Multilayer dielectric reflector using low-index nanolattices.

Dielectric mirrors based on Bragg reflection and photonic crystals have broad application in controlling light reflection with low optical losses. One key parameter in the design of these optical multilayers is the refractive index contrast, which controls the reflector performance. This work reports the demonstration of a high-reflectivity multilayer photonic reflector that consists of alternating layers of TiO2 films and nanolattices with low refractive index. The use of nanolattices enables high-index contrast between the high- and low-index layers, allowing high reflectivity with fewer layers. The broadband reflectance of the nanolattice reflectors with one to three layers has been characterized with peak reflectance of 91.9% at 527 nm and agrees well with theoretical optical models. The high-index contrast induced by the nanolattice layer enables a normalize reflectance band of Δλ/λo of 43.6%, the broadest demonstrated to date. The proposed nanolattice reflectors can find applications in nanophotonics, radiative cooling, and thermal insulation.

Precise control of the optical refractive index in nanolattices.

Recent developments in photonic devices, light field display, and wearable electronics have resulted from a competitive development toward new technologies to improve the user experience in the field of optics. These advances can be attributed to the rise of nanophotonics and meta-surfaces, which can be designed to manipulate light more efficiently. In these elements the performance scales are favorable to the index contrast, making the use of low-index material important. In this research, we examine the precise control of refractive indices of a low-index nanolattice material. This approach employs three-dimensional (3D) lithography and atomic layer deposition (ALD), allowing for precise control of the nanolattice geometry and its refractive index. The refractive indices of the fabricated nanolattices are characterized using spectroscopic ellipsometry and agree well with models based on effective medium theory. By controlling the unit-cell geometry by the exposure conditions and the shell thickness by the ALD process, the effective index of the nanolattice film can be precisely controlled to as low as 5 × 10-4. The proposed index control technique opens a gamut of opportunities and enables better performance in nanophotonic elements used in displays and other integrated devices.