arXiv subjects

Matjaž Humar

Publications and source records attributed to Matjaž Humar.

6 recordsLinked to original sources

arXiv2025

Nanometer-precision tracking of adipocyte dynamics via single lipid droplet whispering-gallery optical resonances

Biophotonics - and more recently, biointegrated photonics - offer transformative tools for probing cellular processes with unprecedented precision. Among these, whispering gallery mode (WGM) resonators - optical microcavities formed in spherical structures - have emerged as powerful biosensors and intracellular barcodes. Lipid droplets (LDs), with their high refractive index and intrinsic spherical geometry, are ideal candidates for supporting intracellular lasing. Although lasing in LDs has been previously demonstrated, it has not yet been harnessed to study live cell biology. Here, we report the first use of WGM resonances in LDs of live primary adipocytes, employing a continuous-wave (CW) laser at powers below the biological damage threshold. By measuring these resonances, we achieved nanometer-scale precision in size estimation, enabling real-time observation of rapid LD dynamics and deformations on the minute scale, far beyond the spatio-temporal resolution of conventional microscopy. We systematically characterized this photonic sensing approach, demonstrating its ability to resolve adipocyte heterogeneity, monitor lipolytic responses to forskolin and isoproterenol, and detect early signs of cell viability loss, well before conventional assays. This proof-of-concept establishes intracellular LD WGM resonances as a robust platform for investigating live single-cell metabolism. The technique enables rapid, cost-effective assessment of adipocyte function, reveals cell-to-cell variability obscured by bulk assays, and lays the foundation for high-throughput analysis of metabolism- and obesity-related diseases at both cellular and tissue levels.

physics.optics↗

arXiv2024

Edible microlasers for monitoring authenticity and quality of food and pharmaceuticals

Traceability, security and freshness monitoring are crucial to the food and pharmaceutical industries. Currently, barcodes and sensors are almost exclusively located on product packaging. Making them edible and introducing them into edible products could significantly enhance their functions. Here, several types of microlasers made entirely out of edible substances were developed. It is striking that olive oil already contains enough chlorophyll to be used as a laser when dispersed in water as droplets. The edible lasers can be embedded directly into edible products and serve as barcodes and sensors. Due to their much narrower spectral lines compared to fluorescent or color-changing sensors, they are significantly more sensitive to various environmental factors. The edible lasers were employed to sense sugar concentration, pH, the presence of bacteria, and exposure to too-high temperatures. They can also encode tens of data bits, such as manufacturer's information and expiration date. The microlasers are entirely safe for consumption, do not change the appearance and taste of food considerably, and are environmentally friendly. The developed barcodes and sensors could also be applied to non-edible items, such as cosmetic and agricultural products, for environmental monitoring and biomedical applications.

physics.optics↗

arXiv2024

Tuneable entangled photon pair generation in a liquid crystal

Liquid crystals, with their ability of self-assembly, strong response to the electric field, and integrability into complex systems, are key materials in light beam manipulation. Recently discovered ferroelectric nematic liquid crystals also possess a considerable second-order optical nonlinearity, making them a perspective material for nonlinear optics. Their use as sources of quantum light could drastically extend the boundaries of photonic quantum technologies. However, spontaneous parametric down-conversion, the basic source of entangled photons, heralded single photons, and squeezed light, has been so far not observed in liquid crystals - or in any liquids or organic materials. Here, we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with the efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is dramatically varied by applying a few volts or twisting the molecular orientation along the sample. A liquid crystal source enables a new type of quasi-phasematching, which is based on molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices, and multi-pixel tunable quantum light sources.

physics.optics↗

arXiv2023

Smectic and soap bubble optofluidic lasers

Soap bubbles are simple, yet very unique and marvelous objects. They exhibit a number of interesting properties such as beautiful interference colors and the formation of minimal surfaces. Various optical phenomena have been studied in soap films and bubbles, but so far they were not employed as optical cavities. Here we demonstrate, that dye doped soap or smectic liquid crystal bubbles can support whispering gallery mode lasing, which is observed in the spectrum as hundreds of regularly spaced peaks, resembling a frequency comb. The lasing enabled the measurement of size changes as small as 10 nm in a millimeter-sized, $\sim$100 nm thick bubble. Bubble lasers were used as extremely sensitive electric field sensors with a smallest measurable electric field of 110 Vm$^{-1}$Hz$^{-1/2}$. They also enable the measurement of pressures up to a 100 bar with a resolution of 1.5 Pa, resulting in a dynamic range of almost $10^7$. By connecting the bubble to a reservoir of air, almost arbitrarily low pressure changes can be measured while maintaining an outstanding dynamic range. The demonstrated soap bubble lasers are a very unique type of microcavities which are one of the best electric field and pressure microsensors to date and could in future also be employed to study thin films and cavity optomechanics.

physics.optics↗

arXiv2023

From coffee stains to uniform deposits: significance of the contact-line mobility

Hypothesis: Contact-line motion upon drying of sessile droplet strongly affects the solute transport and solvent evaporation profile. Hence, it should have a strong impact on the deposit formation and might be responsible for volcano-like, dome-like and at deposit morphologies. Experiments: A method based on a thin-film interference was used to track the drop height profile and contact line motion during the drying. A diverse set of drying scenarios was obtained by using inks with different solvent compositions and by adjusting the substrate wetting properties. The experimental data was compared to the predictions of phenomenological model. Findings: We highlight the essential role of contact-line mobility on the deposit morphology of solution-based inks. A pinned contact line produces exclusively ring-like deposits under normal conditions. On the contrary, drops with a mobile contact line can produce ring-, at- or dome-like morphology. The developed phenomenological model shows that the deposit morphology depends on solvent evaporation profile, evolution of the drop radius relative to its contact angle, and the ratio between initial and maximal (gelling) solute concentration. These parameters can be adjusted by the ink solvent composition and substrate wetting behaviour, which provides a way for deposition of uniform and at deposits via inkjet printing.

physics.flu-dyn↗

arXiv2020

Topological liquid crystal superstructures as structured light lasers

Liquid crystals (LCs) form an extremely rich range of self-assembled topological structures with artificially or naturally created topological defects. Some of the main applications of LCs are various optical and photonic devices, where compared to their solid state counterparts soft photonic systems are fundamentally different in terms of unique properties such as self-assembly, self-healing, large tunability, sensitivity to external stimuli and biocompatibility. Here we show that complex tunable microlasers emitting structured light can be generated from self-assembled topological LC superstructures containing topological defects inserted into a thin Fabry-P\'erot microcavity. The topology and geometry of the LC superstructure determine the structuring of the emitted light by providing complex three dimensionally varying optical axis and order parameter singularities, also affecting the topology of the light polarization. The microlaser can be switched between modes by an electric field and its wavelength can be tuned with temperature. The proposed soft matter microlaser approach opens new direction in soft matter photonics research, where structured light with specifically tailored intensity and polarization fields could be designed and implemented.

physics.optics↗