Plasmonic Analog of Electromagnetically Induced Absorption Leads to Giant Thin Film Faraday Rotation of 14°

We demonstrate the realization of a new hybrid magnetoplasmonic thin film structure that resembles the classical optical analog of electromagnetically induced absorption. In transmission geometry our gold nanostructure embedded in an EuS film induces giant Faraday rotation of over 14° for a thickness of less than 200 nm and a magnetic field of 5 T at $T=20\mathrm{K}$. By varying the magnetic field from $-5$ to $+5\mathrm{T}$, a rotation tuning range of over 25° is realized. As we are only a factor of 3 away from the Faraday isolation requirement, our concept could lead to highly integrated, nonreciprocal photonic devices for light modulation, optical isolation, and optical magnetic field sensing.

Popular Summary

Many modern optical technologies, such as lasers and high-speed data communication, depend on nonreciprocal optical devices, which behave differently for forward and backward traveling light waves. The most important example is optical isolators, which are transparent for forward-propagating light but block light that propagates backward. This prevents, for example, damage to laser diodes upon back reflection. Usually, nonreciprocal devices rely on the Faraday effect, where the polarization of light traveling through a material is rotated depending on the direction of an applied magnetic field. The growing demand for highly miniaturized devices, however, is challenging for Faraday rotators, which typically require large crystals of roughly centimeter size. By modifying a thin film, we show a way that much smaller Faraday rotators could be built.

We use a thin film of europium sulfide (EuS) to demonstrate that the Faraday effect can be substantially enhanced by the inclusion of gold nanoscale metal wire arrays. With the application of an external magnetic field, we measure a Faraday rotation of light of 14 ° transmitted through the film. Varying the magnetic field lets us change the rotation over a range of 25 °. Although our structures are less than 200 nm thick, their performance is only a factor of 3 smaller than centimeter-sized Faraday rotators used in conventional optical isolators.

Modifications to our design could result in a Faraday rotation close to or above 45 °. Hence, our concept could pave the way to novel applications in laser and communication systems.

Figure 1

(a),(b) Schematic drawing of the sample geometry. $p$, nanowire period; $t$, $w$, gold nanowire thickness and width. The wires are buried with a distance $b$ between the glass substrate and their lower edge. The nominal thickness of the EuS magneto-optical waveguide is $h$, which is increased near the position of the gold nanowires. (c) Colorized scanning electron micrograph of the sample cross section. The samples are measured at $T=20\mathrm{K}$.

Figure 2

(a),(b) Simulated transmittance and absorbance ($1-T-R$, with $T$ and $R$ denoting transmittance and reflectance, respectively) for $x$-polarized incident light and different burial parameters $b$. For increasing $b$ there is a gradual transition from the regime of induced transparency to induced absorption. The white dotted line indicates the Rayleigh anomaly. (c),(d) Slice cuts for a clearer view on the line shapes of the spectra. The oval indicates the EIA regime, characterized by very sharp induced absorption peaks.

Figure 3

The left-hand column displays the measured transmittance (a) and Faraday rotation (b) for $x$-polarized incident light for a wire width of 85 nm, burial parameter $b=33\mathrm{nm}$, and different grating periods. The experimental data agree with the performed simulation shown in the right-hand column (c),(d).

Figure 4

The left-hand column displays the measured transmittance (a) and Faraday rotation (b) for $y$-polarized incident light for a wire width of 75 nm, burial parameter $b=33\mathrm{nm}$, and different grating periods $p$. The experimental data agree well with the performed simulation shown in the right-hand column (c),(d). For a period of 505 nm, the Faraday rotation reaches values of up to 14°.

Figure 5

(a) Measurement of the magnetic field dependence of the polarization rotation for a period of 505 nm. (b) Closer view on the rotation spectra for weak magnetic fields. Already for 250 mT the Faraday rotation reaches values of over 4°. (c) Saturation behavior of the Faraday rotation at 737 nm.

Figure 6

(a) Oscillator model representing the magnetoplasmonic nanostructure. A precondition for EIA is that the coupling between the TM waveguide mode and the plasmon mode is weak and includes retardation. This is taken into account by the complex coupling constant $\kappa \exp (i\varphi )$ with phase $\varphi$. (b) Imaginary part of the component ${\chi }_x$, which is proportional to the absorbance of $x$-polarized incident light. (c) Imaginary part of ${\chi }_{xy}$, which induces polarization rotation of both $x$- and $y$-polarized incident light.