Blue-ray-induced optical properties of noble metal oxide thin film in super-rens disk

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1 Blue-ray-induced optical properties of noble metal oxide thin film in super-ens disk Qian Liu, oshio Fukaya, Junji ominaga Center for Applied Near-Field Optics esearch (CAN-FO), National Institute of Advanced Industrial Science and echnology (AIS) 1-1-1, Higashi, sukuba, Ibaraki Japan ABSAC he optical reflectance and transmittance of Platinum oxide (PtO x ) and palladium oxide (PdO x ) mask layer, which are used to super-resolution near-field structure (super-ens) disk, are investigated using Z-scan technique, under blue laser (442 nm) irradiation. he power thresholds of the PtO x and PdO x decomposition are obtained; the reversible and irreversible features for the two kinds of mask layers are cleared. Deformation in the micro irradiation region on surface of the mask samples, which is formed by decomposition of the PtO x or PdO x driving the Z-scan, is analyzed by means of an atom force microscope (AFM). he deformation analyses agree well with the Z-scan results. he optical features obtained at 442nm wavelength are compared with those at 532-nm wavelength, and the power threshold difference between the two wavelengths is also analyzed in detail based on irradiation power density and absorption spectrum of the mask samples. INODUCION A super-resolution near-field structure (super-ens) with resolution ability beyond the diffraction limit has attracted much attention in the ultrahigh-density storage field due to its rapid progress [1-13]. A short time ago, the third generation super-ens disks with a PtO x or a PdO x mask layer have been realized under illumination of red laser (635nm) and to be suitable for practical use [11-13] due to their excellent readout stability, and high carrier-to-noiseratio (CN), namely, more than 40 db for a 100-nm-mark train. In the super-ens systems, the super-resolution effect was recently cleared mainly due to the ferroelectric properties and second phase-transition of the phase-change film, although the mask layer had been thought to play a major role in readout with high CN and stability [14,]. However, it is still important that the mask layer, PtO x or PdO x has a role to record and solidify a very small pit less than 100 nm. he performance of the mask layer in the super-ens disk mainly depends on its optical features, which are not only related to the recording and reading properties but also related to thermal effect induced by laser irradiation in an optical storage system. In the measurement of optical and nonlinear properties of the film materials, Z-scan technique has been widely used since it can provide a high-density incident light in a microscopic region on a sample [16]. Based on the Z- scan technique and thermal analysis, we have investigated that the optical responses and its mechanisms as well as thermal and thermal-optical properties of the PtO x and PdO x thin films under laser irradiation with a wavelength of 532nm [17, 18]. With a rapid development of blue diode lasers, it will become a powerful tool to increase the resolution and storage density in optical disk system. Currently, the super-ens disk suited for blue-ray laser is being developed. herefore, it is necessary for the super-ens disk design to study optical property of the mask layers under the irradiation of a blue laser. In this paper, we report the reflectance and transmittance properties in a microscopic irradiation region of the PtO x and PdO x mask samples, based on a Z-scan technique with the blue laser with λ=442nm. he main optical features of the two mask samples are also compared briefly with those under a laser with a wavelength of 532nm. he wavelength-dependent reason of the mask samples are analyzed and discussed. Deformation in the micro irradiation region on surface of the scanned mask samples, which is generated by decomposition of the PtO x or PdO x, is also analyzed by an atom force microscope (AFM). 2.PEPAAION OF MASK SAMPLES AND Z-SCAN SE-UP he PtO x and/or PdO x mask sample, which is composed of a polycarbonate (PC) substrate with a thickness of 0.6 mm and a multilayer film with PtO x and/or PdO x mask layer is shown in Fig. 1. he (ZnS) 85 ( SiO 2 ) layer and PtO x mask 424 Advances in Optical Data Storage echnology, edited by Duanyi Xu, Kees A. Schouhamer Immink, Keiji Shono, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$ doi: /

2 layer were sputter-deposited on the PC substrate, respectively, by a composite target in a pure-ar atmosphere and by a pure-pt target in gas mixture of Ar and O 2 (he gas-mass-flow ratio of O 2 to the total was 0.20). he PdO x mask sample with the same structure shown in Fig. 1 was prepared with the same process of manufacture, except for which the gasmass-flow ratio of O 2 to the total was 0.50 when PdO x mask layer was deposited. ZnS-SiO 2 (40 nm) PtO x / PdO x (4 nm) ZnS-SiO 2 (130 nm) Polycarbonate (0.6 mm) Fig.1 Schematic diagram of PtO x and/or PdO x mask sample. Sample PD (-mode) Mirror PC Multilayer CW He-Cd Laser (442nm) VA L OL1 OL2 VA PD (-mode) VA Z-scan Diode Laser PD (ef.-mode) Fig.2 Schematic configuration of Z-scan set-up. OL1 and OL2, objective lens with 40x /NA 0.40 and with 20x / NA 0.35, respectively; Laser source, 442nm continuous-wave He-Cd laser. PD, photodetector; VA, valuable attenuator, L, lens; Sample: structure of PtO x and PdO x mask sample. he Z-scan set-up in experiment is shown in Fig. 2. Continuous-wave 442-nm light from a He-Cd laser (IK5651-G-S0) was used as a light source in the experiment. wo objective lenses OL1 with 40 magnification and 0.40 numerical Proc. of SPIE Vol

3 aperture and OL2 with 20 magnification and 0.35 numerical aperture (Nikon long working distance series) were placed face to face in a con-focal configuration to generate high-density incident light in a microscopic region on the sample and to collect the light transmitted through the sample. eferenced, transmitted and reflected light were detected by the Hamamatsu S silicon photodiodes with a C2719 photosensor amplifier, and the detected signals were monitored by a color four channel digitizing oscilloscope (ektrounix DS 744A). he signals detected by sensor system were measured with a gated integrator (Stanford esearch Systems S 250). he starting position of the Z-scan and inclination of the sample were checked by a sidewise system composed of a mirror and diode laser with an aperture, so that the incident beam was set normal to the mask sample surface. he input power at the sample surface was measured by using an optical power meter (Advantest Q 8210), and a computer controlled the whole measurement system and automatically figured the transmittance and reflectance. In the measurement, the scanning speed of 59 mm/s was used, and scanning was made along the arrow direction shown in Fig EXPEIMENAL ESULS AND ANALYSES 3.1 Z-scan esults for PtO x and PdO x mask samples Figures 3 and 4 show reflectance and transmittance of several typical Z-scan examples for the PtO x and PdO x mask samples, respectively. he first reflectance peak (FP), on the left, was produced at the interface between air and the PC disk and the second reflectance peak (SP), on the right, was caused the multilayer deposited on the PC substrate. he crest values of FP and SP are marked as 1 and 2 shown in Fig. 3(b). he appearance of a transmittance absorption peak (AP) and a relative change between 1 and 2 revealed the optical feature induced by decomposition of the PtO x and/or PdO x in mask layer, which are identified by our foregone work [11-13,,16]. Figures 3(a) and 4(a) reflect that the optical property of the PtO x and PdO x mask samples before decomposition of the noble metal oxides, respectively; Figs. 3(b) and (c), and Figs. 4(b) and 4(c) show optical responses caused by decomposition of the PtO x and PdO x in the mask samples, respectively, in good condition; Figs. 3(d) and 4(d) suggest that mask samples have been broken because & (a) 3.0 mw & 3.0 (b) 3.1 mw & (c) 3.7 mw & (d) 3.8 mw Fig. 3 eflectance and ransmittance of several typical Z-scan results for PtO x mask sample. 426 Proc. of SPIE Vol. 5643

4 2.0 (a) 2.4 mw 3.0 (b) 2.6 mw & 1.0 & 3.0 (c) 2.8 mw 3.0 (d) 2.9 mw & & Fig. 4 eflectance and ransmittance of several typical Z-scan results for PdO x mask sample. the locations of the AP s trough value marked as 2 in Fig. 3(b) and 2 are staggered and the sub-reflectance-peaks near to the second reflectance peak appear. Since the obtained transmittance and reflectance in our optical system are relative values, it is difficult to directly compare the decomposition-induced optical response change for the noble metal oxides with the different input powers. For this objective, we introduce a normalized value = 2 / 1 to describe the decomposition-induced change in the reflectance based on the optical linearity of the PC substrate ( 1 is constant with input power). Γ=2/1 represents the decomposition-induced change in the transmittance as well as. Here, 1 is the value of a linear transmittance as shown in Fig. 3(b). Figures 5 and 6 show normalized reflectance and transmittance for the PtO x and PdO x mask samples, and Γ, varying with the input power with a step of 0.1 mw, respectively. Until 3.0 mw of input power in Fig. 5, no change for the PtO x mask sample was observed in and Γ, and the behaviour was reversible. From 3.1 mw to 3.7 mw, the optical response changes in reflectance and transmittance appeared and the scanned sample could no longer retrieve to the original state. his scope of input power should correspond to the decomposition of PtO x (PtO x Pt+O x )[11-13]. At above 3.8 mw, the multilayer in the sample seemed to be damaged as shown in Fig. 3(d). Figure 6 shows that the linearity and reversibility of the PdO x mask sample were kept at less than 2.5 mw, that the decomposition-induced optical change in reflectance and transmittance was within a range from 2.6~2.8 mw [16], and that the sample was damaged over 2.9 mw, as also shown in Fig. 4(d). Proc. of SPIE Vol

5 2.5 & Γ (a. u.) 2 1 eversible Γ Irreversible 0.5 Decomposition-induced optical response scope Input power (mw) Fig. 5 Normalized reflectance and transmittance of PtO x mask sample, and Γ, versus input power. 2 & Γ (a. u.) (b) PdO x sample eversible Decomposition-induced optical response scope Input power (mw) Irreversible Γ Fig. 6 Normalized reflectance and transmittance of PdO x mask sample, and Γ, versus input power. 428 Proc. of SPIE Vol. 5643

6 3.2 Microscopic observations and analyses he microscopic observations demonstrated that the laser-irradiated spots on the surface of the PtO x mask sample in the Z-scan experiment were not found when the input power was up to 3.0 mw, and that the permanent bubble deformations were formed in the multilayer of the sample when input power is more than 3.1 mw. No bubble deformation for the PdO x mask sample was observed until 2.5 mw, and permanent bubble deformation appeared over 2.6 mw. AFM images of permanent bubbles for the PtO x and PdO x mask samples obtained by NanoScope IV System (Digital Instruments Veeco Metrology Group) are shown in Figs.7 and 8, respectively. It is clear from Figs. 7 and 8 that the bubble has formed in multilayer at 3.1 mw for the PtO x mask sample, and at 2.6 mw for the PdO x mask sample. he section analyses shown in Fig. 7 and 8 indicated that the diameter and height of the bubble increased with an increase in input power. At above 3.8 and 2.9 mw, the cover layer of the bubble was exploded and the gas pressure was released for the PtO x and PdO x mask sample, respectively, as shown in Figs. 7(c) and 8(c). he microscopic observations and AFM analyses in formation and damage of the bubble agree well with the optical features from Z-scan experimental results Section Analysis: Diameter:5.36, Height:8 nm 5 1 Section Analysis: Diameter:6.90, Height:386 nm 5 10 Section Analysis: Diameter:8.46, Height:498 nm (a) 3.1 mw (b) 3.4 mw (c) 3.8 mw Fig.7 AFM topping and section analysis of bubbles on the surface of the scanned PtO x mask sample for several input power. 3.0 (a) Section Analysis Diameter:5.06, Height:0 nm (a) 2.6 mw 5 10 Section Analysis Diameter:6.02, Height:3 nm (b) 2.8 mw 5 10 Section Analysis: Diameter:8.06, Height~468 nm (c) 2. 9 mw Fig.8 AFM topping and section analysis of bubbles on the surface of the scanned PdO x mask sample for several input power. Proc. of SPIE Vol

7 4. DISCUSSIONS 4.1hresholds of the decomposition of PtO x and PdO x in the mask samples It is no doubt that a decomposition threshold is an important physical property because the decomposition can produce the recorded marks of the super-ens disk. In our case of the Z-scan measurement, the determination of decomposition power thresholds of the samples is a comprehensive judgement based on the decomposition-induced optical response, bubble formations as well as the sample irreversibility. According to the judgement principle mentioned above and the results obtained from the experiment, the decomposition power thresholds of the PtO x and PdO x in the samples should be 3.1 mw and 2.6 mw, respectively. 4.2 eversible and irreversible features he reversible and irreversible features of the mask layer are very important for a storage system. Generally speaking, reversibility of a scanned mask sample can simply be checked by a repetitive Z-scanning with a much smaller input power (0.5 mw), which cannot result in any thermally induced optical nonlinearities for a virgin position on the sample. If the scanned sample is reversible, the transmittance and reflectance in a repetitive Z-scan are completely consistent with those in a Z-scan with the same power for virgin sample. By means of the method mentioned above we found that the samples can recover to the original state until an input power of 3.0 and 2.5 mw for the PtO x and PdO x sample, respectively, as shown in Figs. 5 and 6. he scanned samples become irreversible and have a memory feature over 3.1 mw for the PtO x sample and over 2.6 mw for the PdO x mask sample. 4.3 hreshold difference between irradiation of 532nm and 442nm Optical features of the PtO x and PdO x mask samples depend on the wavelength of laser irradiation system. In particular, the wavelength-dependent decomposition power thresholds (hp) of PtO x and PdO x in the mask samples are closely related to the reversibility and irreversibility in the third-generation super-ens disk. Based on the results obtained under a illumination of 442 nm wavelength, the threshold powers of the PtO x and PdO x, respectively, are about 47% and 0.51% of those under an irradiation of 532 nm [, 16], i.e., hp(442 nm)/hp(532 nm)=0.47 and 0.51, respectively, for the PtO x and PdO x mask sample. Since the decomposition of PtO x or PdO x is a thermal-induced chemical reaction, the power threshold difference depends mainly on the two parameters: <1> laser power density irradiated in a micro irradiation region on the samples, <2> absorption spectrum of the mask samples. (a) Incident beam (442nm) (b) Incident beam (532nm) 1 sinθ =/(f ) 1/2 sinθ 1 = 1 /(f ) 1/2 θ f θ 1 f.. Focused spot Focused spot Fig. 9 elationship between numeral aperture (NA) and size of incident beam, (a) ideal NA, and (b) effective NA. 430 Proc. of SPIE Vol. 5643

8 For an input power, laser power density irradiated on the mask sample can be decided by the focused spot size. he laser focused spot size passing through objective lens OL1 in Fig.2 follows D=λ/ NA=λ/(nsinθ)=λ/{n[/( 2 +f 2 ) 1/2 ]}, (1) where D and λ denote diameter of focused spot and incident laser wavelength; and f are radius and focal length of OL1; NA and n represent numeral aperture of the OL1 and refractive index of medium between the lens and the focused spot. Since the NA refers to an ideal situation corresponding to one half the acceptance angle (θ) of the lens shown in Fig. 9(a), Eq (1) is effective only when incident beam size is equal to or larger than size of the objective lens. In practice, an incident beam smaller than size of the objective lens is often adopted for effectively utilizing the input power, as shown in Fig. 9(b), and the effective focused spot size in such situation can be described as D 1 =λ/(nsinθ 1 )=λ/{n[ 1 /( 1 2 +f 2 ) 1/2 ]}. (2) In our experiment, the radius of the blue incident beam (λ=442 nm) is equal to as shown in Fig. 9(a), i.e., it is suitable for Eq. (1); the radius of the green laser (λ=532 nm) is about 0.8 for obtaining higher input power as shown in fig. 9(b) and the focused spot size is governed by Eq. (2). Hence, considering <<f and 1 <<f, the ratio of blue laser irradiation area A(442nm) and green laser irradiation area A(532nm) in the focal point can be expressed as A(442 nm)/a(532 nm) 0.69( 1 / ) (3) Without considering absorption difference for the two wavelengths, Eq. (3) suggests that the threshold powers of the PtO x and PdO x in blue laser are about 44% of those under an irradiation of 532 nm, and this is close to experimental values of 47% for PtO x and 51% for PdO x. It indicates that the irradiation power density difference in the 442nm and 532nm laser systems is main reason of the threshold power difference. he differences between the value based on Eq. (3) and experimental values are decided by absorption property of the mask samples for the two wavelengths. 100 ransmittance Pt mask sample Pd mask sample PC substrate Wavelength (nm) Fig. 10 ransmittance spectrums of the PC substrate, PtO x, and PdO x mask samples. Proc. of SPIE Vol

9 Absorption related to wavelength in the mask samples is another important reason of the threshold difference. For the ease of analysis the absorption feature of the mask samples, we measured the transmittance spectrum using UV-VS recording spectrophotometer (UV-2500PC, SHIMADSU) for the PC substrate and for the PtO x and PdO x mask samples, as shown in Fig.10. hey show that for three measurements, the transmittances varies with wavelength ranging from 800~400 nm and has a sharp decline from 400 to 290 nm. It is obvious that in Fig.10 for the PtO x and PdO x mask samples, the wavelength dependency of the transmittance in the range of visible light is mainly contributted to the multilayer film because the PC substrate has little change in this range. In addition, no interference effect on the absorption, which comes from the different layers in multilayer film, is found for the two samples. he transmittance of the PdO x mask sample at λ=442 nm and λ=532 nm are 74.79% and 63.27%, respectively. For the PtO x mask sample, the values are 74.28% at λ=442 nm and 70.29% at λ=532 nm. Since the Protection layer ZnS 85 -(SiO 2 ) and PC substrate are almost transparent for the white light, the transmittance difference, therefore, is mainly caused by the absorption of the PtO x or/and PdO x mask layer for the two wavelengths. In another words, the PtO x mask layer and the PdO x mask layer at λ=442 nm than at λ=532 nm decreases in energy absorption by about 4% and about 11%, respectively. Combining the absorption difference of the mask layers mentioned above with the result from Eq.(3), the final theoretical values of hp(442 nm)/hp(532 nm for PtO x and PdO x mask samples are 48% and 55%, which are very close to the experimental values of 47% and 51%. 5. CONCLUSION he optical reflectance and transmittance of the PtO x and PdO x mask layer are investigated using a blue laser with a wavelength of 442 nm based on Z-scan technique. Optical features caused by the decompositions of PtO x and/or PdO x in the multilayer film are cleared. Surface deformations in the micro irradiation region of the mask samples, which are formed by the PtO x or PdO x decomposition in the multilayer film, are analyzed by an atom force microscope (AFM), and these analyses agree well with the Z-scan results. he reversible and irreversible features for the PtO x and PdO x mask samples are also discussed. he power threshold difference between the 442 nm and 532 nm wavelengths for both the PtO x and PdO x mask samples has been explained by means of the transmittance spectrum analyses and the irradiation power density. hey indicated that the blue laser increases not only in storage density, but decreases in recording and readout power. Since the absorption features of the mask samples depends on the composition of oxygen in noble metal oxide, dielectric layer material, and thickness of each layer in the sample, the super-ens disk design must consider these factors for different laser systems. Authors are grateful to Dr. A. Kolobov for a stimulating discussion EFEENCES 1. J. ominaga,. Nakano, and N. Atoda, "An approach for recording and readout beyond the diffraction limit with an Sb thin film," Appl. Phys. Lett. 73, 2078~2080 (1998). 2. H. Fuji, J. ominaga, L. Men,. Nakano, H. Katayama, and N. Atoda, "A near-field recording and readout technology using a metallic probe in an optical disk," Jpn. J. Appl Phys. 39, 980~986 (2000). 3. J. ominaga and N. Atoda, Appl. Phys. Lett. 75, 1~3 (1999). 4. D. P. sai, and W. C. Lin, "Probing the near fields of the super- resolution near-field optical structure," Appl. Phys. Lett. 77, 1413~14 (2000). 5. Q. Chen, J. ominaga, L. Men,. Fukaya, N. Atoda, H. Fuji, "Superresolution optical disk with a thermo reversible organic thin film," Optics Letters 26, 274~276 (2001). 6. J. ominaga, C. Mihalcea, D. Buechel, H. Fukuda,. Nakano, N. Atoda, H. Fuji and. Kikukawa, "Local plasmon photonic transistor," Appl. Phys.Lett. 78, 2417~2419 (2001). 7. D. Buechel, C. Mihalcea,. Fukaya, N. Atoda, J. ominaga,. Kikukawa, H. Fuji,"Sputtered silver oxide layer for surface- enhenced amanspectroscopy," Appl. Phys. Lett. 79, 620~622 (2001). 8.. Fukaya, D. Buchel, S. Shinbori, J. aming, N. Atoda, D. P. sai and W. C. Lin, "Micro-optical nonlinearity of a silver oxide layer," J. Appl. Phys. 89, 6139~6147 (2001). 432 Proc. of SPIE Vol. 5643

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