Plasmonic cathode research at UCLA P. Musumeci UCLA Department of Physics and Astronomy
Acknowledgements • Zhang Zhe (visiting student from Tsinghua University) • H. To, G. Andonian, Radiabeam Technologies • R. K. Li (now at SLAC), D. Cesar, E. Curry, E. Pirez, S. Custodio, E. Threkheld, UCLA • H. Padmore, A. Polyakov, LBNL • F. Hannon, JLAB
• Funding agencies: DOE STTR, Radiabeam Technologies, NSF
Outline • Review of results – Nanopatterned cathodes for RF photoinjectors
• Research directions – Novel materials / structure tests • Ag substrate • Nano‐grooves resonance
– Transverse structure preservation. • e‐e scattering limit • High gradient 1.4 cell gun development
• Conclusions
Motivation / background •
High‐average power beams from metallic photocathodes – Robust – Prompt emission – Tolerate “poor” vacuum conditions – Easily integrable into existing injectors – limited by low QE – drive laser conversion efficiency
•
Large yield from multiphoton photoemission
•
Ultrashort laser pulses naturally married with ‘blow‐out’ regime
•
Avoid lossy non‐linear frequency conversion
•
How to enhance multiphoton photoemission? – By modifying reflectivity (coating) – By optical field enhancement (Surface plasmon excitation)
Musumeci et al. PRL,100:244801, 2010
Surface plasmon assisted photoemission • •
• • •
The reflectivity of a metal can be controlled by coupling incident linearly polarized light with surface plasmon oscillations. Kretschmann geometry requires back‐illumination.
The coupling can also be done by using periodic nanostructures such grids or arrays of holes. Use Focused Ion Beam technique for nanofabrication FDTD simulations (Lumerical) to optimize pattern (p, d, w for Gaussian holes). Low reflectivity corresponds to optical electric field enhancement.
w d p
Nanopatterned cathodes in RF photoinjectors Charge yield map Ratio pattern/flat > 100 ! 3‐photon emission process Emittance x3 relative to flat Cu Pulse length comparable to flat Cu
3‐photon emission
1.75
Laser spot
2.110 1.847 1.585 1.323 1.060 0.7975 0.5350 0.2725 0.01000
Laser spot 1.50
<0.01
Y(m)
• • • • •
1.25
125 um x 125 um 1.00 1.25
1.50
Pattern area in black
1.75
X(m)
Optical reflectivity measurements
Recent results (after PRL) • •
•
Single crystal wafer integration onto cathode plug Fast solution: recessed cathode plane – Frequency shift 470 KHz – Degrade cathode field/peak field ratio in the gun – Looking at more advanced cathode plug designs 10 mm diameter wafer of any material can be inserted and tested in few hours !
•
Broadband Imaging spectrometer – White‐light source, gratings, slits – CCD + spectrometer
•
Slight polarization dependence – Due to asymmetry of holes – Seen in simulations
Patterned wafer Polarization dependence
Simulations
Charge yield characterization of the nanopattern Charge yield ratio > 3000 times !
Gain x20
Could be further improved by higher extraction field, or shorter laser wavelength
charge density versus incident laser intensity Absorption of 800 nm laser is 81.8% for the nanopattern and 7.8% for the flat surface, Due to absorption is (81.8/7.8)3=1.1×103 5x in yield due to the non‐uniform distribution of laser intensity on the surface (see PRL 110, 074801 (2013)).
Goal of current efforts • Current yield enables 1 J @ 800 nm ‐> 100 pC e_beam equivalent to 10‐3 QE (with caveats of non‐linear process !) • Increase yield ! • Different materials/work function – Silver, Mg, Nb
• Nanobeam dynamics – Preserve structure for emittance exchange “nano‐bunched” beams
• Nanogrooves • Kretschmann configuration – Rear illumination Material
Spacing (nm)
FWHM (nm)
Depth (nm)
Reflectivity @800 nm
Cu
767
200
240
0.4%
Nb
750
280
364
0.9 %
Mg
780
217
261
0.81 %
Ag
770
174
209
0.63 %
Ag nanoholes • Higher absolute yield from nanopattern • Lower relative enhancement ( < 10) • UV measurements indicate lower work‐ function for Ag • Need to measure ! Pattern • Emittance >2 mm‐mrad/mm rms
QE map
1 mm
Nanogrooves on Cu • Investigate role of field enhancement in emission • Use FIB to nanofabricate grooves • Optimize design with Lumerical • Reflectivity data under analysis • Strong polarization dependence • Emission in RF photoinjector ‐ next
Polyakov et al. PRL 2013
Simulations of nanostructured cathodes • Transverse structure at the sub‐ micron scale in the beam • Could exploit in EEX schemes (Graves et al. PRL, 2012) • Understand thermal emittance origin • Can we preserve the structure? ‒ Similar problem to relativistic TEM (Li&Musumeci Phys. Rev. Applied, 2014)
‒ Relativistic photoemission electron microscopy (RPEEM) ‒ Space charge analyzed as sum of smooth and discrete contribution ‒ Extraction field is very important !
Preserve transverse structure •
Tighter requirements on beam quality •
•
Not just second‐order moment but detail of distribution
Different contribution to the spatial resolution vs. current ‒
Rose criterion (need enough electrons to beat Poisson noise)
‒
Gun+solenoid aberrations (small for RF photoinjectors)
‒
Space charge: smooth + stochastic part
•
Scaled simulation (constant density) is usually incorrect !!!
•
Full scale simulation cumbersome and not easy to optimize 100 MV/m = 30 deg z_sol = 0.19 m
1.0e-5
10 pC/mm2
z = 3.0 m Mag = 3.7
0.5e-5 0.0e-5 -0.5e-5 -1.0e-5 -1e-5
0e-5
Initial
1e-5
4e-5
4
2e-5
4
0e-5
4
-2e-5
4
-4e-5 4
-5e-5
0e-5
5e-5
No space charge
Smooth space charge
Point‐to‐point
1.4 cell gun development Traditional (1.6 cell) photocathode rf gun optimized for high charge, high final output energy
Shorten the photocathode cell 1.6 cell type
At Pegasus recently commissioned up to 100 MV/m novel design (not‐brazed) 1.6 cell gun made in Italy
Limited by power output of RF klystron.
Using SPARC‐like 1.2 us‐long RF pulse for driving
1.4 cell gun can give much higher extraction gradient. Under development using similar design.
2 cm
cathode
Shortening the photocathode cell ï higher launch phase, e.g. 70˚, (sin70˚=0.94), ï ×2 improvement in brightness
Decrease effect of e‐e interactions Improved point spread function (Best quantity to quantify imaging performances)
1.4 cell type
Conclusions • Plasmonic photocathodes in operation in high field RF photoinjectors • Promise for further improvements in yield • Can they be useful? Need better understanding: – Emittance growth – Lifetime and damage threshold
• Nanostructure transverse dynamics in high brightness electron beams Thank you for your attention !