Study Revolutionizes 3D Particle Tracking with 1D Photonic Crystals

By Muhammad OsamaReviewed by Laura ThomsonOct 7 2024

In a recent article in Light | Science & Applications, researchers introduced a novel one-dimensional photonic crystal (1D PC) to enhance spin-to-orbital angular momentum conversion for single-particle tracking (SPT). This technique aims to boost the accuracy and efficiency of three-dimensional (3D) SPT, which is crucial for studying dynamic processes in various scientific fields.

The study revealed how the 1D PC improves spin-to-orbital angular momentum conversion, significantly enhancing the interferometric scattering (iSCAT) technique for 3D SPT.

Background

SPT is a powerful microscopy technique that tracks individual particles in mediums or living cells. It provides insights into how particles move dynamically over time. For example, it can track nanoscopic protein movement on cell membranes.

While traditional fluorescence-based tracking methods are effective, they face limitations like photobleaching and short fluorescence emission durations. iSCAT microscopy, a label-free technique, has emerged as a potential alternative. It offers high spatial and temporal resolution without fluorescent labels. However, iSCAT struggles with 3D SPT due to multiple contrast inversions when tracking particles along the axial direction.

Enhancing Spin-to-Orbital Angular Momentum Conversion

This paper addresses the limitations of iSCAT by introducing a 1D PC that enhances spin-to-orbital angular momentum conversion, which is crucial for improving 3D SPT. The authors designed the 1D PC to act as a substrate for iSCAT microscopy and generate a double-helix point spread function (DH-PSF).

The DH-PSF encodes a particle's axial position based on the angular orientation of its lobes. This allows for continuous 3D SPT without multiple contrast inversions.

The 1D PC comprises alternating silicon nitride (Si3N4) layers and silicon dioxide (SiO₂). These layers enhance the conversion efficiency of scattered light from a single particle into a point source with orbital angular momentum (OAM). Common-path interference occurs between the scattered light with OAM and the transmitted non-scattered light. This creates an axial-location-dependent DH-PSF for iSCAT microscopy.

This innovative approach ensures that the DH-PSF iSCAT does not experience multiple contrast inversions. As a result, it is more suitable for long-range and continuous 3D SPT.

Methodology and Experimental Setup

The researchers used numerical simulations with the finite difference time domain (FDTD) method to calculate scattered light's far-field angular-dependent electric field distribution from a single particle on the 1D PC substrate. They compared these results with those from a glass substrate, showing improved spin-to-orbit angular momentum conversion efficiency.

In the experimental setup, a single polystyrene particle (500 nm diameter) was illuminated by a left-circularly polarized (LCP) laser beam. The scattered light was collected using a high numerical aperture oil-immersed objective (NA 1.49).

The study used quarter-wave plates and polarizers to generate and select the circularly polarized light components. The interference between the scattered and non-scattered light created DH-PSF, which was captured using a back focal plane (BFP) imaging instrument.

Key Findings and Insights

The outcomes showed that the 1D PC significantly improved spin-to-orbit angular momentum conversion, resulting in a DH-PSF that provides uniform Fisher information for 3D position estimation. This technique was used to track the 3D trajectory of a single microbead attached to a bacterial flagellum, allowing precise analysis of motor dynamics. 

The authors also tracked the three-dimensional (3D) diffusion trajectories of 20 nm gold nanoparticles in glycerol solution, demonstrating the technique’s ability to track nanoparticles over a long axial range. The DH-PSF iSCAT images maintained nearly consistent contrast throughout the axial range. The axial position of the particle was encoded in the angular orientation of the DH-PSF lobes. This eliminates the problem of multiple contrast inversions, making it more suitable for continuous 3D SPT.

The study also confirmed that the DH-PSF iSCAT technique could track single nanoparticles smaller than 100 nm, such as gold and polystyrene nanoparticles.

Potential Implications

The DH-PSF iSCAT technique has significant potential across various scientific fields. Its ability to track single nanoparticles with high spatial and temporal resolution makes it valuable for studying dynamic processes at the nanoscale.

In biological research, it can be used to explore the dynamics of rotary motors like the F0F1 ATPase and bacterial flagellar motor. In nanotechnology, it can monitor nanoparticle motion to understand behavior and interactions. In material science, it supports research on nanomaterials and their interaction with surrounding media.

Conclusion and Future Direction

In summary, the presented novel approach proved effective for enhancing spin-to-orbital angular momentum conversion using a 1D photonic crystal, significantly boosting iSCAT microscopy's capabilities for 3D single-particle tracking. The DH-PSF iSCAT technique provides high sensitivity, improved image contrast, and a long axial tracking range, making it a valuable tool for various scientific applications.

Future work should refine the 1D PC design for different particle sizes and explore further applications in diverse scientific fields. Additionally, integrating advanced data processing and PSF engineering techniques could further enhance the DH-PSF iSCAT's performance.

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