Circular Dichroism in Higher-Order Diffraction Beams from Chiral Quasiplanar Nanostructures

Christian Kuppe, Calum Williams, Jie You, Joel T. Collins, Sergey N. Gordeev, Timothy D. Wilkinson, Nicolae-Coriolan Panoiu, Ventsislav K. Valev
2018 Advanced Optical Materials  
sensing. [5] [6] [7] [8] [9] [10] Several relevant reviews have been published recently. [11] [12] [13] Circular dichroism (CD) is routinely used to investigate chiral nanomaterials, which absorb and scatter left-and right-circularly polarized light (LCP and RCP) differently depending on the handedness of the nanostructures. Such nanostructures can give rise to much stronger CD than chiral molecules, in part because the pitch of the twist is better matched to optical wavelengths. [14] [15] [16]
more » ... ths. [14] [15] [16] [17] [18] In metal nanoparticles, localized surface plasmon resonances (coherent oscillations of the free electrons at the surface) can greatly enhance the light-matter interaction and, by extension, the chiroptical interactions. [19] [20] [21] [22] In general, chiroptical interactions can be enhanced by increasing the chirality parameter of the nanostructures [23, 24] or of light (optical chirality). [25] Most investigations have been performed on structures with subwavelength dimensions, chiefly because this enables their theoretical treatment within an effective medium approximation. [26] However, diffractive chiral nanomaterials are also of great interest because the CD measured in the diffracted beams can be orders of magnitude larger than that in the zeroth-order beam. [27] Although previous studies have addressed the CD in diffracted beams, those studies were limited to zeroth-and first-order beams. [28] [29] [30] [31] [32] [33] [34] Here, we investigate chiral metal nanostructures that exhibit large CD in the diffracted beams. We show that, for our structures, the third-order diffraction beam gives the strongest CD response. This CD changes sign depending on wavelength. Our results are validated by a good agreement between numerical and experimental data. Moreover, we establish the robustness of our findings by making use of Babinet's principle. In order to identify the origin of the effect, we provide numerical simulations of the near field. These simulations show that for LCP and RCP beams, a difference in the electromagnetic response at the surface can be linked to the far-field CD. The samples measured in this study were fabricated using electron beam lithography (EBL)-a detailed description can be found in the Experimental Section. In Figure 1a , the dimensions and depth profile are schematically shown. The gold U-shaped structures are 1 µm in length, have a separation of 200 nm, and a highly subwavelength thickness. Each square unit cell consists of four U-shaped gold structures rotated by 90° with respect to each other. The dimensions of the actual Miniaturization down to the nanoscale has enabled a new paradigm of ultrathin optical devices, capable of manipulating the direction, polarization, and frequency of light. Great interest is drawn by the promising prospects of deep-subwavelength material dimensions. However, interesting properties and opportunities offered by structures with sizes comparable to the wavelength of light appear to have been overlooked. Here, quasiplanar chiral arrays made of gold are considered and show that higher-order diffracted beams can yield extremely large chiroptical responses for optical frequencies. The chosen sample geometry demonstrates spectrally tunable polarization conversion and extremely large circular dichroism. Experimental and numerical data are in good agreement, for both sample chiral forms, and for the complementary geometries under Babinet's principle. Specifically, the experimental results show that the fractional circular dichroism (CD) can be as high as 20%, in the third-order diffraction beam. Based on the numerical results, a great potential for improvement is anticipated, which makes higher-order diffraction CD a very promising candidate for ultrathin optical applications.
doi:10.1002/adom.201800098 fatcat:tesbppkkpnfbxbi7hkfqcb4u6u