Extraordinarily Large Optical Cross Section for Localized Single Nanoresonator

Ming Zhou, Lei Shi, Jian Zi, and Zongfu Yu

Phys. Rev. Lett. 115, 023903 – Published 10 July 2015

Abstract

Using an optical nanoresonator to realize extreme concentration of light at subwavelength nanoscale dimensions is of both fundamental and practical significance. Unfortunately, the optical cross section of an isotropic nanoresonator is determined by the resonant wavelength, which unfavorably limits the highest concentration ratio. Here we show that the cross section of a localized subwavelength resonator can be drastically enhanced by orders of magnitude. A single microscopic nanoresonator could exhibit a macroscopic optical cross section. We further show that the enhancement can be implemented in simple dielectric structures that are readily compatible with optoelectronic integration. The giant optical cross section of a nano-object provides a versatile platform to create extremely strong light-matter interactions at the nanoscale.

Received 23 April 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.023903

Authors & Affiliations

Ming Zhou¹, Lei Shi^2,3, Jian Zi^2,3,*, and Zongfu Yu^1,†

¹Department of Electrical and Computer Engineering, University of Wisconsin, Madison, Wisconsin 53706, U.S.A
²Department of Physics, Key Laboratory of Micro-and Nano-Photonic Structures (MOE), and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
³Collaborative Innovation Center of Advanced Microstructures, Fudan University, Shanghai 200433, China

^*Corresponding author. jzi@fudan.edu.cn
^†Corresponding author. zyu54@wisc.edu

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Issue

Vol. 115, Iss. 2 — 10 July 2015

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Images

Figure 1
(a) A resonator is excited by an incident plane wave (red arrow) and scatters light to other plane waves (green arrows). (b) Channels are represented by the dots in the projected $k_{xy}$ space. More coupling channels are available in vacuum as enclosed in the gray sphere ( $n = 1$ ) than in index near-zero materials as enclosed in the yellow sphere ( $n = 0.02$ , not drawn to the scale) (c)–(d) Flow of the Poynting vectors around a nanoresonator in vacuum (c) and index near-zero material (d). The resonator (red circle) has a radius of $0.1 μ m$ .
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Figure 2
(a) A Gaussian beam scans across a resonator placed on a perfect mirror in vacuum. (b) The top view of the spatial intensity of the reflected beam measured at three different positions A, B, and C [also labeled in (a)]. Red dot indicates the position of the resonator. (c) Similar to (a), but the resonator is in a slab of an index near-zero material. An antireflection layer is placed on the top of the slab to ensure that there is no external cavity formed by the slab. In both cases, (a) and (c), the reflected beam profile is obtained at a fixed distance from the resonator. (d) Reflected beam profiles at three different positions A, B, and C. A macroscopic shadow created by an extraordinarily large cross section emerges in the reflected beam profile. (e) Upper panel: A matching Gaussian beam with a waist that is the same as the size of the shadow in (d). Lower panel: The reflected beam when shining on the resonator in the index near-zero material. The beam is completely absorbed.
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Figure 3
(a) Schematic of the near-zero phase index structure consisting of two mirrors. The dispersion curve is obtained assuming the mirror has 100% reflection. (b) The spectra of the absorption cross section for mirrors with 80% (black solid line) and 95% (red solid line) reflectance. Insets show the angular response for both cases.
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Figure 4
(a) Schematic of the 2D numerical simulation. The resonator is formed by a nanoslit on a PEC slab. The depth and width of the slit is 110 and 20 nm, respectively. The slit is filled with a dielectric material, which has a refractive index of $3.5 - j 0.07$ . The spacing between the PEC and the DBR is 785 nm. (b) Simulated absorption cross-section spectrum (circles) agrees very well with the coupled-mode theory (solid line). (c) The scattered fields from the nanoresonator without (upper) and with (lower) the top mirror.
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