Electrical & Computer Engineering

The detection of subterahertz (200 GHz) radiation by silicon-oninsulator MOSFETs with submicron gate lengths in the temperature range from $8 to 350 K is reported. The photoresponse measured against gate voltage exhibited a maximum near the threshold voltage with the amplitude decreasing with decreasing temperature. The photoresponse reached the maximum at the gate bias close to the threshold voltage and increased with an increase of the drain-to-source current. This behaviour agrees with the mechanism linking the photoresponse to the excitation of the overdamped plasma waves in the transistor channel. The observed effect could be used for nondestructive, contactless testing of silicon very large integrated circuits in situ.

We experimentally and numerically analyze the charge transfer THz plasmons using an asymmetric plasmonic assembly of metallic V-shaped blocks. The asymmetric design of the blocks allows for the excitation of classical di-polar and multipolar modes due to the capacitive coupling. Introducing a conductive microdisk between the blocks, we facilitated the excitation of the charge transfer plasmons and studied their characteristics along with the capacitive coupling by varying the size of the disk. Direct manipulation of charges via redistribution, transfer, and quantum tunneling has been studied theoretically and experimentally for nanoscale plasmonic systems [1]. With a different mechanism compared with classical capacitive resonance coupling , the direct transfer of excited charges allows for designing advanced plasmonic nanosystems with exquisite features. Metallic subwavelength particles and blocks are particularly promising candidates to support and manipulate ultrastrong plasmonic resonant modes across a wide spectrum range spanning from the ultraviolet to microwave frequencies [2]. The spectral properties of the localized plasmon resonant modes strongly rely on the shape and geometry of the sustaining components. These resonant modes can be classified into superra-diant bright modes [3], subradiant dark modes [4] (recognized as asymmetric line shapes, such as the electromagnetically induced transparency [5], and Fano resonances [6]), and charge transfer plasmons (CTPs) [1]. Classical plasmonic sub-and superradiant modes have been broadly used for several applications , including all-optical and optoelectronic devices [7,8]. In contrast, the applications of CTPs are primarily limited to the fundamental analysis of the quantum physics behind the process at visible spectrum energies [9,10]. The CTP resonant modes that have been observed in nano-scale systems with the reduced offset gaps down to atomic sizes were explained by direct quantum tunneling [10] and indirect Fowler–Nordheim tunneling [11] principles. The CTP modes possess attractive features across low energies below <0.9 eV. However, shifting the CTP resonant modes to the mid-and far-infrared domain with high tunability is a challenge. Realizing this feature across low energies would allow for tailoring novel active plasmonic devices, such as single-molecule sensors and high-responsivity optoelectronic devices. Despite having unique plasmonic features, metallic nanoparticles and assemblies suffer from destructive losses under low-energy beam exposure ranging at mid-and far-infrared and terahertz (THz). This is caused by weak capacitive coupling between the proxi-mal blocks, causing a substantial localization of the plasmons in the micro-gaps. Conversely, the direct transfer of charges between the metallic structures delivers unique abilities to turn on and off the charge shuttling by varying the intensity of the incoming beam, which could be used for development of functional THz plasmonic devices. In the present work, we analyze the plasmonic response of an asymmetric four-member assembly of V-shaped metallic blocks (VSMBs), which are placed head-to-head close to each other. Both numerical and experimental analysis are carried out to investigate and demonstrate the capacitive coupling and direct charge transfer between VSMBs at THz frequencies. To this end, four different regimes are considered to monitor and evaluate excitation of dipolar, quadrupole modes, and CTPs resonances across the THz domain. It is shown that presence of a conductive disk between VSMBs in touching regime facilitates efficient transfer of charges along the blocks leading to a distinguished CTP resonant mode at lower energies. Such an asymmetric design is used to employ its geometrical features to induce dipolar and multipolar modes. In contrast with conventional bowtie antennas and analogous structures without antisymmetry features, the proposed assembly allows for inducing low energy modes such as multipolar dips across the THz spectrum.

We report on numerical study of dispersion properties and frequency dependent absorption characteristics of asym-metric dual grating gate terahertz (THz) plasmonic crystals. The study shows that the dispersion relations of plasmons in a two-dimensional electron gas (2DEG) capped with asymmetric dual grating gates have energy band gaps in the Brillion zones. Depending on the wave vector, the plasmons can have symmetrical, anti-symmetrical, and asymmetrical charge distributions that are different from the ones for uniform gratings case. Plasmons in the studied plasmonic crystal exhibit both tightly confined/weakly coupled behavior and propagating/strongly coupled behavior depending on the plasmonic modes. The responsivity of the plasmonic detector based on asymmetric dual grating gate does not monotonically decrease with the frequency, which is in contrast to the responsivity of uniform grating THz detectors. The cross-section of an asymmetric dual grating gate terahertz plasmonic device under THz illumination is represented, where excited plasmons are shown in red.