Velocity-Map Imaging for Emittance Characterization of Field Emitter Arrays

Electron beams of high quality, e.g., low emittance, are of crucial importance for cutting-edge scientific instruments, such as x-ray free electron lasers (XFELs) and ultrafast electron diffraction (UED) setups. A velocity-map-imaging spectrometer is demonstrated to characterize the normalized transverse emittance of photoemitted electron bunches. The spectrometer operated in both, spatial map imaging (SMI) and VMI modes. Therefore, spatial- and velocity-coordinates were recorded independently and quickly. The technique allows for fast complete emittance measurements, within minutes. The two-dimensional (2D) projected velocity distribution images of photoemitted electrons are recorded by the detection system and analyzed to obtain the normalized transverse emittance. With the presented distribution function of the electron photoemission angles a mathematical method is implemented to reconstruct the three-dimensional (3D) velocity distribution curve. As a first example, multiphoton emission from a planar Au surface is studied via irradiation at a glancing angle by intense 45 fs laser pulses at a central wavelength of 800 nm. The reconstructed energy distribution agrees very well with the Berglund-Spicer theory of photoemission. The normalized transverse emittance of the intrinsic electron bunch is characterized to be 128 and 14 nm·rad in X- and Y -directions, respectively.The 500 nm and 75 μm spacing Au-nanorod arrays, with nanorods itself of dimension 100×30 nm, were studied. For the one spaced by 500 nm, the same 4-th order photon emission has been observed as for the planar Au surface. The broad transverse velocity spread is contributed by the electrons photoemitted from the nanorod sidewalls. A patterned electron bunch was observed from the 75 μm spacing Au-nanorod array, each emitted from a single nanorod within the array. In the tunneling regime, upon photoemission, an electron is steered by the laser field resulting in fascinating physics. An energy plateau due to the re-scattering electrons was observed in the tunneling regime. A polarization dependent photoemission study was performed showing a smaller rms-normalized divergence of 0.8 mrad with the laser polarization normal to the sample surface, compared to 1.15 mrad while the polarization is parallel. The demonstrated imaging spectrometer opens up new opportunities in the study of correlated electron emission from solid state. Meanwhile, the electron emission from a single nanotip or field emitter arrays, which are predicted to provide high-current low-emittance coherent electron bunches, foster the further development of free electron lasers, ultrafast electron microscopy and diffraction instruments