3D optical data storage with two-photon induced photon ... - CiteSeerX

RIKEN Review No. 49 (November, 2002): Focused on Ultrafast Optical Sciences

3D optical data storage with two-photon induced photon-oxidation in C60-doped polystyrene film Satoshi Wada,∗1 Andong Xia,∗2 and Hideo Tashiro∗1 ∗1 ∗2

Fundamental Technology Development Division, Advanced Engineering Center, RIKEN

National Synchrotron Radiation Laboratory, University of Science and Technology of China

A new approach for multilayered optical data storage in C60 -doped polystyrene film was demonstrated. We find that photo-oxidation may increase the fluorescence intensity of C60 molecule by two-photon excitation in a wide wavelength region (780–910 nm) from a 100 fs Ti:sapphire laser. The proposed scheme encodes the digital bits by photo-oxidation of C60 induced by two-photon excitation, and reads the increase in fluorescence from the oxidation area. The high-fluorescence signal can easily be distinguished as bit 1, and the lowfluorescence signal, as bit 0. This change in the fluorescence intensity could be used to encode information for read-only memory.

Introduction Successful isolation and purification of macroscopic quantities of fullerene clusters have generated in a huge amount of interest in the physical and chemical properties of these carbon structures. Fullerenes are now anticipated to have a number of applications in future nano-photonics science and technology.1–4) Recently, we successfully demonstrated the three-dimensional (3D) optical data storage in C60 and read from the enhanced fluorescence of the photo-oxidation5) products. The highly symmetric π-electron density distribution of the icosahedral C60 cage may considerably enhance the twophoton absorption of powerful femtosecond pulse light.6) We further note that the fluorescence of C60 is also enhanced in ambient air after two-photon irradiation in a wide wavelength region (780 nm–910 nm) from 100 fs Ti:sapphire laser. Therefore, the two-photon-induced photo-oxidation of C60 may also be a subject of interest for application to 3D optical data storage. In this review, we introduce a new approach for multilayered optical data storage in C60 -doped polystyrene film. The proposed scheme encodes the digital bits by photooxidation of C60 induced by two-photon excitation in a wide wavelength region. We report that the fluorescence intensity of C60 -doped polystyrene film is significantly increased in areas that have been exposed to a sufficient dose from the 100 fs, 82 MHz Ti:sapphire laser. This change in the fluorescence intensity could be used to encode information for read-only memory (ROM). The fluorescence properties and photo-oxidation dynamics of C60 induced by two-photon excitation have been investigated.

Experimental The investigated films were prepared as described elsewhere.5) The C60 -doped polystyrene film with porous structure was coated to a thickness of about 200 µm on a microscope slide glass in air at room temperature. A mode-locked Ti:sapphire laser pumped by an argon ion laser was used for two-photon excitation, which was focused onto the sam-

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ple with a spot size of 15 µm diamater through an objective (10 x, NA 0.25). Typical intensity of the laser at the focal point is 3.5 GW/cm2 . This objective served as both the focusing lens and fluorescence collecting lens. The pulse width, continuously monitored by an intensity autocorrelator, was about 100 fs, and the pulse repetition rate was 82 MHz as measured with a fast photodiode. The Ti:sapphire laser was used not only for fluorescence measurement of C60 but also for photo-oxidation of C60 . The C60 -doped polystyrene film was mounted upon a computer-controlled programmable X-Y-Z translation stage. This stage permits precise (1 µm) linear translation of the sample with a maximum displacement of 2.5 cm in all directions, and a wide range of velocities. The data writing processes in the film were performed in air by keeping the Ti:sapphire laser beam focused at a fixed position and moving the sample through the focused beam point by point. The irradiation dwell time per exposed point was also controlled by a computer. We obtained the photo-oxidation points in the sample. To read these points, the film was then sealed carefully with a cover slide glass and wax in a N2 environment such that further photo-oxidation of C60 was blocked to enable nondestructive reading. Fluorescence was detected through an emission band-pass filter (D680/32) in order to receive fluorescence wavelengths between 664 nm and 696 nm.

Results and discussion C60 has a complex absorption band structure extending from 400 nm to 650 nm.7) The two-photon-induced fluorescence from C60 is extremely weak, but the relative fluorescence spectral distributions from one- and two-photon-excitation are very similar (data not shown). This suggests that the same excited states are reached regardless of the excitation mode. Figure 1 shows the two-photon excitation fluorescence spectra of C60 -doped polystyrene film under Ti:sapphire laser irradiation (910 nm, 100 fs, 82 MHz, and 50 mW) in air. We find that the fluorescence signal is weak with a broad peak at about 730 nm before extended laser irradiation of the sample in air, and after 30 s irradiation in air, the fluorescence intensity increases 3-fold with a peak shift to about 715 nm. The enhancement of the fluorescence of C60 might

Fig. 1. Fluorescence spectra of C60 doped polystyrene film under 100 fs, 82 MHz, 50 mW Ti:sapphire laser irradiation at 910 nm, for different exposure times of (a) 0 s and (b) 30 s.

result from the decrease in symmetry of the oxidized C60 in the polystyrene induced by two-photon excitation. Figure 2 shows the irradiation time dependence of the fluorescence intensity of C60 -doped polystyrene film in air at different excitation wavelengths. Similar to the case of onephoton excitaion, the relative fluorescence intensities at various excitation wavelengths show a rapid increase followed by a slow decay with the irradiation time. The finding that the photo-oxidation rate of C60 , under irradiation at the same power is much faster at a shorter wavelength than at a longer

Fig. 2. Irradiation time dependence of the fluorescence intensities in the wavelength region between 664 nm and 696 nm for C60 -doped polystyrene film in an air environment with an irradiation power of about 50 mW at various wavelengths. (a) 830 nm, (b) 880 nm, and (c) 910 nm.

wavelength was unexpected. In fact, UV light is highly efficient for C60 photo-oxidation.8) In the one-electron description the one-photon excitation of C60 between the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) is strongly dipole-forbidden due to the centrosymmetric structure of the molecule, while the third-order optical polarizibility is always allowed. Two-photon excitation is a third order effect in nonlinear optics. After photooxidation, some higher excited singlet states are also reached with greater probability by two-photon excitation in the near UV region than in visible region. This may be the reason why the photo-oxidation rate is higher at 830 nm than at 910 nm at the same excitation power. However, the changes of the fluorescence intensity induced by two-photon excitation can be blocked in an air-free environment, and nondestructive reading becomes possible for two-photon excitation.5, 9) When the oxidized areas in C60 -doped polystyrene films are exposed to a reading laser beam, the fluorescence becomes much stronger than in the non-oxidation areas, and the highfluorescence signals can easily be distinguished as bit 1 and the low-fluorescence signals from nonoxidation areas, as bit 0. This change in fluorescence intensity could be used to encode information for read-only memory. Figure 3 shows a demonstration of this 3D optical data storage. Seven points in the sample were selected along the X-direction on the same line in the X–Y plane with the desired distances from one another by moving the computer-controlled, and another six points were chosen along the Z-direction on the same line in the X–Y plane in the same manner as before. All the points were irradiated with focused 880 nm, 100 fs, 82 MHz, 50 mW Ti:sapphire laser for different durations. The recorded bit data was read nondestructively with a low two-photon excitation power of about 30 mW at 880 nm after the film was sealed carefully with a cover slide glass and wax in a N2 environment to block photo-oxidation. For reading these data, the sample stage was scanned at a rate of 50 µm/s along both X- and Z-directions. The results for the 3D data storage in C60 -doped polystyrene film are shown in Figs. 3 (a) and (b), where (a) represents the data written in the X–Y plane and (b), that in the X–Z plane. We obtained the FWHM of about 5 µm for one point in the X–Y plane, and FWHM of about 12 µm for one point in the X–Z plane. The oxidation of C60 was very sensitive to light with energy higher than its LUMO. For two-photon induced fluorescence of C60 , the excitation rate for the process is proportional to the square of the incident intensity, and excitation is hence

Fig. 3. (a) Fluorescence intensity vs distance along the X-direction in the X–Y plane. (b) Fluorescence intensity vs distance along the Z-direction in the X–Z plane. The exposure time for each point is marked above the peak. Fluorescence was observed in the wavelength region between 664 nm and 696 nm.

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confined to the focal volume, where the excitation intensity is extremely high. C60 molecules could reach the excited state by simultaneously absorbing two photons; this simultaneous absorption may also result in the photo-oxidation of C60 . The oxidation area was confined to the focal volume, therefore it is possible to increase the data storage density by two-photon-induced photo-oxidation. We did not attempt any optimization to obtain high-density storage, due to the limitation of the experimental configuration used.

Summary We demonstrated 3D optical data storage with photooxidation of C60 pumped by the fs Ti:sapphire laser. However, the writing speed of our demonstration experiment is still much too slow for practical use. Recently, we noted that some polymer composites with C60 as the photosensitizer could produce highly efficient optical property changes under both one-and two-photon excitation with very fast response times of several microseconds. Thus, very fast writing can be achieved with such a memory system.

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