Fig. 1: Slot waveguide geometry.
From: Compact THz absorption spectroscopy using a LiNbO3 slot waveguide
a Schematic illustration of the slot waveguide structure. Two parallel strips of material with lateral dimensions \({W}_{\text{core}}\) and \({H}_{\text{core}}\) and refractive index \({n}_{\text{core}}\) are separated by a distance \({W}_{\text{slot}}\). A slab of sample material with refractive index \({n}_{\text{slot}}\) is inserted between the two strips and extends outside of the slot. The entire assembly is surrounded by cladding with refractive index \({n}_{\text{clad}}\). b Schematic illustration of the experimental geometry. The slot waveguide structure is fabricated in a 50 µm thick LN slab waveguide (ncore ≈ 5.1, Hcore = 50 µm). A thin slab of sample is inserted into the slot. (The sample height along the z-axis is truncated for clarity.) The optical pump beam, polarized along the LN c-axis, is line-focused outside of the slot waveguide and launches a THz wave via optical rectification that propagates along the x-axis. The optical probe beam passes through one of the LN strips, followed by a quarter-wave plate (QWP) and a Wollaston prism. The probe intensity is measured using balanced photodiodes. The parameters used for the slot waveguide are: \({W}_{\text{core}}=50\,\upmu{\rm{m}}\), \({W}_{\text{slot}}=50\,\upmu{\rm{m}}\). The LN slab is free-standing and surrounded by air (\({n}_{\text{clad}}=1\))
