Threshold response using modulated continuous wave illumination for multilayer 3D
optical data storage
A. Saini, C. W. Christenson, T. A. Khattab, R. Wang, R. J. Twieg, and K. D. Singer
Citation: J. Appl. Phys. 121, 043101 (2017); doi: 10.1063/1.4974867
View online: http://dx.doi.org/10.1063/1.4974867
View Table of Contents: http://aip.scitation.org/toc/jap/121/4
Published by the American Institute of Physics
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JOURNAL OF APPLIED PHYSICS 121, 043101 (2017)
Threshold response using modulated continuous wave illumination
for multilayer 3D optical data storage
A. Saini,1 C. W. Christenson,1 T. A. Khattab,2 R. Wang,3 R. J. Twieg,2 and K. D. Singer1
1
Department of Physics, Case Western Reserve University, 2076 Adelbert Road, Cleveland, Ohio 44106, USA
Department of Chemistry, Kent State University, 800 E. Summit St., Kent, Ohio 44240, USA
3
Hathaway Brown School, 19600 North Park Boulevard, Shaker Heights, Ohio 44122, USA
2
(Received 7 October 2016; accepted 11 January 2017; published online 27 January 2017)
In order to achieve a high capacity 3D optical data storage medium, a nonlinear or threshold
writing process is necessary to localize data in the axial dimension. To this end, commercial
multilayer discs use thermal ablation of metal films or phase change materials to realize such a
threshold process. This paper addresses a threshold writing mechanism relevant to recently
reported fluorescence-based data storage in dye-doped co-extruded multilayer films. To gain understanding of the essential physics, single layer spun coat films were used so that the data is easily
accessible by analytical techniques. Data were written by attenuating the fluorescence using
nanosecond-range exposure times from a 488 nm continuous wave laser overlapping with the single
photon absorption spectrum. The threshold writing process was studied over a range of exposure
times and intensities, and with different fluorescent dyes. It was found that all of the dyes have a
common temperature threshold where fluorescence begins to attenuate, and the physical nature of
the thermal process was investigated. Published by AIP Publishing.
[http://dx.doi.org/10.1063/1.4974867]
I. INTRODUCTION
Commercial optical data storage (ODS) technologies
have found a widespread application for personal data storage and multimedia distribution. However, their application
for enterprise data storage has been limited by the storage
capacity, arising from limited access to the axial dimension
due to a small number of data layers (less than 4) and
the limitations in areal density imposed by the diffraction
limit. There have been efforts to increase the storage capacity, including holographic storage,1,2 spatial3 and polarization multiplexing,4 and DNA-based storage.5 Another widely
studied approach is to increase access to the axial dimension
by involving nonlinear optical processes such as two-photon
absorption and reverse saturable absorption, which confine
the writing of bits only to one layer. These approaches have
demonstrated the technical viability of many layer storage,6,7
however, these nonlinear responses are weak and typically
require high power pulsed lasers that increase the cost, size,
and have safety drawbacks. Thus, a viable many layer ODS
approach has been difficult to commercialize.
To develop an efficient 3D data storage medium, it is
helpful to consider the operating principles of existing technology. Current writable commercial systems rely on the
modulated reflection patterns from metal films due to an
adjacent phase change material, usually an inorganic dielectric or organic dye.8,9 Absorption of the light causes localized heating, which results in a change in phase of the
material (typically crystalline to amorphous) which then
alters the reflection from the adjacent metal layer.10 Disc
capacity can be increased by stacking more than one data
layer, but this approach is difficult because of high attenuation per layer in the reflection-based data scheme11 as well
as the high fabrication complexity12 and resulting costs.
0021-8979/2017/121(4)/043101/6/$30.00
We have previously reported on a method to overcome
these limitations by showing that a co-extruded multilayer
polymer film can be used as a 3D data storage medium. Data
are written by attenuating the fluorescence (FL) intensity of
a fluorophore incorporated in thin active layers separated by
thicker buffer layers.13 In this case, the nanoscopically thin
active layers allowed by the co-extrusion process resulted in
sufficiently low absorption so that the propagating beam
could access 23 layers. Thus, this writing method is suitable
for incorporation into the co-extrusion process, in which dozens of writable layers can be fabricated at low cost, provided
the materials are relatively simple.14
In this previous work, the writing process was seen to
change character from photochemical bleaching on long
time scales to a different, and nonlinear process, on shorter
time scales. The work reported here aims to investigate the
physical nature of this nonlinear writing process by using
thin, spun coat, fluorophore-doped polymer films. Thin films
are used as opposed to multilayer films so that they are more
accessible to a broad range of analytical techniques. The
results will be used to inform later studies on co-extruded
films where the written data is buried inside the polymer.
The writing process is similar to commercial discs in
that it uses a thermal process, but bits are based on FL modulation which, as opposed to reflection, is more suitable for
accessing many layers. The FL of the film is changed using a
photothermally mediated permanent FL attenuation (FLA)
of the fluorophore, which as we describe here, exhibits a
threshold dependence on the laser intensity. Data are written
within the linear absorption band of the dye using a modulated continuous wave (CW) Blu-rayTM-like laser. A threshold process is of great importance in 3D ODS as it localizes
the response to the focal volume eliminating interlayer
121, 043101-1
Published by AIP Publishing.
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Saini et al.
crosstalk and thus allows many layers to be written and read
more often. A threshold response also allows writing below
the diffraction limit leading to high areal data density.15
Furthermore, the process is largely independent of the type
of dye molecule, provided a sufficient temperature can be
achieved, allowing for a large degree of freedom in designing an ODS medium.
II. EXPERIMENT
Three different dyes were chosen: Rhodamine 6G (R6G),
Perylene Orange (PO), and 4-(4-(4-hydroxypiperidin-1-yl)
styryl)-2-(dicyanomethylene)-2,5-dihydro-5,5-dimethylfuran3-carbonitrile (TK01093). R6G was chosen because it is
well studied,16 and PO because it is relatively photostable.17
Since these dyes do not have a strong absorption at 405 nm,
a modulated 488 nm CW laser was used to provide a large
enough single photon absorption in the films. TK01093 is a
custom dye that has a broader absorption spectrum. We have
previously examined the dyes similar to TK01093 containing
the dicyanomethylenedihydrofuran acceptor for use in photorefractive media18 and in single molecule FL studies.19 The
molecular structures and weights of the dyes used in the current study are shown in Fig. 1. Details on the extinction and
FL properties of these dyes can be found in the supplementary material.
Films containing poly(styrene-co-acrylonitrile) (SAN25)
doped with the fluorescent dyes R6G, PO, and TK01093 at
different molar concentrations were used for photoinduced
FLA experiments. R6G was obtained from Sigma Aldrich and
J. Appl. Phys. 121, 043101 (2017)
PO was obtained from Kremer Pigments, Inc. SAN25 Tyril
100 was obtained from the Dow Chemical. This polymer was
chosen as all three dyes are soluble in it to at least 2 to
3 wt. %. SAN25 was first dissolved in dichloroethane at a concentration of 10 wt. % and an appropriate amount of dye was
added to the solution, depending on the desired molar concentration (between 0.022 mM to 0.044 mM). This solution was
then spun coat on to clean glass slides, and the films annealed
at 100 C for 10 min. This yielded 500 nm to 700 nm thick
films, which were then used for all the experiments.
A. Writing and reading
The FL was attenuated in localized spots using
single pulses from a 488 nm wavelength, 200 mW CW laser
(Omicron LuxXþ) externally controlled with a National
Instruments DAQ card. The laser was focused on to the sample using a Leica HCX Plan Fluotar, 100, 0.90 NA objective, which provided a FWHM of around 380 nm in the
radial direction and 1.2 lm in the axial direction. Several
FLA spots were made on the same sample by translating it
along the focal plane of the objective. Spots were made on
different dye samples at various pulse lengths and laser
power. Samples having different dyes had the same molar
concentration. The pulse lengths used to produce the spots
were less than 1 ls. Experiments with a longer pulse length
(>200 ls) were also performed with laser powers up to 9 lW
to consider the non-thermal effects.
The spots were read in a Leica SPE scanning confocal
microscope using a 488 nm wavelength laser and an ACS
apochromatic 100/1.4NA oil-immersed objective. The magnitude of FLA is calculated as the intensity of the background
FL minus the FL of the written spot, divided by the background FL. The width of a single spot was obtained by fitting
the FL profile of the spot to a Gaussian. The data points plotted in the Figs. 2, 3, and 6 are an average of the results from
5–10 spots, written using the same laser pulse parameters, but
in different areas and different samples. The uncertainty in all
the FLA data points is approximately 60.03.
B. Atomic force microscopy (AFM)
Atomic Force Microscopy (AFM) imaging of the spots
was performed on a PO-doped SAN25 thin film at a concentration of 0.033 mM. An Agilent 5500 AFM was used to
image the spots written at two different zoom levels. To
obtain a quantitative topographical image for the bits, scans
were performed with a resolution of 512 pixels and a scanning speed of 0.1 lines/s over an area of 44 44 lm and
17 17 lm. False color for the images was provided using
the open source software Gwyddion 2.43, a scanning probe
microscopy data visualization and analysis tool.
C. COMSOL multiphysics simulations
FIG. 1. Molecular structures and molecular weights of the dyes under study.
The determination of the physical causes for FLA relies
on an estimation of the temperature of the film. However, it
can be difficult to measure the temperature rise directly over
the small laser spot, and so simulations are performed. The
temperature rise in the dye/polymer system caused by the
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J. Appl. Phys. 121, 043101 (2017)
FIG. 2. FLA with sub-microsecond
exposures vs. temperature as calculated by COMSOL simulations at the
center of the laser pulse, for two different
molar
concentrations—(a)
0.033 mM and (b) 0.044 mM. (c) Plot
shows all the data for 0.044 mM near
the threshold region. Plot (d) shows
different FLA levels for dyes at the
same exposure time (500 ns), thus having different intensity thresholds.
laser pulse exposure was estimated using COMSOL
Multiphysics simulations. The problem of temperature rise
by laser radiation incident on an absorbing material has been
well-studied.20 Solutions can be found for specific laser
intensity distributions and sample boundary conditions,21,22
but we chose to use COMSOL for accuracy. In general, the
temperature rise in a polymer film due to the absorption of
light can be determined by solving the heat equation
@Tð~
r; t Þ
jr2 Tð~
r; t Þ ¼ Qð~
r; t Þ;
@t
(1)
where T is the temperature, j is the thermal diffusivity, and
Q is the net energy per unit volume per unit time generated
within the solid from the laser.
For the simulations, the film was modeled as 8 lm by
8 lm square film, 700 nm thick, with infinite boundary conditions on the edges of the film. Thermal radiation into air was
allowed only from the top surface of the film, to simulate the
actual spun coat films.
A Gaussian heat source was placed at the center of the
film
8
r 2
>
< Qpeak ewðzÞ2 eaz t < t
exp
2
(2)
Qðr; tÞ ¼ wðzÞ
>
:
0
t texp ;
where a is the absorption coefficient of the sample, texp is
the exposure time or pulse duration, w(z) spot size of the
beam as a function of distance from the beam focal plane,
given by
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
z2
(3)
w ðzÞ ¼ w 0 1 þ 2 ;
zR
where zR and w0 are the Rayleigh range (380 nm) and diffraction limited spot size of the beam, respectively, and the middle of the sample is defined to be z ¼ 0. In Eq. (2), Qpeak is
the heat energy deposited at the center of the focused spot,
which is
FIG. 3. FLA versus temperature with short (<1 ls and up to 90 MW/cm2),
intermediate (100–300 ls and up to 50 kW/cm2), and long (4 ms and up to
6 kW/cm2) exposure times, showing chemical and thermal mechanisms
on distinct time scales. Temperature is calculated by COMSOL simulations.
X axis represents the change in temperature relative to room temperature
(298 K).
Qpeak ¼ I 0 aEf ;
(4)
where I0 is the peak intensity of the laser beam, and Ef is the
factor of energy deposited as heat. If we define Elaser as the
energy of the laser wavelength and Eband as the energy of the
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J. Appl. Phys. 121, 043101 (2017)
FL band, and QY as the FL quantum yield of the dye film,
then Ef can be expressed as
ð1 QYÞ Elaser þ QY ðElaser Eband Þ
Elaser
Elaser QY Eband
¼
:
Elaser
Ef ¼
(5)
Here, (1–QY)Elaser corresponds to the energy of the photons
that do not cause FL, and QY(Elaser–Eband) is the amount of
energy deposited by the photons that cause FL. For each
intensity, exposure time, and dye utilized in producing the
data in Fig. 2, the temperature rise at the center of the film at
the end of the exposure time is simulated, and plotted as the
independent variable in Figs. 2(a)–2(c). An example, of the
temperature profile in 0.044M PO films at 63 MW/cm2 and
different exposure times, is shown in the supplementary
material.
The heat equation is solved using the Heat Transfer module
of the software package. Heat capacity (2.10 J g1 K1), density
(1.07 g cm3), and thermal conductivity (0.193 W m1 K1)
used for SAN25 came from the supplier’s data sheet (Dow
Chemical).
III. RESULTS AND DISCUSSION
The experiments described in the Writing and Reading
section above were then carried out on the thin films of the
three dyes. The photoinduced attenuation of the FL was measured by writing a series of spots using the CW laser at various exposures times. The results for FLA versus laser
intensity for 300 ns exposure times are shown in Fig. 2(d).
This is data for just one particular time, but the dyes exhibit
different degrees of FLA. Similar data for all three dyes at a
variety of different intensities and exposure times are plotted
in Figs. 2(a) and 2(b) (for samples with two different molar
concentrations). These times and intensities are converted
into temperature rise at the center spot as determined by
COMSOL simulations, and the data are plotted versus sample temperature instead of laser intensity. All the dyes now
exhibit the same degree of FLA (the data fall on one common curve). The FLA of each dye exhibits a different trend
with intensity (Fig. 2(d)), but all follow the same trend with
temperature. It is evident from these plots that there is a
sharp temperature threshold that does not depend upon the
nature of the dye. All of the different dye/polymer films start
to exhibit FLA at around 500–700 K (Fig. 2(c)). The similarity between the threshold temperature for the different dyes
is discussed below.
Since temperatures used to analyze the data in Fig. 2
were simulated and not measured directly, we want to independently confirm that the FLA process involved here is at
least distinct from the usual photochemical bleaching mechanism.16,24,25 This latter process requires an exposure time
much greater than the 700 ns used here because the chemical
reactions usually occur from the triplet state, and the intersystem crossing time from the first excited singlet to the triplet is typically on the order of microseconds.16,26 Previous
studies on photochemical bleaching mechanisms were
performed with lower intensity and longer exposure times,
and so the specific mechanism responsible for the FLA
should be different here. We can verify this by using exposure times much longer than microseconds, where the photochemical bleaching should still be observed with no
temperature rise.
The FLA observed in PO and R6G films for exposure
times from a few hundred ns to 4 ms is shown in Fig. 3. For
exposure times of 100–300 ls and 4 ms, significant FLA is
achieved at a much lower temperature rises compared to the
sub-ls times. At 4 ms, there is approximately 30% FLA with
virtually no theoretical temperature rise. The PO and R6G
curves at 200 ls (filled and hollow squares) exhibit different
levels of FLA. This is because the photochemical bleaching
depends on the chemical structure of the dye, not just on the
temperature generated. This is also true for the two dyes
4 ms: R6G exhibits around 0.29 FLA on average, and PO
around 0.37. These data show that for exposures times less
than about 1 ls, chemical processes are suppressed, and the
thermal effects dominate. The specific thermal effects present are described below. Note that the presence of this effect
should be largely independent of the laser intensity, as long
as the exposure times is less than the intersystem crossing
time, the triplet population will remain small.
As the temperatures simulated by COMSOL are well
above the glass transition temperature of the SAN25 matrix
(380 K),23 the surface of the FLA bits was also examined by
AFM. Figs. 4(a) and 4(b) show the AFM and FL images of
spots written at 90 MW/cm2 and different exposure times in
PO-containing films. For less than 300 ns (Fig. 4(b)), written
spots exhibited a slightly raised bump, and an increased FL
intensity. This suggests that the temperature rise is large
enough to cause the polymer to deform, but the short time
for heating limits the degree of melting or FLA.
For exposure times longer than about 300 ns, the written
spots are characterized by a pit surrounded by a raised ring,
suggesting an apparent flow of the polymer away from
the center of focused beam due to a higher temperature over
a larger area. A small degree of FLA (less than 10%) is
observed at the same times where pit formation begins.
Comparison between the pit depth and FLA is shown in
Fig. 5. The depth of the pits for times less than 1 ls is a few
hundred nanometers, but as the film is around 700 nm thick,
the pits only partially penetrate the film. For 1 ls and 2 ls
times, the FLA is nearly complete (over 80%) and the pits
almost completely penetrate the film (given uncertainties in
the ability of the AFM to measure deep pits and the dark
counts of the FL measurement setup).
The COMSOL calculations of the temperature rise are
only theoretical, but the observation of physical deformation
of the polymer suggests that experimentally the polymer is at
least reaching its glass transition temperature (380 K). The
strong correlation between the magnitude of the FLA and the
pit depth (in Fig. 5), as well as the raised ring, suggest that
mass flow plays a significant role. It is also possible that the
FLA is due in part diffusion of the dye and a direct thermal
decomposition of the dye. Since the FLA is correlated with
pit formation, thermogravimetric analysis (TGA), which measures the weight loss with temperature, was done to determine
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J. Appl. Phys. 121, 043101 (2017)
FIG. 4. (a) Comparing the surface
topography of the films using AFM
(right, R) with the FLA spots (left, L).
These spots are made in 0.033 mM PO/
SAN25 film using the 120 mW power
at different exposure times. (b) Zoomed
in AFM image of the spots made at
sub-300 ns times showing the bumps or
hills.
the degradation temperature of the dye/polymer systems
(supplementary material). For the neat and dye-doped polymers, this is around 700 K. Since we are achieving larger temperatures than this, ablation as well as degradation likely play
a role in the FLA. The similarity of the decomposition and
glass transition temperatures between the different dye samples is likely the reason for the uniformity in the threshold
temperature for the different dyes.
We can look more closely at the effects of thermally
induced mass flow and degradation by examining the spot
sizes. The size of the FLA spot is calculated by fitting the
measured FL profile to a Gaussian. The FWHM of the spot
versus exposure time for PO at 0.033 mM and 90 mW/cm2 is
plotted in Fig. 6. This experimental FL FWHM is compared
to the width of the temperature distribution at the threshold
value of 700 K, as well as the FWHM of the pits in the AFM.
At exposure times less than about 500 ns, the size of the FL
profile corresponds more closely to the temperature profile
than the AFM profile. For longer times, the widths of all
three profiles are similar. These results suggest that the FLA
mechanism is as follows. For times less than about 200 ns,
there is enough heating to cause some slight deformation of
polymer (bumps in Fig. 4(b)), but not enough for outflow.
FIG. 5. FL profile (top) of the time resolved spots shown in Figure 4 compared to the topography (bottom) of the spots.
For slightly longer times but below 500 ns, a direct thermal
degradation of the dye reduces the FL. The high temperature
also melts the polymer slightly, but significant diffusion of
the dye is still limited. For times more than 1 ls, a multitude
of effects are occurring (diffusion, degradation, and possibly
ablation). Understanding the precise degradation products
of the polymer or the dye would require an additional
investigation.
To develop an ideal dye/polymer system for ODS, we
want a dye that can be written at high intensity but will not
photochemically bleach at a low intensity and long time
scales while reading or storing the disc. It was seen in the
FLA experiments that all three dyes showed same response
at sub-ls exposure, meaning that all dyes can be written
equally well if a high enough temperature can be reached.
However, as seen in Fig. 3, the behavior of different dyes
deviated from each other in the low power, long exposure
time regime where temperature is not a factor. This behavior
is more dependent on the properties of the dye and is due to
photochemical degradation. The results here indicate that we
FIG. 6. Predicting the spot size by comparing the FWHM of the FLA spot
size (Circles) with temperature spot size (width of the temperature profile at
threshold in squares) to the AFM pit size (triangles). Uncertainty in the temperature spot size results from the uncertainty in the threshold temperature
(6200 K). These measurements are done for PO/SAN25 0.033 mM sample.
043101-6
Saini et al.
can find a dye which is very stable under low power corresponding to photochemical bleaching, but that is still writable at high powers. This allows a significant degree of
freedom in choosing a dye that is sufficient for both reading
and writing of data.
IV. CONCLUSIONS
In conclusion, we have shown that a photothermal process can be used to write data in a simple dye/polymer system, leading to a threshold response similar to commercial
ODS media. The threshold response limits crosstalk and
allows writing below the diffraction limit. The temperature
required for FLA is independent of the dye. We also showed
that the FLA mechanism shifts away from photothermal to
photochemical at long exposure times as expected. This indicates that the ambient photostability is independent of the
ability to write data with nanosecond pulses.
Temperatures during the writing process as predicted by
the COMSOL Multiphysics simulations and the topographical AFM images indicate a significant role for mass flow as
well as thermal degradation of the dye in the observed FLA.
In multilayer films, we expect differences due to the fact that
the active layer is surrounded by the polymer buffer layer.
This will alter the heat flow and thus change the writing conditions. In addition, one might expect that both active and
buffer polymers could flow so that, instead of pits and
bumps, physical distortions would be observed. Direct thermal degradation and dye diffusion are also possible. Studies
of the mechanisms in multilayer films are underway.
SUPPLEMENTARY MATERIAL
See supplementary material for details on extinction and
fluorescence of the dyes used, thermogravimetric analysis of
the films used, and synthesis of TK01093.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the
National Science Foundation Center for Layered Polymer
Systems (CLiPS) under Grant No. DMR-0423914.
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