Optical Properties of Bulk Single-Crystal Diamonds at 80–1200 K by Vibrational Spectroscopic Methods

Bulk diamonds show great potential for optical applications such as for use in infrared (IR) windows and temperature sensors. The development of optical-grade bulk diamond synthesis techniques has facilitated its extreme applications. Here, two kinds of bulk single-crystal diamonds, a high-pressure and high-temperature (HPHT) diamond and a chemical vapor deposition (CVD) diamond, were evaluated by Raman spectroscopy and Fourier Transform Infra-Red (FTIR) spectroscopy at a range of temperatures from 80 to 1200 K. The results showed that there was no obvious difference between the HPHT diamond and the CVD diamond in terms of XRD and Raman spectroscopy at 300–1200 K. The measured nitrogen content was ~270 and ~0.89 ppm for the HPHT diamond and the CVD diamond, respectively. The moderate nitrogen impurities did not significantly affect the temperature dependence of Raman spectra for temperature-sensing applications. However, the nitrogen impurities greatly influence FTIR spectroscopy and optical transmittance. The CVD diamond showed higher transmittance, up to 71% with only a ~6% drop at temperatures as high as 873 K. This study shows that CVD bulk diamonds can be used for IR windows under harsh environments.

Divergent effects of laser irradiation on ensembles of nitrogen-vacancy centers in bulk and nano-diamonds: implications for biosensing

Ensembles of negatively charged nitrogen vacancy centers (NV-) in diamond have been proposed for sensing of magnetic fields and paramagnetic agents, and as a source of spin-order for the hyperpolarization of nuclei in magnetic resonance applications. To this end, strongly fluorescent nanodiamonds represent promising materials, with large surface areas and dense ensembles of NV-. However, surface effects tend to favor the less useful neutral form, the NV0 centers. Here, we study the fluorescence properties and optically detected magnetic resonance (ODMR) of NV- centers as a function of laser power in strongly fluorescent bulk diamond and in nanodiamonds obtained by nanomilling the native material. In bulk diamond, we find that increasing laser power increases ODMR contrast, consistent with a power-dependent increase in spin-polarization. Surprisingly, in nanodiamonds we observe a non-monotonic behavior, with a decrease in ODMR contrast at higher laser power that can be ascribed to more efficient NV-→NV0 photoconversion in nanodiamonds compared to bulk diamond, resulting in depletion of the NV- pool. We also studied this phenomenon in cell cultures following internalization of NDs in macrophages. Our findings show that surface effects in nanodiamonds substantially affect the NV properties and provide indications for the adjustment of experimental parameters.

Ground-State Depletion Nanoscopy of Nitrogen-Vacancy Centres in Nanodiamonds

The negatively charged nitrogen-vacancy ($${\text{NV}}^{ - }$$ NV - ) centre in nanodiamonds (NDs) has been recently studied for applications in cellular imaging due to its better photo-stability and biocompatibility if compared to other fluorophores. Super-resolution imaging achieving 20-nm resolution of $${\text{NV}}^{ - }$$ NV - in NDs has been proved over the years using sub-diffraction limited imaging approaches such as single molecule stochastic localisation microscopy and stimulated emission depletion microscopy. Here we show the first demonstration of ground-state depletion (GSD) nanoscopy of these centres in NDs using three beams, a probe beam, a depletion beam and a reset beam. The depletion beam at 638 nm forces the $${\text{NV}}^{ - }$$ NV - centres to the metastable dark state everywhere but in the local minimum, while a Gaussian beam at 594 nm probes the $${\text{NV}}^{ - }$$ NV - centres and a 488-nm reset beam is used to repopulate the excited state. Super-resolution imaging of a single $${\text{NV}}^{ - }$$ NV - centre with a full width at half maximum of 36 nm is demonstrated, and two adjacent $${\text{NV}}^{ - }$$ NV - centres separated by 72 nm are resolved. GSD microscopy is here applied to $${\text{NV}}^{ - }$$ NV - in NDs with a much lower optical power compared to bulk diamond. This work demonstrates the need to control the NDs nitrogen concentration to tailor their application in super-resolution imaging methods and paves the way for studies of $${\text{NV}}^{ - }$$ NV - in NDs’ nanoscale interactions.

Cooling Mechanisms in a Rotating Brake Disc With a Wire-Woven-Bulk Diamond Cellular Core

Thermofluidic behaviors governing the enhanced cooling performance of the wire-woven-bulk diamond (WBD) cored brake disc in comparison with the conventional pin-finned brake disc used on heavy vehicles were characterized experimentally. For each type of brake disc, detailed internal thermofluidic data of the two rotating brake discs were obtained using transient thermochromic liquid crystal (TLC) for end-wall heat transfer and particle image velocimetry (PIV) for the inflow field. The results demonstrate that the pin-finned brake disc exhibits a circumferentially periodic curved inline-like passage flow and large dead flow regions, with strong recirculation that reduces its thermal dissipation performance. The cooling advantage of the WBD core is primarily attributed to the combination of enlarged heat transfer surface area (both end-wall and core) and greater utilization of the larger surface due to favorable fluidic behavior developed from the WBD topology. The internal WBD core has approximately three times the surface density of the pin-finned disc which, in combination with the smaller and weaker recirculation zones, leads to more effective usage of the available core surface area for thermal dissipation. The aerodynamic anisotropy of the WBD core induced by its topological anisotropy causes a globally irregular thermofluidic distribution in the brake disc.

The Structural Design of a High-Performance WBD Brake Disc

ABSTRACT A high performance, newly-developed wire-woven bulk diamond (WBD) ventilated brake disc is introduced to reduce the operating temperatures and mass of conventional brake discs. The use of the highly porous material requires a deeper understanding of the mechanical stresses developed within a brake disc to be developed to improve the disc core strength to withstand the high stresses developed during braking. In this study, experimentally determined solid brake disc stress distribution results, separated into the compressive stresses due to the pad clamping force and the shear stresses due to the applied brake torque, were applied to the reinforcement ofthe WBD core brake disc. The analysis was based on the maximum predicted deceleration conditions of a medium sized truck (Mercedes-Benz Atego). While the WBD core material possessed sufficient strength to withstand the shearing due to the braking torque, the pad clamping load was predicted to cause disc failure. Consequently, straight radial ribs were designed to reinforce the ventilated core, with final rib dimensions of 74x14x2.5 mm, manufactured from mild steel (SAE1006). A total of 10 ribs at 36° intervals were added to reinforce the core, increasing the mass by 0.20 kg compared to the original disc. The newly reinforced WBD brake disc remains lighter than a commercially available pin-finned disc, and is expected to maintain superior thermal performance while possessing the required mechanical strength. Additional keywords: Ventilated disc, mechanical stresses, braking, stress distribution

Adjoint-optimized nanoscale light extractor for nitrogen-vacancy centers in diamond

We designed a nanoscale light extractor (NLE) for the efficient outcoupling and beaming of broadband light emitted by shallow, negatively charged nitrogen-vacancy (NV) centers in bulk diamond. The NLE consists of a patterned silicon layer on diamond and requires no etching of the diamond surface. Our design process is based on adjoint optimization using broadband time-domain simulations and yields structures that are inherently robust to positioning and fabrication errors. Our NLE functions like a transmission antenna for the NV center, enhancing the optical power extracted from an NV center positioned 10 nm below the diamond surface by a factor of more than 35, and beaming the light into a ±30° cone in the far field. This approach to light extraction can be readily adapted to other solid-state color centers.