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  • Journal article
    Jeon K-R, Ciccarelli C, Ferguson AJ, Kurebayashi H, Cohen L, Montiel X, Eschrig M, Robinson JWA, Blamire MGet al., 2018,

    Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents

    , Nature Materials, Vol: 17, Pages: 499-503, ISSN: 1476-1122

    Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin.1,2 Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet (FM) spacers, where spin transport can only occur within the FM and in conjunction with a charge current. Ferromagnetic resonance (FMR) injects a pure spin current from a precessing FM into adjacent non-magnetic materials.3,4 For spin-singlet pairing, FMR spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor (SC).5,6 Here we present FMR experiments in which spin sink layers with strong spin-orbit coupling are added to the SC. Our results show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state; although further work is required to establish the details of the spin transport process we show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent.

  • Journal article
    Morozov S, Gaio M, Maier S, Sapienza Ret al., 2018,

    Metal−dielectric parabolic antenna for directing single photons

    , Nano Letters, Vol: 18, Pages: 3060-3065, ISSN: 1530-6984

    Quantum emitters radiate light omni-directionally, making it hard to collect and use the generated photons. Here, we propose a three-dimensional metal–dielectric parabolic antenna surrounding an individual quantum dot as a source of collimated single photons, which can then be easily extracted and manipulated. Our fabrication method relies on a single optically induced polymerization step once the selected emitter has been localized by confocal microscopy. Compared to conventional nanoantennas, our geometry does not require near-field coupling, and it is, therefore, very robust against misalignment issues and minimally affected by absorption in the metal. The parabolic antenna provides one of the largest reported experimental directivities (D = 106) and the lowest beam divergences (Θ1/2 = 13.5°) and a broadband operation over all of the visible and near-infrared range together with extraction efficiency of more than 96%, offering a practical advantage for quantum technological applications.

  • Journal article
    Hamilton I, Chander N, Cheetham NJ, Suh M, Dyson M, Wang X-H, Stavrinou PN, Cass M, Bradley DDC, Kim J-Set al., 2018,

    Controlling molecular conformation for highly efficient and stable deep-blue copolymer light-emitting diodes

    , ACS Applied Materials and Interfaces, Vol: 10, Pages: 11070-11082, ISSN: 1944-8244

    We report a novel approach to the achievement of deep-blue, high-efficiency, and long-lived solution processed polymer light-emitting diodes (PLEDs) via a simple molecular-level conformation change whereby we introduce rigid β-phase segments into a 95% fluorene - 5% arylamine copolymer emission layer (EML). The arylamine moieties at low density act as efficient exciton formation sites in PLEDs whilst the conformational change alters the nature of the dominant luminescence from a broad, charge-transfer like emission to a significantly blue-shifted and highly vibronically structured, excitonic emission. As a consequence, we observe a significant improvement in Commission International de L'Eclairage (CIE) (x, y) co-ordinates from (0.149, 0.175) to (0.145, 0.123) whilst maintaining high efficiency and improving stability. We achieve peak luminous efficiency, η = 3.60 cd/A and luminous power efficiency, ηw = 2.44 lm/W; values that represent state of the art performance for single copolymer deep-blue PLEDs. These values are five-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) (PFO) EML PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a vector to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

  • Journal article
    Mignuzzi S, Mota M, Coenen T, Li Y, Mihai A, Petrov PK, Oulton RF, Maier SA, Sapienza Ret al., 2018,

    Energy-momentum cathodoluminescence spectroscopy of dielectric nanostructures

    , ACS Photonics, Vol: 5, Pages: 1381-1387, ISSN: 2330-4022

    Precise knowledge of the local density of optical states (LDOS) is fundamental to understanding nanophotonic systems and devices. Complete LDOS mapping requires resolution in energy, momentum, and space, and hence a versatile measurement approach capable of providing simultaneous access to the LDOS components is highly desirable. Here, we explore a modality of cathodoluminescence spectroscopy able to resolve, in single acquisitions, the dispersion in energy and momentum of the radiative LDOS. We perform measurements on a titanium nitride diffraction grating, bulk molybdenum disulfide, and silicon to demonstrate that the technique can probe and disentangle the dispersion of coherent and incoherent cathodoluminescence signals. The approach presented raises cathodoluminescence spectroscopy to a versatile tool for subwavelength design and optimization of nanophotonic devices in the reciprocal space.

  • Journal article
    Gartside JC, Arroo DM, Burn DM, Bemmer VL, Moskalenko A, Cohen LF, Branford WRet al., 2017,

    Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing.

    , Nature Nanotechnology, Vol: 13, Pages: 53-58, ISSN: 1748-3387

    Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing-a scanning probe technique-provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy 'monopole-chain' states that exhibit negative effective temperatures.

  • Journal article
    Castro-Lopez M, Gaio M, Sellers S, Gkantzounis G, Florescu M, Sapienza Ret al., 2017,

    Reciprocal space engineering with hyperuniform gold disordered surfaces

    , APL Photonics, Vol: 2, ISSN: 2378-0967

    Hyperuniform geometries feature correlated disordered topologies which followfrom a tailored k-space design. Here, we study gold plasmonic hyperuniformdisordered surfaces and, by momentum spectroscopy, we report evidence of kspaceengineering on both light scattering and light emission. Even if the structureslack a well-defined periodicity, emission and scattering are directional inring-shaped patterns. The opening of these rotational-symmetric patterns scaleswith the hyperuniform correlation length parameter as predicted via the spectralfunction method.

  • Journal article
    Burn DM, Chadha M, Branford WR, 2017,

    Dynamic dependence to domain wall propagation through artificial spin ice

    , PHYSICAL REVIEW B, Vol: 95, ISSN: 2469-9950

    Domain wall propagation dynamics has been studied in nanostructured artificial kagome spin-ice structures. A stripline circuit has been used to provide localized pulsed magnetic fields within the artificial spin-ice (ASI) structure. This provides control of the system through electrically assisted domain wall nucleation events. Synchronization of the pulsed fields with additional global magnetic fields and the use of a focused magneto-optical Kerr effect magnetometer allows our experiments to probe the domain wall transit through an extended ASI structure. We find that the propagation distance depends on the driving field revealing field-driven properties of domain walls below their intrinsic nucleation field.

  • Journal article
    Van DT, Caixeiro S, Fernandes FM, Sapienza Ret al., 2017,

    Microsphere Solid-State Biolasers

    , Advanced Optical Materials, Vol: 5, ISSN: 2195-1071

    Biolasers obtained from biomaterials are attracting a wealth of interest for their potential as future biosensors with enhanced sensitivity, and advanced cell tracking. Here, miniature biolasers are reported, which are formed by bovine serum albumin (BSA) protein and biosourced polysaccharides derived from land plants such as cellulose and pectin. Using a green processing route aided by simple emulsions, solid-state microspheres with diameters of 15–100 µm are fabricated and utilized as whispering gallery mode lasers with thresholds of a few µJ mm–2 and quality factors of up to 3000. Furthermore, BSA microlasers are found to be compatible with cell growth and resistant to the aqueous environment of cell culture media. This environmentally friendly and biocompatible design shows promise for future implantable biosensing devices opening a path between laser science and medicine.

  • Journal article
    Caixeiro S, Peruzzo M, Onelli OD, Vignolini S, Sapienza Ret al., 2017,

    Disordered Cellulose-Based Nanostructures for Enhanced Light Scattering

    , ACS Applied Materials and Interfaces, Vol: 9, Pages: 7885-7890, ISSN: 1944-8244

    : Cellulose is the most abundant biopolymer onEarth. Cellulose fibers, such as the one extracted form cottonor woodpulp, have been used by humankind for hundreds ofyears to make textiles and paper. Here we show how, byengineering light−matter interaction, we can optimize lightscattering using exclusively cellulose nanocrystals. Theproduced material is sustainable, biocompatible, and whencompared to ordinary microfiber-based paper, it showsenhanced scattering strength (×4), yielding a transport meanfree path as low as 3.5 μm in the visible light range. Theexperimental results are in a good agreement with thetheoretical predictions obtained with a diffusive model for light propagation.

  • Journal article
    Gaio M, Caixeiro S, Marelli B, Omenetto FG, Sapienza Ret al., 2017,

    Gain-Based Mechanism for pH Sensing Based on Random Lasing

    , Physical Review Applied, Vol: 7, ISSN: 2331-7019

    Here, we investigate the mechanism of a random-lasing-based sensor which shows pH sensitivity exceeding by 2 orders of magnitude that of a conventional fluorescence sensor. We explain the sensing mechanism as related to gain modifications and lasing-threshold nonlinearities. A dispersive diffusive lasing theory matches the experimental results well, and it allows us to predict the optimal sensing conditions and a maximal sensitivity as large as 200 times that of an identical fluorescence-based sensor. The lack of complex alignment and the high sensitivity make this mechanism promising for future biosensing applications.

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