H. "Nakamura

K. Sawada

T. Sato

T." Saiki

National Institute for Fusion Science (NIFS-Repository)

§2 7. Numerical Analysis for Design of Tapered Structure of Fiber Probes Nakamura, H., Sato, T., Sawada, K. (Shinshu Univ.), Saiki, T. (KAST) Improvement of the optical throughput of metal-coated fiber probes is the important issue in terms of the applica-tions of near-field scanning optical microscopy (NSOM) to the spectroscopic studies and optical recordings. Recent ex-perimental studies l.2) show that the transmission efficiency of light through the fiber probe is subject to the taper structures of metal-cladding region . In Ref. I , for the enhancement of the throughput, it is proposed to shorten the narrow metal-cladding region with strong optical losses by making the double-tapered structure. Recently, with the progress of com-putational work, the numerical analysis has come to be rec-ognized as a powerful tool for deep understanding of the elec-tromagnetic field in the tapered waveguide and the vicinity of an aperture, and for the systematic design of highly effi-cient probes. The finite -difference time-domain (FDTD) method3-S) is one of the most promising methods for the above purposes, because it can be easily applied to actual three-di-mensional problems. We have reported a FDTD simulation for the double-tapered structure in Ref.6, in which the spatial resolution beyond the conventional NSOM is successfully reproduced by the simulation. Here we show a FDTD simulation, which corresponds to the experimental configuration in Ref.I. Figure I illus-trates the FDTD geometry of the probe model. A fiber probe with a double- or single-tapered structure collects lumines-cence (A=I~m) from a quantum dot (x-directed dipole radia-tion) buried 1J40 beneath the semiconductor surface. The ra-diation caught by the aperture with a diameter of 1J5 propa-gate in the tapered part clad with perfect conducting metal. The signal intensity is evaluated by calculating the light power finally coupled to the ordinary waveguide (optical fiber) re-gion . The simulation box consists of 120x 120x360 grid in x, y, and z directions. The space step is 1J40. The calculation is done for three types of probes whose dimension are summa-rized in Table I . The sample No.1 is a single-tapered probe. The sample No.2 is a single-tapered probe which has a shorter 336 tip length than that of the sample No.1. The sample No.3 is a double-tapered probe whose cone angle of the tip is the same as that of the sample No.2. The calculation shows that the ratio of signal intensity for the sample No.I-3 is I :32: 100. It is clear that a large cone angle contributes to high collection ability since this structure minimizes the length of cutoff re-gion. In addition, the double-tapered probe makes the cou-pling of light into the normal waveguide more efficient than the single-tapered probe with the same cone angle. The col-lection efficiency and spatial resolution of the double-tapered probe in Ref.2 is also reproduced by this FDTD simulation. z O.5~ 20J.un core n=L487 quantum a dot d O.5J.U11 semiconductor 2.0~ 7.0~ n=3.5 O~ ____________ ~-L~ x y Fig. I. Cross-section diagram of the simulation model Sample No. hs as hd ad 1 3.6 0.2 2 0.9 0.2 3 2.0 1.0 0.4 0.2 Table I Parameters (in J-lm) in Fig.I. References 1) T.Saiki, et aI, Appl. Phys . Lett. 68 (1996) 2612. 2) T.Saiki and K.Matsuda, AppJ. Phys. Lett. 74 (1999) 2773. 3) K.S.Yee, IEEE Trans. Antennas Propag., AP-14 (1966) 302. 4) Taflove, Computational Electrodynamics: The Finite-Difference Time-Domain Method, Norwood, MA, Artech house, 1995. 5) H. Furukawa and S. Kawata, Opt. Commun. 132 (1996) 170. 6) H.Nakamura, et aI., Supp\. Prog. Theor. Phys. (in press). 

Numerical Analysis for Design of Tapered Structure of Fiber Probes

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