Sculpting electromagnetic fields to induce forbidden transitions in molecules
Yiqiao Tang, Nan Yang
We are working on a new class of spectroscopies in which the 3-dimensional shape of the electromagnetic field is engineered to induce qualitatively new physical effects in molecules. By sculpting the electromagnetic field we hope to:
- photoexcite molecules of a single chirality in the presence of an excess of their mirror-image brethren, with enantioselectivity up to 100 times larger than is achieved by circularly polarized light;
- create photochemical reactions in which the outcome is exquisitely sensitive to an externally applied magnetic field.
The unifying idea behind these spectroscopies is to engineer the spatial degrees of freedom of the electromagnetic field to couple strongly to molecular transitions that are only weakly tickled by far-field plane waves.
An ultimate goal of spectroscopy is to control the electromagnetic field throughout a complex molecule, atom-by-atom and femtosecond-by-femtosecond. The goal of ultrafast spectroscopy is to change the field on timescales comparable to the internal dynamics of a molecule, and thereby to learn about these dynamics. While there is much work on pushing spectroscopy to femtoseconds and below, there is currently less emphasis on creating fields that vary significantly over distances comparable to the size of a molecule. Spatially engineered fields allow one to probe the physical structure of a molecule and its excitations in a manner that is inaccessible to far-field techniques working on any timescale, even ultrafast. These two approaches provide complementary information about the temporal and spatial dynamics of excitations within complex molecules.
1. Superhelical-light-enhanced circular dichroism
It has long been known that chiral objects interact asymmetrically with chiral fields. For example, opposite enantiomers are excited at slightly different rates when exposed to circularly polarized light (CPL). Differential absorption between left- and right-CPL provides abundant information about the structures of chiral molecules. This circular dichroism measurement is widely used to characterize organic and biological compounds.
Chiroptical effects are typically small, due to a mismatch between the wavelength of light and the size of most molecules. To couple more strongly to molecular chirality, one should increase the helicity of the electromagnetic field. We invented a quantity which we call the relative electromagnetic helicity, which measures the degree of chirality, or twistiness, of the electromagnetic field.
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where ε is the permittivity of the medium, ω the frequency, c the speed of light, and E the electric field; the brackets indicate an average over time.
We found a simple geometry in which the relative helicity of light is greatly enhanced, in some regions of space, relative to a circularly polarized plane wave. These super-helical fields are created from two counter-propagating CPL plane waves, of the same frequency and phase, slightly different amplitude, and opposite handedness.
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•Blue: Helicity in superhelical light
•Purple: Helicity in conventional circularly polarized light
•Red: Intensity distribution in superhelical light |
Planar chiral molecules R- and L-Helicene have been synthesized, characterized, and studied via fluorescence-detected circular dichroism (FDCD).
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(a) molecule structure of R- and L-enantiomer (b) Circular dichroism spectrum measured by commercial spectrometer (c) fluorescent images (under UV illumination) (d) FDCD images for R- and L-Helicene. The area in the two green boxes is a racemic mixture, and the regions in red boxes are R-Heliecene (left) and L-Helicene (right).
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To visualize the enhancement circular dichroism in super-helical fields, we used micropatterning scheme and a microscope.
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Microfabrication of alternating stripes of chiral and achiral fluorophores. a) – c) formation of a poly(dimethyl siloxane) (PDMS) stamp by conventional photolithography followed by casting of PDMS; d) – e) inking of the stamp with a solution of chiral fluorophores; f) transferring the pattern to a substrate via microcontact printing; g) – h) repeating the process with achiral fluorophores. i) Illumination scheme for detecting enantioselective excitation in a superhelical standing wave. Left- and right-CPL are alternately introduced into the microscope. A mirror mounted above the sample reflects the light to generate a super-helical standing wave. A coverslip patterned with chiral and achiral molecules is introduced at an angle relative to the mirror. j) Excitation pattern without the top mirror, showing alternating chiral and achiral stripes (the chiral and achiral stripes are different widths so we can tell them apart). Scale bar 10 mm. k) Excitation pattern with the top mirror, showing an optical standing wave (vertical stripes) superimposed on the patterned molecules (horizontal stripes). |