Extremely high pump-to-terahertz (THz) conversion efficiencies up to 0.7% were demonstrated in recent experiments with ZnTe THz pulse sources. Such high efficiencies could be achieved by pumping at an infrared wavelength sufficiently long to suppress both two-and three-photon absorption and the associated free-carrier absorption at THz frequencies. Here, highfield high-energy THz pulse generation by optical rectification in semiconductor nonlinear materials is investigated by numerical

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... ns. Basic design aspects of infrared-pumped semiconductor THz sources are discussed. Optimal pumping and phase-matching conditions are given. Multicycle THz pulse generation for particle acceleration is discussed. Index Terms-Semiconductor materials, Terahertz materials, Terahertz radiation M. I. Mechler is with MTA-PTE High-Field