The intriguing physics of vanadium dioxide (VO2) makes it not only a fascinating object of study for fundamental research on solid-state physics but also an attractive means to actively modify the properties of integrated devices. In particular, the exceptionally large complex refractive index variation produced by the insulator-to-metal transition of this material opens up interesting opportunities to dynamically tune optical systems. This Perspective reviews some of the exciting work done on VO2 for nanophotonics in the last decade and suggests promising directions to explore for this burgeoning field.

The performance of optical devices relying in vanadium dioxide (VO) technology compatible with the silicon platform depends on the polarization of light and VO properties. In this work, optical switching in hybrid VO/Si waveguides thermally triggered by lateral microheaters is achieved with insertion losses below 1 dB and extinction ratios above 20 dB in a broad bandwidth larger than 30 nm. The optical switching response has been optimized for TE and TM polarizations by using a homogeneous and a granular VO layer, respectively, with a small impact on the electrical power consumption. The stability and reversibility between switching states showing the possibility of bistable performance is also demonstrated.

Typically, materials with large optical losses such as metals are used as microheaters for silicon based thermo-optic phase shifters. Consequently, the heater must be placed far from the waveguide, which could come at the expense of the phase shifter performance. Reducing the gap between the waveguide and the heater allows reducing the power consumption or increasing the switching speed. In this work, we propose an ultra-low loss microheater for thermo-optic tuning by using a CMOS-compatible transparent conducting oxide such as indium tin oxide (ITO) with the aim of drastically reducing the gap. Using finite element method simulations, ITO and Ti based heaters are compared for different cladding configurations and TE and TM polarizations. Furthermore, the proposed ITO based microheaters have also been fabricated using the optimum gap and cladding configuration. Experimental results show power consumption to achieve a π phase shift of 10 mW and switching time of a few microseconds for a 50 µm long ITO heater. The obtained results demonstrate the potential of using ITO as an ultra-low loss microheater for high performance silicon thermo-optic tuning and open an alternative way for enabling the large-scale integration of phase shifters required in emerging integrated photonic applications.

Nonvolatile switching is still a missing functionality in current mainstream silicon photonics complementary metal‐oxide‐semiconductor platforms. Fundamentally, nonvolatile switching stands for the ability to switch between two or more photonic states reversibly without needing additional energy to hold each state. Therefore, such a feature may push one step further the potential of silicon photonics by offering new ways of achieving photonic reconfigurability with ultrasmall energy consumption. Here, a detailed review of current developments that enable nonvolatile switching in silicon photonic waveguide devices is provided. Nonvolatility is successfully demonstrated either based on device engineering or by hybrid integration of silicon waveguides with materials exhibiting unique optical properties. Furthermore, several approaches with high potential for evolving toward a nonvolatile behavior with enhanced performance are also being explored. In most cases, many development steps are still necessary to ensure reliable devices. However, this research field is expected to progress in the coming years boosted by current and emerging applications benefiting from such functionality, such as new paradigms for photonic computing or advanced reconfigurable circuits for programmable photonic systems.

A tunable transverse electric (TE) pass polarizer is demonstrated based on hybrid vanadium dioxide/silicon (VO/Si) technology. The 20-μm-long TE pass polarizer exploits the phase transition of the active VO material to control the rejection of the unwanted transverse magnetic (TM) polarization. The device features insertion losses below 1 dB at static conditions and insertion losses of 5.5 dB and an attenuation of TM polarization of 19 dB in the active state for a wavelength range between 1540 nm and 1570 nm. To the best of our knowledge, this is the first time that tunable polarizers compatible with Si photonics are demonstrated.