Multidimensional luminescence microscope for imaging defect colour centres in diamond

Daniel Jones, Sunil Kumar, Peter M. P. Lanigan, Colin D. McGuinness, Matthew W. Dale, Daniel J. Twitchen, David Fisher, Philip M. Martineau, Mark A. A. Neil, Christopher Dunsby and Paul M. W. French

Photonics Group, Imperial College London, London SW7 2AZ, UK

Diamond Science and Technology CDT, University of Warwick, CV4 7AL, UK

De Beers Technologies, Belmont Road, Maidenhead, SL6 6JW, UK

Element Six GIC, Harwell Campus, Didcot, OX11 0QR

Key words – Luminescence, Hyperspectral, Lifetime, Diamond, Silicon Vacancy (SiV), Nitrogen Vacancy (NV), N3, Natural Diamond, CVD Diamond, Spectroscopy

Abstract We report a multidimensional luminescence microscope providing hyperspectral imaging and time-resolved (luminescence lifetime) imaging for the study of luminescent diamond defects. The instrument includes crossed-polariser white light transmission microscopy to reveal any birefringence that would indicate strain in the diamond lattice. We demonstrate the application of this new instrument to detect defects in natural and synthetic diamonds including N3, nitrogen and silicon vacancies. Hyperspectral imaging provides contrast that is not apparent in conventional intensity images and the luminescence lifetime provides further contrast.

Introduction Diamond has been used as a gemstone for over a millennium and more recently has been used in applications exploiting its extreme physical, electronic and quantum properties. Since the advent of chemical vapour deposition (CVD) diamond growth, it has become possible to systematically explore and utilise the optical and quantum properties of diamond. The nitrogen vacancy (NV) has received particular attention due to its potential for quantum optical technologies and there is increasing interest in the silicon vacancy (SiV), which offers further advantages for quantum optical applications. However, there are many more optical defects, and over 500 optical signatures of diamond luminescence have been recorded1. It is interesting to explore the photophysics of this rich diversity of luminescent defects in order to better understand the potential for photonics applications and to characterise natural diamonds. In addition, the spectroscopic signatures can be used to distinguish different compositions and growth conditions of diamond, aiding future engineering of photophysical properties for photonics technologies and enhancing the ability to classify different natural and synthetic diamonds. Thus, there is a need for instrumentation to characterise the broad range of spectroscopic properties of these defects and of their interactions, since these can modify their photophysical properties. Mapping such defects can provide additional information since they are not typically homogeneously distributed, and the distribution of emissive and absorptive defects can change significantly, particularly near strain fields and boundaries, for example of growth planes. Luminescent defects in diamond are typically imaged through conventional epifluorescence microscopes, and the NV- centre has been imaged with super-resolved microscopy techniques including STED2, RESOFLT3 and STORM4. However, there are relatively few

reports of hyperspectral imaging of defect luminescence and lifetime imaging is mostly confined to the NV- centre due to its importance for quantum optical applications.

Previous work on hyperspectral imaging on diamond has employed cathodoluminescence of carbonado, which is a natural form of polycrystalline diamond to measure spatially resolved spectral changes5 and a hyperspectral imaging instrument (HeLIOS6) for diamond is under development. Luminescence lifetimes of diamond defects have historically been measured using ultrafast flashtubes7,8, pulsed cathodoluminescence9, and time-correlated single photon counting (TCSPC)10. More recently, the Hanbury Brown and Twiss interferometry has been used to determine lifetimes via fitting of second-order correlation measurements, e.g.11, although this has not been applied to characterise complex multiexponential decays. Time-resolved luminescence imaging of diamond luminescence is less common but has been realised with limited spectral coverage of NV- centres12, NV0 centres13 and H3 centres14, using TCSPC in laser scanning confocal microscopes. More recently, luminescent lifetimes of SiV- defects have been mapped in microneedles15 and combined with confocal measurements16 . Typically, these lifetime mapping experiments have utilised fixed wavelength excitation lasers and have measured lifetimes in a single spectral band, although lifetime measurements have previously been demonstrated across multiple emission wavelengths17.

Here we report a multidimensional imaging system developed for diamond luminescence imaging, which provides white light transmission, semi-confocal hyperspectral and laser scanning TCSPC imaging, with a tunable supercontinuum-based excitation source, complemented by a frequency-tripled Yb:glass optical fibre laser providing excitation at 355 nm. Hyperspectral imaging of diamond is a powerful way to rapidly map a wide variety of emissive defects in diamond and study their spectral properties. This can be useful for screening bulk diamond and nanodiamonds for desirable or interesting photophysical properties and could be combined with high throughput generation of defects in diamond18 to explore new defects and applications. The ability to tune the excitation enables a wide range of defects to be interrogated and can also provide conventional photoluminescence excitation (PLE) data. We implement hyperspectral imaging using the “push-broom” technique with line illumination and an imaging spectrometer. An EMCCD is used for detection of the x-λ image combined with stage scanning in the y direction to acquire the full x-y-λ hypercube. Spectral detection via an imaging spectrometer can provide superior spectral resolution compared to wide-field approaches using electronically tunable liquid crystal or acoustic-optic filter. We can also implement ‘whisk broom’ imaging and utilise TCPSC after a computer-controlled monochromator to provide spectrally resolved luminescence lifetime imaging. Measuring the luminescence lifetime is important from the perspective of potential applications in colour centre lasers, quantum computing and photonics devices and can also provide spectroscopic information for the identification of specific defects or to elucidate interactions between defects. For example, the quenching of the N3 defect lifetime can indicate the presence of A centres7, which themselves are not thought to emit optical radiation. The implementation of crossed polarisers in white light transmission mode enables visualisation of birefringence in the sample, which is a signature of strain in the lattice19, which is known to affect the photophysics of defects.

Methods The configuration of this multidimensional luminescence microscope is depicted in Figure 1. It employs two excitation sources: an electronically tunable excitation source based on a spectrally filtered optical fibre laser-based supercontinuum source (Fianium, SC400-4) , which provides ~picosecond pulsed radiation over 400-750 nm at 40 MHz pulse repetition rate; and a frequency tripled Yb-doped mode-locked optical fibre laser (Fianium, UVP-355) that provides picosecond pulses at 355 nm at 40 MHz repetition rate.

A series of 4 cold mirrors blocks the infrared radiation above 750 nm from the supercontinuum source to provide ~400 mW total average power over the ~350 nm tuning range. The collimated output beam is directed into a dispersive prism (SF10, P1), and the resulting spectral components are collimated by a 150 mm focal length anti-reflection coated achromatic lens (L1, Edmund Optics, 400-1000 nm) that also focusses each spectral component onto an anodised slit in its back focal plane that is mounted on a motorised translation stage (Newport, ESP100, TRA25CC). As shown in Figure 1, a second matched lens (L2, f = 150 mm) and second SF10 prism (P2) undoes the dispersion of the radiation to provide a collimated excitation beam with spectral properties determined by the width and position of the computer controlled slit. The mode-locked picosecond Yb:glass fibre laser can provide output beams at 1064 nm (fundamental), 532 nm after second harmonic generation and 355 nm after third harmonic generation. The 355 nm beam is separated using a 440 nm short pass filter (Di1) to provide up to ~150 mW average power and is expanded 7.5x. For hyperspectral imaging, line illumination is realised using a 20 mm focal length cylindrical lens followed by a x2 magnification telescope to expand the beam to overfill the back aperture of the objective lens in order to reduce the intensity variation along the illumination line that is due to Gaussian laser beam profile. This telescope also expands the tunable excitation beam from the supercontinuum source.

Figure 1. Optical configuration of the instrument including two excitation arms: the supercontinuum excitation source (SC400-4, Fianium) and the 355 nm ps laser source (UVP-355, Fianium). Each excitation beam is expanded to fill the back aperture of the objective on the microscope frame. A cylindrical lens (Cl1) on a kinematic stage enables switching between point illumination using the photomultiplier (PMT) for lifetime imaging and line illumination for hyperspectral imaging using the EMCCD camera. All lenses were achromatic doublets for visible measurements except the cylindrical lens. L5-6 were replaced with singlets when using 355 nm illumination. Lens focal lengths of L1, L2: 150 mm; L3: 20 mm; L4: 150 mm; Cl1 (cylindrical): 20 mm; L5: 100 mm; L6: 200 mm; O1 (microscope objective lens):4X 0.16NA, 10X 0.3NA, 40x 0.55NA; and L7/8: 180 mm.

The excitation beams are coupled into an inverted fluorescence microscope frame (Olympus IX71) with a motorised z-drive and the sample is mounted on a motorised x-y stage (Marzhauser, SCAN IM 112x74). The emission is coupled to the (10 µm) entrance slit of a dual monochromator/imaging spectrograph (Acton SP-300i), which is electronically switchable to direct the signal to either the

spectrograph imaging port or a monochromator output. At the imaging port the luminescence is detected using an EMCCD (Andor, iXon DV887) and at the monochromator output it is detected using a photon-counting photomultiplier (Becker and Hickl PMC-100-1) coupled to a TCPSC system (SPC-830, Becker and Hickl). The spectrometer contains a high resolution 600 lines/mm and a lower resolution 150 lines/mm blazed gratings to provide either a spectral detection bandwidth of ~40 nm with ~0.25 nm resolution or a ~170 nm bandwidth with ~1 nm resolution respectively on the EMCCD. The microscope also incorporates a colour CMOS camera (Point Grey, Chameleon) for polarisation-resolved imaging using a lamp with Kohler illumination or for low magnification wide-field imaging for sample orientation. The luminescence emission spectra presented in the paper are uncalibrated (they have not been corrected for variations in instrument transmission or detector sensitivity as function of wavelength). The entire instrument is controlled using a custom written LabVIEW (2017) program, which can be used to perform automated measurements. All images generated by the system are co-registered, allowing for links between different phenomena to be easily identified and further examined. Lifetime analysis was achieved using FLIMfit20 for imaging and spectrally resolved lifetime measurements. The fluorescence decay profiles were fitted to the model:

𝐼𝐼(𝑡𝑡) = 𝐼𝐼0 ∑ 𝛽𝛽𝑖𝑖𝑒𝑒− 𝑡𝑡𝜏𝜏𝑖𝑖𝑖𝑖 , where βi is the population fraction of the ith decay component, τi is its

corresponding lifetime and I0 is the intensity at time t = 0.

We present here the exemplar application of this instrument to image two diamond samples. The first sample (0.05 ct, 1.52 X 1.92 X 0.75 mm) is a synthetic diamond grown by chemical vapour deposition (CVD) and includes a type Ib high pressure high temperature (HPHT) grown substrate. The CVD layer was doped with silicon and increasing amounts of nitrogen. After CVD growth, the sample was electron-bombarded and subsequently annealed to create vacancy defects. The second sample (0.22 ct, 3.9 X 3.9 X 2.4 mm) is a natural type IaAB cut and polished diamond. Results from CVD-grown diamond Figure 2 shows images of the CVD diamond acquired using a 10x, 0.3 NA microscope objective lens. Figure 2(a) shows a white light transmission image and Figure 2(b) the same transmission image recorded through crossed polarisers of the HPHT (substrate) and CVD-grown diamond. There are no significant features indicative of the (growth) structure of this diamond sample other than the darker region at the top of the transmission image that corresponds to the bare HPHT substrate (green region) and some strain indicated in the crossed polariser image near a chip towards the bottom edge of the sample where the higher intensity indicates some birefringence in the sample. The CVD diamond (red region) represents the bulk of the image. This diamond has been cut perpendicular to the growth direction and so we do not expect to see significant birefringence in the transverse face imaged.

Figure 2. Images were taken using a 10X, 0.3 NA objective. a) White light transmission image of the CVD (red region) with HPHT substrate (green region) synthetic diamond. b) White light transmission with crossed polarisers image showing birefringence in the sample. c) False colour intensity-modulated median wavelength hyperspectral image acquired in line scanning mode, where the blue and green arrows represent the striation bands. Median wavelengths is represented using hue in HSV colour space from 640 nm (red) to 695 nm (purple); d) Averaged ROIs of the hyperspectral image showing the zero phonon lines of NV0 (575 nm), NV- (637 nm), and SiV- (737 nm) at different locations on the sample. e) White light transmission image of full sample, where the CVD diamond is transparent, and the HPHT is the orange growth substrate, taken with a 1.25X, 0.04 NA objective. f) White light transmission image with crossed polarisers showing birefringence. g) above bandgap (<225 nm) excited ‘DiamondViewTM’ 21 luminescence image of sample with region imaged shown by a blue dashed line. Scale bar is 250 μm.

Figure 2(c) shows an image from a hyperspectral data set acquired using the EMCCD and line excitation at 475 nm with luminescence recorded from 570 to ~740 nm, with an acquisition time of 1 s per spectral slice. The false colour image displays an intensity-modulated plot of the median wavelength (calculated from the acquired spectrum at each pixel) where the luminescence intensity and median wavelength are encoded in the HSV colour space at each pixel, such that hue represents wavelength and value represents intensity, with black corresponding to the lowest luminescence intensity. The bare HPHT substrate appears in purple at the top of the image (region of interest 1). The CVD diamond presents orange bands (white dashed lines show the boundaries, Figure 2(c)) of varying intensity moving away from the HPHT substrate that we attribute to different growth stages with changes in the nitrogen concentration during growth, increasing away from the substrate, approximately every 275 µm. Striations at an angle of ~30° are indicated with blue and green arrows.

The hyperspectral data set can be further analysed to obtain a spectrum at each pixel or averaged over any region of interest (ROI). Figure 2(d) shows the extracted spectra from five ROIs indicated on the hyperspectral image of Figure 2(c). The bare HPHT substrate in ROI 1 indicates a high concentration of NV- defects. This is indicated by the peak at 637 nm, which corresponds to the NV- zero phonon line (ZPL), and by the large phonon side band extending above 740 nm. No other defects are indicated in this spectrum. ROI 2 from the growth region immediately after the substrate presents a largely flat spectrum with a peak at 737 nm which is the ZPL of the SiV- defect. We expect

this growth region to have the lowest nitrogen incorporation, ROI 2. The spectrum from ROI 5 in the furthest growth band from the substrate shows a higher concentration of SiV- defects, together with an indication of NV0 defects revealed by the ZPL peak at 575 nm and broad phonon side band >575 nm. ROI 3 and ROI 4 indicate the presence of both SiV- and NV0 defects, with the relative heights of the ZPLs indicating differing concentrations. The ‘green arrow’ (ROI 3) striations in the hyperspectral image present the two ZPL peaks with a similar peak height, while the ‘blue arrow’ (ROI 4) striations indicate a higher concentration of SiV- relative to NV0. This could be explained by the CVD growth following a step and riser regime with differing quantities of nitrogen being incorporated into the lattice for each step and riser. Silicon is currently not known to be incorporated differentially with respect to growth phase but has been discovered to be hindered by excessive nitrogen incorporation22, which is important because nitrogen is typically used as a growth enhancer for diamond growth23. Figures 2(e) and 2(f) show low (1.25x) magnification white light transmission and birefringence (crossed polariser) images of the whole diamond sample. Figure 2(g) shows a ‘DiamondViewTM’ image that indicates the overall banded CVD growth and the striations, however, due to the excitation method and lack of spectroscopic information, the origin of this emission cannot be deduced.

Luminescence lifetime was imaged over a 0.340 × 1.425 mm region of interest (5 μm pixel size), using the same x10, 0.3 NA microscope objective lens and excitation at 475 nm with emission being detected in three emission wavelength bands using the monochromator. Figure 3(a) is the same hyperspectral image as Figure 2(c) with the region of interest indicated. Figure 3(b) shows the luminescence lifetime image measured with a peak/width emission passband of 560/15 nm and is dominated by the ZPL of NV0 which presents a monoexponential decay profile with an average lifetime of 19.1 ns from a pixelwise fit (Supplementary Figure 1, Supplementary Table 1) averaged over the CVD region, which is in reasonable agreement with the literature13. Consistent with Figure 2, there is no emission from the HPHT substrate in this 560/15 nm emission passband.

Figure 3. Images taken with 10X, 0.3 NA objective. a) False colour intensity-modulated median wavelength hyperspectral image with the ROI used for luminescence imaging shown by a red dashed line. Scale bar is 250 μm. b) Intensity-modulated luminescence lifetime map of emission at 560/15 nm corresponding to NV0

emission. c) Intensity-modulated weighted mean luminescence lifetime map of emission at 621/25 nm corresponding to emission of NV0 and NV-. d) 3x3 binned intensity modulated weighted mean luminescence lifetime map of emission at 721/25 nm corresponding to emission of all 3 defects, with variations in the striations’ lifetimes. e) 3x3 binned intensity weighted mean luminescence lifetime map of a region shifted down under 675 nm excitation to preferentially excite SiV- and not NV0 or NV- at 721/15 nm detection. All lifetime values are in ps.

Figure 3(c) shows the pixelwise fitted lifetime image in an emission passband of 621/25 nm, where the HPHT substrate luminesces strongly, and the CVD diamond emits from the NV0 phonon side band, as indicated by the hyperspectral spectrum data. The emission from the averaged CVD diamond region is well fitted (χ2 = 1.0) to a triple exponential decay profile with spatially averaged lifetime components τ1 of 20.4 ns (attributed to NV0) and τ2 of 3.0 ns (Supplementary Table 1), which has an unknown origin. τ1 is slightly longer than in the 560/15 nm spectral band, and this requires further investigation. The τ3 component for this region presents a β3 of 0.01 and a lifetime τ3 of 530 ps, which is of unknown origin. The false colour plot of figure 3(c) shows the intensity-weighted mean of these lifetime components at each pixel encoded such that colour represents lifetime and brightness represents intensity, with black corresponding to the lowest luminescence intensity.

The emission from the HPHT substrate presents a more complex decay profile that required a triple exponential decay model to fit the data (Supplementary Figure 2b), where the average values comprise of a long (τ1) component of 13.0 ns and faster lifetime components of 4.0 ns (τ2) and 600 ps (τ3) respectively. The τ1 component is slightly longer than the previously reported14 12 ns decay of NV- and we note there are other reports of varying NV- lifetimes24. We do not know the origin of the faster lifetime components and we do not observe any spatial variation in lifetime or intensity of these components across the HPHT substrate (Supplementary Figure 2a). It is possible that this emission originated from defects which overlap spectrally with the NV- defect emission but which become apparent through the lifetime contrast. In future work, these faster lifetime emission components could be studied at low temperatures to remove the phonon side band emission and PLE studies could also be undertaken.

Figure 3(d) shows the 3x3 binned pixelwise fitted lifetime image of the same region of interest with detection in an emission passband at 721/25 nm and presents complex emission profiles with components due to at least the known NV0, NV- and SV- defects, see Supplementary Figure 3. The weighted mean lifetime of the HPHT region is reduced in this emission passband due to a reduced lifetime of the long decay component (τ1) that we attribute to NV- (10.2 ns) due to the emission spectrum and additional lifetime components of 3.2 ns (τ2) and 350 ps (τ3). The change in lifetime, which we attribute to NV-, requires further investigation. In the CVD diamond, a triple exponential decay fit is required to fit the emission profiles with components of 1.1 ns attributed to SiV-, which agrees well with experimental literature25, and of 18.7 ns, which is attributed to NV0. The third component, τ3 is a fast 290 ps decay with unknown origins. The variation in lifetime between the striations of the CVD diamond is attributed to a variation in the NV0 concentration, which is supported by the lifetime component population fraction (β) map shown in Supplementary Figure 3. The variation in NV0 suggests that N concentration would also vary with the striations. Further research is required to reveal the origin of some of these decay components which cannot be spectroscopically identified.

Figure 3(e) shows a 3x3 binned pixelwise fitted lifetime image with a region shifted down acquired with an emission of 721/15 nm under 675 nm excitation where we would expect to excite the SiV- defect but not the NV- or NV0 defects. Here, the HPHT substrate presents a triple decay with a long component (τ1) of 8.9 ns that contributes 40% of the emitter population, see very top left corner of

β1 map of Supplementary Figure 4a and decay shown in supplementary Figure 4b. This contribution decreases over the first 2 CVD layers away from the substrate, which gives rise to the decrease in intensity weighted mean lifetime moving down the field of view seen in Figure 3(e). Due to the excitation wavelength, NV- is unlikely to be the origin of this emission, in addition to the faster than expected lifetime. A lifetime component of 1.1 ns (τ2) is present over the CVD region, which can be attributed to SiV-. The third component (τ3) presents a ~100 ps lifetime over the CVD diamond with higher concentration in the HPHT substrate (Supplementary Figure 4a β3 map), which could be related to previously reported lifetime components associated with SiV- decays 26, in the CVD diamond. In the HPHT substrate, the fastest (τ3) component is 120 ps, which could be related to the previous fastest decay present at 475 nm excitation.

The luminescence decay profiles and tables of fitted lifetime components are presented in Supplementary Table 1.

Results from natural diamond

Figure 4(a) shows a hyperspectral (false colour intensity-modulated mean wavelength) luminescence image, acquired with a 4X, 0.16 NA objective lens and excited at 355 nm, of the table of the natural diamond shown in Figure 4(b). This hyperspectral image indicates some variation in intensity across the field of view, notably a brighter horizontal band one third of the way down from the top of the field of view, but the mean emission wavelength is reasonably uniform across the image. Figure 4(e) shows the emission spectrum resulting from spatial averaging over the whole image. This indicates the presence of the N3 defect with a ZPL of 415 nm. Using a higher NA objective (40x 0.55 NA), we acquired a point-scanned lifetime image with excitation at 355 nm. Figures 4(c,d) show the time-integrated luminescence intensity and lifetime images of this ROI. The improved optical sectioning and signal to noise ratio of the point-scanning microscope results in more features appearing across the field of view, compared to the hyperspectral imaging of Figure 4(a). These features, particularly the sharp cornered features (red arrows in Figure 4(c)), appear to correspond to regions of change in growth of this natural diamond21. As the diamond grew in the Earth, the relative abundance of nitrogen and other elements will have changed, and this could explain the variation in the observed intensity of N3 emission across these features.

Figure 4. a) Intensity modulated false colour mean wavelength hyperspectral image of a natural cut and polished diamond, taken under 355 nm excitation, using a 4X, 0.16 NA objective of a ~170 nm spectral band from 400 nm, with spectral resolution of ~1 nm. b) White light colour image of the sample 0.25 carat cut natural diamond. c) Integrated intensity point-scanned image of the emission at 430/5 nm, acquired with a 40x 0.55 NA objective in a). d) 3x3 binned intensity weighted mean lifetime luminescence lifetime map (ps), fitted pixelwise with a triple exponential decay, showing spatial variations in intensity weighted mean lifetime. e) Luminescence emission spectrum of the sample integrated across the whole hyperspectral image. The ZPL at 415.2 nm indicates this is the emission from the N3 defect and its lifetime has been significantly quenched to an average of 23.6 ns. f) Spectrally resolved intensity weighted mean lifetimes of two regions on the sample that showed a variation in lifetime in the luminescence lifetime map d.

Figure 4(d) shows how the intensity-weighted mean luminescence lifetime measured with an emission passband of 430/5 nm varies between two ROIs. The data was fitted using 3x3 binning to a triple exponential decay model with the faster components fixed at τ3 = 342 ps and τ2 = 2.9 ns, and the slow component determined pixelwise. The values for τ2 and τ3 were determined by spatially integrating the decays from all pixels and fitting the resultant decay profile to a triple exponential decay model. This produced a better fit (χ2 = 1.0) compared to pixelwise fitting of all three lifetime components, although the general trends were similar. The spatial average of the long (τ1) component was 23.6 ns, which we attribute to the N3 defect undergoing significant quenching due to interactions with A centres in the sample, which reduces the N3 defects’ natural lifetime from ~40 ns7. The population contributions of the slowest lifetime components (β1) and of the fastest (β3 = 342 ps) component were found to vary spatially across the FOV while the intermediate lifetime component (β2 = 2.9 ns) appeared to be less spatially variant (Supplementary Figure 5). Thus, variations in the mean luminescence lifetime followed variations in the long component lifetime (attributed to N3 emission). This lifetime was observed to decrease where the luminescence intensity was higher (Supplementary Figure 6), which may be explained by local increases in the concentration of nitrogen during growth resulting in increases in both N3 and A centre defects, which would increase both the total emission intensity and lifetime quenching. To explore this further, two small ROIs were selected, as indicated in Figure 4(d), and spectrally resolved (1 nm sampling) luminescence decay data were acquired. For this ROI lifetime analysis, the faster (342 ps)

component and intermediate (2.9 ns) components were fixed and the intensity weighted mean lifetime is plotted as a function of emission wavelength in Figure 4(f). The similar spectral profiles suggest that the same emissive (N3) defects are present in each ROI but that the degree of quenching (by A centres) is different.

Figure 5. a) Spectrum of a point on the natural cut sample under 355 nm excitation using a 4X, 0.16 NA objective, with a log scale indicating the presence of the 1st and 2nd order Raman shift at 372 and ~392 nm respectively. b) Spectrally resolved decay profiles from same region with a decay plotted every 0.05 nm. c) Spectrally resolved intensity-weighted mean lifetimes globally fitted with 3 exponential decays, binned to 0.5 nm resolution. d) Spectrally resolved β and τ values of each decay component globally fitted, binned to 0.5 nm resolution.

Figure 5 presents a more detailed spectroscopic analysis of a ROI at the centre of the table of the natural diamond for which luminescence was excited at 355 nm and collected using a 4X, 0.16 NA objective lens, every 0.05 nm. Figure 5(a) shows linear and log plots of the emission spectrum in which the first and second order Raman shifts are observed (at 372 nm and ~392 nm respectively), along with the spectrum from ~410 nm attributed to the N3 defect emission. Figure 5(b) shows how the luminescence emission spectrum changes as a function of decay time. Figure 5(c) shows how the intensity-weighted mean lifetimes, calculated from a global fit to a triple exponential decay model, vary as a function of emission wavelength, after binning to 0.5 nm resolution.

Figure 5(d) shows the lifetime component population fractions (β) for each lifetime component of the triple exponential decay model as a function of emission wavelength, after binning to 0.5 nm resolution. The analysis used a triple exponential decay model where the three lifetime values were determined globally across all decays. The long ~23.5 ns component (τ1) is attributed to the N3 defect emission. The intermediate component was found to have a lifetime of 3.0 ns (τ2). Below 400 nm, the τ2 emission has the largest β2 fraction at ~0.7, which reduces to ~0.3 between 400 - 500 nm and to ~0.1 for emission above ~500 nm. The 3.0 ns lifetime of this component

together with the spectral information suggests it can be attributed to A-Band emission, which is thought to originate from dislocations1,27 and which is efficiently excited at 355 nm. The β3 values of the fastest (τ3) lifetime component of 450 ps appear to be inversely correlated with the β2 values of the τ2 component (Supplementary Figure 5), but this needs further investigation. Repeating this study at low temperatures may help elucidate the photophysics since this would enhance the emission from the ZPL of N3 and reduce any phonon side bands.

Conclusion We have demonstrated the capabilities of a multidimensional luminescence microscope designed to characterise the luminescent defects of natural and synthetic diamond and have measured the emission from a selection of defects with known and unknown origins. This work illustrates how multidimensional luminescence measurements can enhance spectroscopic contrast. Future work could include PLE measurements of the emission bands and the acquisition of larger area hyperspectral emission and lifetime maps. Characterising HPHT/CVD diamond emission could inform the optimisation of growth conditions, e.g. to engineer photonic and quantum optical devices. A systematic study of N3 centre emission in diamond and the impact of spatially varying concentrations of N3 and A centres due to quenching of N3 lifetimes could provide characteristic signatures of specific gemstones.

Acknowledgements We would like to acknowledge Element Six who grew the HPHT-CVD synthetic diamond used for the work presented here. We would like to thank to thank Martin Kehoe and Simon Johnson from the Physics Department Optical Instrumentation facility for the design and fabrication of key optical mechanical components and Mark Newton (University of Warwick) for his valuable insights into the interpretation of the data. Daniel Jones acknowledges an EPSRC-funded studentship from the EPSRC Centre for Doctoral Training in Diamond Science and Technology. The authors have declared that no conflicting interests exist.

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