materials. Such more complex structures are difficult to realize using conventional poling methods, for example, poling using electrode masks applied to the opposing first and second sides of each medium.

[0006] It has been proven that the graphic polling is very suitable for providing a complex spatial polling structure in a ferroelectric medium. In this process, the contact surface of the first electrical contact, preferably in the shape of a needle, moves on the first side so as to sweep over individual surface sections that delimit the volume segment to be polled. The second side of the ferroelectric medium, opposite the first side, is usually completely covered by a flat, planar electrode. The polling structure is thus "written" to some extent into the ferroelectric medium.

[0007] This graphic polling known from the prior art gives good results in thin ferroelectric media but fails in the polling of thick media. The ferroelectric medium under consideration clearly exhibits anisotropic polling behavior. This means that the voltage required for polling between the two electrodes varies significantly along the direction of movement of the first contact surface on the first side of the ferroelectric medium.

[0008] Since the anisotropic polling behavior of the ferroelectric medium can be compensated by adjusting the voltage between the first and second electrical contacts as a function of the current flowing between the first and second electrical contacts, a predetermined target current is maintained and the current flowing along the movement path of the first contact surface is basically constant. However, unexpectedly, this compensation is only successful for relatively thin ferroelectric media. Summary of the Invention Problems to be Solved by the Invention

[0009] In contrast, it is an object of the present invention to provide a method and apparatus for polling a ferroelectric medium that is also suitable for thick ferroelectric media. Furthermore, it is an object of the present invention to provide a method and apparatus for polling a ferroelectric medium that can provide a complex polling structure. Means for Solving the Problems

[0010] At least one of the above-mentioned objects is solved by a method for poling a ferroelectric medium according to appended independent claim 1. For this purpose, in a method of the type described initially, the second electrical contact is an ion conductor. Then, the detected measured value of the current flowing between the first electrical contact and the second electrical contact becomes the measured value of the poling current.

[0011] The poling current refers to the current flowing through the ferroelectric medium by re-poling. The poling current is proportional to the reversed polarity per unit time. The region to be considered here is the region of the surface section of the first side bounded by the volume segment of the re-poled and re-poled volume that has been reversed over a certain period. The poling current in complete re-poling is shown as follows, assuming that the original spontaneous polarization and the reversed spontaneous polarization are 180 degrees opposite to each other.

Number

Number

[0012] It has been found that metal conductors used for the second electrical contact in the prior art do not produce the desired result, i.e., a uniform re-poled volume segment, at least in a thick ferroelectric medium. This is due to the fact that metal conductors based on electronic conduction form an ohmic contact with the second side of the ferroelectric medium in the domain wall region of the ferroelectric medium. However, an ohmic contact means that the current flowing between the contacts through the ferroelectric medium and recorded for the adjustment of the poling voltage is mainly determined by the parasitic current along the domain wall formed between the re-poled volume segment and the non-re-poled volume segment. These domain walls are highly conductive. Therefore, the current flowing between the first and second electrical contacts measured in the prior art consists of a poling current and a parasitic current along the domain wall. Therefore, in the case of an ohmic contact in the region of the domain wall, the voltage is controlled not by the poling current but rather by the parasitic current along the domain wall. In this way, it is impossible or insufficient to compensate for the anisotropic poling behavior of the ferroelectric medium.

[0013] Therefore, the present invention is based on the idea of forming the second electrical contact as an ion conductor. The ion conductor forms a non-resistive diode-like contact with the ferroelectric medium, at least in the region of the domain wall. This suppresses the parasitic current caused by the domain wall and makes it possible to control the voltage with respect to the poling current.

[0014] The first and second volume segments of the ferroelectric medium are often referred to as domains of the medium or crystal. Here, a domain of the ferroelectric medium refers to a continuous volume segment of the ferroelectric medium in which the spontaneous polarization of the ferroelectric medium has the same orientation. The first and second side sections of the first volume segment on the opposing first and second sides of the ferroelectric medium define, i.e., delimit, the first volume segment. The third and fourth side sections of the second volume segment on the opposing first and second sides of the ferroelectric medium define, i.e., delimit, the second volume segment.

[0015] To promote the growth of domains, a voltage is applied between two contact surfaces. The polarity of the voltage is determined by the orientation of the medium, and the voltage level is determined by the thickness and coercive force of the polarized ferroelectric medium. According to the present invention, the polarity is also determined by the blocking direction of the diode-like contact between the second electrical contact and the second side surface in the region of the domain wall. Both a steady voltage and a time-varying pulsed voltage can be applied. In one embodiment of the present invention, the voltage is 1000 volts or more.

[0016] It will be appreciated that the voltage must be applied between the first and second electrical contacts at a plurality of or all points swept by the first contact surface within the first side section.

[0017] For the purposes of the present application, the length of the ferroelectric medium or volume segment is defined as its extension in the desired propagation direction of electromagnetic radiation within the ferroelectric medium. This propagation direction is also referred to as the longitudinal direction of the ferroelectric medium. The thickness refers to the extension of the ferroelectric medium in the thickness direction, which is perpendicular to the longitudinal direction and perpendicular to the second contact surface. The thickness direction usually coincides with the orientation of the spontaneous polarization. The width of the ferroelectric medium is its extension in the width direction, which is perpendicular to the longitudinal direction and perpendicular to the thickness direction.

[0018] In one embodiment of the present invention, the surface normals of the first and second side surfaces of the ferroelectric medium are parallel to each other but face in opposite directions.

[0019] In one embodiment, such a ferroelectric medium has a thickness perpendicular to the second contact surface of 500 μm or more.

[0020] The first side section that defines the first volume segment on the first side is understood to be smaller than the first side. The second side section that defines the first volume segment on the second side is understood to be smaller than the second side. Preferably, the first and second side sections of the first volume segment are substantially coincident with each other.

[0021] In one embodiment of the present invention, the relative movement between the first contact surface of the ferroelectric medium and the first side section is continuously performed in two mutually perpendicular directions. In one embodiment of the present invention, the relative movement is performed line by line. In one embodiment, the first contact point with the first contact surface first moves in a first direction along a first line. Next, the first contact surface moves in a second direction perpendicular to the first direction to a second line, and then moves again in the first direction or in the opposite direction along the second line.

[0022] In an alternative embodiment, the relative movement between the first contact surface of the ferroelectric medium and the first side section is performed along a single, preferably straight line. If the ferroelectric medium includes a plurality of first side sections, in one embodiment, the first contact surface performs relative movement along exactly one, preferably straight line, for each first side section. In one embodiment, the preferably straight line extends in the thickness direction of the ferroelectric medium. In one embodiment, the straight line extends at an angle other than 90 degrees with respect to the longitudinal direction. Nevertheless, the relative movement of the first side section along a single line enables the repoling of the first volume segment having a finite and longitudinally defined extension by the path of the electric field lines extending from the first contact surface to the second contact surface.

[0023] In one embodiment, the voltage is controlled such that the measured current along the line of relative movement equals a constant nominal polling current. In an alternative embodiment, the voltage is controlled such that the measured current along the line of relative movement equals a nominal polling current that varies along the line. By changing the nominal polling current along a line in the thickness direction of the ferroelectric medium, it is possible to write a first volume segment having a first side section in a single sweep of the ferroelectric medium, where this first side section includes a length that varies in the longitudinal direction of the ferroelectric medium. For example, in such a way, it becomes possible to re-poll a first volume segment where the first side section is trapezoidal.

[0024] Sequential “writing” of the first side section onto the first side of the first volume segment enables the design of complex polling structures. An example of such a complex polling structure is a polled ferroelectric medium where the polling period varies in the width direction of the medium.

[0025] In one embodiment of the present invention, the relative movement between the first contact surface and the ferroelectric medium is performed such that the first side section becomes trapezoidal. In the case of such a trapezoidal first side section, it is understood that the relative movement between the first electrical contact or its first contact surface and the ferroelectric medium must be performed in two mutually perpendicular directions.

[0026] In one embodiment of the present invention, the first electrical contact is formed by a needle, and preferably the first contact surface is the tip of the needle. In one embodiment of the present invention, the first electrical contact is a needle made of a hard metal.

[0027] In one embodiment, care should be taken to ensure that the needle-shaped element forms the smoothest possible first contact surface.

[0028] In one embodiment, the first side section of the ferroelectric medium that defines the re-polled volume segment extends beyond the region of the first side that is actually swept by the electric field lines, so in one embodiment of the present invention, the first electrical contact only partially sweeps the first side section. In one embodiment of the present invention, the first electrical contact completely sweeps the entire first side section. In one embodiment of the present invention, the measured value of the current is measured indirectly, and the current between the second contact and ground is used as the indirect measurement value. Since there is no current sink or current source in the ferroelectric medium, the current between this second contact and ground also corresponds to the polling current through the ferroelectric medium according to the present invention.

[0029] Bringing the first or second contact surface into contact with the respective first or second side section includes mechanically contacting the respective contact surface with the side section, so when a voltage is applied between the first electrical contact and the second electrical contact, an electric field spreads between the first contact surface and the second contact surface.

[0030] In principle, the method according to the present invention is suitable for generating a ferroelectric medium with exactly two volume segments, namely a first volume segment and a second volume segment, having different spontaneous polarization orientations. In this case, re-polling of the ferroelectric medium is only induced within the first volume segment.

[0031] However, the structure for quasi-phase matching mainly consists of a large number of domains or volume segments. Domains with different spontaneous polarizations are positioned alternately. In one embodiment, such an arrangement of volume segments is periodic, i.e., in the direction of the electromagnetic radiation beam propagating within the ferroelectric medium, the first and second volume segments form a repeating structure. However, embodiments are also possible in which a plurality of first and second volume segments are positioned aperiodically. In one embodiment, the first and second volume segments having different spontaneous polarizations have the same length in the longitudinal direction.

[0032] Thus, in one embodiment, after implementing the method according to the invention, the ferroelectric medium comprises a plurality of first volume segments and a plurality of second volume segments, and the first and second volume segments are arranged alternately in the beam direction. It is understood that in order to generate a polarized ferroelectric medium comprising a plurality of first volume segments and a plurality of second volume segments, the plurality of first volume segments are formed or written by the steps defined in the independent claims.

[0033] In one embodiment, the second contact surface of the second electrical contact covers only the second side section of a specific second volume segment or the second side sections of all second volume segments. In one embodiment, the second contact surface of the second electrical contact further covers the fourth side section of a specific second volume segment or the fourth side sections of all second volume segments.

[0034] In one embodiment of the invention, during the step of performing relative movement with respect to the ferroelectric medium, the first electrical contact begins to vibrate, such that the first contact surface vibrates at least within the plane of the first side or perpendicular to the plane of the first side. Such a vibratory movement reduces the static friction between the electrical contact and the side of the ferroelectric medium during the sweeping of the first side section.

[0035] In one embodiment of the invention, the method further comprises the step of performing relative movement between the first contact surface and the ferroelectric medium, wherein the ferroelectric medium is preferably heated to a temperature above room temperature (21 °C). The temperature of the ferroelectric medium at which repoling is performed is also referred to as the poling temperature. In one embodiment of the invention, the ferroelectric medium is heated to 50 °C or higher, preferably 100 °C or higher. In one embodiment of the invention, the temperature of the ferroelectric medium heated during repoling is lower than the Curie temperature of each ferroelectric medium.

[0036] By writing a poling structure into a high-temperature ferroelectric medium, the anisotropy of poling is reduced. However, without further countermeasures, heating requires that the second contact be a metal conductor capable of withstanding the temperature rise during the period of re-poling. When using an ion conductor according to the present invention, it is important to prevent the evaporation of the ion conductor.

[0037] The anisotropic poling behavior of the ferroelectric medium can be almost completely compensated by adjusting the voltage to the poling current simultaneously with heating.

[0038] In one embodiment of the present invention, the ferroelectric medium can be heated using an electromagnetic radiation source, for example, an infrared lamp, a microwave oven, a hot bath, or a contact heating plate.

[0039] In one embodiment of the present invention, the second electrical contact contains a solid or liquid electrolyte. An electrolyte is a compound that dissociates into ions in a solid or liquid state and moves directionally under the influence of an electric field. The conduction of current in such an electrolyte is due to ions. The conductivity of such an ion conductor is usually lower than that of a metal by electron conduction. Therefore, an electrolyte is also called a second-class conductor.

[0040] All solids and liquids containing mobile ions at the temperature at which re-poling is performed can be used as electrolytes.

[0041] In one embodiment of the present invention, the electrolyte is NaCl, LiCl or CaCl 2 is.

[0042] In one embodiment of the present invention, the second electrical contact contains a liquid solvent.

[0043] In one embodiment of the present invention, the liquid solvent is water or glycerol.

[0044] In one embodiment of the present invention, the electrolyte solution in the liquid has a boiling point higher than the poling temperature.

[0045] Examples of suitable liquid ionic conductors include salt solutions with glycerol as the solvent, salts as electrolytes, such as NaCl, LiCl, CaCl 2 are mentioned. In one embodiment, the ionic conductor is H 2 a saturated solution of CaCl as an electrolyte with O as the solvent. Such salt solutions have a boiling point exceeding 200 °C. 2

[0046] The ferroelectric medium in the sense of this application particularly includes ferroelectric nonlinear optical crystals such as, for example, lithium niobate, lithium tantalate, potassium titanyl phosphate or magnesium barium fluoride.

[0047] In one embodiment of the invention, the ferroelectric medium is an oxide crystal or a fluoride crystal. Examples of oxide crystals include LiNbO 3 , LiTaO 3 , KTiOPO 4 , LaBGeO 5 . Examples of fluoride crystals include BaMgF 4 .

[0048] In one embodiment, lithium niobate or lithium tantalate is doped with MgO. In one embodiment, lithium niobate or lithium tantalate is eutectic or stoichiometric.

[0049] In one embodiment of the invention, the method further includes the step of applying oil to a first side around the first electrical contact. Such oil is used to prevent corona discharge in the vicinity of the first electrical contact. Examples of suitable oils include silicone oils such as transformer oil. Such oils have a boiling point higher than the polishing temperature.

[0050] In an alternative embodiment, the environment of the ferroelectric medium is exposed to a suitable inert gas such as sulfur hexafluoride (SF 6 ).

[0051] ​At least one of the above objects is also solved by a system for poling a ferroelectric medium described in the appended independent claims directed thereto. For this purpose, a system of the type initially described includes an ion conductor as a second electrical contact.

[0052] As long as aspects of the present invention have been described above with respect to a method for poling a ferroelectric medium, these are also applicable to a system for poling a corresponding ferroelectric medium, and vice versa. As long as the method is implemented using a system according to the present invention, this system includes a corresponding device for this purpose. In particular, embodiments of the present system are suitable for implementing embodiments of the method described above.

[0053] The ion conductor of the second electrical contact is formed by impregnating a non-woven fabric with a liquid solution of an electrolyte and a solvent. Thereafter, the ferroelectric medium is placed on this impregnated non-woven fabric. The non-woven fabric is located under the non-woven fabric and prevents direct electrical conductive contact with the metal electrode necessary for applying a voltage to the ion conductor.

[0054] Further advantages, features, and possible uses of the present invention will become apparent from the following description of the embodiments and the associated drawings. In the drawings, similar elements are denoted by the same reference numerals.

Brief Description of the Drawings

[0055]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0056] A method for poling a ferroelectric medium 2 according to the present invention will be described with reference to two embodiments of the system 1. The purpose of the method described here as an illustration is to write a complex poling structure into a non-linear optical lithium niobate crystal 2 as the ferroelectric medium.

[0057] Figures 1 and 2 each show a schematic side view of a system for poling a lithium niobate crystal 2. Each of Figures 1 and 2 shows a top view from the side facing the longitudinal direction of the crystal 2.

[0058] In Figures 1 and 2, in the crystal 2, only a single first volume segment or first domain 3 and two adjacent second volume segments or second domains 10 are shown as examples.

[0059] For the purposes of the present invention, the thickness of the crystal is defined as an extension in the thickness direction indicated by reference numeral 4. In contrast, the length of the crystal is measured in the longitudinal direction 5. The beam axis of the electromagnetic radiation when using the crystal 2 also extends in the longitudinal direction 5. When the poled crystal 2 is used in a non-linear optical process, the electromagnetic radiation will propagate through the crystal 2 from left to right or vice versa in the longitudinal direction 5.

[0060] Each of the crystals 2 comprises a first side surface 6 and a second side surface 7. Each first domain 3 is bounded on the first side surface 6 by a first side surface section 8 and on the second side surface 7 by a second side surface section 9. In other words, the first and second side surface sections 8, 9 limit the volume of the first domain 3 in the longitudinal direction 5 and in the width direction 19 perpendicular to the longitudinal direction 5 and the thickness direction 4.

[0061] After implementing the method according to the present invention, each of the crystals 2 has a periodic poling structure, that is, the first and second domains 3, 10 are regularly repeated. Further, in the illustrated embodiment, in a cross-sectional plane parallel to the longitudinal direction 5 of the crystal 2 and perpendicular to the width direction 19, all the first and second domains 3, 10 have the same length. This is clearly understood from FIG. 3, which is a top view of the crystal 2 in FIG. 3.

[0062] FIG. 3 shows a possible complex poling structure of the crystal 2 that can be generated using the method and the system. In the embodiment of FIG. 3, the poled crystal 2 has a periodic poling structure in which the first and second domains 3, 10 are alternately arranged, and the lengths of the first and second domains 3, 10, and thus the periodicity of the poling structure, also vary in the width direction 7 of the crystal 2.

[0063] For poling, a voltage is applied between a first contact 11 having a first contact surface 12 on a first side surface 6 of the crystal 2 and a second contact 13 on a second side surface 7. By applying a voltage between the two contacts 11, 13, the spontaneous polarization of the crystal in the first domain 3 is permanently oriented so as to be different from the original spontaneous polarization of the crystal 2. However, this original spontaneous polarization is retained in the second domain 10. In the illustrated embodiment, after poling or re-poling of the first domain 3, the spontaneous polarization of the crystal in the first domain 3 and the spontaneous polarization in the second domain 10 are opposite to each other.

[0064] The basic idea of the method and system 1 of the present invention is to enable poling of a crystal having a thickness exceeding 500 μm in the thickness direction 4 so that the anisotropic poling behavior of the crystal 2 can be compensated. Without further measures, the electric field between the first and second contacts 11, 13 is necessary to re-pole the crystal 2 within the volume segment 3 defined by the first and second side sections 6, 7, and varies along the movement path of the first contact surface 12 within the first side section 8.

[0065] When repolling the first domain 3, the first contact surface 12 of the first contact 11 sweeps each first side section 8 of the first side surface 6 of the crystal 2 exactly along one straight line 21. In the illustrated embodiment, the needle forms the first contact 11. Such an embodiment enables a complex polling structure, as described in connection with FIG. 3 for example, to be “written” without the polling anisotropy of the crystal 2 causing a non-uniform repolling of the spontaneous polarization within the first domain 3. To ensure that the first side section 8 has a trapezoidal shape, i.e., a shape with a varying length in the longitudinal direction 5, the nominal polling current changes during the relative movement even though the first side section 8 is swept only once along the line 21.

[0066] To compensate for the polarity anisotropy of the crystal 2, the method according to the invention relies on current control of the voltage applied between the first contact 11 and the second contact 13. The control loop is arranged such that the current is adjusted to match the nominal polling current which varies in the thickness direction 7 of the crystal 2. For this purpose, an ammeter 14 is used as a current measuring device to measure the current flowing between the first contact 11 and the second contact 13. This measurement of the current is taken between the ground 15 and the second contact 13. Assuming that the crystal does not contain a charge carrier source or a charge carrier sink, this current between the second contact 13 and the ground 15 depends directly and exclusively on the polling current flowing through the crystal 2 itself according to the invention. Here, the control and adjustment device 16 adjusts the voltage source 17 such that the measured value of the polling current measured by the ammeter 14 contains a minimum deviation from the specified nominal polling current.

[0067] In the prior art, it has been proven that a problem exists in that a second contact in the form of an electronic conductor, such as a metal layer on the second side 7 of the crystal 2, forms an ohmic contact with the crystal 2 in the region of the domain wall. However, in this case, the measured current is not the polling current, but is strongly distorted because the current conduction in the crystal 2 is determined by the flow of current along the interface between the domain wall, that is, the boundary between the first domain 3 and the adjacent second domain 10. In contrast, the second contact 13 in the system 1 according to the present invention is an ion conductor. Such an ion conductor provides a diode-like contact in the region of the domain wall, so that if the polarity is appropriate, no significant parasitic current flows to the second contact 13 along the domain wall. As a result, the measured current is substantially equal to the polling current.

[0068] In the illustrated embodiment, the ion conductor is a solution composed of lithium chloride as an electrolyte and glycerol as a solvent. Since this solution for the electrical contact 13 has a relatively high boiling point, the crystal 2 can be heated up to 150 °C during the writing of the polling structure without the second electrical contact 13 evaporating significantly during repolling.

[0069] In one embodiment of the system 1 shown in FIG. 2, an oil film 18 is also applied to the first side 6 of the crystal. This prevents the occurrence of corona discharge in the region of the first contact surface 12 of the first contact 11.

[0070] For the purposes of the original disclosure, as will be apparent to those skilled in the art from the present specification, drawings and claims, all features, even if specifically described only in relation to certain further features, are either explicitly excluded or, unless the technical situation makes such a combination impossible or meaningless, can be combined individually or in any combination with other features or groups of features disclosed herein. An exhaustive and explicit description of all possible combinations of features is omitted here only for the sake of brevity and readability of the description.

[0071] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, this illustration and description are shown by way of example only and are not intended to limit the scope of protection defined by the claims. The present invention is not limited to the disclosed embodiments.

[0072] Modifications of the disclosed embodiments will be apparent to those skilled in the art from the drawings, the specification, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. The reference numbers in the claims are not intended to limit the scope of protection.

Description of Reference Numerals

[0073] 1 System 2 Lithium Niobate Crystal 3 First Domain 4 Thickness Direction 5 Longitudinal Direction 6 First Side 7 Second Side 8 First Side Section 9 Second Side Section 10 Second Domain 11 First Contact 12 First Contact Surface 13 Second Contact 14 Galvanometer 15 Ground 16 Control and Adjustment Device 17 Voltage Source 18 Oil Film 19 Thickness Direction 20 Second Contact Surface 21 Straight Line

Claims

1. 1. A method for poling a ferroelectric medium (2) having a first side (6) and a second side (7) opposite the first side (6), such that the ferroelectric medium (2) comprises a first volume segment (3) and a second volume segment (10), comprising: the spontaneous polarization of the ferroelectric medium (2) comprises a first orientation in the first volume segment (3) and a second orientation in the second volume segment (10); the first orientation and the second orientation are different from each other; said first volume segment (3) being defined by a first side section (8) of said first side (6) and a second side section (9) of said second side (7); the second volume segment (10) is defined by a third side section of the first side (6) and a fourth side section of the second side (7); The method comprises: providing the ferroelectric medium (2); abutting a first contact surface (12) of a first electrical contact (11) against said first side section (8), said first contact surface (12) being smaller than said first side section (8); abutting a second contact surface (20) of a second electrical contact (13) against said second side surface (7), said second contact surface (20) covering at least said second side surface section (9); applying a voltage between the first electrical contact (11) and the second electrical contact (13); providing a relative movement between the first contact surface (12) and the ferroelectric medium (2) such that the first contact surface (12) sweeps the first side section (8); sensing a measurement of a current flowing through the ferroelectric medium (2) between the first electrical contact (11) and the second electrical contact (13); and controlling said voltage such that deviation of current from a nominal poling current is minimized. The method as described above, characterized in that the second electrical contact (13) is an ionic conductor.

2. 2. The method of claim 1, further comprising the step of heating the ferroelectric medium (2), preferably heating the ferroelectric medium (2) to a temperature of 50 degrees Celsius or higher, during the step of performing relative movement between the first contact surface (12) and the ferroelectric medium (12).

3. 3. The method according to claim 1 or 2, wherein the relative movement between the first contact surface (12) and the ferroelectric medium (2) is performed such that the first side section (8) is trapezoidal.

4. 4. The method according to claim 1, wherein the relative movement between the first contact surface (12) and the ferroelectric medium (2) is performed such that the first contact surface (12) sweeps the first side section (8) just along a preferably straight line (21).

5. The method according to any one of claims 1 to 4, wherein the nominal poling current varies along the line (21).

6. The method of any of claims 1 to 5, wherein the second electrical contact (13) comprises a solid or liquid electrolyte.

7. The method of any one of claims 1 to 6, wherein the second electrical contact (13) comprises a liquid solvent.

8. The method of any one of claims 1 to 7, wherein the solvent is glycerol.

9. 9. The method of claim 6, wherein the electrolyte is lithium chloride.

10. The method according to any of the preceding claims, wherein the ferroelectric medium (2) is an oxide crystal or a fluoride crystal.

11. The method according to any one of claims 1 to 10, further comprising the step of applying oil to the first side (6) around the first electrical contact (11).

12. 1. A system (1) for poling a ferroelectric medium (2) having a first side (6) and a second side (7) opposite the first side (6), such that the ferroelectric medium (2) comprises a first volume segment (3) and a second volume segment (10), comprising: a mount for holding the ferroelectric medium (2) during operation of the system (1); a first electrical contact (11) having a first contact surface (12), the first electrical contact (11) being arranged and positioned such that the first contact surface (12) can abut a first side section (8) of the first side (6) of the ferroelectric medium (2) held on the mount during operation of the system (1); a second electrical contact (13) having a second contact surface (20), the second electrical contact (13) being arranged and positioned such that the second contact surface (20) can abut a second side section (9) of the second side (7) of the ferroelectric medium (2) held within the mount during operation of the system (1); a voltage source connected to the first electrical contact (11) and the second electrical contact (13) such that a voltage can be applied between the first and second contact surfaces (12, 20) during operation of the system (1); a current measuring device (14) arranged and positioned to sense a measurement of a current flowing through the ferroelectric medium (2) between the first electrical contact (11) and the second electrical contact (13) during operation of the system (1); and an actuator arranged and positioned to cause relative movement between the first contact surface (11) and the mount during operation of the system (1); a control and regulation device (16) operatively connected to the actuators such that, during operation of the system (1), the actuators receive actuator control signals from the control and regulation device (16); operatively connected to the current measurement device (14) to receive measurements of the polling current from the current measurement device (14) during operation of the system (1); operatively connected to said voltage source (17) such that during operation of said system (1), said voltage source (17) receives a voltage control signal from said control and regulating device (16); configured to generate and output the actuator control signals such that, during operation of the system (1), the first contact surface (12) sweeps the first side section (8) of the ferroelectric medium (2) held on the mount; and a control and regulating device (16) for generating and outputting the voltage control signal during operation of the system (1) such that the deviation of the value of the current from a nominal polling current is minimized. The system (1), wherein the second electrical contact (13) is an ion conductor.

13. The system of any one of claims 1 to 12, wherein the first electrical contact (11) is a needle and the first contact surface (12) is a tip of the needle.

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