Vid Agrež

An optical fiber-based Mach–Zehnder interferometer was developed for photothermal detection of gold nanoparticles in aqueous suspensions. The excitation laser locally heats the suspension, inducing a refractive index change that is detected as a phase shift in the interferometer's measuring arm. By employing excitation wavelengths of 450 nm and 525 nm, the wavelength-dependent absorption was analyzed and compared to Mie theory predictions, enabling simultaneous determination of particle size and concentration. The absorption cross-section ratio, calculated from measurements at both wavelengths, depends only on particle size and provides the basis for size determination without prior knowledge of concentration. The system demonstrated high sensitivity, with detection limits as low as 8 × 10⁶ particles per milliliter for 100 nm gold nanoparticles. The optical fiber-based architecture offers simplicity, robustness, and potential for integration into portable sensing systems for nanoparticle characterization and biosensing applications.

One of the hallmarks of a versatile laser is the ability to generate pulses on demand. In this work, we demonstrate a self-stabilizing pulse on demand fiber laser with passive gain control through an implemented gain-clamped oscillator that generates an idler beam that is filtered out from the amplifier output. The amplification gain is defined by the losses in the oscillator, while the pump power determines the maximum repetition rate for stable amplification. The temporal dynamics of the system are analyzed in the context of gain switching theory to determine the maximum frequency at which the stable amplified pulses can be obtained at the output. As a proof of principle, the range from single pulses to repetition rates over 100 kHz is demonstrated with constant pulse energies. For arbitrary seeding of gain clamping oscillator, a laser diode-based seeder with a pulse duration down to 60 ps and repetition rates of up to 200 MHz is used. The high temporal resolution of the seed is also used to investigate the temporal dynamics of the population inversion in the gain clamped amplifying fiber after a single pulse passage. The presented approach is suitable for fiber preamplifier systems where low power signals consisting of arbitrarily spaced pulses need to be amplified to an appropriate level prior to high power amplification.

The collapse of microbubbles near a fiber is an example often encountered in water treatment situations and cavitation fibrillation processes. However, due to the broken symmetry conditions, this process has not been studied in detail experimentally or numerically, making it difficult to precisely measure or simulate the rapid bubble evolution during collapse. In this work, we present a novel experimental method, allowing for precisely repeatable cavitation events observation, combined with numerical simulations offering insight into pressure and velocity fields distribution developments in time. Both experimental and numerical works focused on small distances between the bubble and the fiber, where the physical interaction between subjects is the strongest. Four different bubble offsets were considered within the scope of this work, and very good agreement of numerical simulations with experiments was found in all cases. Two modes of bubble collapse were identified, leading to mushroom-shaped bubbles at positions closest to the fiber and a pear-shaped bubble at the farthest position. It is noteworthy that in all four cases, a planar jet formation toward the fiber was observed. The formed jet initially assumes an elongated shape, whereas its stability depends on the mode of bubble collapse. Numerical analysis of the planar jet as the defining feature of the collapse defined lower bounds for the actual values of peak jet velocities, ranging between 250 and 330 m/s, and the resulting impact pressures, which range from 100to 500 MPa.

Shock wave visual detection was traditionally performed using streak cameras, limited to homogeneous shock wave emission, with the corresponding shock wave pressure measurements available at rather large distances or numerically estimated through equation of state for water. We demonstrate a multi-frame multi-exposure shock wave velocity measurement technique for all in-plane directions of propagation, based on custom-built illumination system allowing multiple illumination pulses within each frame at multi-MHz frame rates and at up to 200 MHz illumination pulse repetition frequency at sub-nanosecond pulse durations. The measurements are combined and verified using a fiber-optic probe hydrophone, providing independent shock wave pressure and time-of-flight measurements, creating a novel all-optical measurement setup. The measured pressures at distances around 100 µm from the plasma center exceed 500 MPa, while camera-based measurements at even shorter distances indicate pressures above 1 GPa.

Precise excitation of cavitation is a promising mechanism for microsurgery procedures and targeted drug delivery enhancement. The underlying phenomenon of interest, jetting behaviour of oscillating cavitation bubbles, occurs due to near-surface interactions between the boundary, liquid, and bubble. Within this study we measured boundary effects on the cavitation bubble dynamics and morphology, with an emphasis on observation and measurement of jetting behaviour near tissue-phantom biointerfaces. An important mechanism of boundary poration has been observed using time-resolved optical microscopy and explained for different tissue-phantom surface densities and Young’s modulus. Below a critical distance to the boundary, around γ = 1.0, the resulting jets penetrated the tissue-phantom, resulting in highly localized few micrometer diameter jets.