Fabrication of Small-Pitch Gratings for Smith-Purcell Radiation from Low-energy Electrons

Swift-moving electrons carry evanescent near-field, which can be coupled to far-field radiation when the electrons move closer to a periodic structure and in parallel to the periodic structure plane. This effect was named after Smith and Purcell, following their first experimental demonstration of the effect. The wavelength of Smith-Purcell radiation depends on the grating pitch and the electron energy. Here, we demonstrate Smith-Purcell radiation in the optical regime by using gratings with 50-60 nm pitch and electrons with 1.5-6 keV kinetic energy. These results have potential applications in tunable nanoscale light sources.

The gratings were fabricated on gold-coated silicon substrates. The 200-nm-thick gold coating layer was used to suppress cathodoluminescence from silicon. The grating patterns were defined using electron beam lithography in PMMA resist, followed by 0 degrees C cold development in 3:1 IPA:MIBK. 20 nm gold was then deposited via electron-beam evaporation and lifted- off in hot NMP.

To measure Smith-Purcell radiation, the grating samples were mounted inside a modified SEM with an optical attachment to collect the radiated light and measure its spectrum. Electrons with 1.5-6 keV kinetic energy were used to induce the Smith-Purcell radiation. Figure 2 shows the measured Smith-Purcell radiation spectra from a 50-nm-pitch grating using electron beams with different kinetic energies. The peaks of the radiation spectra match well with the theoretical predictions (vertical dashed lines). We demonstrate the Smith-Purcell radiation wavelength decreases as we increase the electron kinetic energy or decrease the grating pitch.