Detection of the Faraday Chiral Anisotropy

The connection between chirality and electromagnetism has attracted much attention through the recent history of science, allowing the discovery of crucial nonreciprocal optical phenomena within the context of fundamental interactions between matter and light. A major phenomenon within this family is the so-called Faraday chiral anisotropy, the long-predicted but yet unobserved effect which arises due to the correlated coaction of both natural and magnetically induced optical activities at concurring wavelengths in chiral systems. Here, we report on the detection of the elusive anisotropic Faraday chiral phenomenon and demonstrate its enantioselectivity. The existence of this fundamental effect reveals the accomplishment of envisioned nonreciprocal electromagnetic metamaterials referred to as Faraday chiral media, systems where novel electromagnetic phenomena such as negative refraction of light at tunable wavelengths or even negative reflection can be realized. From a more comprehensive perspective, our findings have profound implications for the general understanding of parity-violating photon-particle interactions in magnetized media.

Figure 1

(a) Arrays of ferromagnetic nanohelices are subjected to the external magnetic field ($\pm B$), collinear to their helical axis (blue arrows). Samples are excited (dark arrow) with linearly polarized light ($\pm k$), which propagates parallel to the helical axis, too. The normal reflectance (white arrow) difference $\mathrm{\Delta }R=R(+B)-R(-B)$ does not vanish due to the FCA ($\alpha \beta$) and has opposite sign for LH and RH enantiomers. (b) Real part of the solutions $b_j$ of the dispersion relation Eq. (2) in the absence (dashed) and presence (continuous) of MOA $\beta B$. The FCA drives some polarization modes to exhibit opposite group and phase velocities at tunable wavelengths (see wavelengths slightly below 800 nm in this panel).

Figure 2

(a) Schematic of the experimental setup. The external $\overrightarrow{B}$ is generated by a coil and is collinear with the light propagation direction and the helical axis. Samples are excited with (linearly polarized) laser wavelengths 440, 540, 633, and 741 nm. The optical reflectance is detected by a silicon photodiode (PD). The intensity difference ($\mathrm{\Delta }R$) between the two magnetic field directions is phase-sensitively (PS) detected by a lock-in amplifier at the frequency $f=19\mathrm{Hz}$ of the alternating magnetic field. (b) (Right panel) optical image showing the array of ferromagnetic (nickel) nanohelices on top of an 80 nm thick silver film. (Left panel) scanning electron micrograph of one of the enantiomers, showing well-defined nickel nanohelices with a pitch of $\sim 320\mathrm{nm}$.

Figure 3

(a) NOA of our samples measured as the normal reflectance difference $R^{\mathrm{RHCP}}-R^{\mathrm{LHCP}}$ between RH and LH circular polarized light in RH (black) and LH (red) helices. Both enantiomers show a clear maximum of $\sim 0.3$ within a region around $\sim 640\mathrm{nm}$. A zero reflectance difference is measured for a nickel thin film (blue). (b) MO signal in reflection $\eta$ for both RH (black) and LH (red) helices plotted as a function of $\lambda$, exhibiting zero-crossing close to the NOA maximum and a change of sign at both sides of this position in each enantiomer. Dashed lines are guide the eye. For reference, an $\eta$ below $5\times {10}^{-7}{\mathrm{T}}^{-1}$ was measured for nickel powder (blue circles), indicating our setup detection limit. Measurements were performed at the external magnetic field strength $B=0.375\mathrm{T}$.

Figure 4

(a) NOA. Calculated reflectance difference between right- ($R^{\mathrm{RHCP}}$) and left- ($R^{\mathrm{LHCP}}$) handed circular polarized light in RH (black) and LH (red) helices and a nickel film (blue). The NOA maximum appears within the range of 550 to 770 nm. (b) Calculated MO signal $\eta$ in normal reflection of both RH (black) and LH (red) helices plotted as a function of $\lambda$, exhibiting a clear Faraday chiral MO behavior (zero crossing close to NOA maximum and corresponding sign change for both enantiomers). Calculations were undertaken with linearly polarized light (as in the experiment). We note that the exact NOA maximum in (a) and zero crossing in (b) depend slightly on the separation distance between individual helices (see Ref. [29]).