Optical Memory Eﬀect by Interference of MultipleScattered Light in a Fluorescent Fulgide Derivative
ATUSI KURITAa, YASUO KANEMATSU a, TAKASHI KUSHIDAb,TOSHIYA SAGISAKAc, and YASUSHI YOKOYAMA ca

Department of Physics, Osaka University, Toyonaka, Osaka 560-0043,
Japan; b Nara Institute of Science and Technology, Ikoma, Nara 630-0101,
Japan; c Department of Materials Chemistry, Yokohama National University, Yokohama 240-8501, Japan

We report an observation of a new wavelength- and angleAbstract
selective optical memory eﬀect caused by interference of multiple-scattered
light in polystyrene powder doped with a ﬂuorescent fulgide derivative.
This eﬀect is applicable to three-dimensional optical data storage.
Keywords fulgide; polystyrene; interference; multiple scattering; opticalmemory; three-dimensional data storage

INTRODUCTION
Recently we have revealed that photoreactive materials combined withstrongly scattering host media cause a novel optical memory eﬀect, whichis potentially applicable to high density optical data storage. We have discovered it in Sm-doped ZnS nanocrystals[1, 2]. The eﬀect was observed in
the excitation spectrum of the ﬂuorescence of Sm ions; a hole was foundto persist in the spectrum after irradiation of monochromatic light. If one
looks at this result alone, one might consider the eﬀect to be nothing but
the persistent spectral hole-burning[3]. However, the hole was observed
when not only the wavelength but also the incident angle of the probe lightcoincided with those of the recording light. This observation indicates that
this eﬀect diﬀers from the conventional persistent hole burning in kind.
We have shown the mechanism of this eﬀect to be as follows[1]. In theA. KURITA ET. AL.
recording process, an interference pattern of multiple-scattered light within
the medium, which is dependent on both the wavelength and the incident
angle of the recording light, is registered by photobleaching of Sm as a
spatial modulation of the absorbance. In the reading process, intensity of
the ﬂuorescence from the sample is measured. The intensity is proportional
to the overlap integral between the spatially modulated absorbance createdby the recording light and the interference pattern formed by the reading,or exciting, light. Thus, the ﬂuorescence intensity shows a dip when both
the wavelength and the incident angle of the reading light coincide withthose of the recording light, because, in this situation, the absorbance andthe intensity of the reading light at each position in the sample alternatewith each other, and the excitation is less eﬀective.
Necessary conditions for this eﬀect are that (1) the recording medium
is photoreactive, (2) the medium is ﬂuorescent, and the ﬂuorescence intensity is changed according to the photoreaction, and (3) the medium
is optically inhomogeneous and causes multiple-scattering of light in it.
Then we inferred that this eﬀect is quite common and can be observed inother combinations of multiple-scattering media and photoreactive materials. Binaphthol-condensed indolylfulgide is a suitable photoreactive material for a possible observation of this eﬀect because it shows photochromism
and its colored form is ﬂuorescent[4]. We designed a recording medium by
combining the fulgide with a scattering host consisting of polystyrene toprove generality of this eﬀect. In this paper, we report the fundamental properties of this recording medium and the wavelength- and anglemultiplexed optical memory eﬀect observed in it.

EXPERIMENTAL
Synthesis of binaphthol-condensed indolylfulgide has been alreadyreported[5]. This fulgide derivative is one of a few ﬂuorescent photochromic
molecules. Both of the colored C-form and the colorless E-form are thermally stable, and the C-form is transformed to the E-form by the irradiation within the absorption band in the visible region. When each form intoluene is excited within respective absorption bands at room temperature,
the C-form (absorption maximum 471 nm) emits ﬂuorescence (ﬂuorescencemaximum 640 nm), while the E-form (absorption maximum 368 nm) does
not[4]. In order to observe the optical memory eﬀect by the recording of theinterference pattern, fulgide must be doped into a solid host which strongly
scatters light within it. Thus we prepared a polystyrene ﬁlm doped withthe fulgide derivative (1.2 wt %) and ground it into ﬁne powder. Obser-OPTICAL MEMORY EFFECT IN FULGIDEÂêæððæìëæëñâëðæñö¥Þïß«òëæñð¦³­±­¯­­°­­±­­²­­³­­:DYHOHQJWK QP´­­µ­­

FIGURE 1 Excitation spectrum of the emission at 630 nm (dashed line),and emission spectrum excited at 500 nm (solid line) of the C-form ofbinaphthol-condensed indolylfulgide in polystyrene powder.
vation by a scanning electron microscope showed that the powder sample
was composed of grains of the order of 1 µm. The polystyrene powder isexpected to act as an eﬀective scatterer for visible light because of thisstructural inhomogeneity.
Excitation and emission spectra were measured by using a spectrophotometer (Hitachi F4500). In the measurement of the memory eﬀect,
the sample was placed on a stage equipped with a computer-controlled micrometer, which rotates the stage around an axis lying in its surface to scan
the angle of the incident beam relative to the surface normal. The minimum step of the rotating angle was 0.0028 ◦. The sample was irradiated byattenuated light from a cw dye laser or an Ar-ion laser, and the wavelength
or the incident angle of the laser light was scanned while the ﬂuorescencewith the wavelengths longer than 590 nm was detected through cutoﬀ ﬁlters
by a photomultiplier; the same laser beam was used for burning the holes,
but the intensity was about 104 times higher than that for the ﬂuorescencemeasurements. All the experiments were made at room temperature

RESULTS AND DISCUSSION
Since the sample is powder, we measured the excitation spectrum insteadof the absorption spectrum. Figure 1 shows the excitation and the emissionÂêæððæìëæëñâëðæñö¥Þïß«òëæñð¦A. KURITA ET. AL.

¯­®²®­²­­¯­±­³­µ­Ñæêâ¥ð¦®­­®¯­

FIGURE 2 Decay of the ﬂuorescence intensity of the C-form fulgide inpolystyrene powder caused by the light irradiation (wavelength 514.5 nm,intensity 0.5 mW/mm2 ).
spectra of the C-form fulgide in polystyrene powder. In polystyrene, Stokes
shift is smaller than in toluene, and the excitation and the ﬂuorescencemaxima are 505 nm and 597 nm, respectively.
Figure 2 shows the decay of the ﬂuorescence intensity around 600 nm
by the irradiation of 514.5 nm light. Irradiation was suspended momentarily from 33 to 40 s in order to check the stability of the product state.
The ﬂuorescence intensity showed a smooth decay curve if the suspension
period was cut out, which indicates that the back reaction in the dark isnegligible. The decay curve is non-exponential as has been often observedin photochromic materials in solid matrices[6]. The curve in Figure 2 can be√ﬁtted by an empirical function a+b/ 1 + ct very well, where t is the irradiation time, a, b and c are adjustable parameters. The ﬂuorescence intensity
does not decay toward zero, which means that the C-form in polystyrene
is not completely converted to the E-form.
We measured the angular dependence of the backscattered light fromthe sample by using 633 nm light of a He-Ne laser. In multiple-scattering
media, a constructive interference of time-reversed counter-propagatingwaves is known to enhance the intensity of the scattered light in the exact
backward direction[7]. The peak observed in the scattered light around thebackward direction is called coherent backscattering cone. In the present
polystyrene powder sample, a coherent backscattering cone similar to thatof Sm-doped ZnS nanocrystals[2] was observed. This result indicates thatOPTICAL MEMORY EFFECT IN FULGIDEßâãìïâßòïëæëä®

±­¯°­Âêæððæìëæëñâëðæñö¥Þïß«òëæñð¦²­°¯­²±®­­ª®«² ª®«­ ª­«² ­«­ ­«² ®«­ ®«²ª®«² ª®«­ ª­«² ­«­ ­«²¾ëäéâ¥áâä¦®«­®«²FIGURE 3 Proﬁle of successively burned ﬁve holes. Emission intensity wasmeasured as a function of the incident angle. After the measurement of theinitial proﬁle before the burning, holes were burned at diﬀerent incident angles indicated by the numerals(wavelength 488 nm, intensity 0.5 mW/mm2 ,burning time 20 s for each hole) and then the proﬁles were measured. The
ﬁnal proﬁle with ﬁve holes is enlarged in the inset.

the polystyrene powder has the scattering strength comparable to ZnS
nanocrystals. The FWHM of the backscattering cone was 0.41◦ , from which
the mean free path of light in the sample is estimated at 15 µm according
to ref. [7].
Now that it was conﬁrmed that the polystyrene powder doped withthe fulgide derivative fulﬁlls the necessary conditions, we tried to observe
the optical memory eﬀect caused by interference of multiple-scattered light.
Figure 3 shows the result of an experiment to burn multiple holes at diﬀer-A. KURITA ET. AL.
ent incident angles. The holes are clearly observed after each irradiation,and the fact that they are dependent on the incident angle signiﬁes that theobserved holes are due to interference. Besides the holes, there exists almost
uniform decrease of the emission intensity after every irradiation, which has
been observed also in Sm-doped ZnS nanocrystals[1]. This uniform decreaseis an indication that this eﬀect stems from a nonlocal interference eﬀect,in which a writing action at any writing angle more or less aﬀects the ﬂuorescence intensity at all the reading angles. With the sequence of the holeburning, previously burned holes are getting shallower but they show nobroadening at all. Therefore, in principle, a large number of holes can berecorded in a single recording area.
The hole width shown in Figure 3 is about 0.33◦ , while the width of
the hole measured as a function of the wave number was about 5 cm−1 .
If the hole can be burned all over the solid angle 2π of the incident light,
the angular multiplicity is about 1.9 × 105 . The wavelength multiplicity
is estimated at about 1500 from the width of the excitation band shownin Figure 1. The size of the recording area is considered to correspondto the distance between the impinging points of the interfering waves on
the sample[2], which is estimated from the angular width of the hole tobe 85 µm. Accordingly, the surface density of the memory amounts to39000 bit/µm2 . This extremely high storage density comes from the factthat information is stored in the body of the medium as a three-dimensional
interference pattern. This method has an advantage over other threedimensional optical data storage methods, because information is storedand retrieved by a simple way of changing the wavelength and the incident
angle of light.

REFERENCES[1] A. Kurita, Y. Kanematsu, M. Watanabe,
Phys. Rev. Lett. 83, 1582 (1999).

K. Hirata,

T. Kushida,[2] A. Kurita, Y. Kanematsu, M. Watanabe, K. Hirata, T. Kushida, J. Lumin.
(Proceedings of ICL’99) (2000) in press.
[3] W. E. Moerner, Ed., Persistent spectral hole-burning : Science and applications. (Springer-Verlag, Berlin, 1988).
[4] T. Inada, S. Uchida, Y. Yokoyama, Chem. Lett. 321 (1997).
[5] Y. Yokoyama, S. Uchida, Y. Yokoyama,J. Am. Chem. Soc. 118, 3100 (1996).

Y. Sugawara,

Y. Kurita,[6] J. Biteau, F. Chaput, Y. Yokoyama, J. Boilot, Chem. Lett. 359 (1998).
[7] E. Akkermans, P. E. Wolf, R. Maynard, Phys. Rev. Lett. 56, 1471 (1986).