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Abstract
We demonstrate the multi-focal fabrication of single $\textrm{N}{\textrm{V}^ - }$ centres at different depths in a bulk diamond. After performing an annealing process, the optically detected magnetic resonance (ODMR) signal is measured to confirm the generation of $\textrm{N}{\textrm{V}^ - }$ centres. These emitters are photo-stable and are imaged by using three different excitations wavelengths at different depths close to the bulk diamond surface. Using two-photon microscopy, the $\textrm{N}{\textrm{V}^ - }$ centres are imaged with a resolution down to 0.36 µm to study the effect of the exposure time of the multi-focal laser writing on the spatial resolution of each fabrication. The Hanbury Brown and Twiss (HBT) test is performed with two-photon excitation to study the number of $\textrm{N}{\textrm{V}^ - }$ centres at the location of each multi-focal fabrication. We demonstrate the formation of single $\textrm{N}{\textrm{V}^ - }$ centres placed between 100 nm and 3 µm below the diamond surface. The ability to control the placement of single $\textrm{N}{\textrm{V}^ - }$ centres with depth is of paramount importance for applications in quantum information, single spin magnetometry, and optical data storage.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to its photostability and unit electron spin that remains controllable at room temperature up to milliseconds [1], the nitrogen vacancy ($\textrm{N}{\textrm{V}^ - }$) centres in diamonds have been extensively implemented for various applications ranging from magnetometry [2,3], quantum information processing [4] superresolution imaging [5–8], and optical data storage [9]. In particular, controlling the placement of $\textrm{N}{\textrm{V}^ - }$ centres in three dimensions in a bulk diamond is relevant for data storage, biomedical applications, and to achieve functional devices based on artificial intelligence and neuromorphic units [10–15]. $\textrm{N}{\textrm{V}^ - }{\; }$centres are usually produced via chemical vapour deposition (CVD) in the presence of nitrogen impurities [16] or by means of ion implantation (mostly nitrogen) [17]. Other methods, also successfully demonstrated the formation of $\textrm{N}{\textrm{V}^ - }$ centers [18,19]. Ultra-short pulse laser beams, in particular femtosecond laser beams, have been used for the creation of$\; \textrm{N}{\textrm{V}^ - }$ centres from ensemble to the single level [20–24] and for nanofabrication in diamonds [25]. The high energy laser pulses induces graphitization in the irradiated zone [20], while $\textrm{N}{\textrm{V}^ - }$ centres are created on the surface [21] and in the bulk [22] due to multi-photon ionization (MPI) of oxygen and nitrogen molecules in air, generating free electrons [21]. Alternately, formation of $\textrm{N}{\textrm{V}^ - }$ centres deeper in the diamond and without damaging the material was achieved with a high numerical aperture (NA) objective and a light spatial modulator (SLM) to compensate for the high refractive index aberration to induce MPI in the bulk diamond [23,26]. More recently, other mechanism to form $\textrm{N}{\textrm{V}^ - }$ centres on demand have also been reported [27–29]. However, these methods were successful to generate $\textrm{N}{\textrm{V}^ - }$ either close to the surface or at 20 µm or deeper from the surface, these last methods with an increased complexity in the experimental setup and at the expenses of longer writing time. The simultaneous fabrication of single $\textrm{N}{\textrm{V}^ - }$ centre in multiple focal zones and different later position was not yet shown. Using a multifocal beam we are able to create $\textrm{N}{\textrm{V}^ - }$ at different depths and at different lateral positions simultaneously with a much shorter writing time and a simpler experimental setup
In this paper we use a multi-focal beam arrays generated with SLMs technology to enable faster and parallel laser writing [30] of colour centres close to the diamond surface. The energy regime is higher than the 60 TW/cm2, which is the maximum peak intensity for the MPI to occur in the bulk diamond for our experimental conditions or in the air. Therefore, single $\textrm{N}{\textrm{V}^ - }$ centre formation occurs in the tunnelling ionization energy regime, where the electric field of the laser suppresses the Coulomb well and bound electrons tunnel from the valence band to the conduction band. Alternatively $\textrm{N}{\textrm{V}^ - }$ centre formation occur in concomitance of graphitization as previously demonstrated [21].
We also demonstrate the multi-focal laser writing of single $\textrm{N}{\textrm{V}^ - }$ centres at different depths in a bulk diamond using two-photon absorption imaging for better imaging localisation. After performing an annealing treatment, the formation of colour centres is observed. The verification of the optically detected magnetic resonance (ODMR) [31–33] confirms the generation of $\textrm{N}{\textrm{V}^ - }$ centres. These bright and fluorescent $\textrm{N}{\textrm{V}^ - }$ centres are imaged with different excitation wavelengths and at three different focal planes in the bulk material. We demonstrate also the detection of $\textrm{N}{\textrm{V}^ - }$ centres at the two-photon excitation wavelength of 980 nm. Finally, the Hanbury Brown and Twiss (HBT) test is performed with a 980 nm laser beam to study the number of $\textrm{N}{\textrm{V}^ - }$ centres formed in the diamond. It is demonstrated that it is possible to fabricate single $\textrm{N}{\textrm{V}^ - }$ centres, respectively, at 100 nm and at 3 µm below the diamond surface. Therefore, our multi-focal fabrication condition enables to place single $\textrm{N}{\textrm{V}^ - }$ centres at different depths in the bulk diamond. The multi-focal laser writing of single $\textrm{N}{\textrm{V}^ - }$ centres at different depths is useful for increasing the speed of fabrication and may find applications in detection and manipulation of single spin states for efficient data storage, quantum information processing and magnetometry [34–37].
2. Laser writing experiment
The laser source was a femtosecond (100 fs pulse width) laser beam at the 800 nm wavelength with a repetition rate of 5 kHz. A fast mechanical shutter (Uniblitz 6 mm) controlled the laser exposure during fabrication. The laser beam was expanded by using a negative focal length first lens (f1) and a positive focal length second lens (f2) onto a SLM (Hamamatsu X10468-02) and then focused into a single crystal plate diamond sample (Element Six Technologies US Corporation, California, USA) through a 40X 0.75 NA lens (Fig. 1(a)). The laser pulse intensity was set at 30 mW average power, corresponding to about 6 µJ/pulse, before entering the SLM. The beam was then split into an array of 4 spatially separated laser pulses by the SLM [30]. These four beams were focused on the diamond plate with different exposure times of the shutter. For each multi-focal array, the exposure time was varied at 1000 ms, 500 ms, 200 ms, 100 ms and 80 ms, respectively. Damages on the surface of the diamond were observed at the location of the 4 incident beams imaged with a widefield optical microscope (NIKON, Tokyo, Japan) with 20X 0.4 NA objective lens (Fig. 1(b)). Under the condition of 30 mW and 80 ms exposure time, no damages were observed on the diamond surface. In our specific experimental conditions to generate $\textrm{N}{\textrm{V}^ - }$ centres in the diamond due to nonlinear photoionization such as MPI, the laser beam peak intensity should be I<60 TW/cm2, using the Keldysh parameter [26], describing the transition between multiphoton ionization and tunnelling ionization. In particular the energies per pulse are 120 nJ, 150 nJ, 300 nJ, 750 nJ and 1500 nJ for the fabrications at 80 ms, 100 ms, 200 ms, 500 ms and 1000 ms, respectively. As we use the higher intensity, we can operate in the regime of tunnelling ionization in the diamond and air because the laser beam is focussed close to the diamond surface. The tunneling rate scales more weakly with the laser intensity than the multiphoton ionisation rate.
Fig. 1. (a) Schematic of the multi-focal laser fabrication system. The beam diameter from the laser is ∼ 11 mm. The the diameter on the SLM set by the phase pattern is ∼ 11.25 mm. (b) Bright field image of the bulk diamond surface after fabrication. The image is taken before the annealing procedure is performed. (c) After annealing, fluorescence from $\textrm{N}{\textrm{V}^ - }$ centres is observed with a confocal microscope. Scale bar is 7 µm. (d) Emission spectrum obtained at the 561 nm excitation wavelength. ZPL is the zero phonon line at 637 nm. (e) ODMR signal detected for the fluorescent $\textrm{N}{\textrm{V}^ - }$ centres.
After the fabrication, irregularities in the shapes and diameters of the fabrications were observed due to optical aberrations (Fig. 1(b)). The bulk diamond was annealed in dry nitrogen for 3 hours at 1000°C [23]. The sample was then imaged with a confocal microscope (NIKON, Tokyo, Japan) with a laser beam at the 561 nm wavelength focused onto the sample through a 1.4 NA oil immersion lens. The fluorescence was collected through the same objective and detected by a single-photon avalanche photodiode (SPAD) to confirm the formation of $\textrm{N}{\textrm{V}^ - }$ centres. No fluorescence was detected for the 1000 ms exposure time used during fabrication. The fluorescence detected for the exposure time of 500 ms for the laser writing process is reported in Fig. 1(c). The $\textrm{N}{\textrm{V}^ - }$ centres generates a fluorescent spot shaped like a ring around the beam focus. The observed fluorescence is stronger at the centre of each fabricated crater [21]Further, the emission spectrum was acquired for the excitation wavelength at 561 nm (Fig. 1(d)).
In order to confirm the formation of $\textrm{N}{\textrm{V}^ - }$ centres, a gold pattern was placed on the bulk diamond to deliver microwaves stimulation for the ODMR experiment [31,38]. The ODMR signal showed a dip at the microwave frequency of 2.87 GHz (Fig.1e), confirming the formation of $\textrm{N}{\textrm{V}^ - }$ centres.
3. Results and discussions
3.1 Imaging $\textrm{N}{\textrm{V}^ - }$ centres at different wavelengths
We next studied the effect of the laser fabrication energy on the fluorescence emission of the fabricated $\textrm{N}{\textrm{V}^ - }$ centres. For this study, a confocal Nikon microscope (NIKON, Tokyo, Japan) with three laser wavelengths was implemented. The arrays of $\textrm{N}{\textrm{V}^ - }$ centres were imaged with a 100× oil-immersion objective of NA 1.40 and wavelengths at 488 nm, 561 nm and 638 nm, respectively. Imaging of $\textrm{N}{\textrm{V}^ - }$ centres generated by laser pulses with the 500 ms exposure time was performed at 3µm below the diamond surface. $\textrm{N}{\textrm{V}^ - }$ centres generated by the beams under the exposure time of 200 ms and 100 ms were imaged at the depth of 430 nm and 100 nm, respectively (Fig. 2(a)). The position accuracy calculated as displacement between each fluorescent $\textrm{N}{\textrm{V}^ - }$ centres and a uniform grid was measured to be between 1.0 and 1.5 µm.
Fig. 2. (a) Confocal images of the arrays of $\textrm{N}{\textrm{V}^ - }$ centres under three different laser sources at the wavelengths of 488 nm, 561nm and 638 nm, respectively. The scale bar is 10 µm (b) Photon counts emission versus excitation wavelengths. An emission peak is observed when a beam at 561 nm wavelength is implemented as an excitation source. (c) Photon counts under a beam at 561 nm versus Energy/pulse.
The fluorescence intensity of $\textrm{N}{\textrm{V}^ - }$ centres was found to increase when switching from the excitation wavelength of 488 nm to 561 nm. A decrease in fluorescence was observed from the wavelengths of 561 nm to 638 nm (Fig. 2(b)). A fluorescent peak was measured for the 561 nm excitation wavelength. No photobleaching was observed at the three wavelengths. In addition, we observed higher fluorescence counts for the fabrications performed with the femtosecond beam at 300 nJ/pulse (Fig. 3(c)). This result depends on the number of $\textrm{N}{\textrm{V}^ - }$ centres generated for each exposure time which we discuss in section 3.3. After demonstrating that bright and fluorescent stable $\textrm{N}{\textrm{V}^ - }$ centres are generated at different depths in the bulk diamond, three $\textrm{N}{\textrm{V}^ - }$ centre locations, each in a given focal plane are selected. As a further step, we introduce two-photon microscopy [39–41] to image the fabricated $\textrm{N}{\textrm{V}^ - }$ centres in depth.
Fig. 3. (a,left) Two-photon confocal images of $\textrm{N}{\textrm{V}^ - }$ centres with the fabrication power of 30 mW (a, left). The smallest $\textrm{N}{\textrm{V}^ - }$ centre has a FWHM of 0.36 µm. The scale bar is 1 µm. (b) FWHM of $\textrm{N}{\textrm{V}^ - }$ centres versus the exposure time of the beams during fabrication. (c) The depths (µm) where each $\textrm{N}{\textrm{V}^ - }$ centres are imaged plotted as a function of the fabrication exposure time (ms).
3.2 Two-photon imaging
For this experiment, we used a home built confocal microscope with a femtosecond beam at the wavelength of 980 nm (pulse width 140 fs) and a repetition rate of 80 MHz focused onto the sample through a 1.4 NA oil immersion lens. Two-photon excitation imaging of $\textrm{N}{\textrm{V}^ - }$ centres were performed at different focal planes in the bulk diamond with better accuracy than using one photon absorption (Fig. 3(a), left). It has been previously demonstrated that $\textrm{N}{\textrm{V}^ - }$ can be excited by two-photon absorption [39,40]. Further, we examined the size dependence of the $\textrm{N}{\textrm{V}^ - }$ centre spot on the multi-focal fabrication exposure time at different depths in the bulk diamond (Fig. 3(a)). The smallest full width at half maximum (FWHM) was measured to be around 0.36 µm for $\textrm{N}{\textrm{V}^ - }$ centres generated with the exposure time of 100 ms. The theoretical FWHM of the two-photon fluorescence microscopy system is 0.35 µm [42]. Advanced imaging methods, like stimulated emission depletion (STED) microscopy would provide the measurement of the size of the $\textrm{N}{\textrm{V}^ - }$ centres with nanoscale resolution [43]. The largest spatial resolution of around 1.24 µm was measured for an $\textrm{N}{\textrm{V}^ - }$ centre spot generated for the exposure time 500 ms. By increasing the exposure time of the laser beams during the writing process, we registered an increase of the FWHM of the diffraction limited size of $\textrm{N}{\textrm{V}^ - }$ centres (Fig. 3(b)). The measured photoluminescence (PL) intensities depend on the numbers of the $\textrm{N}{\textrm{V}^ - }$ centres in each fluorescence spot. This can be verified by the HBT test together with two-photon microscopy (see section 3.3)
Further we measured the depth of the positioning of $\textrm{N}{\textrm{V}^ - }$ centres, with ∼ 0.97 µm axial resolution uncertainty, as distance between the diamond surface and the focal plane at which the fluorescent image of all the fabrications at a given exposure time reached the maximum amount of sharpness. We observed an increase of the in-depth placement of $\textrm{N}{\textrm{V}^ - }$ centres in the diamond with the increase of the exposure time adopted for the multi-focal fabrication (Fig. 3(c)). By increasing the exposure time, the thermal energy diffuses out of the focal volume and a non-thermal ionic motion induces permanent structural changes causing the vacancies to form more in deep in the bulk diamond [44].
3.3 Fabrication of single $\textrm{N}{\textrm{V}^ - }$ centres at different focal plans
The effect of the multi-focal laser fabrication on the number of $\textrm{N}{\textrm{V}^ - }$ centres within the bulk diamond were studied by measuring the second-order auto-correlation function versus the time delay with a confocal microscope with fs fabrication at wavelength 980 nm and a repetition rate of 80 MHz. The beam was focused onto the sample through a 1.4 NA oil immersion lens. The PL was coupled into two separate SPADs via a fibre beam splitter, to measure the correlation data with a time correlated single-photon counting card. In our setup, the fluorescent emission of the fabrications are detected with a signal to background ratio greater than 500000.
For the$\; \textrm{N}{\textrm{V}^ - }$ centres fabricated with exposure time of 100 ms, the second order auto-correlation function at the zero delay measured a value ${g^{(2 )}}$(0) <0.5 that described the presence of a single $\textrm{N}{\textrm{V}^ - }$ centre in the focal region (Fig. 4(a), left). The HBT measurement for the $\textrm{N}{\textrm{V}^ - }$ centres fabricated with a beam of the 200 ms exposure time indicated ${g^{(2 )}}$(0) ≈ 0.6 that represented the presence of 3 $\textrm{N}{\textrm{V}^ - }$ centres (Fig. 4(a), centre). At the 500 ms exposure time, the HBT test clearly showed ${g^{(2 )}}$(0) <0.5 and therefore the presence of a single $\textrm{N}{\textrm{V}^ - }$ centre (Fig. 4(a), right). The formation of single $\textrm{N}{\textrm{V}^ - }$ centres using an exposure time of 500 ms for the laser pulses is justified by the low concentration of N in the bulk diamond, less than 5 parts-per-billion (ppb) [23].
Fig. 4. Second order correlation functions ${g^2}(t )$ of single $\textrm{N}{\textrm{V}^ - }$ centres (left, right) and of 3 $\textrm{N}{\textrm{V}^ - }$ centres (centre).
4. Conclusion
We have demonstrated the multi-focal laser writing of arrays of $\textrm{N}{\textrm{V}^ - }$ centres in a bulk diamond. In particular, our fabrication conditions have enabled to place single emitters at different depths in the diamond material. We have studied the PL properties of the fabricated $\textrm{N}{\textrm{V}^ - }$ centres with different laser wavelengths. Furthermore, the spatial resolution of $\textrm{N}{\textrm{V}^ - }$ centres and their placement in the diamond using different exposure time of the writing beams was revealed with two-photon microscopy. Finally, the HBT test was performed to measure the number of $\textrm{N}{\textrm{V}^ - }$ centres. We have demonstrated that single $\textrm{N}{\textrm{V}^ - }$ centres can be generated between 100 nm and 3 µm below the diamond surface. Our method could be further improved by on demand annealing in situ using femtosecond laser with calibrated annealing pulse energy to control the $\textrm{N}{\textrm{V}^ - }$ centres positioning with nanometer accuracy [27]. The precise control of the fabricated structures could also be achieved if correction for the optimum amount of aberration at each fabrication depth will be performed with the SLMs. The ability to place single $\textrm{N}{\textrm{V}^ - }$ centres in diamond with a multi-focal array at different depths enables to increase the speed of fabrication by a factor of four and can facilitate the centre fabrication in 3D for applications in quantum information processing, magnetic sensing and particularly optical data storage.
Funding
Australian Research Council (DP170101775).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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