Innovative research provides a foundation for NB-Photonics to establish leading-edge technologies and key insights that benefit our collaborating partners. Publications of research involving NB-Photonics members are found across high-impact journals in a variety of scientific fields.

For associated groups where NB-Photonics members contribute to research, detailed publications lists are available for Photonics Research Group, Liquid Crystals and Photonics, INTEC Design, CMST, Physics and Chemistry of Nanostructures, Lumilab, Polymer Chemistry and Biomaterials Group, and CoCooN (links open in new tabs).

Examples of our recent research breakthroughs in each theme of the NB-Photonics research focus is detailed below (links open in new tabs).

Economical routes to colloidal nanocrystals [Science, 2015]

Colloidal semiconductor nanocrystals (NCs) or “quantum dots” offer exquisite control of absorption and emission colors for device applications by tuning their size and shape. The proliferation of NC applications has been enabled by the “hot-injection” synthesis, which can tightly control the particle size to achieve pure colors. However, current hot-injection synthesis recipes are largely incompatible with low-cost, large-area remote phosphor applications of NCs. Hendricks et al. report on a radical improvement of the hot-injection synthesis economics of metal sulfide NCs by introducing a library of inexpensive thioureas as reagents.

Interaction between light and highly confined hypersound in a silicon photonic nanowire [Nature Photonics, 2015]

In the past decade there has been a surge in research at the boundary between photonics and phononics. Most efforts have centred on coupling light to motion in a high-quality optical cavity, typically geared towards manipulating the quantum state of a mechanical oscillator. It was recently predicted that the strength of the light–sound interaction would increase drastically in nanoscale silicon photonic wires. Here we demonstrate, for the first time, such a giant overlap between near-infrared light and gigahertz sound co-localized in a small-core silicon wire. The wire is supported by a tiny pillar to block the path for external phonon leakage, trapping 10 GHz phonons in an area of less than 0.1 μm2. Because our geometry can also be studied in microcavities, it paves the way for complete fusion between the fields of cavity optomechanics and Brillouin scattering. The results bode well for the realization of optically driven lasers/sasers, isolators and comb generators on a densely integrated silicon chip.

Room-temperature InP Distributed Feedback Laser Array Directly Grown on Silicon [Nature Photonics, 2015]
A wide range of applications from various fields requiring low-cost on-chip laser sources could benefit from the long-awaited missing piece of Si photonics: integrated laser sources on Si.
We present the first highly scalable monolithic solution. The in-plane configuration and the use of a selective-area growth technique in combination with a top-down integration scheme provides a route towards the integration of dense arrays of III–V laser sources with Si photonic circuits.
In particular, for on-chip optical interconnects, the demonstrated monolithic laser array, together with the WDM technology, may finally pave the way to terascale computing.

Heterogeneously integrated III-V/silicon dual-mode distributed feedback laser array for terahertz generation [Optics Express, 2014]

We demonstrate an integrated distributed feedback (DFB) laser array as a dual-wavelength source for narrowband terahertz (THz) generation. The laser array is composed of four heterogeneously integrated III–V-on-silicon DFB lasers with different lengths enabling dual-mode lasing tolerant to process variations, bias fluctuations, and ambient temperature variations. By optical heterodyning the two modes emitted by the dual-wavelength DFB laser in the laser array using a THz photomixer composed of an uni-traveling carrier photodiode (UTC-PD), a narrow and stable carrier signal with a frequency of 0.357 THz is generated.

High-speed Plasmonic Phase Modulators [Nature Photonics, 2014]
Demands in optical communications mean that electro-optic modulators should feature large bandwidths, operate across all telecommunication windows, offer a small footprint, and allow for CMOS-compatible fabrication to keep costs low.
We contributed to a group making a new ultra-compact plasmonic phase modulator based on the Pockels effect in a nonlinear polymer. The device has a length of only 29 µm and operates at 40 Gbit s-1. Its modulation frequency response is flat up to 65 GHz and beyond. The modulator has been tested to work across a 120-nm-wide wavelength range centred at 1,550 nm, and is expected to work beyond this range. Its operation has been verified for temperatures up to 85 °C and it is easy to fabricate.
To the best of our knowledge, this is the most compact high-speed phase modulator demonstrated to date.

Nanostructured TiO2/carbon nanosheet hybrid electrode for high-rate thin-film lithium-ion batteries [Nanotechnology, 2014]

Heterogeneous nanostructured electrodes using carbon nanosheets (CNS) and TiO2 exhibit high electronic and ionic conductivity. In order to realize the chip level power sources, it is necessary to employ microelectronic compatible techniques for the fabrication and characterization of TiO2-CNS thin-film electrodes. To achieve this, vertically standing CNS grown through a catalytic free approach on a TiN/SiO2/Si substrate by plasma enhanced chemical vapour deposition (PECVD) was used. The substrate-attached CNS is responsible for the sufficient electronic conduction and increased surface-to-volume ratio due to its unique morphology. Atomic layer deposition (ALD) of nanostructured amorphous TiO2 on CNS provides enhanced Li storage capacity, high rate performance and stable cycling. The amount of deposited TiO2 masks the underlying CNS, thereby controlling the accessibility of CNS, which gets reflected in the total electrochemical performance, as revealed by the cyclic voltammetry and charge/discharge measurements. TiO2 thin-films deposited with 300, 400 and 500 ALD cycles on CNS have been studied to understand the kinetics of Li insertion/extraction. A large potential window of operation (3-0.01 V); the excellent cyclic stability, with a capacity retention of 98% of the initial value; and the remarkable rate capability (up to 100 C) are the highlights of TiO2/CNS thin-film anode structures. CNS with an optimum amount of TiO2 coating is proposed as a promising approach for the fabrication of electrodes for chip compatible thin-film Li-ion batteries.

Stealth Monoolein-based Nanocarriers for Delivery of siRNA to Cancer Cells [Acta Biomaterialia, 2015]
siRNA are a class of therapeutic nucleic acids that allow specific downregulation of disease-related proteins, although efficient intracellular delivery is the major bottleneck. Nanocarriers must enable efficient intracellular delivery and release the encapsulated siRNA in the target cell’s cytosol to produce its intended therapeutic effect.
We evaluate a new liposomal formulation for delivering siRNA to leukemia cells, and find that a metastable protective coating with polyethyleneglycol enhances the stability of the siRNA-liposomes in blood, while the capacity for cell uptake is retained.
With this formulation we could successfully silence a defective gene in leukemia cells.

Chitosan nanoparticles for siRNA delivery: Optimizing formulation to increase stability and efficiency [Journal of Controlled Release, 2014]

This study aims at developing chitosan-based nanoparticles suitable for an intravenous administration of small interfering RNA (siRNA) able to achieve (i) high gene silencing without cytotoxicity and (ii) stability in biological media including blood. Therefore, the influence of chitosan/tripolyphosphate ratio, chitosan physicochemical properties, PEGylation of chitosan as well as the addition of an endosomal disrupting agent and a negatively charged polymer was assessed. The gene silencing activity and cytotoxicity were evaluated on B16 melanoma cells expressing luciferase. We monitored the integrity and the size behavior of siRNA nanoparticles in human plasma using fluorescence fluctuation spectroscopy and single particle tracking respectively. The presence of PEGylated chitosan and poly(ethylene imine) was essential for high levels of gene silencing in vitro. Chitosan nanoparticles immediately released siRNA in plasma while the inclusion of hyaluronic acid and high amount of poly(ethylene glycol) in the formulation improved the stability of the particles. The developed formulations of PEGylated chitosan-based nanoparticles that achieve high gene silencing in vitro, low cytotoxicity and high stability in plasma could be promising for intravenous delivery of siRNA.

Precisely and Accurately Localizing Single Emitters in Fluorescence Microscopy [Nature Methods, 2014]
The 2014 Nobel prize was awarded for the development of super-resolution fluorescence microscopy. Super-resolution information can be obtained by stochastically switching on a sparse subset of fluorescent emitters that are subsequently localized by image processing. Repeating this procedure many times for different sets of emitters leads to a super-resolved image appearing.
We describe the various ways of localizing emitters and explain how this affects emitter localization in terms of precision and accuracy. Based on our previous work, we also discuss how resolution is influenced by the potential movement of emitters during image capture. Taken together, this provides a comprehensive guideline on best practice for emitter localization in super-resolution microscopy.

Three-Dimensional Observation of the Conductive Filament in Nanoscaled Resistive Memory Devices [Nano Letters, 2014]

The basic unit of information in filamentary-based resistive switching memories is physically stored in a conductive filament. Therefore, the overall performance of the device is indissolubly related to the properties of such filament. We report for the first time on the three-dimensional (3D) observation of the shape of the conductive filament. The observation of the filament is done in a nanoscale conductive-bridging device, which is programmed under real operative conditions. To obtain the 3D-information we developed a dedicated tomography technique based on conductive atomic force microscopy. The shape and size of the conductive filament are obtained in three-dimensions with nanometric resolution. The observed filament presents a conical shape with the narrow part close to the inert-electrode. On the basis of this shape, we conclude that the dynamic filament-growth is limited by the cation transport. In addition, we demonstrate the role of the programming current, which clearly influences the physical-volume of the induced conductive filaments.

Broadband Enhancement of Single Photon Emission and Polarization Dependent Coupling in Silicon Nitride Waveguides [Optics Express, 2015]
Single-photon (SP) sources are important for various optical quantum information processing applications. Integrating triggered solid-state SP emitters directly on a photonic chip will require efficient extraction of their emission into a single guided mode.
We investigate the SP emission from dipole-like nanometer-sized inclusions embedded into different silicon nitride (SiNx) photonic nanowire waveguide designs. We elucidate the effect of the geometry. The results show that highly efficient and polarized SP sources can be realized using suspended SiNx slot-waveguides.
Combining this with CMOS-compatible processing technology, fully integrated and complex optical circuits for quantum optics experiments can be developed.

Nanoscale and Single-Dot Patterning of Colloidal Quantum Dots [Nano Letters, 2015]

Using an optimized lift-off process we develop a technique for both nanoscale and single-dot patterning of colloidal quantum dot films, demonstrating feature sizes down to ∼30 nm for uniform films and a yield of 40% for single-dot positioning, which is in good agreement with a newly developed theoretical model. While first of all presenting a unique tool for studying physics of single quantum dots, the process also provides a pathway toward practical quantum dot-based optoelectronic devices.