2, Stanford University, Stanford, California, United States
Optically resonant nanostructures allow for the deterministic design of their optical properties through material properties, dimensions, and dielectric surrounding. Due to their large scattering and absorption cross section, such nanostructures are promising for a wide range of applications, including sensors, photodetectors, and color filters. However, each of these properties is typically static, fixed at the time of synthesis and/or fabrication. The ability to dynamic control the optical response of nanostructured elements enables new functionalities and structures, and is a grand challenge.
One way to create actively-tunable structures is to modulate the optical index of the dielectric media. Electrochemical ion insertion is a well-established method to dynamically tune the relative permittivity, but the spectral response of electrochromics is generally defined by their intrinsic material properties. Here, we combine electrochemical ion insertion in electrochromic tungsten trioxide (WO3) with engineered gap plasmon resonances in subwavelength aluminum nanoparticle arrays to achieve tunable reflection spectra. The aluminum particles have a diameter of 60-100 nm, 30 nm height, and 250 nm periodicity. The gap plasmon resonance of these particles allows for precise control of the spectral response during fabrication, and shows high sensitivity to the refractive index of the material in the gap. Changing the refractive index of the 30 nm WO3 in the gap enables active tuning. To this end, a solid dry polymer electrolyte and a LiXFePO4 counter-electrode were used to enable reversible electrochemical lithium insertion into WO3, modifying the refractive index of the WO3 dielectric media and thereby the resonant response of the gap plasmon. Using this solid-state electrochemical device, we demonstrate continuous tuning of the spectral response by up to 100 nm in visible wavelengths within 10 seconds, giving rise to drastic color changes. Such spectral tuning is obtained with <1 V switching voltages, non-volatile behavior, high reversibility, and the ability to address individual pixels, and provides a powerful platform for displays, sensors, and other actively-tunable plasmonic devices.