High-precision calculation of quasistatic field near a photocathode surface microrelief

Applied Physics Letters

Bazarov

2011

High quantum yield, low transverse energy spread and prompt response time make GaAs activated to negative electron affinity (NEA), an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. We show that a nanoscale surface roughness can affect the transverse electron spread from GaAs by nearly an order of magnitude and explain the seemingly controversial experimental results obtained so far. This model can also explain the measured dependence of transverse energy spread on the wavelength of incident light.The need for a high brightness electron beam is well established 1 . GaAs activated to negative electron affinity via cesiation is a high quantum efficiency (QE) photocathode and can be effectively used for producing such beams 1,2 . Properties of GaAs as a photocathode have been studied for decades [3][4][5] . However, the mechanism of photoemission from these photocathodes is not well understood.Most models follow the Spicer 3-step theory 3 , and they all assume near full thermalization of electrons to the Γ valley minimum when excited with near band-gap energy photons. The difference arises when one considers the effects on the electron going through the surface (band bending and activation regions). To explain the experimental data, one approach argues that the electrons undergo sufficient scattering at the surface so that the transverse energies of the emitted electrons are of the order of 25 meV (thermal energy at room temperature) 2,5 . The other body of work, however, treats the emission process as a refraction of a Bloch wave at an ideal surface while largely ignoring scattering effects at the surface. It predicts the transverse energy of the electrons to be around 1 to 2 meV at room temperature 4,6 and the electrons are emitted in a cone with an half angle of 15 • , which is a result of the small effective mass of the electrons in the Γ valley of GaAs.Furthermore, the experimental measurements of the mean thermal energy (MTE) and thermal emittance are also inconsistent. Some groups report values of MTE close to the room temperature thermal energies of 25meV 2,5 . While others report values of measured MTE near 2meV and the 15 • angular distribution as predicted by the second model 4 . Additionally, measurements show that MTE depends strongly on the wavelength of light used for photoemission 2 . None of the existing models can quantitatively explain this dependence. MTE and normalized transverse rms emittance ( nx ) are related to the spot size of the laser (σ x ) by nx = σ x MTE/ (m e c 2 ) where m e c 2 is the rest mass energy of a free electron.In this paper, we attempt to resolve these discrepancies by considering the effects of nano-scale surface roughness of GaAs on the MTE. The surface roughness effect can explain measurement data 2 and the variation of the MTE with incident wavelength, as well as reconciles seemingly contradictory collection of data in the literature 4,5 .

Section: Fig 1: Afm Images Of Gaas Surfacesmentioning

confidence: 99%

Effect of nanoscale surface roughness on transverse energy spread from GaAs photocathodes

Applied Physics Letters

Bazarov

2011

“…In principle, any finite element or boundary element method can work for this calculation, however due to the small scale of the surface roughness relative to the spatial scale of the problem, these methods become impractical owing to large computational requirements. To circumvent this issue, Gorlov suggested a formalism based on modeling the electric potential using a combination of sinusoidal and exponential functions and applied it to an equipotential surface with nanoscale physical roughness [18]. Here we extend Gorlov's formalism to model electric fields close to physically rough surfaces with varying surface potential.…”

Section: A Description Of Methodsmentioning

confidence: 99%

“…Note that the basis functions ϕ i are not orthonormal as they contain the exponential term K in order to satisfy the Poisson equation. Here we use the Ritz method [18] to find the set of coefficients c i that minimizes the difference between the measured surface potential U 0 ðx; yÞ and the potential obtained from Eq. (5) over the surface.…”

Section: A Description Of Methodsmentioning

confidence: 99%

“…Zhang and Tang extended this formalism to 2-D surfaces with realistic surface roughness by expanding the surface in terms of its Fourier coefficients [17]. Gorlov used a more precise, combined analytic numerical method of the 3-D field calculation close to a realistic 2-D surface [18]. This method has been used to obtain the MTE increase from the physical roughness on realistic photocathode surfaces measured using an AFM [19,20].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Effects of physical and chemical surface roughness on the brightness of electron beams from photocathodes

Gevorkyan

Phys. Rev. Accel. Beams

Emamian

et al. 2018

The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible brightness in such systems. With ongoing improvements in photocathode design and synthesis, we are now at a point where the physical and chemical surface roughness of the cathode can become a limiting factor. Here we show how measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.

“…ny , p nx and p ny are periods corresponding to the Fourier components in x and y directions respectively, E 0 is the longitudinal electric field away from the surface and C n are coefficients obtained by solving the Poisson equation 25 . If p n A n then C n ≈ A n E 0 11 .…”

Section: Fig 4 (A) Afm Image Of a Cathode Grown Using Sequential Dementioning

confidence: 99%

Near atomically smooth alkali antimonide photocathode thin films

Feng

Journal of Applied Physics

Nasiatka

et al. 2017

Nano-roughness limits the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. However there is an urgent need for photocathodes that can produce an order of magnitude or more lower emittance than present day systems in order to increase the transverse coherence width of the electron beam. In this paper we demonstrate a method for producing alkali antimonide cathodes with near atomic smoothness with high reproducibility.Photoemission based electron sources for the next generation x-ray high repetition rate, high brightness light sources such as Energy Recovery Linacs 1 and Free Electron Lasers 2 need to satisfy several criteria, namely: high (>1%) quantum efficiency (QE) in the visible range, smallest possible intrinsic emittance, fast (sub-ps) response time and a long operational lifetime. During the past decade, alkali-antimonides (eg. K 2 CsSb) have emerged as the only class of materials that satisfies all these requirements with a high QE >5% and a low intrinsic emittance in the range of 0.36-0.5 µm per mm rms laser spot size in green (520-545 nm) light 3-5 . Additionally, alkali-antimonides also show promise as sources of ultra-cold electrons for ultrafast electron diffraction 6 applications and Inverse Compton Scattering based Gamma ray sources 7 . Although alkali antimonides have many excellent characteristics, the synthesis process leads to relatively high levels of roughness 8 . K 2 CsSb photocathodes are typically grown as thin films over conducting substrates by thermal evaporation of ∼10-30 nm of Sb followed by sequential thermal evaporation and reaction of K and Cs respectively 3,9 . The films created by this process are not ordered and can have a root mean square (rms) surface roughness as high as 25 nm with a period of roughly 100 nm 8 . Such a surface roughness can distort the electric field near the cathode surface causing the intrinsic emittance to drastically increase. Ignoring the contribution of the slope effect 10,11 , to first order, the intrinsic emittance after accounting for this electric field effect can be given by in = 2 in0 + 2 f , where in0 is the intrinsic emittance of the cathode at near zero electric field and f is the enhancement to the intrinsic emittance at an electric field of f MV/m (typically in the range of 1-20 MV/m) at the cathode surface. In RF/SRF based electron guns, used for high bunch charge applications, the electric field at the cathode surface can be greater than 20 MV/m. In this case the electric field enhancement of the intrinsic emittance can be as high as 2 µm per mm rms laser spot size making these cathodes unusable.12 . The smallest possible intrinsic emittance is limited by the lattice temperature of the cathode to in0 = 0.22 a) Electronic mail: fjun@lbl.gov µm at room temperature and can be obtained by exciting electrons with near threshold photons 13 . However, in alkali-antimonides the smallest possible emittance is limited to a higher value even at photoemission...