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Plasmonic nanostructures enable light to be concentrated into nanoscale ‘hotspots’, wherein the intensity of light can be enhanced by orders of magnitude. This plasmonic enhancement significantly boosts the efficiency of nanoscale light–matter interactions, enabling unique linear and nonlinear optical applications. Large enhancements are often observed within narrow gaps or at sharp tips, as predicted by the classical electromagnetic theory. Only recently has it become appreciated that quantum mechanical effects could emerge as the feature size approaches atomic length-scale. Here we experimentally demonstrate, through observations of surface-enhanced Raman scattering, that the emergence of electron tunnelling at optical frequencies limits the maximum achievable plasmonic enhancement. Such quantum mechanical effects are revealed for metallic nanostructures with gap-widths in the single-digit angstrom range by correlating each structure with its optical properties. This work furthers our understanding of quantum mechanical effects in plasmonic systems and could enable future applications of quantum plasmonics. Plasmonic nanostructures concentrate light into sub-wavelength volumes, enhancing its electric field. In a series of surface-enhanced Raman measurements, Zhu and Crozier show that as the structure size shrinks, quantum-mechanical electron tunnelling places a limit on the achievable enhancement.

The polarization of light plays a central role in its interaction with matter, in situations ranging from familiar (for example, reflection and transmission at an interface) to sophisticated (for example, nonlinear optics). Polarization control is therefore pivotal for many optical systems, and achieved using bulk devices such as wave-plates and beam-splitters. The move towards optical system miniaturization therefore motivates the development of micro- and nanostructures for polarization control. For such control to be complete, one must distinguish not only between linear polarizations, but also between left- and right-circular polarizations. Some previous works used surface plasmons to this end, but these are inherently lossy. Other works used complex-layered structures. Here we demonstrate a planar dielectric chirality-distinguishing beam-splitter. The beam-splitter consists of amorphous silicon nanofins on a glass substrate and deflects left- and right-circularly polarized beams into different directions. Contrary to intuitive expectations, we utilize an achiral architecture to realize a chiral beam-splitting functionality. There is a need for micro- and nanostructures capable of polarization control to cope with the increasing miniaturization of optics devices. Here, the authors propose a beam-splitter consisting of amorphous silicon nanofins on glass that reflect left- and right-circularly polarized light in different directions.

Conventional optical tweezers based on traditional optical microscopes are subject to the diffraction limit, making the precise trapping and manipulation of very small particles challenging. Plasmonic optical tweezers can surpass this constraint, but many potential applications would benefit from further enhanced performance and/or expanded functionalities. In this Perspective, we discuss trends in plasmonic tweezers and describe important opportunities presented by its interdisciplinary combination with other techniques in nanoscience. We furthermore highlight several open questions concerning fundamentals that are likely to be important for many potential applications.Plasmonics: tweezer opportunitiesThe progress, challenges, and potential applications for research in plasmonic-assisted optical tweezers are highlighted in this Perspective. Kenneth Crozier from the University of Melbourne in Australia describes how plasmonic nanostructures that confine and enhance electromagnetic fields on a sub-wavelength scale allow optical tweezers to operate beyond the diffraction limit of light and trap nanoscale particles. Potential applications for such plasmonic tweezers include nanoparticle manipulation and sensing, use in “lab-on-a-chip” devices such as flow cytometers for biological studies, or cold atom trapping for quantum information processing tasks. However, before these opportunities can be fully realized it is important that issues such as heating, which occurs due to absorption from the metal structures used in plasmonics, and a more complete understanding of the plasmonic trapping process are properly addressed.

Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions. Recent work has shown that quantum mechanical effects in plasmonic nanogap structures become important as the gap distances approach the subnanometre length-scale. Here, the authors review the major findings which challenge the classical picture of these structures and discuss future directions for the field.

Smartphones are a potentially powerful platform for scientific instruments. Here, we demonstrate speckle spectroscopy with smartphone-level hardware. This technique promises greater performance thresholds than traditional diffraction gratings. Light is injected into an optical fiber and the emergent speckle patterns are imaged by a reversed-lens smartphone camera. The smartphone then uses an algorithm, running on a mobile computing app, to determine, in less than one second, the (hitherto unknown) input spectrum. We reconstruct a variety of visible-wavelength (470–670 nm) single and multi-peaked spectra using a tunable source. The latter also include a metameric pair, i.e., two spectra that are different, yet represent colors that are indistinguishable to the human eye.

Black phosphorus has emerged as a unique optoelectronic material, exhibiting tunable and high device performance from mid-infrared to visible wavelengths. Understanding the photophysics of this system is of interest to further advance device technologies based on it. Here we report the thickness dependence of the photoluminescence quantum yield at room temperature in black phosphorus while measuring the various radiative and non-radiative recombination rates. As the thickness decreases from bulk to ~4 nm, a drop in the photoluminescence quantum yield is initially observed due to enhanced surface carrier recombination, followed by an unexpectedly sharp increase in photoluminescence quantum yield with further thickness scaling, with an average value of ~30% for monolayers. This trend arises from the free-carrier to excitonic transition in black phosphorus thin films, and differs from the behaviour of conventional semiconductors, where photoluminescence quantum yield monotonically deteriorates with decreasing thickness. Furthermore, we find that the surface carrier recombination velocity of black phosphorus is two orders of magnitude lower than the lowest value reported in the literature for any semiconductor with or without passivation; this is due to the presence of self-terminated surface bonds in black phosphorus. Thickness-dependent photoluminescence quantum yield measurements in black phosphorus reveal a free-carrier to excitonic transition, differing from the behaviour of conventional semiconductors.

Smart, low-cost and portable gas sensors are highly desired due to the importance of air quality monitoring for environmental and defense-related applications. Traditionally, electrochemical and nondispersive infrared (IR) gas sensors are designed to detect a single specific analyte. Although IR spectroscopy-based sensors provide superior performance, their deployment is limited due to their large size and high cost. In this study, a smart, low-cost, multigas sensing system is demonstrated consisting of a mid-infrared microspectrometer and a machine learning algorithm. The microspectrometer is a metasurface filter array integrated with a commercial IR camera that is consumable-free, compact ( ~ 1 cm3) and lightweight ( ~ 1 g). The machine learning algorithm is trained to analyze the data from the microspectrometer and predict the gases present. The system detects the greenhouse gases carbon dioxide and methane at concentrations ranging from 10 to 100% with 100% accuracy. It also detects hazardous gases at low concentrations with an accuracy of 98.4%. Ammonia can be detected at a concentration of 100 ppm. Additionally, methyl-ethyl-ketone can be detected at its permissible exposure limit (200 ppm); this concentration is considered low and nonhazardous. This study demonstrates the viability of using machine learning with IR spectroscopy to provide a smart and low-cost multigas sensing platform.

The generation of optical vortices in compact systems and across different spectral regions can open new horizons for their applications in end-user devices. Latest advances in the design and fabrication of optical metasurfaces made of a quadratically nonlinear material enable highly precise creation of vortices with different topological charges at the second-harmonic frequency, with the potential to obtain various other structured states of light.

Semiconductor lasers play critical roles in many different systems, ranging from optical communications to absorption spectroscopy for environmental monitoring. Despite numerous applications, many semiconductor lasers have problems such as significant beam divergence and polarization instability. External optical elements like objective lenses and polarizers are usually needed to address these issues. This Review will discuss how these issues have recently been dealt with by instead integrating metasurfaces into semiconductor lasers. This necessitates the development of innovative fabrication methods; these will also be the topic of this Review. Metasurfaces can be integrated on the emitting facet of a laser. This can help select the lasing mode or can be used just to modify the output beam properties without affecting the modes. They can also be integrated monolithically with lasers through waveguides, or work in an external cavity configuration. These integrated devices provide novel optical functions, such as direct orbital angular momentum (OAM) mode generation, wavelength tuning and holographic pattern generation. We hope this Review will help extend the use of metasurface-integrated semiconductor lasers to scientific and industrial systems that employ lasers.

Although optical tweezers based on far-fields have proven highly successful for manipulating objects larger than the wavelength of light, they face difficulties at the nanoscale because of the diffraction-limited focused spot size. This has motivated interest in trapping particles with plasmonic nanostructures, as they enable intense fields confined to sub-wavelength dimensions. A fundamental issue with plasmonics, however, is Ohmic loss, which results in the water, in which the trapping is performed, being heated and to thermal convection. Here we demonstrate the trapping and rotation of nanoparticles using a template-stripped plasmonic nanopillar incorporating a heat sink. Our simulations predict an ~100-fold reduction in heating compared with previous designs. We further demonstrate the stable trapping of polystyrene particles, as small as 110 nm in diameter, which can be rotated around the nanopillar actively, by manual rotation of the incident linear polarization, or passively, using circularly polarized illumination. 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.