Optics and photonics

Absorption imaging relies on the capture of photons by an object to create intensity contrasts, allowing for the visualization of small quantum systems. Streedet al. demonstrate the first absorption imaging of an isolated ytterbium ion, with contrast at the limit of semiclassical theory.

Fluorescence imaging is important for biomedical research and applications, but the turbidity of biological material prohibits deep tissue study. By combining ultrasound-encoding with digital time-reversal, Wanget al.perform focussed fluorescence imaging at a tissue depth of 2.5mm.

Graphene's remarkable properties make it ideal for optoelectronic devices, and its two-dimensional nature enables its integration with photonic structures. By combining a graphene transistor with a planar microcavity, Engelet al. control the spectrum of the photocurrent and the light emitted by the device.

High-intensity laser-plasma ion generation is promising as a compact proton source for applications like ion beam therapy. Using a femtosecond table-top laser system, Zeilet al. show that protons efficiently gain energy in the pre-thermal intra-pulse phase of the generation process.

Three-dimensional optical metamaterials provide a range of exciting features, such as broadband circular dichroism, yet their fabrication is challenging. Here, a broadband optical circular polarizer is presented based on twisted stacks of metasurfaces, avoiding the issues of three-dimensional fabrication.

Fibre-optic waveguides are used to provide timing delays for different sensing and signal processing applications, but their transfer to on-chip platforms is a challenge. Here low-loss delay lines based on whispering-gallery spiral waveguides up to 27 m long are produced, presenting a scalable alternative.

Nanoscale optomechanical systems offer a route to using optical forces for a range of devices based on photonic structures. Deotareet al. present a reconfigurable optical filter based on coupled silicon photonic crystal nanobeam cavities that can overcome thermo-optic effects at high frequencies.

Light propagating in a medium can undergo polarization rotation, an effect that depends on light intensity and chiral properties. Renet al. report polarization rotation in a plasmonic metamaterial with million-fold stronger nonlinearity than that found in natural crystals.

As lengthscales in plasmonic structures enter the sub-nanometre regime, quantum effects become increasingly important. Here, a quantum-corrected model is presented that addresses quantum effects in realistic-sized plasmonic structures, a situation not feasible for full-quantum-mechanical simulations.

Micromechanical oscillators present a route to miniaturisation of devices and may be used as frequency references or sensitive sensors, but their small size means that they often behave nonlinearly. Antonioet al. demonstrate frequency stabilisation of nonlinear resonators by coupling two vibrational modes.

Flow lithography is used to synthesize microparticles but relies on polydimethylsiloxane microchannels for oxygen to permeate and inhibit polymerization near channel interfaces. Now, non-polydimethylsiloxane devices have been developed, which allow oxygen-free lithography, increasing the capabilities of flow lithography.

The terahertz spectral region is desirable for applications such as imaging or spectroscopy, but progress is hampered by a lack of efficient terahertz devices. By exploiting intraband transitions in graphene, Sensale-Rodriguezet al. demonstrate a broadband intensity modulator working at terahertz frequencies.

Stable, ultrahigh repetition rate optical clocks are critical for applications in high-speed communications, metrology and microchip computing. Pecciantiet al.present a mode-locked laser based on an integrated microcavity, with repetition rate exceeding 200 GHz and narrow linewidth pulses.

An important goal in optics is to image objects hidden by turbid media, although line-of-sight techniques fail when the obscuring medium becomes opaque. Veltenet al. use ultrafast imaging techniques to recover three-dimensional shapes of non-line-of-sight objects after reflection from diffuse surfaces.

Single-photon sources are important for quantum optical technologies, although achieving efficient light extraction from them with waveguides is limited in top-down approaches. Reimeret al. show a high extraction efficiency using a bottom-up method to grow quantum dots on the axis of nanowire waveguides.

Ultrafast excitation offers new routes to controlling material properties on short timescales, but probes are needed to better understand the changes. By studying the phonon spectrum of VO₂ in the time domain, Wall et al. find a prompt change in lattice potential after a photoinduced structural transition.

Diffractive imaging can deliver wavelength-scale resolution with X-rays, although its use with electrons is hampered by experimental constraints. By applying ptychographic methods to transmission electron microscopy, Humphryet al. demonstrate sub-nanometre resolution using low-energy electrons.

Biological materials efficiently exploit self-assembly of simple constituents to produce complex functional structures such as optical devices. By controlling organic molecules, Leeet al. show fast two-step self-assembly of CaCO₃microlens arrays, reminiscent of their biological counterparts.

Light-emitting diodes in the form of nanocrystals offer promise for environmental and biomedical diagnostics. Brovelliet al. present a method for realizing mechanically robust and chemically stable nanocrystals emitting light in the ultraviolet range.

As quantum information and communication experiments grow in sophistication, the need for efficient sources of entangled photons escalates. Using exciton and biexciton emission in GaAs island quantum dots, Ghaliet al. demonstrate the electric field-induced generation of entangled photons with high fidelity.

By controlling the flow or composition of liquids, optofluidics provides numerous possibilities for devices, and so has great potential for transformation optics. Here, a multi-mode optofluidic waveguide is presented, which manipulates light to produce controllable chirped focussing and interference.

Self-reconstructing laser beams can propagate deep into thick media, making them ideal for light-sheet microscopy of organic matter. By considering the rings of self-reconstructing Bessel beams, Fahrbach and Rohrbach present a technique for improving the contrast and resolution of this approach.

A dual-contrast agent has been developed for combined ultrasound and photoacoustic imaging. This agent uses vaporization for ultrasound contrast enhancement and photoacoustic signal generation, providing significantly higher signals than thermal expansion, the most commonly used photoacoustic mechanism.

Optical vortex traps are appealing for handling delicate particles, but conventional techniques are challenging with objects smaller than the diffraction limit of light. By exploiting plasmonic resonances in gold diabolo nanoantennas, Kanget al. demonstrate low-power vortex trapping of nano-scale objects.

Mid-infrared semiconductor lasers suffer from a high threshold power density, but interband cascade lasers may offer a more efficient alternative. Here, theory and experiments on such emitters demonstrate remarkably low thresholds and power consumption compared to state-of-the-art quantum cascade lasers.

Optical imaging and spectroscopy rely on understanding how light enters and propagates through turbid media, yet its behaviour near the point-of-entry has remained elusive. Now Vitkinet al. report an analytical solution to this problem and demonstrate its agreement with simulations and experiments.

Nanomechanical resonators are attractive as ultra-low concentration sensors of biomolecules, as their small scale allows for sensitive mass detection. Here, using a nanowire array as part of a photonic crystal, such a device is presented for light trapping, absorption and low-concentration sensing.

The night sky viewed from Earth is very bright at infrared wavelengths due to atmospheric emission, making land-based astronomy difficult in this spectral region. Here, a photonic filter is demonstrated to suppress this unwanted light, opening new paths to infrared astronomy with current and future telescopes.

Among the wide range of potential applications of graphene, photodetection is believed to be among the most promising. By combining graphene with plasmonic nanostructures, Duan and colleagues observe dramatic improvements in the efficiency and spectral sensitivity of graphene-based photodetectors.

As quantum information processing continues to develop apace, the need for integrated photonic devices becomes ever greater for both fundamental measurements and technological applications. To this end, Crespiet al.demonstrate a high-fidelity photonic controlled-NOT gate on a glass chip.

When two spatially separated parties flip a coin, it is impossible to choose between two alternatives in an unbiased manner. This study presents a quantum coin-flipping protocol that overcomes this problem and ensures a dishonest party cannot bias the outcome completely.

Generation of multipartite entanglement between quantum states is crucial for developing quantum computation systems, although it has proven harder to achieve for photons than ions. Here, an eight-photon entangled state based on four independent photon pairs is observed, beating the previous record of six.

Photonic alternatives to electrical circuits require low energy demand and fast modulation speed, which has proven difficult for on-chip devices. Using quantum dot photonic crystal nanocavities, Vučkovićet al. demonstrate an electrically-switchable light-emitting diode with such capabilities.

Most quantum communication experiments are performed at visible wavelengths, yet practical, long-range schemes need photons in the telecommunications range. Here, down-conversion of a visible photon to the near-infrared is demonstrated, while retaining its entanglement to another visible photon.

The miniaturization of optical devices is crucial for their on-chip integration with a variety of technological applications. Here, Liuet al. present an ultracompact beam splitter to control the direction of light through the generation of surface plasmon polaritons.

Quasi-three-dimensional plasmonic crystals have potential uses in miniaturized photonics. In this study, a method is described to enhance plasmonic resonance in the crystals by coupling them to optical modes of Fabry–Perot type cavities, with possible applications in photonic and sensor components.

Inertial sensors using atom interferometry have applications in geophysics, navigation- and space-based tests of fundamental physics. Here, the first operation of an atom accelerometer during parabolic flights is reported, demonstrating high-resolution measurements at both 1g and 0g.

Plasmonic nanostructures can be used to manipulate objects larger than the wavelength of light but create thermal heating. In this work, the trapping and controlled rotation of nanoparticles is demonstrated using a plasmonic nanotweezer with a heat sink, predicting a reduction in heating compared with previous designs.

Autostereoscopic three-dimensional displays allow the perception of depth, by presenting offset images to the left and right eye, without the need for specialized glasses. Yoonet al propose a Luciusmicroprism array to control the directionality and intensity of light in three-dimensional displays.

Photodetection is believed to be among the most promising potential applications for graphene. Here, by combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodetectors is increased by up to two orders of magnitude.

Various methods have been investigated to locally control atmospheric precipitation. In this study, field experiments show that laser-induced condensation is initiated when the relative humidity exceeds 70%, and that this effect is largely a result of photochemical HNO₃formation.

Multiple scattering complicates femtosecond optics such that phase conjugation allows spatial focusing and imaging through a multiple scattering medium, but temporal control is problematic. McCabeet al. report the full spatio-temporal characterization and recompression of a femtosecond speckle field.

The development of practical photonic quantum technologies will be aided by the spatial control of entangled photons. Lenget al. achieve on-chip spatial control of entangled photons by using domain engineering, rather than by using external optical elements.

Single atoms can be detected using optical resonators that extend the lifetime of the photon. Here, the authors demonstrate fast, high-fidelity detection of very low atom densities using a microfabricated optical cavity to couple the detection light with the atoms.

The second order correlation functiong⁽²⁾ is used to test quantum correlation properties of light. Here, two-photon counting is used to measure g⁽²⁾and an extrabunching effect is demonstrated, providing evidence that two-photon counting is an appropriate method for measuring light beam photon correlations.

Quantum computing has advantages over conventional computing, but the complexity of quantum algorithms creates technological challenges. Here, an architecture-independent technique, that simplifies adding control qubits to arbitrary quantum operations, is developed and demonstrated.

Brillouin interactions between sound and light can excite mechanical resonances in photonic microsystems, with potential for sensing and frequency reference applications. The authors demonstrate experimental excitation of mechanical resonances ranging from 49 to 1,400 MHz using forward Brillouin scattering.

Optical computing, involving on-chip integrated logic units, could provide improved performance over semiconductor-based computing. Here, a binary NOR gate is developed from cascaded OR and NOT gates in four-terminal plasmonic nanowire networks; the work could lead to new optical computing technologies.

Two-qubit operation is an essential part of quantum computation, but implementation has been difficult. Gotoet al.introduce optically controllable internuclear coupling in semiconductors providing a simple way of switching inter-qubit couplings in semiconductor-based quantum computers.