Investigation of ultra-broadband terahertz generation from sub-wavelength lithium niobate waveguides excited by few-cycle femtosecond laser pulses

The generation of coherent, ultra-broadband terahertz (THz) radiation pulses spanning more than a few octaves is vital to understanding the ultrafast response of elementary excitations, molecules, nanostructures, materials, and explore device functionality across a wide spectrum. In this work, we use 2D finite-difference time-domain simulations to show that ultra-broadband (0.18-106 THz) Cherenkov radiation can be produced from SiO 2 :MgO-LiNbO 3 :SiO 2 waveguides having core dimensions that are sub-wavelength with respect to the optical pump pulse being guided. These sub-wavelength core dimensions allow the ultra-broad Cherenkov radiation to be emitted at an angle between 47.2° and 47.5° (dictated by the Si cladding layer dispersion), making these waveguide structures superior to the THz generation arrangements in bulk MgO-LiNbO 3 crystals. When excited by a 7 fs, 780 nm laser pulse having an energy of 2 nJ, a 300 µm-long waveguide with transverse core dimensions of 500 nm × 2 mm can generate a sub-ps, kV/cm electric field pulse. Unlike THz pulse generation in bulk MgO-LiNbO 3 crystals, having sub-wavelength core dimensions reduce the absorption from the MgO-LiNbO 3 reststrahlen bands. These sub-wavelength SiO 2 :MgO-LiNbO 3 :SiO 2 waveguides are ideal for on-chip applications that require ultra-broadband, compact THz sources.