Holography is the conventional optical technique used to record phase information. This is a method where complex amplitude data is recorded using an interference effect between two coherent beams. Recently, we developed a novel optical recording technique in which phase information of a birefringent object is recorded on a polarization-sensitive optical material with a single beam.1 Known as retardagraphy, this technique records the retardance pattern of an optical signal (the pattern of the phase difference between its two orthogonal polarization components). In holography the recording beam must be split into signal and reference parts. However, in retardagraphy, the beam does not need to be split, which simplifies the optical system.
For example, in retardagraphy, the retardance pattern of a linear birefringent object is converted into a linear polarization pattern and recorded on a polarization-sensitive medium as the azimuth of the linear polarization. As in holography, the horizontal and vertical polarization states are composed of both signal and reference components. In addition, the phase of the signal component (relative to that of the reference component) is modulated by the linear birefringent object. Figure 1 shows the optical setup used for recording and reconstructing phase information. In the experiment, we used a diode laser of wavelength 407nm as the recording laser, and we generated a two-dimensional linear polarization pattern using wave-plates and a spatial light modulator. Because the optical information provided was binary (with phase of zero or one) the azimuth of linear polarization becomes plus or minus 45 degrees. Further, we projected the polarization pattern on an azobenzene copolymer film in a reducing optical system, and we then reconstructed the binary retardance pattern using a 632.8nm He-Ne laser beam. The reconstructed beam was left-circularly polarized using a half-wave plate and a quarter-wave plate. Next, we transmitted this beam through the azobenzene film and the quarter-wave plate and analyzed it using a polarizer. We adjusted the transmission axis of the polarizer in order to maximize the contrast of the two reconstructed values. Finally, we captured the reconstructed pattern using a CCD camera.2
Figure 1. Experimental setup for retardance recording and reading. HWP: Half-wave plate. QWP: Quarter-wave plate. M: Mirror. DM: Dichroic mirror. LCOS-SLM: Liquid crystal on silicon spatial light modulator. SCF: Sharp cut filter. P: Polarizer.
Figure 2 shows the experimental result of retardagraphy on a binary pattern. The recorded pattern is reconstructed, although with some errors. (These errors are caused by the general imprecision of the optical equipment used in the experiment.)
Figure 2. Experimental results: (a) binary phase pattern on the LCOS-SLM, (b) observed reconstructed pattern. The phase values in black and white regions are 0 and π, respectively.
The technique can also be applied to a multi-valued phase pattern. After being recorded on a polarization-sensitive medium, the pattern can be reconstructed by measuring the retardation between two polarization components (see Figure 3). This measurement can be performed using imaging polarimetry. However, polarization modulation devices are required for this technique, and the optical system for the reconstruction is complex.
Figure 3. Experimental results on multi-valued object: (a) two-dimensional retardance pattern on the LCOS-SLM, (b) reconstructed retardance pattern.
Retardagraphy can be applied to various optical memory systems including read-only and read-and-write setups. To increase the information capacity of optical mass-storage systems based on holographic architectures, we suggest polarization multiplexing and angular multiplexing. (These are techniques by which information can be stored in a medium by changing polarization states or recording angles, respectively.) To this purpose, the development of polarization-sensitive photopolymers is essential, and we have done some experimental studies in this area. Furthermore, we have started working on an optical mass-storage material project based on polarization-sensitive photopolymers. This project, which is supported by the Japanese Science and Technology Agency, comprises material development and memory system design including a co-axial holographic memory setup.
In summary, retardagraphy is a promising optical recording technique that can be applied to holographic architectures. In this technique, the pattern of phase difference between the horizontal and vertical polarization components is converted into the phase difference between the orthogonal and circular polarization components. The pattern is then recorded on a polarization-sensitive medium. In principle, an arbitrary combination of polarization components can be selected and suitably converted to match the properties of the polarization-sensitive medium. In the future, we intend to explore further applications of retardagraphy such as the analysis of optical anisotropic structures and optical data storage.
This work is supported partially by the Program for Strategic Promotion of Innovative R&D from the Japanese Science and Technology Agency.
Toyohiko Yatagai, Daisuke Barada
Toyohiko Yatagai graduated from Tokyo University in 1969, was a researcher in RIKEN from 1970 to 1983, and a professor of applied physics at the University of Tsukuba from 1983 to 2007. He has been the director of the Center for Optical Research and Education since 2007.
1. D. Barada, K. Tamura, T. Fukuda, M. Itoh, T. Yatagai, Retardagraphy: a technique for optical recording of the retardance pattern of an optical anisotropic object on a polarization-sensitive film using a single beam, Opt. Lett.
33, pp. 3007-3009, 2008. doi:10.1364/OL.33.003007
2. D. Barada, K. Tamura, T. Fukuda, T. Yatagai, Optical information recording in films of photoinduced birefringent materials and application to retardagraphy, Jpn. J. Appl. Phys.
48, pp. 09LE02–09LE02-4, 2009. doi:10.1143/JJAP.48.09LE02