Professor Joshua D. Caldwell
Mechanical Engineering Dept., Vanderbilt University
The field of nanophotonics is based on the ability to confine light to nanoscale dimensions. Within the mid- to far-infrared, such confinement inherently implies overcoming the diffraction limit due to the long free-space wavelengths. Through the implementation of polaritons one can overcome the diffraction limit through the formation of quasi-particles formed by coupling of coherent oscillating charges with photons. A whole suite of potential polaritons can be realized through careful choice of the charge, with two predominant types being the surface plasmon (free carriers) and phonon (bound charge on ionic lattice) polaritons in the infrared. While each form offers significant advantages, significant restrictions also remain. As such, identifying novel materials with unique optical functionalities and the creation of hybrid materials where the properties and function can be designed are imperative for advanced IR devices to be realized. This talk will discuss recent advancements from our group including low-loss plasmonic conducting oxides for novel infrared sources through hybridization of optical modes, the observation and exploitation of the natural hyperbolic response of hexagonal boron nitride and molybdenum trioxide for on-chip photonics, as well as the implementation of hybridization of polaritonic modes and manipulation of the phonon dispersion and density of states as a means to design infrared nanophotonic materials. Beyond this, methods to improve material lifetime, realize active modulation, control polariton propagation with nanoscale precision and to provide additional functionality will be discussed.
Prof. Joshua Caldwell is the Flowers Family Chancellor Faculty Fellow and Associate Professor of Mechanical Engineering and Director of the Interdisciplinary Materials Science Program at Vanderbilt University. He was awarded his Bachelor of Chemistry from Virginia Tech in 2000 before heading to the University of Florida where he received his PhD in Physical Chemistry in 2004. There he used magnetic resonance methods to investigate electron-nuclear spin coupling within low-dimensional quantum wells and heterostructures. He accepted a postdoctoral fellowship at the Naval Research Laboratory in 2005, using optical spectroscopy as a means of understanding defects within wide-band gap semiconductors. He was transitioned to permanent staff in 2007, where he began work in the field of nanophotonics, investigating coupling phenomena within plasmonic materials. Prof. Caldwell merged his prior work in wide band gap semiconductor materials with his efforts in nanophotonics, leading to his work exploiting undoped, polar dielectric crystals for low-loss, sub-diffractional infrared optics. He is a three-time recipient of the highly competitive NRL Nanoscience Institute grants and was promoted to senior (supervisory) staff at NRL in 2012. He was awarded a sabbatical at the University of Manchester with Prof. Kostya Novoselov in 2013-2014, investigating the use of van der Waals crystals such as hexagonal boron nitride for mid-IR to THz nanophotonics, where he demonstrated the natural hyperbolic response of this material. During his time at NRL he was a 4-time recipient of the Alan Berman Best Pure Science Paper Award and received the Thomas Edison Best Patent Award for his dry transfer technique for 2D materials. In 2017 he accepted a tenured Associate Professorship at Vanderbilt University within the Mechanical Engineering Department. He was elected as a Fellow of the Materials Research Society in 2020 and has published over 160 papers, >7700 citations and 11 patents, with two more pending.