An ultra-fast liquid switch for terahertz radiation

Ultra-fast switches are essential devices for basic research and technological development. Here, we demonstrate that aqueous solutions of sodium iodide can be used for this purpose. When pumped with an intense optical pulse at 400 nm, these water-based liquids display large and fast responses in the terahertz range, around 1 THz. In a 9M NaI solution at a temperature comprised between 10 and 50 °C, the relative variation of the terahertz peak transmission drops by 20% at the pump–probe overlap and recovers with a fast time constant of ∼70 fs. As the optical properties of the liquid vary on a timescale shorter than the terahertz cycle, it is possible to tailor the shape of the transmitted terahertz fields. In this way, we demonstrate the frequency upshifting of terahertz radiation from about 1 to 3 THz and beyond with an efficiency of 4%.