Angstrom-Scale Active Width Control of Nano Slits for Variable Plasmonic Cavity

Abstract

Nanogap slits can operate as a plasmonic Fabry–Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wavenumber. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blueshift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10−5 enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

Strain versus Tunable Terahertz Nanogap Width: A Simple Formula and a Trench below

“…Incidentally, this gap width is almost exactly the same as the strain multiplied by the periodicity, which leads us to make a conjecture about the relationship between the gap width (ω) and the strain: curvature = 1/r c where r c is the radius of curvature of the PET substrate at the center, and the strain ε is given by h sub /(2r c ). This trend has already been predicted by one of our earlier works [ 66 ]. To see if this is indeed the case, at least phenomenologically, we change the strain and perform the gap width measurement using AFM.…”

“…Indeed, the measured gap width precisely follows the simplistic phenomenological Equation (1) ( Figure 2 b). Strain singularities are formed where the gaps are located [ 66 , 67 , 68 ], which would exert enormous force on the PET below the Au gap at which nonlinear tearing will occur, generating trenches effectively canceling the initial singularity [ 69 , 70 ]. This trench will smooth out the opening and closing of the nanogap structure, ensuring repeatability of the performances [ 41 ], forming free-standing gaps on top of the PET trench to ensure the phenomenological validity of Equation (1).…”

“…The black and green solid lines represent the experimental results of the 0.5 THz transmission coefficient for the TM and TE modes, respectively. The discrepancy between these data could be associated with the existence of numerous local contacts made by the salient features of the uneven respective metal layers, generating an imaginary part of the dielectric constant [ 42 , 46 , 66 , 72 ]. …”

Terahertz Electromagnetically Induced Transparency with Electric-Field-Coupled Inductor-Capacitor Resonators on LCP Substrate

“…When the vertical coupling distance was further reduced to 16 μm, the transmission peak was 0.88 at 0.398 THz, showing a significant blue shift of 12 GHz due to the strong electric field coupling between the two ELC resonators. In general, the bending substrate has a remarkable impact on the resonant frequency and the peak value of the transmission peak [32,36]. To estimate the influence of the bending LCP substrate on the transmission curve, a curved model that can be bent with arbitrary curvature radius was built in HFSS using the wrap sheet method, as shown in Figure 7a.…”

Trench Formation under the Tunable Nanogap: Its Depth Depends on Maximum Strain and Periodicity

“…We refer to this spacerless tunable gap technology as zerogap technology. For theoretical study using structural simulation, Ångstrom scale control of nanogaps by means of mechanical strain was explored [21].…”