March 2021
Abstracts of the QSIT/Quantum Center, ETH Zurich Lunch Seminar, Thursday, March 4, 2021
Polaritonic nonlocality in ultra-strongly coupled systems
Shima Rajabali – Quantum Optoelectronics Group (Faist group), ETH Zurich
Increasing the light-matter coupling strength is attracting much interest in quantum systems. A higher coupling rate results in higher performance in electro-optic devices and allows the study of exotic predicted phenomena such as virtual photons in the ground state of the system. In this work, we show that theoretically, there is a physical limit to the increase of light-matter coupling by reducing the resonator’s modal volume. We prove this theory with an experimental system in which some of the highest light-matter coupling ratios (Ω⁄ω) are obtained; A system that consists of metasurfaces of metallic THz resonators coupled to Landau level transitions in a semiconductor quantum well. We show that resonators with sub-micrometer gaps can excite high-momentum magneto plasmons that cause non-local polaritonic effects such as broadening or partial disappearance of the Polaritonic modes. Due to these effects, the coupling rate cannot be calculated for a defined mode and the system enters a regime of bound-to-continuum non-perturbative coupling. Furthermore, in this new regime, some high-order non-linear features emerge that are signatures of the local breaking of Kohn’s theorem due to the extreme subwavelength confinement.
Phononic properties of nanowire heterostructures
Diego de Matteis – Nanophononics group (Zardo group), University of Basel
Nanowires (NWs) recently gained interest as an exciting platform for a wide range of fields. To name a few, lasing, single photon emission, solar cell functionality and peculiar thermal transport have been reported [1-4]. The possibility of fine-tuning the synthesis of NWs to smaller and smaller scales allows control over more and more phenomena. One important achievement in this sense is the creation of heterostructures: alternation of different materials or crystal phases along the NW growth axis (axial heterostructures) or of different materials around one another in a core-shell fashion (radial heterostructures).
Our research is focused around nanostructured materials, with the aim of gaining a better understanding and control of their vibrational properties. In this talk we will present some of our recent studies. In particular, we will discuss the investigation of the lattice dynamics of core-shell GaAs-SixGe1-x alloy NWs by μ-Raman spectroscopy [5]. In this case, the heterostructuring allows the formation of 2H SixGe1-x compounds with a direct band-gap, otherwise unattainable in bulk form [6]. We have also exposed the effect of a peculiar type of structural defects in core-shell GaAs-2HGe NWs on their Raman spectrum and established a route to quantify the amount of defects in a non-destructive way.
Finally, we will discuss our work on superlattices, which offer an interesting way to tailor the phonon dispersion of the materials [7]. We aim at investigating the spatial and temporal decay of coherent phonons with ultra-fast optical and Raman techniques.
References
[1] Feng et al., Nano Lett., 13, 1, 272-275, 2013
[2] Babinec et al., Nat. Nanotechnol., 5, 195-199, 2010
[3] Garnett et al., Annu. Rev. Mater. Res., 41, 269-295, 2011
[4] Vakulov et al., Nano Lett., 20, 4, 2703-1709, 2020
[5] de Matteis et al., ACS Nano, 14, 6845-6856, 2020
[6] Fadaly et al., Nature, 580, 205-209, 2020
[7] De Luca et al., Nano Lett., 19, 7, 4702-4711, 2019