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Research summaries for 2017

Integrated optics: chip-based sources of quantum light

Several new platforms are promising for generating and manipulating complex quantum optical states on a chip. Chip-based sources of quantum states of light are needed to bring quantum technologies out of the lab and into the real world, but such sources are still immature. David Moss at Swinburne University of Technology, Australia, and an international team have reviewed progress in developing and characterizing such sources. Waveguide, cavity and ring resonator devices made from nonlinear materials such as silicon, silicon nitride, silicon oxynitride, Hydex and periodically poled lithium niobate offer scientists a rich variety of sources. Furthermore, many of these technologies can be integrated with silicon CMOS photonics, providing a path for building sophisticated, scalable optical integrated circuits for generating and manipulating quantum optical states for applications in quantum information processing and communications.

Light Sci Appl. Research Summary. Published online 17 November 2017

Synchrotron light sources: high X-ray beam stability achieved

A laser-accelerator-based source of ultrashort X-ray pulses with unprecedented stability has been demonstrated by scientists in Europe. Ultrashort X-ray pulses are invaluable for exploring the dynamic properties of matter at very short time scales. High-peak-power lasers are promising for producing such beams, but their usefulness has been limited by the poor control and stability of X-ray beams they generate. Andreas Döpp from the Ecole Polytechnique in France and co-workers tested a new, tunneling ionization scheme for injecting electrons longitudinally into a laser-wakefield accelerator. They generated a kiloelectron-volt-scale X-ray beam with far superior stability than that obtained using the usual injection approach. In addition, the X-ray emission was highly polarized, and the polarization could be tuned by tilting the polarization axis. Such a source could be useful for applications requiring broadband, femtosecond X-ray pulses.

Light Sci Appl. Research Summary. Published online 17 November 2017

Organic light-emitting diodes: stacked design for color-tunable display

Using a tandem design of stacked organic emitters, color-tunable organic light-emitting diodes (OLEDs) have been made by solution processing. Conventional displays consisting of red, green and blue pixels arranged in a two-dimensional array suffer from limited resolution. Now, scientists in Germany and China have demonstrated a 4 × 4 pixel matrix color-tunable display that uses the color-tunable OLEDs as pixels. Each pixel consists of two OLEDs, which are stacked vertically and emit different colors. A thin layer of conductive silver nanowires is used as a transparent intermediate charge-injection electrode, which allows the emission intensity and color of the tandem device to be controlled. Compared with the conventional approach, the stacked approach is easier to manufacture and allows for smaller pixels and thus a higher resolution display.

Light Sci Appl. Research Summary. Published online 17 November 2017

Multiphoton imaging: Diamond Raman laser realizes deep in-vivo imaging

A diamond Raman laser pumped by a ytterbium-fiber amplifier is a convenient and stable optical source for multiphoton imaging. Andrew Dunn and his co-workers from the University of Texas at Austin in the USA and Macquarie University in Australia used the combined outputs from a diamond Raman laser at 1240 nanometers and a ytterbium-fiber amplifier at 1055 nanometers to perform both two-color, two-photon and two-color, three-photon imaging of biological samples. In particular, they demonstrated multicolor microscopy of cells labeled with a far-red fluorescent protein and a blue-emitting stain. In addition, deep in vivo microscopy of the vasculature in the brain of an adult mouse perfused with a red dye was performed at cortex depths of up to 1 millimeter using the system for two-color, two-photon excitation.

Light Sci Appl. Research Summary. Published online 17 November 2017

Chemical imaging at the submolecular scale

Advanced multivariate analysis can dramatically improve the chemical sensing capabilities of tip-enhanced Raman spectroscopy (TERS). A powerful surface analysis technique, TERS is hampered by inherently weak Raman signals, which have not permitted complex molecular architectures to be distinguished at the subnanometer scale. By employing advanced multivariate analysis, Song Jiang and co-workers from the University of Science and Technology of China used TERS to unambiguously identify adjacent molecules on a surface. The scheme’s spatial resolution of just 0.4 nanometers opens up the possibility of gathering information on a submolecular scale, such as mapping local vibrations and structural units within a single molecule. Experiments with a mixture of ZnTPP and H2TBPP molecules adsorbed on the edge of a silver step confirmed the advantages of the multivariate approach over conventional single-peak spectral analysis.

Light Sci Appl. Research Summary. Published online 17 November 2017

Super-resolution microscopy: Breaking the diffraction limit without dyes

Super-resolution microscopy based on light-activated atomic force microscopy has been demonstrated a team in Korea. The diffraction limit makes it difficult to image structures smaller than about 200 nanometres using optical microscopy. While several super-resolution fluorescence microscopy techniques have been realized, there is a need for a super-resolution technique that does not use fluorescent dyes. Chulhong Kim of Pohang University of Science and Technology in Korea and co-workers have realized a resolution of 8 nanometres by using a pulsed laser beam to optically excite a sample and then observing the response using a conventional atomic force microscope. The researchers demonstrated their system by using it to image single gold nanoparticles as well as cancer and plant cells. They anticipate that it could find use in fields as diverse as physics, biology, chemistry, medicine and material science.

Light Sci Appl. Research Summary. Published online 03 November 2017

Endomicroscopy: Two-photon probe provides functioning imaging

A compact, flexible probe based on nonlinear optics offers doctors and researchers a noninvasive way to directly image internal organs. Histology using optical microscopy has been widely employed for diagnosing disease, but it typically takes a few days to obtain results and it does not provide functional information. Now, Xingde Li at Johns Hopkins University and coworkers have developed a 2-millimeter-diameter probe that uses two-photon imaging to obtain images of organs inside the body that are comparable in quality to those obtained by a laser scanning microscope. They realized this through innovations in double-clad fiber optics, a miniature objective lens and short pulse management. The team demonstrated the potential of their probe by using it to obtain metabolic imaging of a functioning mouse kidney model. The probe is promising for both diagnosing disease and basic research.

Light Sci Appl. Research Summary. Published online 03 November 2017

Nanoscopy: super-resolution imaging of nitrogen-vacancy centres

A new spin-manipulated nanoscopy method can resolve collectively blinking negatively charged nitrogen-vacancy centers (NVCs) on a nanoscale. The exceptional optical and magnetic properties of negatively charged nitrogen-vacancy centers in nanodiamonds make them attractive as fluorescent biomarkers in imaging and sensing applications. By combining wide-field localization microscopy with nanoscale spin manipulation by a microwave signal, Min Gu and colleagues from RMIT University and Swinburne University of Technology in Australia have achieved super-resolution imaging of the blinking of multiple, closely spaced negatively charged NVCs. The nanoscopy approach can resolve NVCs as small as 42 nanometers and separated by as little as 23 nanometers. Two close NVCs often appear to emit coupled fluorescence as they exhibit a collective behavior. The method offers a new way to study and image spin-related quantum interactions at the nanoscale.

Light Sci Appl. Research Summary. Published online 03 November 2017

Two-dimensional materials: silicon waveguide enhances nonlinear effects

Combining two-dimensional materials with a silicon waveguide imparts silicon chips with desirable nonlinear optical effects. Two-dimensional transition-metal dichalcogenides exhibit large second-order responses, making them promising as nonlinear light sources, but their subnanometer thickness limits their nonlinear interaction with light. A European−Australian collaboration found that silicon slab waveguides topped with a layer of the transition-metal dichalcogenide MoSe2, a well-known two-dimensional material, produce enhanced second-harmonic generation. This is due to that the evanescent field of the silicon waveguide modes couple with MoSe2, which enables large second-order nonlinear response. Calculations indicate that a 1-mm-long waveguide could provide a second-harmonic signal that is 500 000 times larger than that obtained by pumping the MoSe2 monolayer from above. Other nonlinear effects such as wavelength conversion, parametric amplification and the generation of entangled photons should also be possible.

Light Sci Appl. Research Summary. Published online 20 October 2017

Raman microscopy: nanoscale samples near surfaces can be probed

A new form of Raman microscopy capable of probing nanoscale samples near surfaces without using enhanced fields has been shown. Raman scattering is a powerful tool for obtaining valuable information about molecular systems, but its application is limited by its inherently weak signals. Now, Diana Serrano and Stefan Seeger of the University of Zurich, Switzerland, have demonstrated that, by collecting Raman light emitted at angles exceeding the critical angle of total internal reflection, Raman microscopy can probe nanoscale specimens without using field amplification or enhancement. This technique exploits the fact that dipoles near an interface with a higher refractive index medium emit some light at angles that exceed the critical angle. While this effect has been applied to fluorescent probes, this is the first time it has been used for Raman spectroscopy.

Light Sci Appl. Research Summary. Published online 20 October 2017

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