1096 ISSN 1063-7761, Journal of Experimental and Theoretical Physics, 2015, Vol. 121, No. 6, pp. 1096–1103. © Pleiades Publishing, Inc., 2015. Original Russian Text © A.H. Gevorgyan, 2015, published in Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2015, Vol. 148, No. 6, pp. 1248–1256. Optical Properties of a Stack of Cholesteric Liquid Crystal and Isotropic Medium Layers A. H. Gevorgyan Yerevan State University, ul. A. Manukyana 1, Yerevan, 025 Armenia Institute of Applied Problems in Physics, Yerevan, 0014 Armenia e-mail: agevorgyan@ysu.am Received June 23, 2015 Abstract—Some new optical properties of a stack consisting of cholesteric liquid crystal (CLC) and isotropic medium layers are studied. The problem is solved by the modified Ambartsumyan method for the summation of layers. Bragg conditions for the photonic band gaps of the proposed system are presented. It is shown that the choice of proper sublayer parameters can be used to control the band structure of the system. In the gen- eral case, the effect of full suppression of absorption, which is observed in a finite homogeneous CLC layer, is not detected in the presence of anisotropic absorption in CLC sublayers. It is shown that this effect can be generated in the system under study if certain conditions are imposed on the isotropic sublayer thickness. Under these conditions, the maximum photonic density of states (PDS) increases significantly at the bound- aries of the corresponding band. The influence of a change in the CLC sublayer thickness and the system thickness on PDS is investigated. DOI: 10.1134/S1063776115120122 1. INTRODUCTION Photonics, i.e., the science that deals with the fun- damental and applied problems of generation, emis- sion, transmission, modulation, switching, amplifica- tion, detection/probing, and processing of light signal, is being extensively developed. Photonics is an analog of semiconductor electronics, which uses photons instead of electrons to transmit signals and deals with photonic processes of processing signals. It is charac- terized by high data transmission rates and very low energy losses and, hence, can have the possibility of a high operation speed and miniaturization. However, electrons have a charge and can be controlled by (elec- tric, magnetic, etc.) fields, whereas photons can only be controlled by changing medium parameters. Therefore, interest in creating new media, in particu- lar, photonic crystals (PCs) and metamaterials with controlled parameters, has quickened in recent years. Based on the aforesaid, PCs and metamaterials can be divided into the following two groups: solid PCs and metamaterials that are characterized by an improper elasticity and controllability by external fields, which substantially restricts their application (once created, the parameters of these media can hardly be changed), and so-called soft PCs and meta- materials. The well-known representatives of soft PCs are cholesteric liquid crystals (CLCs) and blue phases. Apart from the ability of self-organization of its peri- odic photonic structure, CLCs have the properties of wide-scale easy deformability, high sensitivity, and highly elastic ability of phase or morphology modula- tion. These properties explain the fact why such struc- tures with a photonic band gap (PBG) rapidly react to external actions. Moreover, CLCs have the following unique optical properties: they only have a first-order PBG (for normal incidence of light) for light with cir- cular polarization having the sense of rotation that is identical to the sign of medium chirality, and absorp- tion and emission in CLCs also have polarization peculiarities [1, 2]. The effect of anomalously strong (anomalously weak) absorption (emission) takes place outside PBG, near its boundaries, during anisotropic absorption (amplification). On the other hand, the soft character of CLCs lim- its the possibility of creating cells with a homogeneous spiral structure with N = d/p > 10 or more, where d is the thickness and p is the pitch of CLC helix. However, thick cells are necessary to form an ideal PBG, which is important for some applications. To form systems with a high elasticity, easy deform- ability, and a high sensitivity as compared to an ideal periodic structure at a large thickness, we can use a stack of CLC and isotropic medium layers, in particu- lar, isotropic layers subjected to specific treatment to produce homogeneous CLC sublayers. The authors of [3–11] studied the optical properties of such a stack and showed that, in contrast to a homogeneous CLC layer, this system has a higher-order PBG, and such systems can be applied in many fields, such as the pro- duction of monitors, due to multicolor reflection. STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS