Multi-wavelength optical storage of diarylethene PMMA film

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1 Optical Materials 22 (2003) Multi-wavelength optical storage of diarylethene PMMA film Haobo Guo a, Fushi Zhang a, *, Guo-shi Wu a, Fan un a, houzhi Pu a, Xuesong Mai b, Guosheng Qi b a Department of Chemistry, Tsinghua University, Beijing, , PR China b Department of Precision Instruments andmechanology, Tsinghua University, Beijing, , PR China Received 29 March 2002; received in revised form 14 October 2002; accepted 25 October 2002 Abstract Current commercial optical storage technologies are all based on the heat effect of the recording laser, i.e., heatmode optical storage. In the present work, photon-mode optical storage using photochromic diarylethene materials was investigated. Two diarylethene derivatives were dispersed into PMMA solution, and spin-coated on a glass substrate with Al reflective layer as the recording layer. Two laser beams of 532 and 650 nm were used in recording and readout simultaneously, and signals with high =N ratio were detected. Multi-wavelength optical storage was realized with the diarylethene PMMA film. Ó 2002 Elsevier cience B.V. All rights reserved. PAC: Vb; Me Keywords: Optical storage; Multi-wavelength; Diarylethene 1. Introduction Trends in optical storage are higher recording density, higher storage capacity and higher data rate [1]. Current commercial recordable and rewritable optical storage systems are based on laser-induced pit formation in organic dye films, or laser-induced amorphous-to-crystalline phase changes in inorganic alloy films, or magneto-optical recording in inorganic alloy films. These systems are all heat-mode recording systems. The heat effect limits the size of the recorded pit, and * Corresponding author. Tel.: ; fax: address: zhangfs@mail.tsinghua.edu.cn (F. Zhang). each kind of the heat-mode optical storage system has a temperature threshold in the recording process, which limits the writing speed. Photon-mode recording of photochromic materials is a promising storage system, which has various advantages over heat-mode recording in terms of resolution, speed of writing, and multiplex recording capability, such as wavelength, polarization and phase [2]. Photochromism is defined as a reversible transformation in a molecule between two forms having different absorption spectra. The transformation is induced by an electromagnetic irradiation at least in one direction. Diarylethenes are newcomers to the photochromic field, developed by Irie et al., they show no thermalchromism (p-type photochromism), good chemical and thermal stability, and remarkable fatigue resistance /03/$ - see front matter Ó 2002 Elsevier cience B.V. All rights reserved. doi: / (02) 转载

2 270 H. Guo et al. / Optical Materials 22 (2003) These properties are indispensable for applications to optical storage materials [2 4]. Investigations have been carried out on photochromic materials in optical storage, and diarylethenes show the most applicable potentiality [5 10]. Multi-wavelength (or multi-frequency) storage is a promising approach to increase the recording density and data capacity. N-wavelength storage can increase the recording density by N times of the recording density in single-wavelength storage. For example, in single-wavelength storage the unrecorded and recorded dots are coded by {(0), (1)}, but in three-wavelength storage they are coded by {(0 0 0), (0 0 1), (0 1 0), (0 1 1), (1 0 0), (1 0 1), (1 1 0), (1 1 1)}, thus the recording density is three times that of single-wavelength storage. Photochromic materials with different absorption bands can be used in the multi-wavelength storage system. Laser beam corresponding to the absorption of each material will record by photon-induced isomerization independently, and the recorded information is readout by the same laser as recording [11]. In the present work, two diarylethene molecules with different absorption bands of the closed forms were synthesized, and one recording layer obtained by mixing these two molecules in PMMA matrix was prepared by spin coating on a glass substrate with an aluminum reflective layer. 532 and 650 nm laser beams were used in recording and readout. imultaneous writing and reading with both laser beams showed a good result on this film, and photochromic diarylethene molecules showed good performance as multi-wavelength optical storage materials. 2. Experimental Diarylethene 1a and 2a were synthesized following the method of Ref. [4]. Their photochromic reactions were shown in cheme Preparation of the optical sample discs Fifty milligram PMMA was dissolved in 1 ml chloroform, and 60 mg 1a was mixed with this solution, which was stirred ultrasonically to make it homogeneous. Another solution containing 15 mg 2a was prepared by the same procedure. The structure of the optical sample disc is shown in Fig. 1. The substrate of the sample disc was glass, the aluminum reflective layer was over coated on the glass substrate by vacuum evaporation. The thickness of the reflective layer was 100 nm, and the reflectivity of it was regarded as 100% in this paper. Using above PMMA solutions, several kinds of films were spin coated on the glass substrate as the recording layer of the sample disc. The rate of spinning was 2000 rpm, for 40 s. The PMMA film containing both 1a and 2a (using the two PMMA solutions equally mixed) was initiated by a UV lamp for 5 min, to make sure that the open forms 1a and 2a were entirely converted to the closed forms 1b and 2b. Under the UV irradiation, the colorless film turned purple Recording and readout In the recording and readout process, the closed forms 1b and 2b were regarded as unrecorded UV VI H 3 C CH 3 H 3 C 1a CH 3 H 3 C CH 3 1b H 3 CO CH 3 H 3 C 2a OCH 3 UV VI H 3 CO 2b OCH 3 cheme 1. The photochromic reaction of compounds 1 and 2.

3 H. Guo et al. / Optical Materials 22 (2003) Fig. 1. tructure of the sample disc. states, and the open forms 1a and 2a were recorded states. In the recording process, the unrecorded states 1b and 2b were inverted to the recorded states 1a and 2a respectively. The blank (unrecorded) sample disc is purple, which was put on the worktable, and the focus of the two laser beams were adjusted at the same dot onto the surface of the reflective layer. The recording and readout conditions are summarized in Table 1. The power of 532 and 650 nm lasers were and mw respectively; the total recording time of both lasers was 1.6 s; the distance between each two recorded dots was 1 mm. In the readout process, the scanning velocity of the laser beams was 10 mm/s. The reflective light intensity was detected by a photosensitive detector, and the output was transformed into voltage. The reflectivity of the reflective Al layer (a disc without recording layer) was used as standard of 100%. 3. Results and discussion 3.1. Photochromism of the diarylethene PMMA films Three diarylethene PMMA films on glass substrates without Al reflective layers were studied. Table 1 Recording and reading conditions Laser 1 Laser 2 Wavelength (nm) Recording power on focal surface (mw) Readout power on focal surface (mw) Total writing time (s) Fig. 2. The absorption bands of 1b and 2b in PMMA film. The colorless 1a PMMA film turned red under the UV irradiation for 60 s, with a new broad absorption band at k max ¼ 510 nm (in hexane solution k max ¼ 504 nm, e max ¼ 2: l mol 1 cm 1 ) appeared, which was assigned to the formation of the closed form 1b. The colorless 2a PMMA film turned blue under the UV irradiation for 60 s, with a new broad absorption band at k max ¼ 608 nm (in hexane solution k max ¼ 588 nm, e max ¼ 1: lmol 1 cm 1 ) appeared, which corresponded to the formation of 2b. Two PMMA solutions containing 1a and 2a were mixed equally to prepare the third colorless film. Under the UV irradiation for 60 s, this film turned purple and a broad absorption band in the region of nm appeared, corresponding to the closed forms 1b and 2b (Fig. 2). Under the irradiation of visible light, the three films inverted to colorless films slowly; the closed forms reverted to the open forms under the visible irradiation. As shown in Fig. 2, the mixed PMMA film contained 1b and 2b is sensitive to the laser of 532 and 650 nm, thus laser beams of these two wavelengths can be used in recording and readout Recording process of the sample disc The sample disc contained 1b and 2b was prepared and initiated. Table 1 summarizes the recording and readout conditions. Fig. 3 shows the recording process by 532 (a) and 650 nm (b) laser respectively. The shape of the recorded dot was

4 272 H. Guo et al. / Optical Materials 22 (2003) R ¼ I x ¼ R max expð 2:303eCLÞ; ð2þ I R max 6 1 is the maximum reflectivity, which is a constant for the same recording layer of the sample disc. o we can get (t is the recording time): 1 C ¼ 2:303eL ln R ; ð3þ R max Fig. 3. Recording R=t curves of 532 (a) and 650 nm (b) lasers. not like the shape of CD-R or CD-RW pit, and the heat effect did not contribute to the recording process, so the power of the recording laser could be very low in the photon-type recording. The laser light intensity on the surface of the recording layer is regarded as a constant in the recording process, and the ring-open reaction B þ hm! A (B is the closed form, A is the open form) obeys the Beer Lambert rule: I x ¼ I 0 expð 2:303eCLÞ; ð1þ where I 0 is the intensity of the incident light, and I x is the intensity of the reflective light; e (l mol 1 cm 1 ) is the mole absorption coefficient of B, and C is the concentration of B; L is the thickness of the recording layer. Considering the absorption of PMMA and the residual solvent, we can get ) dc dt ¼ 1 or 2:303eL Rot : ð4þ The total energy de of the photons absorbed by B in a time interval dt is de ¼½I 0 ðr max RÞþI 0 RR ref ðr max RÞŠd dt; ð5þ where R ref is the reflectivity of the uncoated reflective layer. Herein we assume R ref ¼ 1, so the number of photons absorbed by B in dt is dn ¼ de hc=k ¼ ½I 0ðR max RÞþI 0 RðR max RÞŠd dt : hc=k ð6þ Due to the tark Einstein rule, the number of reacted B molecules is (/ is the quantum yield of the ring-open reaction): dn ¼ dn/ ¼ I 0ðR max RÞð1 þ RÞ/d dt ; ð7þ hc=k we can easily get (Na is the Avogadro constant): dn ¼ dcldna: ð8þ Comparing the Eqs. (7) and (8), and we will get oc ot ¼ I 0/ LNahc=k ðr max RÞð1 þ RÞ: ð9þ Compare Eq. (9) with Eq. (4), we can get the relation between R and t: or ot ¼ krðr max RÞð1 þ RÞ; ð10þ where k (in the unit of s 1, named recording constant in this paper) is a constant for a particular sample disc under the same recording condition. The expression of k is: k ¼ 2:303eI 0/ Nahc=k ¼ 2:303b P ke/; ð11þ

5 H. Guo et al. / Optical Materials 22 (2003) b ¼ 1=ðhcNaÞ 8:352 mol/j m, and e/ is related to the storage material, which can be regarded as the recording sensitivity of the material. The light intensity I 0 ¼ P=, where P (W) is the power of recording laser, and (cm 2 ) is the irradiated area [5]. Thus we can finally find out the relation between R and t, expressed as the following equation: t ¼ 1 k Z R R min dr RðR max RÞð1 þ RÞ ; ð12þ if we know R max and R min (obtained from the recording curve), the recording constant k can be calculated from the recording curve, and then the reflectivity R at an arbitrary recording time t can be calculated by the definite integral of Eq. (12). As shown in Fig. 4, the real recording curve fit the theoretical recording curve very well. Thus we can conclude that the writing rate was determined by the recording constant k, a larger k value presented a faster writing rate. Eq. (11) shows that k is proportions to I 0, e, and/. For the photon-type recording of photochromic diarylethene materials, we can increase the recording constant by increasing the mole absorption coefficient e together with the ring-closing quantum yield / of the diarylethene materials. Increasing the power of the recording laser will accelerate the writing rate too Readout of the sample disc Fig. 5 shows the simultaneously readout R t curves. Five dots labeled 1 5 were recorded, and the distance between each two dots was 1 mm. The readout laser was the same as recording (532 nm: mw on focal surface and 650 nm: mw on focal surface). The laser beams moved from dot 1 to 5 fast with the scanning velocity of 10 mm/s. The reflectivity of the recorded dot was much higher than that of the unrecorded region. After recording, the closed forms 1b and 2b in the recorded region were converted to the open forms 1a and 2a by the two laser beams. Fig. 5 shows the 1st and the 61st readout by the laser beams at both Fig. 4. The theoretical R=t curves: (a) 532 nm recording, R min ¼ 0:452, R max ¼ 0:658, k ¼ 21:5 s 1 and (b) 650 nm recording, R min ¼ 0:473, R max ¼ 0:780, k ¼ 11:9 s 1. Fig. 5. The 1st and 61st readout R=t curves of 532 (a) and 650 nm (b) lasers.

6 274 H. Guo et al. / Optical Materials 22 (2003) Table 2 Destruction of the readout Wavelength (nm) R r R 1 ur R 61 ur ðr r R 61 ur Þ=ðR r R 1 ur Þ Laser % Laser % wavelengths. Yet the readout process was not a non-destructive readout, after the 61st readout, the reflectivity of the unrecorded region tardily increased. The average of the reflectivity was calculated from the readout curve. As shown in Table 2, R r is the average of the reflectivity at the recorded dots, and R ur is the average of the reflectivity in the unrecorded regions. The destruction of the readout for 60 times can be represented by ðr r R 61 ur Þ=ðR r R 1 urþ. The values at 532 and 650 nm are 73.9% and 81.8% respectively, i.e., at 532 and 650 nm, the reflectivity of the 61st readout were 73.9% and 81.8% of that of the 1st readout. Lack of a non-destructive readout capability is still an unsolved drawback of the photochromic compounds in the use of optical storage. According to Eq. (12), readout by laser beams of lower power will decrease the destruction of readout. Above discussions ignore the crosstalk of the two materials. ince the absorption band of 1b overlaps with that of 2b in the visible region (as shown in Fig. 3), crosstalk would exist in the recording and readout process. ingle wavelength recording and readout had been carried out to check the crosstalk between these two materials. The PMMA film of 1b was recorded by 650 nm laser, and no signal was detected by 532 nm laser. Yet after recording of the 2b PMMA film by 532 nm laser, 650 nm laser detected weak reflectivity peaks, with the height about 1/10ðR r R ur 0:05Þ as that of the recorded dot by 650 nm laser. 4. Conclusions Two-wavelength photon-mode optical storage with diarylethene photochromic materials in PMMA film was carried out. In the recording process, the closed forms 1b and 2b were converted to the open forms 1a and 2a respectively. The ringopen reactions obey the Beer Lambert and tark Einstein rules, and the calculated theoretical R=t curves fitted the real R=t curves very well. The destruction of the readout was also studied; recorded signals could be read out simultaneously for more than 61 times. The recording and readout processes are merely based on photon absorption and reflection, i.e., belong to photon-mode storage, which has many advantages over current heatmode optical storage technologies, and will lead to a higher data capacity and faster data recording optical storage. Acknowledgements We are grateful to the support of the Fund of Fundamental Research of Tsinghua University, and the Projects of Development Plan of the tate Key Fundamental Research Fundamental Research of uper-high Density and uper Fast Optical torage (G ). References [1] H.J. Borg, R. van Woudenberg, J. Magnetism Magnetic Mater. 193 (1999) 519. [2] M. Irie, Chem. Rev. 100 (2000) [3] M. Irie, M. Mohri, J. Org. Chem. 53 (1988) 803. [4] M. Irie, K. Uchida, Bull. Chem. oc. Jpn. 71 (1998) 985. [5] T. Tsujioka, F. Tatezono, T. Harada, K. Kuroki, M. Irie, Jpn. J. Appl. Phys. 33 (1994) [6] T. Tsujioka, Y. himizu, M. Irie, Jpn. J. Appl. Phys. 32 (1993) [7] T. Tsujioka, M. Kume, M. Irie, Jpn. J. Appl. Phys. 33 (1994) [8] T. Tsujioka, M. Kume, M. Irie, Jpn. J. Appl. Phys. 34 (1995) [9] T. Tsujioka, M. Kume, K. Kuroki, M. Irie, Jpn. J. Appl. Phys. 36 (1997) 526. [10] H. Hamano, M. Irie, Jpn. J. Appl. Phys. 35 (1996) [11] T. Tsujioka, M. Kume, M. Irie, Jpn. J. Appl. Phys. 33 (1994) 1914.

Chemistry 524--"Hour Exam"--Keiderling Mar. 19, pm SES

Chemistry 524--Hour Exam--Keiderling Mar. 19, pm SES Chemistry 524--"Hour Exam"--Keiderling Mar. 19, 2013 -- 2-4 pm -- 170 SES Please answer all questions in the answer book provided. Calculators, rulers, pens and pencils permitted. No open books allowed.