BACKGROUND

[0001] Detecting the intensity of photoluminescence emanating from spin ensembles made of nitrogen vacancy (NV) centers in diamond can be useful in a variety of sensor applications. NV center-based sensors have a high degree of sensitivity that enables detection of small magnetic and electric fields, as well as other quantities such as strain. Such sensors can be used in conjunction with fluorescent microscopes to measure high-resolution spatial maps of local magnetic sources, and other applications such as biological process imaging and mechanical stress detection.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] Examples will now be described with reference to the accompanying drawings, in which:

[0003] FIG. 1 shows a block diagram of an example nitrogen vacancy (NV) quantum sensor device;

[0004] FIG. 2 shows a block diagram of another example of a nitrogen vacancy (NV) quantum sensor device;

[0005] FIGs. 3, 4 and 5, show top down views of different examples of a second substrate in an example NV sensor device having differently shaped grating couplers; [0006] FIG. 6 shows a block diagram of another example of a nitrogen vacancy (NV) quantum sensor device integrated into a microfluidic architecture; and, [0007] FIGs. 7 and 8 show flow diagrams that illustrate example methods of sensing a magnetic field using a nitrogen vacancy (NV) center quantum sensor. [0008] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

[0009] Current implementations of nitrogen vacancy (NV) center quantum sensors involve the use of discrete off-the-shelf components that are bulky and not conducive to quantum sensing applications outside of a laboratory environment. For example, NV center quantum sensors can include a discrete RF/microwave signal generator and amplifier, a discrete pump laser (light source) to raise electron energy levels and induce fluorescence, a discrete photodetector or array of photodetectors for measuring NV fluorescence, and several optical filters to filter the pump laser. NV center quantum sensors implemented using such discrete components are generally not mobile, and they are limited in their practical applicability and scalability. Efforts to integrate such components to achieve a more compact NV sensor device are ongoing.

[0010] Accordingly, example integrated quantum sensor devices and methods based on nitrogen vacancy (NV) centers in diamond are described herein. The use of thin, surface-mount illumination/excitation sources (e.g., VCSELs) and light detectors, in addition to thin-film optical metasurfaces, allow for the integration of active and passive optical and electrical components on a planar, compact sensor device. The compact sensor device enables a platform for quantum sensing based on NV centers in diamond that has a form factor conducive to mass production and integration into portable devices. Such mobile quantum senor platforms can provide advantages in both remote and laboratory environments when performing general purpose NV quantum sensing functions, such as measuring the strength and direction of magnetic fields (magnetometry).

[0011] As used herein, an optical metasurface may include and may be referred to as a grating coupler, flat lens, zone plate, Fresnel lens, and the like. In general, optical metasurfaces comprise nanostructured interfaces with sub-micron thickness that can manipulate light, for example, by bending and directing light at engineered refraction angles, focusing light, and dispersing or filtering light. The use of thin, surface-mount illumination sources and detectors, and thin-film optical metasurfaces, enables the integration of components in a co-planar manner onto substrates such as a silicon die, a printed circuit board (PCB), and a diamond layer substrate. The co planarity of these elements allows for the miniaturization of NV center quantum sensor devices.

[0012] In example NV center quantum sensor devices and methods, total internal reflection (TIR) of the source illumination/excitation through a diamond layer is enabled by a sharp angle of refraction caused by a grating coupler disposed on an outer edge of the diamond. The effect of TIR can be exploited to provide illumination coupling into a sensing zone of the diamond where the NV centers are implanted into the diamond volume close to the diamond surface where TIR occurs. TIR additionally provides for different spacing or placement options of components within an integrated quantum sensor device so that the source illumination can reach the components by reflecting (i.e. , bouncing) off the inner edges of the diamond layer.

[0013] In a particular example, a nitrogen vacancy (NV) center quantum sensor includes a first substrate having a surface-mount light source and detector element integrated onto a surface of the first substrate, and a second substrate having embedded NV centers. The sensor also includes a grating coupler and a flat lens integrated onto a surface of the second substrate facing the light source and detector element, the grating coupler to direct light from the light source to the NV centers and the flat lens to focus fluorescent light from the NV centers to the sensor element. [0014] In another example, a method of sensing a magnetic field with a nitrogen vacancy (NV) center quantum sensor, includes emitting excitation light from a surface- mount light source on a first substrate, bending the light through a metasurface grating coupler integrated on a second substrate to direct the light onto NV centers embedded in the second substrate, focusing fluorescent light from the NV centers through a metasurface flat lens integrated on the second substrate coplanar with the grating coupler, and sensing the focused fluorescent light with a detector element integrated on the first substrate coplanar with the light source.

[0015] In another example, a nitrogen vacancy (NV) center quantum sensor includes a first substrate having a surface-mount light source and detector element integrated in a co-planar orientation onto a surface of the first substrate, and a second substrate having embedded nitrogen vacancy (NV) centers. A grating coupler and a flat lens are integrated in a co-planar orientation onto a surface of the second substrate facing the light source and detector element. The sensor also includes a microfluidic architecture coupled to the second substrate to provide a fluid in a microfluidic channel in which an analyte magnetic field is to be measured by detection of fluorescence from the NV centers.

[0016] FIG. 1 shows a block diagram of an example nitrogen vacancy (NV) quantum sensor device 100. The example NV sensor device 100 can include a first substrate 102, such as a silicon die 102 or printed circuit board (PCB). The first substrate 102 can have an illumination/excitation source 104 and a detector element 106 integrated thereon in a co-planar orientation. Thus, both the illumination source 104 and detector element 106 are integrated on the same substrate 102. The illumination source 104 can include any suitable surface-mount or thin-film light source, such as a vertical-cavity surface-emitting laser (VCSEL), an array of VCSELs, a surface-mount diode (SMD), an array of SMDs, and so on. A detector element 106 can include any suitable surface-mount or thin-film photoelectric device to convert light energy into an electrical signal, such as a charge coupled device (CCD), array of CCDs, a photodiode, a silicon photomultiplier, a phototransistor, and so on.

[0017] The example NV sensor device 100 can also include a second substrate 108, such as a diamond layer modified with NV center implantation. The second substrate 108 can be separated by, or spaced away from the first substrate 102 by a spacer 110 or multiple spacers 110. A spacer 110 can comprise an opaque material that can block stray light from contacting the detector element 106, or a transparent material that enables light such as green light and red light to pass through. A spacer 110 can comprise any suitable material such as SU8, polymer, glass, plastic, or the like. The area 112 between spacers 110 can be filled with air, an inert gas, or additional transparent spacer material, for example. The spacer 104 provides separation between the first and second substrates to enable optical communication (i.e. , illumination excitation and coupling) between components integrated onto the surfaces of the first and second substrates. Spacers 104 additionally allow for a compact, robust integration and easy alignment and packaging of the example NV sensor device 100.

[0018] The second substrate 108 can include optical metasurface components such as a grating coupler 114 and a flat lens 116 integrated in a co-planar orientation onto a first surface 118 that faces the illumination source 104 and detector element 106 on the first substrate 102. The second substrate 108 comprises NV centers 120 implanted in the material (e.g., diamond) volume of the substrate 108. In some examples, the NV centers 120 are implanted close to the second surface 122 of the second substrate 108 to improve the exposure (i.e., double the exposure) of the NV centers 120 to excitation illumination 124 reflecting (bouncing) off the second surface 122 of the second substrate 108 in a TIR mode, as discussed below.

[0019] The second substrate 108 can also include a radio frequency (RF) excitation coil/circuit 126 fabricated on the outside of the second surface 122 of the second substrate 108 closest to a sensing zone “A” 127 that includes the implanted NV centers 120. However, the location of the RF coil 126 is not limited to the outside of the second surface 122. In some examples the RF coil 126 can be fabricated elsewhere, such as on the first surface 118 between the flat lens 116 and the first surface 118. In some examples, other coatings and/or substrates can be interposed between the second substrate 108 and the RF coil 126, and/or the second substrate 108 and the grating coupler 114 and flat lens 116.

[0020] During operation of an example NV sensor device 100, illumination/excitation 124 (e.g., green light on the order of 532 nanometer wavelength) emitted from illumination source 104 in a direction indicated by direction arrow 128 impinges the grating coupler 114 and is refracted at a sharp angle 130 through the second substrate 108 toward the NV centers 120 directly or through multiple TIR reflections. The illumination/excitation 124 impinges on and excites the NV centers 120, causing them to fluoresce with red fluorescent light 132. As noted above, the grating coupler 114 enables a TIR mode (total internal reflection) of the illumination 124 such that the illumination 124 reflects without loss off the inner second surface 122 of the second substrate 108. The close proximity of the implanted NV centers 120 to the second surface 122 enables a second exposure of the NV centers 120 to the fully reflected illumination 124 as it bounces off the inner second surface 122 of the substrate 108. The second exposure of the NV centers 120 to the illumination 124 increases the red fluorescent light 132 output from the NV centers 120 which improves the overall sensitivity of the NV sensor device 100.

[0021] Although not specifically illustrated, it is noted that the TIR mode which provides a lossless reflection of the illumination/excitation 124 within the second substrate 108, allows for placement of components of the NV sensor device 100 at various lengths along the substrate 108. For example, the NV centers 120 can be implanted in the substrate at a location that is farther away from the grating coupler 114. Likewise, the flat lens 116 and sensing zone “A” 127 that includes the implanted NV centers 120 can also be located at such a farther distance away from the grating coupler 114. Furthermore, the grating coupler can be designed with a distribution of refracted angle so as to focus illumination light onto a desired location (e.g. the NV center location). Such varying placements of components can be beneficial to accommodate varying sizes, shapes, and configurations of example NV sensor devices 100.

[0022] The flat lens 116 collects and focuses the red fluorescent light 132 output from the NV centers 120 in the direction of arrow 134 and onto the detector element 106. In some examples, an optical metasurface filter 136 can be used to filter unwanted light, such as stray ambient light or scattered green light, and prevent the unwanted light from striking the detector element 106. The detector element 106 can sense the red fluorescent light 132 output from the NV centers 120 and convert it into an electrical signal that provides information about the environment being sensed. For example, the electrical signal from the detector element 106 can provide information that indicates the strength and direction of a sensed magnetic field.

[0023] In some examples, stimulation from an RF coil 126 induced, for example, by application of an oscillating voltage power supply (not shown) to the RF coil inputs 142, 144 (FIGs. 3, 4, 5) can be used to provide RF excitation to the NV centers 120. Applying an oscillating voltage over a frequency range to the RF coil 126 can cause the red fluorescent light 132 output from the NV centers 120 to vary as a function of the RF frequency. The varying fluorescent light 132 output from the NV centers 120 detected by detector element 106 can shape the electronic signal from the detector element 106 to provide additional information about the sensed environment.

[0024] FIG. 2 shows a block diagram of another example of a nitrogen vacancy (NV) quantum sensor device 100. In FIG. 2, three active zones A, B, and C, are illustrated, and are discussed further with respect to FIGs. 3, 4, and 5. Active zone A (127, FIG. 1 ) comprises a sensing zone, while active zones B and C comprise illumination coupling zones. In the Fig. 2 example, an additional illumination/excitation source 138 is integrated onto the first substrate 102 in a co-planar orientation with the source 104 and detector element 106. The source 138 emits illumination 124 in a direction indicated by direction arrows 128 that impinges a second grating coupler 140 integrated in a co-planar orientation with flat lens 116 onto the second substrate surface 118 that faces detector element 106 on the first substrate 102. The illumination 124 from the source 138 is refracted at a sharp angle through the second substrate 108 toward the NV centers 120 directly or through multiple TIR reflections, causing them to fluoresce with red fluorescent light 132. The additional illumination source 138 can increase the fluorescence from the NV centers 120, increasing the sensitivity of the NV sensor device 100. [0025] FIGs. 3, 4 and 5, show top down views of different examples of the second (diamond) substrate 108 in an example NV sensor device 100. The optical metasurfaces (i.e. , grating couplers 114 and 140, and flat lens 116) are shown in FIG. 3 as being generally circular or elliptical in shape, although other shapes are possible. In FIGs. 4 and 5, the grating couplers 114 and 140 are shown with shapes that increasingly surround the area of the substrate 108 containing the NV centers 120. In FIG. 4, the grating couplers 114 and 140 are partly surrounding the NV centers 120, and in FIG. 5, the grating couplers 114 and 140 are joined in a loop that completely surrounds the NV centers 120. These different example configurations of the grating couplers 114 and 140 can provide varying levels of illumination coupling into the NV centers 120 resulting in varying levels of fluorescence, for purposes such as increasing the sensitivity of the NV sensor device 100.

[0026] FIG. 6 shows a block diagram of another example of a nitrogen vacancy (NV) quantum sensor device 100 integrated into a microfluidic architecture 142 for the monitoring and analysis of magnetic fields in bio-fluids with high sensitivity, for example. In this implementation, a microfluidic channel 144, or a channel network comprising fluidic mixers, filters, sources and reaction chambers (not shown), is written into a thin layer 146 (e.g., an SU8 layer) which is coupled to the second surface 122 of the diamond substrate 108 in which the NV centers 120 are implanted. In this example, the RF coil 126 is positioned on the first substrate 102. Flowever, the various components including the RF coil 126, the optical metasurfaces (114, 116, 140), and the sources 104, 138, and detector 106, are not limited to the locations shown in the NV sensor device 100 of FIG. 6. In some examples, varying the locations of some components can be beneficial to accommodate varying sizes, shapes, and configurations of example NV sensor devices 100. In the FIG. 6 example, the NV sensor device 100 functions in a manner similar to that discussed above with reference to FIGs. 1 and 2, to monitor magnetic fields associated with analytes flowing through a microfluidic channel 144 in a microfluidic architecture 142 coupled to the sensor device 100.

[0027] FIGs. 7 and 8 are flow diagrams showing example methods 700 and 800 of sensing a magnetic field using a nitrogen vacancy (NV) center quantum sensor. Method 800 comprises extensions of method 700 and incorporates additional details of method 700. Methods 700 and 800 are associated with examples discussed above with regard to FIGs. 1 - 6, and details of the operations shown in methods 700 and 800 can be found in the related discussion of such examples. The methods 700 and 800 may include more than one implementation, and different implementations of methods 700 and 800 may not employ every operation presented in the respective flow diagrams of FIGs. 7 and 8. Therefore, while the operations of methods 700 and 800 are presented in a particular order within their respective flow diagrams, the order of their presentations is not intended to be a limitation as to the order in which the operations may actually be implemented, or as to whether all of the operations may be implemented. For example, one implementation of method 800 might be achieved through the performance of a number of initial operations, without performing other subsequent operations, while another implementation of method 800 might be achieved through the performance of all of the operations.

[0028] Referring now to the flow diagram of FIG. 7, an example method 700 of sensing a magnetic field using a nitrogen vacancy (NV) center quantum sensor begins at block 702 with emitting excitation light from a surface-mount light source on a first substrate. The method continues with bending the light through a metasurface grating coupler integrated on a second substrate to direct the light onto NV centers embedded in the second substrate (block 704) either directly or through multiple TIR reflections. The method includes focusing fluorescent light from the NV centers through a metasurface flat lens integrated on the second substrate coplanar with the grating coupler (block 706), and sensing the focused fluorescent light with a detector element integrated on the first substrate coplanar with the light source.

[0029] Referring now to the flow diagram of FIG. 8, another example method 800 of sensing a magnetic field using an NV center quantum sensor is shown. Method 800 comprises extensions of method 700 and incorporates additional details of method 700. Accordingly, method 800 begins at block 802 with emitting excitation light from a surface-mount light source on a first substrate, and continues with bending the light through a metasurface grating coupler integrated on a second substrate to direct the light onto NV centers embedded in the second substrate (block 804), focusing fluorescent light from the NV centers through a metasurface flat lens integrated on the second substrate coplanar with the grating coupler (block 806), and sensing the focused fluorescent light with a detector element integrated on the first substrate coplanar with the light source (block 808).

[0030] In some examples, method 800 can also determine the strength and direction of a magnetic field based on detecting the focused fluorescent light (block 810), and applying RF excitation over a frequency range to the NV centers using an RF coil integrated on the second substrate (block 812). In some examples, the RF coil can be integrated on a surface of the second substrate that is opposite a surface of the second substrate on which the grating coupler and flat lens are integrated (block 814). In some examples, the first and second substrates can comprise, respectively, a silicon substrate and a diamond substrate (block 816). In some examples, emitting excitation light can include emitting excitation light from first and second (or multiple) surface-mount light sources oriented on the first substrate in a co-planar orientation (block 818). In some examples, bending the light can include bending light from the first light source through a corresponding first grating coupler, and bending light from the second light source through a corresponding second grating coupler, wherein the first and second grating couplers are positioned with respect to one another on the second substrate in a co-planar orientation (block 820). In some examples, bending the light can include bending light from the first light source and the second light source through a single grating coupler surrounding the NV centers (block 822).