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Optical-thermal and thermal-optical properties of a PdOx mask layer in a system with a superresolution near-field structure are investigated with a Z-scan technique and a heating experiment. The high photothermal stability of the PdOx mask is shown, and the reversible limit of the PdOx mask layer and a weak switch effect are revealed. The PdOx decomposition, which results in a bubble with Pd particles, is confirmed, and the laser-induced physical and chemical mechanisms in the PdOx mask layer are clarified and discussed. Our microscopic studies and heating analysis are consistent with the Z-scan results. The PdOx mask sample is also compared briefly with a PtO2 mask layer that has the same structure.
©2003 Optical Society of America
1. Introduction
A superresolution near-field structure (super-RENS) [1], in which a Sb thin film as a nonlinear optical material layer is inserted into an optical disk, has successfully overcome the low-datatransfer-rate difficulty reported in Betzig et al.’s near-field recording method [2], since the super-RENS needs neither a tip nor complicated equipment to control the nanoscale space between tip and recording medium. The super-RENS, which can be used to overcome the diffraction limit and to increase storage density up to the subterabyte level, has attracted increasing attention in the field of optical ultrahigh-density storage. Many studies [3–9] on the super-RENS have been carried out since the structure was proposed in 1998. Recently, a novel super-RENS disk with a PtO2 mask was realized [10] and for the first time to our knowledge has been shown to be compatible with the levels needed for practical applications, owing to its excellent readout stability and high carrier-to-noise ratio (CNR=40 or 43 dB by 100- or 150-nm mark length, with writing power and readout power set at 12 and 4 mW, respectively [11]). Not long after, another super-RENS disk with a PdOx mask layer was discovered that possesses a proper CNR level for practical applications (CNR=37 or 41 dB by 100- or 150 mark length, with writing power and readout power set at 11 and 4 mW, respectively [11]), only in this case with the PdOx mask instead of the PtO2 mask layer. In super-RENS systems, the mask layer is thought to play a key role in increasing CNR and readout stability and in giving significant insight into the working mechanism of the super-RENS, which has not yet been clear. So far, four types of super-RENS disk with Sb, AgOx, PtO2, and PdOx mask layers have been discovered, and properties of the first three mask layers have been studied in part [12–14]. However, optical and thermal properties of the PdOx mask layer for optical data storage have not been thoroughly investigated. In this paper, we address the optical-thermal and thermal-optical properties of the PdOx mask layer for super-RENS optical data storage, by means of a Z-scan technique [15] and a heating analysis experiment, as well as with microscopic observation studies. The physical and chemical mechanism of the PdOx decomposition induced by laser irradiation is clarified and discussed briefly.
2. Experiments
The experimental setup for our Z-scan is shown in Fig. 1. Two objective lenses, one with 40× magnification and 0.40 numerical aperture and one with 20× magnification and 0.35 numerical aperture (Nikon long working distance series), were placed face to face in a confocal configuration to generate high-density incident light in a microscopic region on the sample surface and to collect transmitted light through the sample. Continuous-wave second-harmonic-generation 532-nm light from a Nd:YAG laser was employed as a light source in the experiment. Referenced, transmitted, and reflected lights were detected by the Hamamatsu S2281-01 silicon photodiodes with a C2719 amplifier, and these detected signals were monitored by a color four-channel digitizing oscilloscope (Tektrounix TDS 744A). The signals detected by sensor systems were measured with a gated integrator (Stanford Research Systems SR 250). Input power at the sample surface was measured with an optical power meter (Advantest TQ 8210), and a computer controlled the whole measurement and automatically figured the transmittance and reflectance.
The PdOx mask sample used in the experiment was fabricated by depositing of a (ZnS)85(SiO2)15[130 nm]/PdOx[4 nm]/(ZnS)85(SiO2)15[40 nm] multilayer onto a 0.6-mm poly-carbonate (PC) disk substrate. The (ZnS)85(SiO2)15 and the PdOx were sputter-deposited, respectively, by a composite target in a pure-Ar atmosphere and by a pure-Pd target in gas mixture of Ar and O2 (0.5:0.5). A value of x=1.1 (PdOx=10%-PdO2+90%-PdO) was determined by Rutherford backscattering spectrometry and nuclear reaction analysis.
In the Z-scan measurement, a scanning speed rate of 17 s/mm was used, and each scan with each different laser power was carried out at a virgin position on the sample surface to avoid photothermal accumulation. The thermal property of the PdOx layer was also investigated by a heating stage equipped with a white-light source and a multichannel photodetector (Hamamatsu Photonics, PMA-11). In the measurement, a sample, which was fabricated by depositing of a PdOx film with a thickness of 15 nm onto a 0.6-mm-thick SiO2 substrate, was placed on the heating stage with a hole; a collimating beam through this hole went through the SiO2 substrate and PdOx film and then reached an intensity detector over the PdOx film. The transmitted light intensity was monitored at a wavelength of 633 nm while the sample was heated from room temperature up to 900 K at a ramp rate of 30° k/min in ambient air.
Fig. 1. Schematic configuration of the Z-scan microscopic measurement of transmittance and reflectance. A, aperture; SP, beam splitter prism; OL1, objective lens with 40× magnification and 0.40 numerical aperture; OL2, objective lens with 20× magnification and 0.35 numerical aperture; L, lens; VA, variable attenuator; PT, photodetector.
3. Results and analyses
Several typical Z-scan examples of the PdOx mask sample are shown in Fig. 2. The first reflectance peak (left) is produced at the interface between air and PC disk, and the second reflectance peak (right) is caused by the multilayer in the sample. The crest values of the first and second reflectance peaks are marked as R 1 and R 2 shown in Fig. 2(c). Figure 2(a) shows an example of the intensity-independent transmittance and reflectance. The appearance of a transmittance absorption peak and a relative change between R 1 and R 2 in Figs. 2(b) and 2(c) indicate intensity-dependent transmittance and reflectance (IDTR) in the PdOx mask layer. Figure 2(d) shows that the sample has deformed. Note that Figs. 2(a)–2(d) cannot be directly comparable, because the reflected and transmitted light intensity incident to the sensors must be adjusted for each different laser power to ensure that they are within a linear response scope. For ease of comparison with cases in different powers, we introduce normalized values ℜ=R 2/R 1 and Γ=T 2/T 0 to describe the change of IDTR in the sample. Here, T 0 is the value of intensity-independent transmittance and T 2 is the peak value of a transmittance absorption peak as shown in Fig. 2(c). Note that we can also normalize R 2 by using R 2/R 0 the same as with normalized T 2 [here R 0 is the value of intensity-independent reflectance shown in Fig. 2(c)], and the approach is essential no different that in the present normalizing method. Here we have selected R 2/R 1 only because it can be used for easily judging the change in R 2 with respect to R 1(constant) in a reflectance curve drawn automatically by the computer.
Figure 3 shows the normalized reflectance and transmittance, ℜ and Γ, varying input power. No change for ℜ and Γ was observed at less than 4.1 mW. From 4.1 to 5mW, we observed weak IDTR, but a scanned sample could still recover its original state. Within the power range, weak upward transmittance peaks existed as shown in Fig. 4. This indicated that there seemed to be a window on the mask that could be opened and closed, as with the optical switch effect in the Sb mask [12]. From 5.1 mW, the strong IDTR appeared, and the scanned sample could no long recover its original state. A strong scattering light was also observed from 5.1 mW when and after the PdOx layer went through the focal point of the focused laser beam, while the light was weak at less than 5.1 mW and rapidly disappeared after the PdOx layer passed the focal point. Further investigation indicated that the above phenomenon still existed when repeating scanning was carried out with a weak beam intensity of much less than 5.1 mW for the same spots, which had been scanned at powers from 5.1 to 5.6mW. The origin of the strong scattering light is most likely the Pd particles generated by the decomposition of PdOx[PdOx→Pd+(x/2)O2], which we identified in previous publications [10, 11]. Above 5.7mW, the outer protection layer should have damaged, but it continued to show a strong IDTR response.
The microscopic observations for the scanned spots in the Z-scan measurement indicated that the mask sample was not deformed when powers of less than 5.1 mW were used, as shown in Figs. 5(a) and 5(b). The microscopic photos in Figs. 5(c) and 5(d) demonstrated clearly that the bubbles, where the decomposed oxygen gas was confirmed, had been formed in the multilayer but could still support the oxygen gas pressure, and the bubble heights were increased with an input power increase from 5.1 to 5.6 mW. Finally, at more than 5.7 mW, the outer protection layer was broken and the gas pressure was released as shown in Fig. 5(e). From the above results, the IDTR features from 5.1 to 5.6 mW depend not only on the decomposition of the PdOx (chemical change) but also on the bubble deformation (physical change) in the multilayer. The IDTR response at more than 5.7 mW is, however, mainly due to the physical deformation generated by the collapse of the multilayer. Therefore it is suggested that the measured IDTR responses can be attributed to two different but interrelated mechanisms: chemical and physical.
Figure 6 shows thermal-optical properties of the PdOx layer sample. It was found that the PdOx layer makes a transition from 473 to 760 K before the thermal explosion (optical constants are n=4.64, k=1.68 and n=6.09, k=0.95, respectively, before and after transition). Since we know that PdO2 starts to decompose to PdO+O from a temperature of 473 K, a transition of PdO2→PdO+O should exist from 473 to 583 K in Fig. 6. When we compare Fig. 3 with Fig. 6, it is easy to see that the temperature range from 583 to 760 K should correspond to power range from 4.1 to 5 mW in the Z-scan experiment. Therefore the PdOx mask layer from 583 K should remain in a PdO state until 760 K, and then a reaction of PdO→Pd+(1/2)O2↑ takes place at greater than 760 K. Note that the transition temperature in the heating experiment is decided by material structure and is not related to the detected wavelength.
4. Discussion
4.1 Thermal decomposition and threshold
From the thermal property analysis, we can assume that the decomposition of the PdOx is a thermal-induced chemical reaction rather than a photochemical reaction. The recording process of the PdOx-type disk should be as follows: The laser-irradiated energy is first converted to heat by absorption of the disk, and then the heat energy results in the decomposition of the PdOx in the mask layer. From studying the mask layer sample, we know that PdOx decomposition generates oxygen gas and Pd particles. The oxygen gas generates the bubble formation in the multilayer, Pd particles in the bubbles result in strong light scattering, and the strong IDTR is the optical response of the decomposition resulting in structural change. Note that the IDTR here refers to a thermal-induced optical response rather than conventional light-induced nonlinearities.
Thermal analysis indicated that the decomposition temperature of the mask sample is over 760 K. Therefore a threshold power for the decomposition of the PdOx in the mask sample should exist in the Z-scan. From 5.1 mW, the strong IDTR, bubbles, and strong scattering light appeared, and in additional the sample became irreversible in the Z-scan measurement. This revealed that 5.1 mW is the threshold of the decomposition of the PdOx and the generation of Pd particles. The decomposition threshold, 5.1 mW, should correspond to a temperature of more than 760 K and reflects that the PdOx mask layer has high thermal stability, which directly results in the readout stability of the PdOx-type super-RENS disk.
4.2 Irreversible feature
The irreversible feature of a mask layer is very important for a super-RENS WORM disk. Generally speaking, a scanned sample (similar to a recorded sample) can be checked by a repetitive scanning with a much smaller input power (similar to a readout process), which cannot result in any nonlinear change for a virgin position on the sample. If the scanned sample is reversible, the optical responses on transmittance and reflectance in a repeating scan are completely consistent with the optical responses in a scan with the same power for a virgin sample. With our method we checked the reversible property of the PdOx mask sample and found that the sample could recover its original state until an input power of 5 mW, as shown in Fig. 3. Figure 7 shows transmittance of repeating scanning with a power of 0.5 mW for the scanned sample at powers from 5.1 to 5.6 mW. It is obvious that these transmittance absorption peaks in Fig. 7 did not exist when a virgin sample was scanned at a power of 0.5 mW, as shown in Fig. 3. Therefore the scanned sample is irreversible or appears as a memory feature from 5.1 mW.
4.3 Weak optical switch
From 4.1 to 5 mW, normalized transmittance Γ has a small increase as shown in Fig. 3 and as the corresponding upwards absorption peaks show in Fig. 4, and the sample is reversible. This indicates that there seemed to be a window on the mask that could be opened and closed, like with an optical switch. The experiment results indicated that the temperature scope from 583 to 760 K should correspond to a power range from 4.1 to 5 mW in the Z-scan experiment, and that the PdOx mask layer from 583 K should keep its PdO state until 760 K. Therefore a reasonable explanation for the switch effect may be a reversible reaction (PdO2↔PdO+O) with the laser on and off. In addition, 10%-PdO2 in PdOx weakens the reversible reaction (switch effect). Until now, it has not been clear whether the effect has practical applications. Determining this will require further study with a pulse laser to clarify the response time of the switch.
4.4 Comparison between the PdOx and PtO2 mask
Differences do exist between the PdOx- and PtO2-type super RENS disks, although they have almost the same structure, recording power and readout power. There is no doubt that the differences come from the different mask layer materials in the two types of disk. From the Z-scan study on the PtO2 with the same structure as PdOx, it is clear that the PtO2 mask sample has a higher decomposition threshold of 6.6 mW [14], and it is consistent with a higher decomposition temperature of PtO2 (870 K) [10]. This should be the main reason why there is a slightly higher CNR and better readout stability for the PtO2-type super-RENS disk.
5. Conclusions
We have studied the optical and thermal properties of the PdOx mask sample for optical data storage with the super-RENS by means of a Z-scan and a heating experiment. The physical and chemical mechanism was clarified before and after the decomposition of the PdOx. The scanned sample was reversible until 5.0 mW; it became irreversible from 5.1 mW and was damaged at above 5.6 mW. A weak switch effect from 4.1 to 5 mW was discovered, and 5.1 mW was determined as a threshold of the PdOx decomposition resulting in Pd particles and bubbles. The Z-scan result was further verified by heating analysis and microscopic studies. It was also revealed for the PdOx mask that the thermal-induced IDTR responses came not only from the chemical change but also from the physical change. Compared with a PtO2 mask with the same structure as the PdOx, the PdOx-type disk has slightly lower CNR and readout stability than does the PtO2-type disk.
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