[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Cherenkov Generation of Wideband Terahertz Radiation using a Sub-Micron LiNbO <sub/>3</sub> Slab Waveguide

摘要： Terahertz (THz) radiation is critical to the areas of security, medicine, communications, and consumer applications. THz generation is often achieved using lithium niobate, LiNbO3, since it has a high nonlinear coefficient in this regime (d33=180 pm/V [1]). Dramatic improvement to the generation efficiency has been achieved by incorporating this material into waveguiding configurations [2,3], since waveguides allow the pump pulse to remain confined over large distance (millimeters or longer). To date, waveguides investigated for THz generation utilize transverse core dimensions larger than a few microns. However, when the waveguide core is sub-wavelength with respect to the pump wavelengths, as well as the generated THz wavelengths, another benefit is observed: substantial broadening of the generated THz bandwidth. Here, we report on a sub-wavelength SiO2-LiNbO3-SiO2 slab waveguide that produces wideband THz radiation and emits it in the form of Cherenkov waves. The investigated LiNbO3 slab waveguide has transverse core dimensions of 720 nm × 4 mm and a length of 100 μm. This waveguide is excited using an electric field pulse having a duration of 50 fs and a central-wavelength of 780 nm. Figure 1(a) shows the generated THz electric field pulse, which has the short duration of <1 ps. The power spectrum of the generated THz pulse is shown in Fig. 1(b), having a central-frequency of 6.9 THz and a full-width half-maximum (FWHM) bandwidth of 2.1 THz. Notably, LiNbO3 has a strong phonon resonance at 7.6 THz [4]. However, since the generated THz radiation is emitted as Cherenkov waves, it exits the sub-wavelength LiNbO3 core after propagating a distance much less than a wavelength. This characteristic minimizes the high loss of the LiNbO3 phonon resonance, allowing for generation to occur directly at this phonon resonance. As such, the sub-wavelength nature of the LiNbO3 core allows for dramatic improvement to the THz radiation bandwidth. The data is Fig. 1 is obtained by performing finite-difference time-domain simulations. Experimental verification of this result is currently underway. In this experimental work, the LiNbO3 slab waveguide instead has a length of 1 cm and the excitation pulses will have energies ranging up to 200 nJ.