Terahertz Switch Using an Array of Subwavelength Metallic Holes-coupled-disks

Broadband switching of terahertz waves at room temperature is demonstrated using a reconfigurable subwavelength metallic hole coupled disk array. The interaction between a metallic membrane featuring periodically arranged circular holes and a substrate bearing a correspondingly periodic array of metallic disks - precisely aligned at their centers - significantly enhances the light coupling within each individual metallic structure, leading to an improved optical transmission and the appearance of a new transmission peak. By mechanical displacement of the metallic membrane with respect to the substrate with metallic disks, the light transmission through the structure can be reconfigured. The device exhibits a polarization-insensitive high-contrast switching performance of 89.4 dB at 942 GHz. The full-width at half-maximum bandwidth of the switch is 288 GHz. By proper design of the device’s geometric dimensions, the operation frequency and bandwidth of the switch can be scaled.

💡 Research Summary

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The paper presents a reconfigurable terahertz (THz) switch that exploits the extraordinary optical transmission (EOT) effect in a dual‑layer metallic structure composed of sub‑wavelength holes coupled to metallic disks. The upper layer is a thin (2 µm) gold membrane perforated with a square lattice of circular holes (radius = 45 µm, period = 100 µm). The lower layer consists of a matching square lattice of metallic disks (radius = 47.5 µm, thickness = 2 µm) fabricated on a low‑index polymer (polystyrene foam) substrate. The holes and disks are precisely aligned so that each hole sits directly above a disk, creating a strong electromagnetic coupling between the two metallic elements.

Finite‑element method (FEM) simulations, performed with periodic boundary conditions to model an infinite array, reveal that when the membrane is suspended 2 µm above the disks (the “ON” state), a pronounced transmission peak appears at 942 GHz with a full‑width at half‑maximum (FWHM) bandwidth of 288 GHz. The insertion loss at this resonance is only 0.56 dB, indicating highly efficient transmission. The coupling mechanism is identified as a hybrid resonance involving spoof surface plasmon polaritons (spoof SPPs) supported by the hole array and localized surface plasmons (LSPs) excited at the edges of the disks. The hybrid mode concentrates the electric field at the hole‑disk interface, dramatically enhancing the EOT effect and generating a new transmission peak beyond that of a solitary hole array.

Actuation is achieved with a MEMS actuator that vertically displaces the membrane. When a voltage is applied, the membrane moves downward until it contacts the disks (the “OFF” state). This contact eliminates the gap‑mediated coupling, suppressing the hybrid resonance. Consequently, the transmission at 942 GHz drops by more than 85 % and the insertion loss rises to ~90 dB, yielding a switching contrast of 89.4 dB. The switch is polarization‑independent because of the four‑fold symmetry of the lattice.

Parametric studies explore the influence of three geometric variables: (i) the membrane‑disk spacing, (ii) the lattice period, and (iii) the overall thickness of the metallic layers. The transmission reaches its maximum at a spacing of ≈2 µm; larger separations weaken the coupling, while smaller separations lead to mechanical contact and loss of the resonance. Increasing the period shifts the resonance to lower frequencies and broadens the bandwidth but reduces peak transmission, offering a trade‑off for frequency scaling. Raising the structure height slightly raises the resonance frequency and further widens the bandwidth. These dependencies demonstrate that the device’s operating frequency, bandwidth, and isolation can be engineered by simple geometric scaling, making the concept adaptable across the THz spectrum.

The choice of polystyrene foam as the supporting substrate is justified by its extremely low refractive index (1.017–1.022) and negligible absorption (0.2–4 THz), ensuring that the substrate does not introduce additional loss. Moreover, the hole array behaves as a high‑pass filter while the disk array acts as a low‑pass filter; together they form an effective band‑pass filter with a transmission higher than the simple sum of the individual arrays, owing to the strong field enhancement at the coupled edges.

In summary, the authors demonstrate a room‑temperature, MEMS‑actuated THz switch that achieves:

• A high‑contrast switching ratio of 89.4 dB,
• A broad 3 dB bandwidth of 288 GHz centered at 942 GHz,
• Polarization‑independent operation,
• Scalability of frequency and bandwidth through geometric design,
• Low insertion loss in the ON state (0.56 dB) and extremely high isolation in the OFF state (~90 dB).

The work advances the state of THz photonic components by providing a practical, mechanically reconfigurable platform that leverages coupled sub‑wavelength hole‑disk resonances. Potential applications include high‑speed THz communication links, dynamic coded‑aperture imaging, beam steering, and on‑chip THz modulators where fast, low‑loss, and broadband switching is essential.