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.

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.

Photopolymerizable nanoparticle-polymer composites (NPCs) have thus far shown their excellent performance in various applications, such as holographic data storage, nonlinear optics and neutron optics. Specifically, for such applications, a high spatial frequency material response is necessary, as it is the response to high spatial frequencies that determines their spatial resolution and diffraction properties. However, it is known that the spatial frequency response of a recorded hologram in multi-component photopolymers including NPCs and holographic polymer-dispersed liquid crystals exhibits a reduction in refractive index modulation at high spatial frequencies. In order to overcome this drawback, an addition of chain transfer agents (CTAs) may be useful as done for all-organic photopolymers to modify their nonlocal response and phase separation characteristics. In our work, we investigate the effect of CTAs on the spatial frequency response in NPCs. Here we employ various chain-transfer agents with three different thiol groups in a photopolymerizable ZrO2 NPC film. A range of CTA concentration is carried out, in order to explore the most effective material combination used in the examination of spatial frequency response. The significant improvement in spatial frequency response of NPCs through the addition of a CTA with the most appropriate concentration is presented.

We have recently demonstrated the enhancement of the saturated refractive index modulation (Δnsat) of ZrO2 nano-particle-polymer composite (NPC) volume gratings recorded at short grating spacing by doping chain transferring a single functional thiol (mono-thiol) as a chain transfer agent (CTA) [1]. We have also achieved substantive shrinkage suppression in this NPC system. Since cross-linking networks can be controlled by introducing multifunctional thiols in photopolymers [2], it is expected that another important property of recorded NPC volume gratings, the thermal stability, may also be improved. Here we report on a thermal stability analysis of ZrO2 NPC volume gratings containing thiols with varied functionalities.

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.

We have performed the freezing pressure and heat capacity measurements for 4He confined in a nano-porous medium FSM16 which possesses a one-dimensional 2.8-nm straight channel, in the pressure region above the bulk freezing pressure. At 2.8 MPa, where 4He in the channel remains a non-superfluid liquid down to the lowest temperature, the heat capacity decreases monotonically with decreasing temperature. It is in contrast to the heat capacity at 0.03 MPa with a broad bump at the temperature higher than the superfluid onset. At around the freezing onset in the channel, no clear heat capacity peak was observed. It suggests that the latent heat of the liquid–solid transition in the channel is quite small.