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Research summaries for 2017

Fluorescent nanoparticles: improved bioimaging and a single-particle secret

Plasmon-enhanced nanostructures can modify both the emission intensity and polarizationstate of light for bioimaging applications. Rare-earth-doped nanocrystals have become sought-after materials for cellular bioprobes because of their long emission lifetimes and low cytotoxicity. Dang Yuan Lei from the Hong Kong Polytechnic University and colleagues have now discovered how to make these probes even brighter by coupling them to gold nanorods materials that can induce field-enhanced fluorescence through surface plasmon resonances. They optimized this effect by systematically varying the thickness of a protective silica coating sandwiched between the gold nanorods and the doped nanocrystals. Intriguingly, by exciting a single hybrid nanostructure with a polarized laser, the team controlled the polarization of the emitted fluorescent light–an unexpected effect that could lead to new applications using polarization-sensitive diagnostic imaging.

Light Sci Appl. Research Summary. Published online 19 May 2017

Nanosphere lithography: quick and easy route to nanoprobes

A simple way is used to make nanopatterned tips for optical fibres by combining self-assembly of nanospheres with nanofabrication processes. Nanostructures fabricated on optical fibre tips can be used to realize exceptional light trapping and manipulation on the nanoscale, which can be harnessed for sensing applications. The ability of nanospheres to self-assemble into hexagonal arrays is promising for achieving such nanostructures. Marco Pisco at the University of Sannio in Italy and co-workers have demonstrated a new approach for realizing nanoprobes. They created a self-assembled hexagonal array of polystyrene nanospheres on a water surface and then transferred it to an optical fibre tip. The team then modified the periodic structure by employing processes based on thermal evaporation, plasma etching and sonication. They demonstrated this method by using it to produce probes for surface-enhanced Raman spectroscopy.

Light Sci Appl. Research Summary. Published online 19 May 2017

Ultra-thin metasurfaces to generate wavelength-independent sub-wavelength Bessel beams

Custom-designed metasurfaces can act as compact and efficient generators of special light beams known as Bessel beams. These beams are of great interest because unlike other kinds of light beams they do not diffract as they propagate. Wei Ting Chen and co-workers from Harvard University in the USA have made a metasurface that functions as an axicon — a special type of lens with a conical front surface that converts plane waves into Bessel beams. The metasurfaces consists of tiny TiO2 fins arranged in concentric ring-like patterns; each fin has a precisely defined orientation. On striking this metasurface axicon, a plane wave of light is transformed into a Bessel beam. The team fabricated and tested two ‘meta-axicons’ designed for operation with blue and green light and found that the beams' intensity profiles were independent of wavelength.

Light Sci Appl. Research Summary. Published online 19 May 2017

Optical-frequency combs: long cavity leads to dense comb

An integrated mode-locked laser that can generate a dense optical comb with 1400 narrow lines is promising for high-resolution spectroscopy. An emerging technology, optical frequency combs enable highly sensitive, near-real-time spectroscopy with high resolution, but they use bulky and costly femtosecond lasers as sources. Now, Zhechao Wang and co-workers from Ghent University in Belgium and Eindhoven University of Technology in The Netherlands have demonstrated a compact, low-cost alternative laser for generating optical frequency combs. They use a long, low-loss silicon spiral waveguide to realize a long laser cavity. This allows the laser operate with a record-low repetition rate of 1 gigahertz, which in turn enables a large number of narrow lines to be generated within a 12-nm wide comb. This laser has the potential to be used in cost-sensitive applications such as mobile spectroscopic analysis.

Light Sci Appl. Research Summary. Published online 19 May 2017

Coherent light sources: sideband scheme offers alternative to lasing

A scheme for generating coherent directional emission without the need for population inversion could be a valuable alternative to the laser. Developed by scientists at Texas A&M University in the USA, the approach can in principle be applied anywhere in the electromagnetic spectrum—from the extreme ultraviolet to the far infrared. It involves pumping a two-level system with a strong detuned optical signal, which induces transient AC Stark splitting of the energy levels and four-wave mixing. Initial tests with a dense rubidium vapour indicated that the near-infrared sidebands generated in the process are coherent pulses that are less than 1 nanosecond in duration. The approach should work with other atomic and molecular systems at other wavelengths and may be an attractive way for generating coherent extreme ultraviolet or X-ray radiation as well as a novel approach for remote sensing.

Light Sci Appl. Research Summary. Published online 19 May 2017

Sleep behaviour: Red light intensity matters for sleep

High-intensity red light is comparable to white light in inducing sleep behaviours in mice. Red light is used to mimic darkness in lab-based nocturnal experiments because retinal cells have a low sensitivity to it. However, variations in data from similar experiments may be a result of using different intensities of red light. By recording the brain and muscle activity of mice, Huang and colleagues from Fudan University, China, examined the effect of different red light intensities on sleep behaviour and compared it with that induced by white light. They found that, like white light, red light intensities over 20 lux induced sleep, whereas intensities below 10 lux did not affect sleep−wake behaviour. These findings provide important guidelines for nocturnal behavioural experiments on rodents and highlight the importance of light intensity for effective sleep.

Light Sci Appl. Research Summary. Published online 05 May 2017

Metamaterials: bird feathers inspire robust colour

The feathers of the South American bird Cotinga maynana have inspired the design of a nanoscale material that exhibits brilliant colour. The colour originates from the material’s intricate structure rather than a pigment or dye. Andrea Fratalocchi of King Abdullah University of Science and Technology in Saudi Arabia and co-workers report that the material provides saturated colours in the visible region. Importantly, the colour can be observed over a very wide range of observation angles (up to 70 degrees) and persists even when the material’s surface is scratched. The nanomaterial consists of 300-nanometre-thick layers of dealloyed PtYAl metallic nanowires, which are sputtered onto SiNx/Si substrates and then coated with 60-nanometre layers of a dielectric coating of Al2O3. The resulting nanomaterial is robust, lightweight and suitable for large-area fabrication.

Light Sci Appl. Research Summary. Published online 05 May 2017

Solar cells: making ions do a little light work

Using light to excite ions in perovskite thin films can improve the conductivity and synthetic deposition of low-cost solar cells. Organometal halide perovskites have a suitable bandgap for photovoltaics and are compatible with solution processing, but tend to degrade after long exposure to sunlight. A team led by Qing Zhao from Peking University now reports that excited state ionic transport is the key to understanding perovskite’s poor photostability. Through video snapshots and quantitative conductivity extractions, their analysis revealed that illumination drops the energy barrier needed to activate ionic transport by almost five fold—an enhancement that may induce disorder of electronic structure in the solar cell over time. Intriguingly, the light-enhanced ionic transport can also catalyze removal of metal halide precipitates during thin film annealing in sequential deposition reaction, boosting the device efficiency from 7.5 to 15.7% after just 10 minutes of light exposure.

Light Sci Appl. Research Summary. Published online 05 May 2017

Quantum optics: photon-pair source on a chip

A chip-based source of photon pairs for applications in quantum information processing has been built by a team of scientists in the US. Xiang Guo and co-workers from Yale University built the quantum light source from an aluminum nitride microring resonator. The strong second-order nonlinearity of the aluminum nitride yields efficient parametric down-conversion. This allows 775-nm-wavelength pump photons to be converted into a pair of entangled photons in the telecommunications window of 1550 nm. Tests indicated that the heralded photons generated by the microring source are anti-bunched and have high visibility and a high purity of state, indicating that they are highly suitable for use in quantum optics experiments. In principle, the on-chip source is compatible with megahertz generation rates and large-scale manufacture of integrated optical circuits.

Light Sci Appl. Research Summary. Published online 05 May 2017

Multiphoton microscopy: three photons reach deeper than two

Three-photon excitation of chromophores deep inside biological tissue will aid research of optogenetics and photodynamic therapy. Two-photon microscopy is widely used for imaging chromophores in biological specimens, but it has limited penetration into tissue. Christopher Rowlands and co-workers from Massachusetts Institute of Technology have shown that threephoton excitation realized by using short near-infrared pulses can excite quantum dots and channelrhodopsins (light-sensitive ion channels containing a retinal chromophore) at depths of up to 700 micrometres into brain tissue — about twice the depth achievable with two-photon excitation. They used three-photon excitation of quantum dots to realize wide-field imaging of the cerebral vasculature of a mouse and stimulation of action potentials from hippocampal neurons expressing channelrhodopsin. In both cases, no photodamage was observed in the tissue following three-photon experiments, indicating its suitability for use with living samples.

Light Sci Appl. Research Summary. Published online 05 May 2017

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