Refractive Index Tuning of Terahertz Photonic Materials Based on a Stretchable Silicon Effective Medium

Abstract

Dynamically tunable terahertz (THz) photonics requires low-loss dielectric platforms with practical, continuous control of refractive index. Here we present a mechanically reconfigurable THz photonic material platform: a monolithic, all-silicon (Si) stretchable effective medium whose refractive index is tuned by deformation. A 200 micrometer-thick high-resistivity single-crystal Si slab was patterned into a subwavelength spiral-spring through-hole lattice, rendering bulk Si mechanically compliant while preserving its low-loss dielectric response. THz time-domain spectroscopy demonstrates high transmission below 0.6 THz and reveals a monotonic decrease in the effective refractive index under uniaxial stretching. At 12.6% elongation, the effective index decreases by 6% and 8% for polarizations perpendicular and parallel to the stretch direction, respectively, thereby demonstrating deformation-induced, controllable anisotropy without a detectable increase in extinction. This structurally engineered bulk-Si approach offers a process-compatible route to mechanically tunable, low-loss THz components for adaptive wavefront and polarization control.

Figures (5)

• Figure 1: All-Si stretchable effective-medium device based on a spiral-spring through-hole lattice. (a) Schematic of the chip layout (patterned area and rigid handles) and the spiral-spring unit cell with a 200 µm pitch and nominal 10 µm line/slot width. (b) Photographs of the fabricated 200 µm-thick high-resistivity Si chip and an optical micrograph of the spiral-spring lattice.
• Figure 2: THz-TDS measurement setup and custom stretching fixture. (a) Photograph of the transmission configuration in the TeraProspector system (Nippo Precision): the THz pulse is linearly polarized by a polarizer, transmitted through the sample mounted in the sample holder, passed through a second polarizer (analyzer) aligned with the first to measure the co-polarized component, and then detected. (b) Photograph of the sample holder (stretching fixture) with two apertures for reference and sample measurements. Uniaxial elongation along the in-plane $y$ direction is applied by gripping the bulk-Si handles and tightening the screw.
• Figure 3: Optical micrographs of the spiral-spring effective-medium region under (a) relaxed (natural-length), (b) 7.3% elongation, and (c) 12.6% elongation along the $y$ direction. The dark regions correspond to through-etched air gaps, which broaden with increasing elongation.
• Figure 4: Measured (solid) and simulated (dashed) spectra and retrieved effective refractive index for different elongation ratios. (a)--(d) $x$-polarized incidence (perpendicular to the stretching direction): (a) transmittance, (b) phase delay, (c) retrieved effective refractive index, and (d) relative change in refractive index referenced to the relaxed state. (e)--(h) $y$-polarized incidence (parallel to the stretching direction): (e) transmittance, (f) phase delay, (g) retrieved effective refractive index, and (h) relative change. Black: relaxed; red: 7.3% elongation; blue: 12.6% elongation.
• Figure 5: FEM-simulated von Mises stress distribution in the spiral-spring periodic structure under 12.6% uniaxial elongation along the $y$ direction. The stress hotspot appears around the midspan of the outermost spiral arm, reaching a peak value of 0.57 GPa.