JP7660879B2 - Method for forming vacancy defects and method for producing diamond - Google Patents

Description

The present invention relates to a method for forming vacancy defects in diamond, a method for producing diamond having vacancy defects, and diamond having vacancy defects formed therein.

Diamond has excellent optical, electrical, and thermal properties, and is expected to be applied to optical elements and electronic devices. In particular, in recent years, nitrogen-vacancy centers called NV centers (Nitrogen Vacancy centers) that exist inside diamond have attracted attention. NV centers consist of pairs of nitrogen impurities inside diamond and adjacent vacancy defects, and when an electron is captured in the vacancy, they exhibit a magnetic property called electron spin. The electron spin in the NV center has a long coherence time even at room temperature, and the spin state can be controlled and detected at room temperature, so it is expected to be used in quantum computing and as a highly sensitive quantum sensor for magnetic and electric fields.

Previous reported examples of forming NV centers in diamond include the following non-patent documents 1 and 2.

V. V. Kononenko, I. I. Vlasov, E. V. Zavedeev, A. A. Khomich, and V. I. Konov, “Correlation between surface etching and NV center generation in laser-irradiated diamond”, Appl. Phys. A 124, 226 (2018). https://doi.org/10.1007/s00339-018-1646-x V. V. Kononenko, I. I. Vlasov, V. M. Gololobov, T. V. Kononenko, T. A. Semenov, A. A. Khomich, V. A. Shershulin, V. S. Krivobok, and V. I. Konov, “Nitrogen-vacancy defects in diamond produced by femtosecond laser nanoablation technique”, Applied Physics Letters 111, 081101 (2017). https://doi.org/10.1063/1.4993751

Ensemble NV centers, which are a collection of many NV centers, are important for applications to quantum computing. Ensemble NV centers have the advantage of improving the performance of quantum sensors by increasing signal strength. In addition, when an ensemble NV center is a collection of NV centers with four different orientations, such an ensemble NV center has the advantage of being able to determine three-dimensional vector components of a magnetic field by utilizing the difference in Zeeman splitting width due to the difference in orientation.

Therefore, in the application to quantum sensors, it is effective to form a large number of NV centers (ensemble NV centers) in diamond to increase the concentration of NV centers in order to improve the measurement sensitivity of the sensor. However, quantitative evaluation of the concentration of NV centers produced by laser irradiation has not been performed so far, and there have been no reports on the formation of NV centers with a high concentration (for example, about 10 ¹⁵ cm ⁻³ to 10 ¹⁶ cm ⁻³ or higher) that can be applied to quantum sensors. In order to apply the NV centers of diamond to quantum sensors, etc., a method for forming NV centers with a high concentration that can be applied to quantum sensors, etc. is required.

The aim of the present invention is to create a high concentration of vacancy defects in diamond.

In order to achieve the above object, the present invention includes, for example, the following aspects.
(Item 1)
A method for forming vacancy defects in a diamond, comprising the steps of:
A step of shaping a pulse waveform of a laser having a wavelength transparent to the absorption wavelength of the diamond to compensate for temporal dispersion in the phase of the pulse light of the laser;
irradiating the dispersion-compensated pulsed laser light into the diamond a plurality of times;
The method includes:
(Item 2)
Item 2. The method according to item 1, wherein the number of pulsed lights focused and irradiated inside the diamond is 5×10 ⁷ or more.
(Item 3)
3. The method according to claim 2, wherein the number of pulsed lights focused and irradiated inside the diamond is 2.5×10 ⁸ or more.
(Item 4)
The pulse energy of the laser is in the range of 60 nJ to 200 nJ;
Item 4. The method according to any one of Items 1 to 3, wherein the pulse width of the laser is 40 femtoseconds or less.
(Item 5)
5. The method of claim 4, wherein the laser pulse energy is in the range of 150 nJ to 200 nJ.
(Item 6)
6. The method according to any one of items 1 to 5, wherein in the step of compensating for the dispersion, a time delay is generated between a first sub-pulse light consisting of a first polarization component and a second sub-pulse light consisting of a second polarization component intersecting the first polarization component by performing phase modulation on the pulsed light.
(Item 7)
Preparing a diamond;
forming vacancy defects inside the diamond by any one of the methods of items 1 to 6;
A method for producing diamond, comprising:
(Item 8)
A diamond having a concentration of vacancy defects formed therein of 1×10 ¹⁵ cm ⁻³ or more.
(Item 9)
Item 9. A diamond according to item 8, wherein the concentration of vacancy defects is 3×10 ¹⁵ cm ⁻³ or more.
(Item 10)
Item 10. A diamond according to item 9, wherein the concentration of vacancy defects is 1×10 ¹⁶ cm ⁻³ or more.

The present invention makes it possible to form a high concentration of vacancy defects in diamond.

FIG. 2 is a diagram showing a schematic configuration of an optical system used when performing a void defect forming method according to an embodiment of the present invention. 2 is an example of a time waveform of pulsed light output from a laser. FIG. 2 is a diagram showing a schematic configuration of the pulse shaper shown in FIG. 1 . FIG. 13 is a diagram showing a schematic configuration when dispersion compensation of pulsed light is set in a pulse shaper. FIG. 13 is a diagram for explaining the results of verification according to the embodiment, and is a graph showing the change in concentration of the NV center with respect to the number of pulsed lights for each pulse energy. 1A and 1B are diagrams for explaining the results of verification according to an embodiment, in which (a) is a pulse sequence using the Hahn echo method based on the optically detected magnetic resonance (ODMR) method used to measure the transverse relaxation time _T2 , and (b) to (e) are measurement signals using the Hahn echo method.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the following description and drawings, the same reference numerals indicate the same or similar components, and therefore, redundant descriptions of the same or similar components will be omitted.

According to a first aspect of the present invention there is provided a method of forming vacancy defects in a diamond. In one embodiment the method comprises the steps of:
A step of shaping a pulse waveform of a laser having a wavelength transparent to the absorption wavelength of diamond to compensate for temporal dispersion of the phase of the pulse light of the laser;
irradiating the inside of the diamond with a plurality of pulsed dispersion-compensated laser beams;
Includes.

In one embodiment of the method, the number of pulsed lights focused and irradiated inside the diamond is 5× ¹⁰ or more. Preferably, the number of pulsed lights focused and irradiated inside the diamond is 2.5× ¹⁰ or more.

In one embodiment of the method, the laser pulse energy is in the range of 60 nJ to 200 nJ and the laser pulse width is 40 femtoseconds or less. More preferably, the laser pulse energy is in the range of 150 nJ to 200 nJ.

In one embodiment of the method, in the step of compensating for dispersion, a time delay is generated between a first sub-pulse light having a first polarization component and a second sub-pulse light having a second polarization component that intersects with the first polarization component by performing phase modulation on the pulse light.

According to a second aspect of the present invention, there is provided a method for producing diamond. In one embodiment, the method comprises the steps of:
Preparing a diamond;
forming vacancy defects within a diamond by a method provided according to a first aspect of the present invention;
Includes.

According to a third aspect of the present invention, there is provided a diamond. In one embodiment of the diamond, the concentration of vacancy defects formed therein is 1×10 ¹⁵ cm ^-3 or more. Preferably, the concentration of vacancy defects is 3×10 ¹⁵ cm ^-3 or more. More preferably, the concentration of vacancy defects is 1×10 ¹⁶ cm ^-3 or more.

In the examples of the present invention described later, even if the number of pulsed light irradiated to the diamond sample is increased to 5×10 ⁵ to 5×10 ⁸ to form NV centers, the transverse relaxation time _T2 of the formed NV centers does not vary significantly within the range of about 1.6±0.2 microseconds to about 2.4±0.2 microseconds, and graphitization of diamond is suppressed.

[Optical system]
FIG. 1 is a diagram showing a schematic configuration of an optical system used when performing a void defect forming method according to an embodiment of the present invention.

The optical system 10 mainly comprises a laser 1 used as a light source for modifying the sample 9, a pulse shaper 2 that shapes the pulse waveform of the laser light, and an objective lens 3 that focuses and irradiates the waveform-shaped pulse light inside the sample 9. The pulse light Lp irradiated from the laser 1 has its pulse waveform shaped and its pulse width controlled in the pulse shaper 2, and is focused by the objective lens 3 and irradiated to a predetermined depth inside the sample 9. The sample 9 is modified by the irradiation of the laser light. The direction of travel of the pulse light Lp irradiated from the laser 1 is appropriately adjusted by a total reflection mirror 4.

The optical system 10 may further include a dichroic mirror 5 and a photodetector 6 such as an avalanche-type photodiode as components for observing the modified sample 9. The optical system that irradiates the sample 9 with the pulsed light Lp output from the laser 1 through the pulse shaper 2, and the optical system that observes the reflected light from the sample 9 through the photodetector 6 are confocal optical systems. Such an optical system 10 may be configured using, for example, a confocal fluorescence microscope. Note that when modifying the inside of the sample 9, the optical system 10 does not need to include the dichroic mirror 5 and the photodetector 6, and does not need to configure a confocal optical system.

2 is an example of the time waveform of the pulsed light Lp output by the laser, and is an example of an intensity autocorrelation waveform. The vertical axis of FIG. 2 indicates normalized light intensity. Unless otherwise specified in the following description of this specification, the pulse width is a value after time waveform shaping by the pulse shaper 2,
Pulse width = full width at half maximum (FWHM) of intensity autocorrelation waveform / 1.5426
It is assumed that the pulse waveform is SECH type ² .

In this embodiment, synthetic diamond is used for the sample 9. In this embodiment, the sample 9 is, for example, an Ib-type HPHT single crystal synthetic diamond with a high concentration of nitrogen impurities, and illustratively, the nitrogen concentration of the synthetic diamond used for the sample 9 is about 33 ppm. The sample 9 is placed on a three-axis stage (not shown), and the area (X-Y plane) on the sample 9 where the laser light is irradiated and the position (Z axis) of the sample 9 along the irradiation axis of the laser light are adjustable. The laser light can be irradiated onto the sample 9 in the atmosphere at room temperature. Exemplarily, room temperature means a temperature range of about 1°C to 30°C, including the normal temperature range of about 15°C to 25°C, but is not limited to these temperature ranges.

The laser 1 outputs pulsed light Lp with a pulse width on the order of femtoseconds. The pulsed light Lp output by the laser 1 has a wavelength that is transparent to the absorption wavelength of the sample 9. As a result, the laser 1 causes modification of the sample 9 near the focal point of the laser by nonlinear absorption, enabling spatially selective position control including the depth direction of the sample 9. The pulsed light Lp having a wavelength that is transparent to the absorption wavelength of the sample 9 means that the wavelength band of the pulsed light Lp is a wavelength band in which no spectral lines appear in the absorption spectrum of the sample 9 even when the sample 9 is irradiated with the pulsed light Lp. The energy of the pulsed light Lp irradiated to the sample 9 is absorbed nonlinearly by the crystal lattice of the sample 9. In this embodiment, the pulsed light Lp is linearly polarized, and the laser 1 uses a regenerative amplification type titanium sapphire laser that outputs pulsed light with a center wavelength of about 800 nm.

In this embodiment, the laser pulse energy is preferably in the range of about 60 nJ to 200 nJ, and the pulse width is about 40 femtoseconds. As described below, the laser pulse width can be shaped by the pulse shaper 2, and the illustrated pulse width of about 40 femtoseconds is the value after time waveform shaping by the pulse shaper 2. In this embodiment, the repetition frequency when outputting the pulsed light is about 250 kHz.

In this embodiment, the laser light is irradiated onto the sample 9 for a longer time than in the past (for example, about 1 second). The time for irradiating the laser light onto the sample 9 is about 2 seconds to about 5000 seconds. That is, in this embodiment, the laser 1 outputs a plurality of pulsed lights Lp of about 5×10 ⁵ to about 1.25×10 ⁹ .

（式１）に示す多項式中、パルスの波形に寄与する項は二次関数以上の項である。よって、（式１）中のａ_２の値を変更することにより、パルス幅を制御することができる。パルス整形器２は、後述する空間光変調器２７と回折格子による分散素子２５とにより、（式１）に記載されている二次の位相の項を制御することによって、照射レーザーパルスの時間波形を変化させて、パルス幅を調整する。パルス整形器２は、レーザーのパルス幅を約４０フェムト秒以下に整形することができる。 The pulse shaper 2 shapes the pulse waveform of the laser light to control the pulse width. The phase φ of the pulse light can be approximated by a polynomial through Taylor expansion.

In the polynomial shown in (Equation 1), the terms that contribute to the pulse waveform are terms of a quadratic or higher function. Therefore, the pulse width can be controlled by changing the value of _a2 in (Equation 1). The pulse shaper 2 adjusts the pulse width by changing the time waveform of the irradiated laser pulse by controlling the quadratic phase term described in (Equation 1) using a spatial light modulator 27 and a dispersion element 25 using a diffraction grating, which will be described later. The pulse shaper 2 can shape the pulse width of the laser to about 40 femtoseconds or less.

The objective lens 3 focuses and irradiates the pulsed light after time waveform shaping inside the sample 9. In this embodiment, the objective lens 3 has a magnification of approximately 50 times (numerical aperture NA = 0.80) and focuses at a depth of approximately 15 μm from the surface of the sample 9.

[Pulse Shaper and Dispersion Compensation]
Pulse Shaper Fig. 3 is a diagram showing a schematic configuration of the pulse shaper shown in Fig. 1. The pulse shaper 2 includes a polarization control unit 21, mirrors 22, 23, and 24, a dispersive element 25, a focusing element 26, and a spatial light modulator (SLM) 27. The pulse shaper 2 passes the input pulse light Lp through the dispersive element 25 and the spatial light modulator 27 in this order, thereby modulating the phase of the pulse light Lp for each wavelength.

The pulse shaper 2 performs phase modulation on the pulse light Lp output from the laser 1, thereby generating a time delay between a first sub-pulse light consisting of a first polarization component and a second sub-pulse light consisting of a second polarization component that intersects with the first polarization component.

The polarization control unit 21 is an optical element that is optically coupled to the laser 1 and rotates the polarization plane of the input pulsed light Lp. The polarization control unit 21 is composed of, for example, a wavelength plate such as a λ/2 plate, a polarizing element, and the like. As a result, the pulsed light Lp includes a first polarization component along a first polarization direction and a second polarization component along a second polarization direction that intersects with the first polarization direction. In one embodiment, the first polarization direction and the second polarization direction are orthogonal to each other.

Mirrors 22, 23, and 24 are optical elements that direct the direction of the input pulsed light Lp toward the dispersion element 25. Mirrors 22, 23, and 24 are, for example, total reflection mirrors. Mirrors 22, 23, and 24 can be omitted depending on the relative positional relationship with the dispersion element 25.

The dispersion element 25 is an optical element that disperses the pulsed light Lp by wavelength. The dispersion element 25 is composed of, for example, a diffraction grating or a prism. Since the diffraction angle differs for each wavelength in a diffraction grating, when pulsed light Lp having a wide band of wavelength components is incident on the diffraction grating, each wavelength component is diffracted in a different direction.

The focusing element 26 is an optical element that focuses each wavelength component of the pulsed light Lp output from the dispersion element 25 while expanding at different positions on the spatial light modulator 27. The focusing element 26 is formed, for example, of a cylindrical lens. The focusing element 26 has a refractive power (lens power) in a plane that includes the wavelength dispersion direction by the dispersion element 25, but does not have a refractive power in a plane perpendicular to the wavelength dispersion direction.

The spatial light modulator 27 is a phase modulation type spatial light modulator that modulates the pulsed light Lp dispersed by the dispersion element 25 for each wavelength. In this embodiment, the spatial light modulator 27 is configured as a liquid crystal type (Liquid Crystal on Silicon: LCOS) spatial light modulator. The modulation surface of the spatial light modulator 27 includes a plurality of modulation regions corresponding to the plurality of wavelength components, and these modulation regions are aligned in the direction of wavelength dispersion by the dispersion element 25. When each wavelength component of the pulsed light Lp is input to the corresponding modulation region, it is modulated independently according to the modulation pattern of that modulation region.

The polarization plane of the pulsed light Lp input to the spatial light modulator 27 is rotated by the polarization control unit 21, and as a result, the polarization direction in which the spatial light modulator 27 provides the modulation effect is rotated. As a result, of the polarization components of the pulsed light Lp, the polarization components along the polarization direction in which the spatial light modulator 27 provides the modulation effect are modulated by the spatial light modulator 27, and the time waveform changes.

Dispersion Compensation FIG. 4 is a diagram showing a schematic configuration when dispersion compensation for pulsed light is set in a pulse shaper.

As shown in FIG. 4, when the dispersion compensation of the pulsed light is set in the pulse shaper, a focusing optical system 81, a wavelength conversion element 82, and a spectrometer 83 are optically coupled to the rear stage of the optical system including the laser 1 and the pulse shaper 2 of the optical system shown in FIG. 1. The dispersion compensation of the pulsed light is set in the pulse shaper 2 by the control unit 84.

The focusing optical system 81 is provided between the spatial light modulator 27 and the wavelength conversion element 82, and focuses the pulsed light Lp toward the wavelength conversion element 82.

The wavelength conversion element 82 changes the intensity spectrum of the light emitted according to the phase of the pulsed light Lp. For the wavelength conversion element 82, for example, a nonlinear optical crystal such as GaAs, GaP, ZnTe, KDP, BBO, BIBO, LiNbO ₃ , KTP, LBO, or CLBO can be used. From the wavelength conversion element 82, several types of light such as second harmonic, third harmonic, and higher harmonic are emitted. Note that these several types of light emitted from the wavelength conversion element 82 can be separated by a wavelength filter.

The spectrometer 83 detects the intensity spectrum of the light emitted from the wavelength conversion element 82 by irradiating the wavelength conversion element 82 with the pulsed light Lp. The detected light intensity spectrum is output to the control unit 84 as a detection signal.

The control unit 84 controls the polarization control unit 21 and the spatial light modulator 27 based on the detection signal output from the spectrometer 83. The control unit 84 is, for example, a calculation device such as a general-purpose computer or a personal computer (PC). The control unit 84 changes the polarization direction of the pulsed light Lp by controlling the rotation angle of the polarization plane in the polarization control unit 21. The control unit 84 also changes the time waveform of the pulsed light Lp by controlling the phase pattern of the spatial light modulator 27.

In this embodiment, the pulse shaper 2 controls and compensates for the temporal dispersion of the phase of the pulsed light based on the MIIPS (Multiphoton Intrapulse Interference Phase Scan) method. This aligns the phase of the pulsed light irradiated from the laser 1, generating a Fourier transform limited pulse with a further shortened laser pulse width.

In the MIIPS method, the pulsed light Lp is passed through a wavelength conversion element 82 such as a nonlinear optical crystal to generate second harmonics of the pulsed light Lp, and the phase dispersion is measured by a spectrometer 83. The pulse shaper 2 generates a Fourier transform limited pulse by compensating for the phase dispersion measured by the spectrometer 83. In this embodiment, the pulse shaper 2 compensates for the phase dispersion and aligns the phase of the pulsed light Lp, for example, by modulating the phase of the pulsed light Lp so that the intensity of the second harmonic is maximized.

Note that it is sufficient to measure the phase dispersion using the wavelength conversion element 82, the spectrometer 83, and the control unit 84 shown in FIG. 4, and to set the dispersion compensation for the pulse shaper 2 once before constructing the optical system 10 shown in FIG. 1.

The following examples of the present invention will make the features of the present invention clearer.

In this example, the optical system according to the above embodiment of the present invention was constructed using a confocal fluorescence microscope. Phase dispersion was measured using the wavelength conversion element, spectrometer, and control unit described in the above embodiment, and dispersion compensation was set for the pulse shaper so as to generate a Fourier transform limited pulse based on the MIIPS method. Using an optical system equipped with a pulse shaper with dispersion compensation settings, pulsed laser light was irradiated onto diamond to form an NV center inside the diamond.

In this example, NV centers were formed inside diamond using the vacancy defect formation method according to the above-described embodiment of the present invention. NV centers were formed using various combinations of laser light irradiation time and pulse energy. The concentration of NV centers was estimated from the measured values of the intensity of the fluorescence emitted from the NV centers. The intensity of the fluorescence emitted from the NV centers was measured using a photodiode.

In this example, two items were verified. In verification 1, it was verified whether the concentration of the formed NV center was improved compared to previous reports. In verification 2, it was verified whether the diamond of the formed NV center was graphitized.

<Verification 1>
Concentration of NV centers was compared between NV centers formed inside diamond by the vacancy defect forming method according to the present embodiment and NV centers formed on the surface or inside diamond in a conventional report as a comparative example. A graph of the comparison results is shown in FIG.

The concentrations in the comparative examples were estimated based on the values of fluorescence intensity described in conventional reports. In estimating the concentrations in the comparative examples, conditions and values not described in the reports were assumed. The conventional reports cited as comparative examples were the two above-mentioned Non-Patent Documents 1 and 2. In the following explanation, 1 ppm = 1.76 x 10 ¹⁷ cm ^-3 .

Comparative Example 1
As Comparative Example 1, an estimated NV center concentration of 7.2×10 ¹⁴ cm -3 was obtained using the maximum fluorescence intensity of about ^2.3 (number of pulsed light: about 1.68×10 ⁷ ) reported in Non-Patent Document 1. In Non-Patent Document 1, the NV centers are formed on the surface by surface ablation.

Regarding Non-Patent Document 1, the concentration of the NV center was estimated as follows.
Nitrogen concentration of diamond sample: 0.5 ppm (calculated from the absorption coefficient at 270 nm)
Fluorescence intensity of non-irradiated area (Original PL signal): approx. 0.28
Maximum fluorescence intensity: about 2.3 (number of pulsed lights: about 1.68×10 ⁷ )
Assuming that the NV concentration in the portion not irradiated with the pulsed light is 0.1% of the nitrogen concentration in the diamond sample, the maximum concentration in the portion irradiated with the pulsed light was estimated to be 7.2×10 ¹⁴ cm ⁻³ .

Comparative Example 2
As Comparative Example 2, an estimated NV center concentration of 5.4×10 ¹⁴ cm -3 was obtained using the maximum fluorescence intensity of approximately ^0.87 (number of pulsed light approximately 8.39×10 ⁴ ) reported in Non-Patent Document 2. In Non-Patent Document 2, the NV centers are formed on the surface by surface ablation.

Regarding Non-Patent Document 2, the concentration of the NV center was estimated as follows.
Nitrogen concentration of diamond sample: 0.5 ppm (calculated from the absorption coefficient at 270 nm)
Fluorescence intensity of non-irradiated area (Original PL signal): approx. 0.14
Maximum fluorescence intensity: about 0.87 (number of pulsed lights: about 8.39×10 ⁴ )
Assuming that the NV concentration in the portion not irradiated with the pulsed light is 0.1% of the nitrogen concentration in the diamond sample, the maximum concentration in the portion irradiated with the pulsed light was estimated to be 5.4×10 ¹⁴ cm ⁻³ .

Figure 5 is a diagram for explaining the verification results of the embodiment, and is a graph showing the change in NV center concentration versus the number of pulsed light for each pulse energy. The data of the four types of line graphs plotted in the figure using four different symbols are the measured values of the concentration of NV centers formed inside diamonds using the vacancy defect formation method of the embodiment. The data plotted in the figure using two different types of enclosed symbols are the estimated values of the concentration of NV centers in the comparative example.

Based on the graph of NV center concentration comparison shown in Figure 5, we investigated the conditions of laser light irradiation time and pulse energy for forming a high concentration of NV center. The target concentration of NV center to be formed was set to about 10 ¹⁵ cm ^-3 to 10 ¹⁶ cm ^-3 , which is a concentration that can be expected to increase the sensitivity of quantum sensors.

First, an overview of the measurement results according to this embodiment will be reported. In this embodiment, as the number of pulsed lights irradiated to the diamond sample was increased, the concentration of the NV center first increased, and then saturated or slightly decreased. Then, the number of pulsed lights irradiated was increased to obtain measurement data, and at the measurement point where the number of pulsed lights was about 2.5×10 ⁷ to about 5×10 ⁷ , the concentration of the NV center increased again from about 3 to 4×10 ¹⁵ cm ⁻³ . The number of pulsed lights irradiated was further increased to obtain measurement data, and in this embodiment, the maximum concentration of the NV center was obtained by irradiating the diamond sample with about ⁵ × ^{10 8} pulsed lights. The NV center was formed without annealing the diamond after irradiating the diamond sample with pulsed laser light.

In addition, in these measurement results, the behavior in which the concentration of NV centers saturates or decreases as the number of pulsed light increases can be interpreted from the perspective of competition between the formation and annihilation of NV centers.

The measurement results of the NV center concentration in this embodiment were compared with the estimated values of the NV center concentrations in Comparative Examples 1 and 2. The observation results were as follows.

According to this embodiment, a diamond sample was irradiated with about 2.5×10 ⁶ pulsed light with a pulse energy of about 60 nJ, and a diamond with an NV center concentration of about 1×10 ¹⁵ cm ⁻³ was obtained. The obtained concentration value was higher than any of the estimated values of the NV center concentration shown in Comparative Examples 1 and 2, and achieved about 10 ¹⁵ , which is the lower limit of the target concentration value at which high sensitivity can be expected in a quantum sensor. In addition, when the pulse energy was about 100 nJ, about 150 nJ, and about 200 nJ, a diamond with an NV center concentration of about 1×10 ¹⁵ cm ⁻³ or more was obtained by irradiating the diamond sample with about 1.25×10 ⁶ pulsed light.

According to this embodiment, a diamond sample was irradiated with about 5×10 ⁷ pulsed light beams, and a diamond with an NV center concentration of about 3×10 ¹⁵ cm ⁻³ or more was obtained. This NV center concentration was obtained at any pulse energy. The obtained concentration value was the concentration at which the NV center concentration, once saturated, started to increase again. This number of pulsed light beam irradiations was a number of pulsed light irradiations that was not achieved in either Comparative Example 1 or 2. The obtained concentration value was a higher concentration than any of the estimated values of the NV center concentration shown in Comparative Examples 1 and 2, and achieved about 10 ¹⁵ , which is the lower limit of the target concentration value at which high sensitivity can be expected in a quantum sensor.

In this embodiment, a diamond sample was irradiated with about 2.5×10 ⁸ pulsed light beams, and a diamond with an NV center concentration of about 1×10 ¹⁶ cm ^-3 or more was obtained. This NV center concentration was obtained at any pulse energy. This number of pulsed light beams was not achieved in either Comparative Example 1 or 2. The obtained concentration value was higher than any of the estimated values of the NV center concentration shown in Comparative Examples 1 and 2, and achieved the target concentration value of about 10 ¹⁶ that can be expected to increase the sensitivity of a quantum sensor.

In this example, a diamond sample was irradiated with a maximum of about 5×10 ⁸ pulsed light with a pulse energy of about 150 nJ or about 200 nJ, thereby obtaining a diamond having an NV center concentration of a maximum of about 3×10 ¹⁶ cm ^-3 . This number of pulsed light irradiations was not achieved in either Comparative Example 1 or 2, and the obtained concentration value was also not achieved in either Comparative Example 1 or 2.

<Verification 2>
Regarding the NV center formed inside the diamond by the vacancy defect forming method according to the present embodiment, it was verified whether the diamond was graphitized or not.

The verification was carried out by measuring the transverse relaxation time _T2 of the NV center by a known optically detected magnetic resonance (ODMR) method. If the diamond is graphitized when the NV center is formed in the diamond, this transverse relaxation time _T2 decreases.

For the verification, multiple diamond samples were used in which NV centers were formed by varying the concentration, i.e., the number of irradiated pulsed light beams, between about 5 × 10 ⁵ and about 5 × 10 ^8. The pulse energy of the laser irradiated to the diamond samples was about 150 nJ.

FIG. 6 is a diagram for explaining the verification results according to the embodiment, in which (a) is a pulse sequence by the Hahn echo method based on the optically detected magnetic resonance (ODMR) method used to measure the transverse relaxation time _T2 , and (b) to (e) are measurement signals by the Hahn echo method.

As shown in the measurement signals of (b) to (e), even when the number of pulsed lights irradiated to the diamond sample was increased to 5×10 ⁵ to 5×10 ⁸ to form NV centers, the transverse relaxation time T ₂ of the formed NV center did not vary significantly within the range of about 1.6±0.2 microseconds to about 2.4±0.2 microseconds, and furthermore, the transverse relaxation time T ₂ of the NV center contained in the original diamond sample was about 1.6±0.2 microseconds, which was almost the same value. This confirmed that the vacancy defect formation method according to this embodiment suppresses graphitization of diamond, which may occur when forming NV centers.

Furthermore, when the number of pulsed lights irradiated onto the diamond sample was increased to 7.5 × ¹⁰ or more, a decrease in the concentration of NV centers was confirmed. When the number of pulsed lights irradiated was further increased, the area near the focal point clearly turned black, confirming that the diamond sample was graphitized.

In addition, the number of pulsed lights that were confirmed to graphitize the diamond sample was confirmed while increasing the number of pulsed lights irradiated to the diamond sample. When the pulse energy was about 60 nJ, it was confirmed that the diamond sample was graphitized when irradiated with about 1.75 x 10 ⁹ pulsed lights, and when the number of irradiated pulsed lights was less than this value, about 1.25 x 10 ⁹ , graphitization of the diamond sample was not confirmed.

Similarly, when the pulse energy was about 100 nJ, it was confirmed that the diamond sample was graphitized when it was irradiated with about 5×10 ⁹ pulsed light, and when the number of irradiated pulsed light was less than this value, about 7.5×10 ⁸ , graphitization of the diamond sample was not confirmed. When the pulse energy was about 200 nJ, it was confirmed that the diamond sample was graphitized when it was irradiated with about 1×10 ⁹ pulsed light, and when the number of irradiated pulsed light was less than this value, about 7.5×10 ⁸ , graphitization of the diamond sample was not confirmed.

As described above, according to the present invention, a high concentration of NV centers can be formed in diamond. Conventionally, it has not been easy to form NV centers in diamond without graphitizing the diamond, and it has been difficult to form a high concentration of NV centers in diamond. According to the present invention, the concentration of the NV centers formed is a high concentration of about 10 ¹⁵ cm ^-3 to 10 ¹⁶ cm ^-3 , which is a concentration that can be expected to increase the sensitivity of quantum sensors, and graphitization of diamond is suppressed even when the NV centers are formed.

Furthermore, according to the present invention, NV centers can be formed in diamond without annealing the diamond after irradiating it with a pulsed laser beam. If the annealing step is no longer necessary, it becomes possible to employ a low-temperature process when industrially manufacturing quantum sensor elements containing NV centers, and it becomes possible to manufacture quantum sensor elements in the atmosphere at room temperature.

[Other forms]
Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

In the above embodiment, diamond is used as sample 9, and nitrogen-vacancy centers called NV centers are formed inside the diamond as color centers, but the color centers formed inside the diamond are not limited to NV centers. Instead of NV centers, silicon-vacancy centers or germanium-vacancy centers may be formed inside the diamond.

In the above embodiment, the pulse width of the pulsed light Lp generated by the laser 1 and the pulse shaper 2 is on the order of femtoseconds, but the pulse width of the laser irradiated to modify the diamond is not limited to the order of femtoseconds. As long as a high concentration of NV centers can be preferentially formed inside the diamond, a laser having a pulse width on the order of femtoseconds ( ^10-15 seconds) or less, for example, attoseconds ( ^10-18 seconds), can be irradiated onto the diamond.

The concentration of nitrogen impurities contained in sample 9 is not limited to the concentrations exemplified in the above-mentioned embodiment. For example, in applications to quantum sensors, the concentration of nitrogen impurities contained in synthetic diamond may be less than about 33 ppm (e.g., less than about 1 ppm) as exemplified above, and the synthetic diamond used is not limited to type Ib but may be type IIa.

1. Laser (light source)
2 Pulse shaper 3 Objective lens 4 Total reflection mirror 5 Dichroic mirror 6 Photodetector (photodiode)
9 Sample 10 Optical system 21 Polarization control section (wave plate)
22, 23, 24 Mirror (total reflection mirror)
25 Dispersion element (diffraction grating)
26 Light-collecting element (cylindrical lens)
27 Spatial light modulator (LCOS-SLM)
81 Light-collecting optical system 82 Wavelength conversion element (nonlinear optical crystal)
83 Spectrometer 84 Control unit Lp Pulsed light

Claims (7)

A method for forming vacancy defects in a diamond, comprising the steps of:
A step of shaping a pulse waveform of a laser having a wavelength transparent to the absorption wavelength of the diamond to compensate for temporal dispersion in the phase of the pulse light of the laser;
irradiating the dispersion-compensated pulsed laser light into the diamond a plurality of times;
The method includes: The method according to claim 1 , wherein the number of the pulsed lights focused and irradiated inside the diamond is 5×10 ⁷ or more. The method according to claim 2 , wherein the number of pulsed lights focused and irradiated inside the diamond is 2.5×10 ⁸ or more. The pulse energy of the laser is in the range of 60 nJ to 200 nJ;
The method according to any one of claims 1 to 3, wherein the pulse width of the laser is 40 femtoseconds or less. The method of claim 4, wherein the laser pulse energy is in the range of 150 nJ to 200 nJ. The method according to any one of claims 1 to 5, wherein in the step of compensating for the dispersion, a time delay is generated between a first sub-pulse light consisting of a first polarization component and a second sub-pulse light consisting of a second polarization component intersecting the first polarization component by performing phase modulation on the pulse light. Preparing a diamond;
forming vacancy defects inside the diamond by any one of the methods of claims 1 to 6;
A method for producing diamond, comprising:

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