Ultrafast Electron Sources for Diffraction and Microscopy Applications UCLA Workshop, December 12-14, 2012 UIC m*: A Route to Ultra-bright Photocathodes W. Andreas Schroeder Joel A. Berger and Ben L. Rickman Physics Department, University of Illinois at Chicago Department of Energy, NNSA DE-FG52-09NA29451 Department of Education, GAANN Fellowship DED P200A070409 Outline UIC Experiment: Direct transverse rms momentum pT measurement Two-photon thermionic emission (2ωTE) from Au (2ħω < ) GaSb and InSb photocathodes Excited state thermionic emission (ESTE); ħω < Electron effective mass (m*) effects … Metal photocathodes (Ag, Ta, Mo, and W) Single-photon photoemission (1ωPE); ħω > More evidence of m* effects … Simulation of photoemission (m*, g(E), T(p1,p2)) Agreement with standard expressions of pT for m* = m0 Significant reduction of pT for m* < m0 Brightness: Transverse Emittance UIC Measure of transverse electron beam (or pulse) quality: T mc x 2 k x2 1 x.pT mc … a conserved quantity in a ‘perfect’ system. ‘Short-pulse’ Child’s Law: x0 Nq ≈ 0.5mm for N = 108 0 E DC Reduce pT Standard theoretical expressions: Single-photon photoemission: pT m( eff ) 3 Thermionic emission: pT mkBT D.H. Dowell & J.F. Schmerge, Phys. Rev. ST – Acc. & Beams 12 (2009) 074201 K.L. Jensen et al., J. Appl. Phys. 107 (2010) 014903 UIC Experiment 2W, 250fs, 63MHz , diodepumped Yb:KGW laser 1W, ~200fs at 523nm ~4ps at 261nm (ħω = 4.75eV) Electron detector at back focal plane of lens system Direct measurement of ΔpT distribution Analytical Gaussian (AG) model UIC DC photo-gun − Extended AG model simulation Fourier plane beam size independent of x0 pT0 Agreement with experiment indicates minimal aberrations Detector Lenses ½pT0 J.A. Berger & W.A. Schroeder, J. Appl. Phys. 108 (2010) 124905 2ħω thermionic emission (2ωTE) UIC – ħω = 2.37eV and Au = 5.1eV EXPECT: e0.35eV Au Isotropic rms momentum pT ~35meV ħ ħ EDC 8kV/cm F Au Vacuum Thermionic emission of tail of two-photon excited Fermi electron distribution I2Laser dependence of emission Increasing pT with ILaser Heating of Fermi electron gas UIC 2ωTE: Au results – 300nm Au film on Si wafer substrate Au ħω = 2.37eV Nonlinear I2 electron yield 2ω process Zero free parameter AG model fit to data: Laser heating of Fermi electron gas I2 pT m0 k BTe … as m ≈ m0 in Au GaSb and InSb photoemission? UIC – ‘Real space’ picture: ħωLaser = 4.75eV (261nm) Electron yield, Y GaSb GaSb InSb InSb Expect minimal (if any) singlephoton photoemission: ħωLaser ħωLaser ħω (eV) G.W. Gobeli & F.G. Allen, Phys. Rev. 137 (1965) A245 ħω eff ≤ 0 … Schottky barrier suppression ~35meV at 8kV/cm UIC GaSb and InSb results − Strong electron emission with ~4ps, 261nm pulses p-polarized UV radiation incident at 60º: InSb GaSb ≈ 4x10-6 InSb ≈ 7x10-6 GaSb GaSb UIC GaSb band structure – Vacuum level at eff = 4.84eV above bulk VB maximum Strong absorption at 261nm: eff = 1.44x106cm-1 -1 ≈ 7nm εF … ‘metal-like’ -valley transitions from VB (HH, LH, and SO bands) to upper 8 conduction band J.R. Chelikowsky & M.L. Cohen, Phys. Rev. B 14 (1976) 556 D.E. Aspnes & A.A. Studna, Phys. Rev. B 27 (1983) 985 UIC ESTE in GaSb − -valley absorption at ħω = 4.75eV E 8 Eelectron CB Eg / τdecay 7 ħω k LH SO Eg/ 3.85eV 0.99eV Initial excess Eelectron Te ~0.35eV 4,200K ħωLO 29meV τLO ~200fs m*(8) ~0.3m0 Initially; exp[-/(kBTe)] ≈ 0.06 Eg HH GaSb properties Excited state thermionic emission Cooling rate of ~1,600K/ps by LO phonon emission AND possible fast decay via 7 band No electron emission latency pT for GaSb UIC − Analysis of Fourier plane momentum distribution Fit to AG model simulation using pT mkBT gives mT ≈ 360m0 (i) For m = m0 with T = 360K: exp[-/(kBT)] ~ 10-15 … no emission !! 480(±50)μm (HWe-1M) (ii) For m = m* ≈ 0.3m0 with T = 1,200K: exp[-/(kBT)] ≈ 5x10-5 … reasonable for TE (c.f. GaSb ≈ 4x10-6) pT m * k BTe m* dependence of pT UIC − Quantum mechanics: Potential step e- p2 Cathode p 22 E2 2m0 p12 E1 2m * p2 p// p1 e- p// p1 Cathode Vacuum Vacuum Momentum parallel to interface is conserved AND for emission; p//max 2m * ( E1 ) An implicit m* dependence for pT UIC 1ωPE: Ag photocathode − Fourier plane data vs. AG model simulation Spot size (mm) ħω = 4.75eV (261nm) Ag E = ħω eff (eV) pT m0 ( eff ) 3 UIC 1ωPE: Metals − Ag, Ta, Mo, and W Spot size (mm) ħω = 4.75eV (261nm) pT Ag Ta W Mo E = ħω eff (eV) m0 ( eff ) 3 pT and m* UIC − Effective mass in metal photocathodes: dH-vA, CR, optical, … Cu pT ,expt. m0 ( eff ) 3 Mo Mg Ag W Ta m* m0 H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102 X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea. UIC Photoemission Simulation − Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K) m* = m0 Transverse momentum distribution (Fourier plane) 0.8 1.0 pz ((m0.eV)) 0.8 0.6 0.6 0.4 0.4 0.2 0.0 1.0 -1.0 0.2 0.5 0.0 -0.5 0.5 0.0 0.5 pT ((m0.eV)) pT ,sim. 0.0 -1.0 -0.5 0.0 pT ((m0.eV)) 0.5 1.0 m0 ( eff ) 3 1.06 1.0 1.0 UIC Photoemission Simulation − ‘Light Fermion’ Ag photocathode (eff = 4.52eV, ħω = 4.75eV, F = 5.5eV, Te = 300K) m* = 0.3m0 Transverse momentum distribution (Fourier plane) 1.2 pz ((m0.eV)) 1.0 max. = 1.0 m* ≈ 33 m0 sin-1 0.8 0.6 0.8 0.4 0.6 0.2 0.0 0.4 0.6 -0.6 0.4 -0.4 0.2 -0.2 0.0 0.0 0.2 0.2 0.4 0.4 pT ((m0.eV)) 0.2 pT ,sim. 0.0 -0.6 -0.4 -0.2 0.0 pT ((m0.eV)) 0.2 0.4 0.6 m0 ( eff ) 3 0.64 m* m0 0.6 0.6 pT and m* UIC − Effective mass in metal photocathodes: dH-vA, CR, optical, … Cu pT ,expt. m0 ( eff ) Te ? 3 Mo Simulation (Te =0) Ag W Oxide? Mg Ta m* m0 H.J. Qian et al., Phys. Rev. ST – Acc. & Beams 15 (2012) 040102 X.J. Wang et al., Proceedings of LINAC2002, Gyeongju, Korea. UIC Summary m* Mean square transverse momentum: pT 2 M ( eff ) 3 3k BTe 1 eff 2 … where M = min (m*, m0) PLUS: small emission efficiency enhancement for m* < m0 A route to high brightness, planar photocathodes UIC Thank you! UIC NEA GaAs − Cesiated NEA GaAs photocathode (GaAs-CsO) 1.8 m* = 0.067m0 ≈ 15 max. sin 1 m* 15 m0 pz ((m0.eV)) 1.6 1.4 1.2 1.0 0.8 -0.3 -0.2 -0.1 0.0 0.1 pT ((m0.eV)) Zhi Liu et al., J. Vac. Sci. Tech. B 23 (2005) 2758 0.2 0.3 UIC m*: Emission efficiency − Quantum mechanics: Potential step ep 22 E2 2m0 p12 E1 2m * Barrier transmission: T 2 p p2 2 1 R 1 1 p1 p 2 |T |2 ≈ 1 for p1 ≈ p2 i.e., for m*E1 ≈ m0E2 Cathode Vacuum … only possible for m* < m0 2 UIC m*: Emission efficiency − Quantum mechanics: Potential step = 4.5eV Barrier transmission: eE2 p12 E1 2m * p 22 2m0 |T|2 T 2 m* = 0.1m0 p p2 2 1 R 1 1 p1 p 2 2 |T |2 ≈ 1 for p1 ≈ p2 i.e., for m*E1 ≈ m0E2 Cathode Vacuum m* = m0 m* = 10m0 … only possible for m* < m0 E = ħω (eV) Emission efficiency enhancement for m* < m0 UIC