Optical sensors are widely used for refractive index measurement in chemical, biomedical, and food processing industries. Due to specific field distribution of the resonances, optical sensors provide high sensitivity to ambient refractive index variations. The sensitivity of an optical sensor is highly dependent on material and structure of the sensor. Here, six major categories of optical refractive index sensors using plasmonic and photonic structures are reviewed: i) metal‐based propagating plasmonic eigenwave structures, ii) metal‐based localized plasmonic eigenmode structures, iii) dielectric‐based propagating photonic eigenwave structures, iv) dielectric‐based localized photonic eigenmode structures, v) advanced hybrid structures, and vi) 2D material integrated structures. Representative configurations working in the wavelength range of 400–2000 nm will be selected and compared in terms of bulk refractive index sensitivities, figures of merit, and working wavelengths. A technology map is established in order to define the standard and development trend for optical refractive index sensors.

This paper reviews the history of multipactor discharge theory, focusing on recent models of multipactor accessibility and saturation. Two cases are treated in detail: That of a first-order, two-surface multipactor, and that of a single-surface multipactor on a dielectric. In both cases, susceptibility curves are constructed to indicate the regions of external parameter space where multipactor is likely to occur, taking into account the dependence on surface materials, and the effects of space charge and cavity loading. In the case of a dielectric, multipactor is found to deliver about 1% of the rf power to the surface. The two cases are contrasted in light of experimental observations.

We study electrical transport properties in exfoliated molybdenum disulfide (MoS ₂ ) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 10 ⁴ s, due to photo-charge trapping at the MoS ₂ /SiO ₂ interface and in MoS ₂ defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 k Ω μ m − 1 , ON current as high as 1.25 nA μ m − 1 , 10 ⁵ ON–OFF ratio, mobility of ∼1 cm ² V ⁻¹ s ⁻¹ and photoresponsivity ℛ ≈ 1 A W − 1 .

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic-, field-, and photoemission) to the space charge limited state (CL) will be addressed, especially highlighting the important simulation and experimental developments in selected contemporary areas of study. We stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion.