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Research summaries for 2018

Quantum information science: Twisted photons open up new possibilities

Photons possessing orbital angular momentum are promising for systems for realizing new quantum information applications. Quantum computing and communications are set to revolutionize information technology, but most systems studied to date are based on qubits —quantum analogs of classical bits that can take one of only two states. Manuel Erhard at the University of Vienna, Austria, and co-workers review progress in higher dimensional systems that use photons with orbital angular momentum, or twisted photons, as ‘qudits’, which can have any number of levels. They look at the advantages of such higher-dimensional systems, which include higher information capacity and greater protection from eavesdropping. The researchers then examine exciting developments in the field in the past two to three years, such as the creation of high-dimensional entanglement and optimal quantum cloning. Finally, they consider future challenges.

Light Sci Appl. Research Summary. Published online 09 March 2018

Optical communications: fiber vector eigenmodes promising for expanding data transmission

The practicality of using fiber vector eigenmodes to expand the data transmission capacity of optical communication systems has advanced. Optical vector vortices have donut-shaped beam profiles and are promising for applications such as manipulation, microscopy and quantum processing. Now, Jian Wang from Huazhong University of Science and Technology, China, and co-workers have shown that chip-integrated emitters of optical vector vortices are suitable for use in a direct fiber vector eigenmode multiplexing communication link over a span of large-core optical fiber. The team’s emitters are fabricated from silicon microring resonators with their inner sidewalls etched with angular gratings in order to convert whispering-gallery modes into radiated optical vector vortices. The researchers encoded fiber-eigenmode-like TM01 and TE01 vector modes from the emitters with data and multiplexed them for transmission over 2 kilometers of a large-core fiber. The measured crosstalk and bit error rates were sufficient for data communication purposes.

Light Sci Appl. Research Summary. Published online 09 March 2018

Orbital angular momentum: device generates beams with variable angular momentum

A device that can generate light beams with continuously variable values of orbital angular momentum (OAM) has been demonstrated. Quantum entanglement between photons with integer or fractional OAM has been realized, but until now it has been difficult to generate light beams that have an arbitrary rational amount of OAM. Now, Cheng-Wei Qiu of the National University of Singapore and co-workers have designed and tested an analog vortex transmitter of rational-order OAM, that consists of two tilted gratings under a circular aperture. Plane waves of green light incident on the aperture are converted into vortex beams with a continuously controllable amount of OAM that is determined by the effective variation in size of the aperture across the device. Potential applications for the device include selection and sorting of microparticles, spiral imaging and high-dimensional quantum entanglement.

Light Sci Appl. Research Summary. Published online 09 March 2018

Computational imaging: neural networks key to faster imaging

A new method that uses neural-network-based deep learning could lead to faster and more accurate holographic image reconstruction and phase recovery. Optoelectronic sensors such as charge-coupled devices and complementary metal-oxide–semiconductor imagers are sensitive to intensity but are unable to directly detect the phase of light waves diffracted from an object. Additional information or measurement is thus needed to recover the missing phase information, which enables reconstructing the image of the sample. Now, Aydogan Ozcan and colleagues from the University of California, Los Angeles in the USA have designed a neural network that can perform phase recovery and holographic image reconstruction from a single intensity-only hologram. Using deep learning, they demonstrated the elimination of twin-image and self-interference-related spatial artifacts arising from missing phase information. The technique could significantly simplify the imaging hardware and speed up the image acquisition and reconstruction processes in various holographic and coherent imaging systems.

Light Sci Appl. Research Summary. Published online 23 February 2018

Ultraviolet LEDs: an alternative to p-doping

Narrow-band ultraviolet electroluminescence has been observed from unipolar GaN/AlN heterostructures. Ever since the realization of GaN light-emitting diodes in the early 1990s, GaN photonics has been growing steadily, but a significant bottleneck has been p-type GaN contacts, which are hard to grow and to realize uniform injection of carriers. Now, Paul Berger of Ohio State University and co-workers have developed devices that eliminate the need for p-type GaN doping by using a bipolar tunneling scheme for charge injection. The light emission of the devices was centered on a wavelength of 360 nanometers and had a narrow spectral width of less than 16 nanometers. While the devices had an optical power on the microwatt scale and a low quantum efficiency, the team anticipates that their performance can be improved by subsequent optimization.

Light Sci Appl. Research Summary. Published online 23 February 2018

Plasmonic sensors: high-throughput biosensor based on phase detection

A phase-sensitive plasmonic biosensor suitable for high-throughput sensing of protein biomarkers has been demonstrated. Plasmonics is promising for sensing biomolecules since metasurfaces can enhance light-matter interactions to unprecedented levels. Plasmonic biosensing has mainly involved detecting spectral shifts and associated intensity changes. A team in Switzerland and Spain has built an optical-based portable sensor that detects the phase response of plasmonic resonances using lens-free interferometric imaging. Their device is capable of reading thousands of microarrays simultaneously by plasmonically enhancing the image contrast. The microarrays are functionalized on a plasmonic gold plate riddled with nanoholes fabricated using scalable techniques. The biosensor, built with low-cost off-the-shelf consumer electronic and optical components, is ideal for point-of-care applications.

Light Sci Appl. Research Summary. Published online 23 February 2018

Bioimaging: ‘windows’ created in the skulls of living mice

A non-invasive approach for creating an optical window in the skull to enable the brains of living mice to be imaged has been demonstrated. Dan Zhu and co-workers from Huazhong University of Science and Technology, China, tested the use of optical clearing agents that they applied to the bare skulls (hair and skin removed) of living mice. The agents, which include collagenase, EDTA disodium and glycerol, were used to soften the skull and reduce optical scattering by refractive index matching. A water-immersion lens was then used to perform in vivo, two-photon imaging of neurons, microglia and the microvasculature in the mouse brain. The easy handling and safety of this method make it promising for use in neuroscience research.

Light Sci Appl. Research Summary. Published online 23 February 2018

Optical chirality: Giant chirality achieved using metasurfaces

Giant optical chirality has been realized at visible wavelengths in planar engineered surfaces. Strong optical chirality is desired for various applications. Metamaterials are promising for realizing this, but three-dimensional ones are difficult to make, while planar ones impart low chirality. Now, Alexander Zhu of Harvard University and co-workers have made metasurfaces consisting of an array of miniature gammadion-cross-shaped dielectric structures that gave a circular dichroism in transmission of 80% for green light, while numerical simulations suggested that 95% should be possible. Furthermore, 600-namometer-thick surfaces can provide a circular birefringence as large 60 degrees of polarization rotation, equivalent to 100,000 degrees per millimeter thickness — much larger than that measured in other media, whether natural or engineered. The metasurfaces could lead to high-performance flat devices for controlling the polarization of light beams in applications such as telecommunications.

Light Sci Appl. Research Summary. Published online 23 February 2018

Femtosecond laser fabrication: realizing dynamic control of electrons

The prospects for improving femtosecond-laser materials processing by controlling the electron dynamics in target materials are reviewed. Lan Jiang of Beijing Institute of Technology and co-workers from China and the US describe how a scheme called electron dynamic control can be implemented by temporally or spatially shaping femtosecond laser pulses. Electron dynamic control permits laser fabrication of microscale and nanoscale structures with enhanced precision, throughput and repeatability. The researchers describe the theory behind electron dynamic control and present various simulations and experimental demonstrations of its capabilities. The panoramic dynamics of femtosecond laser ablation was revealed by a multiscale measurement system. Triumphs include the drilling of high-quality, high aspect ratio microholes using pulses with a Bessel profile, which are employed to fabricate some key structures in one of the 16 Chinese National S&T Major Projects.

Light Sci Appl. Research Summary. Published online 09 February 2018

Flexible photonics: stretchable optical devices

A stretchable material for creating optical components could form the basis of wearable health monitors. Flexible electronics and photonics offer a way of integrating devices into clothing or attaching components directly to the skin. But many applications require optical materials that are both bendable and stretchable. Juejun Hu from the Massachusetts Institute of Technology, USA, and co-workers have incorporated a chalcogenide glass and an epoxy polymer into an elastomer substrate. This choice of materials permits the seamlessly integration of photonic components with both high and low index contrasts in the same platform. While the glass and epoxy are intrinsically brittle, the composite material is stretchy because the optical components are located in ‘islands’ and interconnected by optical waveguides with a serpentine shape. The material exhibited no measurable degradation in optical performance after 3000 stretching cycles.

Light Sci Appl. Research Summary. Published online 09 February 2018

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