Linear size gradient single layers of polymer-dispersed liquid crystal micrometer-sized droplets for diffractive optics Georgi B. Hadjichristov a, * , Yordan G. Marinov b , Alexander G. Petrov b a Laboratory of Optics and Spectroscopy, Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria b Biomolecular Layers Laboratory, Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria article info Article history: Received 5 September 2008 Received in revised form 20 February 2009 Accepted 10 March 2009 Available online 16 April 2009 PACS: 42.25.Fx 42.70.Df 61.30.Gd 61.30.Pq 78.66.Qn Keywords: Polymer-dispersed liquid crystals (PDLCs) Single layers Gradient layers Thin films Coherent light diffraction abstract An experimental study of coherent light diffraction by wedge-formed single layers composed of liquid crystal (LC) micro-sized droplets dispersed in a transparent solid polymer matrix is reported. The micrometer-sized polymer-dispersed liquid crystal (PDLC) material contains prolate-ellipsoid-like LC droplets with a linear-gradient size distribution along the wedge slope. The droplet diameter in the films reaches several tens of micrometers, defined by the wedge. Such a droplet organization in a two-dimen- sional layer provides both spatial and electrical control of the coherent light diffraction by the LC/polymer interface. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The development of modern diffractive optics involves the use of liquid crystals and polymer composite materials. The well estab- lished polymer-dispersed liquid crystal (PDLC) technology for elec- tro-optical (EO) devices [1–6] is also applied in optical engineering of diffractive structures and elements [7–10]. Consisting of liquid crystal (LC) droplets dispersed in an optically transparent polymer binder, PDLCs operate through refractive-index change upon applying an AC electric field. Being electrically-controllable, these hybrid soft-solid smart materials have a great potential for use in diffractive optics for optical switching and beam steering applica- tions, as well as to control the split of the light and the creation of homogenous light pattern. All these are of special interest for a projection displays, photonic switching systems and optical com- munications [11–13]. Recently, several attempts were done in the polymer-dispersed materials possessing simultaneously fullerene nanotubes and organic chromophores [14]. The gradient distribution of droplet size in PDLC systems pro- vides the opportunity their structural and EO properties to be spa- tially controlled and tuned. This is certainly of importance for diffractive optics applications, e.g. Fresnel lens, phase gratings, grating beam splitters, etc. [15]. One technological advantage of the gradient PDLCs is the potential for integrated polymer/LC de- vices. By photopolymerization, variable-intensity photomasks are capable of forming PDLCs with regular gradients [15]. We have ap- plied another approach to produce microscale PDLC single layers of controllable-gradient LC droplets, namely by nanosecond laser photopolymerization [16]. In this way, the formation of the PDLC material, as well as the droplet gradient in the film, are fully and finely controlled by both wedge geometry and UV laser exposure rate. Moreover, single layers of LC micro-droplets of very defined shape can be produced. Further, the electrically switched optical diffractive properties of these composite materials are of interest. Here, we present experiment results on spatially- and electri- cally-controlled coherent light diffraction by such PDLC films, which may be of importance for device applications. 2. Experimental The PDLC films used in our experiments were prepared by tech- nology commonly used for making PDLC devices [4–6]. Briefly, a 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.03.001 * Corresponding author. Tel.: +359 27144464; fax: +359 29753632. E-mail address: georgibh@issp.bas.bg (G.B. Hadjichristov). Optical Materials 31 (2009) 1578–1585 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat