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 (VO 2 /quartz) and electro-optical switch (VO 2 /TiO 2 /ITO/glass) based on the semiconductor-to-metallic phase transition characteristic of vanadium dioxide (VO 2 ) smart coatings. The VO 2 active layer, the TiO 2 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 VO 2 in the case of the all-optical switch and by an external electric field applied between the ITO and the VO 2 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 VO 2 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 VO 2 active layer.