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Research summaries for 2018

Imaging: faster image capture broadens applications for spectroscopic imaging

Increasing image acquisition speed of a spectroscopic imaging technique opens the door for real-time detection of molecules in living cells. Spectroscopic stimulated Raman scattering imaging is a nondestructive, label-free technique for detecting the chemical fingerprints of biological molecules. However, its relatively slow image acquisition speed has limited its use. This led Ji-Xin Cheng and colleagues from Boston University and Purdue University in the United States to develop a method that greatly increases the signal acquisition speed without affecting the signal level. By randomly sampling a small portion of pixels throughout the spectroscopic image stack, the researchers used an algorithm to decompose the sub-sampled image stack into spectral signatures and concentration maps, increasing the image acquisition speed to 0.8 s per image stack, and could lead to real-time metabolic imaging of living organisms.

Light Sci Appl. Research Summary. Published online 04 May 2018

2D materials: high resolution probe

The crystal properties of transition-metal dichalcogenides, a popular type of 2D material, have been revealed with unprecedented resolution. Sotiris Psilodimitrakopoulos and coworkers from the Foundation for Research and Technology and the University of Crete in Greece used nonlinear laser-scanning optical microscopy to reveal information about the crystal lattice orientation and crystallographic imperfections in large area crystals of WS2. The team's all-optical approach, based on polarization-resolved second-harmonic imaging, is fast, non-invasive and allows the mapping of the orientation of the main crystallographic axis (also known as armchair orientation) of monolayer WS2 with a resolution of 120 × 120 nm2. Information extracted from the imaging scheme is enabling researchers to build a new theoretical model for the nonlinear optical properties of WS2 crystals and aiding the fabrication of large-area 2D materials that are free from defects.

Light Sci Appl. Research Summary. Published online 04 May 2018

Image processing: morphology approach

The detection of small, dim targets, such as stars, in 2D images is benefiting from a fast, reliable image processing scheme developed in China. Minsong Wei and coworkers from Tsinghua University in Beijing have developed a row-by-row morphology-based approach that operates in real-time as the data is being read out. The approach uses a 1D morphology-based algorithm to subtract background noise and thus enhance and detect targets. Tests with astronomical images show that it can find targets, such as a dim star, with an accuracy of better than 0.1 pixel under poor sky conditions. The scheme operates with very low latency times on the nanosecond to microsecond scale and is thus well suited to applications requiring high-speed processing. In addition to astronomical images, the scheme could potentially prove useful for medical imaging and surveillance.

Light Sci Appl. Research Summary. Published online 04 May 2018

Metasurfaces: negative reflection control

A clever design of anisotropic-coded metasurface has allowed scientists in China to break Snell’s law of reflection. The metasurfaces fabricated by Shuo Liu and coworkers from Southeast University in Nanjing consist of an array of elliptically shaped metal coding particles on a FR4 substrate with a copper coated rear surface. They are designed to operate at microwave frequencies around 10 GHz and have two important, unique functions. Firstly, they are able to perform so-called negative reflection, whereby both the incident and reflected waves are on the same side of the surface normal, contrary to Snell’s law and usual behavior. Secondly, they are able to split incident propagating waves of orthogonal polarizations into surface waves that travel in different directions. Such metasurfaces are envisaged to have applications in camouflage, radar, wireless communications and microwave circuitry.

Light Sci Appl. Research Summary. Published online 04 May 2018

Entanglement: classical fields get a welcome wobble

A novel optical field engineered to undergo an oscillation in its spin-orbit coupling highlights a hitherto unnoticed property of classically entangled fields. The spin-orbit coupling arises from the non-separability between polarization and spatial shape and has already found a myriad of applications such as laser material processing. By combining two orthogonal non-separable states propagating in opposite directions, a team of German and South African scientists led by Prof Andrew Forbes of South Africa’s University of the Witwatersrand have now engineered a classical field that fluctuates between two states, entangled and non-entangled. To characterize these oscillations without destroying them, the team generated and manipulated each of the orthogonal fields independently, using a single spatial light modulator. This technique allows each beam to be propagated digitally, to simulate the travel of opposing beams, and facilitates phase adjustments to deliver a desired state to a specific target, enabling their transport across arbitrary distances.

Light Sci Appl. Research Summary. Published online 04 May 2018

Metasurfaces: nanobricks strengthen integration

Arrays that convert visible light into surface plasmon polaritons and provide broadband beam steering can help miniaturize optical circuits. Optical metasurfaces, thin-layer devices with subwavelength-scale patterns, offer extraordinary control over light in ultra-compact packages. Now, Fei Ding and co-workers from the University of Southern Denmark have built and tested a metasurface that can perform two distinct functions simply by switching light polarization. The proof-ofconcept device uses customized arrays of tiny silver ‘nanobricks’ to create linear phase gradients that propagate along a single direction, but with different characteristics for orthogonal polarizations. The team reports an average coupling efficiency of more than 25% for unidirectional surface plasmon polariton excitation with an x-polarized beam. Concurrent beam steering of light wavelengths between 580 to 700 nanometers was achieved with y-polarization, with low crosstalk between functionalities.

Light Sci Appl. Research Summary. Published online 20 April 2018

Laser micro-machining: ultrafast scanning

The throughput and tolerance of laser micro-machining can be improved by rapidly scanning the focused laser spot along the optical axis. This is the finding of Ting-Hsuan Chen and coworkers from Princeton University in the USA. The team built a micro-machining system that combines a high repetition rate Nd:YVO4 laser emitting ultraviolet (355 nm-wavelength), 15 ns duration pulses with an ultrafast z-scanner made from an acoustically driven variable focal length lens. The approach means that laser pulses are focused to different positions on the sample, relaxing the demands on the surface flatness and positioning of the sample. It also leads to faster machining rates, with experiments with a silicon sample indicating about a threefold increase in machining speed of a 200 × 200 μm square hole with no degradation in quality.

Light Sci Appl. Research Summary. Published online 20 April 2018

Air quality: sizing up particulate pollution

A new device that accurately measures particulate sizes can improve air quality measurements, say researchers in China. The widely published PM2.5 index shows the prevalence of particles <2.5 μm in diameter, but no detail on the smallest ‘ultrafine’ particles, which are especially dangerous because they can penetrate the lungs. To tackle this problem, Yun-Feng Xiao at Peking University and co-workers developed a simple monitoring device in which circular polarized laser light transmitted through an array of nanofibers. When air is blown over the nanofibers, particles from the air stick onto the nanofibers and change the transmission of light, allowing the sizes of particles as small as 100 nm to be estimated. The team’s measurements of air in Beijing closely matched official government figures for PM2.5, while providing unprecedented new detail that could benefit public health.

Light Sci Appl. Research Summary. Published online 20 April 2018

Bioimaging: breaking limits for deeper tissue views

Using sound signals to spot light-absorbing microspheres flowing through narrow tubes may lead to sharper images of small blood vessels. Optoacoustic imaging turns brief laser pulses into ultrasonic pressure waves that outline shapes of vascular systems below the skin. Xose Luis Dean-Ben at the Helmholtz Center Munich and colleagues now describe a new technique, localization optoacoustic tomography, which overcomes previous limits of sound-based imaging. After capturing rapid sequences of flowing objects that act as point light sources, the team mapped out 3D structural features inside microscale capillaries by analyzing spatial differences between individual absorbers—distances which can be much smaller than ones observable with acoustic waves. The high sensitivity of this technique to blood flow parameters makes it attractive for uncovering disease-induced changes in microcirculation at centimeter-scale depths.

Light Sci Appl. Research Summary. Published online 20 April 2018

Infrared phononic antennas for ultrasensitive vibrational molecular spectroscopy

Infrared spectroscopy is a powerful tool for characterizing materials based on their specific vibrational fingerprints. However, its ability to characterize small amounts or thin layers of molecules is limited by their extremely small infrared absorption cross-sections. This limitation can be overcome by surface-enhanced infrared absorption spectroscopy (SEIRA), which exploits the field enhancement provided by plasmon polaritons on thin metal films or resonant metallic nanostructures. Now, Rainer Hillenbrand from CIC nanoGUNE in San Sebastián (Spain) and co-workers have developed highly sensitive phonon-polariton resonators for SEIRA detection, based on hexagonal boron nitride ribbons, which exhibit quality factors much higher than their plasmonic counterparts. They demonstrated phonon-enhanced molecular vibrational spectroscopy with sensitivity down to femtomolar levels, approaching the strong coupling limit.

Light Sci Appl. Research Summary. Published online 06 April 2018

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