Single pump, broadband probe experiments extract the dynamic permittivity of materials.
Single pump, broadband probe experiments extract the dynamic permittivity of materials.

Photonics relies on controlling the interaction between materials and light. That interaction is determined by the dielectric permittivity of a material. Now researchers have shown that extraordinarily large changes in permittivity can be induced in zinc oxide (ZnO) and used to control light [Saha et al., Materials Today (2020),].

“We reversibly transitioned ZnO from dielectric to metal using an optical pump,” explains Alexandra Boltasseva of Purdue University, who led the effort. “The large changes in permittivity allowed us to make lithography-free all-optical switches with picosecond timescales.”

Transparent conducting oxides (TCOs) like ZnO are a promising class of tailorable, dynamically tunable nanophotonic materials that allow an unprecedented level of control over optical response. The permittivity of TCOs can be altered using dopant atoms or varied spatially and periodically using multilayered structures to create novel metamaterials, but these changes are permanent. The next step in optical control would be dynamic – or reversible – tuning of a material’s permittivity in real time, which could enable new technologies like ultrafast signal transfer and open up new areas of research. TCOs are ideal for this purpose because their deposition methods are well-developed, their optical properties can be readily tailored, and they have a high threshold for laser damage.

“We wanted to test out reversibly controlling the optical properties (permittivities) of a TCO, without using any dopants,” says first author Soham Saha.

Together with colleagues at Northwestern University and Argonne National Laboratory, the researchers achieved this by optically pumping ZnO with ultraviolet light from a pulsed laser. This creates a large number of free carriers in the conduction band, which turn ZnO from a dielectric to a metal with high absorbance in the telecom frequency range.

“The transition is instantaneous – as electrons fall back into the valence band, ZnO reverts to a dielectric. This helped us make fast, large-amplitude electroabsorption switches,” explains Boltasseva.

The researchers show that metal-backed, ZnO dielectric mirrors can achieve broadband all-optical reflectance modulation of up to 70%, which is as good as indium-doped tin oxide or aluminum-doped ZnO. Moreover, TiN-backed ZnO nanostructures can boost reflection modulation at specific wavelengths and polarizations.

“To our knowledge, this is the first time all-optical permittivity modulation has been reported in ZnO without doping with another ion,” says Boltasseva.

The ability of ZnO to control light via permittivity changes establishes the material as a promising candidate for the design of ultrafast all-optical switches, beam-steering devices, and dynamic nanofocusing. The approach could also be used to study other undoped and doped conductive oxides and explore novel optical phenomena.

“We plan on building up a database of dynamically tunable materials, cataloging their laser damage thresholds and permittivity modulation limits,” she adds.