Electro-optical characterization of switchable Bragg gratings based on nematic liquid crystal – photopolymer composites with spatially ordered structure G. Zharkova, I. Samsonova, S. Strel’tsov, V. Kchachaturyan, A. Petrov Key words: flexible materials with layer structure, electro-optical phenomena, switchable Bragg diffraction Institute of Theoretical and Applied Mechanics SB RAS Boreskov Institute of Catalysis SB RAS N. Rudina

the Laboratory of Optical Methods of Gas Flow diagnostics Scientific activities of the LC Group: Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Development of liquid crystal (LC)-polymer composite material (thin films) for Thermography, Panoramic flow visualization, Skin friction measurements, Optoelectronics applications.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Development of fabrication procedure for LC composite materials with the periodic ordered supermolecular structures using holographic method (modification of PIPS) for their possible applications as switchable Bragg gratings, light shutters, tunable photonic components. Current work:

Disadvantages of the transmission-type devices include : - no sufficient response times (tens and hundreds msec) ; -no sufficient on-state transmittance; - a high driving voltage for some applications (telecom applications, thin-film displays); - competition between high contrast ratio (CR) and low operating voltage. Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Current situation in this scientific field In recent years, there has been an active search for advanced electro-optical materials with improved operating characteristics making the materials suitable for opto- and microelectronic applications. Nonetheless, these materials, still do not meet the requirements presently placed upon optical media for display applications, although displaying many useful properties.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Abstract of the paper The composite materials with spatially alternating LC refractive index modulation have been obtained, and electro-optical effects in these materials are studied. The periodic structure of the composites and small sizes of their structural elements allow one to improve the operating parameters of display devices: increased contrast ratio, shorter switching times, and lower reorientation fields.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Composite formation method: -non-uniform-polymerization induced phase separation. Gratings characterization methods: -structure microscopic observations, -Investigation of light scattering and electro-optical effects. The aim of the present study is to investigate the effect of composition of the initial photopolymer mixture and curing conditions on the morphology and electro-optical behavior of the gratings. Presence of a large interface area for the NLC-polymer interaction and micro- and nanoscale structure sizes will change the known electro-optical phenomena in NLC.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Technology approaches include - choice of the pre-polymer composition and variation of the component concentrations, - variation of the parameters (wavelength, laser energy) of the polymerization process. composition PHOTO-INITIATING SYSTEM Dye Co-initiator Binding component MONOMER(S)NLC

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Technology description Gratings formation The pre-polymer composition was prepared as a homogeneous solution. Photo-polymerization induced by interfering laser beams. The laser wavelength corresponds to a maximum of composition photosensitivity. The interference pattern period was 1  m. After the laser curing the electro-optical cell with the material was subjected to post-cure treatment.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Technology description Experimental setup for writing transmission gratings 1 Light source – laser (intensity and exposure time or total energy) 2 Collimator 3 Diaphragms 4 Deflecting mirrors 5 Fresnel mirrors 6 Cell with the pre-polymer mixture 7 Power meter  - the angle between interfering laser beams (control of spacing of periodic structure)

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Technology description A schematic for investigation of light scattering by samples s – sample under study; O – objective lens; A– aperture diaphragm;  – radiation detector, VCS - voltage control system From the measured angular dependences of light transmission, transmittance-voltage characteristics, the grating spacing, the diffraction efficiency  1, the refractive-index modulation amplitude  n, the reorientation field and the contrast ratio (CR) were found.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 1. Grating morphology SEM image of a film with periodic polymer-rich layers. The NLC is removed (dark stripes). The polymer layer width is about 600 nm. The morphology indicates the formation of Bragg planes. polymer-rich layers The initial composition containes 0.1 wt% of dye and 37 wt% of NLC. The curing conditions are as follows: curing energy- 900 mJ/cm 2 ; convergence angle of the laser beams-28°; writing =658 nm, cell thickness – 10  m.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 1. Grating morphology The fine structure of the polymer -rich layer is determined by nanoscale sized elements (30-80 nm). One layer sector is shown. The fine structure of the polymer and NLC layers forming the grating controls the scattering- induced noise. The optical scattering on the structural elements differed from the Bragg scattering, and the sizes of elements in them, smaller than the grating spacing. Substrate fragment. The structure was examined by means of transmission electron microscopy. The initial composition contained 0.1 wt% of dye and 37 wt% of NLC.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 1. Morphology SEM of PDLC film after the removal of NLC. The initial composite material obtained by uniform polymerization process. There is no spatially periodic or layer structure. The initial composition contained 0.4 wt% of dye and 37 wt% of NLC.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 1. Morphology (1) (2) The initial composition contained 0.1 wt% of dye and 50 wt% of NLC. Curing energy: (1)-900; (2)-1800 mJ/cm 2. SEM of films after the removal of NLC. The degradation of the grating structure is observed. NLC concentration is 50 wt%.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 2. Light scattering characteristics of films - with periodic structure (holographic formation technique) - without ordered structure (typical PDLC).. A full-width at half- maximum of 1-2° was measured at nm.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 2. Light scattering characteristics the refractive-index modulation amplitude  n The experiments performed proved the very fact that volume gratings were formed.  n was estimated for the case of thick transmission-type phase holograms with a sinusoidal profile: where  1 is the first-order diffraction efficiency, d is the grating-cell thickness, is the reading wavelength ( =632.8 nm),  is the diffraction angle.  1 =sin 2 ( ), =(  n  d)  ( cos  ), For gratings obtained  1 ~0.45  0.6  n~0.014  0.001

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 Experimental Results 3. Electric Field Switching No electric field is applied Incident light sees an admixture of the ordinary and extraordinary refractive indices of the liquid crystal. This average index differs from a polymer one, and hence produces an index modulation forming a phase grating. When a reorientation field is applied to the cell, NLC-molecules in the liquid crystal-rich layers reorient in a direction parallel to this field. Due to this incident light sees an index approximately equal to the ordinary index. This reduces the index modulation, and decreases the first-order diffraction efficiency

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, Electric Field Switching These data confirm a refraction-index modulation existing in the volume of the film under electric field applied. the zero-order transmittance and first-order diffraction efficiency versus applied voltage

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, 2004 CONCLUSIONS 1. The composite material with spatially periodic structure (holographic films) and with randomly oriented structure domains (traditional PDLCs) have been realized. The holographic film structure represents polymer-rich or NLC-rich layers presenting Bragg planes. Presence of the periodic layer structure of the materials defines its different optical properties as compared with traditional PDLCs. The holographic films have a bulk refractive index modulation. 2. Such material can be considered as a volume phase diffraction grating. Presently, a 10-  m thick transmission grating with a spacing of 1.1  m was written by laser curing at a wavelength of 658 nm. The first-order diffraction efficiency of the grating amounts to 50-60%.

Switchable Bragg gratings, NGCM 2004, Krakow, September 13-17, The change in the first-order diffraction efficiency is accompanied by an equal but opposite change in the zero- order transmittance, clearly indicating that the refractive index was indeed modulated by the field. 4. The gratings obtained can be switched by lower reorientation fields (4 - 7 V/  m ) compared to PDLC-based holographic materials (12-20 V/  m). 5. The grating shows a turn-on time of 200 – 300  s and a turn- off time of about 1.2 – 3 ms depending on the initial composition of the mixture containing 37 wt% of NLC.