1. Workshop on High-Field THz Science High Power THz Generation and THz Field Enhancement in Nanostructures Fabian Brunner 1, Salvatore Bagiante 2, Florian Enderli 1, Justyna Fabianska 1, and Thomas Feurer 1 1 Institute of Applied Physics, University of Bern, Switzerland 2 Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, Switzerland

2. Outline > Introduction: Applications of strong electric and magnetic fields at THz frequencies > Generation of intense THz pulses in nonlinear optical crystals > THz field enhancement in metallic nanostructures > THz SHG in lithium niobate crystals > Conclusions Outline2

3. Applications of strong electric and magnetic fields at THz frequencies > Nonlinear optics at THz frequencies (bulk semiconductors, quantum dots, …) > Direct excitation of transverse optical phonons or anti- ferromagnetic spin waves > Acceleration of charged particles and measurements of electron bunch profiles (THz streak camera) > Single shot imaging > … Introduction3

4. Efficient THz generation in NLO crystals: Requirements for the crystals > A large tensor element of the nonlinear optical susceptibility > Low linear and nonlinear optical absorption > Low THz absorption > Phase-matching: —collinear: (organic nonlinear optical crystals: DAST, OH1,…) —Cherenkov-type: (LiNbO 3 ) THz generation in nonlinear optical crystals4

5. High power THz time-domain spectroscopy 5

6. Comparison of laser-based methods for THz pulse generation 6

7. Peak electric field vs. pulse energy Assumptions: > Single cycle pulse > Focusing with F# = 1 7

8. Non-resonant enhancement: array of slits 8 Gold slit array on silicon Typical field enhancement of a slit

9. Ideal periodicity of slit array > most effective harvesting of incident field 9 > Nonlinear signal: trade-off between volume of high field strength and high field enhancement

10. Resonant enhancement: Split ring resonators 10 31 nm

11. Second harmonic generation in LiNbO 3 Simulated spectra of transmitted pulses 11 Fabrication of the sample

12. Conclusions > Combination of high power THz source with local field enhancement enables new experiments > Fabrication of gold nano-structures on silicon > New fabrication process for gold nano-slits filled with lithium niobate > Simulation of SHG spectra and optimization of geometry parameters for the highest conversion efficiency 12