The plasmonic behavior of dimers of touching semiconductor disks is studied experimentally in the difficult‐to‐realize regime where the disks are only marginally overlapping. Previous theoretical studies have shown that this geometry exhibits a highly efficient broadband response that may be very promising for light harvesting and sensing applications. By taking advantage of the plasmonic character of InSb in the terahertz regime, we experimentally confirm this broadband response and describe the associated strong field enhancement and sub‐micrometer field confinement between the disks.

A strained-modified, single-band, constant-potential three-dimensional model is formulated to study the dependence of electronic states of InAs/ GaAs quantum dots ͑QDs͒ on shape and size variation. The QD shapes considered are ͑i͒ cuboid, ͑ii͒ cylindrical, ͑iii͒ pyramidal, ͑iv͒ conical, and ͑v͒ lens shaped. Size variations include ͑i͒ QD volume ͑ii͒ QD base length, and ͑iii͒ QD height, taking into account aspect ratio variation. Isovolume QD shapes with narrow tips were found to have higher ground-state energies than those with broad tips, and this is attributed to the smaller effective volume. The volume, base length, and height dependencies were obtained and found to tally well with both experimental results and advanced calculations. Hence, upon growth parameter variation, this can provide an alternative to confirm whether the change to the size of the uncapped QDs implies a similar change to the capped ones. Ground-state energy as function of aspect ratio does not follow a monotonic trend. Owing to the competing effect of a decrease in base length and an increase in height, the energy trend exhibits a sharp decrease to an optimum aspect ratio, followed by gentle, almost linear increase. The optimum aspect ratio varies among shapes and is predicted to be smaller for shapes with broad tips. The effective volume ratio of both shapes ͑V eff,CUBOID / V eff,PYRAMID ͒ was determined, and found to vary with aspect ratio. Furthermore, a "cross-over" of lens-shaped QD from "lower energy" to "higher energy" group is predicted due to significant shape transition.

Research on terahertz (THz) electromagnetic radiation has been booming in the past two decades in view of its unique properties and important applications in sensing, imaging, and spectroscopy. [1][2][3] The recent advancement of plasmonics and metamaterials has brought new approaches for manipulating THz waves. Signifi cant progress has been achieved in THz component development, such as ultra-high refractive index THz metamaterials, [ 4 ] beam collimated THz quantum cascade laser, [ 5 ] high effi ciency THz photomixer, [ 6 ] near fi eld on-chip THz detector, [ 7 ] THz modulators, [8][9][10][11][12] high extinction ratio THz polarizer [ 13 ] and THz spoof surface plasmon waveguides. [14][15][16] The electromagnetic response of plasmonic and metamaterial structures arise from the combined contributions of subwavelength structuring and the dielectric materials properties. As the properties of metals can hardly be tuned, THz metamaterial and plasmonic devices based on metallic subwavelength structures are normally tuned or modulated only through adjusting the properties of ambient or active dielectric elements. On the other hand, low-bandgap semiconductors, such as InSb, are excellent candidates for plasmonic materials in the THz range, owing to their small band gap, high electron mobility, small effective mass and low electron density. More importantly, the properties of semiconductors can be easily tuned by thermal, electrical or optical ways. Very recently, broadband THz plasmonic absorption in InSb touching disks was demonstrated, which showed consistency with the theoretical predictions of transformation optics for gold nanostructures at visible frequencies. [ 17 ] InSb was also used as the material to study coherent interference induced THz transparency tuned via an external magnetic fi eld. [ 18 ] By taking advantage of the fast response time of optical processes, semiconductor-based optically tunable THz active plasmonics is a very suitable concept for high-speed THz modulation. In this paper, we report the direct optical tuning of the THz plasmonic response of InSb subwavelength gratings. The potential modulation speed was characterized through carrier lifetime study by optical-pump THz-probe (OPTP) spectroscopy. Theoretical calculations of the subwavelength grating plasmonic structure show full-scale operation agility in modulation frequency and depth.In THz frequency range InSb can be modeled as a classic solid-state plasma, with its permittivity given by g InSb (T) = g ∞ [1 − T 2 p / T 2 + iT( ] and plasma frequency T p = ne 2 / g 0 g ∞ m * , where n is the carrier concentration. [ 19 ] At room temperature, the plasma frequency of InSb lies within the THz frequency range and its permittivity resembles that of noble metals in the optical frequencies, enabling plasmonic resonances of subwavelength structures with incident THz waves. [ 20,21 ] As indicated by the above equations, the permittivity of InSb depends strongly on carrier concentration, which allows the permittivity and consequently plasmonic res...