07-05-2013, 02:49 PM
3D optical data storage with two-photon induced photon-oxidation in C60-doped polystyrene film
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ABSTRACT
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 Tiapphire 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 Tiapphire 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, 82MHz Tiapphire 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
Tiapphire laser pumped by an argon ion laser was used
for two-photon excitation, which was focused onto the sample
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 3.5GW/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 82MHz as
measured with a fast photodiode. The Tiapphire 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 Tiapphire 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 664nm and 696 nm.
Results and discussion
C60 has a complex absorption band structure extending from
400nm 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 Tiapphire laser
irradiation (910 nm, 100 fs, 82 MHz, and 50mW) 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.
Summary
We demonstrated 3D optical data storage with photooxidation
of C60 pumped by the fs Tiapphire 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.