Owing to the chirality of Weyl nodes, the Weyl systems can support one-way chiral zero modes under a strong magnetic field, which leads to nonconservation of chiral currents—the so-called chiral anomaly. Although promising for robust transport of optical information, the zero chiral bulk modes have not been observed in photonics. Here we design an inhomogeneous Weyl metamaterial in which a gauge field is generated for the Weyl nodes by engineering the individual unit cells. We experimentally confirm the presence of the gauge field and observe the zero-order chiral Landau level with one-way propagation. Without breaking the time-reversal symmetry, our system provides a route for designing an artificial magnetic field in three-dimensional photonic Weyl systems and may have potential for device applications in photonics.

Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document. When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive.

The radiation of an electric dipole emitter can be drastically enhanced if the emitter is placed in the nano-gap of a metallic dipole antenna. By assuming that only surface plasmon polaritons (SPPs) are excited on the antenna, we build up an intuitive pure-SPP model that is able to comprehensively predict the electromagnetic features of the antenna radiation, such as the total or radiative emission rate and the far-field radiation pattern. With the model we can distinguish the respective contributions from SPPs and from other surface waves to the antenna radiation. It is found that for antennas with long arms that support higher-order resonances, SPPs provide a dominant contribution to the antenna radiation, while for other cases, the contribution of surface waves other than SPPs should be considered. The model reveals an intuitive picture that the enhancement of the antenna radiation is due to surface waves that are resonantly excited on the two antenna arms and that are further coupled into the nano-gap or scattered into free space. From the model we can derive a phase-matching condition that predicts the antenna resonance and the resultant enhanced radiation. The model is helpful for a physical understanding and intuitive design of antenna devices. R esonant optical nano-antennas are intensively studied in recent years due to their superior properties of generating strong electromagnetic field under far-field illuminations 1-6 and reciprocally, enhancing the radiation of emitters such as molecules or quantum dots in the vicinity of antennas [7][8][9][10][11][12][13][14][15][16][17][18][19] . Plasmonic nanoantennas are widely used in enhanced Raman scattering spectroscopy [20][21][22][23] , nonlinear optical control [24][25][26][27] , and single-emitter fluorescence enhancement [9][10][11][15][16][17][18]28 . Much experimental and theoretical work has been devoted to achieve an understanding of the underlying physics of resonant nano-antennas for guiding the design of relevant devices. For a simple single-wire nano-antenna, it is described as a Fabry-Pérot resonator of surface plasmon polaritons (SPPs) 4,5,12,19 for predicting the resonance frequency. The single-wire nano-antenna is also treated as an equivalent circuit composed of resistors, inductors and capacitors [29][30][31] , and radiation or scattering features such as the resonance frequency and the extinction spectrum can be predicted. Resonant dipole antennas, which are made of two metallic nano-wires separated by a nano-gap 1 , can achieve a much stronger enhancement effect than the single-wire antenna. Concepts of impedance and resistance are proposed for dipole antennas [32][33][34][35] for reproducing quantities such as the resonance frequency, the quantum efficiency and the enhancement of field. The dipole antennas are also modelled as one-dimensional micro-cavities 34,36 , and the enhancement effect is attributed to the resonance of SPPs. In previous literatures, it is commonly believed that the enhancement effect of the antenna radiation or th...

Rapid and convenient biosensing platforms could be beneficial to timely diagnosis and treatment of diseases in virtually any care settings. Sandwich immunoassays, the most commonly used methods for protein detection, often rely on expensive tags such as enzyme and tedious wash and incubation procedures operated by skilled labor. In this report, we revolutionized traditional sandwich immunoassays by providing a wash-free homogeneous colorimetric immunoassay method without requirement of any separation steps. The proposed strategy was realized by controlling the growth of gold nanoparticles (AuNPs) to mediate the interparticle spacing in the protein-AuNP oligomers. We have demonstrated the successful in vitro detection of cancer biomarker in serum samples from patients with high clinical sensitivity and specificity.