Consequences of chirality in second-order non-linear spectroscopy at surfaces

Abstract

Surface second-harmonic generation (SHG) is recognized as a second-order non-linear laser-based spectroscopy offering good surface sensitivity and selectivity. With the goal of developing optical methods to study chiral surfaces, a way to detect dichroic behaviour in SHG in analogy to circular dichroism (CD) spectroscopy has been elucidated. The SHG efficiency from a monolayer of oriented chiral molecules [(R)- or (S)-2,2′-dihydroxy-1,1′-binaphthyl (BN)] has a strong dependence upon whether the excitation beam has left- or right-handed circular polarization. The relative differential SHG signal is much larger than in ordinary CD spectroscopy. The large chiroptical effects in SHG are attributed not to R, the rotational strength responsible for traditional CD (R= Im µ·m), but to the electric dipole-allowed second-order non-linear tensor, χ⁽²⁾. There is a chiral element of χ⁽²⁾, χ_xyz, that must be included in descriptions of the SHG response from chiral surfaces. The ability to measure chiral properties of an adsorbed molecule separately, with no background from its achiral properties, provides another form of chiral spectroscopy, different from CD and having no analogue in linear spectroscopy, but related in a well understood manner.