High repetition rate source of narrowband extreme-ultraviolet harmonics for time-resolved ARPES*

We present a highly efficient table-top source of extreme ultraviolet (XUV) femtosecond pulses operating at 50-kHz repetition rate. A bright XUV source flux of 3x10$^{\mathrm{13}}$ photons/s is generated at 22.3 eV by driving high-harmonic generation with the ultraviolet second-harmonic of a laser amplifier focused tightly into Kr gas. The conversion efficiency (5x10$^{\mathrm{-5}})$ is enhanced by two orders-of-magnitude in this cascaded scheme, exceeding dipole wavelength scaling and evidencing enhanced phase matching conditions as confirmed by simulations. Importantly, the spectral structure enables the direct, high-contrast isolation of a single, narrowband harmonic with 72 meV linewidth. The high repetition rate, narrow bandwidth, and high flux (10$^{\mathrm{11}}$-10$^{\mathrm{12}}$ ph/s at the sample) of this source is ideal for time-resolved photoemission or nanoscale imaging. First applications in time- and angle-resolved photoemission (trARPES) will be discussed.