Giant plasmonically induced circular conversion dichroism in an anisotropic golden slit grating filled by a chiral medium

We report the giant plasmonically induced (PI) circular conversion dichroism in transmission $\left(\mathrm{CC}{\mathrm{D}}_T\right)$ and PI-unconverted CD effect $\left(\mathrm{unCC}{\mathrm{D}}_R\right)$ in reflection from the golden slit grating filled by chiral medium (GSGCM). The $\mathrm{CC}{\mathrm{D}}_T$ and $\mathrm{unCC}{\mathrm{D}}_R$ are the differences between the off-diagonal elements in transmission matrix and diagonal elements in reflection matrix, respectively, and originate from both the chirality and anisotropy. Our research indicates that the $\mathrm{PI}-\mathrm{CC}{\mathrm{D}}_T$ and PI-$\mathrm{unCC}{\mathrm{D}}_R$ in the plasmonic resonance wave band come from the coupling between the inherent chiroptical resonance of chiral medium and the plasmonic resonance of the golden slit grating, where the real part of chiral parameter $\xi$ from chiral medium plays a main role in the coupling process. The isotropic chiral medium slab, the anisotropic chiral medium grating, the golden slit grating, and the GSGCM have been rigorously investigated in this work. Our research clearly reveals the difference of PI-chiroptical effect between the anisotropic and isotropic system, and will find potential applications in biosensor and nanophotonic devices.

Figure 1

(a) Schematic of the chiral medium slab, where the slab height $h$ is set as $h=120$ nm; (b), (c) are the dispersion curves of the permittivity $\varepsilon$ and the chirality parameter $\xi$ of chiral medium, where the blue and red dashed curves are related to the real and imaginary parts, respectively. E1 and $\text{E}2\left(\mathrm{E}{2}^{{}^{\prime }}\right)$ denote the peak of $\mathrm{Im}\left(\varepsilon \right)$ and $\mathrm{Re}\left(\varepsilon \right)$, and W1 and $\text{W}2\left(\mathrm{W}{2}^{{}^{\prime }}\right)$ denote the peak of $\mathrm{Im}\left({\xi }_0\right)$ and $\mathrm{Re}\left({\xi }_0\right)$; (d), (f) are the transmittance and reflectance spectra of different components in ${\widehat{T}}_{\mathrm{circ}}$ and ${\widehat{R}}_{\mathrm{circ}}$, respectively: $T_{--}$ and $R_{--}$ (red), $T_{++}$ and $R_{++}$ (purple), $T_{+-}$ and $R_{+-}$ (blue), and $T_{-+}$ and $R_{-+}$ (green); (e), (g) The unCCD (red), CCD (green) and total CD (blue) spectra in transmission and reflection, respectively. The points E1, $\text{E}2\left(\mathrm{E}{2}^{{}^{\prime }}\right)$, W1, and $\text{W}2\left(\mathrm{W}{2}^{{}^{\prime }}\right)$ correspond to the peaks denoted in (b), (c).

Figure 2

(a) Schematic of the chiral medium grating, where the grating height $h$, period $P$, and width $d$ are set as $h=120$ nm, $P=180$ nm, and $d=30$ nm, respectively. (b), (d) Transmittance and reflectance spectra of different components in ${\widehat{T}}_{\mathrm{circ}}$ and ${\widehat{R}}_{\mathrm{circ}}$, respectively: $T_{--}$ and $R_{--}$ (red), $T_{++}$ and $R_{++}$ (purple), $T_{+-}$ and $R_{+-}$ (blue), and $T_{-+}$ and $R_{-+}$ (green). (c), (e) unCCD (red), CCD (green), and total CD (blue) spectra in transmission and reflection, respectively. W1 and $\text{W}2\left(\mathrm{W}{2}^{{}^{\prime }}\right)$ denote the peak of $\mathrm{im}\left({\xi }_0\right)$ and $\mathrm{re}\left({\xi }_0\right)$, repectively.

Figure 3

(a) Schematic of the designed golden slit grating, where one unit cell is combined by two same gold strips with width of $w=50$ nm, height of $h=120$ nm, and distance of $d=30$ nm, and the grating period is $P=180$ nm; (b), (d) Transmittance and reflectance spectra of different components in ${\widehat{T}}_{\mathrm{circ}}$ and ${\widehat{R}}_{\mathrm{circ}}$, respectively: $T_{--}$ and $R_{--}$ (red dashed), $T_{++}$ and $R_{++}$ (purple), $T_{+-}$ and $R_{+-}$ (blue dashed), and $T_{-+}$ and $R_{-+}$ (green). The inset images denote the plasmonic resonance modes at 522 nm (marked by P1) and 616 nm (marked by P2), which are the charge distribution in the XZ plane upon the injection of RCP light; (c), (e) unCCD (red), CCD (green), and total CD (blue) spectra in transmission and reflection, respectively.

Figure 4

(a) Schematic of the designed GSGCM, where one unit cell is combined by one chiral medium strip with width of $d=30$ and two same gold strips with width of $w=50$ nm, and the grating height $h$ and period $P$ are $h=120$ nm and $P=180$ nm, respectively; (b), (d) Transmittance and reflectance spectra of different components in ${\widehat{T}}_{\mathrm{circ}}$ and ${\widehat{R}}_{\mathrm{circ}}$, respectively: $T_{--}$ and $R_{--}$ (red dashed), $T_{++}$ and $R_{++}$ (purple), $T_{+-}$ and $R_{+-}$ (blue dashed), and $T_{-+}$ and $R_{-+}$ (green); (c), (e) unCCD (red), CCD (green), and total CD (blue) spectra in transmission and reflection, respectively. P1 and P2 denote the nonstandard dipole mode and the electric quadruple mode of plasmonic resonance in golden slit grating at the wavelength of 522 and 658 nm, and W1 and $\text{W}2\left(\mathrm{W}{2}^{{}^{\prime }}\right)$ denote the peak of $\mathrm{Im}\left({\xi }_0\right)$ and $\mathrm{Re}\left({\xi }_0\right)$, respectively.

Figure 5

CD spectra of pure chiral medium grating (a), (b) and GSGCM (c), (d) when only $\mathrm{Im}\left(\xi \right)$ (a), (c) or $\mathrm{Re}\left(\xi \right)$ (b), (d) exists. The $\mathrm{CC}{\mathrm{D}}_T$ and un $\mathrm{CC}{\mathrm{D}}_T$ are shown in blue dashed and solid curves, respectively, and the $\mathrm{CC}{\mathrm{D}}_R$ and $\mathrm{unCC}{\mathrm{D}}_R$ are shown in red dashed and solid curves, respectively. P2 denotes the peak of plasmonic resonance peak, $\text{E}2\left(\mathrm{E}{2}^{\prime }\right)$ denotes the peak of $\mathrm{Re}\left(\varepsilon \right)$, and W1 and $\text{W}2\left(\mathrm{W}{2}^{\prime }\right)$ denote the peak of $\mathrm{Im}\left({\xi }_0\right)$ and $\mathrm{Re}\left({\xi }_0\right)$, respectively. The peak of $\mathrm{Im}\left(\varepsilon \right)$, i.e., E1, is not signed because E1 and W1 locate at the same position.

Figure 6

CD value versus chirality parameter $\xi$ spectra of GSGCM at peaks P1 (a)–(c), W1 (d)–(f), and W2 (g)–(i), which denote the peaks of plasmonic resonance of grating, $\mathrm{Im}\left({\xi }_0\right)$ and $\mathrm{Re}\left({\xi }_0\right)$, respectively. The influences of $\mathrm{Im}\left(\xi \right)$ and $\mathrm{Re}\left(\xi \right)$ are analyzed separately: (a), (d), and (g) both $\mathrm{Im}\left(\xi \right)$ and $\mathrm{Re}\left(\xi \right)$ exist; (b), (e), and (h) only $\mathrm{Re}\left(\xi \right)$ exists; (c), (f), and (i) only $\mathrm{Im}\left(\xi \right)$ exists. The CD signals in transmission and reflection, i.e., $\mathrm{C}{\mathrm{D}}_T$ and $\mathrm{C}{\mathrm{D}}_R$, are shown in black and red curves, respectively, and the converted and unconverted CD signals, i.e., CCD and unCCD, are shown in dashed and solid curves, respectively.