Single-walled carbon nanotubes (SWCNTs) are promising for a staggering array of applications, ranging from thin-film electronics to sporting goods. Unfortunately, bulk manufacture yields a mixture of nanotubes with different properties, and separating them is very difficult. The authors have found that the tiny optical force from a laser beam can be used to sort SWCNTs according to their sizes and electronic properties. By tuning the laser to an appropriate color, any of multiple types of nanotubes can be selected and separated from a mixture—a discovery that could be the key to realizing new and higher quality applications for this wonder material.

Despite great progress in renewables, the vast majority of the world’s energy is still supplied by fossil fuels, releasing huge amounts of carbon dioxide. CO${}_2$ capture and sequestration using tailored materials aim to reduce the impact of fossil fuels on the environment. The authors combine simulations and experiments to understand at the atomic level the mechanisms of CO${}_2$ capture in a series of Li- and K-doped sodium zirconates, to guide materials design, facilitate sorbent synthesis, and mitigate global climate change.

Flows in microfluidic channels are often assumed to be two-dimensional (2D). However, the authors find that the vortical structures of micro bubbles subject to vertically uniform acoustic agitation produce complex 3D trajectories, rather than the 2D streamlines that one would expect. As with Einstein’s observations of 3D trajectories of swirling tea leaves in a cup, the 3D character of the flow can be traced to boundary effects. These results have important consequences for the behaviors of confined vortical microfluidic structures.

Spin-orbit coupling is a well-studied means to change the orientation of a spin, and thus flip a logic bit in a spintronic device. The authors offer an advance by showing how the anomalous Hall effect and the anisotropic magnetoresistance, both discovered in the 19th century, can be used like the spin Hall effect to generate spin-orbit torques. This approach provides greater control over the angular dependence of torque than existing methods, and can switch perpendicularly magnetized samples, or move domain walls efficiently.

For problems ranging in scale from seismology down to nanoelectromechanical systems, many subjects rely on the ability to measure and interpret the mechanical response of a system as a wave passes through it. The challenge is to extract as much information as possible, potentially at length scales much smaller than the wavelengths of the excited modes. The authors show that optical interferometry and spatiotemporal analysis of bending waves can reveal the stress distribution in a thin membrane, by accessing all of the information encoded in a measured mode shape.

Single-walled carbon nanotubes (SWCNTs) are promising for a staggering array of applications, ranging from thin-film electronics to sporting goods. Unfortunately, bulk manufacture yields a mixture of nanotubes with different properties, and separating them is very difficult. The authors have found that the tiny optical force from a laser beam can be used to sort SWCNTs according to their sizes and electronic properties. By tuning the laser to an appropriate color, any of multiple types of nanotubes can be selected and separated from a mixture—a discovery that could be the key to realizing new and higher quality applications for this wonder material.

The operation of tomorrow’s phase-change and resistive memory devices is based on the electric-field-induced nucleation of conducting particles embedded in an insulating host. The authors show how that process is exponentially accelerated in the presence of metal electrodes, because of the energy decrease due to image-charge attraction. Their insight should help to shape these emerging technologies.

For quantum information technologies to become mainstream, they must be integrated into microchips. The authors present all-optical generation of nonclassical squeezed light (i.e. with quantum noise reduced below the shot-noise level) using an ultracompact broadband cavity in CMOS-compatible silicon nitride. Such a source of bright squeezed light, operating at technologically accessible power levels, paves the way for quantum enhanced sensing, quantum interconnects, and quantum information processing via a scalable, on-chip platform.

Hot-carrier solar cells are promising third-generation photovoltaic devices that should be able to beat the Shockley-Queisser limit of about 34% efficiency. This paper provides a detailed view of the recently proposed optical carrier extraction scheme for such devices, focusing on downconversion of carrier energy via impact ionization. Probing the limits of what we can expect under practical operating conditions, the authors conclude that there is great potential for application of optical carrier extraction.

Microscale fluid jets are used in applications ranging from inkjet printing to injection of high-potency drugs. Unfortunately, microscale nozzles tend to clog, especially when processing suspensions. In this work, the authors use nanosecond-long optical excitation of a carbon nanotube composite to produce high-speed liquid jets at a fluid interface, without the need for a nozzle. This optoacoustic approach is demonstrated to be effective for both high-resolution printing and injection into material mimicking human skin.

Amorphous oxide semiconductors like In-Ga-Zn-O (a-IGZO) have attracted much attention, but shifting threshold voltage and other problems are obstacles to successful applications. The authors’ calculations indicate that interstitial oxygen defects in a-IGZO are responsible for the observed instability under positive bias stress. This insight helps clear the way for devices such as transparent, flexible thin-film transistors.

Further advances in the fidelity of quantum information systems will rely on precise measurements of ever smaller amounts of noise. The authors present a technique based on randomized benchmarking that is ideal for measuring noise at the small timescales and low error rates relevant to fault-tolerant error-correction schemes. Their SQUID-based test qubit is found to be limited not by $1/f$ flux noise, but rather by telegraph noise too small to be studied with standard methods.

A judicious choice of material heterostructure could spur a breakthrough in infrared photodetector technology, for diverse uses in astronomy, medicine, and industry. Unfortunately, for this system little is known about the nonequilibrium charge-carrier dynamics, which is critical to device performance. This study outlines an approach to uniquely determine the magnitudes of the various recombination mechanisms that determine carrier lifetime, and ultimately the limitations of photodetector dark currents.

Magnetic nanoparticles are widely applicable in systems ranging from spintronics and data storage to biomedical imaging and cancer treatment. However, studying these particles requires ultrasensitive detectors. Superconducting quantum interference devices (SQUIDs) are the finest detectors of magnetic flux, but for this purpose they must be drastically miniaturized. To this end, the authors have fabricated nanosSQUIDs of exquisite sensitivity that will facilitate both basic and applied research on nanoscale spin systems.

Developing a tunable source of high-power terahertz radiation is an ongoing research challenge, with promise for applications including biosensing and high-speed communication. The authors show that the THz emission from a stack of intrinsic Josephson junctions in a cuprate superconductor embedded between two gold layers can be manipulated by a focused laser beam. The output power can be increased by as much as 75% with laser irradiation, and tuned continuously and rapidly as the laser beam is moved along the length of the stack, locally heating different spots of the sample.

Despite great progress in renewables, the vast majority of the world’s energy is still supplied by fossil fuels, releasing huge amounts of carbon dioxide. CO${}_2$ capture and sequestration using tailored materials aim to reduce the impact of fossil fuels on the environment. The authors combine simulations and experiments to understand at the atomic level the mechanisms of CO${}_2$ capture in a series of Li- and K-doped sodium zirconates, to guide materials design, facilitate sorbent synthesis, and mitigate global climate change.

Current transients in planar optoelectronic devices are of great importance to both the scientific and industrial communities. The authors offer a simple, general theory to describe these transients, with specific examples for an important class of solar cells. Their analysis reveals that the apparent free-carrier concentration obtained via the usual integral approach is altered by a nontrivial factor of two, which could lead to misinterpretations of the charge densities and overall device physics. This fresh perspective could have far-reaching effects on semiconductor research and technology.

Modern biosensors seek to detect a single virion, or even a single molecule. For this purpose optical whispering-gallery-mode resonators are ideal: exquisitely sensitive down to the limit imposed by thermal fluctuations. The authors have devised a way to independently measure local temperature and refractive index at the same time, allowing them to suppress temperature fluctuations dramatically while maintaining full sensitivity. This enables simple millimeter-scale resonators to achieve detection limits previously possible only with finicky micrometer-scale resonators; if implemented in the latter, the method could improve detection limits to record levels.

In ultrafast laser systems, graphene has become a popular saturable absorber to provide passive mode-locking with tunable wavelength. The authors elucidate the pulse width saturated mode-locking mechanism, focusing on the soliton compression that can be achieved by strengthening self-phase modulation in the regime of weakly negative group-delay dispersion. This mechanistic understanding allows for an optimized soliton pulse width of 500 femtoseconds or less from an erbium-doped fiber laser, for photonics and optoelectronics applications.

Stacked intrinsic Josephson junctions in the well studied superconductor Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ are promising as a compact source of coherent terahertz radiation, which would have applications ranging from bioimaging to security screening. Understanding localized heating in such stacks is essential to optimizing their performance, and to this end the authors directly image the complex self-heating behaviors of these devices under bias conditions typical for THz emission. For good heat removal, narrow (compared to device size) filaments of current nucleate hot spots in asymmetric locations, which is consistent with theoretical predictions and suggests a means to enhance emission power.

The impact of a laser pulse on a microdrop of opaque liquid induces a spectacular response: The drop is propelled forward rapidly, and is strongly deformed until it breaks up or even explodes. A detailed understanding of this process is crucial for generating extreme-ultraviolet (EUV) light in nanolithography. The authors reveal that droplet propulsion is driven by localized boiling on the illuminated side and subsequent expansion, in proportion to the energy of the laser pulse.

Deciduous trees shed their leaves each autumn to survive cold weather. A first step in this process causes the leaves to lose their hydrophobic layer–a phenomenon that is hastened by pollution. Little is known about the physical details of raindrop-leaf interaction as hydrophobicity is changed, yet these details affect premature leaf loss and vulnerability to rainstorms. The authors model the dynamic response of a leaf to a raindrop as that of an elastic cantilever beam, and find that a hydrophobic surface can protect against raindrop impact better than a hydrophilic one. These results could also be used to optimize biomimetic piezoelectric devices for harvesting energy from falling rain.

Simple optical microscopy of subwavelength-scale objects is impossible, but clever techniques have pushed the resolution limit to the range of 1–10 nm. Here the authors have imaged the optical near field at a surface with atomic resolution, i.e. about 1 $\mathrm{å}$ngstrom. This advance is promising for the exploration of the physical and chemical interactions between light and individual atoms or molecules, and offers deeper insight into the various photonic processes on surfaces.