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Hyperbolic metamaterials (HMMs) offer unconventional properties in the field of optics, enabling opportunities for confinement and propagation of light at the nanoscale. In-plane orientation of the optical axis, in the direction coinciding with the anisotropy of the HMMs, is desirable for a variety of novel applications in nanophotonics and imaging. Here, a method for creating localized HMMs with in-plane optical axis, based on block copolymer (BCP) blend instability, is introduced. The dewetting of BCP thin film over topographically defined substrates generates droplets composed of highly ordered lamellar nanostructures in hierarchical configuration. The hierarchical nanostructures represent a valuable platform for the subsequent pattern transfer into a Au/air HMM, exhibiting hyperbolic behavior in a broad wavelength range in the visible spectrum. A computed Purcell factor as high as 32 at 580 nm supports the strong reduction in the fluorescence lifetime of defects in nanodiamonds placed on top of the HMM., Comment: This is the pre-peer reviewed version of the following article: I. Murataj et al. Adv. Optical Mater. 2021, 2001933, which has been published in final form at https://doi.org/10.1002/adom.202001933. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions

Single-photon sources represent a key enabling technology in quantum optics, and single colour centres in diamond are a promising platform to serve this purpose, due to their high quantum efficiency and photostability at room temperature. The widely studied nitrogen vacancy centres are characterized by several limitations, thus other defects have recently been considered, with a specific focus of centres emitting in the Near Infra-Red. In the present work, we report on the coupling of native near-infrared-emitting centres in high-quality single crystal diamond with Solid Immersion Lens structures fabricated by Focused Ion Beam lithography. The reported improvements in terms of light collection efficiency make the proposed system an ideal platform for the development of single-photon emitters with appealing photophysical and spectral properties., Comment: 17 pages, 5 figures

We report on the structural modifications induced by a lambda = 532 nm ns-pulsed high-power laser on sub-superficial graphitic layers in single-crystal diamond realized by means of MeV ion implantation. A systematic characterization of the structures obtained under different laser irradiation conditions (power density, number of pulses) and subsequent thermal annealing was performed by different electron microscopy techniques. The main feature observed after laser irradiation is the thickening of the pre-existing graphitic layer. Cross sectional SEM imaging was performed to directly measure the thickness of the modified layers, and subsequent selective etching of the buried layers was employed to both assess their graphitic nature and enhance the SEM imaging contrast. In particular, it was found that for optimal irradiation parameters the laser processing induces a six-fold increase the thickness of sub superficial graphitic layers without inducing mechanical failures in the surrounding crystal. TEM microscopy and EELS spectroscopy allowed a detailed analysis of the internal structure of the laser irradiated layers, highlighting the presence of different nano graphitic and amorphous layers. The obtained results demonstrate the effectiveness and versatility of high-power laser irradiation for an accurate tuning of the geometrical and structural features of graphitic structures embedded in single crystal diamond, and open new opportunities in diamond fabrication., Comment: 24 pages, 7 figures

Due to their outstanding mechanical properties, diamond and diamond-like materials find significant technological applications ranging from well-established industrial fields (cutting tools, coatings, etc.) to more advanced mechanical devices as micro- and nano-electromechanical systems. The use of energetic ions is a powerful and versatile tool to fabricate three-dimensional micro-mechanical structures. In this context, it is of paramount importance to have an accurate knowledge of the effects of ion-induced structural damage on the mechanical properties of this material, firstly to predict potential undesired side-effects of the ion implantation process, and possibly to tailor the desired mechanical properties of the fabricated devices. We present an Atomic Force Microscopy (AFM) characterization of free-standing cantilevers in single-crystal diamond obtained by a FIB-assisted lift-off technique, which allows a determination of the Young's modulus of the diamond crystal after the MeV ion irradiation process concurrent to the fabrication of the microstructures, and subsequent thermal annealing. The AFM measurements were performed with the beam-bending technique and show that the thermal annealing process allows for an effective recovery of the mechanical properties of the pristine crystal., Comment: 15 pages, 5 figures

The control of the charge state of nitrogen-vacancy (NV) centers in diamond is of primary importance for the stabilization of their quantum-optical properties, in applications ranging from quantum sensing to quantum computing. To this purpose, in this work current-injecting micro-electrodes were fabricated in bulk diamond for NV charge state control. Buried (i.e. 3 {\mu}m in depth) graphitic micro-electrodes with spacing of 9 {\mu}m were created in single-crystal diamond substrates by means of a 6 MeV C scanning micro-beam. The high breakdown field of diamond was exploited to electrically control the variation in the relative population of the negative (NV-) and neutral (NV0) charge states of sub-superficial NV centers located in the inter- electrode gap regions, without incurring into current discharges. Photoluminescence spectra acquired from the biased electrodes exhibited an electrically induced increase up to 40% in the NV- population at the expense of the NV0 charge state. The variation in the relative charge state populations showed a linear dependence from the injected current at applied biases smaller than 250 V, and was interpreted as the result of electron trapping at NV sites, consistently with the Space Charge Limited Current interpretation of the abrupt current increase observed at 300 V bias voltage. In correspondence of such trap-filling-induced transition to a high-current regime, a strong electroluminescent emission from the NV0 centers was observed. In the high-current-injection regime, a decrease in the NV- population was observed, in contrast with the results obtained at lower bias voltages. These results disclose new possibilities in the electrical control of the charge state of NV centers located in the diamond bulk, which are characterized by longer spin coherence times., Comment: 28 pages, 8 figures

We report on the fabrication and characterization of a single-crystal diamond device for the electrical stimula- tion of light emission from nitrogen-vacancy (NV0) and other defect-related centers. Pairs of sub-superficial graphitic micro-electrodes embedded in insulating diamond were fabricated by a 6 MeV C3+ micro-beam irra- diation followed by thermal annealing. A photoluminescence (PL) characterization evidenced a low radiation damage concentration in the inter-electrode gap region, which did not significantly affect the PL features domi- nated by NV centers. The operation of the device in electroluminescence (EL) regime was investigated by ap- plying a bias voltage at the graphitic electrodes, resulting in the injection of a high excitation current above a threshold voltage (~300V), which effectively stimulated an intense EL emission from NV0 centers. In addition, we report on the new observation of two additional sharp EL emission lines (at 563 nm and 580 nm) related to interstitial defects formed during MeV ion beam fabrication.

Focused MeV ion beams with micrometric resolution are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk. Their effectiveness has been shown for the fabrication of multi-electrode ionizing radiation detectors and cellular biosensors. In this work we investigate such fabrication method for the electrical excitation of color centers in diamond. Differently from optically-stimulated light emission from color centers in diamond, electroluminescence (EL) requires a high current flowing in the diamond subgap states between the electrodes. With this purpose, buried graphitic electrode pairs with a spacing of 10 $\mu$m were fabricated in the bulk of a single-crystal diamond sample using a 6 MeV C microbeam. The electrical characterization of the structure showed a significant current above an effective voltage threshold of 150V, which was interpreted according to the theory of Space Charge Limited Current. The EL imaging allowed to identify the electroluminescent regions and the residual vacancy distribution associated with the fabrication technique. Measurements evidenced bright electroluminescent emission from native neutrally-charged nitrogen-vacancy centers ($NV^0$); the acquired spectra highlighted the absence of EL associated with radiation damage., Comment: This article has been withdrawn due to the inclusion of additional upgrades and modifications, which significantly improve the paper. A new submission will be performed at a later stage

Focused MeV ion microbeams are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk, as demonstrated in previous works with the fabrication of multi-electrode ionizing radiation detectors and cellular biosensors. In this work we investigate the suitability of the fabrication method for the electrical excitation of colour centres in diamond. Differently from photoluminescence, electroluminescence requires an electrical current flowing through the diamond sub-gap states for the excitation of the colour centres. With this purpose, buried graphitic electrodes with a spacing of 10 micrometers were fabricated in the bulk of a detector-grade CVD single-crystal diamond sample using a scanning 1.8 MeV He micro-beam. The current flowing in the gap region between the electrodes upon the application of a 250 V bias voltage was exploited as the excitation pump for the electroluminescence of different types of colour centres localized in the above-mentioned gap. The bright light emission was spatially mapped using a confocal optical microscopy setup. The spectral analysis of electroluminescence revealed the emission from neutrally-charged nitrogen-vacancy centres ($NV^0$, $\lambda_{ZPL}$ = 575 nm), as well as from cluster crystal dislocations (A-band, {\lambda} = 400-500 nm). Moreover, an electroluminescence signal with appealing spectral features (sharp emission at room temperature, low phonon sidebands) from He-related defects was detected ($\lambda_{ZPL}$ = 536.3 nm, $\lambda_{ZPL}$ = 560.5 nm); a low and broad peak around {\lambda} = 740 nm was also observed and tentatively ascribed to Si-V or GR1 centres. These results pose interesting future perspectives for the fabrication of electrically-stimulated single-photon emitters in diamond for applications in quantum optics and quantum cryptography.

Electronic transport and magnetic properties of Ge1-xMnx/Ge(100) films are investigated as a function of Mn dilution. Depending on x, characteristic temperatures separate different regimes in both properties. Resistivity exhibits an insulator-like behavior in the whole temperature range and, below about 80 K, two distinct activation energies are observed. At a higher temperature value, TR, resistivity experiences a sudden reduction. Hall coefficient shows a strong contribution from the anomalous Hall effect and, at TR, a sign inversion, from positive to negative, is recorded. The magnetic properties, inferred from magneto-optical Kerr effect, evidence a progressive decrease of the ferromagnetic long range order as the temperature is raised, with a Curie temperature TC not far from TR. The transport and magnetic results are qualitatively consistent with a percolation mechanism due to bound magnetic polarons in a GeMn diluted magnetic semiconductor, with localized holes [A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)]., Comment: 16 pages, 6 figures. To be published in Phys. Rev. B