Fig. 4: Tunable SP emission spectrum.
a Voltage dependence of the SP emission spectrum (colored dots) for a RIET device. For clarity, data at the two low voltages 1.7 V and 1.8 V are amplified by a factor of 2 and 4, respectively. Solid colored lines are the respective SP radiation power spectrum calculated based on the model described in the text, where the same scaling factors were used to match the experimental results. Black vertical arrows indicate the respective cutoff frequency \({v}_{{{\max }}}\) with the quantum relation \(h{v}_{{{\max }}}=e{V}_{{\rm{b}}}\). Note that the small portion of emission with \(v \;> \; e{V}_{{\rm{b}}}/h\) could be attributed to multielectron processes or hot-electron light emission50,51. b Calculated vacuum source efficiency \({\gamma }_{{\rm{inel}}}^{0}\) (color lines) and SP radiation enhancement \({\rho }_{{\rm{opt}},{\rm{avg}}}/{\rho }_{0}\times {\eta }_{{\rm{SP}},{\rm{avg}}}\) (black line) at various voltages. Here, \({\rho }_{{\rm{opt}},{\rm{avg}}}\) and \({\eta }_{{\rm{SP}},{\rm{avg}}}\) were obtained under a dipole-source excitation with different polarization states and spatial positions, see Supplementary Note 3 for details.
