Broken-symmetry plasmonic nanostructures that enable circular dichroism have many applications in various fields, from pharmaceutical industry for enantioselectivity enhancement to photonics industry for producing devices such as circular polarizers. These nanostructures respond differently to the right- and left handed circularly polarized (RCP and LCP) lights. The difference is called circular dichroism (CD) and can be measured in both transmission and reflection. Due to the strong interaction between the light and plasmonic nanostructures, the CD obtained from these broken-symmetry nanostructures is usually orders of magnitude higher than that obtained from their molecular counterparts. 2-D and 3-D plasmonic nanostructures have been designed and fabricated to enable huge CDs at mid-IR frequencies with relatively complicated 3-D structures, such as nano-helices, which requires advanced nano-fabrication techniques. Shrinking the size of these nanostructures to have almost the same values of CDs at visible frequencies, where many photonic devices work and many molecules have resonances, has been very challenging due to the fabrication difficulties. Here, we present an array of 3-D plasmonic azimuthally graded-depth ramp-shaped split-ring nano-apertures, which enable a huge CD at visible frequencies in the reflection. We fabricate these nanostructures on gold-coated coverslips using focused-ion beam (FIB) milling with gradient intensity to achieve azimuthally gradient depth, hence breaking the symmetry and achieving a huge CD. We characterize the structure in the reflection regime and show a CD up to 64% and a g-factor up to 1.13 at 678 nm. We also investigate the origin of the CD by comparing the near-fields on the nanostructure in response to RCP and LCP lights. Furthermore, we show that how the depth of the nano-apertures plays a crucial role in achieving the huge CD. Oure nanostructure can be used as a host to boost enantioselectivity of chiral molecules or as a circular polarizer.<br/>

Giant circular dichroism at visible frequencies with broken-symmetry plasmonic nano-apertures

2019 Fall National Meeting

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