The study of brain tissue development requires imaging approaches that ideally provide micron resolution, millimeter imaging depth, and multi-contrast capability.

We will discuss recent approaches for large-scale imaging of uncleared tissues, namely color serial two-photon imaging [1] and three-photon (3P) microscopy. We will also present a novel label-free modality that combines some of these concepts, based on third-order sum frequency generation (TSFG). TSFG microscopy provides label-free imaging of red blood cells while being compatible with deep tissue 3P imaging [2].

References:

[1] Abdeladim et al, Nat Commun (2019), https://doi.org/10.1038/s41467-019-09552-9.

[2] Ferrer Ortas et al, Light Sci App (2023), https://doi.org/10.1038/s41377-022-01064-4.

Fast and sensitive detector arrays make Image Scanning Microscopy (ISM) the natural successor of confocal microscopy. Indeed, ISM enables super-resolution at an excellent signal-to-noise ratio. Optimizing photon collection requires large detectors and so more out-of-focus light is collected. Nonetheless, the ISM dataset inherently contains information on the axial position of the fluorescence emitters. We exploit such information to directly invert the corresponding physical model with a maximum-likelihood approach and reassign the signal in the three dimensions, improving the signal-to-background ratio and resolution. We validated our method on synthetic and experimental images; these latter acquired with a custom setup equipped with a single photon avalanche diode array detector. Moreover, our method is compatible with recent developments in ISM data processing and requires minimal knowledge of physical parameters.

Confocal microscopes have been the workhorses of 3-D biological imaging, but they are slow, offer limited depth penetration and collect only ballistic photons. With their inefficient use of excitation photons they expose biological samples to an often intolerably high light burden. The speed limitation and photo-bleaching risk can be somewhat relaxed in a spinning-disk geometry, due to shorter pixel dwell times and rapid re-scans during image capture. Alternatively, light-sheet microscopes rapidly image large volumes of transparent or chemically cleared samples. Finally, with infrared excitation and efficient scattered-light collection, 2-photon microscopy allows deep-tissue imaging, but it remains slow. Here, we describe a new optical scheme that borrows the best from three different worlds: the speed and direct-view from a spinning-disk confocal, deep tissue-penetration and intrinsic optical sectioning from 2-photon excitation, and a large field of view and a low invasiveness of a selective-plane illumination microscope – all with a single objective lens. We validate the performance of our 2-photon spinning-disk microscope in various applications that have in common to simultaneously require a large depth penetration, high speed and larger fields of view. Beyond biological fluorescence, we demonstrate an application in material science, imaging coherent non-linear scattering from a 3-D nano-porous network

The ability to observe traces of biological material on buildings and stone artworks is of particular importance in understanding how to best deal with them and maybe, in the future, even make use of biofilms for conservation science. We have identified hyperspectral imaging as a viable method for the efficient analysis of such biological materials. Thanks to the high throughput of our approach based on an interferometric method based on Fourier Transform, we were not only able to detect traces of biofilms on stone samples, but also to map in which areas these were found to have higher biological activity.

The study of the interaction of biological tissue with polarized light leads to relevant information of physical properties (dichroism, retardance and depolarization) of samples. Polarimetric analysis of different characteristics in tissues is useful for applications such us tissue classification, contrast enhancement or pathology detection. By means of polarimetric imaging techniques we can characterize the polarimetric signature of biological samples in a noninvasive and nondestructive way. We have found that depolarization information is of special interest in turbid media such as plant tissue. In this manuscript we use polarimetric observables for plant inspection. In particular, we provide enhanced visualization of certain plant pathologies by constructing depolarization based pseudocolored images of pathological leaves where the pathological areas are revealed.

We demonstrate how a novel approach for closed-loop Adaptive Optics (AO) specifically adapted to microscopy enables straightforward integration in Light-Sheet and Multiphoton microscopes, as well as fast aberration correction. We present corresponding experimental setups as well as first demonstrations of the benefits of the correction of sample-induced aberrations in zebrafish and mouse brain tissue, with the ultimate goal to enable high-speed, high-sensitivity functional imaging at large depths.

Rapid prototyping methods for the fabrication of polymeric labs-on-a-chip (LoC) are on the rise, as they allow high degrees of precision and flexibility. In this contest, the flexibility of ultrafast laser technology enables the rapid prototyping and high-precision micromachining of 3D LoC devices with complex microfluidic channel networks. In this paper, we describe the realization process of a microfluidic tool for fully inertial particles sorting. The microfluidic network was realized in polymethyl methacrylate (PMMA), exploiting femtosecond laser technology. The multilayer device was assembled through a facile and low-cost solvent-assisted method. In particular, we studied the particle focusing in curved inertial microfluidic channel with trapezoidal cross section. A particles focusing along the walls of the device, sensitive to particle size and flow rate, was observed based on the principle of Dean-coupled inertial migration in spiral microchannel

Sensors based on photonic crystal (PC) surface mode imaging are promising tools for label-free drug screening and discovery, diagnostics, and analysis of ligand–receptor interactions. Imaging of PC surface modes has been demonstrated to allow simultaneous real-time detection of multiple events at the sensor surface. Here, we report the engineering of a lateral-flow microfluidic assay where PC surface mode imaging is used for multiplexed detection of biomolecular targets (antibodies, oligonucleotides, and a DNA repair protein), as well as kinetic data on their interactions obtained without additional labelling or signal amplification. Our data demonstrate the suitability of the biosensing platform designed for ultrasensitive, quick, and low-cost detection and monitoring of interactions between different biomolecules.

We report on the development of Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technology to detect 8 different air pollutants, namely CH4, NO2, CO2, N2O, CO, NO, SO2 and NH3, with the same acoustic detection module and interchangeable laser sources, to prove the modularity of the technique as well as the adaptability to different lasers. For each gas species, the fine structure of the infrared absorption bands has been simulated by using HITRAN database. Each gas species was detected with an ultimate detection limit well below their typical natural abundance in air even with a signal integration time as low as 0.1 s.

A recent challenge in bioimaging is the observation and imaging of vital, thick, and complex tissues in real time and in non-invasive mode. In the last decade, non-linear excitation microscopy showed several advantages for in-vivo imaging compared to conventional confocal techniques. Nevertheless, deep tissue imaging remains challenging, especially for thick media, due to spherical aberrations induced on focused beams by the tissue. A low numerical aperture objective lens coupled to high dioptric power microlenses, implanted in the tissue, can be beneficial for the reduction of optical aberrations. In this context, we fabricated a system of plano-convex microlenses and microscaffolds on a single chip by means of two-photon polymerization), to be used for non-linear imaging of biological specimens.

THz ATR spectroscopy provides, in a single measurement, the relative number of defects per membrane surface created by oxidative stress generated during photodynamic therapy (PDT), offering early, sensitive real-time information. THz spectroscopy is therefore a complementary technique to established (biological) assays and can be applied to any topic requiring the real-time examination of short-term plasma membrane permeabilization.

We review our recent work on the use of genetic algorithms to control non-stationary nonlinear wave dynamics in ultrafast fibre lasers, including the generation of breathing-soliton dynamics with controlled characteristics, the disclosure of the fractal dynamics of breathers, and the generation of rogue waves with controlled intensity.

We demonstrate for the first time the generation of broadband tunable and synchronized pulses exceeding the microjoule level using the new concept of fiber optical parametric chirped-pulse oscillation (FOPCPO). The oscillator is based on a collapsed-ends photonic crystal fiber pumped in the normal dispersion regime by a fiber laser.

We unveil a strategy for configuring mode-locked fiber lasers by means of a tunable bandpass filter, which allows multi-pulse structures to be generated without distortions in their intensity profiles, with significantly reduced pumping power levels.

Because of the pulse energy quantization in fiber lasers, it is of great importance to find effective

ways to increase the pulse energy directly from a fiber laser. An efficient technique is based on the dissipative

soliton resonance (DSR) effect. The DSR manifest as a square pulse with constant peak power and a linear

increase of both the pulse energy and duration for increasing pumping power. In practice, DSR is favoured

with the use of long cavities. In this communication we propose an overview of DSR in fiber lasers including

general theoretical approaches together with the most recent relevant experimental results

Optical frequency combs are applicable across many fields, especially in metrology. Through accounting for field polarization, the spontaneous symmetry breaking of vector temporal cavity solitons and combs in Kerr Fabry–Perot resonators is presented. This work can improve the generation of frequency combs through its control on the maximum number of soliton-pairs in a round trip and reveals the interesting phenomenon of dominant polarization conformity across all soliton-pairs in the cavity.

In this talk, I will consider optical comb sources characterised by a certain bandwidth, comb line power and background noise level, and discuss requirements to such combs in order to qualify as sources in coherent optical communication systems. I will describe how, that for comb line powers and performances already achievable by integrated micro-ring resonators, a single comb source could act as a highly scalable multi-wavelength (WDM)-source for multi-Pbit/s spatially multiplexed (SDM) systems. I will discuss the possibilities of using a single comb source to support 10s Pbit/s data transmission, and how the scaling to capacities beyond that, is promising. I will discuss using a single comb source for ultra-long-haul transmission systems, such as trans-oceanic reaches of e.g. 8.000 km, and I will show how a comb source performs just as well as a bank of individual lasers, with the added benefit of the possibility to reduce the spectral guard-band, thus increasing the spectral efficiency.

Optical microresonators are attractive comb sources due to their small form factor and stable broad optical spectra. We report on the first demonstration of microcomb-based digital holography. The large line spacing of microcombs promises an unprecedented combination of precision, fast update rate and ambiguity ranges on the scale of a few mm. Using a pulse-driven lithium niobate microcomb of 100 GHz line spacing and a scanning Michelson interferometer, we generate spectral hypercubes of holograms. Our first experimental results show that the amplitude and phase information of the object can be recovered for more than 100 comb lines.

Self-injection locking to a photonic crystal ring-microresonator with synthetic back-reflection is demonstrated for the first time. The on-chip system permits deterministic generation of exclusively single-soliton microcombs and does not rely on random backscattering from defects or imperfections.

In this work, we present a theoretical and numerical study about the generation of double- frequency-comb-like source with Gain Through Filtering (GTF) in passive Polarization Maintaining (PM) fibre ring cavity.

GTF is a nonlinear phenomenon which allows the formation of MI sidebands whose frequencies position is dependant by the central frequency of a spectral filter. By exploiting a PM fibre Bragg grating, and its double central frequency nature, we are able to generate a double-frequency-comb like spectrum, i.e, a spectra on each polarization with a spectral separation equals to the separation of the filter's components.

A phase-modulated frequency-shifting loop is injected by a single-frequency laser at 1.5 µm. In so-called Talbot conditions, i.e., when the modulation frequency is an integer multiple of the inverse of the cavity round-trip time, the loop generates a frequency comb whose temporal trace consists in a train of pulse doublets whose positions in time depend on the frequency of the injection laser. When the modulation frequency is slightly detuned from the Talbot condition, nonlinear frequency chirps are predicted and observed in the output pulse train. We demonstrate that these nonlinear chirps are not restricted to sinusoidal shapes, and also that the loop can be stabilized by exploiting the intracavity phase modulation.

New Ytterbium based femtosecond lasers system are very interesting systems due to their high repetition rate / average power, compactness and long term stability due to direct diode pumping. Associated with nonlinear compression schemes (multipass cell or capillary), few cycle pulses are now available at around 1 µm. This presentation will present our recent work in this field and the extension of these sources in the mid-infrared by using intrapulse difference frequency generation and amplification to cover the 4 – 20 µm spectral range with high conversion efficiency. An optimization of the polarization state and chirp of the input pulses and a temporal synchronization leads to sub-two optical cycles CEP stable pulses at 8 µm with hundreds of mW average power (> µJ energy per pulses at 250 kHz).

The complicated electric field structure of ultrashort pulses as characterized by frequency-resolved optical gating (FROG) algorithms often results in local convergence, which does not accurately represent the actual field pulse. We present an efficient and universal test procedure applicable to any FROG retrieval algorithm that allows recognition of the erroneous local convergence. The 100% efficacy of the procedure is demonstrated using Line-Search FROG algorithm, and comparison is given with the performance of a standard extended ptychographic iterative FROG trace retrieval engine.

The generation of sub-five optical cycle pulses centered at 11.2 µm wavelength with 50 µJ energy at a 1 kHz repetition rate is reported. A GaSe optical parametric chirped pulse amplifier (OPCPA) is driven by the residual 2.0 µm pump and 5 µm idler of a high-energy midwave-IR OPCPA. The latter serves as driver for hard X-ray generation and this makes the achieved fs longwave-IR pulses available for X-ray pump-probe experiments.

I will review our recent work in advanced applications of optical microcombs to optical neural networks, optical communications and other areas.

Thin film aluminum nitride on insulator (AlNOI) has gained attention as a promising material platform for integrated photonic circuits (PICs) due to its ability to operate over a wide spectral range covering the ultra-violet to mid-infrared regions, while enabling a broad range of passive photonic functionalities. This study aims to optimize sputtered AlNOI films for PICs, with an emphasis on the spectroscopic ellipsometry study over a range from 0.19 µm to 25 µm. Furthermore, we discuss our approach for fabricating AlNOI PICs components, with a particular focus on optimizing the etching process to attain smooth sidewall waveguides.

In last years, the introduction of 2-dimensional materials such as graphene has revolutionized the world of silicon photonics. In this work, we demonstrate a new approach for integrating graphene into silicon-based photodetectors. We leverage a thin film of hydrogenated amorphous silicon to embed the graphene within two different photonic structures, an optical Fabry-Perot microcavity, and a waveguide, achieving a stronger light-matter interaction. The investigated devices have shown promising performance resulting in responsivities as high as 27 mA/W and 0.15 A/W around 1550 nm, respectively.

Novel applications in optical coherence tomography (OCT) and LiDAR systems have become possible due to performance characteristics of a state-of-the-art silica-on-silicon planar lightwave circuit (PLC) platform. We have achieved ultra-low propagation losses of <0.009 dB/cm with unmatched phase control in a polarization-insensitive way, enabling a range of real-time advanced vision and imaging applications utilizing k-clocks and analog frequency sampling architectures.

We describe a selection of work carried out within the ECSEL-VIZTA European research project concerning the development of an integrated solid-state 905nm time-of-flight (TOF) LIDAR device. Pseudo two-dimensional, single wavelength beam steering from a 7x32 channel silicon nitride-based optical phased array was achieved, with optimized single-pass thermo-optic phase shifters with Pπ = 30mW. Direct optical coupling of a photonic chip to a 7 W tapered GaInAsP laser diode is also shown to suggest a pathway to achieving medium-to long-range integrated LIDAR using a low-cost light source.

Robust GHz Frequency Combs: a new tool for many applications

We demonstrate a multimode diode-pumped Yb:CALGO laser oscillator based on bandwidth-optimized cross-polarization pumping targeting sub-30-fs operation. In our first proof of principle experiment we achieved mode-locked operation at 83 MHz repetition rate with 0.4 W of average power and a 33-nm-bandwidth optical spectrum supporting sub-40-fs pulses. This concept offers a simple and cost-efficient alternative to green-pumped Ti:sapphire lasers.

We demonstrate generation of bursts that consist of up to 40 ultrashort pulses with 10 μJ pulse energy, 250 fs pulse duration and an ultrashort tunable spacing, from picoseconds to nanoseconds, corresponding to a terahertz intraburst repetition rate. This was achieved by the build-up of a novel thermally-stable sub-mJ Vernier Regenerative Amplifier (RA), whose round-trip detuning is similar to its master oscillator round-trip time. The RA includes two cavities pumped from a common diode, and is able to provide for either 2 bursts (one burst out of each cavity), or for one burst and a synchronous reference pulse for characterization.

Micromachining of various materials with femtosecond lasers operating in the GHz-burst regime has recently attracted great attention. In this contribution, we show our latest results on top-down percussion drilling in different dielectrics in this new operating regime. The dependence on the burst parameters such as burst repetition rate, number of pulses per burst, and burst energy are discussed. Moreover, we will focus on the influence of the burst shape on the drilling process. The quality of the drilled holes and their reachable dimensions are presented.

We employ real-time spectral measurements to investigate the transitions from the stable mode locking to period doubling and long-period pulsations. We highlight the role of self-phase modulation as a general mechanism triggering period-2 bifurcations. We analyze the new frequencies generated during the sequence of bifurcations and report an entrainment where the period of the long oscillation locks to multiples of the cavity roundtrip time.

Thin-disk laser oscillators can nowadays reach few tens of femtosecond pulses at gigawatt-level intracavity powers and megahertz-repetition rates becoming increasingly more powerful sources for intra-oscillator high harmonic generation (HHG). Currently, we can generate high harmonics in neon reaching photon energies of 70 eV, which we expect to increase toward 100 eV in the near future. In parallel, the achievable average and peak output powers of these oscillators in the range of 100 W and 100 MW, respectively, make these sources very promising to drive HHG in single-pass configuration after nonlinear pulse compression. Starting from transform-limited 30 to 50-fs soliton output soliton pulses of TDL oscillators, we will likely see these lasers approaching a single-cycle regime becoming highly attractive sources for attosecond science.

We present a table-top, efficient and power scalable scheme enabling the effective generation of extreme ultraviolet radiation up to 100 eV photon energy. Therefor ultrashort pulses (< 20fs) in the visible spectral range (515 nm) are used to drive high harmonic generation in helium. This allows for a significant efficiency boost compared to near-infrared (NIR) drivers, enabled by the favourable scaling of the single-atom response of λ-6 [1]. The experimental realization of a mulitpass cell delivering 15.7 fs pulses with a peak power close to 25 GW at 515 nm and an overall efficiency (IR to compressed green pulse) of >40 %. In conjunction, preliminary HHG results will be presented, paving the way for mW-class HHG sources at 13.5 nm.

The technological refinements on high-power laser systems of Petawatt class unveil scenarios for light-matter interaction beyond the laser-plasma perspective. In this contribution we explore the possibility of assisting high-harmonic generation (HHG) with the strong magnetic field associated with one of those intense sources. Recently, there has been an interest in developing schemes to employ intense laser beams to define spatial volumes in which strong magnetic fields are found isolated from the electric field. In this conditions, the magnetic field can be used to assist the harmonic generation from atoms from standard drivers. We demonstrate that, using the proper interaction geometry, the magnetic field confines the transversal dynamics of the continuum electrons allowing, on one side, to enhance the efficiency of the electron rescattering that produces the harmonic radiation and, on the other side, to excite the electron transverse dynamics and to convert this energy into phase-locked harmonic photons of few hundreds of eV.

We report on a high energy ultrafast fibre laser architecture designed for high harmonics generation in solids. The laser delivering 50 fs pulses with 2.12 µJ at 1550 nm has enabled the generation of harmonics up to harmonic H5 from a magnesium oxide (MgO) bulk sample. To the best of our knowledge this is the first solid-state HHG source driven by a µJ-class few-cycle fiber laser in the mid-IR region.

A phase-matching free pulse retrieval technique based on plasma-induced defocusing in a rare gas is presented. Based on a pump-probe setup, this technique uses a moderately intense pump laser pulse for ionizing the medium, creating in turn an ultrafast defocusing lens. While a coronagraph blocks out the probe pulse in absence of ionization, the plasma lens leads to increase the probe beam size in the far field. By measuring the spectrum of the probe propagating around the coronagraph as a function of the pump-probe delay t, a bi-dimensional trace (w-t) is obtained. This enables to fully characterize the temporal and spectral characteristics of the probe pulse through a method that is free of phase matching constraints. Demonstrated both in the near-infrared (800 nm) and in the ultraviolet (266 nm), the present technique is potentially suited for characterizing pulses in the whole transparency region of the used gas, i.e., from the deep-ultraviolet to the far-infrared.

We point out the breakdown of two approximations widely used to describe decoherence in open quantum systems, the secular and Markov approximations. We probe their limits by studying the influence of pressure on the alignment revivals (echoes) created in properly chosen gas mixtures (HCl and CO2, pure and diluted in He) by one (two) intense and short laser pulse(s). Experiments, as well as predictions using molecular dynamics simulations, consistently demonstrate in some of the aforementioned systems the break-down of these approximations at very short times (<15 ps) after the laser kick(s).

Quantitative and Material-specific Nanoscale imaging with Table-top High Harmonic Sources

Passive modelocking (PML) of lasers is pivotal in modern science and industry. Here, solving a half-century-long challenge, we present the first master equation (ME) describing PML on all time scales, from Q-switched to fundamental and harmonic modelocking, valid for both slow and fast saturable absorption and short and long cavities. The proposed ME should become the workhorse for analytical and numerical studies of ultrafast lasers in both the photonics engineering and laser physics communities.

We present here an experimental demonstration of a new method to access the multi-pulse regime in mode-locked fibre laser. This method allows us to drastically reduce the pumping power, while having a stable train of multiple pulses.

Predicting complex nonlinear dynamical systems has been even more urgent because of the emergence of extreme events such as earthquakes, volcanic eruptions, extreme weather events (lightning, hurricanes/cyclones, blizzards, tornadoes), and giant oceanic rogue waves, to mention a few. The recent milestones in the machine learning framework offer a new prospect in this area. For a high dimensional chaotic system, increasing the system’s size causes an augmentation of the complexity and, finally, the size of the artificial neural network. Here, we propose a new supervised machine learning strategy to locally forecast bursts occurring in the turbulent regime of a fiber ring cavity.

The targeted generation of fs pulses is essential for a variety of applications and it is routinely carried out by 4f pulse shapers. However, this seemingly simple task is complicated by hidden experimental limitations, such as modulator crosstalk or pixelation. We present an approach to overcome this issue by using a high-precision phase plate with a phase change characterized with /500 precision. We generated pseudorandom pulses using a 4f pulse shaper by using a structured PMMA plate with the high-precision predefined shape made by the SPDT machine. We study the accuracy, reproducibility, as well as the sufficiency, and limits of the method. The generated pulses are characterized using the FROG method. The reconstructed pulses’ shapes and their spectral phases are compared to the results of simulations.

The impact of the amount of nonlinearity per focal pass on the beam quality in gas-filled Herriott-type multipass cells (MPCs) is investigated. Through the variation of the gas pressure, the peak nonlinear phase shifts per focal pass was varied from 0.8 rad up to 5.9 rad, which is the highest value demonstrated in a MPC. Simultaneous monitoring of the effective homogeneity [1] and M² revealed no degradation of either. Supported by numeric simulations the findings suggest that the nonlinearity per focal pass in gas-filled MPCs below the self-focusing limit is not limited by spatio-spectral couplings. The presented findings become especially important for challenging regimes such as high compression factors and few-cycle pulses. Here the reduction of passes can significantly boost the transmission and optimized designs may enable the technology to move towards the single-cycle regime.