# Impact of Cubic Symmetry on Optical Activity of Dielectric 8-srs Networks
## 1 Introduction  
Light can be used to collect environmental parameters and provide information about the material that it passes through. For example, polarization and orbital angular momentum of light represent fundamental optical degrees of freedom that are able to reveal physical mechanisms of light-matter interaction [1]. In order to use the information that light can collect, it is essential to develop optical devices and sensors that are able to interact with optical degrees of freedom [2]. Optical sensors are used in a wide range of research and commercial applications and play a significant role in emerging technologies [2; 3; 4; 5]. For example, systems that implement machine learning and artificial intelligence make large use of optical sensors and require new solutions to provide new functionalities and improved performance. To harvest their full potential, optical sensors need to become smaller, cheaper, and more efficient. Moreover, they need to cover a wide range of wavelengths and incorporate new functionalities. Photonic crystals (PhCs) are engineered structures that are able to manipulate all the degrees of freedom of light [6] and can therefore be employed as optical components for light manipulation and sensing [7; 8; 9; 10]. For these applications, an excellent candidate is the gyroid srs-network [11; 12], a three-dimensional periodic network with both cubic symmetry and chirality [13; 14]. In this work we present the fabrication and characterization of three-dimensional (3D) cubically symmetric 8-srs PhCs derived from a combination of eight gyroid srs-networks. We numerically and experimentally investigate optical properties of this PhC and in particular the impact of cubic symmetry on optical activity (OA), i.e., the ability to rotate the plane of polarization of linearly polarized light. We will work in the mid-infrared (IR) spectral region, a region of tremendous scientific and technological interest [15; 16]. This spectral region contains strong characteristic vibrational transitionsof many important molecules, as well as two atmospheric transmission windows of 3-5 \(\upmu\)m and 8-13 \(\upmu\)m, which make it crucial for applications in fields such as night vision, IR astronomy, chemistry, and meteorology [17; 18]. The unique morphology of gyroid geometry imparts remarkable mechanical strength [19] and a rich variety of optical and topological phenomena, from linear and circular dichroism (CD) (different in transmission for left and right circularly polarized light) to OA [20] and the recent demonstration of type-I Weyl points [2; 3]. Geometrically, it is possible to intertwine more than one like- or opposite-handed gyroid srs-network into cubically symmetric structures, which enables engineering of network transmission and polarization properties. Combination of eight identical and equal handed gyroid srs-networks creates the so-called 8-srs network (Figure 1a-c), a body-centered cubic (BCC) structure belonging to the symmetry group I432 [13]. Despite the highly chiral nature of the geometry, the 8-srs network prohibits CD due to the presence of four-fold rotational symmetry [13]. Secondly, it was shown that the 8-srs network possesses a high degree of OA [13; 20]. This degree of rotation was found to be comparable to that of metallic or plasmonic metamaterials [21; 22], but as the network can be made using dielectric materials, OA is accompanied by low loss and high, almost unity transmission. Experimentally, many micro- and nano-fabrication methods can be used for accurate formation of PhCs and metamaterials from single and double srs-networks [23; 24; 25; 26; 27]. For more complex composite networks such as 8-srs and for a high degree of design flexibility, the direct laser writing (DLW) technique has proven to be particularly valuable [28; 29; 30; 31; 32; 33; 34] for PhC formation. However, the DLW method suffers from an elongated cross-section of the writing focal spot, i.e., unequal sizes in lateral and vertical axes of the voxel.