Sub-wavelength effects in a free electron laser oscillator

Previous simulation studies of cavity based free electron lasers (FELs) have utilised models which average the optical field in the FEL interaction over an integer number of radiation wavelengths. In this paper, two unaveraged simulation codes, OPC and Puffin, are combined to enable modelling, for the first time, of a cavity based FEL at the sub-wavelength scale. This enables modelling of effects such as coherent spontaneous emission from the electron beam and sub-wavelength cavity length detuning. A cavity FEL operating in the mid-infrared is modelled and it is shown that, for small sub-wavelength cavity detunings, the FEL can preferentially lase at the third harmonic of the fundamental FEL wavelength. This novel result suggests other modes of operation may be possible and opens up cavity-based FEL operation to investigation of further, potentially useful, modes of operation.