Coherent superradiant terahertz generation mediated by multidimensional lattices

Context: Producing stable, single tone terahertz (0.1-10 THz) radiation at power levels demanded by applications is a long-standing physics challenge. The “THz gap” arises from the dearth of suitable THz quantum transitions, and the limitations in power and efficiency of electronic methods. Solid state techniques, often requiring complex chains of multipliers, are limited to  hundreds of milliwatts while - with the exception of  very large, high voltage devices dependent on precise and high magnetic fields - the miniaturisation of vacuum electronics at THz frequencies restricts attainable power.
Application Impact and Benefits: The urgent need to bridge this gap is motivated by a wide range of important applications including: instrumentation for techniques in structural biology and drug design (DNP-NMR/ESR), drive sources and turbulence diagnostics for fusion energy, non-destructive testing of composites, communications and advanced high frequency radar systems.
Terahertz waves at the lower end of the spectrum are the only realistic way to deliver energy and particularly to drive current in magnetically confined fusion plasmas. As these develop from research systems to real energy production systems, both the density and the magnetic field appear likely to increase. This increases the frequency requirement for the drive beam. The target power, efficiency and frequencies demanded by fusion are rapidly-approaching technological limits. New steady-state THz sources based on concepts underpinning this work will support the UK initiative to put fusion energy on this grid by 2040.
Challenge: The generation of powerful THz signals using conventional electron beam-driven oscillators is hindered by the requirement for the transverse interaction region size to be comparable to the radiation wavelength ?.  Increasing the interaction space size to enhance the power capabilities (by maintaining a given power density) dramatically increases the number of allowed modes within the system, with undesirable consequences (parasitic mode excitation, loss of efficiency and coherence).
This project exploits multi-dimensional interaction structures composed of periodic subwavelength features. Similar to metamaterials, known for their unique properties, these structures can radically modify the electromagnetic waves to form a dominant coupled "supermode" at any desired frequency
Cherenkov radiation can be generated via the interaction between this supermode and the axial motion of energetic electrons. The relative propagation of the electromagnetic wave and the electrons can be controlled using multidimensional structures, with the potential for superradiant emission.  In superradiance, the wave is initiated by the leading edge of the beam before propagating back through the transient pulse of electrons, growing and propagating in a soliton-like manner, and compressing the energy into a signal substantially shorter in duration than the driving electron pulse.
The novelty lies in the use of the grossly overmoded multidimensional  structures which offer superior frequency control and tuning of the slippage (relative phase velocities of the electrons and electromagnetic wave) with the potential for unprecedented peak power .
Understanding the coupling dynamics as the interaction conditions change from the steady-state to the transient regime, represents an important step towards closing the THz gap, offering substantive environmental, economic and societal benefits.
Aims and Objectives: A self-consistent theory of the complicated electromagnetic and electrodynamic coupling,  informed by and compared with numerical eigenmode calculations and full-wave simulations will be developed.  Experimental measurements of THz structures made using ultra-high-precision machining (RAL) will be undertaken at the EPSRC THz measurement facility to benchmark the theory and simulations.