Materials with artificial refractive index modulation - photonic crystals [1,2] and optically induced refractive lattices [3,4] have become the subject of extensive research in the last three decades. Two- and three-dimensional (2D, 3D) photonic crystals and refractive lattice structures offer new possibilities to route, control and steer light and are of great interest for many applications, including guiding and trapping systems, photonic bandgap materials, all-optical devices, information storage and processing, telecommunication systems, optical computers etc [1 - 4]. The approach of an optical induction of refractive lattices in photosensitive materials (holographic technique) becomes particularly attractive since provides an increased flexibility and the ability to tune and adapt refractive lattices in real-time. The light induced material structuring on the one hand and the influence of structured media on the propagation characteristics of light on the other hand lead to the concept of optically induced photonic refractive lattice structures (photonic lattices).
The nonlinear photonic structures are particularly attractive because the interplay of medium nonlinearity and various lattice geometries and periodicities provide the fascinating nonlinear effects, such as discrete spatial optical soliton formation, laser radiation frequency conversion, nonlinear diffraction effect, nonlinear Talbot effect etc.
The spatial soliton formation in nonlinear media is one of the most interesting nonlinear processes for both their fundamental interest and numerous potential applications. Spatial solitons effectively operate as a waveguides and so can be considered as a means of channeling the optical information along the self-sustaining filaments promising for different applications in optical communication networks, optically controlled devices, for all optical signal processing, signal readdressing and future optical computers. The “bright” and “dark” solitons formation in homogeneous nonlinear media has been studied in numerous works. The concept of optical beam self-trapping in pyroelectric photorefractive crystal lead to the formation of 2D spatial soliton named pyroliton [5]. Among nonlinear materials the nematic liquid crystals (LCs) attract considerable interest because exhibit huge nonresonant nonlinearity. In nematic LCs solitons can be produced at milliwatt power level. The formation and behavior of solitons in nematic LCs have been studied in numerous works and was called nematicon [6]. The formation of optically induced waveguide serves also as channels for guiding of probe beams and was studied only in few works (see [7-10]).
Based on these results and as their further development the soliton formation and its propagation in micro-structured nonlinear media is very interesting task. The light propagation in periodic lattices is fundamentally different from that in homogeneous medium. However, the soliton formation in structured media has been realized experimentally in limited number of works, in particular, in 1D linear lattice [11], in rectangular 2D lattice [12], in square and diamond-type lattices [13] and in periodic ring lattice (created by mask technique) [14] in strontium barium niobate crystals.
For the configurations used it is essential that the periodic structure would be as uniform as possible along the probe propagation direction and stable as possible during homogeneous illumination of the lattices. From this point of view the non-diffracting beams are of special interest for optical induction of micrometric scale photonic lattices in photorefractive media because they provide the formation of non-spreading, high contrast and volumetric lattices. In particular, the Bessel beam technique [15] is obviously very promising for the formation of annular photonic lattices. While 1D Bessel-like refractive lattice has micrometric scale modulation in the radial direction, 2D complex lattices inducted by Bessel standing wave [15] are a combination of annular and planar refractive gratings with micrometric scale periodicity in the radial and standing wave submicrometer period in the axial directions.
The aim of the project is the optical formation of nondestructive, specific micrometric scale photonic structures by Bessel beam technique (developed by our group [15]) in doped lithium niobate crystals and in dye doped nematic liquid crystals that allow new light-guiding properties and on this basis the study of novel features on 2D light localization and soliton formation and behavior in 1D and 2D micro-structured nonlinear media. The nonlinearity will be controlled by pyroelectric effect in photorefractive crystals and by director reorientation in dye doped nematic LCs.
The novelty and originality of the project is the study of novel light-guiding features of specific high contrast 1D annular and 2D complex annular-planar photonic lattices in solid and liquid crystals. Both “dark” and “bright” solitons will be studied. The Bessel beam technique not yet been applied for structuring the LC media. No experiments have yet been performed for light localization and soliton formation in complex structured media modulated also along the beam propagation. The use of different materials (i.e. photorefractive crystals and nematic LCs with different mechanisms of optical induction of photonic lattices and different nature of nonlinearity) for realization of the goals of the project will allow the performing of comparable investigations and revealing the advantages and disadvantages of each of mentioned structured materials as novel waveguiding systems for different practical applications.
Our activity as a whole will develop in five relative directions: (i) preparation of LC cells which meet special requirements, (ii) growth of doped nonlinear lithium niobate crystals, (iii) recording of refractive lattices in photorefractive doped LiNbO3 crystals by Bessel beam technique and their characterization, (iv) recording of nonlinear photonic lattices in nematic LCs with simultaneous optical testing by probe Gaussian beam, which will allow to determine the optimal parameters for induction of photonic lattices with maximal diffraction efficiency, v) study of controllable light propagation and soliton formation in the created Bessel-like waveguiding structures in doped LN crystals and dye doped nematic LCs to demonstrate the influence of photonic lattice structures on the propagation characteristics of the light, as well as computer modeling and numerical simulations of wave propagation through 2D and 3D nonlinear refractive periodic structures. These studies will pave a way for novel guiding and trapping systems, all-optical devices, optical communication systems, information optical storage, optical computers, etc.
Project consists of five tasks. Expected results of research activity may be summarized as follows:
i. Formation of 1D and 2D high contrast micro- and submicro-scale waveguiding photonic structures by Bessel beams (both zeroth and first order) in doped LN crystals with the use of 532 nm cw single mode laser beam and their optical testing.
a) Pyroelectric control of nonlinearity in LN crystal. b) Demonstration of persistence of the structures against erasure during illumination by uniform weak probe beam at the recording 532 nm wavelength, as well as at 633 nm light.
ii. Formation of 1D high contrast micro- scale wave-guiding photonic structures by Bessel beam technique in dye doped nematic LCs with simultaneous testing by probe Gaussian beam.
iii. Controllable light propagation and soliton formation in the created Bessel-like waveguiding structures in doped LN crystals and dye doped nematic LCs. Demonstration of the novel features of photonic lattice structures influence on the propagation characteristics of the light. Description of light propagation in structured media by numerical simulations.
The results of the joint investigation will provide new insights for the control of light and light propagation in novel nonlinear photonic structures promising for different applications, i.e. for novel guiding and trapping systems, miniaturized all-optical devices, optical communication systems, information storage and channeling along self-sustaining filaments, all optical signal processing, signal readdressing which are the composite elements for future optical computers.
Armenian team involved in the project has a considerable experience in the fields of nonlinear optics and photonics. The few tens of papers were published in the leading optical and physical journals. The most important and actual for proposed project fulfillment are latest achievements. During the last years the group by leadership of R. Drampyan performs the experimental studies on elaboration of micro- and nano-structured materials for photonics. In this area the main achievements are the development of three new methods, namely Bessel standing wave method [15,16], combined interferometric-mask method [17], and application of diffraction grating self-imaging phenomenon (Talbot effect) for formation of two- and three-dimensional micro- and nanometric-scale higher rotational symmetry photonic lattices [18].
Role of foreign collaborators: Information exchange and joint discussions in the course of Project implementation; Comments to the annual technical reports; Joint use of the project materials; Shared use of certain sample materials; Joint seminars and workshops.
Realization of the project will promote the solution of problems corresponding to the purposes of ISTC: Reorientation of group of scientists, engineers and technicians employed in the field of defenses, to development of new technologies for civil purposes; Encouragement of CIS scientists integration into the international scientific community; Support of investigations and elaborations with peaceful trends; Creation of long-term outlooks for professional activity in the sector and preventing the migration of Armenian specialists engaged in the field of defenses, to countries of neighboring regions; Promotion of development of market relations in Armenia.
Realization of this project will provide new fundamental knowledge with novel potential applications and will build a bridge between a few extensively but independently developing fields of photonics, namely the field of all-optical formation of photonic lattices, singular optical beams and controllable light propagation and localization in structured media.
References:
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15. A. Badalyan, R. Hovsepyan, P. Mantashyan, V. Mekhitaryan, R. Drampyan, “Bessel standing wave technique for optical induction of complex refractive lattice structures in photorefractive materials”, J. Modern Optics, 60, 617 (2013).
16. A. Badalyan, R. Hovsepyan, P. Mantashyan, V. Mekhitaryan and R. Drampyan, “ Nondestructive readout of holograms recorded by Bessel beam technique in LiNbO3:Fe and LiNbO3:Fe:Cu crystals”, European Physical Journal D, 68, no.4, 82-1-11 (2014). DOI:10.1140/epjd/e2014-40356-8.
17. A. Badalyan, R. Hovsepyan, P. Mantashyan, V. Mekhitaryan, R. Drampyan, “Optical induction of 3D rotational symmetry refractive lattices by combined interferometric-mask method in doped LiNbO3 crystals”, Appl. Phys. B, 116 (2014) in press. Doi: 10.1007/s00340-013-5654-4.
18. A. Badalyan, R. Hovsepyan, V. Mekhitaryan, P. Mantashyan. R. Drampyan, “Talbot effect from rotational symmetry gratings: Application to 3D refractive grating formation”. Mol. Cryst. Liq. Cryst. 561, 57-67 (2012).
