Experimental determination of nanocomposite grating structures by light- and neutron-diffraction in the multi-wave-coupling regime.

We experimentally demonstrate how to accurately retrieve the refractive index profile of photonic structures by standard diffraction experiments and use of the rigorous coupled-wave analysis in the multi-wave coupling regime, without the need for taking any auxiliary data. In particular, we show how the phases of the Fourier components of a periodic structure can be fully recovered by deliberately choosing a probe wavelength of the diffracting radiation much smaller than the lattice constant of the structure. In the course of our demonstration, we accurately determine the slight asymmetry of the structure of nanocomposite phase gratings by light and neutron diffraction measurements.

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.

Nanoparticle-polymer composite volume holographic gratings dispersed with ultrahigh-refractive-index hyperbranched polymer as organic nanoparticles.

We report on volume holographic recording at a wavelength of 532 nm in photopolymerizable polymer nanocomposites that are incorporated with new hyperbranched polymers (HBPs) acting as transporting organic nanoparticles. Since HBPs possess an ultrahigh index of refraction of 1.82 due to the inclusion of triazine and aromatic ring units, high-contrast transmission volume holographic gratings with refractive index modulation amplitudes as large as 2.2×10(-2) are recorded. This value enables us to realize a 10 µm thick transmission volume grating with the diffraction efficiency near 100% in the green.

Measurement of polymerization-shrinkage evolution during curing in photopolymer with a white-light Fabry-Pérot interferometer.

We propose a simple method of measuring polymerization-shrinkage evolution during curing in photopolymer. The real-time spectral fringe analysis of a broadband beam transmitted through a Fabry-Pérot etalon supported by a photopolymer film provides the shrinkage evolution during curing. For the proof-of-principle demonstration a blue-sensitized nanoparticle-polymer composite material is used. It is shown that the measured shrinkage dynamics are well correlated with the photo-calorimetric conversion dynamics of monomer to polymer. We also discuss a discrepancy in steady-state shrinkage between our proposed and holographic Bragg-angle detuning measurements.

Nanoparticle polymer composite volume gratings incorporating chain transfer agents for holography and slow-neutron optics.

We demonstrate twofold enhancement of the saturated refractive index modulation (Δn(sat)) recorded in a photopolymerizable nanoparticle-acrylate polymer composite film by incorporating thiols acting as chain transfer agents. The chain transfer reaction of thiols with (meth)acrylate monomer reduces the polymer crosslinking density and facilitates the mutual diffusion of nanoparticles and monomer during holographic exposure. These modifications provide increased density modulations of nanoparticles and the formed polymer, resulting in the enhancement of Δn(sat) as high as 1.6×10(-2) at a wavelength of 532 nm. The incorporation of thiols also leads to shrinkage suppression and to improvement of the grating's spatial frequency response. Such simultaneous improvement is very useful for holographic applications in light and neutron optics.

Effects of chain-transferring thiol functionalities on the performance of nanoparticle-polymer composite volume gratings.

We report influences of varying functionalities of thiols as chain transfer agents on the spatial frequency response, polymerization shrinkage, and thermal stability of a volume grating recorded in a photopolymerizable ZrO2 nanoparticle-polymer composite film. It is shown that a substantial increase in the saturated refractive index modulation is realized at high spatial frequencies by doping with multifunctional thiols. Moreover, the incorporation of multifunctional thiols considerably suppresses polymerization shrinkage of recorded volume gratings and thermal changes in refractive index and film thickness as compared with the case of mono-thiol. These results indicate that multifunctional thiols provide effective control of the properties of nanoparticle-polymer composite volume gratings for various applications in light and neutron optics.

Effects of symbol modulation coding on readout fidelity of shift-multiplexed holographic digital data page storage in a photopolymerizable nanoparticle-(thiol-ene)polymer composite film.

We report on shift-multiplexed holographic storage of 250 digital data pages in a photopolymerizable SiO2 nanoparticle-polymer composite film being capable of step-growth thiol-ene polymerization in the green. Various two-dimensional symbol modulation codes for the digital data page format were employed to examine the dependence of the readout fidelity on modulation coding schemes. It is found that, as compared to 1:2 and 2:4 modulation codes, higher-order 5:9, 9:16, and 13:25 modulation codes possessing reduced white rates and higher coding efficiencies give lower symbol-error rates of ~1×10⁻³ and higher signal-to-noise ratios (>4).

High-order nonlinear optical response of a polymer nanocomposite film incorporating semiconducotor CdSe quantum dots.

We report on observation of high-order optical nonlinearities in our recently developed photopolymerizable semiconductor CdSe quantum dot (QD)-polymer nanocomposite films at various volume fractions of CdSe QDs as high as 0.91 vol.% (3.6 wt.%). We performed Z-scan and degenerate multi-wave mixing (DMWM) measurements using a 532-nm picosecond laser delivering single 35 ps pulses at a repetition rate of 10 Hz. Using the uniformly cured polymer nanocomposite films, we observed the third- and fifth-order nonlinear optical effects in closed-aperture Z-scan measurements by which it was found that saturable nonlinear absorption (light-induced transparency) and large negative nonlinear refraction were induced. We also measured dependences of the effective third- and fifth-order nonlinear refraction constants on CdSe QD volume fraction. Based on the Maxwell-Garnett model, we estimated the third- and fifth-order nonlinear optical susceptibilities of CdSe QD and discussed a contribution of the third-order effect to the fifth-order one due to the cascaded (local-field) effect. Coexistence of the third- and fifth-order nonlinear refraction was also confirmed by DMWM.

Shift-multiplexed holographic digital data page storage in a nanoparticle-(thiol-ene) polymer composite film.

We demonstrate shift-multiplexed holographic storage of 180 digital data pages with low symbol-error rates in a thick (250 µm) SiO2 nanoparticle-polymer composite film using step-growth thiol-ene photopolymerization. A two-dimensional 2:4 modulation code was employed for formatting digital data pages in order to reduce the average intensity of code block without decreasing the coding efficiency. This study clearly shows the feasibility of the thiol-ene based nanoparticle-polymer composite system as a holographic data storage medium.

Order-of-magnitude polymerization-shrinkage suppression of volume gratings recorded in nanoparticle-polymer composites.

We demonstrate substantial shrinkage suppression of nanoparticle-polymer composite transmission gratings by use of the step-growth polymerization mechanism. It is shown that the polymerization shrinkage can be reduced as low as 0.3% at the nanoparticle concentration of 35 vol. % by which the refractive index modulation and the material sensitivity are maximized to be 8x10(-3) and 1014 cm/J, respectively, in the green. Our results offer a noticeable advance in the development of holographic data storage materials.

Editorial for the Special Issue "Polymeric and Polymer Nanocomposite Materials for Photonic Applications".

Polymer nanocomposites are designed and engineered on a nanometer scale with versatile applications including optics and photonics [...].

Zero-broadening SBS slow light propagation in an optical fiber using two broadband pump beams.

A new method of tailoring stimulated Brillouin scattering (SBS) gain spectrum for slow light propagation is proposed by use of two Gaussian-shaped broadband pump beams with different powers and spectral widths. The central frequency interval between the two pump beams are carefully set to be two inherent Brillouin frequency shift, ensuring that the gain spectrum of one pump has the same central frequency with the loss spectrum of the other one. Different gain profiles are obtained and analyzed. Among them a special gain profile is found that ensures a zero-broadening of the signal pulse independent of the Brillouin gain. This is owing to the compensation between the positive gain-dependent broadening and the negative GVD (group velocity dispersion) dependent broadening. The relationship of two pump beams is also found for constructing such a gain profile. It provides us a new idea of managing the broadening of SBS-based slow pulse by artificially constructing and optimizing the profile of gain spectrum.

Diffractive and Interferometric Characterization of Nanostructured Photopolymer for Sharp Diffractive Optical Elements Recording.

We study the behavior of a nanoparticle-polymer composite (NPC) material, based on a thiol-ene monomer system, working with long grating spacing. Thus, we evaluate the suitability of the NPC for storing complex diffractive optical elements with sharp profiles, such as blazed gratings. Using holographic methods, we measure the "apparent" diffusion of the material and the influence of the spatial period on this diffusion. The applicability of this material in complex diffractive optical elements (DOEs) recording is analyzed using an interferometric method. Supported by the results of this analysis, we record blazed gratings with different grating spacing and measure the maximum diffraction efficiency (DE) achieved. The results show that NPC has a good behavior in this range of spatial frequencies.

Ultraviolet-light-induced absorption changes in highly Zn-doped LiNbO(3).

Ultraviolet-light-induced absorption (UVLIA) in a highly Zn-doped LiNbO(3) crystal is studied at room temperature with various probe wavelengths from the violet to the near-infrared spectral regions. Transient dark buildup of UVLIA is observed in the violet-blue (<500 nm) region, while such a phenomenon is yet absent in the green-red (>500 nm) region. It is found that the temporal evolution of UVLIA in the dark is well described by a sum of two stretched exponential functions. These results are explained in terms of a three-level model involving two types of hole-trapped O- levels and one unintentional impurity FeLi level.

Highly transparent ZrO(2) nanoparticle-dispersed acrylate photopolymers for volume holographic recording.

We demonstrate and characterize volume holographic recording in ZrO(2) nanoparticle-dispersed acrylate photopolymer films that have very low scattering loss. More than thirty-fold reduction in the scattering coefficient, as compared with those of previously reported TiO(2) nanoparticle-dispersed photopolymers, is achieved. It is shown that the refractive index modulation as high as 5.3x10(-3), together with substantive photopolymerization-shrinkage suppression, is obtained at the nanoparticle concentration of 15 vol.%. Dependences of nanoparticle concentration and grating spacing on the refractive index modulation are also investigated.

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

We report on an experimental investigation of spatial frequency responses of anisotropic transmission refractive index gratings formed in holographic polymer dispersed liquid crystals (HPDLCs). We studied two different types of HPDLC materials employing two different monomer systems: one with acrylate monomer capable of radical mediated chain-growth polymerizations and the other with thiol-ene monomer capable of step-growth polymerizations. It was found that the photopolymerization kinetics of the two HPDLC materials could be well explained by the autocatalytic model. We also measured grating-spacing dependences of anisotropic refractive index gratings at a recording wavelength of 532 nm. It was found that the HPDLC material with the thiol-ene monomer gave higher spatial frequency responses than that with the acrylate monomer. Statistical thermodynamic simulation suggested that such a spatial frequency dependence was attributed primarily to a difference in the size of formed liquid crystal droplets due to different photopolymerization mechanisms.

Photopolymerizable nanocomposite photonic materials and their holographic applications in light and neutron optics.

We present an overview of recent investigations of photopolymerizable nanocomposite photonic materials in which, thanks to their high degree of material selectivity, recorded volume gratings possess high refractive index modulation amplitude and high mechanical/thermal stability at the same time, providing versatile applications in light and neutron optics. We discuss the mechanism of grating formation in holographically exposed nanocomposite materials, based on a model of the photopolymerization-driven mutual diffusion of monomer and nanoparticles. Experimental inspection of the recorded grating's morphology by various physicochemical and optical methods is described. We then outline the holographic recording properties of volume gratings recorded in photopolymerizable nanocomposite materials consisting of inorganic/organic nanoparticles and monomers having various photopolymerization mechanisms. Finally, we show two examples of our holographic applications, holographic digital data storage and slow-neutron beam control.

Observation of strong stimulated photorefractive scattering and self-pumped phase conjugation in LiNbO3:Mg in the ultraviolet.

We report on the observation of diffusion-dominant photorefraction and light-induced nonlinear forward and backward scattering in highly Mg-doped LiNbO3 at 351 nm. We also demonstrate what we believe to be the first continuous-wave self-pumped phase conjugation via stimulated photorefractive backscattering in the ultraviolet.

Optical Properties of Electrically Tunable Two-Dimensional Photonic Lattice Structures Formed in a Holographic Polymer-Dispersed Liquid Crystal Film: Analysis and Experiment.

We report on theoretical and experimental investigations of optical wave propagations in two-dimensional photonic lattice structures formed in a holographic polymer-dispersed liquid crystal (HPDLC) film. In the theoretical analysis we employed the 2×2 matrix formulation and the statistical thermodynamics model to analyze the formation of anisotropic photonic lattice structures by holographic polymerization. The influence of multiple reflections inside an HPDLC film on the formed refractive index distribution was taken into account in the analysis. In the experiment we fabricated two-dimensional photonic lattice structures in an HPDLC film under three-beam interference holographic polymerization and performed optical measurements of spectral transmittances and wavelength dispersion. We also demonstrated the electrical control capability of the fabricated photonic lattice structure and its dependence on incident wave polarization. These measured results were compared with the calculated ones by means of photonic band and beam propagation calculations.

Improved thermal stability of volume holograms recorded in nanoparticle--polymer composite films.

We examine thermal distortions of volume holograms recorded in (meth)acrylate photopolymers doped with SiO(2) or ZrO(2) nanoparticles. A holographic method is used to evaluate the temperature-induced Bragg-angle detuning of recorded volume holograms as a result of thermally induced refractive index and dimensional changes. It is found that the incorporation of inorganic nanoparticles into photopolymer leads to the effective suppression of these thermal changes, thereby extending the range of operating temperatures for their use in photonic applications.