Optimizing collimated THz-TDS beam propagation for centimeter scale probing with millimetric lateral resolution

Terahertz time-domain spectroscopy (THz-TDS) enables non-destructive in-situ analysis of subsurface cometary materials. Due to its short wavelengths spanning from 0.03mm to 3mm, THz-TDS can achieve significantly higher spatial resolution than conventional techniques like ground penetrating radar, at the cost of a reduced penetration depth in the centimetre-scale. When deployed in a borehole, such technique enables high resolution access to cometary layers that have undergone minimal thermal processing since the formation of the Solar System.

 

Standard THz-TDS setups rely on a focused beam, which can reach a sub-millimetre resolution on the focal plane to the detriment of the resolution in the out-of-focus areas. In the context of cometary soil analysis, the resolution must be preserved throughout the full penetration depth. However, maintaining optimal lateral resolution throughout a thick sample typically requires refocusing the beam at different depth, leading to an increase of acquisition time, data volume and system complexity. Instead, we considered the use of a collimated beam  to approach depth-invariant resolution in a single measurement.

 

A key limitation arises from Gaussian beam propagation: reducing the beam waist improves spatial resolution but intrisincally increases beam divergence, while minimizing divergence and approaching a perfectly collimated beam leads to a larger beam waist and therefore poorer lateral resolution. This fundamental trade-off limits the ability of THz-TDS systems to probe deeper into samples while preserving resolution. 

 

Our results show that optimized beam collimation of a pulse with a main frequency of 1 THz preserves sub-2mm lateral resolution over 3cm depth, significantly increasing the usable depth range compared with conventional THz-TDS systems.