Polymer-nanoparticle composites for thermally-tunable optical resonator devices

Progressing miniaturization and integration of optical systems requires optical devices that can be fabricated in a wafer-level process. Previously, we experimentally demonstrated a tunable optical aperture based on segmented... [ view full abstract ]

Progressing miniaturization and integration of optical systems requires optical devices that can be fabricated in a wafer-level process. Previously, we experimentally demonstrated a tunable optical aperture based on segmented thin-film resonators [1]. Aperture opening and closing is achieved by thermally tuning polymer resonator segments from on-resonance to off-resonance. Using standard lithography for defining the Joule heating elements, arbitrarily-shaped tunable segments can be achieved. Due to the small thermal conductivity of polydimethylsiloxane (PDMS) of 0.15(W/m/K) tuning speeds are limited. We chose PDMS for its high thermal expansion coefficient of 340e-6(1/K). Here, we propose the use of polymer-nanoparticle composite layers for the resonators combining materials with a high thermal conductivity and a high thermal expansion coefficient.

Titanium dioxide (TiO2) offers a much higher thermal conductivity of around 8.5(W/m/K) compared to PDMS. In previous experiments we demonstrated that blending of a polymer with TiO2 nanoparticles (diameter ~35nm) allows for refractive index tuning with negligible scattering losses in the 500nm to 700nm wavelength range [2]. Here, we present a theoretical study of the performance of tunable apertures incorporating PDMS blended with TiO2 nanoparticles. In finite-element-method (FEM) simulations a 10-µm composite layer with 30%vol TiO2 nanoparticles and 70%vol PDMS on a glass substrate is evaluated. The effective medium is modelled with a thermal conductivity of 3.1(W/m/K) and a thermal expansion coefficient of 240e-6(1/K). The attached figure presents a schematic of the simulated device section. Simulation results are given of the temperature distribution 200µs after setting a 2K temperature increase at the left-resonator bottom interface. Clearly, the resonator device with the nanoparticle-polymer composite layer is fully actuated. The PDMS-only device on the other hand shows a temperature gradient across the cavity layer. As the composite layer combines a high thermal conductivity with a high thermal expansion coefficient ten times faster response times are predicted.

[1] H. Block, P. Metz, J. Adam, M. Gerken, “Thermally tunable optical aperture based on a segmented thin-film resonator,” Proc. SPIE 9130, Micro-Optics 2014, 913002 (2014).
[2] A. Pradana, C. Kluge, M. Gerken, “Tailoring the refractive index of nanoimprint resist by blending with TiO2 nanoparticles,” Opt. Mater. Express 4(2), 329-337 (2014).