Chiral phase memory of twisted light through multiple scattering

Abstract

Chiroptical signals, optical responses sensitive to molecular handedness, are rapidly suppressed by multiple scattering, fundamentally limiting their use in turbid media. Here we show that coupling molecular chirality to the topological structure of twisted light generates a protected phase observable that survives strong scattering. When Laguerre-Gaussian beams carrying orbital angular momentum propagate through chiral media, spin-orbit interaction converts circular birefringence into an azimuthal rotation of the helical wavefront. Remarkably, this chiral phase memory persists at scattering strengths that fully depolarize conventional beams, with the rotation magnitude preserved quantitatively between transparent solutions and strongly scattering tissue. The sign of the azimuthal rotation encodes molecular handedness: opposite enantiomers produce mirror-symmetric phase maps even after multiple scattering. Differential measurements between conjugate topological charges isolate the chiral contribution while cancelling achiral background, enabling the resolution of refractive-index changes of order 10-6. These results establish topological phase observables as robust carriers of weak chiral light-matter interactions in complex media, opening new routes for chiroptical spectroscopy and sensing beyond the ballistic-photon regime.

Figures (3)

• Figure 1: Chiral phase memory persists across scattering regimes. (a) Schematic representation of the central part of the Mach–Zehnder interferometer experimental system Meglinski: the LG beam ($\ell = 3, p = 0$), carrying OAM imparted by a spatial light modulator (SLM), traverses ex vivo skin sample and cuvette containing glucose solution in water; (b) Interference between the transmitted LG beam and a reference plane wave (initially an expanded Gaussian beam), captured by camera for phase retrieval analysis, for the glucose concentration range of $C \in [50,150]~mg/dl$; (c) OAM-related phase shift versus glucose concentration $C$ for: (left) ex vivo porcine skin tissue (strongly scattering, $z/l^* \approx 10.0$) placed before a cuvette containing glucose solution; (right) nearly transparent tissue phantom (weakly scattering, $z/l^* = 0.1$). Linear fits demonstrate that the rate of OAM twist preserves the quantitative relationship despite varying scattering strength. Error bars represent standard deviation from $n = 10$ measurements.
• Figure 2: Phase decomposition for OAM-based chiral detection. Individual phase contributions to total LG beam phase $\Psi(\rho,\phi,z;C,\ell)$ at the detection plane for $\ell = 5$, respectively: Radial wavefront curvature $-k \rho^2 z/2(z^2 + z^2_R)$, arising from beam divergence; Azimuthal helical phase $-\ell \phi$ (carrying topological charge); Longitudinal propagation phase $-kz$; the Gouy phase $G(z) =(2p + |\ell| + 1)arctan(z/z_R)$; Complete geometric phase including radial curvature, helical, longitudinal propagation and Gouy phases at the cuvette entrance ($z = 70~cm$); and Scattering-free medium-induced phase accumulation $\Psi_{\text{medium}}(C,\ell)$, manifesting as coherent rotation of the helical structure by angle $\theta_{\text{OAM}} \propto \Delta n_{\text{CB}}(C)$ relative to reference concentration ($C = 50~mg/dl$). In transparent (non-scattering) media, the achiral contribution $\Psi_{\text{achiral}} = 0$, and the observed phase shift arises purely from glucose-induced circular birefringence. Color scale: phase from $\pi$ to $2\pi$.
• Figure 3: Chiral phase discrimination between glucose enantiomers through multiple scattering. Polar plots of azimuthal rotation $\theta_{\text{OAM}}$ extracted from unwrapped phase maps $\Psi(\rho,\phi)$ at fixed radius $\rho$: (a) ex vivo porcine skin ($z/l^* \approx 10$) and (b) tissue phantom ($z/l^* = 2$) for $D(+)$-glucose; (c) tissue phantom for L(-)-glucose, measured with opposite topological charges; $\ell = +5$ and $\ell = -5$. (d) Phase retrieval workflow: wrapped vortex phase and corresponding weighting matrices (top row), and resulting unwrapped phase maps (bottom row), obtained by off-axis vortex interferometry (referenced to $C=50~mg/dl$). (e) Differential phase maps $\Psi_{\ell=+5}-\Psi_{\ell=-5}$ through ex vivo porcine skin for glucose concentrations $50 - 150~mg/dl$. (f) Differential phase $|\Psi_{\ell = +5} - \Psi_{\ell = -5}|$ versus glucose concentration, isolating the chiral glucose-induced contribution. Symbols show mean over $n = 10$ independent measurements; error bars indicate standard deviation.