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The origin of second harmonic generation (SHG) signal in otoconia was investigated. SHG signal intensity from otoconia was compared to pure calcite crystals, given calcite is the primary component of otoconia and is known to emit surface SHG. The SHG intensity from calcite was found to be ∼41× weaker than the SHG intensity from otoconia signifying that the SHG signal from otoconia is likely generated from the organic matrix. Furthermore, the SHG intensity from otoconia increased when treated with a chelating agent known to dissolve calcite which confirms that calcite is not the source of SHG. Additionally, polarization-resolved SHG microscopy imaging revealed that the arrangement of the SHG emitters is radial and can form highly ordered domains.

In this work we present a novel method for depolarization compensation based on spatially variable wave plate. Thermally induced depolarization losses were reduced from 14.3% to 1.3% and bifocusing eliminated in a double-pass Yb:YAG amplifier.

The Rayleigh collapse time is the time it would take to shrink an empty spherical bubble in an infinite liquid domain to zero size, which is a function of ambient pressure and initial bubble radius. If a solid boundary is located in the vicinity of the shrinking or collapsing bubble, then liquid flow is hindered, such that the collapse time is prolonged. This can be quantified with the Rayleigh prolongation factor k. Here, we provide k for intermediate to smallest bubble to wall stand-off distances. It is measured with single laser-induced cavitation bubbles in water close to a solid boundary. Maximum bubble radii are determined from microscopic high-speed imaging at one million frames per second. Collapse times are measured acoustically via the acoustic transients emitted during bubble seeding and collapse. The experimental findings are compared, with good agreement, to numerical simulations based on a volume of fluid method. As a result, a polynomial fit of k versus stand-off distance is given for the near-wall bubble collapse in water. Then the influence of the viscosity on k is studied numerically in the near-wall regime.

We demonstrate that besides gaseous pockets also a gas supersaturated spot on a substrate can be a nucleus for cavitation. The supersaturation is achieved by either a formerly dissolved bubble or by heating locally the surface below the boiling temperature. The experiments are conducted in a thin film of water; one side of the water film is in contact with a gold coated substrate that is heated by a continuous laser through plasmonic heating. For nucleation of a bubble, the pressure at the heated spot is reduced by a transient rarefaction wave. The experimental findings suggest that the local gas supersaturation is responsible to nucleate cavitation and thus connects the phase transitions of cavitation and boiling. Additionally, the pressure waves in the liquid gap are studied numerically.

Hotothermal therapy (PTT) is an alternative to surgery, which is commonly used to treat tumors in intracavitary organs. PTT involves heating the diseased tissue with radiation energy, resulting in tumor necrosis. In order to improve the safety and effectiveness of PT, it is necessary to monitor the tissue temperature in real time and regulate the laser power during PTT. Photoacoustic imaging (PAI) is a non-invasive and non-ionizing imaging method with high resolution and high accuracy. Due to the dependence of the thermal expansion coefficient on temperature, the Grüneisen parameter is linearly proportional to temperature, and the variation of the amplitude of the photoacoustic signal is related to the variation of the Grüneisen parameter. In this study, we propose a system for laser dose regulation with photoacoustic signal temperature feedback based on PID algorithm. The pulsed laser is irradiated on the sample surface, the ultrasonic probe receives the photoacoustic signal generated by the sample, and the photoacoustic signal is collected by the oscilloscope and transmitted to the computer, which generates the corresponding command to the heating laser according to the signal and changes the output power of the heating laser. The experimental results show that this method can effectively control the photothermal damage range.

Diffuse optical tomography (DOT) use near-infrared light for imaging optical properties of biological tissues. Time-domain (TD) DOT systems use pulsed lasers and measure time-varying temporal point spread function (TPSF), carrying information from both superficial and deep layers of imaged target. In this work, feasibility of nanosecond scale light pulses as sources for TD-DOT is studied. Nanosecond sources enable using relatively robust measurement setups with standard analogue-to-digital converter waveform digitizers, such as digital oscilloscopes. However, this type of systems have some properties, such as variations in source pulses and limited temporal sampling, that could limit their usage. In this work, these different aspects and possible limitations were studied with simulations and experiments. Simulations showed that information carried by TD data of diffuse medium is on low frequencies. This enables usage of relatively slow response time measurement electronics, and image processing using Fourier-transformed TD data. Furthermore, the temporal sampling in measurements needs to be high enough to capture the TPSF, but this rate can be achieved with standard digital oscilloscopes. It was shown that, although variations in light pulses of nanosecond lasers are larger than those of picosecond sources, these variations do not affect significantly on image quality. Overall, the simulations demonstrated the capability of nanosecond sources to be utilised in TD-DOT in diffuse medium. In this work, a prototype TD-DOT experimental system utilising a high-energy nanosecond laser was constructed. The system is relatively robust consisting of a nanosecond Nd:YAG laser combined with optical parametric oscillator for light input and optical fibres for guiding the light, and avalanche photodetector and high-bandwidth oscilloscope for TPSF measurements. The system was used in both absolute and difference imaging of two phantoms. The experiments verified that both absorbing and scattering objects can be reconstructed with good accuracy with TD-DOT using a nanosecond laser.

Photoacoustic tomography (PAT) is an imaging modality that utilizes the photoacoustic effect. In PAT, a photoacoustic image is computed from measured data by modeling ultrasound propagation in the imaged domain and solving an inverse problem utilizing a discrete forward operator. However, in realistic measurement geometries with several ultrasound transducers and relatively large imaging volume, an explicit formation and use of the forward operator can be computationally prohibitively expensive. In this work, we propose a transformation-based approach for efficient modeling of photoacoustic signals and reconstruction of photoacoustic images. In the approach, the forward operator is constructed for a reference ultrasound transducer and expanded into a general measurement geometry using transformations that map the formulated forward operator in local coordinates to the global coordinates of the measurement geometry. The inverse problem is solved using a Bayesian framework. The approach is evaluated with numerical simulations and experimental data. The results show that the proposed approach produces accurate 3-D photoacoustic images with a significantly reduced computational cost both in memory requirements and time. In the studied cases, depending on the computational factors, such as discretization, over the 30-fold reduction in memory consumption was achieved without a reduction in image quality compared to a conventional approach.

A subpicosecond laser system featuring a fiber chirped pulse amplification-based seed laser and a double-pass end-pumped Yb:YAG crystal power amplifier was investigated. The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate. To the best of our knowledge, this method was applied for the first time. The presented laser system produced pulses of 441 fs duration, 116 µJ pulse energy at 116 W average power with a beam quality of M²∼2.1, featured optical-to-optical efficiency of 32% at room temperature (T=20°C), and had residual depolarization level of 2.7%.

FemtoLux 30 is a new 30 W femtosecond industrial grade laser build to work 24/7/365 without any interruptions. Other lasers of similar optical power use water for cooling, which means additional bulky and heavy water chiller is needed which require periodical maintenance (cooling system draining and rinsing, water and particle filter replacement). Moreover, in the unfortunate event of water leakage, not only laser head but also more expensive equipment could be damaged. FemtoLux 30 uses innovative direct refrigerant cooling method that do not contain any water inside the laser head and has much higher cooling efficiency. Laser cooling equipment is integrated together with the power supply unit into a single 4U rack mounted housing with a total weight of just < 15 kg. To tailor laser for specific applications, FemtoLux 30 laser has a tunable pulse duration from < 350 fs to 1 ps and can operate in very broad AOM controlled range of pulse repetition rate from a single shot to 4 MHz. While max energy of >250 µJ, that could be achieved while operating in a burst mode, could ensure higher ablation rates for different materials. FemtoLux 30 is designed as perfect tool for display and microelectronics manufacturing, as well as for micro processing and marking of brittle materials, such as glass, sapphire or ceramics, as well as for highest quality micro processing of different metals and polymers. Innovative laser control electronics ensures easy control of FemtoLux30, thus reducing time and resources required for integrating this laser into different equipment.

In the present work we performed research of supercontinuum generation in several commonly used and new supercontinuum generation crystals for subsequent nonlinear amplification, using 1–3 µJ energy pulses of 300 fs duration at 1 MHz repetition rate. Obtained supercontinuum spectra spanning over 480–1950 nm wavelength range at pump pulse energies as low as 200 nJ in KGW and YVO₄ crystals. We present simple experimental setups of stimulated Raman amplification and optical parametric amplification using supercontinuum seeds obtained from several selected crystals. We achieved total energy conversion efficiencies up to 9% both for optical parametric amplification setup and for stimulated Raman amplification setups. The optical parametric amplifier was tunable in the 680–980 nm spectral range and produced ultrashort pulses of 23–44 fs duration. Raman amplifier produced more than 130 mW average power at 1194 nm wavelength and featured broadened spectrum corresponding to Fourier transform limited ~ 100 fs pulse duration. We demonstrated that low power and low energy femtosecond lasers could be efficiently employed for the nonlinear wavelength conversion.