# Sub-Terahertz Computer Generated Hologram with Two Image Planes
## 1 Introduction  
Relatively long wavelengths corresponding to the terahertz (THz) range of electromagnetic radiation make them safe and allow penetrating different substances, like plastics, polyethylenes, polystyrenes [1], polyamides [2], wood, concrete, foams [3], papers [4, 5], and other materials [6]. These features opens up many possible applications like non-destructive testing [7, 8], non-invasive quality control [9], security screening [10], and many more [11]. Technologies based on THz radiation are quickly evolving, and that is why recent years have brought vast improvements and many new ideas in this field [12, 13, 14, 15]. The main drawback of THz radiation is the fact that it is strongly attenuated by water vapor, and thus, efficient communication using these waves is limited to particular frequencies, often named propagation windows [16]. Nevertheless, in the case of outdoor applications THz waves exhibit significantly lower losses in comparison with visible light while propagating in conditions of fog [17, 18]. Thus, using this range of radiation for communication or automotive systems becomes promising. Taking into account the transferring of THz data or signals, we should focus on multiple-input multiple-output systems (MIMO) [19], which can redirect energy in the desired way. Some effort was put into creating a THz MIMO channel and link that enabled higher transmission speed than a single-channel system [20]. Moreover, designing a graphene-based reconfigurable antenna that can be programmed dynamically allowed forming a MIMO system with higher spectral efficiency [21]. Considerations related to the increasing number of antennas were discussed in the case of massive MIMO systems [22] and their design schemes. Growing interest in transferring data using multiple-input and multiple-output systems makes this research direction desirable. Using opticalstructures to combine and divide input and output signals may also be an interesting application, especially for indoor wireless data transferring. Moreover, using optical diffractive structures (like synthetic holograms) may allow focusing electromagnetic radiation in more than a single plane (which increases the amount of accessible spatial dimensions for signal processing, the main idea of MIMO technique according to [23]). Other optical methods for the implementation of optical MIMO may be carried out by introducing dispersive and modal multiplexing (widely described in [24]). Diffractive structures can be realized for example using computer-generated holographic design. Examples of THz holograms reproducing simple images have been already proposed [25]. However, it has to be indicated that in that paper, only a single image was obtained from one phase plate (the main goal was to prove that a commercially-available 3D printer can be used to produce such complicated structures). Here, the presented paper further develops the topic of terahertz holograms by introducing multiple image planes with different images. Such an approach, however, is not without disadvantages: images have to be simple, and overlapping of images may cause some sort of ghosting or shadowing effect on further image planes. Terahertz holographic structures have been used for the generation of vortices [26] and Bessel beams [27] or to focus radiation in a matrix of spots [28]. However, in the aforementioned cases, relatively more expensive methods of fabrication were used in comparison with the 3D printing technique (here, selective laser sintering (SLS)) and did not allow redirecting the energy for example at different distances. Up to now, the possibility to project in only one image plane limits the application in scanning systems or MIMO devices; thus, using more reconstructed image planes becomes desirable, especially using a single-hologram structure. All previously-mentioned holograms have proven that in the current state-of-the-art, both of these features (multiple image plane coming from one holographic structure) are the novelty of our solution for THz optical structures.