Experimental characterization of photoemission from plasmonic nanogroove arrays

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Christopher M. Pierce,1, 2, ∗Daniel B. Durham,3Fabrizio Riminucci,1Scott Dhuey,1

Ivan Bazarov,2Jared Maxson,2Andrew M. Minor,3and Daniele Filippetto1, †

1LBNL, 1 Cyclotron Road, Berkeley, California 94720, USA

2CLASSE, Cornell University, 161 Synchrotron Drive, Ithaca, New York 14853-8001, USA

3Department of Materials Science and Engineering, University of California, Berkeley, and National Center

for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

(Dated: March 15, 2023)

Metal photocathodes are an important source of high-brightness electron beams, ubiquitous in the operation

of both large-scale accelerators and table-top microscopes. When the surface of a metal is nano-engineered

with patterns on the order of the optical wavelength, it can lead to the excitation and conﬁnement of surface

plasmon polariton waves which drive nonlinear photoemission. In this work, we aim to evaluate gold plasmonic

nanogrooves as a concept for producing bright electron beams for accelerators via nonlinear photoemission.

We do this by ﬁrst comparing their optical properties to numerical calculations from ﬁrst principles to conﬁrm

our ability to fabricate these nanoscale structures. Their nonlinear photoemission yield is found by measuring

emitted photocurrent as the intensity of their driving laser is varied. Finally, the mean transverse energy of

this electron source is found using the solenoid scan technique. Our data demonstrate the ability of these

cathodes to provide a tenfold enhancement in the efﬁciency of photoemission over ﬂat metals driven with a

linear process. We ﬁnd that these cathodes are robust and capable of reaching sustained average currents over

100 nA at optical intensities larger than 2 GWcm−2with no degradation of performance. The emittance of the

generated beam is found to be highly asymmetric, a fact we can explain with calculations involving the also

asymmetric roughness of the patterned surface. These results demonstrate the use of nano-engineered surfaces

as enhanced photocathodes, providing a robust, air-stable source of high average current electron beams with

great potential for industrial and scientiﬁc applications.

I. INTRODUCTION

High brightness electron sources for ultrafast applications

require prompt emission of high-charge electron beams and

direct injection into areas of extreme electromagnetic ﬁeld

amplitudes. Photoemission from metal surfaces has been the

primary means of electron bunch generation, used by the large

majority of user facilities around the world [1–3], owing to

their fast response time and robustness. Despite their broad

use, metal cathodes have a few major disadvantages. First,

the typical quantum efﬁciency for a metal exhibits values in

the 10−5region which, for high charge pulse extraction, re-

quires laser pulse intensities close to the damage threshold

of the material. With time and continuous operation, this

has been shown to lead to partial ablation, increased surface

roughness, and reduced brightness [4]. High intensities may

also cause multi-photon absorption and photoemission, lead-

ing to the generation of unwanted halos, and an overall in-

crease of beam thermal emittance [5]. Furthermore, a typical

metal work function requires UV photons for linear photoe-

mission. The two-stage UV conversion from the initial in-

frared laser pulses has a substantial impact on the size and

complexity of the photocathode laser system. It may also im-

pact the quality of the ﬁnal pulse, resulting in substantial loss

of energy, degradation of transverse pulse shape, and limited

control over longitudinal proﬁle. Altogether, the low quan-

tum efﬁciency and the high work function effectively limit the

maximum average current that can be extracted by metal cath-

∗cmp285@cornell.edu

†dﬁlippetto@lbl.gov

odes and, therefore, the range of applications of the relevant

instrumentation.

High quantum efﬁciency semiconductor ﬁlms provide a

possible path towards higher performance photocathodes. De-

pending on the choice of the material, the quantum efﬁciency

can be orders of magnitude larger for a work function in the

visible or infrared region [6]. Unfortunately, such cathodes

are chemically reactive, and the vacuum levels found in high

ﬁeld photoinjectors often greatly complicate their use as high

brightness electron sources. Further, dark current may be-

come an issue in those same systems for materials with an

extremely low work function.

Nonlinear photoemission may offer another potential solu-

tion to avoid nonlinear wavelength conversion. Depending on

the material and laser parameters, it becomes more efﬁcient to

extract electrons from the cathode directly via multi-photon

photoemission using infrared light, rather than perform wave-

length conversion to the UV [7]. However, as is the case for

linear photoemission, the small nonlinear yield of most ﬂat

metallic surfaces demands laser ﬂuence values close to the

material’s damage threshold (typically on the order of 0.1 to

1 Jcm−2[8]).

One path forward in improving the nonlinear yield of met-

als is by fabricating plasmonic structures by surface nanopat-

terning. Nanoscale grooves formed on a gold photocathode

have been shown to increase its nonlinear yield at 800 nm by

up to six orders of magnitude [9]. A similar concept using a

grid of nanoscale holes showed a dramatic increase in the non-

linear yield of gold and copper photocathodes [10, 11]. On the

other hand many questions remain open before such cathodes

could be effectively considered as a reliable source for ultra-

fast application: Can we produce nano-engineered cathodes

arXiv:2210.05056v2 [physics.acc-ph] 14 Mar 2023

with repeatable properties? How does the mean transverse

energy of the extracted beam depend on the nanostructures?

Can such structures provide stable high average currents for

extended periods with no degradation?

In this work we provide a detailed characterization of

nanogroove array photocathodes that demonstrates under-

standing of both the engineering and the physical aspects of

this advanced class of electron photoemitters. First, in Sec. II

we discuss the theory of plasmonic nanogroove photocath-

odes. In Sec. III we explain the fabrication process, and con-

ﬁrm the design dimensions by direct measurements of their

optical properties. Nonlinear photoemission measurements

performed on a 20 kV electron gun are reported in Sec. IV. We

ﬁnd the non-linear photoemission coefﬁcient for the nanos-

tructured surfaces and are able to correlate its spread in values

with the groove dimensions. We then conﬁrm the polariza-

tion dependence of the photoemission, and perform continu-

ous measurement of average currents in excess of 100 nA to

verify the enhanced electron yield and the photocathode sta-

bility. Lastly, in Sec. V the mean transverse energy of the pho-

tocathode is characterized for different energies and the values

found compared with the cathode’s behaviour at the surface.

The article then concludes by discussing future prospects for

nanopatterned photoemitters.

II. PRINCIPLES OF PLASMONIC NANOGROOVE

PHOTOCATHODES

The ideal nanogroove cathode consists of a periodic ar-

ray of trenches with depth (d) that extend inﬁnitely in one

direction and have nanometric width (w) in the other direc-

tion. We deﬁne a coordinate system used for the rest of this

paper with ˆzpointing normal to the cathode surface, ˆyrun-

ning along the grooves, and ˆxagainst the grooves. Focus-

ing for the moment on a single groove and imagining very

large depth, light incident on the grooves may be coupled into

modes within the gap that are best described by surface plas-

mon polaritons (SPP) within a metal-insulator-metal waveg-

uide [12] (the vacuum is the insulator in this case). These

SPPs require additional momentum to couple with free space

illumination, owing to their dispersion relationship lying at

larger wave-vector for the same energy than the light line. For

the case of the nanogrooves, the sharp edges at the entrance to

the trenches can effectively provide such coupling [13]. The

corner’s proﬁle contains high spatial frequency components

that allow light to diffract around it and onto the plasmon dis-

persion curve.

The ﬁnite depth of the groove acts to form a resonant Fabry-

Perot-like cavity with the allowable modes determined by the

depth, d. The cavity depth that meets the resonance condition

may be surprisingly small, only tens of nm for infrared light.

This is explained by the fact that for the same energy, plas-

mons traveling along the walls of the gap may have an order

of magnitude smaller wavelength than light in a vacuum [12].

The localization of optical energy to a nanometric region has

the effect of ﬁeld enhancement near the gap, which can exceed

factors of one hundred and favor nonlinear photoemission.

Figure 1. (a) Plot (with contours) of the ﬁeld enhancement around

a cross section of the nanogroove structure. Solution was computed

with an FDTD code [14]. The DC accelerating ﬁeld, computed in-

dependently using a ﬁnite difference code [16], is shown as the blue

arrows. Inset (labeled c) shows a magniﬁed view of the groove edge;

(b) Time response of structure to a 15 fs excitation computed with

FDTD (plasmon) compared with ﬂat surface (laser). Estimated cur-

rent proﬁles are shown as dotted lines.

Fig. 1a shows an example of local optical ﬁeld enhance-

ment by a nanogroove cathode computed using a ﬁnite differ-

ence time domain (FDTD) code. Speciﬁcally, Lumerical [14]

is used in this work. Simulations are performed with peri-

odic boundary conditions in the ˆxdirection and excited by

linearly polarized plane waves. The simulated cathode had

grooves 14 nm wide, with a pitch of 680 nm, and was ex-

cited by light with a wavelength of 770 nm; representative

of the cathodes studied in this paper. The same picture also

shows the computed local variation of a static externally ap-

plied electric ﬁeld.

The fact that emission occurs only at the sharp edges of the

grooves may have an impact on the emittance of generated

electron beams [10] and high local optical intensity can dam-

age the gold surface[15]. However, the speciﬁc pattern used,

the type and materials used during nano-fabrication, such as

the sharpness of the pattern have an enormous impact on all

of the above aspects.

The high quality factor (and narrow bandwidth) of the plas-

monic nanogroove also has consequences on the photocathode

response time. When the resonance bandwidth of the grooves

is narrower than the bandwidth of the driving ultrafast laser,

the ﬁeld will continue to oscillate in the nanocavity longer

than the duration of the excitation, effectively broadening the

temporal response time of the cathode. An example of this

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摘要：

ExperimentalcharacterizationofphotoemissionfromplasmonicnanogroovearraysChristopherM.Pierce,1,2,DanielB.Durham,3FabrizioRiminucci,1ScottDhuey,1IvanBazarov,2JaredMaxson,2AndrewM.Minor,3andDanieleFilippetto1,1LBNL,1CyclotronRoad,Berkeley,California94720,USA2CLASSE,CornellUniversity,161SynchrotronDri...

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Experimental characterization of photoemission from plasmonic nanogroove arrays

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