Science

Quantum Materials

THz Domain Control

Using domains to store information is the cornerstone of long term data storage. Most technology is based on magnetic domains, where the orientation of the internal magnetic field is used to encode data. Reading and writing to a magnetic domain requires a combination of magnetic and electrical pulses. However, it would be highly advantageous to achieve domain control by purely electronic means. We have been investigating electronic control of an alternative type of domain which might offer new opportunities for future devices.
Orbital ordering is a phenomenon found in a class of materials known as the manganites in which the electronic levels of the manganese ions form ordered patterns. These orbitals form stripes that run along the diagonals of the crystallographic axes, with different domains corresponding to stripes running along different directions.
We have been using the Infra-red free electron laser called FELBE in Dresden to generate high-intensity polarized electric fields that oscillate at THz frequencies. We recently demonstrated the polarization of the THz field can be used to control the orbital domain orientation. This electronic control arises due to the unique electronic properties found in the manganites and show that new materials present new opportunities for technology.

Publications

THz Field Control of In-Plane Orbital Order in La0.5Sr1.5MnO4
Timothy A. Miller, Ravindra W. Chhajlany, Luca Tagliacozzo, Bertram Green, Sergey Kovalev, Dharmalingam Prabhakaran, Maciej Lewenstein, Michael Gensch, Simon Wall
Nature Communications 6, 8175 (2015) - Open access publication
Light control of orbital domains: case of the prototypical manganite La0.5Sr1.5MnO4
T. Miller, M. Gensch, S. Wall
Physica Scripta 91, 124002 (2016)
High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter
B. Green et al.
Scientific Reports 6 22256 (2016)

Collaborators

Michael Gensch - HZDR, Dresden, Germany
Maciej Lewenstein - ICFO

Sketch of the enhanced coulomb interaction between electrons when an electric field is applied perpendicular to the domain stripe.

Magnetic Dynamics

Anti-ferromagnetic Spin Dynamics

Femto-magnetism is the study and control of magnetic materials with femtosecond light pulses. The goal is to understand the processes that determine how fast magnetic systems can respond to perturbation. These timescales are dictated by the coupling of the magnetic state to the electronic and structural degrees of freedom and by how the system is excited.
Most materials studied to date have focused on systems with a net magnetic moment, which makes monitoring the magnetic state simple through the magneto-optical Kerr effect. However, as the magnetic system carries angular momentum, this momentum has to be transferred to other degrees of freedom, which can slow the evolution of the system down.
We study anti-ferromagnetic systems. These systems, although magnetically ordered, do not have a net magnetic moment. Thus they can potentially demagnetize much faster than ferromagnets, however, probing the magnetic state can be difficult.
We use non-linear optics to measure the magnetic state, which is capable of measuring antiferromagnetic order and the symmetry of the crystal. We study changes in the non-linear signal to reveal how the magnetism melts in response to photoexcitation with different colour pulses and have recently shown that we can control the coupling to the structural degrees of freedom by controlling the colour of the light.

Publications

Resonant optical control of the structural distortions that drive ultrafast demagnetization in Cr2O3
V. G. Sala, S. Dal Conte, T. A. Miller, D. Viola, E. Luppi, V. Veniard, G. Cerullo, S. Wall
Physical Review B 94, 014430 (2016)
Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4
M. P. M. Dean, Y. Cao, X. Liu, S. Wall, D. Zhu, R. Mankowsky, V. Thampy, X. M. Chen, J. G. Vale, D. Casa, J. Kim, A. H. Said, P. Juhas, R. Alonso-Mori, J. M. Glownia, A. Robert, J. Robinson, M. Sikorski, S. Song, M. Kozina, H. Lemke, L. Patthey, S. Owada, T. Katayama, M. Yabashi, Y. Tanaka, T. Togashi, J. Liu, C. Rayan Serrao, B. J. Kim, L. Huber, C.-L. Chang, D. F. McMorrow, M. Först and J. P. Hill
Nature Materials 15 601 (2016)

Collaborators

Giulio Cerullo - Politecnico di Milano

Polarization dependence of the second harmonic singal from Cr2O3 above (green) and below (Blue) the anti-ferromagnetic phase transition temperature.

Phase Change Materials

Phase change materials are found inside rewritable optical storage media like DVDs and BluRay. These materials have a unique capability, their structural state can be modified by light which drastically changes the optical and electronic properties. This contrast can be used encode data.

We have been investigating how light is able to control the state of the material and to find the ultimate speed limit at which these materials can be switched. We have used a combination of femtosecond optical spectroscopy to measure the change in the optical properties and femtosecond electron diffraction (in collaboration with the group of Ralph Ernstorfer in Berlin) to measure the lattice properties.

We find that the optical properties can be controlled on the femtosecond timescale, however the structure responds significantly slower, on a picosecond timescale due to slow electron-phonon coupling. This work places important limitations on the switching speeds of phase change materials for data storage purposes, but present exciting new opportunities for optical modulators that operate at very speeds.

More recently, in collaboration with Valerio Pruneri at ICFO, we have been investigating nano-structured phase change materials can be exploited to produce high-speed optical modulators at telecommunications wavelengths and in collaboration with Robert Simpson in Singapore, we have have been investigating how heterostructures can be used to reduce the threshold energy required for optical switching.

Publications

Time-domain separation of optical properties from structural transitions in resonantly bonded materials
Lutz Waldecker, Timothy A. Miller, Miquel Rude, Roman Bertoni, Johann Osmond, Valerio Pruneri, Robert Simpson, Ralph Ernstorfer, Simon Wall
Nature Materials 14 991 (2015)
Open access version (arXiv)
Ultrafast broadband tuning of resonant optical nanostructures using phase change materials
Miquel Rudé, Vahagn Mkhitaryan, Arif E. Cetin, Timothy A. Miller, Albert Carrilero, Simon Wall, F. Javier García de Abajo, Hatice Altug, Valerio Pruneri
Advanced Optical Materials 4 1060 (2016)
Strain engineered interfacial phase change materials: diffusive atomic switches in 2D
J Kalikka, X. Zhou, E. Dilcher, S. Wall, J. Li, R. E. Simpson
Nature Communications 7, 119838 (2016)
The Ultrafast Optical Response of the Amorphous and Crystalline States of the Phase Change Material Ge2Sb2Te5
T. A. Miller, M. Rudé, V. Pruneri, S. Wall
Physical Review B 94, 024301 (2016)

Collaborators

Ralph Ernstorfer - structural dynamics group - Fritz Haber Institute, Berlin, Germany
Valerio Pruneri - optoelectronic group - ICFO
Robert Simpson - Singapore university of technology and design
Watch the video summary of our Nature Materials paper.

Lens-less imaging

Details coming soon!