We present experimental evidence for the spontaneous formation of discrete $X$ waves in AlGaAs waveguide arrays. This new family of optical waves has been excited, for the first time, by using the interplay between discrete diffraction and normal temporal dispersion, in the presence of Kerr nonlinearity. Our experimental results are in good agreement with theoretical predictions.

Optical systems with gain and loss that respect parity-time ($PT$) symmetry can have real eigenvalues despite their non-Hermitian character. Chiral systems impose circularly polarized waves which do not preserve their handedness under the combined space- and time-reversal operations and, as a result, seem to be incompatible with systems possessing $PT$ symmetry. Nevertheless, in this work we show that in certain configurations, $PT$ symmetric permittivity, permeability, and chirality is possible; in addition, real eigenvalues are maintained even if the chirality goes well beyond $PT$ symmetry. By obtaining all three constitutive parameters in realistic chiral metamaterials through simulations and retrieval, we show that the chirality can be tailored independently of permittivity and permeability; thus, in such systems, a wide control of new optical properties including advanced polarization control is achieved.

Reconfigurable intelligent surfaces (RISs) have been often proposed as a means for restoring non-line-of-sight links in wireless communications. Because the path loss changes with the relative distance of the RIS from the transmitter and the receiver, and their relative orientations, the quality of the communication is expected to depend strongly on the RIS placement in a RIS-aided link. In this work, the optimal placement of the RIS is studied with respect to both position and orientation and it is shown that even for an ideal RIS, i.e. lossless, polarization preserving and with a uniform response with respect to the incident angle, the quality of the communication is greatly affected by the RIS orientation. To provide insight into the optimal choice, the analysis is based on an analytical model that treats the RIS as a continuous surface of finite size that steers the incident beam towards a prescribed direction. By imposing a desired minimum threshold on the received power it is studied how the efficiency of the RIS depends on both its position and orientation. The decisions that have to be made concerning the placement of the RIS are illustrated with examples of D-band indoor scenarios.

A theory of thermotropic nematic liquid crystals in which molecules form internally ordered clusters is presented. The formulation is based on the same molecular-field approximation and form of the anisotropic potential used in the Maier–Saupe theory. One uniaxial nematic and two macroscopically isotropic phases are predicted. The lower-temperature isotropic phase consists of thermodynamically stable clusters with internal orientational order. The transition from this phase to the nematic phase is characterised by the divergence of cluster size whilst the entropy and the order parameter change continuously.