1 × 2 optical switch devices based on semiconductor-to-metallic phase transition characteristics of VO₂smart coatings

We have successfully fabricated two types of 1 × 2 optical switch devices, namely, all-optical switch (VO2/quartz) and electro-optical switch (VO2/TiO2/ITO/glass) based on the semiconductor-to-metallic phase transition characteristic of vanadium dioxide (VO2) smart coatings. The VO2 active layer, the TiO2 buffer layer and the ITO transparent conductive electrode used in these devices were achieved by reactive pulsed laser deposition. The optical switching of the fabricated devices was investigated at λ = 1.55 µm. The semiconductor (on) to metallic (off) phase transition was controlled by photo-excitation of VO2 in the case of the all-optical switch and by an external electric field applied between the ITO and the VO2 layer in the case of the electro-optical switch. The extinction ratio (on/off) is found to be much higher for the all-optical switch than for the electro-optical switch. For the all-optical switch, extinction ratios of about 22 and 12 dB are obtained in the transmission and reflection modes, respectively. In the case of the electro-optical switch, the extinction ratio is about 12 dB in the transmission mode and 5 dB in the reflection mode. Finally, to explain our optical switching results, we propose a simple model based on the energy band diagram of VO2 in which the charge density increases under an external excitation (either photo-excitation or an electrical field), and then induces the semiconductor-to-metallic phase transition in the VO2 active layer.