Graphene-based free-space optical modulators. (A) Schematic diagram of a graphene-PhC modulator. (B) Gate-voltage-induced differential reflection spectra at different voltages [53]. (C) Diagram of a cavity-graphene EO modulator. (D) Normalized EO S₂₁ frequency responses at different DC gate voltages [55]. (E) Schematic of an HCG-graphene based spatial light modulator. (F) Reflection response at 1 MHz [56]. (A) and (B) Adapted with permission from [53]. Copyright 2013, American Chemical Society. (C) and (D) Adapted with permission from [55]. Copyright 2015, American Chemical Society. (E) and (F) Adapted with permission from [56]. Copyright 2016, Optica Publishing Group.