Physicists Create 3D Optical Coil with Intensity-Based Chirality

Researchers have experimentally demonstrated a new method for generating a three-dimensional spatiotemporal optical coil (STOC) whose chirality is encoded directly in the intensity distribution of the light field. Published in Light: Science & Applications, the study introduces a paradigm where optical handedness is physically inscribed into the trajectory of spatiotemporal energy flow, distinct from conventional chirality hidden in phase or polarization. The team, led by Dana Jiashaner and colleagues, achieved this by engineering the non-collinear collision of spatiotemporal optical vortices (STOVs). This approach allows for the direct observation of singularity lines and the evolution of volumetric energy flow, providing a new degree of freedom for topological photonics and ultrafast chiral-selective excitation.

Chirality is a fundamental asymmetry in nature, ranging from the molecular handedness of amino acids to the conformations of biological macromolecules and even the structure of helical galaxies. In optics, the control of chirality has traditionally evolved from spatial properties like polarization to the temporal chirality of local fields. However, conventional approaches often encode chirality in abstract degrees of freedom such as phase or polarization. This limitation restricts the direct and efficient coupling of light with matter on ultrafast timescales. The current study addresses this gap by shifting the focus to the volumetric spatial distribution of field energy density, where mirror asymmetry is explicitly manifested through twisted configurations of the intensity envelope.

The experimental setup utilized a high-stability femtosecond laser source based on Yb-doped gain media, delivering pulses with a central wavelength of 1030 ± 0.5 nm and a duration of 170 fs at a 200 kHz repetition rate. The system comprised a generation module and a characterization module. In the generation module, the incident beam was split into two arms, each coupled into identical 4f pulse shapers. These shapers employed blazed diffraction gratings and cylindrical lenses in a confocal Fourier-transform geometry to achieve linear spatial-spectral mapping. Reflective liquid-crystal spatial light modulators were programmed with specific phase and amplitude mask functions to control the topological charge of the generated STOV pulses and the relative phase between them.

To reconstruct the complex amplitude of the synthesized optical fields, the team employed Fourier Transform Spectral Interferometry. In this technique, the signal field was collinearly interfered with a time-delayed reference pulse, generating a spatio-spectral interferogram. The time delay introduced a spectral fringe pattern carrier that encoded the phase difference into the intensity modulation. By performing a Fourier analysis of the recorded interferograms, the researchers separated the phase-carrying cross-correlation terms as sidebands in the spatial-frequency domain. Isolating one of these sidebands and comparing it to a calibration measurement allowed for the robust extraction of both the spectral phase and amplitude of the signal field, ultimately revealing the full spatiotemporal field and its characteristic helical phase structure.

The generation of the STOC begins with the collision of two STOV pulses carrying different topological charges. When these pulses collide in both space and time, their superposition can exhibit new phase singularities known as secondary vortices. These vortices arise at points where the total field amplitude vanishes and the phase becomes undefined. Geometrically, these singularities correspond to the intersection of the real and imaginary components of the field on sign-changing surfaces. The number of secondary vortices is determined by the difference in input charges, N = |ℓ2 − ℓ1|, with each core carrying a unit topological charge. For instance, when STOV pulses with charges of +3 and −3 collide, six secondary vortices are generated, forming a comb-like structure in the spatiotemporal domain termed the Spatiotemporal Hollow Comb.

The transition from a planar comb structure to a three-dimensional coil occurs when the two STOV beams are oriented at a small non-collinear propagation angle. This angle introduces a linear phase gradient along the transverse direction, effectively coupling the spatiotemporal angle and the spatial coordinate y. The resulting trajectory of the secondary vortices no longer lies within a single plane but instead forms a helical structure. The two primary geometric properties of this optical coil—the number of helical arms and the pitch—are directly governed by the initial collision parameters. The number of distinct helical arms scales linearly with the topological charge difference, while the pitch is inversely proportional to the transverse wavevector component, meaning a larger collision angle produces a tighter, more densely wound coil.

To quantify the chirality inherent to the optical intensity distribution, the researchers introduced the energy-flow helicity density, denoted as hS. This quantity is defined as the dot product of the Poynting vector S and its curl, measuring the helical topology of the energy flow. Unlike conventional doughnut-shaped vortex beams, where the energy flux curl is typically orthogonal to the flow direction resulting in a vanishing helicity density, the non-collinear collision in STOCs induces a braiding of energy flow. This results in a non-zero hS, serving as a robust parameter to distinguish this 3D intensity-twisted topology from conventional vortices. The pseudoscalar nature of hS dictates the selection rules for chiral light-matter interactions, requiring a matching pseudoscalar parameter in the interacting material for a first-order response.

The physical relevance of this intensity chirality is profound, as the mechanical response of a chiral particle is mediated by its magnetoelectric polarizability. The study demonstrates that the component of the enantio-selective force along the direction of energy flow is directly determined by the local chiral density hS. This establishes a direct physical link from the field’s intensity landscape to the mechanical transfer of chirality. By varying the magnitude and sign of the angle between the two STOVs, the chirality of the helical light field can be readily controlled. The local entanglement of energy flow can induce measurable absorption differences in chiral particles and exert deterministic conservative forces and controllable torques, enabling precise manipulation of particles.

The internal energy flow of the spatiotemporal wave packet exhibits a complex structure, comprising a forward-propagating energy flux and a transverse circulation associated with orbital angular momentum. The team performed an orthogonal decomposition of the Poynting vector into a propagation component parallel to the spatiotemporal collision plane and a transverse circulating component perpendicular to this plane. The transverse component directly embodies the intrinsic helicity and twist of the optical field, serving as the physical root of the intensity-encoded chirality. Quantitative analysis revealed that these components exhibit a double-peaked intensity profile with vanishing magnitude at the geometric center, originating from the destructive interference inherent to the singularity generation. Despite the reversal of energy-flow vectors at symmetric positions, the computed local chiral density remains identical, robustly characterizing the field’s chirality as a pseudoscalar.

The study also investigated the optical angular momentum density and its relationship to the secondary vortices formed during the collision. The total orbital angular momentum within a strategically chosen cylindrical volume aligned with the secondary vortex line was computed to quantify the torque the secondary vortex region can exert. Systematic characterization of energy flux and angular momentum properties versus initial collision parameters, such as inter-vortex distance and relative topological charges, provides insights into the efficacy of these structures for particle manipulation. The spectral domain analysis further demonstrated that the core structural information encoded in the relative phases of frequency components is largely preserved, providing a robust fingerprint of the interaction regardless of propagation distance.

This work elevates the study of vortex reconfiguration from schematic two-dimensional pictures to a genuinely three-dimensional, tunable reality. By colliding counter-rotating STOVs, the researchers observed the generation of secondary vortices whose trajectories knit a spatiotemporal hollow comb on the central plane while remaining freely steerable in all three spatial directions. The ability to prescribe vortex orientation at will foreshadows optically driven torque landscapes tailored for chiral nanomechanics. The coil’s chirality, being intrinsic to its intensity, invites further investigations into whether this strong-field chirality can survive nonlinear processes, such as high-order harmonic generation, or if it can selectively address enantiomers in ways inaccessible to conventional circularly polarized light.

In summary, the experimental demonstration of tunable chiral optical fields in three-dimensional spatiotemporal domains represents a significant advancement in topological photonics. By precisely colliding STOVs, the team successfully generated dynamic, inherently chiral helical structures whose chirality is directly encoded in the light field’s intensity distribution. This method provides a powerful platform for ultrafast chiral light-matter interactions and advanced optical nanomanipulation. The findings extend the taxonomy of chiral light beyond conventional spin- or phase-based manifestations, offering a new handle for controlling volumetric angular momentum flow and opening avenues for high-dimensional information encoding and chiral-selective excitation schemes.

Subject of Research: Generation of spatiotemporal optical coil with controllable transverse intensity chirality

Article Title: Generation of spatiotemporal optical coil with controllable transverse intensity chirality

Article References: Jiashaner, D., Zhang, X., Wang, H., Tao, Z., Cao, Q., Liu, D., Luo, D., Yang, L., Geng, K., Tai, Y., Xie, X., Tao, Z., Ahsan, M. S., Fan, G., & Zhan, Q. (2026). Generation of spatiotemporal optical coil with controllable transverse intensity chirality. Light: Science & Applications, 15(1), Article 397. https://doi.org/10.1038/s41377-026-02432-0

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02432-0

Keywords: Generation, spatiotemporal, optical, coil, controllable, transverse, intensity, chirality, scientific research