Non-Dichroic Enantio-Sensitive Chiroptical Spectroscopy

TL;DR

This work introduces Non-Dichroic EnantioSensitive (NoDES) signals that are enantiosensitive but not dichroic, breaking the conventional equivalence between molecular handedness and light ellipticity in chiroptical spectroscopy. The authors generalize NoDES observation to elliptically polarized and orthogonal two-color strong-field ionization of chiral molecules (e.g., $(+)$-fenchone and $\alpha$-pinene), obtaining 3D photoelectron momentum distributions by tomographic reconstruction up to $l=9$. The photoelectron angular distribution is analyzed via spherical-harmonic decomposition, $P(\theta,\phi) = \sum_{l=0}^{2N}\sum_{m=-l}^{l} b_{l,m} Y_{l,m}(\theta,\phi)$, showing that NoDES corresponds to odd $l$ with $m<0$ (enantiosensitive but non-dichroic), while certain coefficients like $b_{3,0}$ and $b_{3,2}$ are enantiosensitive and dichroic. The results demonstrate symmetry-protected enantiosensitivity that is robust to the relative phase between field components, enabling imaging of ultrafast chiral dynamics and suggesting potential use with quantum light such as bright squeezed vacuum states, where dichroic signals would be suppressed but NoDES remains.

Abstract

Chiroptical effects using circularly polarized light produce signals that change sign when switching either molecular handedness (enantiosensitivity) or the light helicity (circular dichroism). Here, we break this enantiosensitive-and-dichroic paradigm by measuring a new type of chiroptical signal which is enantiosensitive but not dichroic. We photoionize chiral molecules using a strong laser field and detect the three-dimensional photoelectron momentum distribution. The non-dichroic, enantiosensitive asymmetry is encoded in octupolar and higher multipolar terms in the photoelectron angular distribution, which appear in multiphoton ionization with elliptically polarized fields or cross polarized two-color fields. The robustness of the enantiosensitivity with respect to the relative phase between the vectorial components of the ionizing field represents an example of symmetry protection, and opens unexplored opportunities for imaging ultrafast dynamics in chiral molecules, such as enantiosensitive photoelectron spectroscopy with bright squeezed vacuum states.

Figures (5)

• Figure 1: Spherical harmonic decomposition of the forward/backward asymmetric part of the 3D photoelectron angular distribution obtained by photoionizing (+)-fenchone (a,b,c) and ($-$)-fenchone (d,e,f) with an elliptically polarized 1030 nm laser field at $2\times10^{13}$$\mathrm{W/cm^{2}}$. The plotted coefficients are $b_{3,0}$ (a,d,g), $b_{3,2}$ (b,e,h) and $b_{3,-2}$ (c,f,i), normalized by the yield $b_{0,0}$ at each photoelectron energy. (g-i) show the results obtained by subtracting the response of opposite enantiomers.
• Figure 2: Forward-backward asymmetry (central plot in each panel) in the photoelectron angular distribution for (+)- (a,b) and ($-$)-fenchone (c,d), and clockwise (a,c) and counterclockwise ellipticities (b,d). The left plot in each panel shows a cut of the 3D FBA in the $(p_x, p_z)$ plane, while the right part is the NoDES distribution, obtained by antisymmetrizing the FBA along the $p_y$ axis.
• Figure 3: Strong-field ionization of (+)-fenchone by an orthogonal two-color laser field. (a) Projected PAD, symmetrized along $p_{x}$ and $p_z$, and evolution of the FBA with $\varphi$ (b-e) when the fundamental field is polarized along $x$. (f) Projected PAD when the laser field polarization is rotated by $45^\circ$. (g) NoDES signal obtained by subtracting the FBAs recorded with $\pm 45^\circ$ polarization rotations of the field. (h) Normalized NoDES signal.
• Figure 4: Spherical harmonic decomposition [Eq. (\ref{['eq:PAD']})] of the photoelectron angular distributions obtained by photoionizing (+)-fenchone (a) and ($-$)-fenchone (b) with a 1030 nm laser field at $2.5\times10^{13}$$\mathrm{W/cm^{2}}$, as a function of the laser ellipticity. The decomposition of the enantiodifferential, forward-backward antisymmetric signal is shown in (c). The inset table recaps the enantiosensitivity (Yes=Y/No=N) and dichroism (Y/N) of the different coefficients.
• Figure 5: Spherical harmonic decomposition [Eq. (\ref{['eq:PAD']})] of the photoelectron angular distributions obtained by photoionizing (+)-$\alpha$-pinene (a) and ($-$)-$\alpha$-pinene (b) with a 1030 nm laser field at $2.5\times10^{13}$$\mathrm{W/cm^{2}}$, as a function of the laser ellipticity. The decomposition of the enantiodifferential, forward-backward antisymmetric signal is shown in (c). The inset table recaps the enantiosensitivity (Yes=Y/No=N) and dichroism (Y/N) of the different coefficients.