We have been studying all-metal field emitter array (FEA) cathode [1-14] as a potential high current and high brilliance electron source for advanced accelerator applications e.g. X-ray free electron lasers such as SwissFEL , requiring stringent cathode specs: high current (200 pC in 10 ps), low normalized transverse emittance (0.4 mm-mrad), and compatibility with the high acceleration electric field in the order of 100 MV/m.
Ultrafast high current & low-emittance FEAs For such ultrafast high current & low-emittance applications, we have proposed the double-gate nanotip FEAs with surface-plasmon-enhanced laser-induced field emission: electrons from 1 million nanotips are produced by near infrared laser pulses, of which optical electric field is enhanced via the surface-plasmon-polariton resonance of the copper-double-gate structure and the lightening rod effect at the molybdenum nanotip apexes, see Mustonen et al. "Efficient light coupling for optically excited high-density metallic nanotip arrays", Helfenstein et al. "Collimated Field Emission Beams from Metal Double-Gate Nanotip Arrays Optically Excited via Surface Plasmon Resonance", and Oh et al. "Numerical study of the laser-tip coupling in surface plasmon assisted stacked-double-gate field emitter arrays" .
Measurement of the intrinsic transverse emittance of FEAs At the same time, we conducted experiment to establish the transverse emittance of FEAs: We demonstrated for the first time the significant reduction of the transverse emittance of FEAs with an optimized double-gate structure by experiment, and the observed value was in good agreement with the intrinsic transverse energy of metal field emitters. Further, the coherence of collimated beamlets of double-gate FEAs was confirmed by the observation of electron diffraction from a suspended ML graphene. These results were reported in an article "Measurement of transverse emittance and coherence of double-gate field emitter array cathodes"  published in Nat. Comm. See also "Can a metal nanotip array device be a low-emittance and coherent cathode?". The result indicate that the double-gate FEAs with the demonstrated collimation capability willopen up new electron beam applications including micro electron microscope, sub-THz and THz vacuum electronic power amplifiers, ultra-compact mass spectrometers, compact X-ray sources, parallel e-beam lithography tools, etc.
Spatial coherence of double-gate single-nanotip molybdenum field emitter The transverse coherence of the highly collimated field emission beam of our double-gate single-nanotip emitter was further studied via the low-energy electron diffraction of a suspended ML graphene in collaboration with R. J. D Miller group at the Max Planck Institute of structure and dynamics of matters : careful analysis of the diffractio image at 0.8 keV showed that the transverse coherence reaches several 100 nm (see "Transmission low-energy electron diffraction using double-gated single nanotip field emitter" by Lee et al. published in Appl. Phys. Lett.) . The result shows that our double-gate single-nano-tip cathode will be applicable to construct a compact electron diffractometer for atomically resolving the structure-function correlation in biological systems to directly observe biology in action at atomic level.
Transverse structure of the wave function of field emission electrons Motivated such observations, we have recently analyzed the wave function of field emisssion in the transverse direction. Despite its fundamental importance for the transverse emittance and transverse coherence of field emission beam, only a handful of research has been reported for the transverse wave function. By extending the WKB approximation familiar for the calculation of longitudinal electron wave function, we showed that the transverse structure can be written in Gaussian form, dictated by the intrinsic transverse energy of field emission electron. From that, we showed that a field emitter with extended spatial coherence area can have a proportionally smaller beam divergence as is known for coherent electro-magnetic wave. The result was published as "Transverse structure of the wave function of field emission electron beam determined by intrinsic transverse energy" in Journal of Applied Physics .
- PSI metallic field emitter arrays : Introduction and Nanotip fabrication
- Ultrafast near infrared laser-induced field emission
- Stacked double-gate field emitter arrays with large collimation gate apertures
- Picosecond electrical switching, high-accelration-compatibility, and relativistic acceleration
- Emittance of single-gate field emitter arrays
- In-situ uniformity control of field emission beam
 S. Tsujino, P. Beaud, E. Kirk, T. Vogel, H. Sehr, J. Gobrecht, and A. Wrulich, Ultrafast electron emission from metallic nanotip arrays induced by near infrared femtosecond laser pulses, Appl. Phys. Lett. 92, 193501 (2008).
 S. Tsujino, F. le Pimpec, J. Raabe, M. Buess, M. Dehler, E. Kirk, J. Gobrecht, and A. Wrulich, Static and optical field enhancement in metallic nanotips studied by two-photon photoemission microscopy and spectroscopy excited by picosecond laser pulses, Appl. Phys. Lett. 94, 093508 (2009).
 A. Mustonen, P. Beaud, E. Kirk, T. Feurer, and S. Tsujino, Five picocoulomb electron bunch generation by ultrafast laser-induced field emission from metallic nano-tip arrays, Appl. Phys. Lett. 99, 103504 (2011).
 A. Mustonen, P. Beaud, E. Kirk, T. Feurer, and S. Tsujino, Efficient light coupling for optically excited high-density metallic nanotip arrays, Scientific Reports, 2, 915 (2012).
 S. Tsujino, P. Helfenstein, E. Kirk, T. Vogel, C. Escher, and H.-W. Fink, Field-Emission Characteristics of Molded Molybdenum Nanotip Arrays With Stacked Collimation Gate Electrodes, IEEE Electron Device Letters, vol. 31, 1059 (2010).
 P. Helfenstein, E. Kirk, K. Jefimovs, T. Vogel, C. Escher, H.-W. Fink, and S. Tsujino, Highly collimated electron beams from double-gate field emitter arrays with large collimation gate apertures, Appl. Phys. Lett. 98, 061502 (2011).
 P. Helfenstein, K. Jefimovs, E. Kirk, C. Escher, H.-W. Fink, and S. Tsujino, Fabrication of metallic double-gate field emitter arrays and their electron beam collimation characteristics, J. Appl. Phys. 112, 093307 (2012).
 P. Helfenstein, V. A. Guzenko, H.-W. Fink, and S. Tsujino, _Electron beam collimation with a 40 000 tip metallic double-gate field emitter array and in-situ control of nanotip sharpness distribution, J. Appl. Phys. 113, 043306 (2013).
 S. Tsujino, M. Paraliev, E. Kirk, T. Vogel, F. Le Pimpec, C. Gough, S. Ivkovic, and H.-H. Braun, _Nanosecond pulsed field emission from single-gate metallic field emitter arrays fabricated by molding, J. Vac. Sci. Technol. B29, 02B117 (2011).
 S. Tsujino, M. Paraliev, E. Kirk, C. Gough, S. Ivkovic, and H.-H. Braun, Sub-nanosecond switching and acceleration to relativistic energies of field emission electron bunches from metallic nano-tips, Phys. Plasmas 18, 064502 (2011).
 S. Tsujino, M. Paraliev, E. Kirk, and H.-H. Braun, _Homogeneity improvement of field emission beam from metallic nano-tip arrays by noble-gas conditioning, Appl. Phys. Lett. 99, 073101 (2011).
 P. Helfenstein, A. Mustonen, T. Feurer, and S. Tsujino, Collimated Field Emission Beams from Metal Double-Gate Nanotip Arrays Optically Excited via Surface Plasmon Resonance, Applied Physics Express, 6, 114301 (2013).
 S. Tsujino and M. Paraliev, _Picosecond electrical switching of single-gate metal nanotip parrays, J. Vac. Sci. Technol. B 32, 02B103 (2014).
 A. Mustonen, V. Guzenko, C. Spreu, T. Feurer, and S. Tsujino, High-density metallic nano-emitter arrays and their field emission characteristics, Nanotechnology 25, 085203 (2014).
 B. Patterson, R. Abela, H.-H. Braun, U. Flechsig, R. Ganter, Y. Kim, E. Kirk, A. Oppelt, M. Pedrozzi, S. Reiche, L. Rivkin, Th. Schmidt, B. Schmidt, V. N. Strocov, S. Tsujino, and A. F. Wrulich, Coherent science at the SwissFEL x-ray laser, New J. Physics, 12, 035012 (2010).
 Y. Oh, A. Mustonen, T. Feurer, S. Tsujino, Numerical study of the laser-tip coupling in surface plasmon assisted stacked-double-gate field emitter arrays, Journal of Vacuum Science & Technology B 33, 03C112 (2015); https://doi.org/10.1116/1.4915252.
 S. Tsujino, P. D. Kanungo, M. Monshipouri, C. Lee, and R. J. D. Miller, Measurement of transverse emittance and coherence of double-gate field emitter array cathodes, Nat. Commun. 7, 13976 (2016); https://doi.org/10.1038/ncomms13976.
 C. Lee, S. Tsujino, and R. J. D. Miller, Transmission low-energy electron diffraction using double-gated single nanotip field emitter, Appl. Phys. Lett. 113, 013505 (2018);https://doi.org/10.1063/1.5030889.
 S. Tsujino, Transverse structure of the wave function of field emission electron beam determined by intrinsic transverse energy, J. Appl. Phys. 124, 044304 (2018). https://doi.org/10.1063/1.5035284,
 S. Tsujion, On the brightness, transverse emittance, and transverse coherence of field emission beam, Journal of Vacuum Science & Technology B 40, 030801 (2022) (featured article); https://doi.org/10.1116/6.0001776.