perylene derivative

Applied Surface Science 197±198 (2002) 808±813

Uniform laser ablation via photovoltaic effect of phthalocyanine/perylene derivative Keiji Nagai*, Hidetsugu Yoshida, Takayoshi Norimatsu, Noriaki Miyanaga, Yasukazu Izawa, Tatsuhiko Yamanaka Institute of Laser Engineering (ILE), Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan

Abstract A uniform laser ablation was observed in a polystyrene ®lm coated with a photovoltaic perylene/phthalocyanine bilayer when an incident took place at an intensity range of 109±1010 W/cm2 (l ˆ 1064 nm, 1.1 ns FWHM). Without the bilayer coating, the laser pulse formed spiky structures in the polystyrene ®lm as self-focusing traces of the laser pulse, while for the coated ®lm, the uniform surface ablation trace without the spiky interior structures was observed. In the case of incident of 532 nm where the coating material exhibits no re¯ection, such difference depending the coating was not observed. These phenomena were coupled with the re¯ection and conduction properties via its photovoltaic effect, and agreed with the required ablation to achieve high-density compression of the fuel capsule for inertial fusion energy (IFE). # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Photovoltaic materials; Inertial fusion energy (IFE); Phthalocyanine; Perylene; Polystyrene

1. Introduction During the investigation of inertial fusion energy (IFE) [1], an ef®cient and symmetric implosion of poly(deuterated styrene) up to 600 times its solid density was demonstrated by irradiation using a high-power (1015 W/cm2) Nd:glass laser, Gekko XII, on a highly spherical capsule with uniform thickness [2]. The fast ignition scheme, a further step in the IFE research, requires compression of the deuterium± tritium fuel at least 2000 times its solid density [3]. A more spherical capsule fabrication with a more uniform thickness was used to investigate this high gain compression objective [4±6].

*

Corresponding author. Tel.: ‡81-6-6879-8778; fax: ‡81-6-6877-4799. E-mail address: [email protected] (K. Nagai).

A severe limitation of the compression ef®ciency is fuel capsule disruption due to hydrodynamic instabilities, which are initiated during the early start-up phase of the implosion as an initial imprint and laser-shine-through, owing to the initial surface perturbation, the areal density perturbation, and momentum perturbation near the outer surface [7±9]. In order to reduce the ®lamentary sparks, controlled ablation has been investigated using not only the laser beam smoothing [10] but also target structure [11] and materials. A recent attraction for material science is to develop systems that exhibit changes in their chromic, magnetic, dielectric, and transport properties by photoirradiation [12±17]. Such functional materials will provide us an ef®cient IFE target capsule to achieve the objective [18±20]. We have found a reduction in damage due to the fundamental wave (o) of the Nd:YAG laser (whose wavelength of 1064 nm is almost

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 4 6 4 - 6

K. Nagai et al. / Applied Surface Science 197±198 (2002) 808±813

809

Fig. 1. Chemical formula of H2Pc and PV.

equivalent to that of Nd:glass laser) by coating the organic bilayer of PV/H2Pc (C36H16O4N2/C32H18N8; Fig. 1) [19,20], which is known as a good photovoltaic material [21±23], and exhibits photo-induced re¯ection due to photogenerated PV [24]. In the present paper, a remarkable reduction will be discussed in comparison with the photovoltaic effect of PV/H2Pc layer. 2. Experiment PV was prepared and puri®ed by a method previously reported [25]. H2Pc was purchased from ACROS Inc., and puri®ed by sublimation at 440 8C. PV/H2Pc/polystyrene ®lm was obtained by vapor deposition of H2Pc and PV sequentially, on a 4 mm polystyrene ®lm. The coating thickness is 0.10 or 0.35 mm. The incident laser of o and 2o (second harmonic wave of Nd:YAG laser, 532 nm) were 1.1 and 1.0 ns FWHM Gaussian pulse, and were focused on to plane target with 1/e2 spot size of 750 and 500 mm, respectively. The ablation traces were observed using a microscope, and their depth was measured using an interferometer (Wyco). 3. Results and discussion The laser ablation of polystyrene by 248 nm has been characterized to be a photothermal process in agreement with absorption coef®cient of polystyrene at 248 nm [26]. In the case of 1064 nm (o) and 532 nm

(2o), their absorption are so weak that another mechanism is expected. Fig. 2 shows the ablation traces after the o laser irradiated, where an incident laser pulse with a peak intensity of 1:4  1010 W/cm2 induced spiky damage inside a 4 mm polystyrene ®lm thickness, while for less than 1:2  1010 W/cm2, no damage was observed. A similar damage pattern and a damage threshold are well known in high-power laser optical materials, such as potassium dihydrogenphosphate and silica glasses, in which the damage threshold values are 1  1010 to 3  1011 W/cm2 [27]. This damage inside the laser optical materials have been characterized to be traces of self-focusing of the laser beam, where both the nonlinear growth of small scale beam break-up and the whole-beam self-focusing alter the focusing properties of the system. Actually, at a crack point in a polystyrene ®lm with a high distortion, sharp spikes were observed in an area irradiated with a ¯uence below the ablation threshold intensity (1:2  1010 W/cm2; Fig. 2b), suggesting the damage generation mechanism due to the self-focusing. The damage observed as the ablation traces is closely related to the initial imprint of the hydrodynamic instability and should be avoided in order to obtain a uniform implosion. The ablated area increased with an increase in the laser intensity. Above 2  1010 W/ cm2, a round trace appeared as shown in Fig. 2c, where its border was sharp suggesting that the round shape ablation was due to integration of the spiky traces. The re¯ection was observed under white-light irradiation as shown in Fig. 3. In the visible region (750 nm), each sample exhibited decrease of the

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K. Nagai et al. / Applied Surface Science 197±198 (2002) 808±813

Fig. 2. Laser ablation trace on polystyrene ®lm (4 mm thickness) in the absence of coating for 1064 nm input beam with a 1.1 ns FWHM pulse and a 1/e2 beam diameter of 750 mm. The peak intensites were 1:4  1010 W/cm2 (a), 1:0  1010 W/cm2 (b), and 3:2  1010 W/cm2 (c).

re¯ection with valleys which are the same as the normal re¯ection for the monochromic light incident. The strong peak of PV at 824 nm should be the PV ¯uorescence [28], while the re¯ection of the PV/H2Pc

Fig. 3. White-light-induced visible-near-infrared re¯ection spectra of (a) H2Pc, (b) PV, and (c) PV/H2Pc layers. The thickness of each layer is 0.10 mm. The spectra were measured using a ¯uorescence spectrophotometer equipped with a halogen lamp as the white-light source. A standard aluminum plate was used as the reference sample to calculate the re¯ectance values. The light from halogen lamp was guided to the sample (80 mW/cm2), and the re¯ection from the sample was through a monochrometer and monitored by a photomultiplier or a Ge bolometer.

bilayer did not show the peak corresponding that the exciton in bulk PV was ef®ciently quenched by photogenerated PV at the interface of PV/H2Pc [24]. In the wavelength in near-infrared region, the p-type semiconducting H2Pc showed little re¯ection, while the single layer of n-type semiconducting PV exhibits broad re¯ection in the near-infrared region. The re¯ectance of PV layer was enhanced by the double-layering to PV/H2Pc in which photogenerated PV is strongly coupled to photovoltaic property. The carrier electron in PV layer should allow absorption of laser light due to inversed bremsstrahlung. An ablation trace on a 4 mm thick polystyrene ®lm coated with a PV/H2Pc (0.35/0.35 mm) bilayer is shown in Fig. 4. For the peak intensity of 1:0  1010 W/cm2, although the coated layers were ablated, the polystyrene ®lm was completely protected from such damage (Fig. 4a). In the absence of bilayer, the polystyrene ®lm did not ablate in the low intensity region. Therefore, the coated layer exhibited just lower ablation threshold for the ablation than the polystyrene ®lm. For the intensity up to 2:0  1010 W/cm2, the ablation diameter of the upstream bilayer was twice as

K. Nagai et al. / Applied Surface Science 197±198 (2002) 808±813

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Fig. 4. Laser ablation trace on polystyrene ®lm (4 mm) coated with PV/H2Pc layers for 1064 nm laser of 1:0  1010 W/cm2 (a), and 2:0  1010 W/cm2 (b).

large as that for 1:0  1010 W/cm2 (Fig. 4b). By considering the Gaussian beam diameter of 750 mm (1/e2), the incident intensities at the border of the ablation are estimated to be so low (3  106 W/cm2 for 1:2  1010 W/cm2 peak intensity and 3  10 4 W/ cm2 for 2:0  1010 W/cm2 peak intensity) that direct laser ablation can be negligible. These wide ablation areas for the bilayer suggest effectively fast electron conduction in the lateral direction due to photoelectron conducting property. The lateral fast conduction is promising for the implosion scenario of the IFE, from the viewpoint of uniform ablation pressure generation, and the ablation from the outer surface. On the other hand, the ablation for the inward direction to the polystyrene ®lm was reduced. Unlike the trace of the non-coated polystyrene (1:4  1010 W/ cm2; Fig. 2a), the trace of the coated polystyrene did not exhibit spiky damage and sharp edge, and a ¯at dimple with a 1.4 mm depth was formed on the front

surface (2:0  1010 W/cm2; Fig. 4b). These ablation behaviors up to 4  1010 W/cm2 indicated the protection by the coating bilayer against the laser-shinethrough into the polystyrene ®lm. With an incident of more than 1:0  1010 W/cm2, the coated layer was completely ablated, but affected the ablation of the polystyrene to protect. To suppress the shine-through into the ®lm, high ef®cient re¯ection and/or absorption should exist as a result of remaining bilayer such as a plume. The analysis of the intensity versus the etch depth will provide further information about the ablation mechanism. Fig. 5 shows the relation between the etching depth and the intensity. Above the threshold ablation of the coating layers (1:0  1010 W/cm2), the etching depth of the polystyrene was simply increased with the high incident intensity. The slope in the relation means a constant ef®ciency from laser incident to ablation. By supposing that the absorption (a(F Fth )) is converted to the thermal ablation of

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K. Nagai et al. / Applied Surface Science 197±198 (2002) 808±813

Fig. 5. The ablation depth of the polystyrene versus the peak intensity of 1064-nm wavelength laser for the coated polystyrene ®lm. The zero thickness means the interface of the polystyrene and the coating layer. The coating layer was (A) PV/H2Pc (*); (B) PV (&); and (C) H2Pc (^). Each coating layer was 0.35 mm thick.

polystyrene (104DgC(Tth Ta )) above the threshold (Fth) by an ef®ciency (f), the following equation is possible. a…F

Fth †f ˆ 104 DgC…Tth

Ta †;

(1)

where a, F, Fth, f, and Ta are the absorption coef®cient, the laser ¯uence (J/cm2), its threshold starting thermal ablation of polystyrene, the ef®ciency from the absorption to the thermal ablation, and the room temperature (20 8C), respectively. The values of D, g, C, and Tth concern the polystyrene as the etching depth (mm), the density (1.05 g/cm3), the speci®c heat (1.21 J/(K g)), and thermal decomposing temperature (370 8C), respectively [26]. The F value is the product of the peak intensity (I) and the pulse length to be F ˆ I=…2  1:1  10 9 † (J/cm2). The Fth adopted here corresponds to the complete ablation of the coating materials H2Pc and PV (1:0  1010 W=cm2  1:1 ns). By substituting the constants, Eq. (1) was rewritten as af ˆ 8:1  107 D=I:

(0.1 mW/cm2), the ordering of the re¯ectivity is the same as the af values to be PV=H2 Pc > PV > H2 Pc (Fig. 3). If the coating layers were fully ionized, similar re¯ectivity due to similar solid density and the average atomic number would be observed. The fact depending on the materials shows that the different plume character of H2Pc and PV in terms of af value, implying the incomplete decomposition of the molecules and therefore the insuf®cient evolution of plasma. This can be interpreted such that the ablated PV molecules keeps the carrier electrons of the n-type after the evaporation, while p-type H2Pc does not have carrier electrons. The PV/H2Pc does not exhibit re¯ection in the visible region (Fig. 3), and the ablation traces after 2o irradiation had spiky damages either with or without coating (Fig. 6). Especially, the trace in the presence of coating has narrow area (Fig. 6b), while wide trace was observed in the case of o irradiation (Fig. 4). Therefore, the present material cannot be adopted to the updated Gekko XII laser with 2o or 3o light. But the difference of the ablation trace was shown as a model experiment, depending on the laser wavelength coupled with the difference of the re¯ectivity which is tunable due to its photovoltaic effect. In conclusion, a clear difference concerning photovoltaic effect was revealed in laser ablation of polystyrene coated with the organic photovoltaic bilayer of PV/H2Pc, which exhibits a re¯ection in the nearinfrared region. The feature can be attributed to the re¯ection for laser, and conduction due to photovoltaic effect. The property is indicating a functionality of

(2)

From the estimation of the slope in Fig. 5 (D/I) applying Eq. (2), af values are evaluated to be 0.21, 0.14, and 0.03% for (A), (B), and (C), respectively. These ef®ciencies are so low that the shine-throughblock cannot be considered only by absorption due to coating layer. Therefore, the contribution of the re¯ection of the coating layer should be ef®ciently high. In spite of the photon ¯ux revel is completely different

Fig. 6. Laser ablation trace on polystyrene ®lm (4 mm) for 532 nm (2o) laser beam of 2:2  1010 W/cm2 ¯uence in a 1.0 ns pulse, without coating (a), and with PV/H2Pc bilayer coating (b). All the trace contains spiky spots inside the ®lm as a result of damage.

K. Nagai et al. / Applied Surface Science 197±198 (2002) 808±813

organic materials to reduce the initial imprint caused by the laser irradiation and to control the laser implosion with a higher compression.

[9] [10]

Acknowledgements

[11]

The work was supported in part by a Grant-in Aid for Scienti®c Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

[12] [13]

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