The size of a detector is the primary consideration to obtain a high-resolution imaging quality. A THz quasi optic can combine an optical component of a hemispherical dielectric lens and an antenna-based sensor to capture effectively incoming radiation. The use of the hemispherical lens can contribute to sensor size. This paper investigates an immersion technique for dielectric lens size reduction to provide radiation performances of gain and radiation efficiency on the purpose of antenna size miniaturization at Terahertz (THz) frequency. This investigation is using the CST Microwave Studio simulation software. Gain and radiation efficiency show a decreasing pattern as the dielectric thickness increases. The obtained gain is still 30 dB by adding thickness until half of the hemispherical radius once combined with matching layers and 0.6 of the radius once without matching layers. Therefore, a smaller size than a hemispherical structure can still provide excellent radiation performance. This information is useful to design as small as a THz detector to obtain high-resolution imaging.