Within the Landau paradigm, phases of matter are distinguished by their symmetries. There exists however a finer level of classification based on the topological entanglement of their electronic wavefunctions. This gives rise to new quantum phases such as topological insulators (TIs), Dirac and Weyl semimetals (DSMs, WSMs) which are the natural 3-Dimensional extension of graphene. In DSMs conduction and valence bands cross near the Fermi energy forming 3D degenerate linearly dispersive electronic bands (Dirac cones). This degeneracy is removed in WSMs, due to broken inversion or time reversal symmetry, and each Dirac cone splits into two cones separated in energy or in momentum. The low-energy electronic excitations in WSMs are Weyl electrons which are massless chiral fermions considered a building block of quantum field theory. Although Weyl fermion was intended as a model of elementary particles, in nearly 90 years, no candidate Weyl fermions have ever been established in high-energy experiments, so WSMs represents the first example of Weyl physics.
DSMs and WSMs show exotic electronic properties as highly conductive, non-dissipative surface states and extremely non linear optical response functions stemming from geometric (Berry) phase effect. In this project, we plan to measure the linear and unlinear optical properties of high-quality Weyl and Dirac thin-films from microwaves (1 GHz) to ultraviolet (1000 THz). These high-quality films will be obtained from already established collaborations (S. Oh, Rudgers University, USA; A. Molle, CNR-IMM, Italy). In particular, we will investigate the electromagnetic response of WTe2 and LaAlGe Weyl and Cd3As2 Dirac semimetals and their exploitation for photonic and plasmonic devices.
The project, strongly innovative in the international scenario, is based on a well composed group combining recognized experts in topological materials, microwaves, terahertz, infrared spectroscopies and non linear optics.
This project is aimed at experimentally investigating and using in smart THz devices new classes of non trivial topological materials, in particular Dirac and Weyl semimetals. In order to get this ambitious goal, a strategy plan based on distinct but mutually reinforcing scientific objectives will be adopted:
1) Scientific excellence;
2) Moving physics and material science forward;
3) Training;
4) Exploitation;
Scientific Excellence.
WSMs and DSMs were experimentally reported on 2015 and these new low-dimensional materials open possible path to engineer topological system on demand. Nonetheless, no achievement has been so far reported on the integration of these highly advanced systems on technology forefronts. Our goal is to focus on WSM/DSM emerging materials to overcome intrinsic limitation of more conventional TIs, investigating prototype systems like WT2, LaAlGe and Cd3As2 thin films and proving a device application in lab for THz photonics. This ambition is solidly sustained by the PI as leading scientist in THz plasmonics and photonics based on TIs and 2D materials and the interconnected competences of the other group members. Their expertise guarantees the proper approach to the study.
Moving research forward.
This project is at the frontier in the field of topological materials. The step forward in the present proposal is to move TM from outstanding concept to a concrete exploitation in the highly demanding field of THz photonics. Indeed, THz harmonic generation and photogalvanic effect will be produced and used for frequency converting devices (WP2). Efficiency could be increased by tuning the Fermi energy by electrostatic gating and then enhancing the light-matter coupling. The whole results should give rise to a new generation of photonics devices in which the 3D nature of Weyl/Dirac electrons provides a step forward with respect to existing materials.
Dissemination.
We have in mind a dissemination effort at two different levels. One intended for rigorous peer-reviewed publications on high-impact journals and participations to material science and photonics international conferences. Second, the project is also based on young researchers and contains the training of a post-doc student to be hired through the funding request.
Exploitation.
It is anticipated that patentable ideas will be generated within the project. Indeed, WP2 proposes to study THz harmonic generation that should be used for a highly efficient photonic device. A further possibility is to rectify THz radiation in a DC current through the chiral anomaly effect in Weyl films (photgalvanic effect). Possible patent proposals will be exploited following the rules of Sapienza.