Abstract
The full development of mono- or multi-dimensional time-resolved spectroscopy techniques incorporating optical activity signals has been strongly hampered by the challenge of identifying the small chiral signals over the large achiral background. Here we propose a new methodology to isolate chiral signals removing the achiral background from two commonly used configurations for performing two-dimensional optical spectroscopy, known as BOXCARS and gradient assisted photon echo spectroscopy (GRAPES). It is found that in both cases an achiral signal from an isotropic system can be completely eliminated by small manipulations of the relative angles between the linear polarizations of the four input laser pulses. Starting from the formulation of a perturbative expansion of the signal in the angle between the beams and the propagation axis, we derive analytic expressions that can be used to estimate how to change the polarization angles of the four pulses to minimize achiral contributions in the studied configurations. The generalization to any other possible experimental configurations has also been discussed.
Highlights
Chirality is a structural property of systems lacking mirror symmetry
The full development of mono- or multi-dimensional time-resolved spectroscopy techniques incorporating optical activity signals has been strongly hampered by the challenge of identifying the small chiral signals over the large achiral background
In a truly colinear/coplanar 2D electronic spectroscopy (2DES) geometry, these three polarization schemes would result in three linearly-independent chiral signals
Summary
Chirality is a structural property of systems lacking mirror symmetry. Almost all biomolecules are chiral (proteins, nucleic acids, sugars, etc.) and many artificial materials currently employed in advanced photonic applications are chiral (carbon nanotubes and graphene [1] and metamaterials [2, 3], for example). [4] The optical activity of a chiral sample strongly depends on the intimate details of its molecular stucture and it represents a fine tool to probe electronic and molecular structure For this reason, optical techniques, such as Circular Dichroism (CD) have been routinely employed to determine conformational and structural properties of these systems, for example in the assessment of secondary structures of proteins and other biologically relevant molecules [5, 6] as well as the non-symmetric arrangement of pigments in light harvesting complexes. These signals provide independent information, which can be used to better determine chromophore structures and remove ambiguity about the origins of signals observed in achiral time-resolved experiments Despite their recognized potential, the full development of ultrafast time-resolved chiral techniques and the two-dimensional (2D) analogous has been strongly limited by the fact that chiral signals are typically three to four orders of magnitude weaker than achiral contributions [4,7].
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