Third-harmonic generation Stokes-Mueller polarimetric microscopy

Unified Stokes-Mueller polarimetry for multi-photon processes at varying wavelengths

Existing polarimetry, mainly focusing on harmonic generations, overlooks the differences in retardance (DRs) caused by illuminations with different wavelengths in nonlinear processes, consequently falling short in accuracy beyond frequency doubling. In this Letter, with DRs considered, we propose a universal nonlinear Stokes-Mueller (NSM) polarimetry design involving illuminations with different wavelengths. Then, we optimize the NSM measurement model, applied to sum-frequency generation (SFG) and difference frequency generation. To demonstrate the necessity of consideration of DRs, the processes of polarization measurement for SFG are simulated, where the condition number decreases by 51.2%, and the root mean square error of the nonlinear Mueller matrix decreases by 20.48%.

Third harmonic imaging contrast from tubular structures in the presence of index discontinuity

Accurate interpretation of third harmonic generation (THG) microscopy images in terms of sample optical properties and microstructure is generally hampered by the presence of excitation field distortions resulting from sample heterogeneity. Numerical methods that account for these artifacts need to be established. In this work, we experimentally and numerically analyze the THG contrast obtained from stretched hollow glass pipettes embedded in different liquids. We also characterize the nonlinear optical properties of 2,2[Formula: see text]-thiodiethanol (TDE), a water-soluble index-matching medium. We find that index discontinuity not only changes the level and modulation amplitude of polarization-resolved THG signals, but can even change the polarization direction producing maximum THG near interfaces. We then show that a finite-difference time-domain (FDTD) modeling strategy can accurately account for contrast observed in optically heterogeneous samples, whereas reference Fourier-based numerical approaches are accurate only in the absence of index mismatch. This work opens perspectives for interpreting THG microscopy images of tubular objects and other geometries.

Optimal nonlinear Stokes-Mueller polarimetry for multi-photon processes

In this Letter, we present an optimization model for nonlinear Stokes-Mueller polarimetry (SMP) to improve the precision in estimating the nonlinear Mueller matrix (MM) for two- and three-photon processes. Although nonlinear polarimeters can measure the polarization properties of multi-photon processes or materials, existing methods are suboptimal, leading to low measurement precision. Based on the model and its solution, we have designed a new measurement strategy to substantially reduce the estimation variance of nonlinear MM coefficients by approximately 58.2% for second-harmonic generation polarimetry and 78.7% for third-harmonic generation polarimetry. The model and measurement method can be directly applied to multi-photon processes to improve the precision of SMP.

Recent Advancements in Optical Harmonic Generation Microscopy: Applications and Perspectives

Second harmonic generation (SHG) and third harmonic generation (THG) microscopies have emerged as powerful imaging modalities to examine structural properties of a wide range of biological tissues. Although SHG and THG arise from very different contrast mechanisms, the two are complimentary and can often be collected simultaneously using a modified multiphoton microscope. In this review, we discuss the needed instrumentation for these modalities as well as the underlying theoretical principles of SHG and THG in tissue and describe how these can be leveraged to extract unique structural information. We provide an overview of recent advances showing how SHG microscopy has been used to evaluate collagen alterations in the extracellular matrix and how this has been used to advance our knowledge of cancers, fibroses, and the cornea, as well as in tissue engineering applications. Specific examples using polarization-resolved approaches and machine learning algorithms are highlighted. Similarly, we review how THG has enabled developmental biology and skin cancer studies due to its sensitivity to changes in refractive index, which are ubiquitous in all cell and tissue assemblies. Lastly, we offer perspectives and outlooks on future directions of SHG and THG microscopies and present unresolved questions, especially in terms of overall miniaturization and the development of microendoscopy instrumentation.

Contrast optimization in broadband passive polarimetric imaging based on color camera

Broadband polarimetric imaging consists of forming an image under spectrally wide illumination after having optimized the polarization state analyzer (PSA) to maximize the target/background discriminability. In previous works, the image sensor was monochrome, and only the intensity contrast was optimized. However, due to its spectrally varying response, the PSA not only changes the light's intensity, but also its color. This color information can serve as a further parameter to improve discrimination. In this paper, we employ a color camera in a broadband Stokes (passive) polarimetric imaging system and take into color difference's contribution to discrimination ability in optimizing the PSA setting. We show through experiments that a significant improvement of discrimination ability over monochrome imaging is obtained, especially when there are multiple objects in the scene.