Proceedings of ICL'99

preprint
Journal of Luminescence (2000)

Wavelength- and angle-selective properties of optical memory e®ect
by interference of multiple-scattered lightin Sm-doped ZnS nanocrystals
Atusi Kurita,1 Yasuo Kanematsu,1 Masahito Watanabe,1Kuniko Hirata,1 Takashi Kushida2
12Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan

Abstract
Our recent experiments have shown that both the wavelength and the angle of incident light are memorized in Smdoped ZnS nanocrystals by photobleaching coupled with interference of the multiple-scattered light. We measured
the angle-selective properties by scanning the incident angle, as well as the wavelength-selective properties, anddiscuss the density of data storage that can be derived from this e®ect.
Keywords : optical memory e®ect; interference; multiple scattering; samarium
Recently we have discovered a new kind of opticalmemory e®ect in Sm-doped ZnS nanocrystals[1]. Thee®ect was observed in the excitation spectrum of °uorescence, and a `hole' was found to persist in the excitation spectrum after irradiation of monochromaticlight. Thus this e®ect appears to be similar to the persistent spectral hole-burning e®ect. However, the hole
was only observed when not only the wavelength but
also the incident angle of the probe light coincided withthose of the recording light. This observation indicatesthat this e®ect di®ers from the conventional persistenthole burning in kind. We have shown that the recording
process of this e®ect is that the interference pattern ofmultiple-scattered light, which is dependent on both thewavelength and the incident angle of the recording light,
is registered by photobleaching of Sm as a spatial modulation of the absorbance. The optical inhomogeneityof the nanocrystal sample probably makes it a multiplescattering medium. In the reading process, the intensity of the °uorescence from the sample is proportional
to the overlap integral between the spatially modulated
absorbance and the interference pattern formed by thereading, or exciting, light. Thus, the °uorescence intensity shows a dip when both the wavelength and theincident angle of the reading light coincide with thoseof the recording light. In this paper, we report angle selective properties, which are most characteristic of thise®ect, measured by continuously scanning the incident

angle of the light beam and compare them with thewavelength selective properties.
The samples employed were Sm-doped ZnS nanocrystals coated by penta°uorothiophenol.
Samplepreparation procedure and fundamental optical properties are described in a separate paper[2].
Thepenta°uorothiophenol coating prevents the nanocrystals from aggregation and makes them soluble in alcohol
while insoluble in water. The sample was obtained froma ethanol solution as powder by evaporating the solvent.
Each powder particle was made up of many nanocrystals and appeared milky with slightly pink color. Thepowder was ground and sorted by particle size by using a sequence of sieves with the mesh sizes of 125, 75
and 45 ¹m. We also prepared a sample by drying precipitates in water. This type of sample was obtained
as a rather solid lump. The sample was placed on astage 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 tothe surface normal. The minimum step of the rotating
angle was 0.0028± . The powder particles were dispersedon the stage sparsely enough so as not to touch eachother. The sample was irradiated by attenuated lightfrom a cw dye laser with a linewidth of 0.2 cm¡1 , andthe laser wavelength (»580 nm) or the incident angle
was scanned while °uorescence around 630 nm was detected; the same laser beam was used for burning the2A. Kurita et al. / Journal of LuminescenceFig. 1. Pro¯le of three holes in the two-dimensional
space of the wave number and the incident angle.
The vertical axis represents decrease of the emission
intensity. Burning time and power were 15 s and
0.6 mW/mm2 , respectively, for each hole. Sample was
Sm-doped ZnS nanocrystals with the diameter larger
than 125 ¹m.
holes, but the intensity was about 104 times higher thanthat for the °uorescence measurements.
Fig. 1 shows a pro¯le of three holes measured byraster-scanning in the two-dimensional space of thewave number and the incident angle. It is evident thatthe hole memorizes both the wave number and the incident angle of the burning light, and the hole pro¯lesalong the wave-number axis and the angle axis look very
similar. Holes are well isolated and there is no crosstalk
between the holes within the measured region.
We compared the hole pro¯le in sample particles withdi®erent sizes. The size of samples sorted by sieves was,(a) smaller than 45 ¹m, (b) between 45 and 75 ¹m, (c)between 75 and 125 ¹m, and (d) larger than 125 ¹m.
In addition to these powder samples, (e) lump sample
from precipitates, which looked whiter than the particles of the powder samples but looked uniform when it
was observed by an optical microscope, was also measured. We burned a hole and measured its pro¯le asfunctions of two kinds of variable, i.e., by scanning thewavelength of the measuring laser while the incident
angle ¯xed at the burning angle, and by scanning theincident angle while the wavelength ¯xed at the burning
wavelength. We also measured the angle dependence ofthe coherent backscattered light from each sample. InFig. 2. Hole pro¯le as a function of the wave number(a), hole pro¯le as a function of the incident angle (b),and coherent backscattering cone (c) of particles of Smdoped ZnS nanocrystals with the diameter between 75
and 125 ¹m.
multiple-scattering media, a constructive interferenceof time-reversed counter-propagating waves is known
to enhance the intensity of the scattered light in theexact backward direction. Fig. 2 shows a result of thethree kinds of measurement in sample (c). In Table 1,
the width of the hole pro¯le as a function of the wave
number, the width of the hole pro¯le as a function of
the angle, and the width of the coherent backscattering
cone of the samples with di®ering sizes are summarized.
All of the three kinds of width narrow in order of increasing particle size.
The interference pattern that causes this memory effect results from waves which are scattered many times
after impinging upon the sample at di®erent points in
the incident plane and encounter at a point within thesample. Therefore, the width of the hole pro¯le as afunction of the incident angle is determined by the distance l between the impinging points of the interferingwaves, and is approximately given by ¸=l, where ¸ is

A. Kurita et al. / Journal of Luminescence
Table 1. Widths of, the hole pro¯le as a function of the wave number, the hole pro¯le as afunction of the incident angle, and the coherent
backscattering cone.
Sample size(¹m)(a) <45(b) 45»75(c) 75»125(d) >125(e) lumpHole(cm¡1 )
18.5
12.9
10.6
6.3
8.6

Width (FWHM)
Hole Backscattering
(deg)
(deg)
0.35
0.88
0.310.74
0.22
0.60
0.17
0.59
0.44
1.15

the light wavelength. The distance l is limited by theparticle size, which accounts for the particle-size dependence of the angular width of the hole shown in Table 1.
The width is comparable to the width of Fraunhoferdi®raction by a circular aperture, which is estimatedat 0.27± (FWHM) when the diameter is 125 ¹m and ¸
is 0.58¹m. In Table 1, the width of the backscattering cone is also larger for smaller particles. However,
if it is compared with the angular width of the hole ofthe sample with the same particle size, we notice thatthe backscattering cone is broader than the hole. Thebackscattering cone stems from the interference of timereversed counter-propagating waves which are scatteredmany times after impinging upon the sample and thenexit from it. Thus the width of the backscattering coneis determined by the distance between the impinging
point and the exit point[3]. This distance is also limited by the particle size. However, probably it is shorter
than l, and the width is larger than that of the hole,which explains the discrepancy between the two kindsof angular width. As to sample (e), both the width of
the hole pro¯le as a function of the incident angle andthe width of the backscattering cone are larger thanthose of sample (a). This result indicates that, althoughsample (e) is not small particles, di®usion length of thescattered light within the sample is less than 45 ¹m
probably because of stronger scattering in sample (e)compared with other samples.
The width of the hole pro¯le as a function of thewavelength, on the other hand, is determined by thedi®erence of the optical pathlength between the interfering waves. This di®erence also is probably smaller forsmaller particle, although it is not directly bounded by
the particle size, because the light path is folded manytimes in the particle. Let us estimate the pathlengthfrom the measured hole width by comparing it with thehole width that is obtained by an interference fringein a uniform non-scattering medium with a mirror at
one end of it[1]. If the hole width 10.6 cm¡1 of sample(c), for example, was obtained from a fringe in a uniform medium with the refractive index 2.37 (of ZnS),

3

the length of the medium would be 120 ¹m. Thus, inthe powder samples, the total length of the folded pathis comparable to the particle size. The hole width determines the density of data when this e®ect is utilized
for optical data storage. The hole width of the uniform
medium with a mirror corresponds to the storage density of conventional methods which is limited by thewavelength of light. The above comparison suggeststhat the density of data that can be stored by using thepresent e®ect in the powder samples might be nothing more than that with conventional methods. However, the hole of sample (e) is much narrower than thehole of sample (a), although the e®ective volume for therecording is smaller than sample (a), as is known fromthe angular width of the hole and the backscattering
cone. This high storage density surmounting the wavelength limit presumably comes from the strong scattering of light in sample (e), where weak localizationof light probably occurs[4]. If light is weakly localized
and stays around the incident point for a long time, thepath di®erence, or the time di®erence, between the interfering waves can be very long, and the hole becomesnarrow.
In conclusion, we have compared the angle selective
properties and the wavelength selective properties ofthe optical memory e®ect that is due to interference ofmultiple-scattered light in Sm-doped ZnS nanocrystals.
The hole width has given information about scatteredwaves in the samples. The density of data that can be
stored by utilizing this e®ect is possibly very high instrongly scattering media. Further studies are necessary for quantitative understanding of this e®ect.
References[1] A. Kurita, Y. Kanematsu, M. Watanabe, K. Hirata, T. Kushida, Phys. Rev. Lett. 83 (1999) 1582.
[2] T. Kushida, A. Kurita, M. Watanabe, Y. Kanematsu, K. Hirata, N. Okubo, Y. Kanemitsu, Proceedings of ICL'99 (submitted).
[3] E. Akkermans, P. E. Wolf and R. Maynard, Phys.
Rev. Lett. 56 (1986) 1471.
[4] D. S. Wiersma, P. Bartolini, A. Lagendijk and R.
Righini, Nature 390 (1997) 671.