Summary: We produced a new system for measuring the small photoelastic constant of a polymer thin film with a small birefringence. Using our mesurement system, we evaluated the photoelastic constant of a polymer film in real time by quantitative analysis. Photoelastic constants of 11.30 Â 10 À12 Pa À1 for a cellulose triacetate film and 78.38 Â 10 À12 Pa À1 for a polycarbonate film were obtained. Furthermore, we obtained a small photoelastic constant of 0.12 Â 10 À12 Pa À1 for a cycloolefin film for liquid crystal displays, using our new measurement system. This value is very small. We emphasize that, if a small change in retardation and stress cannot be detected simultaneously using our system, then we cannot obtain such a small photoelastic constant.
The possibility has been indicated that a piezoelectric polymer with helical chirality (chiral polymer) such as poly-L-lactic acid (PLLA) shows a large linear electrooptical constant (Pockels effect). However, the linear electrooptical constant of a PLLA film may be very small because such a film fabricated by the conventional method has a complex high-order structure with intermingled crystalline and amorphous regions. In order to measure the small linear electrooptical constant of a PLLA film, we developed a new measurement system, which is based on the heterodyne interferometry principle. In this system, the accuracy of retardation is 0.08 nm and the measurement time is 0.1 s. In our attempt to realize a PLLA film with a large linear electrooptical constant, we fabricated a PLLA film, which was heated to 120°C under 320 MPa. Finally, using our new measurement system, we obtained a linear electrooptical constant of 0.070 pm/V in the PLLA film, which is very small compared with those of other famous Pockels materials. However, the linear electrooptical constant of the PLLA film is clarified in this study for the first time.
Given a bipartite system, correlations between its subsystems can be understood as the information that each one carries about the other. In order to give a model-independent description of secure information disposal, we propose the paradigm of private quantum decoupling, corresponding to locally reducing correlations in a given bipartite quantum state without transferring them to the environment. In this framework, the concept of private local randomness naturally arises as a resource, and total correlations are divided into eliminable and ineliminable ones. We prove upper and lower bounds on the quantity of ineliminable correlations present in an arbitrary bipartite state, and show that, in tripartite pure states, ineliminable correlations satisfy a monogamy constraint, making apparent their quantum nature. A relation with entanglement theory is provided by showing that ineliminable correlations constitute an entanglement parameter. In the limit of infinitely many copies of the initial state provided, we compute the regularized ineliminable correlations to be measured by the coherent information, which is thus equipped with a new operational interpretation. In particular, our results imply that two subsystems can be privately decoupled if their joint state is separable.
We propose and demonstrate a novel visual encryption device composed of higher-order birefringent elements. When an optical material with higher-order birefringence is placed between a pair of polarizers and illuminated by white light, it appears only white. In contrast, when it is illuminated by monochromatic light, the transmitted intensity varies depending cosinusoidally on the wavelength. An array of such materials can express information (e.g., letters and/or images) by controlling the birefringence of each pixel. If birefringence phase retardation can be adjusted for a specific wavelength, the information will be clearly displayed when it is illuminated at this wavelength. We denote this wavelength a key wavelength. The encryption device was fabricated by controlling the amount of higher-order birefringence to achieve high contrast only by using polarized illumination at the key wavelength. Thus, the information stored in the encryption device can be decoded only by illuminating it at the key wavelength. To demonstrate the validity of this encryption principle, we constructed a 3 × 3 pixel device in which commercial retarder films were laminated. The device was illuminated by a monochromatic light. When a readout experiment was performed using the monochromatic light at the key wavelength, the stored letter was clearly visible. On the other hand, when pixel brightness was randomly distributed with illumination at the wavelength other than the key wavelength, the letter could not be recognized. Furthermore, the stored information can be easily distributed to multiple physical keys that display arbitrary images. In this case, the birefringence phase retardation is obtained by summing the values of retardation of each pixel of the physical keys. In the experimental device, the observed image was decoded by superimposing the two images using different physical keys.
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