RU2607840C1 - Optical magnetometer - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measuring magnetic fields and concerns an optical magnetometer. Magnetometer comprises a low frequency generator, a condenser, at least one electromagnet coil, an active material of the form of a silicon carbide crystal containing at least one spin center based on silicon vacancy with the main quadrupole state placed inside the coil, a direct current source, a synchronous detector, a control unit, an optical system of a semitransparent mirror, a mirror, a light filter, a lens and an objective lense, a laser radiating in the near-infrared region, and a photodetector.

EFFECT: technical result is simplification of the device and enabling its operation in the biological objects transmissions band.

5 cl, 3 dwg

Description

The invention relates to nanotechnology and can be used in the field of the development of materials based on silicon carbide for magnetometry, quantum optics, biomedicine, as well as in information technologies based on the quantum properties of spins and single photons.

The detection of weak magnetic fields with high spatial resolution at the level of micro- and nanometers is an important problem in various fields, from fundamental physics and materials science to data storage and biomedical science. For example, at a distance of 10 nm, the spin of one electron creates a magnetic field of about 1 mT, and the corresponding field of one proton is several nT. A sensor capable of detecting such magnetic fields with nanometer spatial resolution will find wide applications ranging from the detection of magnetic resonance signals from individual electronic or nuclear spins in complex biological molecules to the reading of classical or quantum bits of information encoded in electronic or nuclear spin memory. A special role in magnetometry is played by optical magnetometers.

After the discovery of the unique emitting properties of NV centers in diamond, which allows optical detection of magnetic resonance in the ground state of NV centers at room temperature until the detection of magnetic resonance on single defects (see A. Gruber, A. Drabenstedt, C. Tietz, L. Fleury, J. Wrachtrup, C. Von Borczyskowski. - Scanning Confocal Optical Microscopy and Magnetic Resonance on Single Defect Centers. - Science, v. 276, pp. 2012-2014, 1997 ;; J. Wrachtrup, F. Jelezko, Processing quantum information in diamond. - J. Phys .: Condens. Matter, v. 18, S807, 2006), it became possible to create optical quantum magnetometers for measuring magnetic fields with nanoscale th resolution. The NV center, which is a carbon (V) vacancy, in the nearest coordination sphere of which one of the four carbon atoms is replaced by a nitrogen (N) atom, has a ground triplet spin state, the populations of the spin levels of which are selectively populated by optical radiation. The principle of magnetometry with NV centers is based on the optical detection of magnetic resonance (ODMR) in an external magnetic field, which must be measured.

Known optical magnetometer using NV centers in diamond, operating at room temperature (J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, PR Hemmer, A. Yacoby, R. Walsworth, and MD Lukin. - Nat. Phys. V. 4, 810, 2008), including a microwave generator operating in the 2.5-3.0 GHz band, a low frequency generator, modulating microwave generator power, a laser with a wavelength of 532 nm, a focusing optical a system in the form of a system of lenses, mirrors and filters, a registration system in the form of an avalanche photodiode, an active material in the form of a nanoscale diamond crystal with NV centers (nanocrystal ) placed on a probe of an atomic force microscope. The magnetic field is measured by optical magnetic resonance detection (ODMR) using the luminescence intensity emitted by NV centers. The magnetic field is determined by measuring the frequency of magnetic resonance, which depends on the Zeeman shift of spin levels in a magnetic field.

The disadvantages of the known optical magnetometer is the use of expensive active material, the production technology is relatively poorly developed. In addition, the known optical magnetometer operates in the visible region of the optical range, which is poorly combined with silicon-based fiber optics, as well as with the transparency band of biological systems. The use of a microwave radiation generator complicates the design, creates additional noise, and also leads to heating of the object of study. The splitting of the fine structure of the NV centers is highly dependent on the ambient temperature; therefore, additional devices are needed to repay undesirable temperature effects. The use of a diamond nanocrystal requires preliminary determination of the crystal orientation in space in order to determine the orientation of the NV centers.

A known optical magnetometer based on electron spins in a solid-state medium, such as defects in crystals and semiconductors, which uses separate electron spins or electronic spin systems (see US Pat. No. 8547090, IPC G01R 33/02, published October 1, 2013), including a generator Microwave radiation, with a system for creating pulsed sequences of microwave radiation, an optical system for collecting and transmitting photons of optical radiation, an active material in the form of a diamond crystal, including one or more NV centers having one Do some of the electron spins, the source of optical radiation, for example laser detector. At high spin densities, methods and systems are needed to decouple electronic spins from each other and from the local medium. In a magnetometer, electronic spins are controlled by applying microwave pulses to electronic spins that dynamically reduce spin-spin interactions and interactions with the lattice.

The disadvantages of the considered optical magnetometer are the use of diamonds with NV centers as the active material of the magnetometer, the technology for which is extremely expensive and relatively poorly developed. In addition, the known optical magnetometer is poorly combined with silicon-based fiber optics, as well as with the transparency band of biological systems. The use of a microwave radiation generator complicates the design, creates additional noise, and also leads to heating of the object of study. In the known optical magnetometer, additional devices are needed to repay unwanted temperature effects.

Known optical magnetometer on NV defects in diamond (see patent US 8947080, IPC G01R 33/02; G01R 33/00; G01V 3/08, published 02/03/2015), which coincides with this solution for the largest number of essential features and adopted as a prototype . The prototype optical magnetometer includes a microwave radiation generator, a microwave radiation modulation unit, a synchronous detector, a laser with a wavelength of 532 nm, a focusing optical system in the form of a system of lenses, mirrors and filters, a photodetector in the form of an avalanche photodiode, a control unit, an active material in the form of a crystal diamond containing at least one spin NV center, the axis of the NV center is oriented along one of the four <111> crystallographic axes. Active material is placed inside an electromagnet coil to measure the magnetic fields generated by extended or distant objects. The magnetic field is measured by an optical prototype magnetometer using the method of optical magnetic resonance detection (ODMR) by the luminescence intensity emitted by NV centers. The magnetic field is determined by measuring the frequency of magnetic resonance, which depends on the Zeeman shift of spin levels in a magnetic field.

The disadvantages of the considered optical magnetometer is the use of diamonds with NV centers as the active material of the magnetometer, the production technology of which is expensive and relatively poorly developed. The prototype optical magnetometer works in the visible region of the optical range, which does not fit well with silicon-based fiber optics, as well as with the transparency band of biological systems. The use of a microwave radiation generator complicates the design of the magnetometer, creates additional noise, and also leads to heating of the object of study. The splitting of the fine structure of the NV centers is highly dependent on the ambient temperature; therefore, the prototype optical magnetometer contains additional devices to suppress undesirable temperature effects.

The present invention is the development of such an optical magnetometer, which would be simpler and cheaper to manufacture, worked in the optical range compatible with fiber optics and the transparency band of biological objects.

The problem is solved in that the optical magnetometer includes a low frequency generator (LF), a capacitor, at least one coil of an electromagnet, an active material in the form of a silicon carbide crystal, containing at least one spin center based on a silicon vacancy with a fundamental quadrupole state, source direct current for powering the electromagnet coil with direct current, synchronous detector, control unit, laser emitting in the near infrared (IR) region, optically coupled through a translucent mirror A mirror, a lens and an active material lens, a photodetector optically connected to the active material through a lens, a mirror, a translucent mirror, and a light filter. The first output of the low-frequency generator through a capacitor is connected to an electromagnet coil, to which the DC output is also connected, the second output of the low-frequency generator is connected to the first input of the synchronous detector, the second input of the synchronous detector is connected to the output of the photodetector, the output of the synchronous detector is connected to the input of the control unit, the output which is connected to the input of a DC source.

The active material can be placed on a scanning table of a confocal microscope with a piezoelectric element capable of reciprocating movement in three mutually perpendicular directions.

The active material can be made in the form of a nanoscale crystal of silicon carbide. A nanosized crystal of silicon carbide can be placed on a probe of an atomic force microscope or on a probe of a near-field microscope.

New in the present optical magnetometer is the implementation of the active material in the form of a silicon carbide crystal containing at least one spin center based on a silicon vacancy with the ground quadrupole state; introducing from the composition of the magnetometer a direct current source and an LF generator connected to an electromagnet coil, as well as replacing a visible laser with a laser emitting in the near infrared region.

In the present optical magnetometer, instead of optical detection of magnetic resonance using microwave radiation, the physical phenomenon of antisection of spin levels in spin centers based on a silicon vacancy with the ground quadrupole state is used, which leads to a strong change in the luminescence intensity in the region of magnetic fields close to the intersection point of spin levels .

Spin centers based on a silicon vacancy with a quadrupole ground state in silicon carbide (SiC) are known (see H. Kraus, V. A. Soltamov, F. Fuchs, D. Simin, A. Sperlich, PG Baranov, GV Astakhov, V. Dyakonov; Magnetic field and temperature sensing with atomic-scale spin defects in silicon carbide, Scientific Reports, 2014). The presence of the physical effect of optical alignment of the spins of spin centers based on a silicon vacancy with a ground quadrupole state when a silicon carbide crystal is irradiated with near infrared (IR) light at room temperature makes it possible to optically record magnetic resonance in the ground state of spin centers at room temperature, up to recording magnetic resonance on single spin centers based on a silicon vacancy. The axes of the ensemble of spin centers based on a silicon vacancy with a quadrupole ground state in silicon carbide are oriented along the hexagonal crystallographic axis c, in contrast to the ensemble of NV centers in diamond, in which the axis of the NV centers are oriented along one of the four <111> crystallographic axes. The spin center based on a silicon vacancy with a quadruplet ground state is a negatively charged silicon vacancy (V _Si ^- ) with spin S = 3/2, interacting with a neutral carbon vacancy (V _C °) located along the hexagonal crystallographic axis (c - axis ) with respect to the silicon vacancy and not having a molecular bond with the silicon vacancy. Optical excitation in the near infrared range (750–850 nm) results in alignment of the spins of the spin centers on the basis of a silicon vacancy with the ground quadruple state, which creates a nonequilibrium filling of the spin levels.

This technical solution is illustrated by drawings,

 where in FIG. 1 is a block diagram of a true optical magnetometer;

in FIG. 2 is a schematic perspective view of an optical microscope assembly with a scanning table (PL - photoluminescence);

in FIG. Figure 3 shows the curves of the dependences of the luminescence intensity of spin centers based on a silicon vacancy with the main quadruplet state of a silicon carbide crystal of the 6H-SiC polytype on the value of the constant magnetic field (19 - in the absence of the test sample; 20 - in the presence of the test sample, the magnetic field of which is measured; ΔВ - measured magnetic field of the test sample).

The present optical magnetometer (see Fig. 1, Fig. 2) contains an LF generator (LFO) 1, a capacitor 2, an electromagnet coil 3 for modulating the magnetic field and for creating a constant magnetic field, the active material 4 in the form of a silicon carbide crystal containing spin centers 5 based on a silicon vacancy with a basic quadrupole state, a direct current source (DC) 6 for powering an electromagnet coil, a lens 7, a scanning stage 8 of a confocal microscope with a piezoelectric element 9, capable of performing a reciprocating movement in three mutually perpendicular directions under the action of control voltages of the piezoelectric element 9, on which the active material 4 is located, containing spin centers 5 based on a silicon vacancy with the ground quadruplet state, located near the crystal surface, a laser (L) 10 emitting light in the near infrared region 750-900 nm, a translucent mirror 11 and a mirror 12, a light filter 13, a lens 14, a synchronous detector (SD) 15, a photodetector (FP) 16, made, for example, in the form of a photomultiplier or a photodiode, and a control unit (BU) 17. L10 optically is engaged with the active material 4 through a translucent mirror 11, a mirror 12 and a lens 7. FP 16 is optically connected to the active material 4 through a lens 7, a mirror 12, a translucent mirror 11 and a filter 13 and a lens 14. The first output of the LFO 1 through a capacitor 2 is connected to an electromagnet coil 3, to which IPT 6 output is also connected. The second LFO output 1 is connected to the first input of LED 15, the second input of LED 15 is connected to the output of FP 16, the output of LED 15 is connected to the input of BU 17, the output of BU 17 is connected to the input of IPT 6 . The test sample 18 during measurements is placed on the active ma eriale 4. Optical excitation and luminescence spin centers register 5 based on the silicon vacancies with the main quadrupole state can be carried out using a standard confocal microscope, if required 2D or 3D scan optically excited small amount (up to 0.2 microns). It is also possible to use STED (stimulated emission depletion microscopy) technology - (Willig, KI, Rizzoli, SO, Westphal, V., Jahn, R., Hell, SW: STED microscopy reveals that synaptotagmin remains clustered after synaptic vesicle exocytosis. Nature 440, 935-939, 2006), where the optically excited and radiated volume of the active material 4 can be limited up to several nm. Active material 4 in the form of a silicon carbide crystal cut out in the form of a plate with a plane perpendicular to the hexagonal crystallographic axis c (the standard form for silicon carbide plates) containing spin centers 5 based on a silicon vacancy with the ground quadrupole state located near the surface of the active material plate 4 , placed on a scanning table 8 with a piezoelectric element 9, capable of reciprocating movement in three mutually perpendicular directions (Fig. 2). In this case, the region of the active material 4 with spin centers 5 based on the silicon vacancy with the ground quadrupole state, excited by the focused beam of the laser 10, will be scanned in a plane perpendicular to the laser beam. The active material 4 can be made in the form of a nanoscale crystal of silicon carbide containing spin centers 5 based on a silicon vacancy with a ground quadrupole state.

This optical magnetometer operates as follows.

Alternating current from the LFO 1 is supplied through a capacitor 2 to an electromagnet coil 3, creating an alternating magnetic field around the active material 4 and the test sample 18. To obtain a constant component of the magnetic field (bias) to the coil 3 of the electromagnet, a constant voltage is supplied from the IPT 6. This makes it possible to obtain both separately variable or constant magnetic fields, and their combination, while using the same coil 3 of the electromagnet, which eliminates mismatch of the directions of alternating and constant magnetic fields. Modulation of the magnetic field allows the use of synchronous detection to record the magnetic response of the signal in the form of a derivative. Optical pumping is carried out by L 10 (for example, with a wavelength of 795 or 805 nm), the radiation of which is directed by a translucent mirror 11 and mirror 12 and focused on the active material 4 using a microscope objective 7. Using the beam L 10 focused by the lens 7, an excited volume of active material 4 (silicon carbide crystal) is extracted, which is close to the upper surface of the active material 4 and contains spin centers 5 based on a silicon vacancy with the ground quadrupole state, conventionally shown in FIG. 2. This allocated volume, which can be reduced to a transverse diameter of 0.3 μm using confocal optics, and using the stimulated emission depletion microscopy STED method to nanometer sizes, is in a thin layer of spin centers 5 in the active material 4, which are located in close contact with the test sample 18, the distribution of local magnetic fields in which it is supposed to be measured. The luminescent radiation of the spin centers 5 through the same lens 7 and the mirror 12 and the translucent mirror 11 enters the filter 13, which cuts off the laser radiation, and then with the help of the lens 14 focuses on the FP 16. Three-coordinate (capable of reciprocating in three mutually perpendicular directions) a scanning table 8 with a piezoelectric element 9 allows precise focusing of laser radiation on the active material 4, as well as scan both in the XY plane and in the XZ or YZ planes and so on to adjust the optical magnetometer precisely when working with active material 4 with a low concentration of spin centers 5. The signal from FP 16, for example, in the form of a photomultiplier, photodiode, or avalanche photodiode, is fed to LED 15, which also receives the reference frequency from LFO 1. BU 17 sets the necessary values of the alternating and constant magnetic fields and registers the change in photoluminescence at the moment of antisection of the spin levels from the output of LED 15. In the region of variation in the luminescence intensity, the first curve of the dependence of the intensity luminescence on the constant magnetic field. Then, the operations described above are performed with the test sample 18 being placed on the active material 4, and in the region of variation in the luminescence intensity, a second curve of the dependence of the luminescence intensity on the constant magnetic field is taken. The magnitude of the magnetic field generated by the test sample 18 at the focal point of the laser radiation L 10 is determined by the horizontal shift of the second curve relative to the first curve. When measuring the gradient of the magnetic field in the test sample 18, a spatial scan of the test sample 18 is performed in the plane transverse to the laser radiation L 10 and the magnitude of the constant magnetic field is measured at each scanning point. Then build a graph of the spatial distribution of local magnetic fields in the sample 18 and calculate the gradient of the magnetic fields.

The operation of this optical magnetometer is carried out using synchronous detection, using an oscillating magnetic field with a low modulation frequency in the range from tens of hertz to tens of kilohertz, the modulation amplitude can vary depending on the signal width from 0.01 mTl to 0.1 mT, and as a result, the PD 16 signals are modulated at the first harmonic using synchronous detection.

The optical magnetometer was tuned by applying a constant magnetic field in such a way that the zero signal from LED 15 was at the center of the resonance caused by the anti-intersection of magnetic levels, and this signal from LED 15 would give the highest magnetic response when the local magnetic field changes.

In FIG. Figure 3 shows the first and second curves of the dependence of the luminescence intensity of spin centers based on a silicon vacancy with the ground quadrupole state of a silicon carbide crystal of the 6H-SiC polytype on the value of the constant magnetic field. The measurements were carried out at room temperature by changing the luminescence intensity in the region of 850-950 nm excited by a laser, the magnitude of the magnetic field modulation amplitude was 0.01 mT, and the frequency was 80 Hz. The possible value of determining local magnetic fields in an optical excitation spot with a diameter of about 0.3 μm, achieved with a confocal microscope, is about 500 nT with a signal measurement time of 1 s (500 nT / Hz).

The main advantage of this magnetometer is the absence of a high-frequency unit in the form of a microwave radiation generator, a system for supplying microwave radiation to the active material, and an ODMR registration system at the frequency modulation power of the microwave generator. This eliminates the interference caused by the microwave generator, as well as the heating of the active material and the object of study by microwave radiation. The absence of a microwave system allows you to place the active material on metal substrates, as well as to study magnetic fields in conductive media.

Claims (5)

1. An optical magnetometer including a low-frequency generator (LF), a capacitor, at least one coil of an electromagnet, an active material in the form of a silicon carbide crystal, containing at least one spin center based on a silicon vacancy with a fundamental quadrupole state, placed inside the coil, source direct current, synchronous detector, control unit, laser emitting in the near infrared region, optically coupled to the active material through a translucent mirror, mirror and lens, photodetector, optical connected to the active material through a lens, a mirror, a translucent mirror and a filter, while the first output of the LF generator through a capacitor is connected to an electromagnet coil, to which the output of the DC source is also connected, the second output of the LF generator is connected to the first input of the synchronous detector, the second input the synchronous detector is connected to the output of the photodetector, the output of the synchronous detector is connected to the input of the control unit, the output of which is connected to the input of the DC source. 2. The magnetometer according to claim 1, characterized in that the active material is placed on a scanning table of a confocal microscope with a piezoelectric element capable of reciprocating movement in three mutually perpendicular directions. 3. The magnetometer according to claim 1, characterized in that the active material is made in the form of a nanoscale crystal of silicon carbide. 4. The magnetometer according to claim 3, characterized in that the active material in the form of a nanoscale crystal of silicon carbide is placed on a probe of an atomic force microscope. 5. The magnetometer according to claim 3, characterized in that the active material in the form of a nanoscale crystal of silicon carbide is placed on a near-field microscope probe.

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