Interdigitated terahertz metamaterial sensors:design with the dielectric perturbation theory

Designing terahertz sensors for highly sensitive detection of nanoscale thin films and a few biomolecules poses a substantial challenge but is crucial for unlocking their full potential in scientific research and advanced applications.This work presents a strategy for optimizing metamaterial sensors in detecting small quantities of dielectric materi-als.The amount of frequency shift depends on intrinsic properties(electric field distribution,Q-factor,and mode volume)of the bare cavity as well as the overlap volume of its high-electric-field zone(s)and the analyte.Guided by the simplified dielectric perturbation theory,interdigitated electric split-ring resonators(ID-eSRRs)are devised to significantly enhance the detection sensitivity compared with eSRRs without interdigitated fingers.ID-eSRR’s fin-gers redistribute the electric field,creating strongly localized enhancements,which boost analyte interaction.The periodic change of the inherent antiphase electric field reduces radiation loss,leading to a higher Q-factor.Experiments with ID-eSRR sensors operating at around 300 GHz demonstrate a remarkable 33.5 GHz frequency shift upon depositing a 150 nm SiO_(2)layer as an analyte simulant,with a figure of merit improvement of over 50 times compared with structures without interdigitated fingers.This rational design offers a promising avenue for highly sensitive detection of thin films and trace biomolecules.

Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor

Plasma waves play an important role in many solid-state phenomena and devices.They also become significant in electronic device structures as the operation frequencies of these devices increase.A prominent example is field-effect transistors(FETs),that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies,where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time.Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave,collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode.In this paper,we present the first direct visualization of these waves.Employing graphene FETs containing a buried gate electrode,we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions.Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting.The plasma waves,excited at 2 THz,are overdamped,and their decay time lies in the range of 25-70 fs.Despite this short decay time,the decay length is rather long,i.e.,0.3-0.5μm,because of the rather large propagation speed of the plasma waves,which is found to lie in the range of 3.5-7×10^(6)m/s,in good agreement with theory.The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1/4 power law.