18.6.2021 New nanofabrication method for studying the thermal conductance of three-dimensional phononic crystals (Heiskanen)

Phononic crystals are periodic structures, which alter the flow of vibrational energy or heat and sound through a material. Interest in such structures has grown lately, since the control of thermal transport has become vital for many applications, e.g. ultrasensitive radiation detectors. In his dissertation, Samuli Heiskanen developed a new fabrication method, which allows the fabrication of measurement elements on three-dimensional (3D) structures. Using samples fabricated with this method he showed that 3D phononic crystals can be used for controlling thermal transport.
Published
18.6.2021

The study of the thermal conductance of phononic crystals has so far mostly focused on two-dimensional membrane structures. The dissertation of Samuli Heiskanen is one of the first thermal conductance studies focusing on 3D crystals. In this study, the phononic crystals were fabricated with 3D lithography in which a laser is used to print 3D nanostructures.

Fabrication of tunnel junctions on 3D structures

In the thesis, a fabrication method based on 3D lithography was developed, which allows the fabrication of superconducting tunnel junctions on 3D nanostructures. These junctions can be used to measure temperatures close to absolute zero. This is very important as the thermal conductance of the crystals fabricated in this study needs to be studied at very low temperatures (below 100 mK).

“The developed method produced high quality junctions, which shows that this method can be used even for the fabrication of complex devices”, says Samuli Heiskanen.

The change in thermal conductance depends on the crystal structure

The effect of the phononic crystal on the thermal conductance of a material depends on the structure of the crystal. By changing the structure, the thermal conductance of the material can usually be reduced but with certain structures it is also possible to increase the thermal conductance. Simulations predicted that also the crystals studied in this thesis would reduce the thermal conductance, but unexpectedly measurements showed that the crystals increased the thermal conductance substantially instead.

“This result is very surprising as the two-dimensional crystals studied so far have followed the simulations and reduced the thermal conductance”, Heiskanen highlights.

The research is published in JYU Dissertations series, number 395, Jyväskylä, 2021.
ISBN 978-951-39-8696-4 (PDF), URN:ISBN:978-951-39-8696-4 and ISSN 2489-9003
Link to publication:

M.Sc. Samuli Heiskanen defends his doctoral dissertation "Application of Direct Laser Writing for the Fabrication of Superconducting Tunnel Junctions and Phononic Crystal Structures" at the Ģֱ on Friday 18 of  June, 2021 starting at noon. Opponent is Dr. Olivier Bourgeois (Neel Institut, CNRS, France) and Custos is Professor Ilari Maasilta from  Ģֱ. The doctoral dissertation is held in English.

The audience can follow the dissertation online.
Link to the Zoom Webinar event (Zoom application or Google Chrome web browser recommended):

Phone number to which the audience can present possible additional questions at the end of the event (to the custos): +358 40 805 4098