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High-index dielectric subwavelength structures and metasurfaces are capable of enhancing light-matter interaction by orders of magnitude via geometry-dependent optical resonances. This enhancement, however, comes with a fundamental limitation of a narrow spectral range of operation in the vicinity of one or few resonant frequencies. Here we tackle this limitation and introduce an innovative and practical approach to achieve spectrally tunable enhancement of light-matter interaction with resonant metasurfaces. We design and fabricate {\it resonance-gradient metasurfaces} with varying geometrical parameters that translate into resonant frequencies dependence on one of the coordinates of the metasurface. The metasurfaces are composed of bone-like nanoresonators which are made of germanium, and they support high-Q optical resonances in the mid-IR spectral range. We apply this general concept to observe the resonant enhancement of the 3rd and 5th harmonics generated from the gradient metasurfaces being used in conjunction with a tunable excitation laser to provide a wide spectral coverage of resonantly-enhanced tunable generation of multiple optical harmonics.

Exogenous contrast agents in photoacoustic imaging help improve spatial resolution and penetration depth and enable targeted molecular imaging. To screen efficient photoacoustic signaling materials as contrast agents, we propose a light absorption-weighted figure of merit (FOM) that can be calculated using material data from the literature and numerically simulated light absorption cross-sections. The calculated light absorption-weighted FOM shows that a Ti nanodisc has a photoacoustic conversion performance similar to that of an Au nanodisc and better than that of a Pt nanodisc. The photoacoustic imaging results of Ti, Au, and Pt nanodiscs, which are physically synthesized with identical shapes and dimensions, experimentally demonstrated that the Ti nanodisc could be a highly efficient contrast agent.

A new versatile patent-pending technology enabling new operation regimes and a unique set of features in the industrialgrade 30 W-level average power femtosecond hybrid laser is introduced in this work. The developed technology, based on the use of an all-in-fiber active fiber loop (AFL), enabled to form GHz bursts of ultrashort laser pulses with any desired pulse repetition rate and any number of pulses in a burst with identical intra-burst pulse separation. Furthermore, the AFL allowed to tune pulse duration from a few hundred femtoseconds to picoseconds and even up to the nanosecond range.

Two-dimensional matrix transducer arrays are the most appropriate imaging probes for acquiring dual-modal 3D photoacoustic (PA)/ultrasound (US) images. However, they have small footprints which limit the field-of-view (FOV) to less than 10 mm × 10 mm and degrade the spatial resolution. In this study, we demonstrate a dual-modal PA and US imaging system (using a 2D matrix transducer array and a motorized 2D scanning system) to enlarge the FOV of volumetric images. Multiple PA volumes were merged to form a wide-field image of approximately 45 mm × 45 mm. In vivo imaging was demonstrated using rat sentinel lymph nodes (SLNs) and bladders stained with methylene blue. We believe that this volumetric PA/US imaging technique with a 2D matrix transducer array can be a useful tool for narrow-field real-time monitoring and wide-field imaging of various preclinical and clinical studies.

Laser sources which produce GHz bursts of ultrashort pulses attract a lot of attention by demonstrating superior performance in material processing. Flexibility of the laser source in a selection of parameters for custom application is highly preferable. In this work, we demonstrate a very versatile method for burst formation using the active fiber loop (AFL). It allows forming GHz bursts containing from 2 up to approximately 2200 pulses in a burst (1000 ns burst width) with identical pulse separation and any predefined intra-burst pulse repetition rate (PRR). The burst pre-shaping by the amplification conditions in the AFL and by the modulation of transmission of the acousto-optic modulator was demonstrated. Industrial-grade ultrafast laser system was able to operate in the single-pulse and GHz-burst regimes. The laser system delivered high-quality 368 fs duration (FWHM) pulses of 15.3 µJ pulse energy and 30.6 W average output power at 2 MHz PRR in the single-pulse regime. In the GHz-burst operation regime, bursts of 2.2 GHz intra-burst repetition rate were formed and amplified to more than 30 W average output power with a burst energy up to 135 µJ at a burst repetition rate of 200 kHz. The sub-picosecond duration of pulses was obtained in the GHz-burst regime at different burst widths.

Subwavelength dielectric resonators assembled into metasurfaces have become a versatile tool for miniaturizing optical components approaching the nanoscale. An important class of metasurface functionalities is associated with asymmetry in both the generation and transmission of light with respect to reversals of the positions of emitters and receivers. The nonlinear light–matter interaction in metasurfaces offers a promising pathway towards miniaturization of the asymmetric control of light. Here we demonstrate asymmetric parametric generation of light in nonlinear metasurfaces. We assemble dissimilar nonlinear dielectric resonators into translucent metasurfaces that produce images in the visible spectral range on being illuminated by infrared radiation. By design, the metasurfaces produce different and completely independent images for the reversed direction of illumination, that is, when the positions of the infrared emitter and the visible light receiver are exchanged. Nonlinearity-enabled asymmetric control of light by subwavelength resonators paves the way towards novel nanophotonic components via dense integration of large quantities of nonlinear resonators into compact metasurface designs.

The collapse of a laser-induced vapor bubble near a solid boundary usually ends in a liquid jet. When the boundary is from a soft material the jetting may pierce the liquid-solid interface and result in the injection of liquid into it. A particular impulsive jet flow can be generated when a laser pulse is focused just below the free surface of a thin liquid layer covering a gelatin sample used as a surrogate of biological tissue. Here, a downwards jet forms from a liquid splash at the free surface and then penetrates through the liquid layer into the soft boundary. In the present manuscript we report on the use of this novel jet, termed “bullet” jet, to pierce soft materials and we explore its potential to become an optical needle-free injection platform. The dynamics and depth of the injection is studied as a function of the elasticity of the solid and the liquid properties. Injections of up to 4 mm deep into 4 %w/w gelatin within 0.5 ms are observed. The advantages of the bullet jet over other kinds of impulsively generated jets with lasers are discussed.

Optoacoustic (photoacoustic) imaging has gained tremendous attention in research and in clinical practice as a point-of-care system for noninvasive, fast, and safe tests. The first optoacoustic (OA) tomograph has recently passed the Food and Drug Administration (FDA) approval stage for clinical applications aimed at early breast cancer diagnostics. Furthermore, a broad application of OA imaging for Biomedical and Materials Science fields requires a proper tool to test the equipment and verify the quality of the measurements on a daily basis. In the present work, we propose fibers based on single-walled carbon nanotubes (SWCNTs) as a material for designing a stable and reliable calibration grid. The main advantage of the developed test system is the broad optical absorption of SWCNT-based fibers, ranging from visible to mid-infrared regions. Inspired by stringed instruments, we elaborate a grid to calibrate and verify spatial resolution in three projections and sensitivity of OA imaging systems. Thus, the real calibration grid parameters, such as fiber length and diameter, could be translated to the OA signal measurements. This proof-of-the-concept study evaluates the geometry of fibers, that is, the length/diameter and design of fibers, such as free-standing/twisted, and shows the fabrication procedure of the calibration grid prototype toward the successful validation of the OA imaging system, including raster-scanning optoacoustic mesoscopy (RSOM) at one wavelength and tomography at several wavelengths, which have grand prospects in preclinical and clinical practices. Besides, the more advanced geometry based on double-twisted fibers, or twistrons, applied here provided us with a chance to reach the lower resolution limit for RSOM because of the difference in diameter between the thin and thick parts in the morphology is verified by scanning electron microscopy.

The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.

Acoustic cavitation, generated by a piezo-driven transducer, is a commonly used technique in a variety of processes, from homogenization, emulsification, and intensification of chemical reactions to surface cleaning and wastewater treatment. An ultrasonic horn, the most commonly used acoustic cavitation device, creates unique cavitation conditions under the horn tip that depend on various parameters such as the tip diameter, the driving frequency of the horn, its amplitude, and fluid properties. Unlike for hydrodynamic cavitation, the scaling laws for acoustic cavitation are poorly understood. Empirical relationships between cavitation dynamics, ultrasonic horn operating conditions, and fluid properties were found through systematic characterization of cavitation under the tip. Experiments were conducted in distilled water with various sodium chloride salt concentrations under different horn amplitudes, tip geometries, and ambient pressures. Cavitation characteristics were monitored by high-speed (200,000 fps) imaging, and numerous relations were found between operating conditions and cavitation dynamics. The compared results are discussed along with a proposal of a novel acoustic cavitation parameter and its relationship to the size of the cavitation cloud under the horn tip. Similar to the classical hydrodynamic cavitation number, the authors propose for the first time an acoustic cavitation parameter based on experimental results.