In this study, we present a new design for an optical near-field probe with a slot-waveguide structure and evaluate it using a finite-difference time-domain simulation. Our model, with a 50-nm slot core, enables illumination around the tip of the probe using a small optical spot 50–250 nm wide with 20%–30% transmission efficiency. Based on the high-index-contrast structure in a slot waveguide, a nanosized optical spot is easily generated, which is impossible with a normal slab waveguide. Similar properties of optical spot and transmission efficiency are obtained for different geometric configurations of flat-faced and tapered dielectric slot waveguides in illumination mode. The transmission efficiency of our models is the same or higher than that in conventional metallic tapered optical probes. When operating in illumination and collection modes, a near-field light reflected at 50–200-nm-wide measured objects is clearly observed, and a spatial resolution of ~50 nm is obtained. These findings suggest the potential for slot-waveguide structures to expand the versatility of nanosized optical probes.
© 2015 Optical Society of America
Scanning near-field optical microscopy (SNOM) enables observation with a spatial resolution beyond the diffraction limit [1–4]. A tapered optical probe with metallic cladding and an aperture of ~10 nm enables the successful detection of the optical near field around target objects to be measured. However, intrinsic transmission loss is inevitable because the tapered nanosized core is much smaller than the wavelength of light, in addition to the difficulty of nanofabrication. Moreover, in a dielectric waveguide, the fundamental mode, HE11 with no cutoff yields high throughput but poor confinement in a very thin core. Several optical probes, such as single- to triple-tapered structures, have been proposed to enhance the transmission and brightness of an optical spot [4–6]. Instead of a regular circular aperture, a bowtie-shaped aperture was also proposed for enhanced transmission, and it achieved a one-order magnitude improvement in transmission compared with the probes with a regular aperture . In addition, extraordinary optical transmission due to waveguide resonance in a tapered probe with a metallic nano-hole provided more than 100 times higher transmission than conventional SNOM probes . However, the power transmission, given as the ratio of output to total input light power through a probe, still remain large, 10−5–10−2 for a 10–160-nm aperture, because the transmission significantly declines in a relatively long tapered region.
Another approach is a metal-dielectric-metal (MDM) structure that incorporates the surface plasmon and the antenna concept [9–13]. Seamless transition between dielectric slab waveguides and nanosize MDM plasmonic waveguides has been achieved with high power-coupling efficiency of 70%–90% [9,10], and ~70% power transmission is obtained in a plasmon gap waveguide with a combination of tapered dielectric core and metal cladding . These results indicate superior transmission efficiency compared with conventional waveguides, wherein no plasmons are excited. Although a plasmonic waveguide would offer an advantage for a nanosized probe, it needs high precision fabrication techniques for both metal and dielectric materials. Also, relatively-large absorption in metallic coat or cladding has a risk of heat damage to high-input light power .
Recently, Jamid and Almeida proposed a slot-waveguide structure to efficiently confine light power into a very thin core of width typically less than 100 nm [14,15]. In spite of this nanosized core, transverse magnetic (TM) mode light can propagate with low loss along the waveguide axis because of its high-index-contrast structure [16–18]. Moreover, several studies have demonstrated very high power coupling efficiencies of 60%–90% between the slot and normal slab waveguides [19–21]. These results suggest the possibility of resolving the abovementioned issues of large intrinsic transmission loss and low coupling efficiency in the conventional tapered probes used in SNOM. Thus far, several slot-waveguide structures have been reported: photonic nanowire circuits, couplers, amplifiers, and sensors [22–26], which include optical field measurements in a slot waveguide using SNOM . However, to the best of our knowledge, there has been no study on the application of slot waveguides to the optical probe tips of scanning near-field microscopes. The major concern of this study is to provide the possible application of a simple dielectric waveguide without a metal nanostructure.
In the present study, we propose a probe tip comprising a dielectric slot waveguide, and evaluate the transmission efficiency and lateral resolution of the probe tip in both illumination and collection modes using a finite-difference time-domain (FDTD) simulation . For simplicity, we construct two two-dimensional models with slot-waveguide and conventional slab-waveguide structures and compare their characteristics.
2. Optical spots output of waveguide probes
First, we focus on the optical spot output of the slot-waveguide probe tip. Two probe tip models of (a) a slot waveguide with a high-index-contrast structure and (b) a normal slab waveguide for reference are compared in Fig. 1. Here, we also analyze the tapered tip cut off along the dotted red lines with taper angle θT, as shown in Fig. 1(a), to compare the validity of the tapered shape generally used in conventional metallic probes. The geometric size of thewaveguides is 2 μm (length) × 1.25 μm (width). We suppose a Si–SiO2–Si slot waveguide sandwiched by SiO2 claddings and their geometrical parameters, i.e., the width of the slot core (dL), the side core (dH), and the cladding (dCL) are 50, 200, and 400 nm, respectively, similar to the typical size of the tips of scanning near-field microscopes. The refractive indices for the low-index (nL) and high-index (nH) layer regions are 1.46 and 3.48, respectively. The normal slab waveguide comprises a 50-nm Si core sandwiched by 600-nm SiO2 claddings. The probes are surrounded by free space or an index-matching fluid with a refractive index of nSU = 1 or 1.46, respectively. These parameters are summarized in Table 1. Because of the low-loss window for the two materials, the light wavelength is assumed to be 1550 nm and the source width is roughly comparable to the diffraction limit. The source thickness is zero. Moreover, the y- or x-polarized light with a Gaussian profile is launched to the top face of the waveguide, as shown in Figs. 1(a) and 1(b), because the TM and transverse electric (TE) modes can effectively confine the light within the center core regions in the slot and normalslab waveguides. As easily predicted in the normal slab waveguide with a high refractive-index difference, the mode field distribution of the TM mode has a dip at x = 0, which reduces the power confinement.
Figures 2(a), 2(b), and 2(c) show the contour maps of the electric field components Ex in the flat-faced slot waveguide with θT = 90° and the tapered slot waveguide with θT = 45°, and Ey in the normal slab waveguides, respectively. Here, Ex,y is normalized by the peak amplitude of the input light at z = −L and nSU = 1. In Figs. 2(a) and 2(b), Ex is observed to be well confined in the slot core region, whereas in Fig. 2(c), Ey is observed to spread throughout the waveguide. Although the waveguide length, L, is restricted because of the computing time in our simulation, the light field shown in Fig. 2(b) is supposed to spread more widely for a larger L because of the very small V-value of ~0.3 for the slab waveguide. Regarding the field distribution around the bottom end of the tip, both Ex and Ey are rapidly spread against the distance z because of the scattering effect; however, in Figs. 2(a) and 2(b), an optical spot of 50–200 nm in diameter with an amplitude of 0.5–1.0 is observed within ~100 nm from the bottom surface. Though the optical spot observed in the tapered slot waveguide looks slightly smaller than that in the flat-faced slot waveguide, they spread in almost the same power distribution, as described below. This result indicates the effective illumination by an optical spot with a few tens of nanometers width in the near-field area.
Next, we examine the power distribution along the x axis to estimate the conversion efficiency for a nanosized optical spot. Figure 3 shows the normalized power distribution defined as Sz(x,z)/Sz(0, −L), where Sz(x,z) is the axial component of the time-averaged Pointing vector at (x,z), and the denominator corresponds to the output of the light source.Figures 3(a)–3(d), 3(e)–3(h), and 3(i)–3(l) are the power distributions obtained at four different positions through the flat-faced slot, tapered slot, and normal slab waveguides, respectively. It is obvious that high power confinement within a center core is kept in the slot waveguides and nanosized optical spots are generated at the near field region of z ≤ 10 nm, as shown in Figs. 3(a)–3(c) and 3(e)–3(g). In contrast, the optical power widely disperses both inside and outside the normal slab waveguide because of low confinement in the core region. These results agree well with the contour maps of the electric field components shown in Fig. 2. To clarify the results, we introduce two parameters: the optical spot size δxFWHM(z) given as the full width at half maximum (FWHM) of the power distribution curve and the confinement power ratio, , where the numerator and denominator are integrated over the optical spot within δxFWHM(z) and the light source, respectively. Light power density in an optical spot width is written as Γ(z)/δxFWHM(z).
Figures 4(a) and 4(b) show δxFWHM(z) and Γ(z)/δxFWHM(z) inside and outside the three kinds of waveguides. In the flat-faced and tapered slot waveguides, δxFWHM(z) is almost constant at 50 nm, corresponding to the width of the slot core, and significantly increases outside the waveguide because of diffraction, e.g., δxFWHM(z) = 61, 72, and 247 nm for the flat-faced slot waveguide and δxFWHM(z) = 57, 75, and 322 nm at z = 5, 10, and 50 nm, respectively. The spreading due to the diffraction effect is almost the same in the near field region for these slot waveguides. In the normal slab waveguide, a similar increasing tendency of δxFWHM(z) is observed in Fig. 4(a), but δxFWHM(z) begins to spread within the waveguide and its width exceeds 300 nm at the end surface. In Fig. 4(b), the power density Γ(z)/δxFWHM(z) for the normal slab waveguide tends to moderately decrease both inside and outside of the waveguide, by contrast with the slot waveguides whose curves rapidly decline near the end face. We note that the power density of the optical spot out of the slot waveguides is still completely larger than that out of the normal slab waveguide over the near field region. We also see a large difference of Γ(z)/δxFWHM(z) between the flat-faced and tapered slot waveguides inside the waveguides, because Γ(z) contains both downward-traveling and upward-reflecting light fields in our simulation, and a smaller reflection at the end surface of the tapered waveguidemay cause a smaller loss due to interference between the two light fields. The slight variability of symbols in Fig. 4(b) inside the waveguide regions occurs because of the same interference effect that depends on position z. The normalized power Γ(z) at z = 5, 10, 50 nm is calculated to be 0.18, 0.18, and 0.31 for the flat-faced slot waveguide and 0.16, 0.17, and 0.40 for the tapered slot waveguide, respectively. These results indicate that 50–250-nm-wide optical spots with high power transmission efficiency of 20%–30% are successfully obtained in the near field region for z ≤ 50 nm. Moreover, a very high power coupling ratio of greater than ~30% between the slot and normal waveguides or the optical fiber has been reported [17,24]. We believe that the transmission efficiency presented here is higher than the typical range of 10−5–10−2, which is obtained for conventional metallic-coated taper probes.
3. Optical probe operated in illumination and collection modes
Next, we analyze an optical probe operated in both illumination and correction modes. Figure 5 shows the schematic of the probe system with a slot waveguide and a nanosized target object. The geometric configuration of the slot waveguide is identical to that shown in Fig. 1(a), except for the photo detector (PD) embedded in the top part of the waveguide. Since there is no remarkable difference in optical spots and transmission properties between flat-faced and tapered slot waveguides, hereafter we evaluate only the flat-faced slot waveguide. For simplicity, we assume that the 50-nm-wide PD can monitor the light power propagating upward through the waveguide without absorption and scattering over the light source area. A target object made of Au with a box shape (width wTG = 50–200 nm, height hTG = 50 nm) is set on a stage of SiO2 and scanned along the x axis. The target distance dTG between the bottom surface of the waveguide and the top surface of the object is 10 nm. According to the abovementioned result, an optical spot with δxFWHM(z) = 72 nm and Γ(z) = 0.18 illuminates the surface of the object. Here, we suppose two cases: the optical system is surrounded by free space or an index-matching fluid. Contour maps of the normalized field component, Ex, in the case of nSU = 1 and nSU = nL = 1.46, are shown in Figs. 6(a) and 6(b), respectively. A 100-nm- wide target object is fixed at x = 0. Similar to Fig. 2(a), the light power is well confined within a slot core area, and a small optical spot effectively illuminates the target object. Optical near-field components can be observed to obviously appear in the immediate vicinity of the target object shown in Figs. 6(a) and 6(b). Magnified images of Figs. 6(a) and 6(b) around the target are shown in Figs. 7(a) and 7(d), respectively. Figures 7(b) and 7(c) indicate contour maps when the target position xTG is shifted to 50 nm and 125 nm in the case of nSU = 1.46, respectively. The maps in Figs. 7(e) and 7(f) are similar in the case of nSU = 1. In Figs. 7(a), 7(b), 7(d), and 7(e), light components reflected and widely scatterd at the target object can be observed to propagate upward in the center and side cores of the slot waveguides, even when the target object is in a slightly off-centered position. We also see that the internal reflectioncomponent is obviously caused at the end surface of the waveguide in Fig. 7(f) under the index unmatching case of nSU = 1; on the other hand, in Fig. 7(c) similar reflection does not appear, which can significantly improve the contrast of an object image, as discussed below.
Figure 8 shows the received power of PD normalized by the input power as a function of the target object position xTG and the object width wTG. All curves indicate obvious peaks at xTG = 0, and larger received power is obtained for larger values of wTG. This is because the reflected and scattered light components at the target object increase with the target size within this range. Although the received power contains a background component of ~0.05, the FWHM of the curve peaks coincide well with the target widths of 50, 100, and 200 nm under the condition of index matching, i.e., nSU = 1.46. For the case without index matching, the skirt part of the curve for wTG = 100 nm tends to heave for z > 150 nm. From the curves in Fig. 8, the contrast, described as the ratio of the peak power of the normalized received power to the power of the background, are roughly estimated to be 6.3, 2.7, and 1.5 for the cases of wTG = 200, 100, and 50 nm, respectively, under the condition of nSU = 1.46. For the case of wTG = 100 nm and nSU = 1, the contrast decays to be 1.5. This large degradation is mainly because of the internal reflection at the bottom surface of the waveguide in the case without index matching. Since a small internal reflection at the bottom surface of the side cores remains even when nSU = 1.46, the abovementioned background component stems and decays the contrast as the received power decreases. This is one of the intrinsic issues for an optical probe incorporating both a light source and photodetector. To suppress the background noise, for example, the slot core with a smaller refractive index, such as air gap, may be effective in a free space environment, and moreover, other detection techniques such as lock-in detection and the dithering of a probe tip would be required to reduce this noise.
Finally, we focus on the spatial resolution of our probe for the observation of two closely-aligned target objects, as shown in the inset of Fig. 9. Here, a pair of target objects of Au with a box shape (width wTG = 30, 50, or 80 nm, height hTG = 50 nm) is arranged with gap δwGAPand scanned along the x axis. Moreover, dTG = 10 nm and nSU = 1.46. The normalized received power of PD is plotted in Fig. 9 as a function of position xTG of the midpoint between the two objects. We can see obvious double peaks when δwgap = wTG = 50 and 80 nm, and the peak positions coincide well with the center of the object. For the smaller gap condition of δwgap = wTG = 30 nm, however, the central dip is absent and the gap is not resolved. This is because an optical spot of ~70 nm in width illuminates the target objects in this model, and the 30-nm gap is buried by broad illumination. As a result, the spatial resolution as the resolvable gap distance is ~50 nm, equal to the center core diameter dL in our model. This result implies that a smaller dL of the slot waveguide makes it possible to provide higher resolution; however, in our simulation for dL = 30 nm and target objects with δwgap = wTG = 30 nm, we cannot observe the separated peaks because the received light power is too small to obtain sufficient contrast. Given that a minimum optical spot size or spatial resolution is found in the 20–300 nm range [3–6], our result provides a fully comparable spatial resolution with conventional optical probes where no plasmons are excited.
In this study, we have presented an optical probe design with slot-waveguide structure. Using numerical FDTD simulation, we demonstrated that an optical spot of 50–250 nm in width is obtained with a transmission efficiency of ~20%, and a spatial resolution of ~50 nm, which corresponds to the slot core width in our model; this is achieved by a probe equipped with both illumination and collection modes. Although some conditions vary, the transmission efficiency presented here is improved compared with that of conventional metallic tapered tip structures. A simple layered structure without a metallic coating is advantageous for easy fabrication and tolerance to the heat-induced damage due to high-input light power, but may have some issues, such as enhancement of the contrast, which will be undertaken in our future study.
Part of the research presented in this paper has been done under JSPS KAKENHI Grant Number 25420346.
References and links
1. L. Novotny and B. Hecht, Principles of Nano-Optics, 2nd ed., Chap. 6.3 (Cambridge Univ. Press 2012).
2. D. W. Pohl, W. Denk, and M. Lanz, “Optical stethoscopy: Image recording with resolution λ/20,” Appl. Phys. Lett. 44(7), 651–653 (1984). [CrossRef]
5. T. Saiki, S. Mononobe, M. Ohtsu, N. Saito, and J. Kusano, “Tailoring a high-transmission fiber probe for photon scanning tunneling microscope,” Appl. Phys. Lett. 68(19), 2612–2614 (1996). [CrossRef]
6. T. Yatsui, M. Kourogi, and M. Ohtsu, “Increasing throughput of a near field optical fiber probe over 1000 times by the use of a triple-tapered structure,” Appl. Phys. Lett. 73(15), 2090–2092 (1998). [CrossRef]
7. L. Wang and X. Xu, “High transmission nanoscale bowtie-shaped aperture probe for near-field optical imaging,” Appl. Phys. Lett. 90(26), 261105 (2007). [CrossRef]
8. L. Neumann, Y. Pang, A. Houyou, M. L. Juan, R. Gordon, and N. F. van Hulst, “Extraordinary optical transmission brightens near-field fiber probe,” Nano Lett. 11(2), 355–360 (2011). [CrossRef] [PubMed]
9. G. Veronis and S. Fan, “Theoretical investigation of compact couplers between dielectric slab waveguides and two-dimensional metal-dielectric-metal plasmonic waveguides,” Opt. Express 15(3), 1211–1221 (2007). [CrossRef] [PubMed]
12. A. Weber-Bargioni, A. Schwartzberg, M. Cornaglia, A. Ismach, J. J. Urban, Y. Pang, R. Gordon, J. Bokor, M. B. Salmeron, D. F. Ogletree, P. Ashby, S. Cabrini, and P. J. Schuck, “Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes,” Nano Lett. 11(3), 1201–1207 (2011). [CrossRef] [PubMed]
13. W. Bao, M. Melli, N. Caselli, F. Riboli, D. S. Wiersma, M. Staffaroni, H. Choo, D. F. Ogletree, S. Aloni, J. Bokor, S. Cabrini, F. Intonti, M. B. Salmeron, E. Yablonovitch, P. J. Schuck, and A. Weber-Bargioni, “Mapping local charge recombination heterogeneity by multidimensional nanospectroscopic imaging,” Science 338(6112), 1317–1321 (2012). [CrossRef] [PubMed]
16. Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Opt. Lett. 29(14), 1626–1628 (2004). [CrossRef] [PubMed]
17. S. T. Lim, C. E. Png, and A. J. Danner, “Embedded air core optical nano-waveguides,” J. Opt. Soc. Am. B 27(10), 1937–1941 (2010). [CrossRef]
19. J. Blasco and C. A. Barrios, “Compact slot-waveguide/channel-waveguide mode converter,” in Proceedings of IEEE Conference on Conference on Lasers and Electro-Optics (IEEE, 2005), 607.
20. V. Nguyen, T. Montalbo, C. Manolatou, A. Agarwal, C. Hong, J. Yasaitis, L. C. Kimerling, and J. Michel, “Silicon-based highly-efficient fiber-to-waveguide couplare for high index contrast systems,” Appl. Phys. Lett. 88(8), 081112 (2006). [CrossRef]
21. F. L. Xiaowen, S. J. Wu, X. Hu, L. Zhou, and Y. Su, “Efficient fiber-to-slot-waveguide grating couplers based ona double-strip waveguide,” IEEE Photon. Technol. Lett. 25(23), 2377–2380 (2013). [CrossRef]
22. T. Yatsui, M. Kourogi, K. Tsutsui, M. Ohtsu, and J. Takahashi, “High-density speed optical near-field recording reading with a pyramidal silicon probe on a contact slider,” Opt. Lett. 25(17), 1279–1281 (2000). [CrossRef] [PubMed]
23. L. Novotny, D. W. Pohl, and P. Regli, “Light propagation through nanometer-sized structures: the two-dimensional-aperture scanning near-field optical microscope,” J. Opt. Soc. Am. A 11(6), 1768–1779 (1994). [CrossRef]
24. Y. Ruan, S. Afshar, and T. M. Monro, “Light enhancement within nanoholes in high index contrast nanowires,” IEEE Photon. J. 3(1), 130–139 (2011). [CrossRef]
26. T. Monro, S. W. Smith, E. P. Schartner, A. Francois, S. Heng, and H. E. Heideprien, “Sensing with microstructured optical fibers,” Opt. Fiber Technol. 16(6), 343–356 (2010). [CrossRef]
27. K. Foubert, L. Lalouat, B. Cluzel, E. Picard, D. Peyrade, E. Delamadeleine, F. de Fornel, and E. Hadji, “Near-field modal microscopy of subwavelength light confinement in multimode silicon slot waveguides,” Appl. Phys. Lett. 93(25), 251103 (2008). [CrossRef]
28. FullWave, http://optics.synopsys.com/rsoft/