Saturated Refractive Index Modulation Research Articles

Thermal stability of fiber Bragg gratings fabricated by 193 nm excimer laser

The thermal stability of uniform fiber Bragg gratings (FBGs) inscribed in single mode double cladding fiber (DCF), standard single mode fiber (SMF-28) and dispersion shifted fiber (DSF) by 193 nm excimer laser was experimentally investigated. When the temperature rises from 30 °C to 800 °C, the grating contrast of FBGs with the saturated refractive index modulation drops slowly, and the corresponding 3 dB bandwidth gradually decrease After annealing at 400 °C for 2000 h, it is found that the resonance wavelength shift of FBGs mainly occur during the first 200 h. It is also observed that the FBG inscribed in DCF requires a longer time to stabilize wavelength than the FBG written in SMF-28, about 1500 h, which suggests that long-term pre-annealing is required to ensure accurate wavelength measurement. After 2000 h of annealing treatment, the FBGs can still have excellent sensing performance in the temperature range of 0–400 °C.

Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

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.

Volume holographic recording in nanoparticle–polymer composites doped with multifunctional chain transfer agents

We report on an experimental investigation of the properties of volume holographic recording in photopolymerizable nanoparticle–polymer composites (NPCs) doped with chain transferring multifunctional di- and tri-thiols as chain transfer agents. It is shown that the incorporation of the multifunctional thiols into NPCs more strongly influences on volume holographic recording than that doped with mono-thiol since more chemical reactions involve in the polymer network formation. It is found that, as similar to the case of mono-thiol doping, there exist optimum concentrations of di- and tri-thiols for maximizing the saturated refractive index modulation. It is also seen that recording sensitivity monotonically decreases with an increase in multifunctional thiol concentration due to the partial inhibition of the photopolymerization event by excessive thiols.

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.

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.

Holographic nanoparticle-polymer composites based on step-growth thiol-ene photopolymerization

We report on the photopolymerization kinetics and volume holographic recording characteristics of silica nanoparticle-polymer composites using thiol-ene monomers capable of step-growth polymerization. Real-time Fourier transform spectroscopy and photocalorimetry are used to characterize the visible light curing kinetics of a thiol-ene monomer system consisting of secondary dithiol with high self-life stability and low odor and triene with rigid structure and high electron density. It is shown that while the nanoparticle-(thiol-ene)polymer composites exhibit high transparency, their saturated refractive index modulation (Δnsat ) and material sensitivity (S) are as large as 1×10−2 and 1615 cm/J, respectively. The polymerization shrinkage is reduced as low as 0.4% as a result of the late gelation in conversion. These values meet the acceptable values for holographic data storage media (i.e., 5×10−3, 500 cm/J and 0.5% for Δnsat, S and shrinkage, respectively). It is also shown that because of the dispersion of inorganic silica nanoparticles and the use of the triene monomer having the rigid structure of the triazine functional group, the thermal stability of recorded holograms is much improved over our previously reported nanoparticle-polymer composites using organic nanoparticles and primary mercaptopropionate trithiol/allyl ether triene monomers [Opt. Lett. 35, 396 (2010)].