Alexander Bykov

We report the implementation of a flow-imaging platform suitable for characterization of cerebral vascular function of small animals. Consisting of an ultra-high-speed streaming camera that ensures throughput up to 200 000 frames per second and high performance PC workstation with an in-house developed processing software, this platform is currently being deployed as part of the infrastructure at the Biocenter Oulu, allowing researchers to conduct detailed examination of cerebral circulation alterations which can be caused by various factors. By utilizing top level hardware and the most recent developments in imaging with dynamic light scattering methods, such as Diffuse Correlation Spectroscopy (DCS), the spatial and temporal resolution of transcranial blood flow imaging is significantly enhanced, allowing to perform close to real-time data visualization and analysis with high efficiency. The proposed platform is designed to serve a diverse range of target groups, each of whom can benefit from its innovative technology, such as pharmaceutical companies, HealthTech startups, research institutions, medical device manufacturers and wearable technology companies. By catering to these key sectors, the platform aims to drive advancements in healthcare, research, and technology.

SignificanceBiophotonics has advanced through many discoveries, yet challenges remain, including label-free biomolecular specificity, quantitative imaging, and single-molecule detection. Progress is further constrained by the need for cheaper, lighter, miniaturized materials that still meet strict optical, electrical, and mechanical specifications. This limitation can be overcome if bioinspired structures are developed. One of the developed areas in which solutions in nature are used is micro and nanostructures including nanosurfaces. It offers a way to increase biomolecular specificity and develop lightweight, low-cost devices for biomedicine. However, it requires measuring phenomena in materials and testing these materials in applications, e.g., sensing systems.AimWe offer a concise, authoritative overview of biophotonics—from nanoscale light–biomolecule interactions to bioinspired materials, phantoms, test methods, and sensor development.ApproachA coherent and comprehensive analysis of the crucial problems related to the development of bioinspired materials and devices was carried out. Recent advances in light scattering by biological surfaces enable structure characterization, disease diagnosis, red-blood-cell analysis, drug discovery, and optical imaging and sensing. Structural and genetic bases of biological photonic surfaces were examined, alongside key performance factors in bio-inspired materials—biocompatibility, biodegradability, structure-optics coupling (e.g., dynamic color change), and scalability limits. We survey chiral nanomaterials, silica frustules, and artificial surfaces that emulate peacock feathers, butterfly wings, iridescent fruits, plant petals, and beetle cuticles, highlighting complementary diagnostics—omics, hyperspectral, and terahertz imaging—for structural analysis and material innovation. We examine bio-inspired phantoms for medical calibration, recent advances in Monte Carlo tissue light-transport modeling, and the resulting applications of these materials and diagnostic tools.ResultsResults confirm a broad set of tunable bio-inspired materials: key optical phenomena were mapped, structures fabricated and modeled, phantoms validated, and strong sensor potential demonstrated.ConclusionsWe survey emerging biophotonics, review material and system requirements, and emphasize simplifying and miniaturizing sensors for biomedical use.

Polarimetry with quantum light promises improved measurements for various scenarios. However, fundamental understanding of quantum photonic state transport in complex, real media, and tools to interpret the state after interaction with the sample are still lacking. Here, we theoretically and experimentally explore the evolution of polarization‐entangled states in a turbid medium on example of tissue phantoms. By elaborating mathematical relationship between Wolf's coherency matrix and density matrix, we introduce a versatile framework describing the transfer of entangled photons in turbid environments with polarization tracking and resulting quantum state representation with the density operator. Experimentally, we reveal a robust trend in the state evolution depending on the reduced scattering coefficient of the medium. Our theoretical predictions correlate with experimental findings, while the model extends the study by photonic states with different degrees of entanglement. The presented results pave the way for quantitative quantum photonic sensing enabling applications ranging from biomedical diagnostics to remote sensing.

Recent advancements in wavefront shaping techniques have facilitated the study of complex structured light’s propagation with orbital angular momentum (OAM) within various media. The introduction of spiral phase modulation to the Laguerre–Gaussian (LG) beam during its paraxial propagation is facilitated by the negative gradient of the medium’s refractive index change over time, leading to a notable increase in the rate of phase twist, effectively observed as phase retardation of the OAM. This approach attains remarkable sensitivity to even the slightest variations in the medium’s refractive index (∼10⁻⁶). The phase memory of OAM is revealed as the ability of twisted light to preserve the initial helical phase even propagating through the turbid tissue-like multiple scattering medium. The results confirm fascinating opportunities for exploiting OAM light in biomedical applications, e.g. such as non-invasive trans-cutaneous glucose diagnosis and optical communication through biological tissues and other optically dense media.

Recent advancements in wavefront shaping techniques have facilitated the study of complex structured light's propagation with orbital angular momentum (OAM) within various media. The introduction of a spiral phase modulation to the Laguerre-Gaussian (LG) beam during its paraxial propagation is facilitated by the negative gradient of the medium's refractive index's temporal change, resulting in an accelerated retardation in OAM twist. This approach attains remarkable sensitivity to even the slightest variations in the medium's refractive index (10^(-6)). The phase memory of OAM is revealed as the ability of twisted light preserving initial helical phase even propagating through the turbid tissue-like multiple scattering medium. The results confirm fascinating opportunities of the exploiting OAM light in biomedical applications, e.g. such as non-invasive trans-cutaneous glucose diagnosis and optical communication through biological tissues and other optically dense media.

Recent advances of the biomedical polarimetry have clearly demonstrated that circularly polarized light (CPL) can be effectively used for overall characterization of biological tissues with optical anisotropy,^{1 – 3} including detection of the senile Alzheimer’s plaques^4,5 and quantitative evaluation of the cervical intraepithelial neoplasia.^6,7 Proper exploration of the CPL-tissue interaction requires accurate self-consistent descriptive simulation tools.^1,8,9 Monte Carlo (MC) based approaches are widely recognized as efficient tools for analyzing light scattering by biological tissues and turbid medium.^{10 – 14} In biophotonics, MC methods, such as MCML,¹⁵ created by L. Wang and S. Jacques, were originally designed to simulate scalar light transport within turbid scattering medium^16,17 and were fundamentally relying on the radiative transfer equation (RTE).^{18 – 20} As significant role of polarized light in extending diagnostic capabilities of biomedical tools became apparent,^21,22 MC methods evolved accordingly resulting in many practical and popular tools particularly developed by Ramella-Roman, Prahl, and Jacques,^23,24 Hielscher,^25,26 Wang,²⁷ and Xu.²⁸ Fundamental ground for these polarized MC approaches was established by the vector radiative transfer equation (VRTE), which represents a system of equations for each Stokes parameter and can be rigorously derived from the Maxwell electromagnetic theory.^{29 – 31} At the same time, an approach based on the iterative solution to Bethe–Salpeter (BS) equation ^{19,32 – 34} utilizing Jones vector formalism has been demonstrated to be effective for polarization tracking of MC-photons within turbid tissue-like medium and simulation of coherent backscattering.^{13,14,35 – 39} Recently, it has been shown on the fundamental level that VRTE- and BS-based approaches are equivalent under certain conditions.⁴⁰ Advantages of the BS-based approach involve a direct relation to the analytic Milne solution and intuitive physical interpretation of the multiple scattering process via ladder diagrams.

SignificanceThe study of the effect of aging on the optical properties of biological tissues, in particular polarization, is important in the development of new diagnostic approaches.AimThis work aims to provide a comprehensive analysis of the factors and mechanisms that contribute to the alteration of skin polarization properties caused by aging, using polarization-sensitive hyperspectral imaging measurements and Monte Carlo simulation.ApproachOur investigation involved both experimental studies of in vivo human skin of volunteers of different ages and computational modeling that accounted for changes in the absorption and scattering properties of the skin model. Specifically, we analyzed alterations in the degree of linear polarization (DOLP) to better understand the impact of aging on skin polarization properties.ResultsA statistically significant increase in the DOLP was found for the elderly group. At the same time, there was no correlation between changes in polarization and the calculated blood volume fraction parameter for different ages. According to the simulation results, it was also found that a change in the scattering properties of biological tissues has a more significant effect on the change in polarizing light compared to the change in absorption.ConclusionsThe results of the work prove that the sensitivity of polarization imaging to age- or pathological-related skin changes may be primarily due to changes in scattering, which in turn is associated with changes in the collagen fibers of the dermis. The proposed technique shows promise for in vivo non-invasive real-time assessment of age-associated skin changes and can also be extended to monitor changes associated with the development of age-related pathologies.

We consider the origin of unpolarized light resulting from the backscattering of circularly polarized light by a random turbid tissuelike disperse medium. We reveal the dynamics of the backscattered fraction of unpolarized light, disclosing its meticulous decomposition into two rigorously polarized components characterized by opposing helicities, with fully defined polarization states. Concurrently, their superposition, driven by multiple scattering within the medium, leads to the appearance of a fraction of linear polarization. We emphasize that the in-depth binding of circular polarization memory of light when the helicity flips occurs within the scattering medium, meaning the conservation of spin angular momentum. We anticipate that the results obtained hold significant implications for future studies, particularly in the field of tissue polarimetry and light vortices.

SignificanceMueller-matrix polarimetry is a powerful method allowing for the visualization of malformations in biological tissues and quantitative evaluation of alterations associated with the progression of various diseases. This approach, in fact, is limited in observation of spatial localization and scale-selective changes in the poly-crystalline compound of tissue samples.AimWe aimed to improve the Mueller-matrix polarimetry approach by implementing the wavelet decomposition accompanied with the polarization-singular processing for express differential diagnosis of local changes in the poly-crystalline structure of tissue samples with various pathology.ApproachMueller-matrix maps obtained experimentally in transmitted mode are processed utilizing a combination of a topological singular polarization approach and scale-selective wavelet analysis for quantitative assessment of the adenoma and carcinoma histological sections of the prostate tissues.ResultsA relationship between the characteristic values of the Mueller-matrix elements and singular states of linear and circular polarization is established within the framework of the phase anisotropy phenomenological model in terms of linear birefringence. A robust method for expedited (up to ∼15 min) polarimetric-based differential diagnosis of local variations in the poly-crystalline structure of tissue samples containing various pathology abnormalities is introduced.ConclusionsThe benign and malignant states of the prostate tissue are identified and assessed quantitatively with a superior accuracy provided by the developed Mueller-matrix polarimetry approach.