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Optical spectrometry is a tool to investigate wavelength-dependent light–matter interactions and is widely used in astronomy, physics and chemistry. Integration and miniaturization of the currently bulky spectrometers will have an impact on applications where compactness and low complexity are key, such as air- and spaceborne missions. A high-resolution spectroscopy principle based on the near-field detection of a spatial standing wave inside a subwavelength waveguide has shown great promise to accomplish some of the aforementioned demands. However, small-scale devices based on this principle face strong bandwidth limitations due to undersampling of the standing wave. Here, we demonstrate an integrated single-waveguide Fourier transform spectrometer with an operational bandwidth of 500 nm in the near- and short-wavelength infrared, not relying on any moving components. The prototype device, with a footprint of less than 10 mm2, exploits the electro-optic properties of thin-film lithium niobate in order to retrieve the complete spatial interferogram.

Aiming at the problem of limited transmission energy of liquid crystal tunable filter (LCTF), a dual-wavelength transmission system with high signal-to-noise ratio (SNR) is proposed in this paper. The proposed transmission factor Qp is the main influence on the number and location of transmission wavelengths as well as the bandwidth of each transmission wavelength for dual-wavelength systems. Dual-wavelength LCTF can improve the effective transmission energy of the system by increasing the number of filtering channels, and the transmission energy can be increased by about 1.8 times and 70% at short and long wavelengths, respectively, which improves the signal-to-noise ratio (SNR) of the system. Moreover, the dual-wavelength LCTF system is even possible to increase the transmission energy by about 7% at a 33% increase in spectral resolution. Therefore, the dual-wavelength LCTF transmission method not only can improve the SNR of target detection with dual-wavelength response features, but also can effectively solve the problem of contradiction between spectral resolution and spectral transmission energy.

Ambitions to reach atomic resolution with light have been a major force in shaping nano-optics, whereby a central challenge is achieving highly localized optical fields. A promising approach employs plasmonic nanoantennas, but fluorescence quenching in the vicinity of metallic structures often imposes a strict limit on the attainable spatial resolution, and previous studies have reached only 8 nm resolution in fluorescence mapping. Here, we demonstrate spatially and spectrally resolved photoluminescence imaging of a single phthalocyanine molecule coupled to nanocavity plasmons in a tunnelling junction with a spatial resolution down to ∼8 Å and locally map the molecular exciton energy and linewidth at sub-molecular resolution. This remarkable resolution is achieved through an exquisite nanocavity control, including tip-apex engineering with an atomistic protrusion, quenching management through emitter–metal decoupling and sub-nanometre positioning precision. Our findings provide new routes to optical imaging, spectroscopy and engineering of light–matter interactions at sub-nanometre scales.Through the use of a plasmon-active atomically sharp tip and an ultrathin insulating film, and precise junction control in a highly confined nanocavity plasmon field at the scanning tunnelling microscope junction, sub-nanometre-resolved single-molecule near-field photoluminescence imaging with a spatial resolution down to ∼8 Å is achieved.

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications. Raman spectroscopy has become an invaluable tool for graphene characterisation, yet the nature of the broadening of the Raman 2D line remains unclear. Here, Stampfer et al . show that the Raman 2D line width is a measure of nanometre-scale strain variations in graphene on insulating substrates.

The development of a spectroscopy device on a chip that could realize real-time fingerprinting with label-free and high-throughput detection of trace molecules represents one of the big challenges in sensing. Dual-comb spectroscopy (DCS) in the mid-infrared is a powerful technique offering high acquisition rates and signal-to-noise ratios through use of only a single detector with no moving parts. Here, we present a nanophotonic silicon-on-insulator platform designed for mid-infrared (mid-IR) DCS. A single continuous-wave low-power pump source generates two mutually coherent mode-locked frequency combs spanning from 2.6 to 4.1 μm in two silicon microresonators. A proof-of-principle experiment of vibrational absorption DCS in the liquid phase is achieved acquiring spectra of acetone spanning from 2900 to 3100 nm at 127-GHz (4.2-cm −1 ) resolution. These results represent a significant step towards a broadband, mid-IR spectroscopy instrument on a chip for liquid/condensed matter phase studies. Dual-comb spectroscopy is a powerful tool for realizing rapid spectroscopic measurements with high sensitivity and selectivity. Here, Yu et al. demonstrate silicon microresonator-based dual comb spectroscopy in the mid-infrared region, where strong vibrational resonances of many liquids exist.

Thermal quenching, in which light emission experiences a loss with increasing temperature, broadly limits luminescent efficiency at higher temperature in optical materials, such as lighting phosphors1–3 and fluorescent probes4–6. Thermal quenching is commonly caused by the increased activity of phonons that leverages the non-radiative relaxation pathways. Here, we report a kind of heat-favourable phonons existing at the surface of lanthanide-doped upconversion nanomaterials to combat thermal quenching. It favours energy transfer from sensitizers to activators to pump up the intermediate excited-state upconversion process. We identify that the oxygen moiety chelating Yb3+ ions, [Yb···O], is the key underpinning this enhancement. We demonstrate an approximately 2,000-fold enhancement in blue emission for 9.7 nm Yb3+-Tm3+ co-doped nanoparticles at 453 K. This strategy not only provides a powerful solution to illuminate the dark layer of ultra-small upconversion nanoparticles, but also suggests a new pathway to build high-efficiency upconversion systems.

Optical manipulation of plasmonic nanoparticles provides opportunities for fundamental and technical innovation in nanophotonics. Optical heating arising from the photon-to-phonon conversion is considered as an intrinsic loss in metal nanoparticles, which limits their applications. We show here that this drawback can be turned into an advantage, by developing an extremely low-power optical tweezing technique, termed opto-thermoelectric nanotweezers. By optically heating a thermoplasmonic substrate, a light-directed thermoelectric field can be generated due to spatial separation of dissolved ions within the heating laser spot, which allows us to manipulate metal nanoparticles of a wide range of materials, sizes and shapes with single-particle resolution. In combination with dark-field optical imaging, nanoparticles can be selectively trapped and their spectroscopic response can be resolved in situ. With its simple optics, versatile low-power operation, applicability to diverse nanoparticles and tunable working wavelength, opto-thermoelectric nanotweezers will become a powerful tool in colloid science and nanotechnology.

Circularly polarized light is utilized in various optical techniques and devices. However, using conventional optical systems to generate, analyse and detect circularly polarized light involves multiple optical elements, making it challenging to realize miniature and integrated devices. While a number of ultracompact optical elements for manipulating circularly polarized light have recently been demonstrated, the development of an efficient and highly selective circularly polarized light photodetector remains challenging. Here we report on an ultracompact circularly polarized light detector that combines large engineered chirality, realized using chiral plasmonic metamaterials, with hot electron injection. We demonstrate the detector’s ability to distinguish between left and right hand circularly polarized light without the use of additional optical elements. Implementation of this photodetector could lead to enhanced security in fibre and free-space communication, as well as emission, imaging and sensing applications for circularly polarized light using a highly integrated photonic platform. Analysis and detection of circularly polarized light involves the use of multiple optical elements. Here, the authors demonstrate an ultracompact circularly polarized light detector using chiral plasmonic metamaterials with hot electron injection, realizing its implementation on an integrated photonic platform.

Advances in the understanding of Raman processes in graphene have made it an essential tool for studying the properties of this one-atom-thick carbon material. Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp 2 -bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

Surface-enhanced Raman spectroscopy (SERS) harnesses the confinement of light into metallic nanoscale hotspots to achieve highly sensitive label-free molecular detection that can be applied for a broad range of sensing applications. However, challenges related to irreversible analyte binding, substrate reproducibility, fouling, and degradation hinder its widespread adoption. Here we show how in-situ electrochemical regeneration can rapidly and precisely reform the nanogap hotspots to enable the continuous reuse of gold nanoparticle monolayers for SERS. Applying an oxidising potential of +1.5 V (vs Ag/AgCl) for 10 s strips a broad range of adsorbates from the nanogaps and forms a metastable oxide layer of few-monolayer thickness. Subsequent application of a reducing potential of −0.80 V for 5 s in the presence of a nanogap-stabilising molecular scaffold, cucurbit[5]uril, reproducibly regenerates the optimal plasmonic properties with SERS enhancement factors ≈10 6 . The regeneration of the nanogap hotspots allows these SERS substrates to be reused over multiple cycles, demonstrating ≈5% relative standard deviation over at least 30 cycles of analyte detection and regeneration. Such continuous and reliable SERS-based flow analysis accesses diverse applications from environmental monitoring to medical diagnostics. SERS is a powerful analytical technique, but achieving reproducibility for continuous analysis a challenge. Here, the authors report a SERS substrate recycling method that enables direct analysis of complex samples without substrate contamination.