Spatial Frequency Responses of Anisotropic Refractive Index Gratings Formed in Holographic Polymer Dispersed Liquid Crystals

Tunable Waveguides Couplers Based on HPDLC for See-Through Applications

Photopolymers have become an important recording material for many applications, mainly related to holography. Their flexibility to change the chemical composition together with the optical properties made them a versatile holographic recording material. The introduction of liquid crystal molecules in a photopolymer based on multifunctional monomer provides us the possibility to generate tunable holograms. The switchable holographic elements are a key point for see-through applications. In this work, we optimize the holographic polymer-dispersed liquid crystals composition to improve the performance of tunable waveguide couplers based on transmission gratings and specifically their response under an applied electric field. A variation around 60% in the transmission efficiency was achieved.

Numerical Technique for Study of Noise Grating Dynamics in Holographic Photopolymers

Although the angular distribution of noise gratings in holographic photopolymer is understood to arise from Bragg matching, the details of scatter strength and dynamics are not fully understood. This confounds development of materials and recording techniques that minimize haze. Here, the kinetics are studied using a multi-physics numerical approach coupling diffraction of light from the dynamic material including scatter centers, reactions of chemical species initiated by this light, diffusion and swelling of these constituents, and the formation of the refractive index from the resulting composition. The approach is validated in the case of two-beam transmission holography by comparison to traditional harmonic series and rigorous coupled-mode approaches. Two beam holography in the presence of scatter is then used to study haze development. This reveals that haze due to weak noise gratings grows significantly above initial scatter only in reaction-limited materials, consistent with proposed Bragg-matched amplification mechanisms. Amplified haze is found to be proportional to initial scatter, quantifying the impact of clean sample fabrication. Conversely, haze is found to grow super-linearly with sample thickness, illustrating the significant challenge for applications requiring low haze in large thickness.

Very high contrast volume holographic gratings recorded in photopolymerizable nanocomposite materials

Volume holographic phase gratings possessing the saturated refractive index modulation amplitudes as large as 4.5×10^-2 were recorded at a wavelength of 532 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. This prominent result was achieved by a combination of the HBP nanoparticles with triazine and aromatic ring units and an electron donor/acceptor photo-initiator system doped in an acrylate monomer blend with low viscosity. As a result, efficient mutual diffusion of HBP nanoparticles and monomer having their very large refractive index difference took place. Obtained results suggest a potentiality of our newly developed HBP-dispersed NPC gratings as efficient volume holographic optical elements for various photonic applications including wearable headsets for augmented and mixed reality.

Spatial Frequency Response of Epoxy-Based Volume Holographic Recording Material

Holographic volume phase gratings are recorded in an epoxy-based, free-surface, volume holographic recording material. Light-induced gratings are formed by photo-triggered mass migration caused by component diffusion. The material resolution enables a wide range of pattern spacings, to record both transmission and reflection holograms with many different spatial frequencies. An optimum spatial frequency response is found between the low spatial frequency roll-off and the high spatial frequency cut-off. The influence of the energy density of exposure on the spatial frequency response is investigated. Secondary volume holographic gratings (parasitic gratings) are observed in the high frequency range. The possibility of distinguishing the regular grating from the secondary grating is discussed in the form of probe wavelength detuning.

Low-Threshold-Voltage and Electrically Switchable Polarization-Selective Scattering Mode Liquid Crystal Light Shutters

Low-threshold-voltage (Vth) and electrically switchable, polarization-selective scattering mode light shutters (PSMLSs) using polymer-dispersed liquid crystals (PDLCs) are demonstrated in this work. The optimized weight ratio of the nematic liquid crystals (LCs) to the adopted monomer (NBA107, Norland Optics) in the low-Vth PDLCs based on NBA107 is 7:3, [7:3]-PDLCsNBA107. The properties of the low-Vth PDLCsNBA107, such as light-scattering performance, initial transmission, Vth, and droplet size were investigated. Experiment results show that the surface anchoring (threshold-voltage) of NBA107 is weaker (lower) than or equal to that of the common NOA65. The cost is that the response time of the proposed PDLCsNBA107 is relatively long. A method to reduce the decay time, which can be applied to all other PDLC devices, will be elucidated. In addition to the low Vth of the proposed PDLCsNBA107, the operation voltage (~6 Vrms) to approach the maximum transmission is relatively low in a 7 μm-thick PDLCsNBA107 cell. Moreover, the polarization-selective light-scattering performances of the proposed PSMLSs based on the [7:3]-PDLCsNBA107, mainly driven by in-plane and vertical fields, are also demonstrated.

Low-voltage-driven and highly-diffractive holographic polymer dispersed liquid crystals with spherical morphology

Low-voltage driven and highly diffractive HPDLC gratings were formed by dialing the phase separated microphology through simple LC mixing.