Fig. 1 (a) Selected transition metal oxide O-PCMs and the temperatures at which they demonstrate a change in their optical properties. Figure reprinted with permission from [21]. © 2011 Annual Reviews. (b) Ternary phase diagram for Te, Ge, and Sb, showing selected chalcogen-based O-PCMs. Figure reprinted with permission from [22]. © 2008 Nature Publishing Group.

Fig. 2 Atomic structures and optical properties of VO₂ and GST. (a) Three-dimensional schematics of the low temperature (T < 68°C), monoclinic (left) and high temperature (T > 68°C), rutile (right) crystal structures of VO₂. Vanadium atoms are shown in light blue. The orange shadows highlight the V-V dimers exhibited in the monoclinic structure. Oxygen atoms are not shown. The monoclinic and rutile states of VO₂ are labeled VO₂:M and VO₂:R, respectively. Figures adapted and reprinted with permission from [32] © 2012 American Physical Society. (b) Two-dimensional schematics (Te atoms in blue; Ge and Sb atoms in gold) of the amorphous (left) and crystalline (right) states of GST. The amorphous and crystalline states of GST are labeled GST:A and GST:C, respectively. Figures adapted and reprinted with permission from [33] © 2015 Nature Publishing Group. (c) Refractive indices of VO₂ and GST. (d) Extinction coefficient of VO₂ and GST. For (c) and (d), optical properties were taken and replotted from [34] and [28] for VO₂ and GST, respectively. In both cases, the change in optical properties was thermally induced.

Fig. 3 (a) Schematic and scanning electron microscopy (SEM) images of VO₂ coated silicon ring resonator. (b) Temperature-dependent transmission of Si/VO₂ ring resonator in (a), demonstrating the change in optical response as VO₂ undergoes its OPC. Figures in (a) and (b) reprinted with permission from [42] © 2010 The Optical Society. (c) Optical transmission of 1.5 µm radius Si/VO₂ ring resonator (SEM inset top left with VO₂ false colored maroon). At the selected wavelength (dashed line), optical transmission is low with no laser-induced photothermal heating (“laser off” inset) while transmission is high with laser induced photothermal heating (“laser on” inset) due to the resonance shift induced by the OPC of VO₂. Small scale bar in SEM image inset is 250 nm. Figures adapted and reprinted with permission from [44] © 2012 The Optical Society. (d) Proposed 2 × 2 Si/VO₂ microring switch. Figure reprinted with permission from [46] © 2016 Institute of Electrical and Electronics Engineers. (e) SEM images of design showing VO₂ (false colored green) embedded within a silicon waveguide. Each bifurcated silicon waveguide (false colored navy) splits into a control waveguide (blue box) and VO₂ embedded waveguide (orange box). The left side of the figure shows tilted (top) and normal incidence (bottom) SEM images of the VO₂ embedded waveguide. The integrated heaters are false colored gold. Figure reprinted with permission from [45] © 2017 The Optical Society. (f) Schematics of proposed pass polarizer using VO₂ on a silicon waveguide (blue). Purple and grey blocks represent VO₂:M and VO₂:R, respectively. Quasi TE and TM light are represented by blue and red arrows, respectively. Figure reprinted with permission from [47] © 2015 The Optical Society.

Fig. 4 (a) SEM image of Si/VO₂ electro-optic waveguide device. VO₂ and Au are false colored purple and gold, respectively. Figure reprinted with permission from [48] © 2015 American Chemical Society. (b) Optical microscope image of Si/VO₂ electro-optic waveguide device which delocalizes the optical mode to increase interaction with VO₂:R. Figure reprinted with permission from [49] © 2015 The Optical Society. (c) Proposed Si/VO₂ electro-optic modulator design based on directional coupler theory. Figure reprinted with permission from [57] © 2014 The Optical Society. (d) Proposed Si/VO₂ electro-optic design including a vertically embedded VO₂ section within the silicon waveguide. Figure reprinted with permission from [58] © 2017 Institute of Electrical and Electronics Engineers. (e) Proposed Si/Au/VO₂ electro-optic modulator design based on near field plasmonic coupling. Figure adapted and reprinted with permission from [59] © 2015 The Optical Society. (f) Proposed Si/GST electro-optic device whereby a thin ribbon of GST embedded within a silicon waveguide is electrically actuated. Figure reprinted with permission from [60] © 2015 Institute of Electrical and Electronics Engineers.

Fig. 5 (a) Transient response of Si/VO₂ ring resonator as a function of increasing pump fluence from 0.45 to 4.74 mJ/cm² (blue to red). SEM image of Si/VO₂ ring resonator in top right (VO₂ is colored maroon). Small scale bar in SEM image inset is 250 nm. Figure adapted and reprinted with permission from [44, 67] © 2012, 2013 The Optical Society. (b) Schematic and optical microscope image of Si/GST ring resonator. Figure reprinted with permission from [68]. © 2013 American Institute of Physics. (c) Schematic of Si/GST multimode waveguide device. Figure reprinted with permission from [69] © 2012 The Optical Society. (d) Schematic (top) and SEM images (bottom) of a 2 µm long GST patch embedded inside of a silicon waveguide. Bottom left SEM shows device cross section (A-B) perpendicular to the direction of propagation. Bottom right SEM shows device cross section (C-D) parallel to the direction of propagation. Out-of-plane optical pulses (660 nm, 89 mW peak power, 500 nanoseconds) crystallize the GST, and the change in optical propagation through the silicon waveguide is measured. Figure reprinted with permission from [70] © 2010 Institute of Engineering and Technology. (e) Schematic for proposed 2 × 2 switch implementing a chalcogen-based O-PCM with low optical loss (GSST) as the active component. Figure reprinted with permission from [73] © 2018 The Optical Society. (f) Schematic for proposed Si/Au/VO₂ all-optical modulator. Figure reprinted with permission from [74] © 2018 Institute of Electrical and Electronics Engineers.

Fig. 6 (a) Schematic of device used for probing voltage-induced electrical dynamics of VO₂. (b) Current density response of device in (a), demonstrating the increase in current in response to a voltage pulse. Figures reprinted with permission from [51] © 2013 Institute of Electrical and Electronics Engineers.