Dynamic Manipulation of THz Waves Enabled by Phase-Transition VO₂ Thin Film

The reversible and multi-stimuli responsive insulator-metal transition of VO₂, which enables dynamic modulation over the terahertz (THz) regime, has attracted plenty of attention for its potential applications in versatile active THz devices. Moreover, the investigation into the growth mechanism of VO₂ films has led to improved film processing, more capable modulation and enhanced device compatibility into diverse THz applications. THz devices with VO₂ as the key components exhibit remarkable response to external stimuli, which is not only applicable in THz modulators but also in rewritable optical memories by virtue of the intrinsic hysteresis behaviour of VO₂. Depending on the predesigned device structure, the insulator-metal transition (IMT) of VO₂ component can be controlled through thermal, electrical or optical methods. Recent research has paid special attention to the ultrafast modulation phenomenon observed in the photoinduced IMT, enabled by an intense femtosecond laser (fs laser) which supports "quasi-simultaneous" IMT within 1 ps. This progress report reviews the current state of the field, focusing on the material nature that gives rise to the modulation-allowed IMT for THz applications. An overview is presented of numerous IMT stimuli approaches with special emphasis on the underlying physical mechanisms. Subsequently, active manipulation of THz waves through pure VO₂ film and VO₂ hybrid metamaterials is surveyed, highlighting that VO₂ can provide active modulation for a wide variety of applications. Finally, the common characteristics and future development directions of VO₂-based tuneable THz devices are discussed.