Tunable Nanophotonics Enabled by Defect-Engineering of VO₂ Using a Focused Ion Beam

Vanadium dioxide (VO₂) has been widely exploited for reconfigurable photonics because of the large changes in its optical refractive index across its insulator-metal phase transition (IMT) at ~70 °C. The IMT temperature of VO₂ is determined by the stability of electron hybridization, which is sensitive to the strain environment, providing various modulation routes such as strain engineering via substrates, electrical gating, chemical doping, and defect engineering via ion implantation. Previously, we demonstrated that the IMT can be shifted close to the room temperature via introduction of structural defects into the VO₂ film by a high-energy ion implanter. Here, we advance this technique by engineering the defect density of VO₂ using a commercial focused ion beam (FIB) system, enabling us to locally tune the IMT temperature via a single-step mask-free lithography process.<br/><br/>First, we experimentally characterized the dependency of IMT temperature on the FIB fluence. A ~50-nm VO₂ film was deposited by sputtering on a sapphire substrate. Then, twelve 200-by-200-µm regions were irradiated using focused 30-keV Ga ions at room temperature with different ion fluences ranging from 0 to 20×10¹³cm^-2. Note that we fabricated 200-by-200-µm regions to enable far-field optical characterization. In principle, much smaller resolution in irradiation can be achieved using a commercial FIB system, which we confirmed via microscopic Raman spectroscopic mapping. The modulated IMT temperature and complex refractive indices of all irradiated regions were extracted using a combination of temperature-dependent Fourier-transform infrared (FTIR) reflectance measurements and spectroscopic ellipsometry.<br/>We found that with FIB fluences &lt; ~7×10¹³ cm^-2, the IMT temperature can be continuously lowered by ~15 °C with no substantial changes in the refractive-index contrast between the insulating and metallic phases. For the higher fluences, though the IMT temperature can be further reduced, it is accompanied by a significantly broadened transition width and smaller refractive-index contrast. The low-fluence (i.e., &lt; 7×10¹³ cm^-2) modulation is generally favorable for reconfigurable photonics because it can simply tune the IMT temperature (i.e., the triggering threshold of a VO₂-based device) without trading off the optical contrast between the device states.<br/>As a proof of concept, we designed a dual-band transmission filter. Our filter consists of periodic arrays of gold aperture antennas on top of a 50-nm VO₂ film on a sapphire substrate. The apertures were designed into two sub-groups with different aperture lengths of 0.9 µm and 1.15 µm, thus supporting two resonant transmission modes centered at 3.6 µm and 4.8 µm, respectively. For the pristine VO₂ film (i.e., with no FIB engineering), this filter features two transmission bands when the VO₂ is in the insulating phase, and both transmission bands are simultaneously suppressed when the VO₂ is in the metallic phase. Using FIB irradiation, VO₂ areas adjacent to different sub-groups of gold apertures can be engineered to have distinct IMTs. For example, if the VO₂ areas around the shorter apertures are irradiated with ion fluence of 6.7×10¹³cm^-2, those areas will be transformed to the metallic phase at ~65 °C while the other VO₂ areas stay in the insulation phase, resulting in suppression of the transmission band at 3.6 µm while leave the other band unaffected—a multi-state optical filter. We fabricated the designed dual-band transmission filter via an e-beam lithography process, and the experimental transmittance spectra are in good agreement with our simulations. FIB-assisted defect engineering on these fabricated filters is in process, and we expect to show experimental characterizations of these optical filters with individually tunable transmission bands in our presentation.