Surface Enhanced Spectra Of Chiral Molecules

Chiral molecules are pervasive in natural world.Molecular chirality determines their physical and chemical properties.Detection and characterization of these chiral enantiomers are of considerable importance for biochemical and pharmaceutical industries.In the thesis,we systematically discuss various methods of enhancing the sensitivities of spectroscopies of chiral molecules with nanostructures.Specifically,this article have investigated the capabilities of plasmonic nanoparticles,silicon nanoparticles with high refractive indices and dielectric photonic crystal slabs in enhancing the intensities and the signal-to-noise ratios of various kinds of chiral spectra.Besides,the possibility of using orbital angular moment beams to probe chiral molecules has also been discussed.First,two different aspects of chirality in plasmon enhanced CD(Circular Dichroism)have been discussed: the molecule-induced chirality and the structural chirality.The influence of interaction between different chiral molecules on the molecule-induced chirality has been investigated.We have found that when the resonance frequency of the chiral molecules is set to be equal with that of the plasmon resonance frequency of the metallic cluster,interaction between different molecules leads to shifts of the peaks of the CD.Furthermore,two types of structural chirality have been found through numerical calculation,which are either caused by dislocation and rotation of the nanoparticles.By comparison,it is found that the dislocation-induced CD can completely suppress the molecule-induced CD,while the rotation-induced CD can be spectrally distinguished from the CD caused by chiral molecules.Second,whether lights carrying orbital angular moment can be used in detection of molecular chirality has been investigated.To this end,the orbital dichroism(OD)has been defined to characterize the sensitivity of orbital angular moment to the chirality of nano systems.According to a theory based on the T-matrix method and through numerical calculation,it is found that giant OD signals induced by the excitation of plasmon resonances can be observed,when molecule-nanoparticle composites are located in some fixed regions.On the other hand,it can be proved analytically that,if the samples are randomly distributed in space,OD signal will be vanished.Thus,in experiments,the orbital angular momentum beam is not suitable for the identification of chiral molecules.Third,a giant chiroptical effect caused by the electric quadrupole is discussed.So far,all the theories on plasmon-induced circular dichroism(CD)have been based on the dipole approximation;the electric quadrupolar contribution is generally considered to be relatively small and neglected.This approximation is reasonable when the samples are randomly oriented.However,the chiral molecules adsorbed on the surfaces of nanostructures possess preferential molecular orientations.Thus,the contribution of molecular electric quadrupole transitions to the total CD signal can play an important role.Through further developing the T-matrix method,we have investigated the effect of the electric quadrupole on the CD of the molecule-nanoparticle system.Here we demonstrate that the electric quadrupolar contribution not only cannot be ignored,but it also plays a key role in many cases.It is found that the electric quadrupole induced CD is closely related to the electric field gradients,the enhancement of electric quadrupole emission caused by the metallic nanoparticles and the symmetry of the nanocluster.Fourth,nanoparticles with high refractive indices can act as efficient light concentrators,which pave a new way for ultrasensitive detection of chiral molecules.Based on an analytical theory of dipole approximation,and through magnitude analysis,we have demonstrated that the intensities and signal-to-noise ratios of Raman optical activity can be strongly enhanced by silicon nanoparticles with high refractive indices.According to analytical derivation,the phenomena originate from large magnetic fields concentrated near the nanoparticle and boosted magnetic dipole emission of the molecule.Furthermore,the symmetric breaking of the electric fields caused by the magnetic dipole response of the nanoparticle also plays an important role.At last,we demonstrate that photonic crystal slab can be used in ultrasensitive detection of chiral molecules.By fine-turning the parameters,a slab with a fourfold degeneracy can be found.We discovered that the slab can achieve high enhancements of optical chirality in the region of nano holes when its state with the fourfold degeneracy is excited by a circular polarized wave.Taking advantage of such a slab,ultrasensitive detections of circular dichroism,fluorescence-detected circular dichroism and vibrational circular dichroism can be realized.