Samples of amorphous indomethacin (C 19 H 16 ClNO 4 ), a popular active pharmaceutical ingredient (API), were prepared by melt quenching from a molten liquid and were exposed to the focused terahertz beam (1.5 mm diameter) with an average power of 200 mW (power density of 3 MW cm -2 ) operating at a frequency of 1.56 THz, chosen due to the presence of an absorption feature in the crystalline form near this frequency. After 1 minute of exposure to the terahertz beam, visual inspection showed that a clear transparent region had formed at the focal point of the beam, which did not extend beyond this region (Figure ). Subsequently, after an additional 4 minutes of exposure (5 minutes total), a crystalline region had formed within the beam spot (Figure ), with the transparent region extending slightly beyond the focal point of the beam. Continued exposure (10 minutes total) resulted in the entire focus region of the beam converting to a crystalline phase (Figure ). It is important to note that other than the focal point of the beam and the extension region (which is in direct contact with the beam focus spot), there was no discernible change in the rest of the sample. Further characterisation with THz-TDS showed that the crystalline areas, both within the focus of the beam and the area that extended outwards, contained a mixture of α and γ polymorphs of indomethacin. However, the region within the focal point of the beam contained a larger proportion of the γ form, indicating that the intense terahertz pulses were preferentially promoting one form over the other. It is important to note that while temperature is known to have an effect on the polymorphic outcome of indomethacin, it is not believed to be the origin of the observed phenomena, as these effects were also observed when the sample was placed under active cooling, helping to highlight that the terahertz radiation, rather than thermal effects, was responsible.