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Understanding and controlling collective quantum phenomena
In correlated electron materials, electrons have a tendency to organize themselves at low temperature, giving rise to exotic quantum orders with remakable properties, such as high temperature superconductivity and magnetism, for instance. Understanding the origin and the nature of these quantum orders, in particular when they coexist and are intertwinned together, is one of the major challenges of consended matter research nowadays. In the TeraX-lab, we not only try to understand the nature of these collective quantum phenomena, but we also seek to manipulate them coherently (i.e. without destroying these fragile orders) with light.
Phase diagram of some correlated materials showing the coexistence of various electronic orders (SC : superconductivity, CDW/SDW : charge/spin density wave)
Studying such collective phenomena in quantum materials allows to explore fundamental questions in condensed matter physics (out of equilibrium collective quantum phenomena, light induced phase transitions, cooperative effects in quantum systems etc.) and may potentially lead to future technological breakthroughs (ultrafast magnetic information storage, electronic devices with enhanced functionnalities and exotic properties « on demand » etc.)
Collective quantum matter in the (Terahertz !) spotlight
In order to probe and manipulate these quantum orders, we use light, mostly in the Terahertz frequency range (1THz=1012 Hz) as it is resonant with the collective modes of these orders (see below).
Additionally, we explore a wide range of the electromagnetic spectrum depending on the type of excitation and perturbation we wish to achieve : from low energy excitations resonant with collective modes of quantum orders (Terahertz), to phonons (mid-infrared) and up to high energy electronic excitations in the visible range (e.g. charge transfer excitations). This approach gives access to most of the relevant excitations that allow to manipulate the properties of these materials with light.
A large spectrum of excitations/perturbations is made possible depending on the frequency of the photons used, from the Terahertz to the visible range.
For instance, an interesting case of coherent manipulation of THz collective modes corresponds to that of Josephson plasma waves in high-Tc cuprate superconductors. These modes consist in collective plasma waves of Cooper pairs in between the CuO2 planes of superconducting cuprates and are highly nonlinear at THz frequencies (Y. Laplace & A. Cavalleri, Advances in Physics X Vol. 1 , Iss. 3 (2016); S. Rajasekaran & al. Nature Physics, 12, 1012–1016 (2016)). In this context, it was possible to manipulate coherently these collective modes and to utilize them so as to parametrically amplify THz light: this phenomenon is rather unique and potentially of interest for the development of the technology in this frequency range. Indeed, the THz technology remains to date underdeveloped despite the rising interest from the scientific community as it could potentially solve some of nowadays societal challenges.
Artistic view of a Josephson plasma wave propagating between neighbouring CuO2 planes in high-Tc cuprates (J. Harms, MPSD Hamburg)
In our laboratory, we generate THz pulses based on nonlinear optical methods (optial rectification, difference frequency generation). The experimental approach used is pump-probe spectrosopy, where the excitation of the material is realized via an initial intense pulse (pump pulse) and whose ensuing dynamics is measured by a secondary pulse (probe pulse) delayed in time, so as to take snapshots of the state of the system as a function of time.
Romain and Jingwei working on the THz setup
With the financial support of :