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We report on the realization of solid immersion lens (SIL)-based near-field (NF) optics with an annular aperture, which is targeted to achieve high optical resolution. A numerical aperture (NA) = 1.84 hemisphere SIL-optics with an annular aperture achieves higher optical resolution than the conventional NA = 2.0 SIL-optics. The designed aperture is fabricated by photo-lithography and dry-etching technique. Experimental verification of the designed optics was performed through beam spot profile measurement under NF imaging conditions. A 15% smaller full-width-at-half-maximum spot diameter is obtained by the aperture. We verified that this method gives an improvement of the resolution in the optical imaging systems requiring higher resolution.
©2010 Optical Society of America
1. Introduction
In optical imaging systems, such as microscopes, exposure systems with a high resolution, and optical storage devices, higher optical resolution is requisite. As one of the promising technologies that is able to satisfy this requirement with relatively simple construction and reliable performance, solid immersion lens (SIL)-based near-field (NF) optical systems with an actuated servo have been extensively researched [1–5]. Consistently pursued technical issues include achieving higher numerical aperture (NA) and increasing the number of recording layers, especially in the field of optical data storage solutions to enhance data storage capacity [2–4]. Although there have been excellent studies on exceeding a NA = 2.0 through aplanatic imaging in a high refractive index super-hemispherical SIL or hemispherical SIL [2–6], there is a technical limitation that applies to multilayered recording by the achievable NA, which arises from the refractive index of a recording medium or a measurement sample that is placed just beneath a SIL. This is because it is impossible to create a micrometer-thick cover- or spacer-layer from a single optical material whose refractive index is as high as 2.0 that also possesses a high transmittance (over 80%) and spreadable fabrication characteristics. Currently, T. Ishimoto et al. reported that the NA of the optics inside the recording medium can reach up to 1.83 by generating a cover-layer with a composite material that is composed of nano-particles with an extremely high refractive index, and a binder material with a lower refractive index that induces nano-particle binding [7]. Through this study, they reported fairly excellent recording and readout results. However, better optical resolution is strongly required to achieve a smaller beam spot inside a recording medium or a measurement sample.
To obtain higher optical resolution and to improve the NF optical characteristics in optics with an extremely high NA, there has been much research on the modulation of amplitude and phase in the entrance pupil of the SIL optics through the use of a discrete annular zone plate. H. Wang et al. showed surprising focal depth improvement in free space through the optimization of phase modulation of the annular aperture with the circularly- and the radially-polarized illumination [8–10]. Even though these studies showed a decrease in the focused spot size and a greatly extended focal depth, they only showed improvement in free space; the optical resolution was considerably lower than that in the SIL-based NF optics with a NA = 1.4. S. F. Pereira et al. introduced an analysis method of achieving high resolution by means of polarization, phase, and amplitude pupil masks [11]. C. Liu et al. and Y. J. Zhang et al. presented a physical model of an optimized annular aperture with a SIL [12,13]. However, they studied only the electric-field characteristics on the bottom surface of the SIL and did not consider the effects of the air-gap distance or the measurement sample. In addition, previous studies have not considered the case of a multilayer structure. Although higher resolution and a longer focal depth were achieved, there was also high side-lobe intensity. For optics exceeding that are composed of a multilayered medium structure with a NA > 1.5, our research group recently demonstrated the possibility that an annular aperture improves the optical characteristics in a SIL-based NF recording system with a NA = 1.8. Although the analytical work of our research group is proven over an annular aperture with amplitude and phase modulation for the following two cases: NA = 1.8 without a cover-layer and NA = 1.45 with a cover-layer, to the best of our knowledge, there has not been any experimental research reported for the case of high NA optics [14–16].
Thus, through this research, we suggest an optical configuration that can achieve a higher optical resolution than the optics with a NA = 2.0. We also verify the theoretical results of the optics with the high NA of 1.84 by performing an experiment. First, we are going to report on the design of the annular aperture that performs amplitude and phase modulation in the entrance pupil plane. This design yields a higher resolution and better optical characteristics in NA = 1.84 SIL-based NF optics whose recording media is composed of stacked layers that includes 2μm-thick cover-layer. Then we analyze the electric-field structures in a focal region inside of multilayer media by using the vectorial diffraction integral formula, which includes a finite number of annular aperture regions, to investigate the performance of the designed annular aperture. Finally, using the directely measured the spot profile results, we will verify that the designed annular aperture improves the optical performance by comparing it to the conventional NA = 1.84 SIL-based NF optics without the annular aperture.
2. NA = 1.84 near-field optics with an annular aperture
We consider the cover-layer protected NA = 1.84 SIL-based NF optical configuration that enables imaging inside the media. Therefore, this study is focused on improving the optical performance of conventional NA = 1.84 SIL-based NF optics by amplitude and phase modulation with an annular aperture. The conventional NA = 1.84 SIL-based NF optics is composed of a NA = 0.77 pre-focusing lens, a SIL with a high refractive index of 2.38, and a cover-layer protected inside recording media. A detailed description of the structure of the media is given in Ref. 17. Because the NA of the optics is much greater than unity, the electric-field distribution near the focal plane of the optics, generally inside the GeSbTe (GST) layer in the rightmost part of Fig. 1 , should be investigated using the full vectorial electromagnetic field behavior that considers the diffraction characteristics of high NA optics and multiple beam interference inside the multilayer media. For this reason, precise calculation of the electric-field near the focal plane of the optics was essential to determine the amount of modulation on the amplitude and phase in the entrance pupil. In this study, we used the vectorial diffraction integral formula following the theoretical basis described in Refs. 18 and 19 to examine the electric-field distributions near the focal plane inside the media. Figure 1 shows the diagram of the designed annular aperture and the detailed optical configuration in which the modulating element is placed in the incident part of collimated light. The annular aperture has three annular zones that each yield a different binary transmittance and phase retardation to enhance the focused spot characteristics. The reason why phase and amplitude modulation are applied simultaneously in the annular aperture is that it is impossible to reduce both the spot size and side-lobe intensity simultaneously using only a pure amplitude modulation aperture or only a pure phase modulation aperture. The final annular aperture design consists of zone 1 (0 ≤ NA ≤ 0.6), zone 2 (0.6 < NA ≤ 1.0) without a phase retardation and zone 3 (1.0 < NA ≤ 1.84) with a phase retardation of up to π/2. The details of the design procedure and the physical meaning of the design factors are described by Y. J. Yoon et al., in Ref. 15.
Fig. 1 Designed annular aperture for the NA = 1.84 SIL-based NF optics with the cover-layer protected multilayer media. The aperture works the binary transmittance and phase retardation of the incident beam in front of the entrance pupil. The multilayer media is assumed to have a cover-layer of refractive index 2.0, and the upper limit of an effective NA is set to 2.0.
Figure 2 shows the radial intensity profiles at the focal region and the beam behaviors along the optical axis through the media with λ/8 air-gap distance, where λ represents the optical wavelength. We calculated the normalized radial intensity at the middle of recording layer ( = GST layer) for three cases, which are compared in Fig. 2(a): the first case is the conventional NA = 1.84 NF optics, the second case is the NA = 1.84 NF optics with an annular aperture, and the third case is the optics with NA = 2.0 without an annular pupil modulation. As summarized in Table 1 , applying the annular aperture to the conventional NA = 1.84 optics, the full-width-at-half-maximum (FWHM) spot size of the focused spot has been reduced from 0.292λ by the conventional NA = 1.84 optics to 0.271λ. Note that the designed optics shows better optical performance than the third case of the NF optics with NA = 2.0; the FWHM spot diameter is 0.285λ.
Fig. 2 Focused beam behaviors in the focal region given a λ/8 air-gap distance for the NA = 1.84 near-field optics in the cases with and without and annular aperture and the NA = 2.0 near-field optics in the case without an annular aperture. (a) Spot profiles along the radial axis, (b) FWHM spot radius of the normalized radial intensity behaviors, and (c) normalized intensity distributions along the optical axis.
Table 1. Optical characteristic comparison between the cases with and without an annular aperture in multilayer media
This fact implies that it is feasible to achieve a higher optical resolution than those of NA = 2.0 NF optics, using only NA = 1.84 optics. Moreover, through Figs. 2(b) and 2(c) we can see the focused beam behaviors of the NA = 1.84 optics along the optical axis. Figure 2(b) compares the normalized FWHM spot radius distributions, which calculated at each position of the propagation axis, between the case with and without an annular aperture, and we can know that those are identical results of the Fig. 2(a). However, the radius behaviors describe the step-like changes. This is because the field distributions inside the media which satisfying the boundary condition have a state of the continuous and the discontinuous at the medium transition. Among the field component, the longitudinal component plays a dominant role to generate the discontinuity at each interface of the materials [18,19]. Figure 2(c) shows the penetrated focused beam toward the optical axis; the annular aperture case maintains a beam intensity of about 10% higher than the case without an aperture. From this result showing the effect of an annular aperture, we can consider the following example case. In an optical data storage system, if the position of the focused beam relative to the optical axis strays due to any uncertainties, it could degrade the performance of reading and writing. However, if the intensity of the beam is high within the acceptable spot resolution, a margin for the focusing stability relating to the data quality gets larger. Note that the zero point on the optical axis in Figs. 2(b) and 2(c) represents the position of minimum wavefront aberration of the focused beam. These calculated results are meaningful in that an annular aperture technique is used to improve optical resolution.
3. Fabrication of the designed annular aperture and experimental verification
In the previous section, we showed an improvement of the optical characteristics by introducing an optimized annular aperture based upon a numerical calculation of the diffraction integral. This section describes the fabrication of the annular aperture and experimental verification of the enhanced optical characteristics of the designed optics.
Figure 3 shows an image of the fabricated annular aperture. After the shape of the aperture is generated by photo-lithography and dry-etching processes on a glass substrate, which has a transmittance higher than 98% and a refractive index of 1.52, a nickel (Ni) thin film is deposited on the substrate with a photo-resist (PR) pattern. Finally, fabrication of the designed annular aperture is completed after the PR pattern and the unnecessary part of nickel film is lifted off by wet-etching. For a phase modulation of π/2, the glass substrate is etched by 195nm.
Fig. 3 Fabricated annular aperture arrays and main annular aperture. Transmittance modulation annular aperture with zone 2 = 1.30mm (NA = 1.0) and zone 1 = 0.78mm (NA = 0.6). Annular aperture arrays are fabricated by a photolithography process; nickel is then deposited onto the glass substrate.
We encountered practical difficulties when attempting to carry out the experiment under conditions similar to use with the multilayer media. It is still very difficult to obtain a 2μm-thick cover-layer protected media with a high refractive index of 2.0. Moreover, to observe the focused beam spot profile inside the media, an exact pair structure of the medium has to be prepared. For this reason, we decided to investigate the beam spot profile on the tip surface of the SIL, and to use the SIL optical head, which was designed and assembled for focusing on the bottom surface of the SIL, i.e. surface-focusing-type SIL optical head. In this case, it is straightforward to observe the beam spot profile by contacting in reverse the conjugate of the SIL to the tip of the designed SIL optical head.
Figure 4 shows the experiment set-up to measure the beam spot profile, which is constructed based on the Twyman-Green interferometer [20]. In the measurement set-up, we used a beam profile analyzer (Spiricon, LBA-500). To observe the focused beam spot profile, which has NA > 1, the conjugate SIL optical head is required because it is otherwise impossible to observe the evanescent wave due to the total internal reflection occurring at the critical angle. Based on the set-up composed of the annular aperture and the NA = 1.84 SIL-based NF optics, we were able to analyze the beam spot profile as depicted in Fig. 4(b). The optical axis between the SIL optical head and annular aperture is aligned by observing the images on the CCD1, where the images change due to the alignment condition between the SIL optical head and the annular aperture. Thus, we know that the CCD1 plays a role to confirm the optical alignment between the Twyman-Green interferometer and the first SIL optical head. When the first SIL optical head, as shown in Fig. 4(a1), is properly aligned with the interferometer, we can observe a straight fringe in the CCD1. At this time, the reference mirror is used to generate undistorted reference beam and phase shift through the use of a highly accurate nano-stage (PI, P-517.2CL). Using CCD1, both near-field and far-field pupil images are observed. In addition, the straight fringe in the CCD1 can only be obtained when the beams focuses on the bottom surface of the SIL. If the beam is not focused on the bottom surface of the SIL, a spherical fringe due to spherical aberration is generated in the CCD1. After we confirmed optical alignment of the first SIL optical head, the second SIL optical head was aligned to measure the beam spot profile on the CCD2, as shown in Fig. 4(a2). The optical axis between the first and the second SIL optical heads is aligned by monitoring CCD1. The image changes according to the contact condition between the two SIL optical heads, as shown in Fig. 4(b). The second SIL optical head and CCD2 are only used to measure the profile of the focused beam at the boundary between the first and the second SIL optical head. Note that, as we introduced in this paper, we applied two identical SIL optical heads to measure the beam spot profile of the focused beam on the bottom surface of the SIL. In our research, we assume that the SIL is perfectly fabricated without any thickness error. Moreover, there is no additional occurrence of spherical aberration due to the index mismatch between the SIL and air, when the two SIL optical heads are in contact. Because the conjugate SIL optical heads, having identical refractive indices and optical characteristics, were assembled and evaluated to satisfy the Maréchal’s criterion [21], and the contact condition between the two SILs was checked. Thus, by using this conjugate SIL optical head, it is possible to observe the beam focused on the surface of the SIL at the CCD2. Note that, to obtain the exactly same beam profile with the calculated one, the zoom lens including CCD2 should be positioned to observe exactly bottom tip of the SIL.
Fig. 4 Schematic diagram and experiment set-up to observe the focused beam spot profile. The reflected exit pupil images corresponding to the distance between two SIL optical head are presented, and the optical alignment and the contact condition of the conjugate SIL optical head can be confirmed.
Because the experiment conditions were different from those of the simulations, we performed a numerical calculation on a beam spot profile identical to that of experimental optical configuration to confirm the results of experiment.
The calculation results based on the vector field theory are plotted in Fig. 5 . It demonstrates that the FWHM spot size is reduced from 0.290λ to 0.272λ. From this result, it can be confirmed that the annular aperture applied to the surface-type SIL optical system also plays a role to improve the resolution in a manner similar to the case of the media inside the imaging configuration depicted in Fig. 1. However, the side-lobe intensity is increased by the use of an annular aperture, which is different from the result of Fig. 2(a) that depicts an inside focusing SIL optical configuration with the cover-layer protected multilayer media. This is because the model applied in both the experiment and the simulation is for the case of focusing onto the bottom surface of the SIL. Therefore, we know that the effect of phase modulation, which reduces the side-lobe intensity, is less effective when focusing directly onto the bottom surface of the SIL. We can deduce that there are some physical phenomena inside the media associated with the phase modulation, such as multiple transmission and reflection at the interfaces of between each layers. Therefore, we intended to compare the focused beam spot size between the experiment and the simulation. By following the above mentioned experiment procedures, we measured the focused beam intensity distributions as depicted in Fig. 6 .
Fig. 5 Calculation results in the cases without and with an annular aperture. The simulation model is the surface-focusing near-field optics which is the same configuration as the experiment.
Fig. 6 Results of the focused beam spot profile on the bottom surface of the SIL optical head without and with the annular aperture. The results of the calculation and the experiment are compared: the dashed line and the solid line show the measurement results and the calculation results, respectively.
We confirmed that the FWHM beam spot size was improved from 0.324λ to 0.280λ. Table 2 compares the experiment results with the calculated values. From Fig. 6, it can be seen that the shapes of the beam spot profiles from the experiments are almost the same as those of the simulations. However, the FWHM spot sizes and the side-lobe intensities of the experiment results are slightly different from the simulation values. We think that these differences may come from the experimental factors such as imperfections in contacting a pair of SILs, slight error in optical alignment, focusing adjustment error, etc. In other words, it is required to minimize effect of those experimental factors. In our experiments, among the various experimental factors, we considerd two major experimental factors which can give effect on the shape and amount of side-lobe of the focused beam profile. Those are unwanted NF air-gap which is induced by the imperfect contact between two SILs and defocus produced by position of the zoom lens including the CCD2 which positioned at the back of the conjugate SIL optical head. To minimize occurrence of these experimental errors, we observed and checked the reflected exit pupil images to ensure contact between the SIL optical head and the conjugated SIL optical head and tried to locate the zoom lens on the best position. However, it was really hard to ensure perfect optical contact between two SILs and exact position of the zoom lens in the static optical configuration without any dynamic NF air-gap servo. Although there is a little deviation from the calculated values, both the experiment and the simulation results show certainly the improvement of the optical resolution. Thus we think that the designed annular aperture plays a key role in improving the optical resolution and overall shape of the beam spot profile.
Table 2. Optical resolution comparison of simulation and experiment results between the cases with and without an annular aperture
4. Conclusion
In this study, we have realized an optical configuration that exceeds the resolution of present SIL-based NF optics by using amplitude- and phase-modulating annular aperture. We designed an annular aperture consisting of three concentric regions based upon numerical calculations of the electric-field distributions inside of multilayer media. The designed SIL-based NF optics with the annular aperture had a NA = 1.84 and showed improved optical performance compared with the conventional SIL-based NF optics, whose NA = 2.0. The improved optical performance was verified by an experiment results that were in very good agreement with the numerical calculations. However, the side-lobe intensity was as much as 7% too high in the calculated results to apply this technique to high precision optical systems, such as optical data storage systems or exposure systems, under present practical conditions. Moreover, in the surface-type SIL optical configuration, the effect of phase modulation is relatively low compared with the case of inside-type SIL optics.
Consequently, it is possible to improve the optical spot resolution, which is much higher than the conventional SIL-based NF optics, by applying a properly designed annular aperture to SIL-based NF optics. To achieve more enhanced optical characteristics, the side-lobe intensity needs to be reduced in the high NA optical system. We hope that research that considers the physical interactions associated with amplitude and phase modulation of the beam will proceed. Therefore, the designed SIL-based NF optics with an annular aperture has much potential for applications in high resolution optical imaging systems.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010-0000769).
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