Enhanced Extraction of Silicon-Vacancy Centers Light Emission Using Bottom-Up Engineered Polycrystalline Diamond Photonic Crystal Slabs

Traceable Optical Physical Unclonable Functions Based on Germanium Vacancy in Diamonds

Physical unclonable functions (PUFs) have emerged as an unprecedented solution for modern information security and anticounterfeiting by virtue of their inherent unclonable nature derived from distinctive, randomly generated physical patterns that defy replication. However, the creation of traceable optical PUF tags remains a formidable challenge. Here, we demonstrate a traceable PUF system whose unclonability arises from the random distribution of diamonds and the random intensity of the narrow emission from germanium vacancies (GeV) within the diamonds. Tamper-resistant PUF labels can be manufactured on diverse and intricate structural surfaces by blending diamond particles into polydimethylsiloxane (PDMS) and strategically depositing them onto the surface of objects. The resulting PUF codes exhibit essentially perfect uniformity, uniqueness, reproducibility, and substantial encoding capacity, making them applicable as a private key to fulfill the customization demands of circulating commodities. Through integration of a digitized "challenge-response" protocol, a traceable and highly secure PUF system can be established, which is seamlessly compatible with contemporary digital information technology. Thus, the GeV-PUF system holds significant promise for applications in data security and blockchain anticounterfeiting, providing robust and adaptive solutions to address the dynamic demands of these domains.

Enhancement of dual zero phonon line emissions in nanodiamonds using quasiperiodic photonic structures

Color centers in nanodiamonds (NDs) have been largely explored by coupling to a photonic structured matrix (PSM) to amplify visible range emission features, enhancing their use in quantum technologies. Here, we study the emission enhancement of dual near-infrared zero phonon line (ZPL) emission from silicon-boron (SiB) and silicon-vacancy (SiV-) centers in NDs using a spontaneously emerged low index-contrast quasiperiodic PSM, having micron-scale air pores. An intensity enhancement factor of 6.15 for SiV- and 7.8 for SiB ZPLs is attained for the PSM sample compared to a control sample. We find Purcell enhancement of 2.77 times for the PSM sample using spatial-dependent decay rate measurements, supported by localized field intensity confinement in the sample. Such cavity-like emission enhancement and lifetime reduction are enabled by an in-plane order-disorder scattering in the PSM sample substantiated by pump-dependent emission measurements. The results put forward a facile approach to tailor the near-infrared dual ZPL emission from NDs using nanophotonic structures.

Metasurface Enhanced Sensitized Photon Upconversion: Toward Highly Efficient Low Power Upconversion Applications and Nanoscale E-Field Sensors

Large-scale nanoimprinted metasurfaces based on silicon photonic crystal slabs were produced and coated with a NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticle (UCNP) layer. UCNPs on these metasurfaces yield a more than 500-fold enhanced upconversion emission compared to UCNPs on planar surfaces. It is also demonstrated how the optical response of the UCNPs can be used to estimate the local field energy in the coating layer. Optical simulations using the finite element method validate the experimental results and the calculated spatial three-dimensional field energy distribution helps us to understand the emission enhancement mechanism of the UCNPs closely attached to the metasurface. In addition, we analyzed the spectral shifts of the resonances for uncoated and coated metasurfaces and metasurfaces submerged in water to enable a prediction of the optimum layer thicknesses for different excitation wavelengths, paving the way to applications such as electromagnetic field sensors or bioassays.

Photonic crystal cavity-enhanced emission from silicon vacancy centers in polycrystalline diamond achieved without postfabrication fine-tuning

Diamond optical centers have recently emerged as promising single-photon sources for quantum photonics. Particularly, negatively charged silicon vacancy (SiV-) centers show great promise due to their narrow zero-phonon emission line present also at room temperature. However, due to fabrication tolerances it is challenging to prepare directly photonic structures with optical modes spectrally matching the emission of SiV- centers. To reach the spectral overlap, photonic structures must typically undergo complicated post-processing treatment. In this work, suspended photonic crystal cavities made of polycrystalline diamond are engineered and more than 2.5-fold enhancement of the SiV- center zero-phonon line intensity via coupling to the cavity photonic mode is demonstrated. The intrinsic non-homogeneous thickness of the diamond thin layer within the sample is taken as an advantage that enables reaching the spectral overlap between the emission from SiV- centers and the cavity modes without any post-processing. Even with lower optical quality compared to monocrystalline diamond, the fabricated photonic structures show comparable efficiency for intensity enhancement. Therefore, the results of this work may open up a promising route for the application of polycrystalline diamond in photonics.

Nanosphere Lithography for Structuring Polycrystalline Diamond Films

This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various arrays of diamond nanostructures—close-packed and non-close-packed monolayers over substrates with various levels of surface roughness, noble metal films over nanosphere arrays, ordered arrays of holes, and unordered pores—were created using reactive ion etching, chemical vapour deposition, metallization, and/or lift-off processes. The size and shape of the lithographic mask was altered using oxygen plasma etching. The periodicity of the final structure was defined by the initial diameter of the spheres. The surface morphology of the samples was characterized using scanning electron microscopy. The advantages and limitations of the fabrication technique are discussed. Finally, the potential applications (e.g., photonics, plasmonics) of the obtained nanostructures are reviewed.

Enhanced SiV photoluminescence by oxidation-induced nano-structures on diamond particle surfaces

We successfully constructed nanostructures on the surface of diamond particles by oxidation, which drastically enhanced their silicon-vacancy (SiV) photoluminescence (PL) intensity. The {100} plane of the diamond crystal initially had a smooth surface and strong anti-oxidation ability, which converted to a nano-pyramid structure with the sides resembling a {111} crystal plane orientation after oxidation. The {111} plane originally presented vertically layered or scale-like structures, but exhibited irregular nanoporous structures with some ridges perpendicular to the {111} plane in the edge area after oxidation. Since the crystal orientation of these nano-structures matches the <111> aligned split-vacancy structure of the SiV center, the collection efficiency of SiV luminescence increased, such that the SiV emission intensity increased by 27-fold and 4-fold for the nano-pyramid and the irregular nano-porous structure, respectively. Oxidation also significantly improved the crystal quality of diamond, such that the lattice stress around the SiV color center reduced, narrowing the linewidth of the SiV PL peak to nearly 3 nm. This study provides a feasible way to optimize SiV photoluminescent properties by building nanostructures on the surface of diamond particles.

Manipulating Luminescence of Light Emitters by Photonic Crystals

The modulation of luminescence is essential because unwanted spontaneous-emission modes have a negative effect on the performance of luminescence-based photonic devices. Photonic crystals are promising materials for the control of light emission because of the variation in the local density of optical modes within them. They have been widely investigated for the manipulation of the emission intensity and lifetime of light emitters. Several groups have achieved greatly enhanced emission by depositing emitters on the surface of photonic crystals. Herein, the different modulating effects of photonic crystal dimensions, light-emitter positions, photonic crystal structure type, and the refractive index of photonic crystal building blocks are highlighted, with the aim of evaluating the fundamental principles that determine light propagation. The applications of using photonic crystals to manipulate spontaneous emission in light-emitting diodes and sensors are also reviewed. In addition, potential future challenges and improvements in this field are presented.

Two-dimensional photonic crystals increasing vertical light emission from Si nanocrystal-rich thin layers

We have fabricated two-dimensional photonic crystals (PhCs) on the surface of Si nanocrystal-rich SiO₂ layers with the goal to maximize the photoluminescence extraction efficiency in the normal direction. The fabricated periodic structures consist of columns ordered into square and hexagonal pattern with lattice constants computed such that the red photoluminescence of Si nanocrystals (SiNCs) could couple to leaky modes of the PhCs and could be efficiently extracted to surrounding air. Samples having different lattice constants and heights of columns were investigated in order to find the configuration with the best performance. Spectral overlap of the leaky modes with the luminescence spectrum of SiNCs was verified experimentally by measuring photonic band diagrams of the leaky modes employing angle-resolved spectroscopy and also theoretically by computing the reflectance spectra. The extraction enhancement within different spatial angles was evaluated by means of micro-photoluminescence spectroscopy. More than 18-fold extraction enhancement was achieved for light propagating in the normal direction and up to 22% increase in overall intensity was obtained at the spatial collection angle of 14°.

Silicon-Vacancy Centers in Ultra-Thin Nanocrystalline Diamond Films

Color centers in diamond have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report the optoelectronic investigation of shallow silicon vacancy (SiV) color centers in ultra-thin (7⁻40 nm) nanocrystalline diamond (NCD) films with variable surface chemistry. We show that hydrogenated ultra-thin NCD films exhibit no or lowered SiV photoluminescence (PL) and relatively high negative surface photovoltage (SPV) which is ascribed to non-radiative electron transitions from SiV to surface-related traps. Higher SiV PL and low positive SPV of oxidized ultra-thin NCD films indicate an efficient excitation-emission PL process without significant electron escape, yet with some hole trapping in diamond surface states. Decreasing SPV magnitude and increasing SiV PL intensity with thickness, in both cases, is attributed to resonant energy transfer between shallow and bulk SiV. We also demonstrate that thermal treatments (annealing in air or in hydrogen gas), commonly applied to modify the surface chemistry of nanodiamonds, are also applicable to ultra-thin NCD films in terms of tuning their SiV PL and surface chemistry.

Bright nanowire single photon source based on SiV centers in diamond

The practical implementation of many quantum technologies relies on the development of robust and bright single photon sources that operate at room temperature. The negatively charged silicon-vacancy (SiV^-) color center in diamond is a possible candidate for such a single photon source. However, due to the high refraction index mismatch to air, color centers in diamond typically exhibit low photon out-coupling. An additional shortcoming is due to the random localization of native defects in the diamond sample. Here we demonstrate deterministic implantation of Si ions with high conversion efficiency to single SiV^- centers, targeted to fabricated nanowires. The co-localization of single SiV^- centers with the nanostructures yields a ten times higher light coupling efficiency than for single SiV^- centers in bulk diamond. This enhanced photon out-coupling, together with the intrinsic scalability of the SiV^- creation method, enables a new class of devices for integrated photonics and quantum science.

Fabrication of silicon-vacancy color centers in diamond films: tetramethylsilane as a new dopant source

A diamond film featuring a structure of nano-crystals separated with (100) micro-grains displays a maximum in the PL intensity of SiV centers.

Fabrication of tough photonic crystal patterns with vivid structural colors by direct handwriting

Patterned photonic crystals (PCs) have attracted considerable attention due to their great potential in practical applications. Direct writing is an important and convenient method to fabricate patterned PCs. However, due to the limited interaction among spheres and the evaporation of ink, the obtained patterns usually suffer from poor structure strength, and non-uniform and unstable structural colors. In this work, an in situ embedding and locking strategy for fabricating tough PC patterns in one step was demonstrated. With properly dried polymer films as "paper" and dispersions of CdS spheres as "inks" to write on the "paper", the self-assembly of CdS spheres and locking of the PC structure can be achieved simultaneously, which gives rise to tough composite patterned PCs with uniform, stable and permanent structural colors. Based on this simple method, tough PC patterns can be easily and quickly created by direct hand writing or drawing without special treatment, equipment, masks or templates. The vivid structural colors of the tough PC patterns and this simple method show great potential for practical applications.