A review on numerical methods for thickness determination in terahertz time-domain spectroscopy

The use of numerical methods for thickness extraction in terahertz time-domain spectroscopy (THz-TDS) is highly deterministic in improving the accuracy of optical parameters significantly. Here, several commonly used thickness determination methods employing THz-TDS, both in transmission and reflection geometry, are discussed and reviewed. The effectiveness of these methods is compared experimentally in this tutorial type review by measuring transmitted THz field through both optically thick (∼ 427 μ m thick sapphire wafer) and thin (∼ 65 μ m thick polymer film) samples, and then extracting their thickness values by using these numerical methods separately. Furthermore, we propose a modification to one of the numerical methods (i.e., quasi-space method) to further improve the accuracy of thickness measurement in transmission geometry.