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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Randomly Distributed Plasmonic Hot Spots for Multilevel Optical Storage Yuhui Chu, Hongmei Xiao, Guang Wang, Jin Xiang, Hai Hua Fan, Haiying Liu, Zhongchao Wei, Shaolong Tie, Sheng Lan, and Qiao-Feng Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03710 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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The Journal of Physical Chemistry

Randomly Distributed Plasmonic Hot Spots for Multilevel Optical Storage Yuhui Chua, Hongmei Xiaoa, Guang Wanga, Jin Xianga, Haihua Fana, Haiying Liua, Zhongchao Weia, Shaolong Tieb, Sheng Lana, Qiaofeng Daia,* a

Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices,

School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China b

School of Chemistry and Environment, South China Normal University, Guangzhou 510006,

China

ABSTRACT

Multilevel optical storage is regarded as one efficient way to achieve higher capacity. In this paper, a kind of multilevel optical storage is presented by encoding the plasmonic hot spots among coupling GNRs. The hot spots not only lower the recoding energy but enhance two-photoninduced luminescence (TPL) intensity of the GNRs adjacent to hot spots significantly. From the numerical simulations based on finite-difference time-domain (FDTD) technique, it can be seen that the existence of hot spots expands the range of TPL intensity and makes a steeper function of TPL intensity distribution than isolated GNRs. The multilevel optical storage are performed experimentally in the GNR-PVA films with optical density (OD) = 3, 12 and 24 respectively. The

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six-level optical storage with high quality could be fulfilled with ultralow energy of only a few picojoule per pulse in the GNR-PVA films of OD = 12. This work can be the building blocks for the cold data storage in Big Data era.

1. Introduction The 21st century is considered as the era of Big Data.1, 2 The massive amounts of data are being created at an exponential rate with the rapid development of science and information technology. The rapid growth of data are produced by various sources of government and military institutions, public education system, medical and health records, social media and so on, and most of them required to be recorded for a long time. Owing to the features of high-capacity, high-security, highthroughputs, and long-term storage, optical data storage draws lots of attention over the past decades,3-9 and it is also regarded as one of the most promising solutions to meet the challenge of cold data storage.10 Many materials are adopted as the recording media of optical data storage, including polymer binders,3 photochromic materials,5 and metal nanoparticles6-9 et al. Among the recording media, the gold nanorod (GNR) is an attractive candidate because of its simple synthesis method11, 12 and high productivity.13

Moreover, GNRs exhibit the excellent

properties of the longitudinal surface plasmon resonance (LSPR) which is sensitive to wavelength and polarization. On the one hand, the longitudinal surface plasmon wavelength is tunable easily from the visible to infrared regions by controlling the aspect ratio of GNR.14-17 On the other hand, the peak intensity of the longitudinal surface plasmon has strong polarization dependence.18, 19 Based on such special optical properties of GNR, P. Zijlstra and co-workers20 have successfully fulfilled the five-dimensional optical data storage by exploiting the sensitivity of wavelength and polarization of LSPR on the basis of optical data storage in the three spatial domains. Recently,


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the long-term optical data storage is also developed based on nanoplasmonic hybrid glass composites.21 Five-dimensional optical data storage based on GNRs has special mechanism for recording and readout. Firstly, the data is recorded via melting GNRs. The certain GNRs are irradiated by a femtosecond (fs) laser pulse which can induce the rise of the temperature for irradiated nanorods. When the pulse energy of fs laser light rises to the value which can lead the GNRs to heat up above the threshold of the melting temperature, the irradiated nanorods transform their shape into shorter rods or spherical particles.16,

17, 20

Subsequently, the recorded sample is detected by using

longitudinal SPR-mediated two-photon-induced luminescence (TPL) which is sensitive to the polarization and wavelength of single GNR. The irradiated pattern exhibits a lower TPL signal than the part isn’t be irradiated.20 With the contrast of TPL signal intensity, the non-destructive, crosstalk-free readout with the longitudinal SPR-mediated TPL can be exhibited. In recent years, hot spots attract much interest because of the enormous enhancement in the intensity of Surface-Enhanced Raman Scattering (SERS) signal.22-28 This superior property makes the advances in biosensing,29, 30 high-resolution bioimaging31 and single molecule detecting.32, 33 Moreover, the TPL of hot spots and the optical data storage based on the hot spots of gold nanoflowers have been researched.34, 35 By encoding the random hot spots, Dai et al. realize the five-dimensional optical data storage with ultralow recording energy.36 The ultralow recording energy can reduce the crosstalk between different recording channels, which also makes it possible to improve the quality and the capacity of optical data storage significantly. Comparing with the isolated GNRs,8, 20, 37 the recording energy using for encoding hot spots is two orders of magnitude lower, even reduces to only a few picojoule (pJ) per pulse.36


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Over the last few decades, multilevel optical data storage is envisioned as the promising way for increasing the capacity. Many techniques, including Pit Depth Modulation,38 Single Carrier Independent Pit Edge Recording,39 and Electron Trapping Optical Memory40 et al, are used to fulfill the multilevel storage. Since hot spots have superior properties in decreasing recording energy, the multilevel optical storage based on hot spots with ultralow energy is anticipated. In this paper, the multilevel optical storage based on hot spots is studied. The melting of the GNRs adjacent to hot spots is firstly researched in experiments. Based on TEM image, a physical model is established to reveal the special TPL intensity distributions of GNRs assembly with hot spots. We also fabricate three kinds of GNR-PVA films as storage media with different coupling strength among GNRs. Finally, the six-level optical storage with superior quality is fulfilled with the GNRPVA films of optical density (OD) = 12. The optical storage with other films of OD = 3 and OD = 24 are also performed and discussed. 2. Experiment and Simulation The GNRs with an average diameter of ~9 nm and an average length of ~35 nm are synthesized

by using a modified seedless method. They are used to fabricate the storage media by dispersing them homogeneously in a 5% polyvinyl alcohol (PVA) and then spin-coating the solution on the glass cover slip. It has been proved that the coupling strength between GNRs is controlled by adjusting the volume density of GNRs and it depends weakly on the length (or aspect ratio) of GNRs.34, 36 The volume density of GNRs in GNR-PVA films is determined by the OD of the GNRs aqueous solution. The formula used to calculate OD is OD = lg (1/T), where T is the transmission of light through the aqueous solution of GNRs contained in a sample cell with a thickness of 1 cm. The concentrations of GNRs in the GNR-PVA films range from 330 ± 10 to 5280 ± 10 × 10-9 M, ACS Paragon Plus Environment

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corresponding to volume densities of 200-3200 GNR μm-3. Herein, three kinds of GNR-PVA films with OD = 3, 12 and 24 are prepared.

The experimental setup is illustrated in Figure 1. The femtosecond (fs) laser light is delivered by a Ti: sapphire oscillator (Mira 900S, Coherent) with the duration of ∽130 fs and a repetition rate

of 76 MHz. The fs laser light firstly goes through a chopper of the lock-in amplifier (SR850, Standford). The intensity of the fs laser light in experiments is controlled by an attenuator. After that, a combination system of a half-wavelength plate and a quarter-wavelength plate is used to adjust the line polarization of the fs laser light. The exposure time of the fs laser light can be controlled via the shutter. Finally, the fs laser light goes into an inverted microscope (Observer A1 Zeiss) and is focused on the sample by the 60 × objective (NA = 0.85). The sample is placed on a

3D positioning system (P-563.3CD, Physik Instruments) with an accuracy of 1 nm in x, y and z axis and moved by the 3D positioning system. The TPL signals of the sample are collected by the same 60x objective and directed to a spectrometer (SR500i-B1, Andor) equipped with a photomultiplier tube (H7244-40, Hamamatsu). The output signals from the photomultiplier tube (PMT) are directed to a data collector system (BNC-2120, NI) after going through the lock-in amplifier. A program compiled by the LabVIEW is used to control the synchronous operation of the shutter, the 3D positioning system and the data collector system. The finite-difference time-domain (FDTD) method software developed by Lumerical Solution, Inc. (http://www.lumerical.com) is performed to simulate the distribution of the electric field of hot spots. In the FDTD simulations, non-uniform girds with the smallest grid of 0.5nm, the background index of 1 and perfectly matched layer conditions are employed. 3. Results and Discussion


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Compared with the isolated GNRs, the local electromagnetic field enhances significantly in the gaps between the adjacent GNRs. The intensely localized modes are referred to as hot spots generally.41 In contrast with Figures 2(a1) and 2(b1), Figures 2(a2) and 2(b2) show the obvious change of shape after the GNRs are irradiated by fs laser light with a ultralow pulse energy of ~4 ± 0.4 pJ (~0.25 ± 0.025 mJ∕cm2) and an exposure time of 20 ms. The wavelengths of the fs laser

light are chosen to be 808 nm and 792 nm respectively, which are close to the resonant wavelength

of the isolated GNRs indicated with the white dashed rectangles in Figure 2. The direction of polarization is shown with the black arrows which are nearly parallel to the longitudinal axis of the isolated GNRs. The physical models of GNR assembly based on the transmission electron microscope (TEM) images in Figures 2(a1) and 2(b1) are simulated by using FDTD technique. The incident light used in simulations is on the same polarization and wavelength as those used in our experiments. The simulation results of the field intensity (|E|2) distribution are shown in Figures 2(a3) and 2(b3), in which the red dotted circles indicate the locations of the higher electric intensity. In Figures 2(a4) and 2(b4), the statistics of length shortening rate extracted for the GNR assembly based on the TEM image before and after the irradiation of fs laser light are shown. Herein, the length shortening rate is defined as ∆L/L, where L is the original length of a certain GNR before irradiation while ∆L is the length shortening of the GNR after the irradiation of fs

laser light. From Figures 2(a4) and 2(b4), we can clearly get the result that all GNRs with large ∆L/L appear at the vicinities of the hot spots with higher electric intensity in Figures 2(a3) and 2(b3), while the isolated GNRs in the white dashed rectangles remain unchanged.

It is demonstrated clearly that the GNRs at the vicinities of hot spots possess a lower melting energy compared with isolated GNRs. In other words, the GNRs around hot spots may melt and deform while the isolated GNRs remain unchanged by controlling the pulse energy of fs laser light.


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In the process of GNRs melting, heat accumulation is generally supposed to increase the background temperature of GNR assembly and to lower the melting energy accordingly. However, Dai et al have proved that hot spots rather than heat accumulation play a key role in lowering the melting energy of GNRs with plasmonic coupling.36 The existence of hot spots enhances TPL of the adjacent GNRs significantly and dominates the TPL of the GNR assembly by the plasmonic coupling. The GNR assembly with randomly distributed hot spots expands the range of TPL intensity obviously compared with the GNR assembly without hot spots. Moreover, hot spots are very sensitive to the gap widths between GNRs. Based on the special properties of hot spots, the small deformations of the adjacent GNRs induced by ultralow energy pulse may lead to the dissociation of hot spots and result in the remarkable reduction in the TPL of the GNRs assembly. In general, there are many hotspots in a recorded bit. The GNRs adjacent to hotspots with different coupling strengths possess different TPL intensity and different deformation energy, which allow for a multilevel memory by precisely controlling the stepwise recording pulse energy. To illustrate the mechanism of multilevel optical storage, the physical model of GNR assembly is established in Figures 3(b)-(f), which is based on the TEM image in Figure 3(a). There are 128 gold nanoparticles (some gold nanoparticles of incomplete shapes are not included) in the TEM image of GNR assembly shown in Figure 3(a). The distribution of |E|4 in Figures 3(b)-(f) are simulated by using FDTD technique. Although the physical mechanisms for the TPL of GNRs remain controversial, the TPL intensity has been found to exhibit a quadratic dependence on localfield intensity.42-43 The distribution of |E|4 shown in Figures 3(b)-3(f) are used to represent the TPL for GNR assembly. In Figure 3(b), the GNRs with top two TPL which are enclosed by red dashed ellipses appear at the vicinities of hot spots. When the GNRs with top two TPL are turned into


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gold nanospheres with the constant volume before and after irradiation, it is clearly observed that the old hot spots disappear in Figure 3(c) with the deformation or melting of the two GNRs. Meanwhile, we can observe that the GNRs with new top six TPL which are enclosed by red dashed ellipses in Figure 3(c) nearly appear at the vicinities of the new hot spots. An exception is found for an isolated GNR located at the under right corner where there is no hot spot. The reason lies in that the orientation and the resonant wavelength of the isolated GNR match well with the polarization and wavelength of the fs laser light (the wavelength of the incident light is set as 800 nm in the physical model). Similarly, when the GNRs with top six TPL melt into nanospheres (or disappearance of hot spots), it is also observed that the old hot spots disappear with the deformation or melting of the six GNRs in Figure 3(d). At the same time, we can observe that the GNRs with new top ten TPL which are enclosed by red dashed ellipses in Figure 3(d) almost appear at the vicinities of the new hot spots except an isolated GNR. The exception of an isolated GNR is found at the middle right of total GNR assembly where there is no hot spot. And the reason has explained above. Likewise, the old hot spots disappear in Figure 3(e) with the deformation or melting of the ten GNRs when the GNRs with top ten TPL are turned into gold nanospheres. Meanwhile, the GNRs with new top fourteen TPL are enclosed by red dashed ellipses in Figure 3(e). Unlike in Figures 3(b)-3(d), the most of the GNRs with new top fourteen TPL are isolated GNRs except several GNRs which appear at the vicinities of the new hot spots. The reason is that the most of hot spots have already quenched with the deformation or melting of GNRs which appear at the vicinities of the hot spots in Figures 3(b)-3(d). In Figure 3(f), the distribution of |E|4 is shown when the GNRs with top fourteen TPL are turned into gold nanospheres. The TPL intensity of total 128 GNRs in Figures 3(b)-3(f), which correspond to five-level channel, is showed in Figure 3(g). The contrast for TPL intensity of total 128 GNRs between Figures 3(b) and 3(c), 3(c) and 3(d), 3(d)


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and 3(e), 3 (e) and 3(f) are 0.33, 0.22, 0.36 and 0.35 respectively. Here, contrast = (I n I n+1 )∕(I n +I n+1 ) and I n is the nth level of TPL intensity. In Figure 3(h), the normalized TPL intensity distribution of GNRs assembly in Figures 3(b)-3(f) are showed by red, yellow, blue, purple and green dot line respectively. The black dot is the function of cos4θ which presents the normalized TPL intensity of a single GNR depended on the polarization angle θ. By comparison with the function of cos4θ, the normalized TPL intensity distribution of GNRs assembly in Figures 3(b)-3(f) exhibit functions much steeper than cos4θ owing to the existence of hot spots which significantly enhance the TPL intensity of the GNRs adjacent to the hot spots. Besides, the TPL intensity distribution of GNRs assembly in Figures 3(b)-(f) illustrate that the higher density of hot spots leads to much steeper functions. This feature makes it possible to fulfill the multilevel optical storage. In order to realize the possibility of multilevel optical storage based on hot spots, the PVA films doped with GNRs are fabricated. Herein, three kinds of GNR-PVA films with OD = 3, 12 and 24 are prepared. TEM images of the GNR-PVA films with different OD values are shown in Figures 4(a)-4(c). In Figure 4(d), the normalized extinction spectra of the GNR-PVA films with OD = 3, 12 and 24 are presented. The extinction spectra which are derived from the measured transmission spectra reflect the liner absorption of the GNR-PVA films. With the increasing coupling strength, a remarkable broadening of the extinction spectrum is observed. Transmission spectra of the GNRPVA films with OD = 3, 12 and 24 at different locations (p1 and p2) are also shown in Figure 4(e). The insets of Figure 4(e) are the photographs of GNR-PVA films with OD = 3, 12 and 24 respectively.


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In the data recording and readout experiments, the wavelength of the fs laser used to irradiate the GNR-PVA films is chosen to 800 nm, which is close to the resonant wavelength of the three kinds of samples with OD = 3,12 and 24. The exposure time of recording is set to be 20 ms. In Figure 5(a1), a pattern is recorded in the sample with OD = 12. The four parts of the pattern are irradiated by fs laser light with different recording pulse energy of 8.6 pJ, 5.8 pJ, 3.7 pJ and 0 pJ from left to right respectively. After that, all parts are read by fs laser light with the pulse energy of 1.25 pJ. It is obvious that the parts of pattern irradiated with lower pulse energy exhibit higher TPL intensity except acceptable noises in Figure 5(a1). Figure 5(a2) shows the distribution of TPL intensity in Figure 5(a1). In Figure 5(b1), a complex pattern composed of 50×50 pixels is recorded on the sample with OD = 12. The four parts of pattern in deep blue, green, yellow and red are irradiated by fs laser light with different pulse energy of 8.6 pJ, 5.8 pJ, 3.7 pJ and 0 pJ respectively. The recording information is read out with pulse energy of 1.25 pJ as before. In this way, the four-level optical storage with high quality can be achieved with ultralow energy of only a few picojoule per pulse. Figure 5(b2) shows the distribution of TPL intensity in Figure 5(b1). It also can be seen that the distribution of TPL intensity is divided into four levels significantly. With repeated reading, there is no obvious data degradation appearing. Compared with the binary optical storage, the multilevel optical storage increases the capacity. The two-level TPL intensity distribution are regarded as the two states of information, just like “0” and “1” in binary data storage. Accordingly, the four-level TPL intensity distribution are regarded as the four states of information, just like “0”, “1”, “2” and “3” in quaternary data storage. In order to reveal the effects of hot spots (or coupling strength), the samples with OD = 3 and 24 are also used for the multilevel optical storage. In Figures 5(c1) and 5(d1), the pattern is recorded on the samples with OD = 3 and 24 respectively. The more obvious crosstalk can be observed in


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Figures 5(c1) and 5(d1). Moreover, there are only two peaks appear in the distribution of TPL intensity in Figure 5(c2), and four unclear peaks appear in the distribution of TPL intensity in Figure 5(d2). It is obvious that the multilevel optical storage in Figures 5(c) and 5(d) exhibit inferior quality compared with that in Figures 5(a) and 5(b). The causes are diverse. For the sample with OD = 3, the weak coupling strength (or slight density of hot spots) induced by slight volume density of GNRs as shown in Figure 4(a) is responsible for inferior quality of multilevel optical storage. While for the sample with OD = 24, it can be see that there are several clusters appearing in Figure 4(c) owing to large volume density of GNRs in the sample. The clusters are generally composed of tens of closely touching GNRs. As we known, the distance of the gap among GNRs is very important to form hot spots. Too far or too near distance all will make the hot spots disappear. The connected GNRs in clusters are not good to produce enough hot spots for multilevel storage. In addition, owning to the superior quality, we also perform the five-level in Figure 5(e) and the six-level in Figure 5(f) of optical data storage in the sample of OD = 12. In Figure 5(e), the five parts of the pattern are irradiated by fs laser light with recording pulse energy of 8.2 pJ, 3.8 pJ, 2.8 pJ, 2.3 pJ and 0 pJ from left to right respectively. Then the recording information is read out with pulse energy of 0.7 pJ. In Figure 5(f), the six parts of the pattern are irradiated by fs laser light with recording pulse energy of 16 pJ, 8.2 pJ, 3.8 pJ, 2.8 pJ, 2.3 pJ and 0 pJ from left to right respectively. After the irradiation, the recording information is read out with pulse energy of 0.7 pJ. The five dimensions of wavelength, polarization and three space dimensions have been multiplexed for optical storage based on hot spots, 36 and the energy is another dimension as shown in multilevel storage. In five-dimensional optical storage, the dissociation of one hot spot in certain channel wouldn’t take obviously crosstalk into the other wavelength and polarization channels. In


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multilevel optical storage, the level of TPL intensity corresponding to different energy is decided by the number of dissociated hot spots with certain wavelength and certain polarization. It is reasonable to deduce that the dissociation of several hot spots in certain channel wouldn’t take obviously crosstalk into the other wavelength and polarization channels. In other words, the multilevel storage could integrate with five-dimensional storage to achieve higher recording density in principal. Nevertheless, it may be challengeable in experiment to fulfill the integration because of the dissipation of hot spots in a certain channel especially for higher-level storage. Moreover, the ultralow recording energy allows us to replace the big Ti: sapphire oscillator laser in the experiment with compact laser such as fs fiber laser, which would reduce the cost and occupied space of the experimental equipment. This work would be beneficial to application of optical storage based on hotspots. 4. Conclusions In conclusion, a concept of multilevel optical data storage based on plasmonic hot spots with ultralow recording energy is presented. The simulations indicate that the TPL of the GNRs adjacent to hot spots is enhanced significantly and dominates TPL of the GNRs assembly by the plasmonic coupling. The TPL intensity distribution of coupling GNRs assembly shows steeper curve than that of isolated GNRs, which makes it possible to fulfill the multilevel optical storage with an ultralow energy. Finally, the multilevel optical storage is experimentally fulfilled with an ultralow energy of only a few picojoule per pulse. It is anticipated that the multilevel optical data storage based on hot spots would promote further development of data storage in the near future.

Corresponding Author


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*E-mail: [email protected] ACKNOWLEDGMENT Zhongchao Wei and Qiaofeng Dai thank the financial support from the National Natural Science Foundation of China (Grant Nos. 61774062 and 60908040). Sheng Lan and Qiaofeng Dai would like to thank the financial support from the Natural Science Foundation of Guangdong Province, China (Grant Nos. 2016A030308010 and 2016A030313851). Qiaofeng Dai thanks the financial support from the Guangzhou Science and Technology Project (Grant No. 2011J2200080). REFERENCES 1. Boyd, D.; Crawford, K. Critical Questions for Big Data Provocations for a Cultural, Technological, and Scholarly Phenomenon. Inf. Commun. Soc. 2012, 15, 662-679. 2. Gantz, J.; Reinsel, D. The Digital Universe in 2020: Big Data, Bigger Digital Shadows, and Biggest Growth in the Far East. Framingham, MA: IDC. 2012, https://www.emc.com/leadership/digital-universe/2012iview/index.htm (accessed June 12, 2018) 3. Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I. Y. S.; McCord-Maughon, D.; et al. Two-Photon Polymerization Initiators for Three-Dimensional Optical Data Storage and Microfabrication. Nature. 1999, 398, 51-54. 4. Parthenopoulos, D. A.; Rentzepis, P. M. Three-Dimensional Optical Storage Memory. Science. 1989, 245, 843-845.


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Figure 1. Schematic drawing of the setup for optical data storage.


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Figure 2. In (a) and (b), (a1) and (b1) show the TEM images of the GNR assembly without irradiation of fs laser pulses, and (a2) and (b2) show the TEM images after irradiation. The black arrows in (a1) and (b1) reveal the polarizations of the fs laser light, which are nearly parallel to the longitudinal axis of the isolated GNRs in the white dashed rectangles. (a3) and (b3) show the field intensity (|E|2) distribution of the GNR assembly. The red dotted circles indicate the hot spots which enhance the electric intensity. (a4) and (b4) show the length shortening rate (∆L/L) for the GNR assembly represented by the color scale. The length of the scale bar in (a) and (b) is 100 nm.


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Figure 3. (a) TEM image of GNRs assembly. (b)-(f) Distribution of |E|4, which represent the TPL of GNRs. (g) TPL intensity of total GNRs in (b)-(f). (h) The normalized TPL intensity distribution of GNRs assembly in (b)-(f) by red, yellow, blue, purple and green dot respectively. The black dot is the function of cos4θ which presents the normalized TPL intensity of a single GNR depended on the polarization angle θ. The polarization of the incident light is indicated by the red arrows. The length of the scale bar in (a)-(f) is 100 nm.


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Figure 4. (a)-(c) TEM images of the GNR-PVA films with different OD values. The content of PVA in the aqueous solutions is 0.5% (wt) and the thickness of the films is 100 nm. (d) Extinction spectra of the GNR-PVA films with OD = 3, 12 and 24. (e) Transmission spectra of the GNR-PVA films with OD = 3, 12 and 24 in different position (p1 and p2). The length of the scale bar in (a)-(c) is 100 nm. The pictures of GNR-PVA films are shown in the insets of (e).


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Figure 5. In (a)-(f), subfigures (a1)-(f1) show the patters recorded in the GNR-PVA films, and subfigures (a2)-(f2) show the distribution of TPL intensity of (a1)-(f1) respectively. The patters of (a1), (b1), (e1) and (f1) are recorded in the GNR-PVA films with OD = 12, and the patters of (c1) and (d1) are recorded in the GNR-PVA films with OD = 3 and 24 respectively. Here, the pseudo colors on the patters and the color bars are used to represent the intensity of TPL.


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