Tunable Optical Bistability and Optical Switching by Nonlinear Metamaterials

We demonstrate a nonlinear metamaterial in microwave frequency regime with hysteresis effect and bistable states, which can be utilized as a remotely controllable micro second switching device. A varactor loaded split-ring resonator (SRR) design which exhibits power and frequency dependent broadband tunability of the resonance frequency for an external control signal is used. More importantly, the SRR shows bistability with distinct transmission levels. The transition between bi-states is controlled by impulses of an external pump signal. Furthermore, we experimentally demonstrate that transition rate is in the order of microseconds by using a varactor loaded double split-ring resonator (DSRR) design composed of two concentric rings.

💡 Research Summary

In this work the authors present a microwave‑frequency nonlinear metamaterial that exhibits optical bistability, hysteresis, and microsecond‑scale switching. The basic meta‑atom is a split‑ring resonator (SRR) fabricated on a 0.2 mm FR‑4 substrate with copper traces. One of the SRR gaps is bridged by a hyper‑abrupt varactor diode, providing a voltage‑dependent capacitance that makes the resonator’s effective RLC parameters nonlinear.

Measurements were performed with a PNA‑X vector network analyzer (VNA) while an N5181A MXG analog signal generator supplied a pump signal. With the pump power fixed at –25 dBm, the pump frequency was swept up and down. The resonance frequency of the SRR shifted over a 60 MHz range and a hysteresis loop wider than 20 MHz was observed, indicating two stable transmission states (high‑ and low‑transmission) for the same pump frequency. The hysteresis originates from the varactor’s capacitance decreasing sharply when the pump frequency coincides with the resonator’s natural frequency, causing an abrupt jump to a higher resonance frequency.

When the pump power was varied instead of the frequency, a similar bistable behavior was found. Below a certain power threshold the hysteresis disappears; above it the loop widens. Increasing pump power drives the resonance upward (lower to higher frequency), whereas increasing pump frequency drives the resonance downward, reflecting the opposite dependence of the varactor voltage on the two control parameters.

Polarization dependence was also investigated. When the electric field is parallel to the SRR gap, the bistability is pronounced; rotating the field reduces the component across the gap, shrinking the hysteresis width and eventually eliminating it. This confirms that the nonlinear response is driven by the electric field component that directly biases the varactor.

To accelerate the switching, a 1 MΩ resistor was placed in parallel with the varactor. The resistor provides a discharge path for the charge accumulated on the diode, shortening the transition time. However, the up‑transition (low‑to‑high transmission) and down‑transition (high‑to‑low) times become asymmetric and depend on the resistor value. The authors note that the minimum observable transition time was limited by the latency of the measurement instruments.

For a more precise measurement of the transition dynamics, a double split‑ring resonator (DSRR) was designed. The inner ring resonates at 1.2 GHz and is pumped, while the outer ring resonates at 0.8 GHz and is probed. The pump power is about 10 dB higher than the probe power. By recording the S‑parameters of the outer ring while pulsing the pump on the inner ring, the authors captured spectrograms that clearly show the moment of state change. Varying the parallel resistor value allowed the authors to tune the transition time from roughly 5 µs to 30 µs, demonstrating controllable microsecond‑scale switching.

The paper concludes that varactor‑loaded SRRs provide a versatile platform for broadband, power‑ and frequency‑tunable metamaterials with intrinsic bistability. Adding a parallel resistor enables fast, remotely controllable switching suitable for applications such as reconfigurable filters, adaptive antennas, and dynamic cloaking devices. The experimental demonstration of microsecond switching speeds and the clear mapping of transition dynamics represent a significant step toward practical nonlinear metamaterial devices.