Coherent Excitation-Selective Spectroscopy of Multipole Resonances

Xu Fang, Ming Lun Tseng, Din Ping Tsai, and Nikolay I. Zheludev

Phys. Rev. Applied 5, 014010 – Published 27 January 2016

Abstract

Thin films of functional materials, from graphene to semiconductor heterostructures, and from nanomembranes to Langmuir-Blodgett films play key roles in modern technologies. For such films optical interrogation is the main and often the only practical method of characterization. Here, we show that characterization of the optical response of thin films can be greatly improved with a type of coherent spectroscopy using two counterpropagating beams of light. The spectroscopy is selective to particular types of multipole resonances that form the absorption spectrum of the film, and therefore can reveal lines that are hidden in conventional absorption spectroscopy. We explicitly demonstrate selectivity of this spectroscopy in a series of proof-of-principle experiments with plasmonic metamaterial arrays designed to exhibit different multipole resonances. We further demonstrate the analytic potential of this spectroscopy by extracting the hidden resonance from the spectrum of a complex nanostructure.

Received 16 August 2015

DOI:https://doi.org/10.1103/PhysRevApplied.5.014010

Physics Subject Headings (PhySH)

Plasmonics

Metamaterials

Spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Xu Fang^1,*, Ming Lun Tseng², Din Ping Tsai^2,3, and Nikolay I. Zheludev^1,4,†

¹Optoelectronics Research Centre and Centre for Photonic Metamaterials, University of Southampton, Southampton SO17 1BJ, United Kingdom
²Department of Physics, National Taiwan University, Taipei 10617, Taiwan
³Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
⁴Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore 637371, Singapore

^*To whom all correspondence should be addressed. x.fang@soton.ac.uk
^†To whom all correspondence should be addressed. niz@orc.soton.ac.uk

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Vol. 5, Iss. 1 — January 2016

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Images

Figure 1
Selective excitation of coherent spectroscopy for ultrathin films: principle and experimental setup. (a) In a standing wave formed by two counterpropagating normal incident laser beams, the absorption of a subwavelength thin film depends on its position. At the electric antinode and magnetic node (case I), the electric dipole absorption is at its maximum, while the magnetic dipole and electric quadruple absorption vanishes. On the contrary, at the electric node and magnetic antinode (case II), the electric dipole resonance is suppressed and the magnetic dipole and electric quadrupole resonances are selectively excited. (b) Schematic illustration of the coherent excitation spectrometer (see the Supplemental Material [17] for details) with an enlarged view around the sample shown in (a).
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Figure 2
Coherent control of electric dipole resonance in metamaterial. (a) SEM image of the slit nanoantenna metamaterial. Unit cell of the metamolecule is marked with the white dashed line. The polarization of the incident light is depicted. (b) Dimensions of a unit cell: $p = 430 nm$ , $h = 180 nm$ , and $w = 60 nm$ . (c),(d) Experimental and numerically simulated absorption spectra of the slit nanoantennas at the $E$ antinode (red line) and the $E$ node (blue line) of a standing wave, and in a traveling wave (green line). (e)–(g) Corresponding surface charge-density distribution on the inner walls of the slit in the Au layer. The wavelength is 870 nm [highlighted by the dashed line in (d)]. All panels are shown at the same color scale.
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Figure 3
Coherent control of magnetic dipole and electric quadrupole resonances in metamaterial. (a) SEM image of the nanowire metamaterial. Polarization of the incident light is depicted. Unit cell of the metamolecule is marked with the white dashed line. (b) Dimension of a unit cell, $t = 100 nm$ and $b = 150 nm$ . The periodicity is 220 nm and the wire length is $30 μ m$ . (c),(d) Experimental and simulated absorption spectra of the nanowires at the $B$ antinode (blue line), the $B$ node (red line), and in a traveling wave (green line). (e)–(g) Corresponding magnetic field (color map) and induced displacement current (white arrows) at the cross section of the metamolecule at the resonance wavelength [850 nm, highlighted in (d)].
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Figure 4
Recovery of the hidden resonance and manipulation of near-field hot spots in the metamaterial. (a) SEM image of the multilayered asymmetric ring slit metamaterial. The polarization of the incident light is depicted. The unit cell of the metamolecule is marked with the white dashed line. (b) Dimensions of a unit cell: $p = 520 nm$ , $b = 210 nm$ , $t = 220 nm$ , $g = 70 nm$ , and $w = 50 nm$ . (c),(d) Experimental and simulated absorption spectra of the sample at the $E$ antinode (red line), at the $E$ node (blue line), and in the traveling wave (green line). (e)–(f) Leading terms of multipole radiation power at (e) the $E$ antinode and (f) the $E$ node. ED, electric dipole; MD, magnetic dipole; TD, toroidal dipole; EQ, electric quadrupole; and MQ, magnetic quadrupole. (g),(h) Electric field in the middle of the ${Si}_{3} N_{4}$ layer at the resonance wavelength of the hidden peak [910 nm, highlighted in (d)].
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Figure 5
Interrogating complex traveling-wave spectra using coherent spectroscopy. Numerically simulated absorption spectra are shown for three split ring slit metamaterials of different dimensions. The planar nanostructure dimensions are the same as in Fig. 4. The thickness of the middle ${Si}_{3} N_{4}$ layer is (a) 30 nm, (b) 50 nm (the same sample as in Fig. 4), and (c) 60 nm. As different multipole resonances have a different dependence on the metamolecule size, the hidden resonance in (b) moves away from the Fano resonance and therefore can be seen in the traveling-wave spectra in both (a) and (c) (highlighted by arrows).
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