Adaptive impedance matching in microwave and terahertz metamaterial absorbers using PIN diodes and GaN HEMTs

Abstract

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.