Thermal nanophysics and superconducting devices

The group has currently three main research directions:

-Nanoscale thermal transport, especially focusing on phononic crystals and near-field transport

-Development of superconducting materials and devices, especially ultrasensitive superconducting radiation detectors and superconducting tunnel junctions for quantum technology

-Utilizing novel nanofabrication and imaging techniques for interdisciplinary projects, such as nanoscale biological imaging with helium ion microscope (HIM) and 3D laser lithography

-We are open to collaborative projects in the above areas both with academic and corporate partners. We could also facilitate research-for-hire for the use of the NSC Nanofab (clean room infra) facilities

Nanoscale phononic crystal device that can be used to reduce thermal conductance by orders of magnitude

Research group type

Academy Project

Research group

Core fields of research

Basic natural phenomena and mathematical thinking

Research areas

Nanoscience Center

Materials physics

Second quantum revolution

Faculty

Faculty of Mathematics and Science

Department

Department of Physics

Research group description

Research direction I: Nanoscale thermal transport

Past decades have seen a tremendous increase of interest in the physics and possible technological applications of fabricated structures in the nanoscale. A lot of it has concentrated on the electronic and optical properties of these devices. On the other hand, we are focusing on the not as widely studied thermal properties (controlling thermal conductivity and heat capacity), funded for a long time by the Research Council of Finland. Applications can be found for example in the development of ultra-sensitive bolometric low-temperature radiation detectors, for which the sensitivity is ultimately limited by the thermal coupling to the substrate (phonon noise), and in the improvement of the efficiency of thermoelectric power generation. The links below describe in more detail the research topics in this direction:

Electron-phonon interaction

Research direction II: Development of superconducting devices and materials for quantum technology

We have a strong history in superconducting device research. First area is the development of bolometric radiation detectors, particularly using transition edge sensors (TES) and superconductor-ferromagnet thermoelectric junctions, where the focus has been in single quantum X-ray detection. The detector research has been funded by the European Space Agency (ESA), Business Finland and EU (FET-open). Another focus area is superconducting tunnel junction device physics and its applications for quantum technology, including the use of novel materials and techniques. Examples are the use of superconducting nitrides, grown in-house with pulsed laser deposition (PLD), and He ion-beam direct writing of Josephson junction devices. We are a member of the Finnish Quantum Flagship.

Research direction III: Novel nanofabrication and imaging techniques for interdisciplinary projects

Our group has strong expertise in applying the most modern nanofabrication and imaging techniques to various applications, including areas outside of physics. In addition to the standard 2D electron-beam and photolithography, we are experts in 3D direct-write laser lithography (Nanoscribe) and helium and neon ion-beam lithography and milling. Examples of our application areas for 3D lithography are microfluidics, thermal transport and micro-optics. In imaging, we have particular expertise in helium ion microscopy (HIM), for example in collaboration with biologists.

Complete tunneling of acoustic waves between piezoelectric crystals
Zhuoran Geng and Ilari J. Maasilta

In this paper, we show that in certain situations a sound wave can jump or “tunnel” fully (100 % efficiency) without any reflections across a vacuum gap between two solids, if the materials in question are piezoelectric. In such materials, vibrations (sound waves) produce an electrical response, as well, and since an electric field can exist in vacuum, it can transmit the sound waves across. The requirement is that the size of the gap is smaller than the wavelength of the sound wave. This effect works not only in audio range of frequencies (Hz-kHz), but also in ultrasound (MHz) and hypersound (GHz) frequencies, as long as the vacuum gap is made smaller as the frequencies increase.

Effective medium theory for the low-temperature heat capacity of a metasolid plate
Tuomas A. Puurtinen, Ilari J. Maasilta

This work developed a theory for a thermal metamaterial, which can be used to strongly modify the phononic heat capacity. The derived analytic theory describes a nanoscale periodic holey plate with the help of effective elastic parameters. It describes the thermal response in the temperature range where the holey material looks continuous, but with modified elastic properties and density because of the structuring. The modified elastic properties in turn change the vibrational spectrum of the material (phonon energies), which directly affects the capacity to store heat.

Controlling thermal conductance using three-dimensional phononic crystals
Samuli Heiskanen, Tuomas A. Puurtinen, and Ilari J. Maasilta

We fabricated for the first time 3D phononic crystals with integrated heater and thermometer structures using the 3D direct laser write lithography, and measured the thermal conduction of such structure at low temperatures. A puzzling enhancement of conduction was observed, which is not theoretically understood yet.

Imaging Bacterial Colonies and Phage–Bacterium Interaction at Sub-Nanometer Resolution Using Helium-Ion Microscopy
Miika Leppänen, Lotta-Riina Sundberg, Elina Laanto, Gabriel Magno de Freitas Almeida, Petri Papponen and Ilari J. Maasilta

This paper, in collaboration with the biology department, demonstrates the first helium ion microscope (HIM) imaging of bacterial colonies interacting with viruses of bacteria, or bacteriophages. Furthermore, by controlling the He beam dose or by using heavier Ne ions, we milled out material in the samples, allowing for subsurface imaging and in situ sectioning. HIM seems to offer great opportunities to advance the studies of microbial imaging.

Broadband Ultrahigh-Resolution Spectroscopy of Particle-Induced X Rays: Extending the Limits of Nondestructive Analysis
M. R. J. Palosaari, M. Käyhkö, K. M. Kinnunen, M. Laitinen, J. Julin, J. Malm, T. Sajavaara, W. B. Doriese, J. Fowler, C. Reintsema, D. Swetz, D. Schmidt, J. N. Ullom, and I. J. Maasilta

PIXE spectroscopy is a widely used accelerator-based method for elemental analysis, down to trace level impurities. This work, performed in collaboration with the JyU Accelerator laboratory and NIST Boulder, demonstrates for the first time that practical PIXE spectroscopy can be performed with a cryogenic X-ray transition-edge sensors array, with over an order of magnitude improved energy resolution to previous state-of-the-art. We show that the technique offers great promise in the elemental analysis of thin-film and bulk samples, especially in the difficult cases where tens of different elements with nearly overlapping emission lines have to be identified down to trace concentrations. In the demonstrated cases, standard silicon drift detectors could not resolve all elements.

Engineering thermal conductance using a two-dimensional phononic crystal
Nobuyuki Zen, Tuomas A. Puurtinen, Tero J. Isotalo, Saumyadip Chaudhuri, and Ilari J. Maasilta

Here we show for the first time that phonon thermal conductance can be strongly modified coherently, by orders of magnitude, using two-dimensional hole array phononic crystals. Calculations performed using finite element method agreed with the experiment and demonstrated that the effect arises from strong reduction of the group velocities of the modified phonon modes.

Research group

Group leaders

Ilari Maasilta

Professor, Vice Head of Department

Members

External members

Ilmo Räisänen

Doctoral student

Toivo Hakanen

Master's student

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