Electron Dynamiχ MPRG Fritz-Haber-Institut der Max-Planck-Gesellschaft

Electron Dynamiχ MPRG Fritz-Haber-Institut der Max-Planck-Gesellschaft How exciting! 2016 Berlin, 3-6 August laura.foglia@elettra.eu

1 Current research challenges V Light Harvesting Light Emission Energy level alignment at the interface determines the functionality

2 Current research challenges V material A i n t e r f a c e material B electron hole photon Light absorption Efficiency depends on:

2 Current research challenges V material A i n t e r f a c e material B electron attraction hole Efficiency depends on: Light absorption Exciton formation and depletion

2 Current research challenges V material A i nt material B e r f a c e Efficiency depends on: Light absorption Exciton formation and depletion Diffusion rate

2 Current research challenges V material A i n t e r f a c e material B Efficiency depends on: Light absorption Exciton formation and depletion Diffusion rate Charge separation and transfer at the interface

3 Linear optical probes of optoelectronics Polarization CB FX Photoluminescence (PL) VB

3 Linear optical probes of optoelectronics Polarization CB FX Photoluminescence (PL) Excited state absorption VB

3 Linear optical probes of optoelectronics Polarization CB FX Photoluminescence (PL) Excited state absorption Monitor the signal as function of Δt Electron and exciton dynamics VB

3 Linear optical probes of optoelectronics Polarization CB FX Photoluminescence (PL) Excited state absorption Monitor the signal as function of Δt Electron and exciton dynamics VB Interfacial response buried into bulk signal No chemical specificity

4 Non linear optical spectroscopy At high incident intensities Second order non linearity Leads to generation at Sum Frequency only for non-centrosymmetric materials or broken inversion symmetry interface specific

4 Non linear optical spectroscopy At high incident intensities Second order non linearity Leads to generation at Sum Frequency Enhanced for photon energies matching intermediate-state or final-state transitions only for non-centrosymmetric materials or broken inversion symmetry interface specific

4 Non linear optical spectroscopy At high incident intensities Second order non linearity Leads to generation at Sum Frequency Enhanced for photon energies matching intermediate-state or final-state transitions Time-resolved electronic sum-frequency generation spectroscopy: tr-esfg only for non-centrosymmetric materials or broken inversion symmetry interface specific [1] Foglia et al., in preparation, (2016)

5 Setup 50% of RegA output generates pump 5% generates white light continuum (WLC) WLC is compressed with deformable mirror 10% used as upconversion pulse (800 nm) 1 Pump drives sample out of equilibrium 2 Time overlapped WLC and 800 nm generate E SFG at the sample 3 SFG is detected in EMCCD 4 Control of time delay between pump and E SFG monitor transient changes of χ (2)

6 The system ZnO (1010) 3.4 ev wide band gap [1] High carrier mobility Wurtzite structure Space group C 6 4v χ (2) 0 in the bulk 3 split valence bands [2,3] C-axis parallel to non-polar surface ZnO surface calculation: courtesy of O. Hofmann and P. Rinke [1] Reynolds et al., Phys. Rev. B 60, 2340 (1999) [2] Özgür et al., J. Appl. Phys. 98, 041301 (2005) [3] Thomas, J. Phys. Chem. Solids 15, 86 (1960)

7 Spectral characterization Luminescence Sharp feature starting at 3.4 ev Excitonic emission LOP-scattered replicas Defect band (1.7 2.7 ev)

7 Spectral characterization Luminescence Sharp feature starting at 3.4 ev Excitonic emission LOP-scattered replicas Defect band (1.7 2.7 ev) Structure visible in the high resolution spectrum VB A and B

8 Spectral characterization Luminescence

8 Spectral characterization Static esfg

8 Spectral characterization Static esfg Signature of valence bands A, B and C [1] [1] Thomas, J. Phys. Chem. Solids 15, 86 (1960)

8 Spectral characterization Static esfg Signature of valence bands A, B and C [1] Polarization dependence [1] Thomas, J. Phys. Chem. Solids 15, 86 (1960)

8 Spectral characterization Static esfg Signature of valence bands A, B and C [1] Polarization dependence esfg couples directly with ALL excitonic transitions [1] Thomas, J. Phys. Chem. Solids 15, 86 (1960)

8 Spectral characterization Static esfg Signature of valence bands A, B and C [1] Polarization dependence esfg couples directly with ALL excitonic transitions UV alone induces luminescence background [1] Thomas, J. Phys. Chem. Solids 15, 86 (1960)

Electron dynamics How exciting! 2016 Berlin, 3-6 August 9

Electron dynamics tr-esfg proof of principle! How exciting! 2016 Berlin, 3-6 August 9

9 CB relaxation

9 CB relaxation Final state exciton-polariton resonances: superimposed to CB continuum

9 CB relaxation Final state exciton-polariton resonances: superimposed to CB continuum CB electron relaxation: scattering with acoustic phonons [1] [1] Zhukov, Phys. Rev. B 82, 094302 (2010)

10 IGS dynamics Photoinduced signal at A+B energy to static SFG

10 IGS dynamics Photoinduced signal at A+B energy to static SFG step-like enhancement @ t=0 Non-zero signal at t<0 τ REL > 25 μs

10 IGS dynamics Photoinduced signal at A+B energy to static SFG step-like enhancement @ t=0 Non-zero signal at t<0 τ REL > 25 μs Defect-related in-gap state dynamics

11 exciton formation

11 exciton formation Signal increase

11 exciton formation Signal increase At A and LOP energy

11 exciton formation Signal increase At A and LOP energy Spectral shape coincides with luminescence Emission increase due to stimulated emission from esfg photons

11 exciton formation Signal increase At A and LOP energy Spectral shape coincides with luminescence Emission increase due to stimulated emission from esfg photons exciton ground state forms in 200 ps

12 Summary Proof of principle of electronic sum-frequency generation spectroscopy Excitonic signatures related to all valence bands Conduction band electrons relax within few ps In-gap states contribute to esfg dynamics At low excitation densities: exciton ground state formation within 200 ps esfg is sensitive to excitonic signatures First step towards interface specific exciton spectroscopy

13 Non linear optical spectroscopy At high incident intensities Third order non linearity non-zero in ALL materials [1] Bencivenga et al., Nature 520, 205 (2015)

13 Non linear optical spectroscopy At high incident intensities Third order non linearity non-zero in ALL materials Leads to four-wave mixing processes: e.g. Coherent Antistokes Raman Scattering (CARS) three incoming beams,, generate with and. B A Δt 12 Δt 23 Δt 34 ω 4 ω 1 ω 2 ω 3 ES GS t=0 ω ex = ω 1 - ω 2 [1] Bencivenga et al., Nature 520, 205 (2015)

13 Non linear optical spectroscopy At high incident intensities Third order non linearity non-zero in ALL materials Leads to four-wave mixing processes: e.g. Coherent Antistokes Raman Scattering (CARS) three incoming beams,, generate with and. B New generation seeded XUV Free Electron Laser FERMI @ Elettra allows four-wave mixing at high energies [1] A ω 1 Δt 12 ω 2 Δt 23 ω 3 Δt 34 ω 4 ES GS t=0 ω ex = ω 1 - ω 2 [1] Bencivenga et al., Nature 520, 205 (2015)

14 XUV coherent Raman Scattering ω 1 ω 2 exciton CB VB Δt ω 2 ω 1 Δt ω 3 ω out ω 3 ω out= ω 3 +ω ex P-site N-site Larger incident ω high energy excitations, e.g. valence band excitons larger wavevectors (FWM signal sensible to the structure) relaxed dipole selection rules Resonant enhancement of χ (3) close to core resonances chemical specific excitation and probing [1] Tanaka and Mukamel, Phys. Rev. Lett. 89, 043001 (2002)

14 XUV coherent Raman Scattering ω 1 ω 2 exciton CB VB Δt ω 2 ω 1 Δt ω 3 ω out ω 3 ω out= ω 3 +ω ex P-site N-site Larger incident ω high energy excitations, e.g. valence band excitons larger wavevectors (FWM signal sensible to the structure) relaxed dipole selection rules Resonant enhancement of χ (3) close to core resonances chemical specific excitation and probing Direct observation of excitation energy transfer within a molecule [1] Tanaka and Mukamel, Phys. Rev. Lett. 89, 043001 (2002)

Thanks to Electrondynamiχ group @ FHI Berlin Julia Stähler Martin Wolf Selene Mor Sarah King Lea Bogner Jan-Christoph Deinert Marc Herzog Daniel Wegkamp Sesha Vempati EIS & DiProI Beamlines at FERMI Claudio Masciovecchio Filippo Bencivenga Flavio Capotondi Emanuele Pedersoli Riccardo Mincigrucci Alberto Simoncig Riccardo Cucini Andrea Calvi Emiliano Principi Maya Kiskinova How exciting! 2016 Berlin, 3-6 August