Fabrication and Modelling of Three-Dimensional Sub-kelvin Phononic Crystals

We have investigated the fabrication and computational modelling of threedimensional phononic crystals for the observation of full band gaps for thermal phonons at sub-kelvin temperatures. Self-assembled arrays of monodisperse polystyrene nanospheres have been fabricated using a vertical deposition technique. Optimal conditions for increasing crystal domain size and crystalline quality have been studied. In addition, the phononic band structure has been computed using the finite element method for the simple cubic lattice. The dependence of band structure on contact area between spheres has also been studied. For small enough contact area a large band gap is observed, predicting a strong influence on sub-Kelvin thermal transport.peerReviewe

Minimizing Coherent Thermal Conductance by Controlling the Periodicity of Two-Dimensional Phononic Crystals

Periodic hole-array phononic crystals (PnCs) can strongly modify phonon dispersion relations and have been shown to influence thermal conductance coherently, especially at low temperatures where bulk scattering is suppressed. One very important parameter influencing this effect is the period of the structure. Here, we measure the subkelvin thermal conductance of nanofabricated PnCs with identical hole-filling factors but three different periodicities, of 4, 8, and 16μm, using superconducting tunnel-junction thermometry. We find that all the measured samples can suppress thermal conductance by an order of magnitude and have a lower thermal conductance than the previously measured smaller-period 1-μm and 2.4-μm structures. The 8-μm-period PnC gives the lowest thermal conductance of all the above samples and has the lowest specific conductance per unit heater length observed to date in PnCs. In contrast, coherent transport theory predicts that the longest period should have the lowest thermal conductance. Comparison with incoherent simulations suggests that diffusive boundary scattering is likely the mechanism behind the partial breakdown of the coherent theory.peerReviewe

Optimization of a Ge₂Sb₂Te₅-Based Electrically Tunable Phase-Change Thermal Emitter for Dynamic Thermal Camouflage

Controlling infrared thermal radiations can significantly improve the environmental adaptability of targets and has attracted increasing attention in the field of thermal camouflage. Thermal emitters based on Ge2Sb2Te5 (GST) can flexibly change their radiation energy by controlling the reversible phase transition of GST, which possesses fast switching speed and low power consumption. However, the feasibility of the dynamic regulation of GST emitters lacks experimental and simulation verification. In this paper, we propose an electrically tunable thermal emitter consisting of a metal–insulator–metal plasmonic metasurface based on GST. Both optical and thermal simulations are conducted to optimize the structural parameters of the GST emitter. The results indicate that this emitter possesses large emissivity tunability, wide incident angle, polarization insensitivity, phase-transition feasibility, and dynamic thermal camouflage capability. Therefore, this work proposes a reliable optimization method to design viable GST-based thermal emitters. Moreover, it provides theoretical support for the practical application of phase-change materials in dynamic infrared thermal camouflage technology

Intregrating metallic wiring with three-dimensional polystyrene colloidal crystals using electron-beam lithography and three-dimensional laser lithography

We demonstrate a method to fabricate narrow, down to a few micron wide metallic leads on top of a three-dimensional colloidal crystal self-assembled from polystyrene (PS) nanospheres of diameter 260 nm, using electron-beam lithography. This fabrication is not straightforward due to the fact that PS nanospheres cannot usually survive the harsh chemical treatments required in the development and lift-off steps of electron-beam lithography. We solve this problem by increasing the chemical resistance of the PS nanospheres using an additional electron-beam irradiation step, which allows the spheres to retain their shape and their self-assembled structure, even after baking to a temperature of 160 degrees C, the exposure to the resist developer and the exposure to acetone, all of which are required for the electron-beam lithography step. Moreover, we show that by depositing an aluminum oxide capping layer on top of the colloidal crystal after the e-beam irradiation, the surface is smooth enough so that continuous metal wiring can be deposited by the electron-beam lithography. Finally, we also demonstrate a way to self-assemble PS colloidal crystals into a microscale container, which was fabricated using direct-write three-dimensional laser-lithography. Metallic wiring was also successfully integrated with the combination of a container structure and a PS colloidal crystal. Our goal is to make a device for studies of thermal transport in 3D phononic crystals, but other phononic or photonic crystal applications could also be envisioned. Keywords: self-assembly, colloidal crystal, cross-linking, electron-beam lithography, three-dimensional lithography, phononic crystal, photonic crystal.peerReviewe