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Electron Gun
Chuanxiang Tang
!
at OCPA Accelerator School (Basic), Xiuning, Anhui Province, PR China
July 28, 2014
Outline
•
Introduction: what’s an e-gun? who needs an egun? what’s important for an e-gun?
•
Concept of emittance
•
Cathode physics and technology: photocathode
•
Electron guns: photocathode RF Gun
•
Summary
What’s an electron gun?
A device to generate electron?
An electric component in some vacuum
tubes which produces narrow,
collimated electron beams with a
precise kinetic energy.
1880
Classification
Electric field: DC and RF
Cathode: thermionic, photocathode, field
emission, explosive emission, plasma source,
secondary electron emission…
Number of electrodes: diode, triode,…
1895
Electron
Discovery: in 1897, by J J Thomson
Composition: elementary particle
Statistics: Fermionic
Interactions: Gravity, Electromagnetic, Weak
Mass: 9.109 382 91(40)x10-31kg
0.510 998 928(11)MeV/c2
Electric charge: -1.602 176 565(35)x10-19C
Magnetic moment: -1.001 159 652 180 76(27)uB
Spin 1/2
What can we do with electrons?
as an energy carrier
to generate other new particles, such as Higgs, by colliding with its
antiparticle positron. —high energy physics.
to generate different electromagnetic radiation (light), such as
bremsstrahlung, FEL, SR, ICS and magnetron, klystron…
to change properties of materials.—Irradiation.
as a probe to see atomic scale structures or changes, such as
SEM, UED, UEM.—material, solid state physics, IC and IT,
biology…
as a bullet to kill cancer or bacteria.—radiotherapy and irradiation.
Electron Guns in Accelerators
•
Electron-Positron Collider: nanosecond
DC gun, polarized electron beam (GaAs).
•
Synchrotron Light Source: DC or RF,
thermionic or photocathode.
•
Free Electron Laser: photocathode or
thermionic, RF or DC.
•
Low energy linacs or other application
electron accelerators : DC, thermionic,
diode or triode.
•
HPM: DC and plasma, explosive,or
thermionic, photocathode?
•
Vacuum tubes: thermionic, dc
Gun
200 kV
High
Voltage
DC and RF Guns
cathode
and
heator
DC Gun
RF resonance
cavity and
TM010 mode
anode
RF Gun
DC Type Guns
DC – always on
current
DC - pulsed
DC – amplitude,
frequency
modulated
time
RF Type Guns
CW – bunch of e- in every RF bucket, typically from 100’s of MHz to GHz, up
to 100’s of pC per bunch
pulsed – not every RF bucket is filled, RF frequencies of 100’s of MHz to GHz,
up to ~nC per bunch, with bunch rep rates of Hz to 1 MHz
What kind of electron gun do we
want?
•
High average current—high repetition rate, high duty
factor, high bunch charge, high peak current.
•
High peak current—high bunch charge, high current
density at cathode, high QE, high gradient at cathode,
large emission area.
•
Low emittance—high gradient at cathode, high current
density, low temperature, good cathode quality, low QE,
small emission area, suitable transversal and longitudinal
distribution.
•
High reliability, small size, low cost…
Why is low emittance important?
Gain Length
Wavelength
Requirement
5/6
n
1/ 2
1/ 2
[ µm]g [cm]
( Lg ) min [m] ≅ 20
2/3 !
I [kA]λr [ A]
ε
3 ε
!
(λr ) min [ A] ≅ 3 × 10
λS
ε<
4π
E.L.Saldin et al , Optics Communications 235
5/ 4
n
3/ 4
[ µm]g [cm]
3/ 2
I [kA]LW [m]
εn=βγε$
2004
3/ 4
415-420
L∝
η RF PRF
E
3/ 2
cm
δ BS γ σ z η RF PRF
∝
ε n, y β y
Ecm
δ BS σ z
ε n, y β y
Some Basic Concepts of
Emittance
Concept:
Emittance is used to describe the super-volume
in the 6-dimentional phase space, as
coordinates (q1,q2,q3) and
momenta(p1,p2,p3) in Cartesian coordinate
systems.
If the particles inside any closed surface s(t) in
the phase space, surfer only conservative
forces, the volume v(t) enclosed by the
surface s(t) will be invariant with time.
Emittance is the volume v(t) occupied by the
particles of a bunch.
!
Without coupling between q1,q2,q3
Phase space: (q1, p1), (q2, p2), (q3, p3)
Emittance is a area of the phase space
RMS Emittance
RMS-root of
mean square
RMS Normal
Emittance
ε n = π ⋅ me c ⋅
x
2
px
2
− x ⋅ px
2
Here mec means that px=βxγ. In most case, εn is expressed in π .mec .µm
RMS Geometrical
Emittance
ε =π⋅
x
2
x$
2
− x ⋅ x$
2
x!=px/pz=βx/βz is the divergence angle, and ε is expressed in
π .mm .mrad
Other expressions of
RMS Emittance
ε n = 4 ⋅ π ⋅ me c ⋅
ε = 4 ⋅π ⋅
x
2
x
2
x$
px
2
2
− x ⋅ px
− x ⋅ x$
2
2
ε n = βγ ⋅ ε
Assume all the particles
have same Pz
A gaussian phase-space distribution may be specified as:
px
with
〈 xp x 〉 = 0
To simplify the analysis, let
and rms normal emittance is
ε n = π ⋅ me c ⋅
ψ ( x, p x ) =
1
2π
x2
px 2
x
x2
px 2
1 x2
exp[ − ( 2 +
)]
2
2 x
px
px 2
And consider an ellipse in (x, px) space defined by:
The area of this ellipse is:
A( K ) = π ⋅ me c ⋅ K 2 ⋅
x
2
x2
+
px
2
px 2
x 2 px 2
The fraction of the bunch within this
ellipse is readily computed:
px
For K=l, the ellipse has an area equal to the
normalized RMS emittance, and contains 39.35%
of the particles. The maximum x coordinate of the
ellipse is the RMS value of x, while the maximum
px coordinate of the ellipse is the RMS value of px.
For K=2, the ellipse has an area equal to four times
the normalized RMS emittance, and contains
86.47% of the particles.
*M.Borland ,A High-Brightness Thermionic Microwave Electron Gun , SLAC-402(1991) ,Ph.D.Thesis
= K2
Statistical Definition of Beam Emittance -From P. Lapostolle, IEEE Trans. Nucl. Sci NS-18, 1101(1971)
J.Buon, CERN 91-04, 30(1991)
N non-interacting particles in phase space
Define distribution function ρ ( x, x")
1
xi = ∫ x ρ ( x, x#)dxdx#
∑
N
1
#
x = ∑ xi# = ∫ x#ρ ( x, x#)dxdx#
N
1
σ x2 = ∑ ( xi − x ) 2
N
1
σ x#2 = ∑ ( xi# − x# ) 2
N
1
σ xx# = ∑ ( xi − x )( xi# − x# ) = rσ xσ x#
N
x =
( x, x!)
∫ ρ ( x, x")dxdx" = 1
ε rms = σ x 2σ x#2 − σ xx#2 = σ xσ x# 1 − r 2
Assuming that particles are Uniformly
distributed in an ellipse:
x 2 x!2
+ 2 =1
2
a
b
Total phase-space area:
A = π ab = 4πε rms
Full emittance : ε = 4ε rms
A = πε
RMS Geometrical & Normal Emittance
dx
dx dz
p x = mvx = m = m ⋅ = mvz ⋅ x! = p z ⋅ x!
dt
dz dt
The transverse momentum
px & the divergence angle x!
x
The RMS geometrical emittance
will decrease with the particales
being accelerated, but the RMS
normal emittance will remain the
same.
dx
x# =
= tan(θ ) ≈ θ
dz
px
v
θ"
dz
z
x#
10MeV
x
x
40MeV
x
z
x#
x#
x#
x
x
x#
x#
x
x
x
dx
Projected Emittance
y
x
x
Real Space
py
px
y
x
y
Phase Space: Projected Emittance
Slice Emittance and Correlated Emittance
Slice Emittance: A transverse cross section
of the beam is called a slice, and the
emittance of a slice is called slice emittance.
Slice emittance consists of two parts, one
part is the thermal emittance and the other
part is due to nonlinear space charge force.
y
x
Correlated Emittance: The growth of
projected emittance is due in large part to
the correlation between the phase space
angle and the longitudinal position of slices.
Normally, this part of projected emittance is
called correlated emittance.
px
px
px
x
x
Slice Emittance
px
x
x
Correlated Emittance
Electron emitted from cathode
w
T=TK
T=0K
w0
U
U
xc
E=0
w0!
wm
T=0K
E=E0
x
dN/dw
E=0
V0!
Vm
E=E0
x
dN/dw
•  Field emission:
•  Thermionic emission:
Flower -Nordheim’s Eqn.
Richardson’s Eqn.
2
J = AT e
−
w0 − wm
kT
J = CE 2 e
J # = Je
e
kT
eE
4πε 0
1952
−
D
E
(A/cm2 )
6.2 ×10 −6 Vm 1/ 2
C=
( )
V0
φ
Schottky effect:
*
xc
V0
D = 6.8 ×10 7 φ 3 / 2
35
103
ron
e
erial,
ed.
tric
(image source:
Masao Kuriki, ILC school)!
14
Photo-electron emission:
photo-current is proportional to photon
number (if photon energy not
changed)
The maximum kinetic energy of the
photo-electron is proportional to
photon energy
QE: Quantum Efficiency
!
Metal: Mg 0.4%, Cu 0.05%, at 266nm
!
Semiconductor:
alkali-based Na2KSb:Cs, K2CsSb,
Cs2Te 266nm 6%~12%,
251nm 16%
GaAs 2.55eV(486nm) 14%
GaAs:Cs 2.3eV 0.26%
δ
Secondary emission factor
δmax
1
0
E1
Emax
E2
Secondary emission:
When the primary electron hits the cathode, atoms will be shocked strongly.
Some electrons can cross the potential well and become secondary electrons.
δ r ( E0 ) =
−B
ε
min( R ( E0 ), d )
∫
0
dE ( x) −α x
e dx
dx
Ep
Schottky effect and the abrupt change in electron
the Fermiangle
energy in Eq. (1) should be replaced by the
!w % !Schottky ¼ !w %
eff & FERMIII. QUANTUM
AND !
THE
chemical potential or Fermi
level. However,EFFICIENCY
at low temacross the metal-vacuum interface.
peratures
areFOR METALS
qffiffiffiffiffiffiffiffiffiffiffiffiffi
DIRAC potential
MODEL
Previously we developed analytic expressions
forthetheFermi energy and chemical
nearly
and at absolute zero theyDirac
are identical.
In
¼ !w temperature
% 0:037 947 Fa ðM
function
at finite
he Pauli
exclusion
the based
lackidentical
of the
unoccupied
quantum
efficiencyprinciple
and thermal and
emittance
on
this paper we only consider
low fermions,
temperatures
and thus the
Being
electrons
uniformly fill all energy states
three-step
ignoringas
electron-electron
scattering
[4]. energy
is the
thethan
image
fielt
l states,
alsomodel
known
Pauli blocking.
As
a upresult,
use of the Fermi
is
Incident
photons
withcharge
energies above
toappropriate.
the Fermi level,
EF , sum
with noof
more
two opposing
work [5]
expanded
the quantum
efficiency
model
The electron
density
of occupied
thestate.
cath- Therefore
function electron-electron
are absorbed by electrons near
Phys.
Rev.
ST
Accel.
Beams
12,
074201
(2009)
spin
electronsstates
perFinside
energy
ctronsFurther
move
freely
through
the
metal
with
long
mean.
The
Schottky
work
!e
ode and
the electric potentials experienceda by a single
which then
migrate function,
to the surface and
to include electron-electron scattering, resulting
in excelscattering
theareFermi
level
suppressed
duein terms of th
electron immediately
outside thebelow
cathode
plotted
in is strongly
The QE can
be expressed
lent
agreement
with
the
measured
quantum
efficiency
of
an
e paths which contributes to the good
electrical
conofforthe
Schottky
potential
is th
Fig. 2. The
occupation number is shown
a Fermithese steps
to occur whereand
it is assumed
Dirac
function
at
finite
temperature.
The
Schottky
potential
atomically clean copper surface. Here we apply the same
tivity
ofThe
metals.
The
distribution
ofthe
occupied
states as a
emission potential barrier typicall
is the
sum
of the
imageof
charge
and
field,
Fermi-Dirac
distribution
with
thefield
three-step
model
of phodistribution
occupied
states
asapplied
a function
R1
R1
Fa .toemission
The
Schottky
work
function,
!
,
is
the
peak
value
to
give
a
consistent
theory
of
the
emittance
and
ction
of
energy
is
given
by
the
Fermi-Dirac
function,
dE½1
%
f
ðE
þ
@!Þ)f
ðEÞ
dðcos#ÞF
Schottky
meters
outside
the
cathode.
The
ze#
FD
FD
e-e ðE; !;
EF þ!eff %@!
of energy is given by the Fermi-Dirac function:
cos#
max ðEÞ
R
R
R
QE
ð!Þ
¼
½1
%
Rð!Þ)
1
[6] of
generalizes
of the
the quantum
Schottkyefficiency.
potentialOther
and recent
is thework
height
the photodE½1 % fFD ðE þ @!Þ)fFD ðEÞ 1%1 dðcos#Þ 2"
%@!
E
0 d!
F
shifted
downward
at
high
electric
this basic
phenomenological
process
to give aa common
1 located
emission
potential
barrier typically
few nanoemission
theory
for
thermionic,
photoelectric,
fFDtheðEÞ
¼
;and field
(1)
work function reducing the barr
meters
outside
cathode.
The zero
field
vacuum
state
is
ðE%E
Þ=k
T
F
B
sorption length, $opt , and the electron-e
Here Rð!Þ
is the cathode optical reflectivity. The Fermi1 þ one the connection
emission. Here we concentrate
between
shifted downward at high electric field byDirac
thefunction,
Schottky
quantum
effective
path, $eThe
states, and yield.
systemwor
is sho
-e . The coordinate
quantum efficiency and the photoelectric thermalfFD ðEÞ, is the density of initial
% fFD ðE þ @!Þ]
Unlike a semiconductor photocath
is the density of final states with the
reducing the
and [1increasing
the
T function
is the electron
gasbarrier
thermal
energy,
E
is
the
ere work
kBemittance.
fined
as
F
electron-phonon scattering can be igno
product of these two functions giving the transition probaquantum
yield.
The
effective
work
function,
!
,
is
deeff
The
effective
work
function
is
defined
as
The derivation
forthe
the quantum
efficiencyTo
(QE)be
and
the
electron scattering dominating. Since th
bility
for
the
excitedprecise,
electron to escape. Fe-e ðE; !; #Þ is the
mi energy,
and
E
is
state
energy.
more
fined
as emittance begins with the electron gas
probability
thermal
theorythe
forexcited electron reaches the surface without less than twice the work function, any
scattering.
This
Fermi
energy
in Eq.
(1)model
should
replaced
byis determined
the by the photon ab- scattering event eliminates both electro
metals
and Spicer’s
three-step
for photoemission
as function
sbe
ffiffiffiffiffiffiffiffiffiffiffi
!eff & !w % !Schottky ¼ !
eFa [5], at
in Fig.
1.Fermi
In a previous
publication
we low temmicalillustrated
potential
or
level.
However,
!eff &
!Schottky
!w % eusing the work
w %using
calculated
the!QE
this ¼
approach
4""0
aturesfunction
the Fermi
energy
and
chemical
potential
are1. (Color) Three-step modelApplied
as the only free parameter.
In this model,
the
FIG.
of photoemission.
qffiffi
Field Potential
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
3-step model of photoemission
1.5
0.5
þ @!Þ)fFD ðEÞ
1
vacuum
0.5
w
Voltage (volts)
Energy relative to Fermi Energy (eV)
Energy (arb.)
rly identical and
they eV:
are identical.
In
Potential
¼ !wat
% absolute
0:037 947 Fzero
(2)
¼ !w Image
% Charge
0:037
947 F
a ðMV=mÞ
074201-1
! 2009 The American Physical Society
paper1098-4402=09=12(7)=074201(10)
we only consider low temperatures and
thusE the
Incident photons
above the effective work
Fa iswith
the energies
applied field.
Schottky Work
Material
of function
the Fermi
energy
is
appropriate.
Incident
photons
with
energies
ab
Function, φ
are absorbed by electrons near the Fermi energy
Work Function, φ
Effective
then migrate
the occupied
surface and escape.
The which
electron
densityto of
states inside the cathfunction are absorbed
by
electrons
E
Work Function,
The
Schottky
potential
is
shown
forprobabilities
an applied for
The
QE
can
be
expressed
in
terms
of
the
φ = φ -to
φ
and the
electric
potentials
experienced
by a single
which then migrate
the surface
field
of
100
MV=m
where
the
peak
of
the
these steps to occur where it is assumed @! ' EF ,
ctron immediately
thenmcathode
are plotted
in
The QE can be
expressed in term
photoemission outside
barrier is 1.9
from the surface
F-D Distribution Electron Density of States
Distance from Cathode (nm)
. 2. The occupation
number is shown
forTheaenergy
Fermithese steps to occur where it is ass
R
R FIG. 2. (Color)
distribution of occupied states for a metal (left) and the electric potentials next to
0
Schottky
− φ work
0.5
F0
occupied states
eff
1
0.5
Electron Density of States
0
2
w
Schottky
4
6
Distance from Cathode (nm)
8
10
1
2" d!
dðcos#ÞF
ðE;
!;
#Þ
e
e
0(right). The Schottky potential (green curve) is the sum of the applied field potential (blue) and the image charge
cos#max ðEÞ
R1
R2"
(3) is the amount the material work function is reduced at the peak of the Schottky pote
Schottky :work function
fDavid
ðE
þ
@!Þ)f
ðEÞ
dðcos#Þ
d!
is shown
for an applied
field of- ACCELERATORS
100 MV=m where the peak
of the
photoemission
barrier is(2009)
1.9 nm fr
FD
%1F. Schmerge0, PHYSICALpotential
H. DowellFDand John
REVIEW
SPECIAL
TOPICS
AND
BEAMS
12, 074201
16)
collimated by the surface is intriguing [12] and its impact
on beam dynamics will be investigated in our future
studies.
sion is found to be
In addition we have assumed the temp
quation
(9) can be integrated to obtain
ndsreplacing
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi Fermi-Dirac distrib
the
allow
the
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s
B. Derivation of the photoelectric emittance
for
metals
!
#2
1$
Rð!Þ inðE
@! # function.
"eff
The rms
emittance
terms
the moments ofEthe
F þof@!Þ
F þ "eff
Heaviside-step
The electron tem
1
$
:
QE
ð!Þ
¼
!pfined
;
(3
¼ as the temperature
electron distribution
is defined as [13]
!opt ð!Þ
17)
x
2
of
the
electro
2@!
E
þ
@!
F
1 þ !! ð!Þqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
3mc
e"-e' hx2 ihx02 i # hxx0 i2
(22)
absorption of a photon. Numerical integr
the
x
(10)
for the parameters
in Table
with the slope of the electron trajectory given by
and therefore the normalized
emittance
is I the electron t
be approximately 2000 K to increase the
The
18) QE can be expanded as a function of @! $ ’eff in
s
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
over
7000
K
to
increase
the
QE by a factor
e following Taylor series which is evaluated at photo@! 300
# "Keffwhere most ph
ode
temperatures near
mission
threshold
where
@!
is
only
slightly
larger
than
x
ally
"n ¼ !operated,
! px ¼ !
:QE is an app
(3
x
x
2
= p sin θ cos Φ
p
the
effect
on
the
DAVID H. DOWELL AND JOHN F.3mc
SCHMERGE
ff :
rreincrease. The temperature has little effect
$
Φ
$
ω
=
+
)
p
2m(E
$
dQE
$
kB T=ðh! $ "emittance
unity.
Fe
θ $
z
eff Þ approaches
Eð!Þ ¼ QEj@!¼"eff $
ð@! $ "6effplots
Þ
$
Figure
the
photoelectric
for
copper
as
$
$
ene
d@! @!¼"eff
We have also assumed a monoenergetic
function of photon energy
for
applied
electric
fields
of
$
du
$
y
the
derivation
of
the
QE.
The
photons
ca
1 dQE $
$
2
ver
$
the
parameters
given
in
Table
ð@! $ "50,
Þ and
$ & &100
& : MV=m
(11) using
þ
$
eff
monoenergetic
unless
the
photon
energy
$
con
2 d@! $@!¼"eff
The emittance approaches
0 asto the
comparable
the photon
electron energy
temperatur
Dia
he first twometal
terms vacuum
are zero verifying
the well-known
255 work
nm photon
and transform
dec
proaches
the effective
function
and thelimited
reduc
and
2 great
adratic
upon
the system
photon
energy
for the which
lengthwe
must
be <13
fsthe
to produce
FIG. dependence
5. (Color) Definition of
the coordinate
and compobeam
brightness,
define
as
QE=!
,
al
px int
mennents
of
the
momentum
just
inside
the
cathode
surface
used
to
antumderive
efficiency
near threshold result [10]:
energy spread. Thus, the energy spread o
the photoelectric emittance.
shi
be
ignored
even
for
pulses
with
100
fs
ris
2
abo
1 $ Rð!Þ ð@! $ "eff Þ
074201-5 QE ð!Þ '
:
(12)
em
!opt ð!Þ 8" ðE þ " Þ
eff
F
eff
1 þ !! ð!Þ
e-e
III. THE PHOTOELECTRIC EMIT
the
METAL CATHODES
abs
As described earlier, multiple assumptions are required
D. Dowell, this
et al analytic
, Nucl. Instrum. Methods Phys. Rwe
es. Aalso
622, FIG.
685, 2010. The quantum efficiency and the normalized
firs
7. (Color)
produce
result.
Therefore
numeriThis section derives the thermal emit
sio
emittance
per rms beam
size plotted asAND
functions
of the
effective
H. Dowellthe
andabove
John F. Schmerge
, PHYSICAL
SPECIALof
TOPICS
ACCELERATORS
BEAMS
12,
074201
(2009)
llyDavid
integrate
equations
to test REVIEW
the accuracy
electric emission by applying the same Fe
Quantum efficiency and thermal
emittance of metal photocathodes
x
total
total
in Figure 3.
Appl. Phys. Lett. 101, 231103 (2012)
emittance. Here, the measured QEs are analyzed to obtain
We have shown that a QE ¼ 4.5 & 10"4 can be obtained
the effective work functions for (100) copper and copper
with a CsBr coated copper cathode at 257 nm with a very
1.1.Parallel
analyzer
with
mesh.
FIG.
Parallel
plate
analyzer
withAu
Au
mesh.The
Theanalyzer
analyzer measures
measures the
the
with a FIG.
2.5
nm
thickplate
layer
of CsBr,
CsBr/Cu.
These
work
low power density of a few Watts/cm2. The QE for higher
photoemission
current
over
a a2p2psolid
angle
asasaafunction
ofofthe
retarding
photoemission
current
over
solid
angle
function
the
retarding
functions
are then
used to compute
the intrinsic emittance.
potential
totodetermine
potential
determinethe
theelectron
electronenergy
energyspectrum.
spectrum.
power density10 is >10"3 for a laser power density of
The effective work function can be obtained from the
4 & 105 W/cm2. Therefore, the QE in a CsBr coated sample
relation resolution
for
the QE
metal-like
photoemitters
near
ofofof
the
analyzer
be
eV
based
resolution
the
analyzertoto
beless
lessthan
than0.1
0.1
eVthreshbased upon
uponis increased by focusing the laser beam or increasing the
1
13
13 it is no lonvalid forwith
UV
CsBrfilm
(since
old assumed
low
measurements
low energy
energy spread
spreadlaser power, an effect which is due to additional activation.
measurements
withaactivated
adiamondoid
diamondoid
film
ger an insulator
UV
activation).
electronafter
source.
addition,ininaapaper
papertotobe
bepublished
published elseelse-Thus, it is possible to obtain an order of magnitude QE
electron
source.
InIn
addition,
where,we
wehave
havevalidated
validatedthe
the energy
energy spread
spread method
method with
with
where,
increase relative to UV cleaned Cu targets since the QE of
2
ð!hx films
"
/efwith
Þ aaparallel
1 in
"inRðxÞ
dataobtained
obtained
diamondoid
films
parallel plate
plate anaanadata
diamondoid
fwith
with laser power
: al.
(2) hem-uncoated clean Cu targets does not change
QE
¼
231103-3
Maldonado
et
Appl. Phys. Lett. 101, 231103 (
kitto
lyzercomparing
comparing
data
obtained
with
a
conventional
optto
lyzer
it
data
obtained
with
a
conventional
hem8/
ðE
þ
/
Þ
F
1 þ !k
ef f
ef f
e"e
isphericalanalyzer.
analyzer.The
Theelectron
electronenergy
energyspectra
spectra for
for aa single
singledensity. In addition, we14 have also observed that the experiispherical
not to
change
energy. for
TheCsBr/Cu
QE curves does
correspond
two setswith
of value
crystal(100)
(100)Cu
Cuphotocathode
photocathodeare
are shown
shown inin Figure
Figure 22 and
andmental energy spread
crystal
reflectivity,
and the optical
and the electron-electron
Here, the
QE is given
assame
the
number
ofcoated
electrons
incident
density.
Therefore,
the increase
in QE with
˚ thick
˚A
compared
the
same
samplecoated
withaper
a25
25A
thick CsBr
CsBrincreasing powerthe
compared
totothe
sample
with
tering lengths have been used in the first factor of Eq
photon. film.
R(x)
isNote
thethat
reflectivity
for
a photon
of frequency
film.Note
thatthe
theCsBr
CsBr
film
broadens
the energy
energyx,
spreadpower density while the energy spread remains constant
film
broadens
the
spread
These curves are useful for estimating the systematic u
and
the
electronthe 1/e somewhat
optical
absorption
depth
is
k
appears
to
be
due
to an increase in the electron density of
somewhat
as
shown
in
the
normalized
data
of
Figure
2(b),
opt
as shown in the normalized data of Figure 2(b),
tainty of the effective work function. Specifically, the
, the Fermiasas
energy
isinin
EFthe
,the
and
electronbut
scattering
length
isQE
ksubstantially
than toofany
decrease
of
butincreases
increases
theQE
substantially
shown
Figurestates with activation
e-e
the
shown
Figure
tematicrather
uncertainty
the further
effective
work functio
2(a)spectra
spectra
normalized
toQE.
should
be mentioned
mentioned
thatthe work function
.QE.
Figure
3 shows
a plot of that
the effective
work
function
is /toeff
since the the
energy
spread
is unchanged.
approximately
horizontal
distance
between the red
2(a)
normalized
ItItshould
be
FIG.
1.
Parallel
plate
analyzer
with
Au
mesh.
The
analyzer
measures
the
when
thelaser
laser
powerdensity
density
isincreased
increased
byphoton
5!the
the change
changeThus, the increase
blue
the QE when
for
a normal
incidence
257 nm
(4.826 eV)
as
inlines.
QE with power density without affectthe
power
is
by
5!
photoemission current over a 2p solid angle as a function of the retarding
The QE for
CsBr/Cu
gives
an effective
work functi
energy
spreadisisless
less
than
10%and
andassuming
operationwith
with aa large
largeing appreciably the energy
a potential
function
of
thespread
effective
work
function
typical
spread
is an
interesting
phenomintoinenergy
than
10%
operation
determine
the
electron energy
spectrum.
"5
3.9þ/"0.1 eV. Copper’s QE of 2.8 & 10 correspon
QE
enhancement
ispossible
possible
ataaabsorption
currentdensity
density
greater
thanenon that deserves
QE
enhancement
is
at
current
greater
than
values for
the
reflectivity,
the
optical
length
and
more study and will be discussed in a
4.55þ/"0.05 eV for its effective work function. These
2 2.
100
A/cm
A/cm
the
e-e 100
scattering
length,toand
using
eVeVforbased
the upon
Fermi
future paper.
resolution
of
the .analyzer
be less
than70.1
functions along with the photon energy of 4.826 eV are
231103-2
Maldonado et al.
Two
analysis
techniques
are
used
extract
theintrinsic
intrinsic
13used
Two
analysis
techniques
are
extract
the
lowtoto
energy
spread
measurements
with
a diamondoid
film
used in the three-step model expression for the int
emittance
from
the
measurements.
In
the
first
method,
the
emittance
from
the measurements.
In published
the first method,
the
electron
source. In
addition,
in a paper to be
elseemittance1
widthofofthe
themeasured
measured
spectrum
used
to directly
directly
FIG. 3.energy
The Cu
quantum
efficiencyisphotocathodes.
a function
of
the effective work function
TABLE I.width
intrinsic
emittance
for
and
CsBr/Cu
spectrum
is aseV.
used
to
where, Calculated
we have
validated
theanenergy
energy
spread
method
with
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
for
incident
photon
of
4.826
The
QE
has
been
computed with two
obtain
the
intrinsic
emittance.
In
the
second
method,
the
hffiffiffiffiffiffiffiffiffiffiffiffi
x " ffi/ef f
obtain the
intrinsic sets
emittance.
Inafor
the
second
method,
thelength, and electron
eintrinsic
of parameters
the reflectivity,
q!
ffiffiffiffiffiffiffiffiffi
data obtained
in diamondoid
films
withq
parallel
plateoptical
ana-absorption
FIG.
2.
Comparison
of
the
energy
spread
of
an
uncoated
(100)
:crystal
¼uncoated
h!x"/single
Eexcess
measuredQE
QEgives
givesscattering
theeffective
effective
work
function,
which
isthe effective
ef f 2
FIG.
2.
Comparison
of
the
energy
spread
of
an
single
crystal
(100)
length to
estimate
the
systematic
error of
work
(lm/
mm-rms)
from
(lm/
mm-rms)
measured
the
work
function,
which
is
r
3mc
2
2
lyzer comparing
it toenergy
data obtained
with
a
conventional
hemx
Excess
(eV)
3mc
3mc
copper
photocathode
with
the
single
crystal
(100)
Cu
photocathode
coated
function.
photocathode with the single crystal (100) Cu photocathode coated
thenused
usedelectron
withthe
thethree-step
three-step
modelelectron
compute
the intrinsic
intrinsic QEcopper
then
model
totocompute
the
with
a 25 A CsBr
a 8 mm
CWexpt.
257 QE
nm laser
spectrum
expt.
/ film
from
(eV)when
from illuminated
expt. QE with
from with
ispherical
analyzer.
The spectrum
electron energy
spectra
for
a single
with a 25 A CsBrefffilm when illuminated
2 with a 8 mm CW 257 nm laser
emittance.
The
results
of
the
two
analysis
methods
are
comrawintrinsic
data corbeam and a power density
of &3 W/cm . (a) Energy spread
The experimental
and the
resultsspectra
of the
two
analysis
methods
are
com-is 0.77
crystalemittance.
(100) Cu The
photocathode
areshow
shown
inexcess
Figure
2 and
(a) Energy
spread
raw data cor-em
beam
and
a power density
of &3 W/cm2. parameters
#4
the
energy
spread
eV
and
Cu 100in T
rected by the ratio
offound
QE’s with
of CsBr/Cu
(4.5 !methods
10 #4) and
Cathode pared with measurements of the energy spectrum
andQE
QE for
for
cesof
these two
areuncoated
summarized
˚
)
and
uncoated
Cu 100
rected
by
the
ratio
QE’s
of
CsBr/Cu
(4.5
!
10
pared
measurements
of
the
energy
spectrum
and
#5
compared
to with
the same
sample
coated
with
a
25
A
thick
CsBr
eV for CsBr/Cu and
copper, respectively.2.8Using
these
"5
).
(b)
Same
as (a)
but with peaks
normalized
to unity. (c)
crystal
(2.8 ! 10#5
Cu
&
10
4.55þ/"0.05
0.42þ/"0.04
bare copper 0.31
and for0.31
CsBr-coated
copper0.45
cathodes.
The exI.
There
is
good
agreement
between
the
emittances
mea
crystal (2.8 ! 10 ). (b) Same as (a) but with peaks normalized to unity. (c)
bare that
copper
forfilm
CsBr-coated
cathodes.
The emittance
exfilm. Note
theand
CsBr
broadens
the
energy
excess
energiescopper
in Eq.
(1),
thespread
intrinsic
for "4
single
Error
function shaped data obtained with the energy spread analyzer shown
with the
electron
those
determined
with the
CsBr/Cu perimental intrinsic
0.77
0.71the two methods 4.5 &
10 function shaped
3.9þ/"0.1
0.77þ/"0.05
Error
data
obtained spectra
with the and
energy
spread
analyzer shown
emittances found using
Figure
1. The energy spread of the uncoated copper sample is 0.31 eV,
crystal
copper
is 0.45þ/"0.05
lm/mm-rms
emitperimental
intrinsic
found
using
the two
methods and ininthe
somewhat
as shown
in theemittances
normalized
data
of Figure
2(b),
Figure 1. TheThis
energy
spread
of the uncoated
copper
sample is 0.31 mode
eV,
result
confirms
the validity
of the
are in good agreement.
and the energy spread
of the CsBr/Cu
sample
is 0.77 eV,
boththree-step
obtained from
tance
is
0.71þ/"0.05
lm/mm-rms
for
CsBr/Cu.
are
in
good
agreement.
but increases the QE substantially as shown in the Figure
and
the energy
spread
of thesimple
CsBr/Cuassumptions.
sample is 0.77The
eV, both
obtained
from
itsderivative
rather
intrinsic
emittance
the
FWHM
of the
curve.
In
the
energy
spectrum
analysis,
the
intrinsic
emittance
Juan
R.
Maldonado
,
et
al,
APPLIED
PHYSICS
LETTERS
101,
231103
(2012)
The
second
analysis
method
uses
the
quantum
efficiency
derivative
curve.copper and CsBr/Cu cathodes
s article
copyrighted
indicated
in the
Reuse
ofintrinsic
AIP content
is subject tothe
theFWHM
termsofat:thehttp://scitation.aip.org/termsconditions.
Downloaded
to
Innormalized
theasenergy
spectrum
analysis,
emittance
2(a)is spectra
to
QE.
It article.
should
be the
mentioned
that
listed
for bare
as determ
, of the emitted
is given in terms of the excess energy, E
Experimental results observed at
magnesium cathode is different from
copper and 3-step model
Surface photoemission mediated by surface
Plasmon (SP)
Surface roughness (20 nm) enhances SP
coupling from UV laser (4.66 eV)
Surface oxidation (MgO) lower SP energy
Strong Ez from SP field cause surface
photoemission
SP model: suface
emission ?
H. J. Qian,J. B. Murphy,Y. Shen,C. X. Tang,and X. J. Wang,,
predicted by
3-step model
3-step model:
bulk emission
APPLIED PHYSICS LETTERS 97, 253504 (2010)!
H.J. Qian,J.B. Murphy, Y. Shen, C.X. Tang, X.J. Wang , Nuclear Instruments and Methods in Physics Research A 646 (2011) 22–26 st accurate and controllable parameter in a FIB
photoemission is clearly seen in the charge yield
map
surface.
Because of the third p
machine. As an experimental benchmark, we show inphotoemission
is
clearly
seen
in
the
charge
yield
mapthe laser intensi
e. AsFig.an2(b)
experimental
benchmark,
we
show
in
density versus
[Fig.
3(a)].
IR
laser
pulses
(150
fs
FWHM)
were
focused
that by decreasing the spacing p from
745
to
Selected for
a Viewpoint
in Physics
intensity
in a small local regio
[Fig.
3(a)].
IR
laser
pulses
(150
fs
FWHM)
were
focused
week
ending
b) that
by
decreasing
the
spacing
p
from
745
to
to
120
!m
rms
and
scanned
around
the
nanopattern
at
the charge yield. We estimate
710 nmPRL
the 110,
resonance
shifted
P Hfrom
Y S I 840
C A Lto R E V I E W L E T T E R S !
15 FEBRUARY 2013
074801 wavelength
(2013)
nanostructured
to
120
!m
rms
and
scanned
around
the
nanopattern
at surface co
m the813
resonance
wavelength
shifted
from 840
to
normal incidence (< 1 ) with a piezo-controlled the
mirror.
nm, in good
agreement with
the simulation
prediction.
equal absorbed intensity b
incidence
(<laser
1! ) spot
withonathepiezo-controlled
mirror.
The position
of the
cathode was monitored
, in good
agreement
with the
prediction.
Polarization
dependence
of simulation
the reflectivity
at normal inci-normal
where the integration is per
surface,
and Sf and If are th
a virtual
cathode
the beam
charge
was
Surface-Plasmon
Resonance-Enhanced
Emission
High-Brightness
Electron
The by
position
of
the
laserofscreen,
spot
onand
the cathode
was
monitored
was not
observed
in experiment
to theincisymmetryMultiphoton
ationdence
dependence
of the reflectivity
at due
normal
flat surface, respectively. Simu
by
a calibrated
high
efficiency
beam ofprofile
of the
nanopattern,
in agreement
simulations.
from a Nanostructured
Copper
Cathode
by ameasured
virtualFIG.
cathode
screen,
the
beam
was
3 (color online).
(a) Charge and
yield map
of the
nanopat- charge
A % 14
for our nanostru
was not
observed
in experiment
duewith
toBeams
the
symmetry
terned
cathode.
The
black
square
indicates
the
nanopatterned
camera.
The
maximum
signal,
obtained
when
the
laser
observed enhancement of A ¼
The bandwidth
of thewith
SP resonance
is also a criticalmeasured by
a rms
calibrated
high efficiency
profile
nanopattern,
in agreement
simulations.
laser spot
(b) 2
The charge density
1
1
1
2
2area. The
1 size is illustrated.
2 beam
the
simulation
R. K. Li, H.
G. the
Andonian,
J. Feng,
A. Polyakov,
M.theScoby,
K.nanopattern,
Thompson,
W.
spot fully
covered
the
isbyWan,
Yaveraging
# 102 prediction cons
feature since it is important
to To,
match
resonance
widthcamera.
exp > 1:2
$C.
versus
absorbed
laser
intensity.
$ is obtained
shapes
the nanostructures.
The
maximum
signal,
obtained
when
the oflaser
2
1,beam
bandwidth
of
the
SP
resonance
is
also
a
critical
the measured
chargethe
over laser
the full pattern
area.
$ can
be
*
H.
A.
Padmore,
and
P.
Musumeci
times
larger
than
when
was
only
hitting
the
flat
The increased absorption c
with the bandwidth of the photocathode driver laser
expressed as $ ¼the
J%, where
we choose % to be equal
to Y
the pulse> 1:2 # 102
spot
fully
covered
nanopattern,
is
1 the resonance width
enhancement
has an importan
since("it20
is nm).
important
to
match
exp
Department
ofmeasurement
Physics and Astronomy,
UCLA,
Losduration
Angeles,
of theCalifornia
drive IRbetween
laser90095,
of 150 the
fsUSA
FWHM
and Jmaximum
is an
surface.
The
transition
signal
and
Comparison between
the
and
threshold properties. For t
2
equivalent
current
density.
Fitting
of the low
charge
density hitting
part
Advanced
Lightresonance
Source
LBNL,
Berkeley,
California
94720,
USA
larger
than
when
the
laser
was
only
the flat
he bandwidth
of thein photocathode
laser Division,
simulation results
Fig. 2 shows
thatdriver
the
oftimes
employed
in multiphoton pho
yields a slope of 3:05 + 0:07.
(Received 12 November 2012; published 11 February 2013)
shorter than electron-phonon
1.0
signal
maximum
nm). the
Comparison
between
theandmeasurement
nanopattern
1, p=745 nm and
actual patterns
is wider
shallower thanand
what issurface. 1.0The transition between the
threshold is set by the absorbed
nanopattern
2, p=710 nm
We
experimentally
investigate
surface-plasmon
assisted
photoemission
to
enhance
the
efficiency
of
flat
surface
is
due
to
the
partial
illumination
of
the
nanothe damage threshold of t
ion results in Fig. 2 shows that the resonance of
0.8simulated
0.8
measured
Interestingly,
this large
increase
cannot
be fully
metallic photocathodes for high-brightness electron1.0
sources. pattern.
A nanohole
array-based
copper
surface
was
changes were observed wh
1.0 The generalized
explained
by
the
change
in
IR
absorption.
nanopattern
1, p=745 nm10 mJ=cm2 . The r
ual patterns is wider designed
and shallower
than
what
is
p
approached
to exhibit a plasmonic response at 800 nm, 0.6
fabricated using the focused ion0.6beam milling
0
-100
-200
0
(a)
= J = CI
10
x
A/mm2
C = 1.44±0.05 (GW/cm2 3
x = 3.05±0.07
measurement
1
2
100 200
20
2
absorbed laser intensity (GW/cm
Fowler-Dubridge theory [14] predicts only an increase
of
nanopattern
2, p=710 on
nma flat copper surfac
reported
p characterized and tested as a photocathode
Þ3
0.8
technique, optically
inð1#R
a phigh
power radio frequency
0.864% andphotoRf ¼ 88%
into account the &4:5 times e
0.4
0.4 YFD ¼ ð1#Rf Þ3 ¼ 27 times, where Rp ¼
injector. Because of the larger absorption and localization ofarethe
field
intensity,
theand
charge
yield
face nanopatterning.
theoptical
reflectivity
of the
nanopattern
the flat
surface,
p
w
h
0.6
0.6 0.2 respectively.
It is important to distinguish
(b)
0.2
hole
observed under ultrashort laser pulse illumination is increased by more than square
100 times
compared
to a flat
Gaussian
hole
(a)To understand the
(b) in charge
sion regimes from strong optic
additional
enhancement
surface.pWe also present the first beam characterization
results
(intrinsic
emittance
and
bunch
length)
from
[16–18]. A signature of strong
0.0
0.4
0.4 0.0 emission we look into the details of
the optical intensity
795
810
825
840
795
810
825
840 excess energy spread of a
2
large
a nanostructured photocathode.
distribution. In Fig. 4(a) we show the simulated I / jEj
wavelength (nm)in contrast with the multiphot
wavelength (nm)
h
profile at resonant
wavelength
on 0.2
the nanohole surface.
(b)
0.2
square
hole
energy is typically % 1 eV.
Two lineouts
ofGaussian
the 41.75.Ht,
intensity
the (b)
metal-vacuum
DOI: 10.1103/PhysRevLett.110.074801
PACS
29.25.Bx,
78.67.!n,
79.60.!i
hole along
(a) numbers:
reflectivity
w
100
reflectivity reflectivity
reflectivity
-200 -100
(b)
2
280
240
200
160
120
80
40
2
100
(pC/mm
rms laser
spot size
charge density
(a)
200
(c)
the excess energy should be
boundaryonline).
are illustrated(a)
in Figs.
4(b)0.0
and 4(c),
respectively, at ofnormal
FIG.
2 (color
Simulated
reflectivity
0.0
normalized
by
the
intensity
over the
entire 825
electron source. The K
795 both
810
840
795
810
840
incidence
of
two825
arrays
ofaverage
Gaussian
(solid
curve)
andtance
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week
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(="½#
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wavelength (nm)
wavelength
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P
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S
I
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A
L
R
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I
E
W
L
E
T
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FIG. 1 (color
online).
(a) (2013)
Scanning
electron microscopy
im-fine (dashed
holes
with the high
sameintensity
p, h, and
values.
The ! is2013
15 wavelength,
FEBRUARY
PRL
110,
074801
In curve)
a recent
experiment,
IRwlaser
pulses
Recent
progress
in nanotechnologies
has enabled
the work f
ages of the
nanohole
Zoomed-in
cutsurfaces
view ofbythe
areasoutshow
results of
changing
h and
$5%.is used to evaluate
intensity,
directly
of a the
Ti:sapphire
amplifier
were
usedwtoby
illumicontrol
of the array.
opticalInset:
properties
of metal
shap- shaded
(a)
a) nanoholes. (b) Nanohole
(c) an optical microscopy
FIG. (b)
2 nate
(color
online).
(a) of
Simulated
reflectivity
atofstrong
normal
the
regime (! )
(b)
(a)with suboptical-wavelength
rms laser
array
under
with
Measured
reflectivity
two
nanopatterns
fabricated
on field
surface.
Because
of
the
power
the
current
a copper
cathode
[7].
Inpthird
this
case
the scaling
electrons
are
ing them
features
[1]. At
nanospot size
regime
(!
*
1).
For our nano
200 visible light. (c) A nanopattern
100
off-resonance
consisting
ofincidence
single
crystal
substrates
with
2 nm
FWHM
bandwidth
lasers.
of
twothrough
arrays
Gaussian
(solidconcentration
curve)
square
generated
three-photon
process
and the and
current
structured
interfaces, strong coupling
can
occur between
density
versusaof
the
laser
intensity,
of
the
laser
intensity ! stays >10 after taki
280
6-by-6
25
!m
square
patches
illuminated
at
resonant
laser
The
two
patterns
have
the
same
hole
width
and
depth
and
areThe
localization
on the surface,
h
(color online).
(a)
electron
microscopy
im- plasmons
(dasheddensity
curve)
holesin
the
same
p,ofh,the
and
w values.
100
scales
aswith
third
power
absorbed
laser
light
and Scanning
the metal electron
oscillations
or surface
c
intensity
athe
small
local
region
can
significantly
enhance
240
regime.
x
wavelength.
onlyintensity.
different
in spacing.
b experimental
CI
= Jby=shaded
The
results benefited
from
the
(SPs).
ByInset:
tailoring
the SP200
properties, controlled
the
the nanohole
array.
Zoomed-in
of
the
areas
show
the
results
of
changing
h
and
w
by
$5%.
w
the
charge
yield.
We
estimate
the
enhancement
factor
of the m
160 cut view
0
In Fig.
3(b) we show
p/2
10
A/mm
120
increased
IR
absorption,
up
to
"85%,
due
to
a
thin
MgF
physical
dimensions
of
the
nanostructures,
one
can
greatly
C = 1.44±0.05
2of the
es. (b) Nanohole array under an optical
microscopy with
(b)
of surface
two nanopatterns
fabricated
onabsorbed
the reflectivity
nanostructured
compared toR
a function
flat cathode
at laser
(GW/cm Measured
80
-100
low
extraction3 field of 25
x = 3.05±0.07 at
absorption,
antireflective
coating.
Asintensity
alternative
nano(c)
(b)
40transmission, or reflection
nance visibleenhance
light. light
(c) A
nanopattern
consisting of
single crystal
substrates
with
2annm
FWHM
bandwidth
lasers.
equal
absorbed
by Aapproach,
¼ ð I3the
dsÞ
=ðIaf slope
Sf Þ, of 3:05
074801-2
curve
has
nano
2
selected
wavelengths
and
localize
the
optical
field
intensity.
plasmonics
concepts
could
be
applied
to
engineer
the
measurement
three-photon
process is domi
1
25 !m square-200patches illuminated at resonant
laser
The two patterns
have
the same hole
width and over
depth
andnanohole
are
where
the of
integration
isandperformed
the
These -200
phenomena
have
sparked
great
interest
in
research
optical
response
a
cathode
obtain
high
efficiency
slope
occurs
above 40–50
2
20
-100 0 100 200
gth.
only
different
in
spacing.
2
are the
area and
intensity
surface,
and Sf and
observed
cathode [7]
related to laser-material interactions
on intensity
nanometerphotocathodes,
bothIfthrough
a control
of the
metal on aofflatthe
absorbed laser
(GW/cm metallic
cathode effects [20]. The high
flat surface,
respectively.
Simulation
results sity
predict
factoris closel
(nm)
femtosecond scales and many entailing exotic applications
reflectivity
and through
the effect
of the localization
of from
the aacathode
in, for
example,
optics,(a)magneto-optic
and
optical
This
idea
was explored
Tsang beam brightness [21
FIG.
3 (color
online).
Charge yield data
map storage,
of the nanopatof
A4 (color
%intensity.
14
for (a)our
nanostructure.
Theby achievable
experimentally
FIG.field
online).
Intensity
I distribution
on the nano074801-2
ized
of photoelectro
hole
surface.
lineouts
of I alongof
the A
metal-vacuum
boundasubwavelength
biomolecules
[2–4].
[8,9]
andThe
more
recently
by
Polyakov
et FD
al.&
[10,11].
terned
cathode. detection
The blackofsquare
indicates
the nanopatterned et al.observed
5 isemission
well within
enhancement
¼ Yexp =Y
z
incident
laser
(pC/mm
2
laser
polarization
y
x
160
140
120
100
60
40
2
20
2 3
3
normalized intenstiy
normalized intenstiy
charge density
80
2
1
0
0
200 400 600 800 1000
3
2
1
0
0.0
0.5
1.0
1.5
2.0
1/--pages
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