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Selected Papers:	Growth Technologies and Device Applications of Electro-optic Crystal KTa_1−xNb_xO₃

KTaO₃ Solid Immersion Lens for Near-field Optical Disk System

Kazuo Fujiura^† and Masahiro Sasaura

Abstract

We have developed KTaO₃ single crystal growth and solid immersion lens (SIL) fabrication technologies for a near-field optical disk system. KTaO₃ has a refractive index of 2.381 at 405 nm, which makes it possible to make an SIL with a high numerical aperture. We fabricated a KTaO₃ SIL with an anti-reflection coating on its surface. High-density near-field optical readout of a 150-GB disk (104.3 Gbit/in²) has been successfully demonstrated by Sony Corporation using an SIL made of our KTaO₃ single crystal.

†	NTT Photonics Laboratories Atsugi-shi, 243-0198 Japan Email: fujiura@aecl.ntt.co.jp

1. Introduction

Several optical data storage technologies have been proposed with a view to making a next-generation optical disk system for applications related to high-definition television. Of these, near-field optical disk technology with a capacity of over 50 GB has already been reported [1], [2]. To increase the capacity further, we must develop a solid immersion lens (SIL) with a higher numerical aperture (NA). This requires a material with a higher refractive index. An SIL has a higher numerical aperture than common lenses because the object space^*1 is filled with a high-refractive-index solid material and its spot size is λ/n sinθ. By using an SIL, we can achieve a spatial resolution better than the diffraction limit in air. Up to now, high-refractive-index glasses, s-LAH79 and LaSF35, and a Bi₄Ge₃O₁₂ single crystal have been investigated as materials for an SIL. These materials have relatively high refractive indices compared with conventional optical glasses, but they are lower than 2.25. In the case of a super-hemisphere^*2 SIL, the disk capacity is quadratically proportional to the NA of the SIL. Therefore, a material having a higher refractive index is better for the next-generation optical disk system. A synthesized diamond has been used for the SIL and 190-GB disk readout has been demonstrated [3]. Diamond has an extremely high refractive index, but its hardness makes it difficult to fabricate a high-quality SIL in terms of sphericity. In this paper, we describe the development of KTaO₃ single crystal with a refractive index of over 2.38 and a KTaO₃ SIL with practical optical quality.

*1	Object space: the space within which an object can be imaged by the lens.
*2	Super-hemisphere: a hemispherical shape that subtends an angle of more than 180°.

2. KTaO₃ single crystal

Disk capacity is quadratically proportional to the NA of the SIL, as shown in Fig. 1. Single-crystalline materials are required for the SIL because the refractive indices of glass materials are insufficient for a disk capacity of over 100 GB. In addition, an SIL requires extremely high optical homogeneity, so the crystals should exhibit no birefringence.

Fig. 1. Capacity of disk at 405 nm
(compared with the lens used for a 25-GB Blu-ray disk player with NA = 0.85).

We have developed the KTaO₃ single crystal and its growth technology to meet the above demands, thus enabling us to produce a high-quality lens material applicable to SILs. The refractive index of KTaO₃ crystal in the visible region is shown in Fig. 2. The absorption cutoff wavelength is around 365 nm and KTaO₃ has relatively high refractive index dispersion. The refractive index is 2.381 at a reading/writing laser wavelength of 405 nm. This is higher than that of high-refractive-index glasses such as s-LAH78 and LaSF35, but slightly smaller than that of diamond. We also confirmed that the fluctuation in the birefringence in the KTaO₃ crystal is sufficiently small to meet the SIL specifications.

Fig. 2. Refractive index of KTaO₃ crystal in the visible region.

In addition to these optical properties, we require excellent mechanical properties. Even though KTaO₃ is a single crystal, it is resistant to cleavage. This means that the chipping does not occur during polishing. Moreover, the hardness of KTaO₃ is almost isotropic. Therefore, we can treat this crystal just like glass, and of course we can use the same polishing techniques as those used for optical glasses. This enables us to undertake commercial production without the development of any additional polishing technology.

3. SIL fabrication process

The SIL fabrication process is shown in Fig. 3. First, we grow a KTaO₃ single crystal by using the top seeded solution growth technique. Then, KTaO₃ crystal is cut into plates and both surfaces are polished to allow us to evaluate the optical quality. After eliminating any defects, we cut the plates into cubes and then fabricate ball lenses by polishing. This process is exactly the same as that used for glass ball lenses. We then fabricate SILs with a cone shape. The cone shape is designed to accommodate tilt between the SIL and the disk surface. Finally, the lens surfaces are covered with an anti-reflection coating.

Fig. 3. KTaO₃-SIL fabrication processes.

3.1 Quality evaluation of KTaO₃ crystal

During the above process, we eliminated regions with refractive index fluctuation caused by internal stress during the crystal fabrication by using a polarized microscope and a birefringence apparatus. After cutting the KTaO₃ crystal into plates and polishing both surfaces, we measured the birefringence distribution in the plates in a 1-mm2 area. The criterion for practical use was defined as having birefringence of less than 1 × 10⁻⁶ for a thickness of 1 mm [4].

3.2 KTaO₃ ball lens fabrication

Ball lenses obtained by polishing the prepared KTaO₃ crystal are shown in Fig. 4. Since KTaO₃ has excellent machinability, we can use the well-established conventional glass ball lens fabrication technique. This enables us to obtain excellent sphericity and surface roughness. In terms of dimensional accuracy, we have obtained sphericity of better than 0.05 µm (a deviation from the target diameter of less than 0.05 µm) and a surface roughness Ra of less than 0.2 µm for a KTaO₃ ball with a diameter of 1 mm. These accuracies are better than those of commercial ball lenses.

Fig. 4. KTaO₃ ball lenses (diameter: 1 mm).

3.3 KTaO₃-SIL fabrication

After fabricating a KTaO₃ ball lens, we polish one side of it to obtain a super-hemisphere or hemisphere with a thickness accuracy of better than 0.1 µm. Then we form a conical shape by polishing. The hemispherical KTaO₃ lenses are individually polished into a cone shape. An SIL that we fabricated and the dimensional shape measured with an interferometer (Zygo Corporation) are shown in Figs. 5 and 6, respectively. In this case, the top radius facing the optical disk surface was 25 µm. There was no chipping around the edge. The decentering accuracy was better than 5 µm.

Fig. 5. KTaO₃ solid immersion lens.

Fig. 6. Dimensional shape of KTaO₃-SIL measured with the Zygo interferometer.

Finally, we prepare an anti-reflection coating on the SIL surface. We can deposit both an SiO₂ single layer and a dielectric multilayer with good adhesion and achieve a homogenous thickness in the region where in angle of incidence is more than 60° from normal incidence. Simulation results for the wavelength-dependence of the reflectivity for various incidence angles are shown in Fig. 7. In this simulation, the wavelength was 405 nm and the coating was an SiO₂ single layer. The results reveal that reflectivity of less than 5% can be achieved by using an SiO₂ single-layer coating.

Fig. 7. Simulation results for wavelength-dependence of the reflectivity for various incident angles.

4. Near-field readout experiment

The NA of the KTaO₃-SIL was designed to be 2.2. A near-field readout experiment with a 150-GB optical disk, corresponding to a density of 104.3 Gbit/in², was performed by Sony Corporation [4]. The results indicate that our KTaO₃ crystal has sufficient optical quality for use as the SIL of a near-field optical disk system.

5. Conclusion

We developed KTaO₃ single crystal and a solid immersion lens made of it for a near-field optical disk system. KTaO₃ has a high refractive index at a reading/writing laser wavelength of 405 nm. We prepared an SIL with practical optical quality, which was confirmed by near-field readout experiments with a 150-GB disk. We think that the KTaO₃-SIL is a promising component for a next-generation optical disk system.

References

[1]	M. Shinoda, K. Saito, T. Kondo, T. Shimoto, and A. Nakaoki, “High-Density Near-Field Readout over 50 GB Capacity Using Solid Immersion Lens with High Refractive Index,” Jpn. J. Appl. Phys., Vol. 42, pp. 1101–1104, 2003.
[2]	F. Zijp, J. M.A. van den Eerenbeemd, P. Urbach, and C. A. Verschuren, “Effects of Polarization on Wave Front Measurements and Manufacturing of Solid Immersion Lenses for Near-Field Optical Recording,” Jpn. J. Appl. Phys., Vol. 45, No. 2B, pp. 1341–1347, 2006.
[3]	M. Shinoda, K. Saito, T. Kondo, A. Nakaoki, M. Furuki, M. Takeda, M. Yamamoto, T. J. Schaich, B. M. van Oerle, H. P. Godfried, P. A. C. Kriele, E. P. Houwman, W. H. M. Nelissen, G. J. Pels, and P. G. M. Spaaij, “High-Density Near-Field Readout Using Diamond Solid Immersion Lens,” Jpn. J. Appl. Phys., Vol. 45, No. 2B, pp. 1311–1313, 2006.
[4]	M. Shinoda, K. Ssaito, T. Kondo, M. Furuki, M. Takeda, A. Nakaoki, M. Sasaura, and K. Fujiura, “High-Density Near-Field Readout Using Solid Immersion Lens Made of KTaO₃ Monocrystal,” Jpn. J. Appl. Phys., Vol. 45, No. 2B, pp. 1332–1335, 2006.

Kazuo Fujiura: Senior Research Engineer, Supervisor, Advanced Opto-electronics Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees in applied chemistry from Kyushu University, Fukuoka, in 1983 and 1985, respectively. Since joining NTT Laboratories in 1985, he has been engaged in research on fluoride glass fiber fabrication technology. In 1993, he received the Ph.D. degree from Kyushu University for his work entitled “Studies on Synthesis and Properties of ZrF₄-Based Fluoride Glasses by Chemical Vapor Deposition.” From 1996 to 1997, he was a visiting scholar at Stanford University. Since 2001, he has been engaged in research on KTN optical devices. He received the 49th Award of the Ceramic Society of Japan and the Joint Ceramics Award of the Japanese and Australian Ceramics Societies. He is a member of the Institute of Electronics, Information and Communication Engineers of Japan, the Japan Society of Applied Physics, the Ceramics Society of Japan, the Chemical Society of Japan, and the Materials Research Society.

Masahiro Sasaura: Senior Research Engineer, Advanced Opto-electronics Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in applied physics and crystalline materials science from Nagoya University, Aichi, in 1985 and 1987, respectively. He joined NTT Atsugi Electrical Communication Laboratories in 1987. Since then, he has been engaged in the development of crystal growth methods. The grown crystals include Si, GaAs, high-Tc superconductor, gallates (oxides) for high-Tc superconductor substrates, and niobates (oxides) for electro-optic devices.

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