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INVESTIGATION OF DIVERSE SCATTERING MATERIALS USING DIGITAL TERAHERTZ HOLOGRAPHY

Lukas Stacevičius1, 2, Linas Minkevičius1, Ignas Grigelionis1, Agnieszka Siemion3, Rusnė Ivaškevičiūtė-Povilauskienė1, Gintaras Valušis1

1 Center for Physical Sciences and Technology, Department of Optoelectronics, 10257 Vilnius, Lithuania

2 Vilnius University, Faculty of Physics, 10222 Vilnius, Lithuania

3 Warsaw University of Technology, Faculty of Physics, 00-662 Warsaw, Poland

[email protected]

Terahertz (THz) radiation has attracted a great scientific interest due to its ability to penetrate materials that are opaque to visible light [1]. However, conventional imaging cannot resolve low-absorbing materials, which require the employment of coherent recording methods. Digital holography [2], a rapidly advancing imaging technique, has been explored for its high sensitivity and potential to distinguish transparent, weakly absorbing materials, giving insight into light scattering effects.

In this work we have investigated diverse scattering materials by numerically evaluating their scattering properties and performing digital holography at 600 GHz frequency. Investigated samples consisted of four different materials, such as sugar, soda, coffee and polymer called TX-151 (see inset in Fig.1(a)). Substances were sifted and divided into different categories by grain sizes: less than 125 µm; 125-250 µm; 250-500 µm and 500-1000 µm.

Since the sizes of investigated grains are comparable to the wavelength of used THz radiation, Mie scattering properties of investigated materials were evaluated, using the discrete dipole approximation (DDA) method [3] for non-spherical particles. The extinction, scattering, and absorption properties were evaluated, where the highest extinction efficiency of 2.12 was found for sugar grains within the 250-500 µm size range, correlating with the lowest value in the hologram reconstruction of amplitude distribution. Moreover, scattered light intensity dependence on scattering angle was evaluated for each material, shown in Fig.1(a). The values of scattered light intensity of different substances matched the experimental recordings, with the least transmitted light corresponding to the highest scattering.

In the reconstructed hologram image shown in Fig.1(c), the signal amplitudes of different materials were better resolved compared to direct imaging (see Fig.1(b)), with an intensity difference of 0.81 compared to 0.65, respectively. It was demonstrated that digital THz holography technique enables a more effective evaluation of various scattering materials compared to conventional THz imaging methods.

Figure 1
Fig. 1. Normalized intensity dependence on scattering angle for different materials, with the y-axis in logarithmic scale (a). Image of the investigated samples is depicted in the inset of this graph. The enlarged picture of TX-151 material is depicted on the left side of the graph. Amplitude distribution of investigated sample using direct THz imaging (b) and reconstructed amplitude distribution obtained using THz holography (c). The color scale is normalized to the maximum value.

[1] Mittleman D. M., Optics Express, vol. 26, no. 8, pp. 9417–9431 (2018).

[2] Sheridan J. T. et al., J. Opt., vol. 22, 123002 (2020).

[3] Draine B. T. et al., J. Opt. Soc. Am. A., vol. 11, no. 4, pp. 1491-1499 (1994).