Jan 30, 2020 3:45 pm - Nielsen Hall 170 (Neal F. Lane Auditorium) - Colloquium
Thomas G. Folland - Vanderbilt University
Infrared Polaritonics: Coupling Long Wavelength Light to Quantum Systems

Generating, manipulating and detecting quantum states is a key challenge for practical quantum computing and sensing. Photons make a natural choice for such efforts, as their relatively weak interaction with the surrounding environment enables them to maintain quantum coherence. Whilst microwaves and visible/near-infrared light have been widely used to probe quantum systems, the mid-infrared to terahertz region of the electromagnetic spectrum (wavelength of 3-300 microns) could offer a host of advantages. Optically active transitions in the infrared (including phonons and intraband states) often have relatively strong dipole moments and optical non-linearities, and can be controlled through multiple techniques. However, it is extremely challenging to explore these phenomena due to the length-scale mis-match between long wavelength infrared light and quantum systems. To overcome this mis-match and achieve strong, coherent interactions, we can turn to surface polaritons. These quasiparticles occur when light couples to coherently oscillating charges in a material, commonly electrons or polar phonons. Surface polaritons take the form of deeply sub-wavelength evanescent waves that propagate on surface of a polaritonic medium, offering a platform to couple to an adjacent material. 

In this colloquium I will demonstrate methods for using surface polaritons present in bulk semiconductors and layered 2D materials to produce coherent coupling to different systems. First, I will introduce the materials and properties that allow the formation of polaritons in the mid infrared to terahertz. In particular, I will discuss recent work on polaritons in 2D materials including graphene, hexagonal boron nitride, and molybdenum trioxide. The highly anisotropic crystal structure of these materials imparts important consequences for the optical and polaritonic properties. Additionally, 2D materials can be transferred onto arbitrary substrates or devices, enabling them to be exploited for new types of coupled systems unachievable with traditional epitaxy. These include frequency tunable semiconductor lasers, reconfigurable nanoscale optics, and combined surface polariton-vibrational polariton systems. Whilst these demonstrations of coupling are classical in nature, these experiments demonstrate a route towards exploiting quantum phenomena, as well as demonstrating platforms towards advanced infrared optical components and devices.