Magnetometer comprising a sensor crystal

Abstract

The invention relates to a magnetometer (1) for detecting a magnetic field, comprising an excitation light source (2) which is designed to emit an excitation beam (3), a sensor crystal (4) which has at least one magneto-optical defect, and a device (8) for generating a microwave field. The sensor crystal (4), the device (9) for generating the microwave field, and the excitation light source (2) are designed and arranged relative to one another such that the excitation light beam (3) emitted by the excitation light source (2) produces an emission of fluorescence radiation in the at least one magneto-optical defect of the sensor crystal (4), and the microwave field can be detected in the sensor crystal (4). The magnetometer (1) additionally has a detector (5), which is designed and arranged such that the detector detects the emitted fluorescence radiation, and at least two permanent magnets (6), which are arranged and designed so as to generate an at least approximately homogenous static magnetic field in the sensor crystal (4). The sensor crystal (4) and the at least two permanent magnets (6) are arranged on a common positioning element (7).

Description

[0001] Description

[0002] Title MAGNETOMETER WITH A SENSOR CRYSTAL

[0003] The invention relates to a magnetometer for detecting a magnetic field and a method for operating such a magnetometer.

[0004] State of the art

[0005] To measure a magnetic field, so-called NV magnetometers are used. These consist of a diamond whose crystal lattice has defects in the form of NV centers. In an NV center, a nitrogen atom occupies the lattice site of a carbon atom, with a defect located directly adjacent to the nitrogen atom - again on the lattice site of a carbon atom. If such a crystal lattice is irradiated with excitation radiation with a wavelength between 490 nm and 575 nm, an electronic transition from a ground state to a 3 A2 into an excited state 3E induced. From the excited state 3 E relaxes the NV center back to the ground state by emitting fluorescence radiation in a wavelength range between 650 nm and 750 nm 3 A2. The ground state 3 A2 has three magnetic substates with m s =0, m s =±1. The states with m s =0 and m s =±1 are distinguished by an energy difference of 2.87 GHz (zero field splitting). The excited state 3 E also has three magnetic substates with m s =0, m s =±1. If the NV center in the ground state 3 A2 is exposed to a microwave field with a frequency of 2.87 GHz, the NV center oscillates between the m s =0, 3 A2 - Ground state and the m s =±1 , 3 A2 -ground state. Upon irradiation with the excitation radiation, the NV center is now partially removed from the m s =±1 , 3A2- Ground state in the excited m s =±1 , 3 E state. From there, it relaxes back to the ground state, predominantly without radiation. If the amplitude of the fluorescence radiation is measured as a function of the frequency of the microwave field, a sudden drop in the amplitude of the fluorescence radiation (a so-called dip) occurs at a frequency of 2.87 GHz. The drop in the amplitude of the fluorescence radiation can be explained by the fact that - when a microwave field with a frequency of 2.87 GHz is applied - fewer NV centers in the m s =0, 3 A2 ground state are available, which are optically excited and emit fluorescence radiation into the m s =±1 , 3 A2 ground state can relax.

[0006] In an external magnetic field, the m s =±1 , 3 A2 -ground state into two states with spin quantum number m s =1 and m s=-1 (Zeemann effect). If the amplitude of the fluorescence radiation is now measured while changing the frequency of the microwave field, two dips are obtained. The frequencies at which these dips occur depend on the size of the splitting of the m s =±1 , 3 A2 ground state and thus on the field strength of the external magnetic field. In this way, the field strength of an external magnetic field can be determined.

[0007] Such a magnetometer can also be used to determine the direction of the external magnetic field. This takes advantage of the fact that the NV diamond lattice has NV centers in four different orientations, each of which has a different angle to the external magnetic field and thus, at different frequencies of the microwave field, enters the two states with the spin quantum number m s =1 and m s=-1. Thus, in the presence of an external magnetic field, a spectrum with eight dips results. The vector components of the external magnetic field can be calculated from the relative positions of the dips. To obtain a well-resolved spectrum, it is necessary to generate an additional, preferably homogeneous, static magnetic field in the NV diamond to measure the direction of the external magnetic field. This field runs at a polar and an azimuthal angle relative to the edges of the NV diamond, which is usually shaped as a cube or cuboid.

[0008] This is described, for example, in US 2019 / 0018091 A1. Here, the static magnetic field is generated by several permanent magnets arranged in a Halbach configuration. The permanent magnets are held by a frame attached to a base. The NV diamond is also attached to this base. The permanent magnets are inserted into the frame along the circumference of the frame, with the frame aligned perpendicular to the base along its main plane.

[0009] Disclosure of the invention

[0010] The invention relates to a magnetometer having the features of claim 1 and a method for operating such a magnetometer. Further embodiments of the invention are the subject of the dependent claims and the following description.

[0011] The magnetometer according to the invention comprises an excitation light source, a sensor crystal, a device for generating a microwave field, a detector, and at least two permanent magnets. The excitation light source is configured to emit an excitation beam. The excitation light source may be a diode laser. In this case, the excitation beam is in the form of a laser beam. The sensor crystal has at least one magneto-optical defect. The sensor crystal may have a crystal lattice. The crystal lattice may be the crystal lattice of a diamond, which has at least one NV center as a defect, i.e., a defect and a nitrogen atom adjacent to the defect, which occupies a lattice site of a carbon atom.The sensor crystal, the device for generating the microwave field, and the excitation light source are designed and arranged relative to one another such that the excitation light beam emitted by the excitation light source causes the emission of fluorescent radiation in the at least one magneto-optical defect of the sensor crystal, and the microwave field is detectable in the sensor crystal. The excitation light beam can be a laser beam with a wavelength between 490 nm and 575 nm, and the fluorescent radiation can be radiation in a wavelength range between 650 nm and 750 nm.

[0012] The magnetometer further comprises a detector which is designed and arranged such that it detects the emitted fluorescent radiation. The detector can be a photodetector which is sensitive in a wavelength range between 650 nm and 750 nm. Furthermore, the magnetometer has at least two permanent magnets which are arranged and configured such that they generate an at least approximately homogeneous, static magnetic field in the sensor crystal. The magnetic field generated by the at least two permanent magnets can in principle have homogeneous and less homogeneous regions. It is advantageous to position the sensor crystal in a region of the magnetic field which is as homogeneous as possible. For this purpose, the sensor crystal and the at least two permanent magnets are arranged on a common positioning element. The relative positions between the permanent magnets and the sensor crystal can thus be defined in a simple manner, e.g.by applying fiducials to the positioning element. These can then serve as reference points for referencing during assembly of the individual components. The positions of the sensor crystal and the permanent magnets can be determined and maintained very precisely in this way. Furthermore, the at least two permanent magnets and the sensor crystal can be arranged in a space-saving manner, allowing the magnetometer to be designed compactly.

[0013] The detector can be arranged between the sensor crystal and the positioning element. Particularly preferably, the detector is attached to the sensor crystal and / or to the positioning element using an adhesive. The excitation beam is coupled into the sensor crystal, in particular, at a side surface of the sensor crystal opposite the detector. The sensor crystal can be designed as a polyhedron, in particular as a cube, cuboid, or truncated pyramid. Alternatively or additionally, further detectors can be arranged on the same or other side surfaces of the sensor crystal. Such an arrangement enables a particularly compact design and very high sensitivity of the magnetometer.

[0014] The sensor crystal and the detector can be arranged on a common support element. The support element is arranged on the common positioning element. This arrangement allows for simple assembly of the magnetometer, as individual components can be assembled first, which can then be combined into a complete structure.

[0015] The at least two permanent magnets and the sensor crystal can be arranged on the positioning element in such a way that the homogeneous, static magnetic field generated by the at least two permanent magnets runs at a polar angle relative to a first edge of the sensor crystal and at an azimuthal angle relative to a second edge of the sensor crystal, wherein the polar angle is preferably between 5° and 20°. The sensor crystal is formed as a diamond crystal with the crystal planes {100}, {010} and {001}. The polar angle is then defined as the angle between the normal of the {001} crystal plane and the static magnetic field. The azimuthal angle is defined as the angle between the normal of the {100} crystal plane and the projection of the magnetic field vector of the static magnetic field onto the {001} crystal plane.To obtain the best possible resolution, the polar angle can be between 5° and 20°, preferably between 8° and 15°, particularly preferably 11.2°. The azimuthal angle can be between 17° and 35°, preferably 26.6° (i.e., within an angular range of approximately ±9° around 26.6°). Such an arrangement of the sensor crystal with respect to the static magnetic field, and thus with respect to the orientation and position of the permanent magnets, enables a particularly compact design of the magnetometer.

[0016] The positioning element can be designed as a flat component. The permanent magnets can be configured and arranged such that the at least approximately homogeneous, static magnetic field runs in or parallel to a plane of the positioning element. In this case, the edges of the sensor crystal can be aligned at a polar and / or azimuthal angle to the plane of the positioning element. The polar angle can assume a value between 5° and 20°, preferably between 8° and 15°, particularly preferably 11.2°. Such a positioning element designed as a board or plate, in turn, enables a particularly compact design of the magnetometer.

[0017] The at least two permanent magnets can be arranged and aligned according to a Halbach configuration. A Halbach configuration is an arrangement of permanent magnets or permanent magnet segments along a preferred direction. The magnetization direction of adjacent permanent magnets or permanent magnet segments is tilted relative to one another along the preferred direction by an angle of 90° or less than 90°. The preferred direction can run along a straight line or along a circular line. If the preferred direction runs along a circular line, the permanent magnets are arranged along such an imaginary circular line. In order to generate the most homogeneous magnetic field possible, in such a planar, annular Halbach arrangement the magnetization direction of adjacent permanent magnets rotates once by 360° along a ring section of 180°.An arrangement of at least four permanent magnets in a Halbach configuration, with the sensor crystal positioned at the center of the circle, is advantageous. Such an arrangement can generate a very homogeneous, static magnetic field in the sensor crystal.

[0018] The sensor crystal and the detector can be arranged on the common support element, with the support element extending between two adjacent permanent magnets. Alternatively, recesses can be provided in the support element through which one or more permanent magnets can protrude through the support element. In this case, in an arrangement of, for example, four permanent magnets, three of the permanent magnets are arranged outside the support element and one of the permanent magnets is arranged in such a recess.

[0019] In such arrangements, both the positioning element and the support element can be designed as flat, planar components. The positioning element, on which the permanent magnets are arranged, can be positioned relative to the support element such that the plane of the support element and the plane of the positioning element are aligned at an angle between 5° and 20°, preferably between 8° and 15°, particularly preferably 11.2°.

[0020] Such an arrangement enables very precise positioning of the individual components relative to one another while maintaining a very compact structure.

[0021] The permanent magnets can be arranged in two layers. This requires at least four permanent magnets. A first of the two layers can be arranged in a plane below the sensor crystal, and a second layer can be arranged in a plane above the sensor crystal. To position the second layer, the permanent magnets of the second layer can be attached to an additional receiving element, with the permanent magnets of the second layer being arranged between the sensor crystal and the receiving element, so that the receiving element holds the permanent magnets from above. Such an arrangement enables the generation of a particularly homogeneous magnetic field.

[0022] The positioning element can have recesses in which the permanent magnets are arranged. The permanent magnets can be secured in these recesses, for example, by a layer of adhesive. Providing recesses in the positioning element enables the permanent magnets to be positioned very precisely with respect to the sensor crystal, as well as the permanent magnets relative to each other.

[0023] The positioning element can be wedge-shaped and / or stepped, at least in sections. Providing a wedge angle makes it possible to arrange the permanent magnets at different heights of the positioning element, thus allowing the angle between the magnetic field plane and the edges of the sensor crystal to be easily adjusted. The same can be achieved by a stepped design of the positioning element.

[0024] The support element and / or the positioning element can be designed as a printed circuit board. This allows for a simplified arrangement, in particular of the detector, on the support element and / or the positioning element.

[0025] The permanent magnets can be designed as cylinders, cubes, or cuboids. The provision of at least one flat surface enables particularly simple production of the permanent magnets and trouble-free attachment of the permanent magnets to the positioning element.

[0026] The sensor crystal can be designed as a polyhedron with at least one angle other than 90° between two adjacent surfaces. This can increase the sensitivity of the magnetometer.

[0027] The invention also includes a method for operating a magnetometer with the aforementioned advantages. Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0028] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0029] Short description of the drawings

[0030] Fig. 1 shows a magnetometer in which a sensor crystal and two permanent magnets are arranged on a common positioning element, in a side view.

[0031] Fig. 2 shows the magnetometer from Fig. 1 in a top view.

[0032] Fig. 3 shows a magnetometer in which a sensor crystal is arranged on a support element and the support element and a plurality of permanent magnets are arranged on a positioning element, in a plan view.

[0033] Fig. 4 shows a possible embodiment of a magnetometer with a support element and a positioning element.

[0034] Fig. 5 shows another possible embodiment of a magnetometer with a support element and a positioning element.

[0035] Fig. 6 shows another possible embodiment of a magnetometer with a support element and a positioning element.

[0036] Embodiments of the invention

[0037] Figure 1 shows a magnetometer 1 with a sensor crystal 4, a

[0038] Excitation light source 2, a detector 5, two permanent magnets 6, a device 9 for generating a microwave field, and a positioning element 7. It is understood that the magnetometer 1 can have more than two permanent magnets 6. The excitation light source 2 is configured to generate an excitation light beam 3, in particular a laser beam in a wavelength range between 490 nm and 575 nm, and can be designed, for example, as a diode laser. The excitation light source 2 is arranged relative to the sensor crystal 4 such that the excitation light beam 3 can be directed onto the sensor crystal 4 directly or via one or more deflecting mirrors and / or beam splitters. The sensor crystal 4 can be designed as a diamond having defects in the form of NV centers. An NV center is understood to be a defect in the immediate vicinity of a metal atom that occupies a lattice site of the diamond crystal lattice.The sensor crystal 4 then emits fluorescent radiation in a wavelength range between 650 nm and 750 nm, which is detected by the detector 5. The sensor crystal 4 is designed as a cuboid with six side surfaces. The detector 5 is arranged between the sensor crystal 4 and the positioning element 7. The excitation beam 3 is coupled in through a side surface of the sensor crystal 4 that is opposite the detector. Additional detectors can be arranged on the other side surfaces of the sensor crystal 4. In this exemplary embodiment, the device 9 for generating the microwave field is also arranged on the positioning element 7. It is aligned and configured such that it can generate a microwave field in the sensor crystal 4 in a frequency range in which the spin states of the various NV centers are energetically split.The permanent magnets 6 are arranged on the positioning element 7 in such a way that they generate a largely homogeneous, static magnetic field, at least in certain regions. The sensor crystal 4 is arranged in a largely homogeneous region of the magnetic field. The edges of the sensor crystal are arranged at a polar and an azimuthal angle to the plane of the magnetic field. The polar angle is between 5° and 20°. A polar angle between 8° and 15°, in particular 11.2°, has proven particularly advantageous. The plane of the magnetic field runs parallel or approximately parallel to the plane of the flat positioning element 7, which is designed, for example, as a printed circuit board. The sensor crystal 4 can be arranged in a recess made in the positioning element 7. Alternatively, the sensor crystal 4 can also be arranged on the positioning element 7, for example with the aid of a wedge element.It is understood that the permanent magnets 6 can also be arranged at an angle to the sensor crystal 4, so that the magnetic field and the sensor crystal 4 are aligned at a polar and azimuthal angle to each other.

[0039] Figure 2 shows the magnetometer 1 from Figure 1 again in a top view. Four permanent magnets 6 are arranged in a Halbach configuration, for example, along an imaginary circular line 10. The sensor crystal 4 is located in the center of the circle, and thus in a largely homogeneous area of ​​the static magnetic field.

[0040] Figure 3 shows a plan view of a magnetometer 1 which, in addition to the positioning element 7, has a support element 8. The sensor crystal 4 is arranged on the support element 8. The detector 5 (not shown in Fig. 3) is arranged between the support element 8 and the sensor crystal 4. The permanent magnets 6 are arranged along a circular line on the positioning element 7 according to a Halbach configuration. The sensor crystal 4 is positioned at the center of the associated circle. The support element 8 is flat, e.g., as a circuit board, and is arranged between two adjacent permanent magnets 6. The support element 8 is arranged tilted about an axis 10 relative to an upper surface 12 of the positioning element 7, wherein the axis 10 runs centrally between two permanent magnets 6 through the sensor crystal 4 and thus through the center of the circle.The support element 8 and the upper surface of the positioning element 12 form an acute angle of preferably 11.2°. Due to the alignment of the support element 8 and the positioning element 7 to each other, two of the permanent magnets 6 are arranged above the plane of the support element 8, and two further permanent magnets 6 are arranged below the plane of the support element 8.

[0041] Figures 4 and 5 each show such a magnetometer 1 in a side view. For reasons of clarity, the detector 5 has been omitted from these illustrations. Figure 4 shows a magnetometer 1 with a partially wedge-shaped positioning element 7. The positioning element 7 has sections on its upper surface 12 that are aligned parallel to the plane of the support element 8. The support element 8 is positioned on these sections. Likewise, the positioning element 7 has sections on its upper surface 12 that are arranged at a wedge angle, an acute angle of, for example, 11.2°, to the support element 8, which is designed as a flat surface. These sections can also be designed as base surfaces 14 of depressions 11.The permanent magnets 6 are shaped like cubes, cylinders or cuboids and are arranged with one of their side surfaces on the partial sections of the upper surface 12 of the positioning element 7 aligned at the wedge angle to the support element 8, in particular in the recesses 11.

[0042] Figure 5 shows a magnetometer 1 with a stepped positioning element 7. The positioning element 7 has surfaces aligned exclusively parallel to the plane of the support element 8. These can be designed as recesses 11 in which some of the permanent magnets 6 are arranged. The recesses 11 can be designed with different depths compared to the upper surface 12. Another part of the permanent magnets 6 is arranged on the upper surface 12 of the positioning element 7. Due to the arrangement of the permanent magnets 6 at different heights, a magnetic field is generated that is aligned at an angle relative to the support element 8.

[0043] Figure 6 shows a plan view of a magnetometer 1, which also has a support element 8 in addition to the positioning element 7. The sensor crystal is arranged on the support element 8. The detector 5 (not shown in Figure 6) is arranged between the support element 8 and the sensor crystal 4. The permanent magnets 6 are arranged on the positioning element 7 according to a Halbach configuration. The support element 8 is flat, e.g., designed as a circuit board. The sensor crystal 4 is arranged centrally relative to the permanent magnets 6 arranged along a circular line. The support element 8 is tilted by an angle of 11.2° relative to the upper surface 12 of the positioning element 7 along the axis 10. The axis 10 runs - unlike the magnetometers 1 from Figures 3 to 5 - through two of the permanent magnets 6 and through the sensor crystal 4.In this arrangement, one of the permanent magnets 6 is arranged above or in the plane of the support element, and another permanent magnet 6 is arranged below the plane of the support element 8.

[0044] Figures 7 to 9 each show such a magnetometer 1 in a side view. For reasons of clarity, the detector 5 has again been omitted from these illustrations. Figure 7 shows a magnetometer 1 with a partially wedge-shaped positioning element 7. The magnetometer 1 from Figure 7 again has 12 subsections on its upper surface that are aligned parallel to the plane of the support element 8. The support element 8 is positioned on these subsections. Likewise, the positioning element 7 has 12 subsections on its upper surface that are arranged at a wedge angle, an acute angle of, for example, 11.2°, to the support element 8, which is designed as a flat surface. These subsections can also be designed as depressions 11.The permanent magnets 6 are cube-, cylinder- or cuboid-shaped and are arranged with one of their side surfaces on the partial sections of the upper surface 12 of the positioning element 7 aligned at the wedge angle to the support element 8.

[0045] The positioning element 7 of the magnetometer 1 shown in Figure 8 again has surfaces aligned exclusively parallel to the plane of the support element 8. These can be formed as recesses 11 in which some of the permanent magnets 6 are arranged. Another part of the permanent magnets 6 is arranged on the upper surface 12 of the positioning element 7. Due to the arrangement of the permanent magnets 6 at different heights, a magnetic field is generated that is oriented at an angle relative to the support element 8.

[0046] Figure 9 shows the magnetometer 1 from Figure 8 with the difference that the permanent magnets 6 have ground undersides. The permanent magnets 6 can alternatively or additionally also have ground upper sides. The upper and lower sides of the cuboid, cube, or cylindrical permanent magnets 6 are not aligned parallel to one another. The permanent magnets 6 are arranged at different heights. This results in an arrangement as in Fig. 7, except that the surfaces on which the permanent magnets 6 are arranged, e.g. the recesses 11, are parallel to the support element 8 designed as a flat surface, while the upper sides of the permanent magnets 6 are aligned at a wedge angle, an acute angle of e.g. 11.2°, to the support element 8 designed as a flat surface.

Claims

Claims 1. A magnetometer (1) for detecting a magnetic field, comprising an excitation light source (2) configured to emit an excitation beam (3), a sensor crystal (4) having at least one magneto-optical defect, a device (8) for generating a microwave field, wherein the sensor crystal (4), the device (9) for generating the microwave field, and the excitation light source (2) are configured and arranged relative to one another such that the excitation light beam (3) emitted by the excitation light source (2) causes an emission of fluorescent radiation in the at least one magneto-optical defect of the sensor crystal (4), and the microwave field is detectable in the sensor crystal (4), a detector (5) configured and arranged to detect the emitted fluorescent radiation;at least two permanent magnets (6) which are arranged and configured such that they generate an at least approximately homogeneous, static magnetic field in the sensor crystal (4), wherein the sensor crystal (4) and the at least two permanent magnets (6) are arranged on a common positioning element (7); 2. Magnetometer (1) according to claim 1, wherein the detector (5) is arranged between the sensor crystal (4) and the positioning element (7) 3. Magnetometer (1) according to claim 1 or 2, wherein the sensor crystal (4) and the detector (5) are arranged on a common support element (8) and wherein the support element (8) is arranged on the common positioning element (7) 4. Magnetometer (1) according to one of the preceding claims, wherein the at least two permanent magnets (6) and the sensor crystal (4) are arranged on the positioning element (7) such that the generated homogeneous, static magnetic field runs at a polar angle relative to a first edge of the sensor crystal (4) and at an azimuthal angle relative to a second edge of the sensor crystal (4), wherein the polar angle is preferably between 5° and 20° 5. Magnetometer (1) according to claim 4, wherein the positioning element (7) is designed as a flat component and wherein the generated, at least approximately homogeneous, static magnetic field runs in a plane of the positioning element (7).

6. Magnetometer (1) according to one of the preceding claims, wherein the at least two permanent magnets (6) are arranged and aligned according to a Halbach configuration.

7. Magnetometer (1) according to one of the preceding claims, which has at least four permanent magnets (6) arranged along an imaginary circular line, wherein the sensor crystal (4) is arranged in the center of the circle.

8. Magnetometer (1) according to one of the preceding claims, wherein the sensor crystal (4) and the detector (5) are arranged on the common support element (8) and wherein the support element (8) is guided between two adjacent permanent magnets (6).

9. Magnetometer (1) according to one of the preceding claims, which has at least four permanent magnets (6), wherein the permanent magnets (6) are arranged in two layers, one of which is arranged above and one below the sensor crystal (4).

10. Magnetometer (1) according to one of the preceding claims, wherein the positioning element (7) has recesses (9) in which the permanent magnets (6) are positioned.

11. Magnetometer (1) according to one of the preceding claims, wherein the positioning element (7) is at least partially wedge-shaped and / or stepped.

12. Magnetometer (1) according to one of the preceding claims, wherein the support element (8) and / or the positioning element (7) is designed as a circuit board, in particular for receiving the detector (5).

13. Magnetometer (1) according to one of the preceding claims, wherein the permanent magnets (6) are designed as cubes or cuboids.

14. Magnetometer (1) according to one of the preceding claims, wherein the sensor crystal (4) is designed as a polyhedron with at least one angle deviating from 90° between two adjacent surfaces.

15. Method for operating a magnetometer (1) according to one of the preceding claims.

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