1st Topical Workshop on Laser Based Particle Sources Photocathode electron INFN Milano INFN D.Sertore
forhigh beams -
LASA
brightness
1st Topical Workshop on Laser Based Particle Sources • a bunch Beam The requestThe of small emittances isa MUST alsoforFEL – – linac The The intervalthe t Peak The beam brightnessgenerated at theInjector of
Brightness w.r.t. emittance
acceleratoris the ultimatevalue and can only be current beam
퐼 spoiled along the accelerator
= termdifferent of sum isthe terms measuresthe
divergence Beam Brightness Δ Δ 푄 t
where Q is the extractediswherethe Q charge fromcathode the in 휖 = 퐵 휖 = 푅퐹 2
ϵ 푁 γ + n 퐴 푝푎푟푡 number 휖
< Ω 푆퐶 2
λ 4 + FEL ≈
π 휖 푡ℎ 2 휖 푥 + 퐼
휖 of 휖 푦 푟표푢푔ℎ 2
generated + ⋯
per
1st Topical Workshop on Laser Based Particle Sources • • • Photoemission (for metal) Field Emission Thermoionic – – – 푗 푗 푗 푝ℎ 퐹퐸 푇ℎ • • • ( ( ( 퐵 퐴 퐴 휔 퐸 푇 Electron production 퐹푁 푅퐿퐷 = ) ) ) = = = 4 = 3 =
ℎ 8 2 퐴 퐴 ℏ휔
(
푒 푚 푞 ℎ 푘 2 푅퐿퐷 푒 퐹푁
퐵
2 휋 3 휋
푒 2 휙
1
ℏ 퐸 푚
3 푇 − ) 2 2 푒 = 푒 푅 − 120 − 퐵 푘 휔 휙 퐵 휙 3 퐸 푇 .
173
2 퐹
휆 Ganter
푐 푚 휔 퐴 2 퐾 et al. NIMA 565 (2006)423 2 ℏ휔
− 휙 2 effects 퐼 휆
1st Topical Workshop on Laser Based Particle Sources • • • byEfficiencyQuantumits The «efficiency» photocathodeof a is measured PrizeNobelthe 1921 in wasand der Phys. awarded132) (6) (Ann. 17 by Einsteinpostulated the Photoelctriceffect in1905 them lighton when shines Photocathodesare materials thatemit electrons
푄퐸 = 퐾 Photocathodes # # 퐸
푖푛푐푖푑푒푛푡 푒푚푖푡푡푒푑 = ℏ휔
−
휙
푒푙푒푐푡푟표푛 푒푓푓 푝ℎ표푡표푛푠
1st Topical Workshop on Laser Based Particle Sources QE • • • 푄 and hencewe need For a typical 4 In Often, specially for calculation, an “Internal” QE is defined
퐸 pratical 𝑖푛푡 = # # 푎푏푠표푟푏푒푑
푒푚푖푡푡푒푑 units QE is th Quantum Efficiency
harmonic harmonic of a 푄퐸
푄퐸 푒푙푒푐푡푟표푛
푝ℎ표푡표푛푠 1 %
휇J % =
to produce = 푄 퐸 퐸 =
푛퐶 푙푎푠푒푟 푄 푙푎푠푒푟 Nd 푛퐶 1
퐸
−
휇퐽 휇퐽 laser ( laser 푝ℎ 4 # . 72 푅 푒푚푖푡푡푒푑 10 0 10 푒푉 .
472 #
푖푛푐푖푑푒푛푡 l
= 262 nm)
푛퐶
푒푙푒푐푡푟표푛
having 1%
푝ℎ표푡표푛푠
1st Topical Workshop on Laser Based Particle Sources • Spicer’s “Three Step Model” Spicer’s“Three Wemetala assume photocathodethe use and 3. 2. 1.
A deeper lookin the photoemission Electron emission Electron drift to the surface Light absorption and electron process for metal*
excitaction *D . . Dowell etal. PRSTAB 12, 074201
1st Topical Workshop on Laser Based Particle Sources • • The potentialThe at the surface isgiven boundary Inside metal the electrons obey Fermi Dirac distribution 휙 푒푓푓 푒푉 휙 QE calculation 푒푓푓 = 푓 휙 = 퐹퐷 푤 휙 푒푉 푤 = − − 휙 1 푆푐ℎ표푡푡푘푦 0
+ . 037947 e 1 퐸 − 푘푇 = 퐸 휙 퐹 푤 퐹
푎 −
푀푉 푒 푚 4
[ 푒퐹 휋 푒푉
휖 0
]
by
1st Topical Workshop on Laser Based Particle Sources 2. 1. • 3. generalThe expression for isthe QE given by 푄퐸 – – –
Escape over barrier Transport to surface Absorptionof photon 휔 the barrier the barrier is The electron can overcome the surface the energybarrier if above is The it. forcriterion escaping = probability to reach thesurface without F collision is Electrons, path their in to the surface, scatter mainly due hits electrons. other with The energy accessible range startingis from It on depends the density of states of thestarting state to the ending states (JDOS). The 1 − 푅 휔
퐸 ∞ 퐹
+
휙 푒푓푓
−
ℏ휔 푑퐸 QE calculation
퐸 ∞ Absorption
퐹
−
푝 1 ℏ휔 푛표푟푚푎푙 2 − 2 푑퐸
푓 푚 퐹퐷
1 퐸 ≥ − + 푓 퐸
ℏ휔 퐹퐷 퐸 퐹 퐹 + 퐸 + 푓 휙 + 퐹퐷 휙
푒푓푓 ℏ휔 푒푓푓 퐸
− 푓 cos 1 e 퐹퐷 - ℏ휔 e . This This thecase is . formetals. 휃 퐸 푚푎푥
− 퐸 1 1 푑
푑 cos cos 휃 휃
퐹 0 푒 2 −
휋 푒 푑 퐸 Φ ,
휔
, 휃
0 2
휋 푑 Φ
1st Topical Workshop on Laser Based Particle Sources • • form The above formulacan be reduced to amore manageable If we expand QE as function of – – 푄퐸 휆 refraction 휆 푒 표푝푡 휔 −
푒
is the electronis the scattering length 휔 = 푄퐸 = 1
+ 휆 4 휆 푙푎푠푒푟 휆
푒 휋 휔 표푝푡 −
푘 푒
퐸
remembering thatremembering = 휔 푚 QE Calculation 1
ℏ휔 1 1 − + 퐸 푅 − 휙 푚 2 3 휆 휆 휔 푒푓푓 푒 표푝푡 푅 −
푒 휔 1 휔 휔 + 휙 ℏ휔 8 푛 푒푓푓
ℏ휔 휙 = 푒푓푓 ℏ휔 푛 퐸 2 퐹 + −
+ ℏ휔 − 퐸 푖푘 ℏ휔 휙 퐹 휙
is the complexis the indexof + 푒푓푓 푒푓푓
1 휙 −
푒푓푓 we obtain 2 2
퐸 퐸 퐹 퐹 + + 휙 ℏ휔 푒푓푓
2
1st Topical Workshop on Laser Based Particle Sources • • charge trend at low charge fitted QE QE corresponding to maximumextracted charge It measuredis at nominal phase (38 Cs 2 QE TeQE is % % @ 262 nm @ 100 0 n n . ph el 47 routinelly Q
E Q C cath E cath nC E ph J J Cs eV
measured RF the GUN inFLASH 2 Te QE@FLASH
Charge (nC)
0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 QE = 9.2 % QE = 1 2 0 0 deg 0.05
for 푄 w.r.t. zero crossing), not =
gaussian 휋 Graph from
푟
푚 2 퐸 0.1
푙푎푠푒푟 laser laser beam Laser Energy (uJ)
휖 S.Lederer 0 퐹
0.15 sin
휙
PPP 2012 Workshop 2012 PPP + 푄퐸 0.2 푒 퐸 ℏ휔 푙푎푠푒푟 space charge 푒 − effect 0.25 푟 푚 2 2 퐸
휎 푙푎푠푒푟 푟 2
0.3
1st Topical Workshop on Laser Based Particle Sources QE A h Cs E 2 G Te QE E A q e vs q e 4
Field @ FLASH F sin 0 → → → m
QE decreased
field enhancement increased E G +E QE @zero gradient 11.2 % = S.Lederer QE @zero gradient 4.5 % = for semiconductor Generalizedformula A
increased
PPP 2012 Workshop 2012 PPP W = E W W = E W
G
G +E
+E A A
= 3.8 eV =
= 3.5 eV =
= 12.7
= 4.7
1st Topical Workshop on Laser Based Particle Sources • • • • • havenormalized the We have then to calculatethe variance of the electronmomentum to In termof If we now define the normalized square root vanishes and we get for and momentum the second term under the If we assumeno correlation between position space The emittancethe area is
rms
quantity we can express emittance 휎 Thermal 푝 휖 푥 푥 emittance = ≡ 휖 훽훾푚푐 푛 푚푐
o 1 = 푝 ccupied ccupied by the electron beam inthe phase 휖 푥 2 훽훾 푛
=
휖 푥 푥 2 휎 = Emittance 푥 푝
휎
휎 푥 2 푝 푥 푥 emittance −
푚푐 푝 푥 푥 2
푝 푥
2
as
p x
x
1st Topical Workshop on Laser Based Particle Sources • • • • • The thermal Aftersome mathwe get fromelectron the No dependence energy in the direction normal to the barrierto be larger thatbarrieritself As for the QE derivation, the the variance of electron momentum To derive(electronthermal distribution related)
휎 푝 2 푥 Thermal =
퐸 ∞ 푓 푚푐 +
emittance
휙 푒푓푓 2
− 퐸 ℏ휔 ∞ 푓 −
푑퐸 ℏ휔 푑퐸
1
is then then givenis by
−
1 휖 emittance 푓 − 퐹퐷 푡ℎ 푓 퐹퐷 휎 퐸 = 2 maximum emission angle 푝 푝
+ - 푚 푧 푥 2 퐸 electronscattering probability 휎 ℏ휔 = 푥 + ≥
ℏ휔 퐸 ℏ휔 푓 푓
ℏ휔 퐹퐷 + 3 푓 3
퐹퐷 −
푚 퐸 휙
− 푚 휙
푒푓푓 푐 푒푓푓 퐸
2
cos 1 푐 휙
2
푒푓푓 cos 1
휃 푚푎푥
휃
derivation 푚푎푥 퐸 emittance ( 퐸 푑 ) cos 푑 cos
휃 q max 휃
, we, calculate 0 2
is givenis by the 휋
0 2 푑
휋 Φ 푑
Φ 푝 푥 2
1st Topical Workshop on Laser Based Particle Sources • • • Angular ResolvedAngular SpectroscopyPhoto Emission Momentron ApproachGun RF – – – Thermal Correlate position versus laser spotsize emittance Minimize all contributions to Time of Flight (T. and momentum Vecchione x 2 1 c
et al. etal. FEL2011)
r and scanand
2 - Emittance 2 INFN Milano LASA m E
0 Kin
cos emittance 2
q
Measurement
M . Otevrel
et et al., FEL
2011
1st Topical Workshop on Laser Based Particle Sources 푄퐸 휔 = 1 1 + − QE 휆 휆 푒 표푝푡 푅 − 푒 휔 휔 휔 vs 8
휙
푒푓푓 ℏ휔 thermal thermal 휙 − 퐸 푒푓푓 퐹 휙 + 푒푓푓 = 휙 휙 푒푓푓 2 푊 −
푒 emittance 4
푒퐹 휋
휖 0
. . . 휖 푡ℎ the material!!! parameters and optical properties Free knobs are the QE. expense of verylow emittance We can reduce photon energy. dependency on havesame the QE and = 휎
scattering 푥 ℏ휔 emittance 3
− 푚
휙 at the
푐 푒푓푓 inside 2
1st Topical Workshop on Laser Based Particle Sources • • Cathode roughness(D. Xiang et al., PAC07) uniformityCathode “not” Other cathode contributions to cos ( x ) intrinsic 휖 푛푠 = 휎 l 푒 n 휋
( x 2 F. Zhou et al., PRSTAB 5, 094203 푎 2 푛 a emittance
n 퐸 푚
y 푅퐹 n
(x)=a 푐 sin 2 휆 푛 n
cos(2 휃 푅퐹
/ l n
)
)
1st Topical Workshop on Laser Based Particle Sources • • • Operation Intrinsic QuantumEfficiency – – – – – – – – – – Low dark currentLow dark Constant chargetrain the along Uniform(spaceemission charge effects) Fast(<100’s response time Highestat longestwavelengths Reliable installation and replacement Possible cleaningand/or rejuvenation Operationallifetime > months Eventuallyconflict tunable(see with QE) possible low as As • UHV requiredUHV Emittance
Photocathode
fs )
Request
1st Topical Workshop on Laser Based Particle Sources • • • of the Here Semiconductor Metal – – – – – PEA Superconductor Mg Cu NEA • • • • • • after
GaAs Cs K Cs Pb Nb
materials
2 CsSb 3 2
Sb Te
and strainedand
I made a «
Photocathodes and
labs very
» personal D. Dowell et al., selection
NIM
of some A 622(2010) 685
1st Topical Workshop on Laser Based Particle Sources • • • Pros repetition These Cons – – – – – – – No Low High work function UV light Low Very Low Unlimited
polarization are the
QEs field sensitivity fast
rate micro
response emission
lifetime Metal Photocathode cathode
to
possible vacuum
time bunch
used
condition
RF RF in normal gun
.
conducting D. Dowell et al.,
NIM
low A 622(2010)
685
1st Topical Workshop on Laser Based Particle Sources Emittance Bunch minimum QE Spot on Laser Parameter wavelength QE
Charge cathode
[
m]
[ pC
[mm]
] [nm]
Low Low QE after 500 Electronimage < 0.5 < 250 10 5 1.2 253 Value
- 5
Cu cathodeCu @ LCLS
• • • high high repetitionrates. operation of the machine, due to decreaseof QE at CleaningLaser to has necessary been allow diamond turning. The Cu cathode surfaceprepare is by optical quality From 2007 to 2011, three cathodes have beenused.
pC crystall Uniformity
run Last Cathode FirstCathode
gain
dominated
Microscopy Laser Cleaning Laser
by
x20
Emittance Laser Cleaning Laser
QE
1st Topical Workshop on Laser Based Particle Sources • • measurements a dedicatedhas PSI facility for intrinsic Ozonetechnique cleaningfor at Cu FERMI Before After X 20 Metal Photocathodes
QE trend C. Hauri et al., PRL 104, 234802(2010) C. Hauri et 104, PRL al., emittance M. M. Trov
ò,EUROFel 2011 Workshop
1st Topical Workshop on Laser Based Particle Sources • • Cons Pros – – – – – – second, each nearly of 1 European given High QE. Complex handling «Slow» response ( Semiconductor photocathode the the average laser power some of tens Wattsof Suitable for XFEL require to produce thousands to ten high high nC charge . , long long pulses and and high high repetition rate - D. Dowell et al., thousands of per bunches . For example NIM FLASH A 622(2010) machine machine or or 685 1st Topical Workshop on Laser Based Particle Sources PEAs Cs 2 Te Vacuum Sensitivity x 10 K 2 3 CsSb 1st Topical Workshop on Laser Based Particle Sources Low averageLow currentoperation Gun DC in QE mapon 02Oct 2012 QE mapon 03Apr 2012 QE mapon 03Feb 2012 KCsSb Cornell ERL @ QE~10% QE~5% QE~8% High L. Cultrera average currentoperation Gun DC in – 2012 PPP Workshop PPP 2012 damaged Central area manytrips RF hours2 mA ~20 No RF trips RF No hours2 mA ~10 1st Topical Workshop on Laser Based Particle Sources Measured in HV DC gun using solenoid scansolenoid using gun DC MeasuredHV in Thermal EmittanceMeasurement KCsSb @Cornell ERL L. Cultrera CsK Cs – 3 2 Sb 2012 PPP Workshop PPP 2012 Sb I. L. Bazarov Cultrera et al., etal., et al., etal., Appl. Phys. Appl. Appl. Phys. Lett Lett . . 98 99 (2011) 224101 (2011) (2011) (2011) 152110 1st Topical Workshop on Laser Based Particle Sources Photocathode made atBNL Transported to KCsSb BNL/JLAB @ coll. JLab actually,to theQE seems increase No QE decrease inany of the runs; QE scanQE after several runs T.Workshop 2012 PPP Rao 1st Topical Workshop on Laser Based Particle Sources QE change plot showsatplotQE change center QE decline of 20 mA, mA, 20 at Recordcurrent JLab at16mA weThen see aslightQE decline KCsSb DC Gun 100 GunDC the laser spot the @ BNL/JLAB coll kV,500 nm, 532 Thingswere “normal” at10mA . μ T.Workshop 2012 PPP Rao m spot m 1st Topical Workshop on Laser Based Particle Sources . . of 4 cathodes, to due the differentwavelength and the absence For Cs Lifetimes similar are and withinCLIC specifications. Cs th harmonics harmonics conversion stage 3 3 Sb a factor Sb afactor 6 less of QE asforis needed Cs Sb above 0.5% QE) (corresponds toh 270 1/e lifetime168h Cathode Cathode (Cs #189 & Cs 2.3 3 Sb) nC 2 , 350ns Te inRFGun Cathode Cathode (Cs #185 C. Hessler above 3% QE) (corresponds toh 300 @ CERN PPP 2012 Workshop 2012 PPP 2.3 2 Te) nC 2 Te , 350ns 1st Topical Workshop on Laser Based Particle Sources • • INFN Milano support theFLASH user facility at DESY. Cs 2 Teare used inmany laboratories around the world and Preparation Systems INFN Milano INFN Milano Fermilab Fermilab DESY HZB HZB (XPS) Cs photocathodes – – HH - Lab7 – A0 2 LASA TeMilano @INFN and systems Transfer System Fermilab DESY DESY DESY LBNL - - - REGAE FLASH around PITZ – NML the world 1st Topical Workshop on Laser Based Particle Sources LASA preparation system PreparationChamber @ FNAL Cathode systems RF Gunand FLASH Transfer FNALTransfer system Chamber linac 1st Topical Workshop on Laser Based Particle Sources • the Cathodes are transported under UHV condition from INFN Milano to Labs Labs since 1997. Loading Cathode SAES D100NEG Pump Getter Pump Ion Ion TransportSystem Triax C.C. OK for airplane No powersupplyneeded. 1st Topical Workshop on Laser Based Particle Sources Bajonet Carriage Evolution Carriage New SlidingNewBlocks IncreasedSpace A0 Carrier 2 nd 4 3 th rd gen. gen. gen. 1st Topical Workshop on Laser Based Particle Sources Prep. chamber viewport chamber Prep. Photocurrent signal Photocurrent • diameteris 5 mm layer and center maskingit. The actual maskingsystem the shapes During cathode depositiona circular Deposition and and DiagnosticDeposition Shutter photoemissive controller Remote Hg lamp Hg Photodiode wheel Filter photocurrent measurement • 405 nm, 436nm 254 nm, 334nm, 365nm, interference filters ( controlled • the film reflected light power from • Picoammeter A Calibrated photodiode motorized Active Area filter wheel 5 mm 5 and and remotely for ) 239 nm, with with for 1st Topical Workshop on Laser Based Particle Sources QE (%) 104.4 1.E+00 1.E+01 1.E+02 1.E 1.E 1.E 1.E 1.E - - - - - 05 04 03 02 01 2.00 Spectral response reproducibility 3.00 Photon Energy (eV) Photon Energy 417.1 4.00 123.2 tr.box, (sp.resp, 6 77.3 tr.box, (sp.resp, 21 107.3 tr.box, (sp.resp, 29 417.1 tr.box, (sp.resp, 4 104.4 tr.box, (sp.resp, 29 5.00 - - - Jul Jul Apr - - Dec Jun - - 2010) 2009) - 2011) - - 2011) 2010) 107.3 6.00 • • • No more low energy shoulder. The largely improved. cathode spectral responsesis The growing. better control The on Cs excess under is control reproducibility - line diagnostic 77.3 of the cathode of produced allows 123.2 . 1st Topical Workshop on Laser Based Particle Sources area • cathode has a clear darker area • Cathode 77.2 Lack Lack of coating After usage, central part of the – Post Usage Diagnostic Post Usage 176 176 days of operation on the right upper film expected Decrease QE Initialwas 11%. QE @ 254nm Measurementdoneat LASA Cathode 13.4 QE map:cat77 QE -4 -3 -2 -1 0 1 2 3 4 -4 -3 2 a ofastep0.500000mm +/-4.000000mm,cathode#77.2,(254) production ofthe -2 After -1 0 – = 1 lowenergythreshold 157days of operation 3.5% 2 3 4 QE map:cat77 QE 0 1 2 3 4 5 6 7 8 9 10 -4 -3 -2 -1 0 1 2 3 4 -4 -3 2 o ld a ofstep0.500000mm +/-4.000000mm,cathode#77.2,(254) After usage -2 -1 0 1 2 as 3 4 0 0.5 1 1.5 2 2.5 1st Topical Workshop on Laser Based Particle Sources 36 QE [%] 10 15 20 0 5 bins QE[%] 8.6 = 10 12 14 16 0 2 4 6 8 13 mm 13 0 Cs QE [%] = 8.6 +/- 2.7 8.6 = +/- [%] QE 2 10 mm 10 4 ± 2 2.7% Teproduction QE after 6 8 QE [%] 10 12 14 16 Cathode 18 QE[%] QE[%] 11.2 = 20 bins 10 0 2 4 6 8 0 QE [%] = 11.2= 1.6 [%] +/- QE 2 5 mm 4 ± 1.6% 6 8 10 12 14 16 18 20 1st Topical Workshop on Laser Based Particle Sources QE [%] 10 15 20 25 0 5 0 50 days days of operation QE duringoperation 100 150 90.3 109.2 91.2 114.2 98.2 76.3 77.2 200 • • cathodeThe if: ischanged operation. 100 daysthan continuous of The operationlast for more operation.QE during LINAC operation dark currentis limiting 0.5 % QE < 1st Topical Workshop on Laser Based Particle Sources Todecreaseoperation during DC notice gun. be of the Trenddark current of for differentguns. Darkcurrent (A) 1000 1200 1400 1600 1800 2000 2200 200 400 600 800 10 0 - Dec - 02 DESY DESY Gun2 #57.2 #60.1 Dark current and Guns 17 - #500.1 Aug #61.1 - 03 23 #60.1 - Apr #61.1 - #500.1 04 DESY DESY Gun1 29 - Dec #500.1 - 04 #58.1 5 - Sep - 05 #80.1 DESY DESY Gun3.2 13 - May #53.1 - #58.1 06 1st Topical Workshop on Laser Based Particle Sources • • Diamond AmplifierDiamond Plasmonic – – R. Li et al. PRL 100, 074801 (2013) Li 074801 PRL et al. R. 100, surface plasmon havestrong metaland betweenlightcoupling electronoscillationor With nanostructurestructuredengineered interface, it ispossible to electronsThe are acceleratedthen field RF bythe that as barrier electronacts an thin multiplier. QE photocathodea diamond An high sits behind – New trends and Enhanced PhotoemissionEnhanced A. A. Polyakov et al. PRL 100, 076802 (2013) 076802 et al. 100, PRL ideas T. Rao,BNL - 73169 X 10 - 2004 6 - CP 1st Topical Workshop on Laser Based Particle Sources Final Cathode S. S. Schreiber2011 Overview Eurofel Photocathode Workshop 1st Topical Workshop on Laser Based Particle Sources Final Laser S. S. Schreiber2011 Overview Eurofel Photocathode Workshop 1st Topical Workshop on Laser Based Particle Sources • • • • • • • a ...... CERN Photocathodes Cs Eurofel2011 Photocathode Workshop WorkshopP3 series – – – nd manyothers 2 http http://wwwlasa.mi.infn.it/ttfcathodes http:// Te :// Photocathode Photoemission photoinjector.web.cern.ch/photoinjector/default.htm photocathodes.chess.cornell.edu/wiki/Main_Page Wiki References Database Laboratory 1st Topical Workshop on Laser Based Particle Sources • Investigationshomogeneityon of electron emission #613.1 #613.1 operatedat PITZ 60 MV/m, 600 bunches approx. 1 Standard Cathode analysis: than a week – possible possible life time problem!! QE nC - , 10 , 10 Hz maps – less less 1st Topical Workshop on Laser • Based• Particle Sources 푄퐸 > [1] R 푊 in Preparation recipe results Theory [1]: 3.5 W no low energy part technique from LASA => New preparation 𝑖 Quantum EfficiencyQuantum measurementsin (QE) Standard Cathode analysis 2 . PowelPhys.etal., Rev. B > > 퐸 = = 3.5 푝ℎ QE versusQE photonenergy afterQE preparation after/ usage 퐸 푔 - 𝑖 = 3.6 + 퐴 퐸 eV 1 eV 푎 𝑖 퐸 푝ℎ − 푊 8 1 (1973), 3987. (1973), QE [%] 푚 1E-4 1E-3 0.01 100 1 0.1 10 1 + 2.5 퐴 2 퐸 푝ℎ 3.0 after usage after preparation − 푊 3.5 2 photon energyphoton [eV] 푚 : 2 cw 4.0 cw mode 4.5 mode 5.0 5.5