Patent Application
20240410083
The present invention relates to a method for forming a vacancy defect in diamond, a device therefor, and a method for producing diamond with a vacancy defect.
Due to its excellent optical, electrical, and thermal properties, diamond is expected to find applications in optical elements and electronic devices. In particular, the nitrogen-vacancy center, known as NV center, which exists inside diamond, has attracted much attention. The NV center is composed of a pair of nitrogen impurity inside the diamond and an adjacent vacancy defect, and exhibits magnetic properties called “electron spin” when electrons are captured in the vacancy. Because the electron spin in the NV center has a long coherence time even at room temperature, and the spin state is controllable and detectable at room temperature, diamond shows promise for application in quantum computing and highly sensitive quantum sensors, for example, for magnetic and electric fields.
Recent studies report that the NV center is formed in diamond by using a femtosecond laser, as disclosed in PTL 1 listed below.
An ensemble of NV centers, which is a collection of many NV centers, is vital for application of diamond in quantum computing. Ensembles of NV centers have an advantage in achieving improved performance of quantum sensors due to their increased signal intensity. If an ensemble of NV centers is an assembly of four differently oriented NV centers, such an ensemble is advantageous in its capability of determining the 3D vector components of a magnetic field by using the differences in Zeeman splitting width due to their different orientation. In applications of diamond in quantum sensors, forming numerous NV centers in diamond (an ensemble of NV centers) to increase the concentration of NV centers is thus effective in improving the measurement sensitivity of sensors.
The method disclosed in PTL 1 is a technique for forming NV centers so as to map them in a desired pattern within the crystal lattice of diamond. The method disclosed in PTL 1 does not desirably increase the temporal formation efficiency or yield in the formation of NV centers, nor does it desirably increase the concentration of NV centers. To apply NV centers of diamond in, for example, quantum sensors, a technique of efficiently forming diamond vacancy defects in diamond is required.
An object of the present invention is to efficiently form vacancy defects in diamond.
The present invention to achieve the object includes, for example, the following aspects.
A method for forming a vacancy defect in diamond, comprising the step of concentrating pulsed light from a pulsed laser to irradiate the diamond with the pulsed light, wherein a fluence of the pulsed laser in a focal region on the diamond is 1.8 J·cm−2 or more.
The method according to Item 1, wherein in the step of concentrating pulsed light from a pulsed laser to irradiate the diamond with the pulsed light, the diamond is repeatedly irradiated with one to four shots of a single beam of the pulsed light per cycle.
The method according to Item 1 or 2, wherein the fluence of the pulsed laser is 54 J·cm−2 or less.
The method according to any one of Items 1 to 3, wherein the focal region has an area of 175 μm2 or more.
The method according to any one of Items 1 to 4, further comprising the step of controlling a shape of the focal region.
The method according to any one of Items 1 to 5, wherein the fluence of the pulsed laser is 5.1 J·cm−2 or more.
The method according to any one of Items 1 to 6, wherein
An apparatus for forming a vacancy defect in diamond, comprising
The apparatus according to Item 7, wherein the optical system has an aperture configured to control a shape of the focal region.
A method for producing diamond having a vacancy defect, comprising
The present invention efficiently forms vacancy defects in diamond.
The following describes in detail embodiments of the present invention with reference to attached drawings. In the following description and drawings, the same reference numerals indicate the same or similar component; therefore, redundant descriptions of the same or similar components will be omitted.
In the first aspect of the present invention, a method for forming a vacancy defect in diamond is provided. The method according to an embodiment comprises the step of concentrating pulsed light from a pulsed laser to irradiate diamond with the pulsed light, wherein the fluence of the pulsed laser in a focal region on the diamond is 1.8 J·cm−2 or more. More preferably, the fluence of the pulsed laser is 54 J·cm−2 or less. Still more preferably, the fluence of the pulsed laser is 5.1 J·cm−2 or more.
In the method according to an embodiment, in the step of concentrating pulsed light to irradiate diamond with the pulsed light, the diamond is repeatedly irradiated with one to four shots of a single beam of the pulsed light per cycle. More preferably, diamond is irradiated with one shot of a single beam of pulsed light in the method.
In the method according to an embodiment, the focal region has an area of 175 μm2 or more.
In the method according to an embodiment, the method further comprises the step of controlling the shape of the focal region. More preferably, the shape of the focal region is substantially elliptical.
In the second aspect of the present invention, an apparatus for forming a vacancy defect in diamond is provided. The apparatus according to an embodiment comprises a pulsed laser and an optical system configured to concentrate pulsed light from the pulsed laser to irradiate diamond with the pulsed light, wherein the fluence of the pulsed laser in a focal region on the diamond is 1.8 J·cm−2 or more. More preferably, the fluence of the pulsed laser is 54 J·cm−2 or less. Still more preferably, the fluence of the pulsed laser is 5.1 J·cm−2 or more.
The apparatus according to an embodiment repeatedly irradiates diamond with one to four shots of a single beam of pulsed light per cycle. More preferably, the apparatus irradiates diamond with one short of a single beam of pulsed light.
In the apparatus according to an embodiment, the optical system has an aperture configured to control the shape of the focal region. More preferably, the shape of the focal region is substantially elliptical.
In the third aspect of the present invention, a method for producing diamond having a vacancy defect is provided. The method according to an embodiment comprises the step of preparing diamond and the step of forming a vacancy defect in the diamond according to the method provided in the first aspect of the present invention.
The vacancy defect formation apparatus 10 according to an embodiment mainly comprises a laser 1 for use as an optical source configured to modify a sample 9 and a light-collecting element 3 configured to collect pulsed light 2 emitted from the laser 1 and irradiate the sample 9 with the pulsed light 2. The pulsed light 2 emitted from the laser 1 has its focus diameter controlled in at least either the vertical direction or the horizontal direction, or both directions on the plane perpendicular to the travelling direction of the pulsed light 2 due to an aperture 4 placed in the light path. The travelling direction of the pulsed light 2 is adjusted by a total-reflection mirror 5, and the pulsed light 2 is condensed by the light-collecting element 3. Then, the sample 9 placed on a stage 8 is irradiated with the pulsed light 2. The sample 9 is modified by the irradiation with the pulsed light 2, and vacancy defects are formed in the sample 9. The aperture 4, total-reflection mirror 5, and stage 8 are optional elements.
In this embodiment, the sample 9 is subjected to convergent beam irradiation with the pulsed light 2 under the condition that the fluence of the laser 1 at a focal region (laser spot) on the sample 9 is about 1.8 J·cm−2 or more and about 54 J·cm−2 or less. “Fluence” indicates the degree of light collection and is expressed in terms of energy per unit area.
The vacancy defect formation apparatus 10 may further comprise a beam profiler 91 as an optional element for measuring the profile of laser light at an irradiation site. For example, the beam profiler 91 may be a CCD camera.
Various types of diamond may be used for the sample 9. In this embodiment, the sample 9 is, for example, an HPHT type-Ib single-crystalline synthetic diamond with a high concentration of nitrogen impurities and is [001]-oriented. By way of example, the nitrogen concentration of synthetic diamond used as the sample 9 is about 100 ppm. The sample 9 is placed on the 3-axis stage 8. The region of the sample 9 on which the pulsed light 2 is emitted (X-Y plane) and the position of the sample 9 along the irradiation axis of the pulsed light 2 (Z axis) are adjustable. The sample 9 can be irradiated with the pulsed light 2 at room temperature in the atmosphere. By way of example, room temperature means, but is not limited to, a temperature range of about 1° C. to 30° C., including the normal temperature range of about 15° C. to 25° C.
The laser 1 outputs the pulsed light 2 at a pulse width on the temporal order of femtoseconds (10−15 seconds). To achieve the output of high-intensity laser light with an extremely short pulse on the order of femtoseconds, the present embodiment uses the chirped pulse amplification (CPA) method for the laser 1, in which the pulse width is stretched, and the laser is amplified, followed by compressing the pulse width. The pulse compressor may contain various optical devices or optical elements to shorten the pulse width of the laser. Examples of such optical devices or optical elements include grating pairs, prism pairs, and chirped mirrors. The laser apparatus 1 according to this embodiment can compress the pulse width to about 35 femtoseconds (fs) by using a grating pair.
In the present embodiment, the pulsed light 2 is linearly polarized to s-polarized light or p-polarized light. The laser 1 for use is a titanium-sapphire laser that outputs pulsed light with a center wavelength of about 800 nm (regenerative amplifier). The laser 1 has a pulse width of about 35 femtoseconds (fs) in full width at half maximum (FWHM) of the intensity autocorrelation waveform and has a pulse energy of about 0.4 J. In other words, the peak power, which represents energy per unit time, of the laser 1 is about 1×1013 W. Because the laser 1 can concentrate light to a diameter of about 3 μm, the maximum electric field amplitude corresponding to the intensity of this peak power is about 3.2×1013 V/m. The contrast of the laser 1 is about 1011, and the energy stability is about 2%. The peak power increases with a decrease in pulse width.
The pulsed light 2 output from the laser 1 has a wavelength transparent with respect to the absorption wavelength of the sample 9. This allows the laser 1 to modify the sample 9 in the vicinity of the focus point of the laser due to nonlinear absorption. The pulsed light 2 having a wavelength transparent with respect to the absorption wavelength of the sample 9 means that the pulsed light 2 has such a wavelength band that no spectral line appears in the absorption spectrum of the sample 9 even when the sample 9 is irradiated with the pulsed light 2. The energy of the pulsed light 2 emitted to the sample 9 is absorbed nonlinearly by the crystal lattice of the sample 9.
The light-collecting element 3, aperture 4, and total-reflection mirror 5 constitute an optical system for applying convergent beam irradiation to the sample 9 with the pulsed light 2. In this embodiment, a parabolic mirror is used for the light-collecting element 3, and gold is deposited on the surface of the total-reflection mirror 5. The aperture 4 is placed on the optical path and controls the focus diameter of the pulsed light 2 emitted from the laser 1 in at least either the vertical direction or the horizontal direction, or both directions. The spatial distribution of laser light at an irradiation site is controlled by the aperture 4. The aperture 4 can be referred to as an “aperture for space control.”
In the present embodiment, controlling the size of the aperture 4 in at least either the vertical direction or the horizontal direction, or both directions on the plane perpendicular to the travelling direction of the pulsed light 2 enables the pulsed light 2 to focus on the sample 9 such that the spot region has a substantially elliptical or circular shape. By way of example, if the focal region has a horizontally elongated, substantially elliptical shape as shown in
Measurement System for Use in Evaluation
The vacancy defects formed in the sample 9 can be evaluated according to the known optically detected magnetic resonance (ODMR) method using a measurement system 20 shown in
In performing measurement according to the optically detected magnetic resonance (ODMR) method, a static magnetic field of about 30 gauss is applied to the sample 9 by using a magnet 30 to define a quantization axis. Excitation light 22 output from an excitation light source 21 is used to cause the NV center to transition from a ground state to an excited state, and is laser light with a wavelength of 532 nm (green). The excitation light 22 is reflected by a dichroic mirror 23 and collected by an objective lens 24A; then, the sample 9 is irradiated with the excitation light 22. If an NV center is present in the sample 9, the NV center in a ground state transitions into an excited state and then relaxes to a ground state. When relaxing into a ground state, the NV center emits red fluorescence 25 with a wavelength of around 700 nm. The intensity of the emitted fluorescence 25 is measured by using a photodiode 26. Microwave MW is applied to the sample 9 from a microwave source 28 by using a copper wire 27 (diameter: 30 μm). The frequency of the microwave MW being swept is measured by using a microwave detector 29. The optically detected magnetic resonance (ODMR) method can detect a magnetic resonance signal as the point at which the red fluorescence intensity has changed when the microwave frequency is swept at around 2.87 GHZ.
A confocal image of the sample 9 can be captured by using the photodiode 26. The position of the objective lens 24A is adjusted by a 3-axis piezoelectric stage 24B, and this enables convergent beam irradiation of the excitation light 22 to different positions on the sample 9.
Examples of the present invention are given below to further clarify the characteristics of the invention. Unless otherwise noted in the description of each Example, the diamond used for samples and measurement conditions for the laser, such as fluence and peak power, are the same between the Examples.
In Example 1, vacancy defects (NV centers) formed in diamond were evaluated according to various measurements. An HPHT type-Ib single-crystalline [001]-oriented synthetic diamond was used as a sample. The fluence of the laser was set to 20 J·cm−2, and the peak power was set to 1.1×1013 W. The shape of the focal region (laser spot) at a laser irradiation site was substantially circular, and the frequency of pulse irradiation N was one (one shot).
The results of various measurements of NV centers shown in
The measurement results of
In Example 2, the dependence of fluence on the concentration of NV centers was evaluated. The fluence was changed from about 1.8 J·cm−2 to about 54 J·cm−2.
In Example 2, the fluence was first set to about 1.1 J·cm−2, and NV centers were formed at the region indicated by reference numeral 52 in
The measurement results shown in
Referring to
As shown in the measurement results in
In Example 3, the effect of the frequency of pulse irradiation on graphitization of diamond was evaluated. NV centers were formed in diamond with different combinations of the shape of the focal region (laser spot) and fluence, while the frequency of pulse irradiation N was changed from 1 (one shot) to 60.
In
In
When the diamond substrate 61 shown in
The measurement results shown in
The measurement results shown in
As shown in the measurement results in
In Example 4, the concentration of the formed NV centers was calculated. The concentration was approximated from the luminescence intensity at a single NV center and the spatial resolution of a confocal fluorescence microscope.
First, the luminescence intensity of a single NV center was measured with a microscope. The laser power during the measurement was 100 ρw.
The resolution of the microscope used for the measurement was the following:
The focal volume V was calculated assuming a rotating cylinder with a radius of DFWHM/2 and a height of ZFWHM.
In the measurements shown in Examples 1 to 3, the maximum luminescence intensity from a focal region (laser spot) was roughly 500 kcps at the same laser power of 100 μW. Because the luminescence intensity of a single NV center was 70 kcps, 500 kcps/70 kcps≈7. It was inferred that roughly 7 NV centers were formed in a focal volume on a converted value basis. Thus, the concentration of NV centers was approximated to be 7/(2×10−13 cm3)=3.5×1013 cm−3. As shown in
The depth direction of the diamond substrate on which the formation of NV centers was confirmed was about the same as the resolution of the microscope in the thickness direction. Thus, the thickness of the region in which NV centers were actually present would be thinner than the resolution of the microscope in the thickness direction, and the actual concentration would be higher than the approximate value. Current research reports that NV centers actually appear to be present only near the surface of diamond. In this case, the focal volume V would be even smaller, and the actual concentration would thus be higher than the approximate value.
In Example 5, attempts were made to form NV centers in a wider region on diamond in view of findings from Examples 1 to 4. A diamond substrate was irradiated with pulsed light in a vacuum created around the diamond substrate. The diamond substrate and laser optical system were placed in a vacuum chamber, and the pressure in the vacuum chamber was adjusted to about 5×10−2 Pa by using a vacuum pump. The pulse energy of the laser was set to 166 mJ (about 0.2 J) with the fluence set to about 33 J/cm−2. The focal region (laser spot) at a laser irradiation site on diamond was substantially circular in shape and about 1.1 mm in diameter in size. The frequency of pulse irradiation N was one (one shot).
The measurement results shown in
According to Example 5, NV centers were able to be formed over a wide region on diamond. The size of the region in which NV centers could be formed was as large as about 1.1 mm in diameter on the millimeter order, as shown in
In Example 6, the relationship between the size of the focal region at a laser irradiation site and the laser fluence was examined for various kinds of pulse energy. Spatial distribution of laser pulse was assumed to be Gaussian. The shape of the focal region was assumed to be substantially circular.
In the graph shown in
In view of the findings from Examples 2 and 5, the fluence of laser required to form an NV center on the millimeter order on diamond was studied. With focus on the actual measurement results indicated by the solid black square “▪,” measurement was assumed with a pulse energy ranging from 0.064 mJ to 500 mJ along the arrow in the figure. As indicated by the open white square “□,” the predicted diameter of the focal region was about 3.6 mm (about 3640 μm) on the millimeter order, and the predicted diameter of fluence required to form an NV center was about 9.6 J/cm−2. The predicted measurement values indicated by the open white square “□” were higher than the horizontal rectangular region shown at the bottom of the graph; thus, it was inferred that NV centers could be formed based on the results of Example 2.
From the above, it was inferred that the fluence of the laser that can form NV centers in a large region on the millimeter order is not limited only to about 33 J/cm−2 (solid black diamond “♦”), which is illustrated in Example 5. In view of the findings from Examples 5 and 2, it was inferred that even a fluence of about 9.6 J/cm−2 (solid black square “▪”) could enable the formation of NV centers in a large region on the millimeter order.
The present invention enables the efficient formation of vacancy defects in diamond. In the past, it was not easy to form NV centers in diamond without graphitizing diamond, and it was difficult to form NV centers in diamond efficiently. The present invention enables the efficient formation of high-concentration NV centers in a large region of diamond without graphitizing the diamond.
The area of the diamond in which NV centers are formed is about 175 μm2 to about 452 μm2 in full width at half maximum (FWHM); NV centers can be formed in a wide region of 102 μm2 or more. Depending on the value of fluence, NV centers can be formed in a region wider than a focal region (laser spot). Furthermore, when diamond is irradiated with pulsed light from a laser, the pressure of the space around the diamond can be set to lower than atmospheric pressure to allow NV centers to form in a wide region on the millimeter order.
The frequency of pulse irradiation in forming NV centers is in the range of one to four, and more preferably one (one shot). This allows the formation of NV centers in diamond while preventing the graphitization of diamond.
In the present invention, the shape of the focal region can also be controlled to be substantially elliptical. An elliptical focal region allows NV centers to efficiently form in a wide region by sweeping the focal region by the present invention, and allows multiple elements to be cut out of a diamond substrate when pieces of diamond having NV centers are cut from the diamond substrate and mounted as elements of a quantum sensor. This improves productivity in the manufacture of elements.
The present invention also enables the formation of NV centers in diamond without annealing the diamond after irradiating the diamond with pulsed light of a laser. Eliminating the need for an annealing step makes it possible to use a low-temperature process in the industrial production of quantum sensor elements including NV centers, and allows quantum sensor elements to be manufactured at room temperature in air.
Another method for forming NV centers in diamond is, for example, irradiating diamond with a high-energy electron beam on the order of MeV. However, such an electron-beam irradiation method has problems on an industrial scale, such as the radiation of X-rays during electron-beam irradiation, which results in limited facilities for forming NV centers and high manufacturing costs. However, the method for forming NV centers by irradiating diamond with pulsed light of a laser according to the present invention does not involve X-ray radiation and allows the formation of NV centers in a relatively large number of facilities as compared with the electron-beam irradiation method.
Although the present invention is described above with reference to a particular embodiment, the invention is not limited to the above embodiment.
In the embodiment above, diamond is used as the sample 9, and a nitrogen-vacancy center, called “NV center,” is formed as a color center, inside the diamond. However, the color center formed inside diamond is not limited to NV centers. Instead of NV centers, silicon-vacancy centers or germanium-vacancy centers may be formed inside diamond.
In the embodiment above, the pulsed light 2 output from the laser 1 has a pulse width in the time order of femtoseconds. However, the pulse width of the laser for irradiation to modify diamond is not limited to the time order of femtoseconds. As long as NV centers can be formed in diamond, diamond may be irradiated with a laser beam with a pulse width on the temporal order of, for example, attoseconds (10−18 seconds), which is shorter than femtoseconds (10−15 seconds). The peak power of a pulsed laser is calculated by dividing pulse energy (unit: J) by pulse width (unit: s). Thus, if pulse energy is constant, then as the pulse width of the pulsed light 2 becomes even shorter than the temporal order of femtoseconds, the peak power of the laser 1 also increases more.
The concentration of nitrogen impurities contained in the sample 9 is not limited to the concentration described in the embodiment above. For example, the concentration of nitrogen impurities in synthetic diamond may be less than the concentration described above (e.g., less than about 1 ppm) in the application in quantum sensors, and the synthetic diamond for use is not limited to type Ib, and may be type IIa. The synthetic diamond for use may also have a plane orientation other than [001].
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-199843 | Dec 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/045265 | 12/8/2022 | WO |
