High-Efficiency, High-Energy THz Generation

Intense ultrafast THz fields are of great interest for electron acceleration, beam manipulation and measurement, and pump-probe experiments with coherent soft/hard X-ray sources based on FELs or inverse Compton scattering sources. Acceleration at THz frequencies has an advantage over RF in terms of accessing high electric-field gradients (>100 MV/cm), while the beam delivery can be treated quasi-optically. This holds the promise of reducing the size and cost of conventional accelerators and X-ray sources by many orders of magnitude, thereby making this indispensable scientific tool available to a much broader range of users. However, high-field THz pulse generation is still demanding when compared with conventional RF generation. The generation of high efficient and high energy THz wave and proof-of-principle acceleration and inverse Compton scattering experiments are the subject of this research.

Recently, we generated highly efficient, single-cycle, 0.45-THz pulses by optical rectification of 1.03-µm pulses in cryogenically cooled lithium niobate (LN). Using a near-optimal duration of 680 fs and a pump energy of 1.2 mJ, we achieved record-high conversion efficiencies above 3% [1], >10 times higher than previous report (0.24%) [2]. Cryogenic cooling of lithium niobate significantly reduces the THz absorption, which will enable the scaling of THz pulse energies to the mJ. Comprehensive 3D simulation efforts to understand and optimize the THz generation system are also being made.

Initial experiments on the THz streaking of photo-emitted electron beams showed an evidence of electron beam acceleration [3]. We also generated radially-polarized TEM01 modes via polarization and mode conversion, suitable for THz electron acceleration in a dielectric waveguide.

References

1. S.-W. Huang, E. Granados, W. R. Huang, K.-H. Hong, L. E. Zapata, and F. X. Kärtner, “High conversion efficiency, high energy terahertz pulses by optical rectification in cryogenically cooled lithium niobate,” Opt. Lett. 38, 796 (2013). [Highlighted in Nature Photonics 7, 343 (2013), “Cool generation”]
2. J. A. Fülöp, L. Palfalvi, S. Klingebiel, G. Almasi, F. Krausz, S. Karsch, and J. Hebling, Opt. Lett. 37, 557 (2012).
3. X. Wu, S. Carbajo, K. Ravi, F. Ahr, G. Cirmi, Y. Zhou, O. D. Mücke, and F. X. Kärtner, “Terahertz generation in lithium niobate driven by Ti:sapphire laser pulses and its limitations,” Opt. Lett. 39, 5403 (2014).
4. K. Ravi, W. R. Huang, S. Carbajo, X. Wu, and F. X. Kärtner, “Limitations to THz generation by optical rectification using tilted pulse fronts,” Opt. Express 22, 20239 (2014).
5. K.-H. Hong, W. R. Huang, K. Ravi, S.-W. Huang, E. Granados, L. E. Zapata, and F. X. Kärtner, “Highly Efficient, High-Energy THz Pulses from Cryo-cooled Lithium Niobate for Accelerator and FEL Applications,” Proceeding of FEL 2013 (Manhattan, NY, Aug. 26-30, 2013), MOPS034.
6. K. Ravi, M. Hemmer, G. Cirmi, F. Reichert, D. N. Schimpf, O. D. Mücke, and F. X. Kärtner, “Cascaded parametric amplification for highly efficient terahertz generation,” Opt. Lett. 41, 3806 (2016).