Chirality, a lack of the mirror symmetry of an object, exists in DNA, many amino-acids, sugars, enzymes, and drugs. Specifically, two nonsuperimposable mirror images of the same chiral molecule are called enantiomers. The two enantiomers have the same chemical formula, but the spatial symmetry breaking leads to different effects on human body; e.g. while one enantiomer acts as a drug, the other one can be inactive or even lead to side-effects. Thus, it is important to have a sensor able to detect and distinguish ultra-low enantiomer concentrations (i.e. enantioselectivity), which is difficult as they have most of physical properties equal. Luckily, the chirality leads to different interaction with left and right circularly polarized light (LCP and RCP, respectively), thus photonic components are used for characterization of chiral solutions. However, conventional techniques usually require big volumes, high concentrations and long post-processing. Recently, there has been an interest in mimicking the chiral effects at the nanoscale. Special designs with broken symmetry in nanomaterials lead to their own chirality, and to the enhancement of chirality when they interact with chiral molecules. In this work, we study chiral substrates fabricated by low-cost nanosphere lithography. Arrays of tilted elliptic nanoholes are numerically designed at SBAI Department, produced in collaboration with University of Padova (prof. G. Mattei and T. Cesca), and characterized in our laboratory. Characterization involves measurements different LCP and RCP extinction and absorption by means of transmission and photo-acoustic spectroscopy, with white light and laser sources. We characterize chiral substrates alone, chiral molecules on glass, and, finally, chiral molecules on chiral substrates, and monitor the change in enantioselectivity. The ultimate goal of the research is to use these low-cost substrates as chiral detectors able to distinguish low concentrations of both enantiomers.
Intrinsically chiral nanostructures are usually produced by expensive and slow EBL-based steps, and their intrinsically highly asymmetric shapes make the coupling with the chiral molecules more difficult and unpredictable. Namely, one usually deposits the solution with some low concentration of enantiomers on the chiral substrate, and waits for the solvent to evaporate. Due to the surface pressure, the chiral molecules remain on the substrate. Next, when it comes to the interaction of the substrate (now with the molecules) with the light, at the nanoscale hot spots of high electromagnetic field enhancement and confinement occur, thus enhancing the interaction between the molecule and the substrate. If the nanostructure in the unit cell of the substrate is highly asymmetric itself, the hot-spots will be present only in small parts of the volume, and only the part of the molecules present in that exact volume will have the enhanced interaction. This would finally lead to the decrease of sensitivity. Moreover, for the enantioselectivity measurements, the possibility to enhance the interaction for both right and left enantiomer is required. This is usually done by fabricating the two metamaterials making the enantiometric pair themselves, i.e. with the same dimensions, but inverted geometry; the first is then used for measurements of coupling with one type of molecule enantiomer, and the second for the coupling with the other enantiomer. Both high-cost and complicated coupling drawbacks can be partially avoided by using low-cost, self-assembling technique to produce dielectric nanostructures, and then cover then asymmetrically by a plasmonic thin layer. One type of such nanostructures are semiconductor nanowires asymmetrically covered by Au. However, another issue occurs due to the mechanic instability of 3D grown nanostructures, with higher aspect ratios. One must ensure that they remain stable, with unchanged geometric parameters after the liquid solvent with chiral molecules is deposited on them. Our proposal does not have any of these issues, because the mechanically unstable parts of the nanostructure (the polystyrene nanospheres, reactive with the solvent) are removed by the end of the fabrication process. The main advantage is that we use the NSL as a low-cost, self-assembling technique to produce plasmonic chiral substrates that are planar and, as such, easier to couple with the molecules. Our approach offers many degrees of freedom that we can optimize for the optical range of interest for the specific molecule. Namely, once the chiral enantiometric pair of molecules is chosen, their intrinsic CD and its dependence on concentration is known in the visible spectral range (we characterize it by the same set-up that will be further used for the substrate-molecule measurements). Then, the chiral nanohole substrates' CD is optimized in that range by means of numerical simulations. The dimensions for NSL fabrication are then chosen by controlling the starting polystyrene diameter choice (which defines the periodicity), RIE etching time (which defines the nanohole diameter), and the metal deposition angle offsets from the surface normal and in-plane hexagonal geometry, which finally defines the chirality. After this, the chiral molecules (in solution) are deposited on our planar substrates, and the CD of the substrate-molecule combination is measured. Apart from extinction-based CD measurements, we moreover propose the photo-acoustic technique, which is a photothermal technique that directly gives the absorption response, without scattering and complicated post-processing of the results (as in transmission/reflection-based measurements). In order to ensure the sensibility and stability of our measurements, especially in terms of the circular polarization entering the chiral sample, white-light lamp source will be later replaced by laser sources working in the range of the CD of the chiral molecule. In particular, for nanoholes in Au, lasers at 532nm, 633nm and 980nm will be used, as well as the new tunable laser with fundamental wavelength tunable from 680nm to 1080nm. As a proof of principle we will first buy conventionally available enantiometric pairs, and characterize CD as presented above. We strongly believe that these steps can lead to low-cost, simple way of enhanced chiral detection. Finally, as the future outlook, we plan on investigating the emission properties of specific chiral molecules. This idea is based on the enhanced near chiral field at the resonance of the plasmonic substrate, which controls the absorption rate of the enantiomer that exhibits photoluminescence at higher wavelengths. There is a plenty of room in the experimental research of chirality governed emission phenomena, which might be important applications regarding enhanced enantioselectivity, polarization control and conversion, and circularly polarized emission.