Sensor element and optical sensor having a sensor element

Abstract

The invention relates to a sensor element (1), comprising: a crystal with colour centres, in particular a diamond crystal (2) with NV centres (7). The sensor element (1) has a lens (3) on which the crystal is fixed, wherein the lens (3) has an orientation aid (5) for fixing the crystal to the lens (3) with a predefined orientation, and wherein the lens (3) is transmissive for excitation light (9) for exciting the colour centres, in particular the NV centres (7), and / or is transmissive for fluorescent light (6) which is generated during the excitation of the colour centres, in particular the NV centres (7). The invention also relates to an optical sensor, in particular a magnetometer (12), comprising: a sensor element (1) which is designed as described above, and a magnetic field generator (14) for generating an offset magnetic field (B0) in the region of the crystal, preferably in the region of the diamond crystal (2), wherein the offset magnetic field (B0) is oriented along a magnetic field axis (15).

Description

[0001] Sensor element and optical sensor with one sensor element

[0002] The present invention relates to a sensor element comprising: a crystal with color centers, in particular a diamond crystal with NV centers, and an optical sensor with such a sensor element.

[0003] Sensor elements play a central role in many technical applications and can be used in optical sensors, e.g., in the form of quantum sensors, to determine various physical quantities. Sensor elements for quantum sensors, especially solid-state quantum sensors, typically comprise crystals doped with color centers. In particular, such a sensor element can utilize a diamond crystal doped with color centers, usually (negatively charged) nitrogen vacancy (NV) centers. NV centers in diamond exhibit a characteristic electronic structure that changes when certain quantities, such as an external magnetic field, temperature, pressure, or electric current, change.

[0004] In an optical sensor featuring such a crystal, the color centers are irradiated with excitation light in the optical, typically visible, wavelength range. When exciting NV centers, excitation light with a wavelength between 515 nm and 570 nm is typically used. Depending on their initial state, the NV centers are excited into a state that decays with the emission of fluorescent light with wavelengths predominantly between 600 nm and 900 nm, or into a state that, with a greater probability than the initial state, decays through several intermediate states with the emission of infrared light, particularly with a wavelength of 1042 nm. The initial state of the NV centers can be manipulated using a resonantly radiated microwave.The resonance frequency of the radiated microwave depends on the projection of the external magnetic field onto the respective NV axis of the NV centers, as well as on the temperature, among other factors. The resonance frequency of the microwave can therefore be used to determine the magnitude or field strength of the external magnetic field. For this purpose, the dip in the fluorescent light during resonant excitation with the microwave is detected by capturing the fluorescent light with a detector, e.g., a photodiode, and monitoring the voltage generated by the fluorescent light, as described, for example, in US 2016 / 0356863 A1.

[0005] Each NV center in the diamond crystal exists in one of four different orientations. More precisely, each NV center in the unit cell is aligned along one of four NV axes. The NV axes are defined by the imaginary connecting axis between the nitrogen atom and the vacancy of the respective NV center. As described above, each NV center is sensitive to the projection of the external magnetic field onto the respective NV axis, i.e., to the portion of the external magnetic field that is aligned parallel to the NV axis. Therefore, in general, a different magnetic field strength is measured for each of the four different NV axes. This fact can be exploited to conduct vector magnetometry, i.e., to determine not only the field strength but also the direction of the external magnetic field.

[0006] Without the presence of an external magnetic field, the various transitions of all four orientations of the NV centers are superimposed. However, stresses in the crystal and temperature gradients can lead to a slight shift of the states of the various orientations relative to each other. This slight shift leads to a broadening of the resulting linewidth of the transition, which reduces the sensitivity of the optical sensor. Furthermore, with a very small external magnetic field, the transitions only shift by values ​​that are small compared to the linewidth of the individual transitions and correspond to the projection of the external magnetic field onto the NV axis. The latter is usually different for the different orientations. This also leads to a broadening of the transitions and degrades the sensitivity of the sensor.One way to solve this problem is to generate an additional, homogeneous, static offset magnetic field with a defined field strength along a defined magnetic field axis, e.g., along one of the four NV axes. This eliminates the degeneracy of the different orientations of the NV axes. Removing the degeneracy requires a defined and reproducible alignment of the crystal and the offset magnetic field.

[0007] Object of the invention

[0008] The invention is based on the object of providing a sensor element with a defined orientation of an optical crystal and an optical sensor with such a sensor element.

[0009] Subject of the invention

[0010] This object is achieved by a sensor element of the type mentioned above, which comprises an optic to which the crystal is fixed, wherein the optic has an alignment aid for fixing the crystal to the optic with a predefined orientation, and wherein the optic is transmissive for excitation light for exciting the color centers, in particular the NV centers, and / or is transmissive for fluorescent light which is generated upon excitation of the color centers, in particular the NV centers.

[0011] To align the axes of the color centers, especially the NV centers, with the magnetic field direction of an offset magnetic field, it is generally possible to fix the crystal to a flat surface of a support element by gluing it to the flat surface. However, a defined and reproducible alignment of the axes of the color centers in space is not achievable in this case, or only if the crystal is ground in such a way that the axes of the crystal are correctly aligned when the crystal is glued to the flat surface. However, grinding a crystal along arbitrary angles is complex in terms of manufacturing. In addition, with a cube-shaped crystal, precise attention must be paid to the orientation of the axes. Therefore, a mechanical adjustment of the alignment of the offset magnetic field with respect to the crystal or to the axes of the color centers is usually necessary.For this purpose, for example, a three-dimensional coil system, a pair of coils that are mechanically tilted relative to the crystal, or permanent magnets that are mechanically adjusted in their orientation are required.

[0012] In the sensor element according to the invention, the alignment aid is designed such that the crystal can be fixed to the optics in exactly one orientation specified by the alignment aid when the crystal is inserted into the alignment aid. In the simplest case, the alignment aid can be a contour on a surface of the optics, for example, an edge that serves as a stop for the crystal. In the sensor element according to the invention, the axes of the color centers can be reproducibly and precisely aligned in space with the aid of the alignment aid, whereby a crystal with a standard cut can be used for aligning the axes.

[0013] Unlike when glued to a flat surface, the crystal, in particular the axes of the color centers, can be aligned relative to the optics in virtually any direction specified by the alignment aid. This allows a magnetic field generator with a predetermined magnetic field direction of the offset magnetic field to be used in an optical sensor in which the sensor element is housed, meaning no mechanical adjustment of the magnetic field direction is required.

[0014] The optics on which the alignment aid is formed are also transmissive for excitation light used to excite the color centers, e.g., in the form of NV centers, and / or for fluorescent light generated upon excitation of the color centers, e.g., in the form of NV centers. It is advantageous if the excitation light and / or fluorescent light are transmitted by the optics to which the crystal is attached, as this simplifies the assembly and connection technology of the optical sensor.

[0015] In the simplest case, the optics can be a transparent alignment adapter that serves only to align the crystal appropriately and transmit the excitation light and / or the fluorescent light. For example, the optics can be a disk that is attached directly to an optical component, e.g., a photodiode, in order to supply the fluorescent light to it or to supply the excitation light to the crystal. In this case, the optics perform no other optical function apart from transmitting the excitation light or the fluorescent light. However, it is understood that the optics can also perform another optical function, e.g., an imaging function.

[0016] In one embodiment, the alignment aid forms a recess in the optic into which the crystal is inserted. The recess is shaped so that the crystal is accommodated in a predetermined orientation. The crystal is generally not inserted completely into the recess, but typically protrudes beyond it. If the crystal is inserted into the recess and fixed in the recess, e.g., by gluing or some other means, the axes or orientations of the color centers relative to the optic are also precisely determined. The size and geometry or shape of the recess is adapted to the crystal used.

[0017] The orientation of the axes of the color centers, especially the NV centers, relative to the unit cell of the crystal, e.g., a diamond crystal, is generally known. If necessary, deviations in the orientation of the axes relative to the unit cell, e.g., caused by stresses, can be determined for a specific crystal through preliminary experiments, e.g., through polarization-dependent measurements. Since the cutting plane under which the crystal was cut to form the unit cell is also known, the orientation of the axes of the color centers relative to the lateral surfaces of the crystal is also known, and the indentation can be adjusted to the desired orientation of the crystal's axes.

[0018] In a further development, the indentation has at least three preferably flat contact surfaces for contacting at least three side surfaces of the preferably cuboid-shaped or cubic crystal. The contact of a side surface of the crystal with the contact surface means that the respective side surface of the crystal bears partially or completely against the respective contact surface. In the first case, the side surface bears against the contact surface with only part of its surface; in the second case, the respective side surface of the crystal bears completely, i.e., with its entire surface, against the respective contact surface of the indentation.

[0019] In a further development, the recess has (exactly) three preferably flat contact surfaces for the attachment of three side surfaces of the preferably cuboid or cube-shaped crystal. In this case, the cuboid or cube-shaped crystal protrudes into the recess with one corner. The cuboid or cube-shaped crystal can be precisely aligned in space by engaging the three flat contact surfaces, which in this case are aligned perpendicular to each other.

[0020] In a further development, the recess has (exactly) four preferably flat contact surfaces for contacting four side surfaces of the preferably cuboid or cube-shaped crystal. In this case, the cuboid or cube-shaped crystal protrudes into the recess with one edge. The cuboid or cube-shaped crystal can be precisely aligned in space by contacting the four flat contact surfaces, two of which are aligned perpendicularly and two parallel to each other in this case.

[0021] In a further development, the recess has (exactly) five preferably flat contact surfaces for the engagement of five side surfaces of the preferably cuboid or cube-shaped crystal. In this case, the cuboid or cube-shaped crystal protrudes completely into the recess with one side surface and at least partially with four other side surfaces. The cuboid or cube-shaped crystal can be precisely aligned in space by engaging the five flat contact surfaces.

[0022] In a further embodiment, the optic is formed from a material having a refractive index that deviates from the refractive index of the crystal by no more than 1.1, preferably no more than 0.7. To guide the excitation light or the fluorescent light in the optic, it is advantageous if the refractive index of the material of the optic deviates as little as possible from the refractive index of the crystal. In the case of a diamond crystal, which has a refractive index of approximately 2.4, the material of the optic can, for example, have a refractive index of approximately 1.5. The material of the optic can, for example, be a polymer.

[0023] In a further embodiment, the optic is manufactured by additive manufacturing or forms an injection-molded part. The optic can be manufactured, for example, by 3D printing. In this case, the material of the optic is typically a light-curable polymer or resin. Alternatively, it is possible to produce the optic using an injection molding process. In both cases, the alignment aid, particularly in the form of the recess, can be formed precisely and reproducibly during the manufacture of the optic. The crystal can be fixed to the optic during production and inserted into the recess to increase the precision of the alignment or positioning of the crystal. After inserting the crystal into the recess, the crystal can be additionally cast or over-molded with additional optic material, or over-printed or over-molded. In principle, the optic can also be manufactured in other ways, e.g.by casting a polymer material or the like.

[0024] In a further embodiment, the crystal fixed to the optics is embedded in a material that is preferably transmissive to excitation light for exciting the color centers and / or to the fluorescent light, wherein the material is designed in particular in the form of an additional optic. The additional optic is typically designed to increase the coupling and coupling efficiency into or out of the crystal. The crystal fixed to the optics and inserted into the alignment aid, e.g. in the form of the indentation, can be completely enclosed by a material or completely covered by a material in order to protect the crystal. In this case, it is not necessary for the material to be transparent to the excitation light and / or the fluorescent light.

[0025] However, it is also possible for the material to be transmissive to the excitation light and / or the fluorescent light, and for the material to form an additional optical system that increases the coupling efficiency of the excitation light and / or the coupling efficiency of the fluorescent light. In the first case, the additional optical system can, for example, be an input coupling system for coupling excitation light into the crystal. In the second case, the additional optical system can, for example, be a collecting system that collects a portion of the fluorescent light to transmit it to a detector, thus increasing the coupling efficiency.

[0026] The material in which the crystal is embedded can be the same material as the optics, but this is not mandatory. For example, liquid glass can be used to fix the crystal to the optics, which is applied in such a way that the crystal is completely covered by the liquid glass. The surface of the cured liquid glass can be post-processed to form optical elements, such as lenses or the like, on the surface of the material. The material can, in particular, be transmissive to excitation light in order to enable the excitation light to be coupled into the crystal. As described above, the material can, for example, be in the form of a coupling optic, e.g. in the manner of a lens or the like. By forming a coupling optic directly on the crystal, the fluorescence yield can be increased.Alternatively or additionally, the additional optics can also increase the efficiency of fluorescence collection or transmission and be designed as a collection or collection optic. In this case, the additional optics can be designed, for example, in the form of a "hood" over the crystal, which serves to reflect the fluorescent light and utilizes total internal reflection, collecting the fluorescent light over the entire solid angle as much as possible, especially to guide it along the fluorescence axis (see below).

[0027] In a further embodiment, at least one component, in particular at least one optical element, is embedded in the material of the optics. The component can be, for example, a detector, e.g., a photodiode, structures for transmitting microwaves, magnetic field-generating elements, etc. The optical element can be a lens, an optical grating, an optical filter, e.g., a wavelength filter, etc. In principle, it is also possible for these or other components or optical elements not to be embedded in the optics, but rather to be connected to the optics during or after production.

[0028] In a further embodiment, the optics are designed to guide the fluorescent light along a fluorescence axis, wherein the optics preferably form a body of revolution with the fluorescence axis as the axis of symmetry. The fluorescence axis typically runs through the center of the crystal fixed to the optics. The optics can in principle have any shape, but a shape has proven advantageous with which the fluorescent light is guided and, if necessary, collected as effectively as possible along one direction (see below), which is referred to in the present application as the fluorescence axis. The optics themselves can be designed to collect the fluorescent light (see below), but it is also possible for the optics to which the crystal is fixed to serve as an adapter that guides the fluorescent light to another optic to which the optics serving as an adapter are attached. In this case, the additional optics can bring about the collection of the fluorescent light.To maximize the fluorescence yield, it is advantageous if the width of the optic increases perpendicular to the fluorescence axis, starting from the crystal or the surface to which the crystal is fixed. The optic can have rotational symmetry around the fluorescence axis, but this is not mandatory. The optic can, in particular, be designed as a body of rotation.

[0029] In a further development, the optics are designed as non-imaging optics, preferably as a concentrator, in particular as a compound parabolic concentrator. For the collection of the fluorescent light, it is not necessary for the optics to be an imaging optic; rather, a non-imaging optic is sufficient for this purpose. The optics can in particular be designed as a (e.g. parabolic) concentrator that collects the fluorescent light. The concentrator can in particular be a compound parabolic concentrator (CPC), which enables particularly efficient collection of the fluorescent light. As described above, it is alternatively possible to use the optics only to guide the fluorescent light and to connect it to another optics, which can for example be designed as a concentrator, in particular as a CPC.

[0030] In a further development, the crystal is cubic and has a spatial diagonal that is aligned essentially parallel to the fluorescence axis of the optics. An alignment essentially parallel to the fluorescence axis is understood to mean an alignment of the spatial diagonal of the cubic crystal that deviates by no more than + / - 15° from the fluorescence axis of the optics. It has proven advantageous for the fluorescence yield if the crystal, which can in particular be a diamond crystal, approximates a retroreflector for guiding the fluorescent light. This can be achieved by aligning the spatial diagonal of the crystal parallel to the fluorescence axis, whereby the spatial diagonal of the crystal typically runs along the fluorescence axis of the optics or extends it.

[0031] In a further development, the optics have a first angle marking for indicating a first angular position of the crystal, preferably of the diamond crystal, in a plane perpendicular to the fluorescence axis of the optics, wherein the first angle marking is preferably used to align the sensor element for the uniaxial operation of an optical sensor, in particular a magnetometer. In this case, the optics are typically designed as a body of revolution, so that the shape of the optics does not provide any indication of the angular position of the crystal and thus of the axes of the color centers, in particular the NV axes of the diamond crystal, in a plane perpendicular to the fluorescence axis. The angle marking makes it possible to correctly position the optics in the mechanical structure of an optical sensor with respect to the fluorescence axis. The first angle marking can be used, for example, for the alignment of the sensor element orthe optics for the single-axis operation of a magnetometer in which one of the NV axes is aligned parallel or essentially parallel to a magnetic field direction of an offset magnetic field (see below).

[0032] In a further embodiment, the optics have a second angle marking for indicating a second angular position of the crystal, preferably the diamond crystal, in a plane perpendicular to the fluorescence axis of the optics. The second angle marking preferably serves to align the sensor element during multi-axis operation in an optical sensor, in particular in a magnetometer. The second angle marking can be used, for example, to use the sensor element in a multi-axis operation of a magnetometer, in which the projections of the four NV axes onto the magnetic field direction of the offset magnetic field are separated from each other as evenly as possible, in particular as strongly and evenly as possible (see below).

[0033] It is possible for the optics to have only the first or only the second angle marking, but it is also possible for the optics to have both the first angle marking and the second angle marking. In this case, the optics can be aligned depending on the desired operation (single-axis or multi-axis) of the optical sensor by using either the first angle marking or the second angle marking to align the sensor element relative to the mechanical structure of the optical sensor or relative to the offset magnetic field. In this case, it is advantageous if the first angle marking and the second angle marking differ from each other by at least one feature (shape, color, etc.) in order to simplify the assignment of the angle marking to the single-axis or multi-axis operation of the sensor.

[0034] If the magnetic field axis of the offset magnetic field runs in the plane perpendicular to the fluorescence axis of the optics, the crystal is typically aligned such that one of the NV axes is essentially parallel to the plane perpendicular to the fluorescence axis of the optics. In this case, for uniaxial operation, the crystal can be rotated into a first angular position around the fluorescence axis, in which the NV axis is essentially parallel to the magnetic field axis of the offset magnetic field, and into a second angular position in which the projections of the magnetic field axis onto the NV axes are as separated as possible. By rotating the optics, it is therefore possible to switch between operating the optical sensor, typically a magnetometer, as a scalar magnetometer or as a vector magnetometer, without requiring any adjustment of the optical path or the remaining mechanics of the optical sensor.

[0035] The first angle marking and / or the second angle marking are preferably formed on a lateral surface of the optic, in particular in a region facing away from the crystal (i.e. at the base of the optic). The optic can have a flat surface on its side facing away from the crystal, which is aligned perpendicular to the fluorescence axis and with which the optic is integrated into the structure of the optical sensor. The angle markings can protrude beyond the circumferential lateral surface of the optic, which in this case is typically rotationally symmetrical to the fluorescence axis. However, it is also possible for the lateral surface to have a circumferential edge or the like in a region facing away from the crystal, which protrudes beyond the remaining lateral surface and to which the first and / or second angle marking are applied in order to integrate the optic with the desired angular orientation into the optical sensor.

[0036] A further aspect of the invention relates to an optical sensor, in particular a magnetometer, comprising: a sensor element configured as described above, and a magnetic field generator for generating an offset magnetic field in the region of the crystal, preferably in the region of the diamond crystal, wherein the offset magnetic field is aligned along a magnetic field axis or a magnetic field direction. The offset magnetic field is a magnetic field that is as homogeneous, static, and defined in its field strength as possible at the location of the crystal.

[0037] The optical sensor typically includes additional components, such as an excitation light source for generating excitation light that is irradiated onto the crystal to excite the color centers, and a microwave generator that serves to generate a microwave field in the region of the crystal to determine a measurand, such as the strength of an external magnetic field, in the manner described above. It is also possible, in principle, to operate the optical sensor without a microwave generator, for example, if an offset magnetic field with a magnetic field strength on the order of approximately 100 mT is used.

[0038] If the measured quantity is a magnetic field, the optical sensor is a magnetometer. As described above, the magnetometer can be used to determine the magnetic field strength (scalar operation) or to additionally determine the direction of the magnetic field (multi-axis or vector operation). The axes of the crystal's color centers should be aligned differently in the two operating modes. The appropriate alignment of the axes of the color centers for the respective operation can be achieved by rotating the sensor element (see above). When using the sensor element described above, either a scalar magnetometer or a vector magnetometer can be realized simply by adjusting the angle of the optics with respect to the offset magnetic field, without the mechanical structure of the magnetometer having to be modified for this purpose.In particular, with appropriate alignment of the crystal or the axes of the color centers, it is not necessary to change the magnetic field direction of the offset magnetic field for this purpose.

[0039] In one embodiment, a magnetic field axis of the offset magnetic field is aligned at an angle between 70° and 120°, preferably between 80° and 100°, to the fluorescence axis of the optics. It has proven advantageous if the magnetic field axis of the offset magnetic field is aligned substantially perpendicular to the fluorescence axis of the optics. For optimal fluorescence collection, it is generally advantageous for the magnetic field axis and the fluorescence axis to be not aligned exactly perpendicular to each other, ensuring optimal separation of the projections of the magnetic field axis of the offset magnetic field onto the axes of the crystal's color centers.

[0040] In a further embodiment, the magnetic field axis of the offset magnetic field is aligned at an angle between -30° and 30°, preferably between -10° and 10°, to the fluorescence axis of the optics. It is also possible for the magnetic field axis of the offset magnetic field to be aligned approximately parallel to the fluorescence axis of the optics.

[0041] In one embodiment, one of the NV axes of the NV centers of the diamond crystal is aligned substantially parallel to the magnetic field axis of the offset magnetic field. In this embodiment, the optical sensor can be operated as a scalar magnetometer. An alignment of one of the NV axes substantially parallel to the magnetic field axis is understood to mean that the magnetic field axis is aligned at an angle of no more than + / - 15° to the NV axis. An NV axis of an NV center is defined as an imaginary connecting line between the nitrogen atom N and the defect V of the NV center. In an alternative embodiment, an amount of a projection of the vector of the offset magnetic field (iethe magnetic flux density of the offset magnetic field) onto any one of the four NV axes of the NV centers of the diamond crystal and a magnitude of a projection of the vector of the offset magnetic field onto any other of the four NV axes of the diamond crystal have a difference of at least 230 pT. In this embodiment, the optical sensor can be operated as a vector magnetometer. For vector operation, the projections of the vector of the offset magnetic field onto the NV axes of the diamond crystal should be separated from each other as strongly and as evenly as possible, i.e. the minimum distance or the minimum difference of the magnitude BNV.A, BNV.B, BNV.C, BNV.D of the projections onto the respective axis NVA, NVB, NVC, NVD should be as large as possible. The distance between the magnitudes BNV.A, BNV.B, BNV.C, BNV.D of the projections and zero should also be as large as possible. In other words, the minimum differences of all components of (0, BNV.A, BNV.B, BNV.C, BNV.D) be as large as possible. This can be achieved by appropriately aligning the crystal when attaching it to the optics, as well as by appropriately aligning the optics relative to the mechanical and optical structure of the optical sensor, the magnetic field axis, or the offset magnetic field vector.

[0042] To switch between the two operating modes described above, it is sufficient to replace the sensor element or to align it appropriately with respect to the fluorescence axis, i.e., to rotate it. It is possible, but not mandatory, for the optical sensor to have a rotation device for manually or automatically changing the angular position of the optics from the first angular position described above to the second angular position and vice versa. Alternatively, the offset field can be rotated (mechanically by rotating the generating elements or by operating another coil).

[0043] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further listed features can be used individually or in combination. The embodiments shown and described are not intended to be exhaustive, but rather serve as examples for describing the invention.

[0044] They show:

[0045] Fig. 1 is a schematic representation of a magnetometer having a sensor element with an optic comprising an alignment aid in the form of a recess into which a diamond crystal is inserted,

[0046] Fig. 2 is a schematic representation of a unit cell of a diamond crystal of Fig. 1 with an NV center defining an NV axis,

[0047] Fig. 3 is a schematic representation of an ODMR spectrum generated in a scalar operation of the magnetometer of Fig. 1,

[0048] Fig. 4a, b schematic representations of a magnetometer in the manner of Fig. 1 with a sensor element having an optic with two angle markings, and

[0049] Fig. 5a-c schematic representations of the magnitudes of the projections of a magnetic field axis of the offset magnetic field onto the four NV axes of the diamond crystal as a function of a respective rotation angle of the diamond crystal.

[0050] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0051] Fig. 1 shows a sensor element 1 which has a crystal, more precisely a diamond crystal 2, and an optic 3. In the example shown, the optic 3 is designed as a rotationally symmetrical truncated cone which is rotationally symmetrical with respect to an axis of symmetry 4 which forms an x-axis of an xyz coordinate system. On an upper end face 3a of the optic 3, an indentation 5 is formed which, in the example shown, is designed as a three-sided pyramid. The diamond crystal 2, which in the example shown is designed as a cube, is inserted into the indentation 5. The indentation 5 serves as an alignment aid and makes it possible to fix the diamond crystal 2 with a predetermined orientation to the optic 3. For this purpose, the diamond crystal 2 can be glued in the indentation 5 or permanently connected to the optic 3 in some other way.

[0052] In the example shown in Fig. 1, a spatial diagonal 2a of the diamond crystal 2 is aligned parallel to the fluorescence axis 4 of the optics 3 and forms the extension of the fluorescence axis 4 of the optics 3, which runs through the center of the diamond crystal 2. The orientation of the diamond crystal 3 shown in Fig. 1 is advantageous because, in the orientation shown or in an orientation slightly deviating from this, it serves as a retroreflector, on whose three side surfaces projecting beyond the indentation 5, total reflection occurs.

[0053] In the example shown, the optic 3 is formed from a polymer material produced by additive manufacturing, more precisely by 3D printing. During production of the optic 3, the liquid or viscous polymer material is selectively cured by irradiation with light and brought into the shape shown in Fig. 1. The optic 3 can alternatively be produced by injection molding or in another way. The diamond material of the diamond crystal 2 has a refractive index nK of approximately 2.4. The refractive index n0 of the optic 3 deviates from the refractive index nK of the polymer material of the optic 3 by no more than 1.1, in the example shown by no more than 0.7.

[0054] The optics 3, or more precisely the material of the optics 3, is transmissive for fluorescent light 6, which is generated upon excitation of color centers in the form of NV centers 7, which are illustrated in Fig. 2, which shows a unit cell 8 of the diamond crystal 2. To collect the fluorescent light 6, an optical element 16 in the form of a converging lens is embedded in the optics 3, or more precisely in the material of the optics 3, which concentrates the fluorescent light 6 in the direction of the fluorescence axis 4. It is understood that other optical elements or non-optical components can also be embedded in the optics 3. In the event that no optical element is integrated into the optics 3, it can serve merely to guide the fluorescent light 6 and be fixed to another optic, which serves to collect the fluorescent light 6 in order to increase the fluorescence yield.

[0055] In the example shown, the fluorescent light 6 is generated by excitation light 9, which is irradiated by an excitation light source 10 onto the diamond crystal 2 and excites the NV centers 7. In the example shown, the diamond crystal 2 is fixed to the optics 3 with the aid of an optically transparent adhesive, which is applied in such a way that the diamond crystal 2 is held in the recess 5. The diamond crystal 2 was subsequently enclosed in liquid glass or a polymer, i.e., the diamond crystal 2 is embedded in a composite of optics 3 and additional optics in the form of a collection optic 11. The collection optic 11 is transmissive to the excitation light 9 for exciting the color centers 7 and serves to collect and transmit the fluorescent light from the diamond crystal 2 to a detector 13.The collection optics 11 is designed in the manner of a hood and serves to reflect a part of the fluorescent light 6 by total internal reflection in order to redirect it towards the detector 13.

[0056] The excitation light source 10 is arranged above the diamond crystal 2, for example, and is part of an optical sensor into which the sensor element 1 is also integrated. In the example shown, the optical sensor is designed as a magnetometer 12. The magnetometer 12 has the detector 13 described above, which serves to detect the fluorescent light 6, which propagates in the optics 3 essentially along the fluorescence axis 4. The magnetometer 12 also has a magnetic field generator 14 for generating an offset magnetic field Bo, which is aligned along a magnetic field axis 15, which in the example shown corresponds to the y-axis of the xyz coordinate system. The magnetic field generator 14 is designed to generate a homogeneous, static offset magnetic field Bo with a defined field strength and, for this purpose, has two coils in the example shown.

[0057] The excitation light source 10 can also be arranged at a different location, provided that it is ensured that the excitation light 9 hits the diamond crystal 2.

[0058] For example, the excitation light 9 can be guided through the optics 3, or the excitation light 9 can propagate parallel to the magnetic field axis 15 of the offset magnetic field Bo. In the example shown, the magnetic field axis 15 is aligned perpendicular to the fluorescence axis 4 of the optics 3, but this is not mandatory. For example, the magnetic field axis 15 of the offset magnetic field Bo can be aligned at an angle between 70° and 120° or between 80° and 100° to the fluorescence axis 4 of the optics 3. The magnetic field axis 15 of the offset magnetic field Bo can also be aligned at an angle of, for example, between -30° and 30°, for example, between -10° and 10°, to the fluorescence axis 4 of the optics 3. The magnetometer 12 has further components not shown, for example an evaluation device for evaluating a signal that is proportional to the intensity of the fluorescent light detected by the detector 13.The evaluation device makes it possible to use the magnetometer 12 to determine the field strength and, if applicable, the orientation of an external magnetic field, which is not illustrated in Fig. 1. For this purpose, the magnetometer 12 typically also has a microwave generator for generating a microwave field in the diamond crystal 2.

[0059] In the example shown in Fig. 1, the magnetometer 12 is a scalar magnetometer used to measure the magnetic field strength without determining the direction of the external magnetic field. For this purpose, it is advantageous if the magnetic field axis 15 of the offset magnetic field Bo is aligned parallel to one of four NV axes of the NV centers 7 of the diamond crystal 2. As can be seen in Fig. 2, which shows a unit cell 8 of the diamond crystal 2, the NV center 7 consists of a nitrogen atom N and a vacancy V. An imaginary connecting line between the nitrogen atom N and the vacancy V defines an NV axis of the NV center 7 shown in Fig. 2. The NV centers 7 are present in the diamond crystal 2 in four different orientations, ie there are four differently aligned NV axes, of which a first NV axis NVA is shown as an example in Fig. 2.

[0060] Relative to the coordinate system of unit cell 8, the four (normalized) NV axes NVA, NVB, NVC, NVD are aligned as follows, with both alignments (+ and -) being possible: NV A : (-1 , -1 , 1) / V3 NV B : (1 , 1 , 1 ) / V3 NVc: (1 , -1 , -1 ) / V3 NV D : (-1 , 1 , -1 ) / V3

[0061] For example, it is assumed below that the diamond crystal 2 was cut along a section in the

[0100] direction, i.e. the coordinate system of the diamond crystal 2 cut in the shape of a cube corresponds to the coordinate system of the unit cell 8. In the orientation of the diamond crystal 2 shown in Fig. 2, the NV axis NVA of the diamond crystal 2 runs in the direction of a spatial diagonal of the unit cell 8. In the example shown, the diamond crystal 2 is inserted into the indentation 5 such that the first NV axis NVA runs parallel to the magnetic field axis 15.

[0062] The energy states of the NV centers 7 are shifted in energy under the influence of a magnetic field, in this case the offset magnetic field Bo, due to the Zeeman effect, which leads to a change in the resonance frequency of the microwave radiated into the diamond crystal 2. Fig. 3 shows an ODMR spectrum of the intensity I of the fluorescent light 6 detected by the detector 13 as a function of the microwave frequency fw of the radiated microwave. The hyperfine structure splitting results in three resonances located close to one another. The dependence of the distance between the two resonance frequencies Af on the magnetic field, which in the example shown is aligned along the [-1 -1 1] direction of the unit cell 8 of the diamond crystal 2, is given by Af = 2 y eB[-1 -1 1] is given. In the representation of Fig. 3, the first NV axis NVA is aligned parallel to the magnetic field B[-1 -1 1], as described above in connection with Fig. 1. As can be seen in Fig. 3, the ms = 0 <-> ± 1 resonances of the first NV axis NVA have a maximum projection onto the magnetic field axis 15 of the offset magnetic field Bo, while the resonances of the other three crystallographic orientations of the second, third and fourth NV axes NVB, NVC, NVD have a smaller projection onto the magnetic field axis 15 of the offset magnetic field Bo and coincide in Fig. 3. The offset magnetic field Bo thus cancels the degeneracy of the four different orientations near the so-called zero-field splitting at a microwave frequency fw of 2.87 GHz. This makes it possible to measure the external magnetic field with a smaller linewidth of resonance, which is directly related to the sensitivity of the magnetometer 12.In this case, the sensitivity of the magnetometer 12 is determined only by the laser power of the excitation light source 10, by the microwave power and by the stability of the offset magnetic field Bo.

[0063] Fig. 4a, b show a magnetometer 12 which differs from the magnetometer 12 shown in Fig. 1 in the type of sensor element 1. As can be seen in Fig. 4a, the optics 3 of the magnetometer are designed in the form of a concentrator, more precisely in the form of a compound parabolic concentrator, in order to collect the fluorescent light 6 and increase the fluorescence yield on the detector 13. As can be seen in Fig. 4b, the indentation 5 has three flat contact surfaces 17a-c, which serve to contact three of the six side surfaces, more precisely partial regions of the three side surfaces of the cube-shaped diamond crystal 2, in order to fix the diamond crystal 2 in a predetermined orientation to the optics 3.

[0064] The orientation of the diamond crystal 2 of Fig. 4a, b with respect to the optics 3 and with respect to the magnetic field axis 15 differs in the sensor element 1 of Fig. 4a, b from the orientation of the sensor element 1 of Fig. 1. The orientation shown in Fig. 4a, b is obtained by first rotating the diamond crystal 2 around the x-axis, by an angle ai of 45°, starting from an orientation in which the three edges of the unit cell 8 of Fig. 2 are aligned along the three axes xyz of the xyz coordinate system shown in Fig. 4a, b. As can be seen in Fig. 5a, which shows the (normalized) magnitude Bo.proj of the projection of the magnetic field axis 15 onto each of the four NV axes NVA, NVB, NVC, NVD as a function of the rotation angle a around the x-axis, the magnitude Bo.proj of the projection of the magnetic field axis 15 onto the first NV axis NVA is at its maximum when rotating through the angle ai of 45° (± 180°). At the angle ai of 45°, the distance orthe difference to the amount Bo.proj of the projection of the magnetic field axis 15 onto the other three NV axes NVB, NVC, NVD is maximum, ie the angle ai is the optimal angle of rotation when the magnetic field axis 15 is aligned in the y-direction.

[0065] For alignment, the diamond crystal 2 is rotated again, specifically by an angle ßi of approximately 35.3° (±180°) around the z-axis. As can be seen in Fig. 5b, at angle ßi, on the one hand, the projection Bo.proj of the magnetic field axis 15 onto the first NV axis NVA is almost maximum, and on the other hand, the difference to the absolute value Bo.proj of the projection of the magnetic field axis 15 onto the other three NV axes NVB, NVc, NVD is also very large. The indentation 5 shown in Fig. 4a, b is designed to accommodate the diamond crystal 2 rotated at the two rotation angles ai and ßi. In this case, the first NV axis NVA is aligned exactly or almost parallel to the magnetic field axis 15. The orientation of the diamond crystal 2 shown in Fig. 4a, b differs from the orientation shown in Fig. 1 in that in Fig. 1 both the rotation angle ai about the x-axis and the rotation angle ßi about the z-axis are 45° each.

[0066] To correctly align the sensor element 1 in the mechanical structure of the magnetometer 12 with respect to the magnetic field axis 15 running in the y-direction, a first angle marking 18a is applied to the base of a lateral surface 3b of the optics 3 (see Fig. 4b). The first angle marking 18a serves to align the sensor element 1 for single-axis operation of the magnetometer 12, in which the magnetic field axis 15 and the first NV axis NVA of the NV centers 7 are aligned essentially parallel.

[0067] As can be seen in Fig. 4b, the optics 3 also has a second angle marking 18b on its circumferential surface 3b, which serves to align the sensor element 1 in the mechanical and optical structure of the magnetometer 12 during multi-axis or vector operation. The first and second angle markings 18a, 18b differ in their design, in the example shown, in their shape, to facilitate the operator's assignment of the respective angle marking 18a,b to scalar or vector operation.

[0068] During vector operation of the magnetometer 12, the magnetic field direction of the external magnetic field is measured in addition to the magnetic field strength. For vector operation, the projections Bo.proj of the magnetic field axis 15 onto the four NV axes NVA, NVB, NVC, NVD of the diamond crystal 2 should be separated from each other as much as possible, i.e., the minimum distance or the minimum difference between the absolute values ​​Bo.proj of the projections of the magnetic field axis 15 onto the four NV axes NVA, NVB, NVC, NVD should be as large as possible. The distance from zero of the absolute value Bo.proj of the projection of the magnetic field axis 15 onto the four NV axes NVA, NVB, NVC, NVD should also be as large as possible.

[0069] As shown in Fig. 5c, this can be achieved by rotating the diamond crystal 2 fixed to the optics 3 once again around the fluorescence axis 4 of the optics 3, which corresponds to the x-axis, after the two rotations shown in Fig. 5a and Fig. 5b. As can be seen in Fig. 5c, the above condition is optimally fulfilled at a rotation angle yi of approximately 50.8° (±180°). At the rotation angle yi shown in Fig. 5c, the magnitude Bo.proj of the projection of the vector of the offset magnetic field Bo (the vector of the offset magnetic field corresponds to the magnetic flux density) onto any one of the four NV axes NVA, NVB, NVC, NVD of the NV centers 7 of the diamond crystal 2 and the magnitude Bo.proj of a projection of the vector of the offset magnetic field Bo onto any other of the four NV axes NVA, NVB, NVc, NVD of the diamond crystal 2 have a difference of at least 230 pT. In the example shown, the vector of the offset magnetic field Bo runs in the direction of the magnetic field axis 15.The offset magnetic field Bo is understood to be the magnitude of the magnetic flux density or the vector of the offset magnetic field Bo in the direction of the magnetic field axis 15. As can also be seen in Fig. 5c, the distances between each two of the NV axes are NVc, NVA. ; NVA, NVB; NVB, NVD; NVD and zero are each approximately equal.

[0070] The optimal angles ai, ßi, yi of 45°, 35.3°, and 0° for single-axis operation and of 45°, 35.3°, and 50.8° for multi-axis operation described above are not the only angle combinations that enable optimal scalar or vector operation of the magnetometer 12, i.e., there are also other angle combinations that have the same or a similar effect. The greater the deviation from the optimal angle combinations, the greater the field strength of the offset magnetic field Bo typically needs to be in order to achieve sufficient separation of the resonances of the four NV axes NVA, NVB, NVC, NVD of the diamond crystal 2 and thus sufficient sensitivity when measuring the external magnetic field. A deviation of a few degrees, e.g. up to approximately 10° from the optimal angles in single-axis operation or up to approximately 5° from the optimal angles in multi-axis operation is generally acceptable.

Claims

Patent claims 1. Sensor element (1), comprising: a crystal with color centers, in particular a diamond crystal (2) with NV centers (7), characterized by an optic (3) to which the crystal is fixed, wherein the optic (3) has an alignment aid (5) for fixing the crystal to the optic (3) with a predefined orientation, and wherein the optic (3) is transmissive for excitation light (9) for exciting the color centers, in particular the NV centers (7), and / or is transmissive for fluorescent light (6) which is generated upon excitation of the color centers, in particular the NV centers (7).

2. Sensor element according to claim 1, wherein the alignment aid forms a recess (5) on the optics (3) into which the crystal is inserted.

3. Sensor element according to claim 2, wherein the indentation (5) has at least three preferably planar contact surfaces (17a-c) for contacting at least three side surfaces of the preferably cuboid-shaped or cube-shaped crystal.

4. Sensor element according to one of the preceding claims, in which the optics (3) are formed from a material having a refractive index (no) which deviates from a refractive index (nx) of the crystal by not more than 1.1, preferably by not more than 0.

7.

5. Sensor element according to one of the preceding claims, in which the optics (3) are manufactured by additive manufacturing or form an injection-molded part.

6. Sensor element according to one of the preceding claims, in which the crystal (2) fixed to the optics (3) is embedded in a material which is preferably transmissive for the excitation light (9) for exciting the color centers (7) and / or for the fluorescent light (6), wherein the material is designed in particular in the form of an additional optics (11).

7. Sensor element according to one of the preceding claims, in which at least one component, in particular at least one optical element (16), is embedded in the material of the optics (3).

8. Sensor element according to one of the preceding claims, in which the optics (3) are designed to guide the fluorescent light (6) along a fluorescence axis (4), wherein the optics (3) preferably forms a rotational body with the fluorescence axis (4) as the axis of symmetry.

9. Sensor element according to claim 8, wherein the optics (3) are designed as non-imaging optics, preferably as a concentrator, in particular as a compound parabolic concentrator.

10. Sensor element according to one of claims 8 or 9, wherein the crystal is cube-shaped and has a spatial diagonal (2a) which is aligned substantially parallel to the fluorescence axis (4) of the optics (3).

11. Sensor element according to one of claims 8 to 10, wherein the optics (3) has a first angle marking (18a) for indicating a first angular position of the crystal, preferably the diamond crystal (2), in a plane (y, z) perpendicular to the fluorescence axis (4) of the optics (3), wherein the first angle marking (18a) preferably serves to align the sensor element (1) for the uniaxial operation of an optical sensor, in particular a magnetometer (12).

12. Sensor element according to one of claims 8 to 11, in which the optics (3) have a second angle marking (18b) for indicating a second angular position of the crystal, preferably the diamond crystal (2), in a plane (y, z) perpendicular to the fluorescence axis (4) of the optics (3), wherein the second angle marking (18b) preferably serves for aligning the sensor element (1) during multi-axis operation of an optical sensor, in particular a magnetometer (12).

13. Optical sensor, in particular magnetometer (12), comprising: a sensor element (1) according to one of the preceding claims, and a magnetic field generator (14) for generating an offset magnetic field (Bo) in the region of the crystal, preferably in the region of the diamond crystal (2), wherein the offset magnetic field (Bo) is aligned along a magnetic field axis (15).

14. Optical sensor according to claim 13, wherein the magnetic field axis (15) of the offset magnetic field (Bo) is aligned at an angle between 70° and 120°, preferably between 80° and 100°, to the fluorescence axis (4) of the optics (3).

15. Optical sensor according to claim 13, wherein the magnetic field axis (15) of the offset magnetic field (Bo) is aligned at an angle between -30° and 30°, preferably between -10° and 10°, to the fluorescence axis (4) of the optics (3).

16. Optical sensor according to one of claims 13 to 15, wherein one of the NV axes (NVA) of the NV centers (7) of the diamond crystal (3) is aligned substantially parallel to the magnetic field axis (15) of the offset magnetic field (Bo).

17. Optical sensor according to claim 13 or 14, wherein a magnitude (Bo.proj) of a projection of the vector of the offset magnetic field (Bo) onto any one of the four NV axes (NVA, NVB, NVC, NVD) of the NV centers (7) of the diamond crystal (2) and a magnitude (Bo.proj) of a projection of the vector of the offset magnetic field (Bo) onto any other one of the four NV axes (NVA, NVB, NVC, NVD) of the diamond crystal (2) have a difference of at least 230 pT.

Images