Selective electric and magnetic sensitivity of aperture probes

We report the effect of geometrical factors governing the polarization profiles of near-field scanning optical microscope (NSOM) probes. The most important physical parameter controlling the selective electric or magnetic field sensitivity is found to be the width of the metal rim surrounding aperture. Probes with metal rim width w < λ/2 selectively senses the optical electric field, while those with w > λ/2 selectively senses the optical magnetic field. Intensity variation of optical Hertz standing wave formed upon reflection at oblique incidence shows a phase difference of π/2 between electric and magnetic probes: an analogue of the classical Wiener’s experiment. Our work paves way towards electromagnetic engineering of nanostructures. ©2015 Optical Society of America OCIS codes: (260.2110) Electromagnetic optics; (050.6624) Subwavelength structures; (180.4243) Near-field microscopy References and links 1. M. Burresi, D. van Oosten, T. Kampfrath, H. Schoenmaker, R. Heideman, A. Leinse, and L. Kuipers, “Probing the magnetic field of light at optical frequencies,” Science 326(5952), 550–553 (2009). 2. H. W. Kihm, J. Kim, S. Koo, J. Ahn, K. Ahn, K. Lee, N. Park, and D. S. Kim, “Optical magnetic field mapping using a subwavelength aperture,” Opt. Express 21(5), 5625–5633 (2013). 3. T. H. Taminiau, S. Karaveli, N. F. van Hulst, and R. Zia, “Quantifying the magnetic nature of light emission,” Nat. Commun. 3, 979 (2012). 4. D. Denkova, N. Verellen, A. V. Silhanek, V. K. Valev, P. Van Dorpe, and V. V. Moshchalkov, “Mapping magnetic near-field distributions of plasmonic nanoantennas,” ACS Nano 7(4), 3168–3176 (2013). 5. B. L. Feber, N. Rotenberg, D. M. Beggs, and L. Kuipers, “Simultaneous measurement of nanoscale electric and magnetic optical fields,” Nat. Photonics 8, 43–46 (2014). 6. T. Grosjean, M. Mivelle, F. I. Baida, G. W. Burr, and U. C. Fischer, “Diabolo nanoantenna for enhancing and confining the magnetic optical field,” Nano Lett. 11(3), 1009–1013 (2011). 7. S. Koo, M. S. Kumar, J. Shin, D. Kim, and N. Park, “Extraordinary magnetic field enhancement with metallic nanowire: role of surface impedance in Babinet’s principle for sub-skin-depth regime,” Phys. Rev. Lett. 103(26), 263901 (2009). 8. S. Karaveli and R. Zia, “Spectral tuning by selective enhancement of electric and magnetic dipole emission,” Phys. Rev. Lett. 106(19), 193004 (2011). 9. D. J. Park, S. B. Choi, K. J. Ahn, D. S. Kim, J. H. Kang, Q. Park, M. S. Jeong, and D. K. Ko, “Experimental verification of surface plasmon amplification on a metallic transmission grating,” Phys. Rev. B 77(11), 115451 (2008). 10. H. W. Kihm, S. M. Koo, Q. H. Kim, K. Bao, J. E. Kihm, W. S. Bak, S. H. Eah, C. Lienau, H. Kim, P. Nordlander, N. J. Halas, N. K. Park, and D. S. Kim, “Bethe-hole polarization analyser for the magnetic vector of light,” Nat. Commun. 2, 451 (2011). 11. M. Burresi, T. Kampfrath, D. van Oosten, J. C. Prangsma, B. S. Song, S. Noda, and L. Kuipers, “Magnetic lightmatter interactions in a photonic crystal nanocavity,” Phys. Rev. Lett. 105(12), 123901 (2010). 12. R. L. Olmon, M. Rang, P. M. Krenz, B. A. Lail, L. V. Saraf, G. D. Boreman, and M. B. Raschke, “Determination of electric-field, magnetic-field, and electric-current distributions of infrared optical antennas: a near-field optical vector network analyzer,” Phys. Rev. Lett. 105(16), 167403 (2010). 13. S. Vignolini, F. Intonti, F. Riboli, L. Balet, L. H. Li, M. Francardi, A. Gerardino, A. Fiore, D. S. Wiersma, and M. Gurioli, “Magnetic imaging in photonic crystal microcavities,” Phys. Rev. Lett. 105(12), 123902 (2010). 14. A. Asenjo-Garcia, A. Manjavacas, V. Myroshnychenko, and F. J. García de Abajo, “Magnetic polarization in the optical absorption of metallic nanoparticles,” Opt. Express 20(27), 28142–28152 (2012). #240313 Received 5 May 2015; accepted 28 Jun 2015; published 31 Jul 2015 (C) 2015 OSA 10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020820 | OPTICS EXPRESS 20820 15. S. Y. Lee, I. M. Lee, J. Park, S. Oh, W. Lee, K. Y. Kim, and B. Lee, “Role of magnetic induction currents in nanoslit excitation of surface plasmon polaritons,” Phys. Rev. Lett. 108(21), 213907 (2012). 16. M. Kasperczyk, S. Person, D. Ananias, L. D. Carlos, and L. Novotny, “Excitation of magnetic dipole transitions at optical frequencies,” Phys. Rev. Lett. 114(16), 163903 (2015). 17. H. U. Yang, R. L. Olmon, K. S. Deryckx, X. G. Xu, H. A. Bechte, Y. Xu, B. A. Lai, and M. B. Raschke, “Accessing the optical magnetic near-field through Babinet’s principle,” ACS Photonics 1(9), 894–899 (2014). 18. N. Liu, H. Liu, S. Zhu, and H. Giessen, “Stereometamaterials,” Nat. Photonics 3(3), 157–162 (2009). 19. S. N. Sheikholeslami, A. García-Etxarri, and J. A. Dionne, “Controlling the interplay of electric and magnetic modes via Fano-like plasmon resonances,” Nano Lett. 11(9), 3927–3934 (2011). 20. W. Cai, U. K. Chettiar, H. K. Yuan, V. C. de Silva, A. V. Kildishev, V. P. Drachev, and V. M. Shalaev, “Metamagnetics with rainbow colors,” Opt. Express 15(6), 3333–3341 (2007). 21. N. A. Mirin and N. J. Halas, “Light-bending nanoparticles,” Nano Lett. 9(3), 1255–1259 (2009). 22. N. Liu, S. Mukherjee, K. Bao, Y. Li, L. V. Brown, P. Nordlander, and N. J. Halas, “Manipulating magnetic plasmon propagation in metallic nanocluster networks,” ACS Nano 6(6), 5482–5488 (2012). 23. D. W. Brandl, N. A. Mirin, and P. Nordlander, “Plasmon modes of nanosphere trimers and quadrumers,” J. Phys. Chem. B 110(25), 12302–12310 (2006). 24. S. N. Sheikholeslami, H. Alaeian, A. L. Koh, and J. A. Dionne, “A metafluid exhibiting strong optical magnetism,” Nano Lett. 13(9), 4137–4141 (2013). 25. I. Staude, A. E. Miroshnichenko, M. Decker, N. T. Fofang, S. Liu, E. Gonzales, J. Dominguez, T. S. Luk, D. N. Neshev, I. Brener, and Y. Kivshar, “Tailoring directional scattering through magnetic and electric resonances in subwavelength silicon nanodisks,” ACS Nano 7(9), 7824–7832 (2013). 26. M. R. Shcherbakov, D. N. Neshev, B. Hopkins, A. S. Shorokhov, I. Staude, E. V. Melik-Gaykazyan, M. Decker, A. A. Ezhov, A. E. Miroshnichenko, I. Brener, A. A. Fedyanin, and Y. S. Kivshar, “Enhanced Third-Harmonic Generation in Silicon Nanoparticles Driven by Magnetic Response,” Nano Lett. 14(11), 6488–6492 (2014). 27. N. Liu, S. Mukherjee, K. Bao, L. V. Brown, J. Dorfmüller, P. Nordlander, and N. J. Halas, “Magnetic Plasmon Formation and Propagation in Artificial Aromatic Molecules,” Nano Lett. 12(1), 364–369 (2012). 28. V. M. Shalaev, W. Cai, U. K. Chettiar, H. K. Yuan, A. K. Sarychev, V. P. Drachev, and A. V. Kildishev, “Negative index of refraction in optical metamaterials,” Opt. Lett. 30(24), 3356–3358 (2005). 29. J. B. Pendry, “Negative refraction makes a perfect lens,” Phys. Rev. Lett. 85(18), 3966–3969 (2000). 30. I. I. Smolyaninov, C. C. Davis, V. N. Smolyaninova, D. Schaefer, J. Elliott, and A. V. Zayats, “Plasmon-induced magnetization of metallic nanostructures,” Phys. Rev. B 71(3), 035425 (2005). 31. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314(5801), 977–980 (2006). 32. A. Nazir, S. Panaro, R. Proietti Zaccaria, C. Liberale, F. De Angelis, and A. Toma, “Fano coil-type resonance for magnetic hot-spot generation,” Nano Lett. 14(6), 3166–3171 (2014). 33. S. Karaveli, A. J. Weinstein, and R. Zia, “Direct modulation of lanthanide emission at sub-lifetime scales,” Nano Lett. 13(5), 2264–2269 (2013). 34. T. H. Taminiau, S. Karaveli, N. F. van Hulst, and R. Zia, “Quantifying the magnetic nature of light emission,” Nat. Commun. 3, 979 (2012). 35. N. Rotenberg and L. Kuipers, “Mapping nanoscale light fields,” Nat. Photonics 8(12), 919–926 (2014). 36. U. Schröter and A. Dereux, “Surface plasmon polaritons on metal cylinders with dielectric core,” Phys. Rev. B 64(12), 125420 (2001). 37. E. Dereux, A. Dereux, E. Bourillot, J. Weeber, Y. Lacroute, J. Goudonnet, and C. Girard, “Local detection of the optical magnetic field in the near zone of dielectric samples,” Phys. Rev. B 62(15), 10504–10514 (2000). 38. D. C. Kohlgraf-Owens, S. Sukhov, and A. Dogariu, “Discrimination of field components in optical probe microscopy,” Opt. Lett. 37(17), 3606–3608 (2012). 39. J. H. Kang, D. S. Kim, and Q. H. Park, “Local capacitor model for plasmonic electric field enhancement,” Phys. Rev. Lett. 102(9), 093906 (2009). 40. H. A. Bethe, “Theory of diffraction by small holes,” Phys. Rev. 66(7-8), 163–182 (1944). 41. O. Wiener, “Stehende Lichtwellen und die Schwingungsrichtung polarisirten Lichtes,” Ann. Phys. Chem. 38(6), 203–243 (1890). 42. H. W. Kihm, K. G. Lee, D. S. Kim, and K. J. Ahn, “Dual mode near-field scanning optical microscopy for nearfield imaging of surface plasmon polariton,” Opt. Commun. 282(12), 2442–2445 (2009).

Here we report on what physical factors govern selective sensitivity of apertured metal coated NSOM probes to either optical electric or magnetic field. The selective electromagnetic field sensitivity is primarily decided by the metal rim width. The physics lies in the lambda-zone: when the thickness of the metal is larger than λ/2, the probe is magnetic; when it is less, it becomes electric. Essentially, the finite speed of light guarantees that anything larger than λ/2 can be considered infinite, bringing the probe into the Bethe hole regime-a single hole lying on an infinite plane. As a proof of principle measurement, an analogue of classical Wiener's experiment ascertains one's freedom to tune NSOM tips to independently map electric and magnetic fields.

Results and discussions
We fabricated tapered optical fibers of various diameters by fusion splicing single mode optical fiber (Thorlabs 780HP -780 -970 nm with Ø125 µm cladding) with commercial CO 2 laser splicing machine (P-2000, Shutter Instrument) and coated with varying width of aluminum layer using thermal evaporation. For uniform metal coating we had used tilted fiber mount 30° inclined with respect to the horizontal on a rotating stage. Adhesion of aluminum to glass was enhanced using 5 nm layer of chromium deposited using e-beam evaporation. The fabricated probes were milled using focused ion beam (Model No. Quanta 3D (FEI))) to achieve nearly flat bottom with different aperture size by orienting probe axis perpendicular to the ion-beam direction. The desired aperture size a (nm) is controlled by position of the milling from tip apex, while the metal rim width w (nm) is controlled by amount of metal evaporated. Using light at oblique incidence angles (θ = 72°), the effect of incident electric ( i E  ) and magnetic field ( ) i H  components projected onto the reflecting plane ( t E  and t H  ) on scattering polarization was monitored to access the selective electric and magnetic field sensitivity of the NSOM probes. We measured scattering of light of wavelength 788 nm through NSOM probes to access their selective EM field sensitivity layout shown in Fig. 1(a). The direction of the incident polarization (φ) was varied monotonically using λ/2-wave plate. Direction and intensity of the scattered field was measured by using analyzer and Avalanche photo-diode (SPCM-AQR-16) coupled to photon counter from Standford research systems (SR400 -Gated photon counter 2 channel). The λ/2-wave plate and analyzer was precisely rotated using Thorlabs 3-axis piezo controller (Thorlabs -MDT693A). Analyzer was rotated from 0 to 360° in steps of 2° to measure the scattered radiation lobe. The direction of scattered polarization is decided by maximum intensity points of radiation lobe. Oblique incidence with asymmetric polarization (0 < φ < 90°) states on the metal plane enable separate quantification of effect of electric and magnetic fields on NSOM probes.  Figure 1(b) shows the plots of scattered dipoles for Probe-1 with varying incident polarization direction at normal (θ = 0°) and at oblique incidence angle (θ = 72°). At normal incidence angle the direction of scattered dipole is simultaneously parallel to the incident electric field dipole (blue arrow) and perpendicular to the magnetic field dipole (red arrow). The scattered radiation due to oscillating charge i.e incident electric field is always along the electric field direction while, the direction of the emitted radition due to induced current led magnetic dipole is always perpendicular to magnetic dipole direction. At normal incidence, since the scattered dipole due to incident electric field and magnetic field are in same direction, it is hard to distinguish the source of scattered radiation whether it is due to electric dipole or, pseudo-magnetic dipoles. At large oblique incidence angle, projected component of incident electric dipole ( t E  ) and of magnetic dipole ( t H  ) onto the aperture apex, no longer remains orthogonal to each other opening up the possibility to distinguish the source of scattered dipole. For Probe-1, at oblique incidence at asymmetric incident polarization states (for 0 < φ < π/2), the scattered polarization angle Sc ψ is smaller than the incident polarization angle (φ). A closer look to the Fig. 1(b) shows that, the scattered polarization follows the projected incident electric field ( t E  ). Simultaneously, this probe shows stronger collection intensity for TE polarization (φ = 0°) than the TM polarization (φ = 90°) as represented by larger size of the lobe, and hence are termed as an "Electric NSOM probe". Figure 1(c) shows the change in the polarization of scattered dipole of Probe-II with change in the incident polarization ( ) φ . For Probe-II ( Fig. 1(c)), the polarization orientation of collected scattered radiation is perpendicular to the projected incident magnetic field ( t H  ) for all arbitrary polarization directions (φ), consistent with Bethe's pseudo magnetic dipole behavior. Again, contrary to the Probe-I, Probe-II shows weaker intensity for TE polarized light and stronger intensity for the TM polarized state and, hence Probe-II is "Magnetic NSOM probe". where, The theoretical fit to the experimental data is shown by solid line in Fig. 2(a) and Fig. 2(b) for Probe-I and Probe-II. The estimated values of field coupling co-efficients α and β are normalized with condition α 2 + β 2 = 1 and are represented as α n and β n . The dashed red and blue curves indicate the perfect Electric probe (α = 1; β = 0) and perfect Magnetic probe (α = 0; β = 1). For Probe-I, the estimated value of normalized electric field coupling co-efficient (α n ) is higher than the normalized magnetic field coupling co-efficient (β n ), reaffirming the selective coupling of electric field to Probe-I ( 17.49 n n α β = ). In contrast to Probe-I, the normalized magnetic field coupling coefficient (β n ) for Probe-II is higher than the α n ( 0.24 n n α β = ). While predominantly electric or magnetic tips will be highly desirable for investigation of near field phenomena, the capability to engineer them on demand has not been realized [5]. Guided by our previous experimental results that a Bethe hole in an infinite metal plane behaves as polarization analyzer for magnetic field of light [10], we fabricated number of NSOM probes with varying aperture diameter and metal rim width expecting geometrical factors aperture diameter and metal thickness predominantly decides the selective electric or magnetic field sensitivity of probes. The diameter of the aperture a (in nm) is decided by milling position from the probe bottom, while the metal rim width w (in nm) is controlled through the deposited metal width. The polarization characteristics of collected radiation by NSOM probes of different geometrical parameters were studied as shown in Fig. 3(a) and 3(b). Figure 3(a) shows the effect of metal rim width on the collected intensity ratio of TE and TM incident polarizations. The NSOM probes with w < 400 nm (i.e < λ/2) shows TM/TE ratio << 1 analogous to Probe-I: the electric probe, while probes with w > 400 nm (i.e > λ/2) shows TM/TE ratio > 1 similar to magnetic probe: Probe-II. In agreement to this, the induced normalized magnetic dipole coupling constant (β n ) obtained by fitting the Sc ψ versus φ curve shown in Fig. 3(b), indicates monotonic increase with increasing w.
Our results give the specifics of how to make a tip over 90% electric or magnetic; it is a matter of w being larger or smaller than λ/2. In contrast, selective field sensitivity of NSOM probes is largely insensitive to aperture diameter (a), see Fig. 5 (Appendix). The dominant role of w was not considered in [5], leading to the conclusion that in all cases, symmetric metal coated probes of various aperture diameters simultaneously sense the electric and magnetic field [5]. Such general conclusion is in principle not wrong; but to be useful, deciding factors to fabricate probes with predefined field sensitivity must be revealed through experiments.  Fig. 3(c). The capacitive coupling of electric field to the NSOM probes with thinner metal rim width leads to selective sensitivity to the incident electric field direction. This is in agreement with our experimental observation of selective electric field sensitivity of NSOM probes with thinner metal rim width (w < 400 nm). This shows that electric field coupling efficiency is dominant for NSOM probes with rim width w < 400 nm. Figure 3(d) shows the surface current density induced around the aperture of same diameter 0.1 λ but having metal rim width of 0.5 λ. For such probes, the induced current is mostly aligned normal to the direction of incident magnetic field. This indicates selective sensitivity of large metal rim width probes to the magnetic field. The dominant magnetic field sensitivity of NSOM probes with w > λ/2 is indicative of the probe realizing its Bethe limit [39,40] at the limit of the λzone [39].  Fig. (d) shows FESEM image of probe-II: the magnetic probe and the collection and scattering measurement of the vertical standing wave by this probe using dual mode NSOM setup is shown in Fig. (e). Phase difference of π/2 between collected and scattered signals from magnetic probes affirms the selective field sensitivity of NSOM probes.
The functionality of electric and magnetic NSOM probes was finally affirmed by mapping the field vector components of a z-standing wave. The interference of incident wave and reflected wave generates standing wave pattern of both fields in vertical direction schematically shown in Fig. 4(a). Analogous to Wiener's experiment, using our selective probes we mapped uniquely the electric and magnetic field maxima located out of synch in space. Otto Wiener's in (1890) successfully mapped the nodes and antinodes of electric field of Hertz standing waves at visible frequency using photographic plates exposed to standing wave formed upon reflection [41]. For standing wave formed upon reflection from metal film, electric field maxima is expected at heights odd multiple of λ z /2 = λ/(2cosθ), while magnetic field maxima are shifted in phase by π/2. We measured collection signal and scattering signal simultaneously using dual mode NSOM [42] with the electric and magnetic probes, at an incident angle of 45°. The scattering signal serves as a reference; the scattering process by probe apex is mainly caused by the electric field. Figure 4(b) shows the FESEM image of Probe-I: the Electric probe. Figure 4(c) plots the z-dependence of collection and scattering signals from the electric probe and they are completely in phase. FESEM of Probe-II: the Magnetic probe used is shown in Fig. 4(d). For the magnetic probes, the relative phase difference between two signals is π/2 (out-of-phase). These results show the possibility of selective detection of electric field and magnetic field with the electric probes and the magnetic probes in random distribution of electromagnetic fields.

Conclusions
In conclusion, metallic rim width decides the nature of electromagnetic radiation collected through metal coated sub-wavelength apertured dielectric probes, whether it is electric or, magnetic field. NSOM probes with metallic width w < 400 nm (< λ/2) primarily collects the electric field, while the probes with thick flat bottom collects dominantly the Bethe's pseudomagnetic dipole. Our experimental observations are explained in terms of capacitive coupling of the electric dipole for electric probes and in terms of eddy currents within λ-zone for magnetic probes and results are in agreement with FDTD simulations. These results establish the method of fabrication of NSOM probes sensitive to magnetic field direction independent of the electric field direction and vice versa. Measurements using these selective field sensitive NSOM probes using simultaneous scattering and collection mode NSOM provide experimental evidence of separate electric and magnetic Hertz standing waves at visible frequency.

Appendix A: Effect of aperture diameter (a)
The variation of intensity ratio of collected light using NSOM probes of various aperture size a (in nm) for TM and TE polarization states (I TM / I TE ) is shown in Fig. 5. Apparently, the selective field sensitivity is largely independent of the aperture diameter (in the range 80 to 400 nm).