Manipulation of optical signals for data processing in optoelectronic circuits requires the development of ultrafast optical switches. In this context, vanadium dioxide (VO₂) has been extensively studied because it undergoes first-order insulator-to-metal (IMT) and metal-to-insulator (MIT) phase transitions that are characterized by a large change in optical and DC conductivity. The phase change can be induced both thermally and by ultrafast fs laser irradiation. VO₂ thin films have shown an IMT faster than 100 fs. However, these films show a large recovery time of the insulating phase (larger than 20 ns); this limits the application of VO₂ for reversible ultrafast ON/OFF/ON transitions. With the aim to identify materials with enhanced optical switching, in this Optical Materials Express article Melissa R. Beebe and co-authors investigate the ultrafast induced phase transition of a related material, Niobium dioxide (NbO₂), with structural and phase change properties similar to VO₂, but with a much higher critical temperature for the phase transition (1080 K). The article describes and compares the dynamics of the IMT transition of VO₂ and NbO₂ monoclinic insulating thin films irradiated by ultrashort laser pulses (approximately 120 fs). Measurements of the transient relative change in reflectivity (&Delta;R/R) associated to the IMT transition, obtained using an ultrafast probe pump-probe set-up, show indeed an ultrafast change in reflectivity in both NbO₂ and VO₂ films. More remarkable is the demonstration of a clear electronic response in NbO₂, with a recovery time of a few picoseconds. Furthermore, in NbO₂ the fluence to induce the IMT is lower, and the initial response time is shorter than in VO₂. These results show that NbO₂ is a robust material for the implementation of ultrafast optoelectronic switches with some potential advantages over VO₂. Finally, the authors suggest that the combination of both VO₂ and NbO₂ films in a single device can be advantageous by implementing double-function switch architectures that would profit from the different time response associated to each material.
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