Basic and Clinical Science Applications Using SLIM and GLIM

Light scattering limits the quality of optical imaging of unlabeled specimens: too little scattering and the sample is transparent, exhibiting low contrast, and too much scattering washes the structure information altogether. As a result, current instruments, target specifically either the thin (low-scattering) specimens or the optically thick (multiply scattering) samples. In 2011 we developed spatial light interference microscopy (SLIM) as a high-sensitivity, high-resolution quantitative phase imaging method, which open new applications for studying structure and dynamics. Color SLIM (cSLIM) is a recent development that allows the phase imaging of stained tissue slices. Using specimens prepared under the standard protocols in pathology, cSLIM yields simultaneously the typical image that the pathologist is accustomed to (e.g. immunochemical stains, etc.) and a quantitative phase image, which provides new information, currently not available in bright field images (e.g., collagen fiber orientation).

However, SLIM works best for thin specimens, such as single cell layers and tissue slices. To expand this type of imaging to thick, multiply scattering media, we developed gradient light interference microscopy (GLIM). GLIM exploits the principle of low-coherence interferometry to extract phase information, which in turn yields strong, intrinsic contrast of transparent samples, such as single cells. Because it combines multiple intensity images that correspond to controlled phase shifts between two interfering waves, GLIM is capable of suppressing the incoherent background due to multiple scattering. We demonstrate the use of GLIM to image various samples, including standard micron size beads, single cells, cell populations, thick bovine embryos, and live brain slices. GLIM operates as an add-on to a conventional microscope and overlays seamlessly with the existing channels (e.g., fluorescence).

Pushing Charged Microparticles with an Electron Beam in a Plasma Crystal

We demonstrate pushing of charged microparticles by a pulsed collimated electron beam (EB) over tens of millimeters for electron energies in the 10-15 keV range. The microparticles are spheres made of plastic and are levitated inside a weakly ionized plasma forming a plasma crystal. The plasma crystal is locally irradiated by a pulsed EB with a diameter in cross section of 5 mm. When the EB is turned on the microparticles are entrained in a flow moving with several mm/s in the direction of the EB. Far from the irradiation zone the plasma crystal preserves its spatial structure. The peak kinetic energy of the dust flow is in the few hundred eV.

Two types of flow regimes are identified: laminar at the beginning of irradiation in the first hundred milliseconds, and turbulent later on, when the flow speed and width increase. Vortices formed initially at the entrance of the EB in the plasma crystal give rise to subsequent eddies which propagate downstream the flow. Laser illumination combined with high speed imaging is employed to track the evolution of the microparticle flow in time. The particle image velocimetry (PIV) technique is implemented for deducing the flow characteristics. Spatio-temporal maps of the microparticle flow speed, kinetic energy and vorticity give insights into the flow dynamic regime.

Interaction of Laser Exposed Phenothiazine Droplets with Target Surfaces Approached in View of Microgravity Applications

The interaction of UV laser radiation with pendant medicine droplets and their wetting properties on target surfaces is proposed to be investigated under the aegis of the United Nations Office for Outer Space Affairs (UNOOSA) within the DropTES Fellowship Programme. Experiments will be conducted in microgravity conditions at the Bremen Drop Tower, sponsored by the Center of Applied Space Technology and Microgravity (ZARM) and the German Aerospace Center (DLR) Space Administration. The current paper is approached within the preparation phase of this application.

Since pathogens develop drug resistance on Earth and evolve into more virulent forms in space, new strategies for treating astronauts and decontaminating spacecraft surfaces are required when executing long-lasting space missions.

The alternative solution proposed for DropTES 2018 project consists in utilizing multifunctional drugs and an unconventional method to make them acquire antimicrobial properties by exposure to UV laser radiations.

The concept of optically induced structure modification of existing medicines regards the transformation of a single parent-compound into new and more efficient photoproducts that may have – either individually or in mixture – increased antimicrobial activity, if compared to the corresponding unirradiated medicines.

The proposed experiment aims to bring a new insight on the behavior of medicine droplets in reduced gravitational environment. A novel aspect of the project represents droplets real time exposure and modification by UV laser beam. Such obtained droplets will be then used to impregnate target surfaces, which might serve as unconventional tools in developing new drug delivery systems. Therefore, the evolution of wetting phenomena is the key feature of proposed studies.

Coaxial Plasma Gun Used in Space Propulsion

Initially projected for nuclear fusion thought achievement of high density plasmas, the coaxial plasma gun became rapidly a useful tool for investigation of fundamental plasma physics proprieties. It is also used successfully to accelerate dust particles at hypervelocity, to operate as a helpful device for cleaning different surfaces covered with dust, or to produce dense plasma jets for testing fusion materials. Coaxial plasma gun can be also used in space propulsion. With the exploration of the cosmos beyond the moon's orbit, space thrusters need great impulses for the new distances. Unlike ionic propulsors that have a high specific impulse, but a low propellant density, pulsed plasma propellant fulfills both necessary conditions: a high specific pulse and increased density, knowing that coaxial plasma jets were created for plasma fusion and implicitly have high densities (10¹⁹-10²¹ m^-3). Also, propulsion with pulsed plasma has advantages over chemical missiles, the last ones having high fuel density, but a small specific impulse. In addition, unlike ionic propulsion engines, the pulsed plasma propellant consumes fuel more slowly, which leads to greater autonomy.

The coaxial gun had two electrodes made of stainless steel, a long centered rod and a coaxial outer cylindrical shell. The axial JxB force ejected plasma out of the gun at a speed of a few km/s using a ballistic pendulum placed in front of the gun’s muzzle we could measure the force and pressure of the plasma jets whose values are 23N and 10⁴ Pa in conditions of a mass of pendulum of 4,64 g and a distance of 9 cm between plasma gun and target. The plasma parameters were measured with a triple probe made in our laboratory. The discharging current is on the order of kA and the temperature of the electrons is about 11 eV.

Photon Induced Electron Dynamics in Diatomic Molecules by XUV Laser Pulses

An efficient numerical method was implemented in order to theoretically investigate the interaction between intense, ultrashort (tens and hundreds of attoseconds), few-cycle laser fields and diatomic molecules considered within the single active electron (SAE) approximation. The presented method is based on the ab initio numerical solution of the time-dependent Schrödinger Equation (TDSE), and the benchmark of the code was done for the H₂+ and HHe++ molecules irradiated with XUV pulses.

Accurate results for the electronic wave packet (EWP) dynamics of the irradiated molecules were obtained by the implementation of a less computer resource demanding numerical code. A low CPU simulation time was achieved by the use of the prolate spheroidal coordinates and by using finite-element discrete variable representation (FE-DVR) grids for representing the time-dependent wave function and the Hamiltonian, resulting in a sparse numerical representation of the Hamiltonian matrix (having a high proportion of zero values). The numerical calculations were further speeded by the use of the Scalable Library for Eigenvalue Problem Computations (SLEPc) package and by the proper parallelization of the code (Open MPI). After the detailed description of the model, the convergent results obtained for the EWP dynamics are presented and discussed for different laser field amplitudes and frequencies, and for different internuclear distances (R).

Here we focused our attention on the H₂+ molecule, since it provided the simplest case to test our code, and to investigate the physics behind the laser induced phenomena. The photo-excitation processes as a function of time were studied by calculating the occupation probabilities of the low-lying electronic bound states. Finally, the investigation of the photoionization was carried out, by the calculation of the ionization probability densities and by the comparison of the obtained spectra of the ejected electrons for different field parameters.

Measuring Everything You've Always Wanted to Know About a Light Pulse

The vast majority of the greatest scientific discoveries of all time have resulted directly from more powerful techniques for measuring light. Indeed, our most important source of information about our universe is light, and our ability to extract information from it is limited only by our ability to measure it.

Interestingly, most of the light in our universe remains immeasurable, involving long pulses of relatively broadband light, necessarily involving ultrafast and extremely complex temporal variations in their intensity and phase. As a result, it is important to develop techniques for measuring, ever more completely, light with ever more complex submicron detail in space and ever more complex ultrafast variations in time. The problem is severely complicated by the fact that the timescales involved correspond to the shortest events ever created!

Extreme Light Infrastructure – Nuclear Physics and the High Power Laser System

Extreme Light Infrastructure – Nuclear Physics (ELI-NP) will be a new European Center for Scientific Research built in Bucharest-Magurele, Romania. The ELI-NP facility will host two state-of-the-art machines: a very high intensity laser with two 10 PW arms (HPLS) and a very intense ~10¹³ γ/s variable energy γ beam (GBS).

The ELI-NP HPLS is a Ti:Sapphire, hybrid CPA system and it has six optical outputs: two 0.1 PW outputs running at 10 Hz repetition rate, two 1PW outputs running at 1 Hz and two 10 PW outputs running at 1 shot per minute. This laser system is produced by a consortium formed by THALES Optronics and THALES Systems Romania. The Laser Beam Delivery (LBD) system interfaces the HPLS with the Nuclear Physics facility and with the experiments and it is built by the LBTS consortium.

TThe ELI-NP GBS is a very intense ~10¹³ γ/s, brilliant γ beam, ~ 0.1 % bandwidth, with E γ > 19 MeV, which is obtained by incoherent Compton back scattering of a laser light off an intense electron beam (Ee> 700 MeV) produced by a warm LINAC. The GBS is built by EuroGammaS Association.

During this talk, I will introduce the facility and some of its challenges with accents on the High-Power Laser System, the Laser Beam Transport System and the envisaged experimental program.

Construction and Optimization of a SESAM - Mode Locked NdYVO4 Laser

Picosecond lasers having emission in infrared range are often employed when applications as high resolution microscopy, nonlinear optics, medical surgery, micro-processing of materials or educational research are envisaged. Most of the picosecond laser sources combine the continuous wave laser emission at 1064 nm of a Nd doped crystal together with the Semiconductor Saturable Absorber Mirror (SESAM) technology in order to obtain a powerful and efficient picosecond, infrared laser. Various papers report on the NdYVO4 laser, mode-locked by SESAM, in different cavity configurations, giving ultrashort pulses with repetition rates from tens of MHz until few tens of GHz.

From researchers and laser developers point of view, understanding and solving the resonator cavity design issues while maintaining laser performances at a high level, by choosing the proper SESAM parameters and proper gain medium, is very demanding.

In this work emphasis on the resonator design and appropriate SESAM employing is made for construction of an efficient mode locked NdYVO4 laser. At first, laser emission at 1064 nm for the Nd:YVO₄ inserted in two different optical cavities is presented. The cavity optimization is followed before using the SESAM. The second part of the work highlights the results concerning laser emission characteristics when two different SESAMs are utilized, SESAM1- 10 ps recovery time, 1.2% modulation depth and SESAM2 -0.5 ps recovery time and 0.4% modulation depth.

The resonator cavity geometry is very important in order to obtain simultaneous effects as perfect mode matching in the gain medium, high nonlinearity at the SESAM and TEM00 operation of the laser, by working near the stability limit of the resonator. The SESAM modulation depth and recovery time are directly influencing the ultimate pulse duration and the mode locking threshold. In our case, SESAM1 has more Q-switching instabilities than SESAM2. The continuous mode locking threshold is lower when SESAM 2 is used, but the nonlinearity is firstly observed (QML – Q-switch mode locking) when SESAM1 is employed, at a very low pump power of 0.5 W. The slope efficiency of the Nd:YVO₄ laser is higher for SESAM2, 42%, and the optical efficiency in ML regime is 40.4%, at 5.03 W, pump power. The optical efficiency of the NdYVO₄ laser with SESAM1, in ML operation is 34% at 5.17 W.

Additive Manufacturing of Microstructures for Laser-Driven Particle Acceleration

Laser-driven particle acceleration opens the path to compact “table-top” accelerators with many applications, such as fast ignition, tumor therapy and radiation hardening for space industry. When ultra-intense laser pulses interact with a target, accelerated energetic particles such as ions, electrons, protons or coherent electromagnetic radiations (XUV, X-ray) are produced. Among the established methods in laser-driven particle acceleration are Target Normal Sheath Acceleration (TNSA) and Radiation Pressure Acceleration (RPA)

Both methods require ultra high laser intensity at the target surface, which further determines the energy of the accelerated particles. Laser intensity is determined not only by the incident pulse peak power, but also by appropriately designing the target. The importance of 3D micro-targets is supported by theoretical and experimental studies. In the case of particle acceleration, 3D targets can provide significantly higher energy absorption compared to flat targets. This in turn results in the production of particles with higher energies and lower divergence for a given laser intensity.

Apart from increasing ejected particle energies, micro-structure targets potentially lower costs for laser-driven particle beams. In the case of flat-surface target TNSA and RPA, the overall cost is greatly influenced by the required laser pulse intensity, especially in the ultra-intense regime. This can also result in better time efficiency, as lower intensity laser systems can usually provide higher frequencies.

We present and discuss the design and fabrication of 3D targets intended for particle acceleration through the RPA method.

In order to design the targets, we developed a specialized CAD software using Python and Qt Designer. The software takes the geometry and fabrication method into consideration in order to optimize the time efficiency of both the fabrication and design processes. We obtained a 3-fold increase in fabrication time using a spiral-like writing method, rather than slice-by-slice method commonly employed in 3D printing systems.

Fabrication is realized using high precision, two-photon polymerization Additive Manufacturing (AM). Resulting microstructures were characterized using Scanning Electron Microscopy (SEM) micrographs.

Laser Sources for Optofluidics

Lasers became the most utilized optical radiation emitters in optofluidics. This paper shows a synthesis of new data about the roles of lasers in optofluidics.

Microfluidic data are presented about microliter droplets that contain laser dyes in water and that emit lasing radiation when optically pumped. Specifically, results are shown when the pumping unit is a nanosecond laser emitting at 532 nm and Rhodamine 6G is the dye contained in the water solution presented as droplets that are pendant in open air and have volumes between 1 µl and 10 µl. Characteristics are shown of the lasing radiation emitted by microdroplets, such as: emission spectral range, beam spectral width and time structure, emission in 4π, properties variations function of pumping conditions and emitted beam collection.

Data are also shown about the correlation between laser-induced fluorescence (LIF) and lasing spectral emissions by microliter droplets and their time evolutions, during optical pumping with laser beam at 532 nm. The optically pumped droplet acts as an optical resonator amplifying the fluorescence signal to the stage in which light scattering is simultaneously produced with the lasing effect. High resolution analysis of lasing peak shows its complex structure, with four oscillation bands (mode-clusters), and periodic resonances forming a ripple structure superimposed on one of oscillation peaks. Ripple spacing value is in good agreement with data reported for partial-wave resonances from Mie calculations of light scattering for large sphere, if its absorption is considered.

Advantages of Photogrammetry as a Competitive Technology for Cultural Heritage 3D Documentation

Almost as old as photography, photogrammetry is still evolving today along with the great developments in hardware technology and software algorithm efficiency. Now we are witnessing the development of real-time basic 3D reconstruction and motion tracking in video-gaming industry (Microsoft’s Kinect sensor) or online 3D reconstruction for design purposes.

Not long ago, ALS (aerial laser scanning) and TLS (terrestrial laser scanning) seemed to completely overcome photogrammetry with certain advantages: fast data collection (hundreds of thousand points per second), range, accuracy, data rate etc. But with the development of automatic block orientation and the improvements in image matching, digital image correlation and stereo correspondences problem in digital image processing (coming from computer vision area) photogrammetry established itself as “the most complete and flexible technique for collecting and archiving 3D information”.

In terms of accuracy, photogrammetry is precise and comparable to other large-volume, high accuracy coordinate measurement systems. Photogrammetric accuracy depends on several factors: camera resolution, measured area size, numbers of photographs and not least the methodology of image recording. But usually accuracies of 25 to 50 microns can be easily achieved.

In archaeology survey and cultural heritage documentation close range photogrammetry is usually employed. Close range photogrammetry usually refers to all non-topographic mapping photogrammetry. It is applied on objects, excavations, buildings, statues etc. This technique is used to document features with complex geometries or large numbers of inclusions, including walls, pavements, rubble collapse and architectural elements, cracks or other physical defects in artifact surfaces, textiles, detailed woodworks etc. Photogrammetric recording became time-saving but also a reliable accurate method of documentation for conservation and preservation of cultural heritage assets. One of the major advantages of this method is its non-contact and non-invasive character, while providing high precision measurements.

In this paper we will present several case studies to emphasize our workflow for photogrammetry in cultural heritage 3D documentation but also some comparisons with several 3D laser scanning projects.

Laser Ignition - A Review of Laser Spark Plug Development and Achievements in Engine Ignition

The concern of humanity over exhaust gas pollution and greenhouse gas emission have stimulated research on finding alternative methods for improving the performances of combustion as the main process of energy conversion. Nowadays the high-voltage spark plugs are commonly used to initiate combustion in various engines. These devices are simple and quite inexpensive and therefore it is largely accepted that they will remain the main tools of ignition for a long time to come. Still, a classical spark plug has some disadvantages, such as: a) the electrodes suffer from wetting and erosion; b) the flame kernel development can be influenced by the spark plug protrusion into the engine cylinder; c) a limited capability to ignite lean air-fuel mixtures; d) reduced performances at higher pressures, or e) the fixed position of ignition inside the combustion chamber. Therefore, alternative techniques of ignition are investigated in order to address the limitations of an electrical spark plug.

One potential candidate for this purpose is ignition with a laser system. Laser ignition (LI) offers several advantages in comparison with ignition by electrical park plug. There is no quenching of the combustion flame kernel due to the absence of electrodes. Furthermore, the laser beam focus can be positioned at an arbitrary point inside the engine cylinder thus offering possibility for further improvements of the engine performances. In addition, the laser beam can be delivered simultaneously to different locations (for spatial multi-point control of LI) or in a train of pulses within a short time span (thus realizing the temporal control of LI). Lean air-fuel mixtures or combustible at high pressure can be ignited in these ways.

In this talk a review of the research performed on LI in internal combustion engines, especially of gasoline engines, will be presented. The path from the first demonstration of LI in an internal combustion engine to the first operation by LI of a 4-cylinder gasoline test engine and to the implementation of LI in a real automobile will be discussed. Steps taken toward developing a spark-plug-like LI system for automobile, stationary gas engines for energy cogeneration or for space applications will be described. The delivery of the laser beam to the engine cylinder by optical fiber as a complementary technique to that of placing the laser spark plug directly on the engine will be introduced. Aspects regarding the fouling of the window that is used to introduce the laser beam into the engine cylinder will be addressed. It is concluded that LI has reached a quite high degree of technical maturity and that the advantages of using LI technique versus ignition by an electrical spark plug were demonstrated

Biomaterial 2D and 3D Processing by Innovative Laser Technologies: Application to Cancer Cell Study

Material engineering is current strategy for discovering new characteristics with potential biological use. Material choice and design are the initial approaches when the research meets a concrete application. Biomaterials are often expensive and difficult to configure within micro- and nano-spaces.

Due to processing at micro- and nano-scale, lasers emerged as versatile tools for fabricating innovative devices for tissue engineering, biomimetic delivery systems for local release or lab-on-chip applications. Laser processing is an exclusive technique that flexibly allows downsizing device dimensions and create 3D hierarchical small details in biochips.

Herein, we present new laser approaches to process and tailor biocompatible transparent glass and polymeric based materials at micro- and nanoscale. Specifically, we are introducing the laser fabrication of innovative: i) microfluidic systems consisting of complex 3D channel structures embedded in glass volume connecting open reservoirs in a desired configuration, ii) glass and polymeric platforms consisting of thousands of unit chambers for single cell immobilization and analysis; iii) photolithography masks for sequel additive manufacturing and iv) various molds for casting/spin casting processes.

In addition, defect-free 3D biomimetic nanoconfigurations were proposed for the evaluation of cancer cell invasion and migration in confined spaces. Specifically, polymeric channels with widths of ~900 nm, which is more than one order of magnitude smaller than the cell size, are integrated by femtosecond laser inside glass microchannels. The cells are responsive to an in-channel gradient of epidermal growth factor and can migrate a distance greater than 20 µm. After migration, the cells suffer partial cytokinesis, followed by fusion of the divided parts back into single cell bodies.

Pulsed Laser Deposition: a Versatile Technique for Growing Thin Films to Investigate Radiation Induced Effects

Many thin films have important applications in advanced nuclear reactors, fusion installations or space exploration, where there are strong radiation fields. It has been recently reported that thin films, which are nanostructured or amorphous, having grain sizes usually smaller than 20 nm, behaved differently under irradiation than polycrystalline or single crystal films and materials. The high concentration of grain boundaries allows for very short diffusion paths of irradiation generated defects towards the grain boundaries, which act as defect sinks. Therefore, the structure and properties of such thin films are not strongly affected by exposure to radiation. Also, dislocations could not usually form in such small crystalline grains, which affects the films mechanical properties. The observed increase of mechanical hardness after irradiation of single crystals or large grain films caused by the generation of arrays of dislocations that become entangled and therefore immobile, was not observed in these nanostructured films.

To investigate in detail the radiation effects on properties, chemical composition and structure we used the Pulsed Laser Deposition (PLD) technique to grow nanocrystalline ZrN or ZrC thin films from inexpensive targets. By simply changing the deposition parameters, films possessing different chemical compositions and/or structures could be readily obtained. The effects of 800 keV Ar and 1.0 MeV Au ions on the microstructure of nanocrystalline ZrC and ZrN thin films were investigated using high resolution transmission electron microscopy, nanoindentation, optical reflectometry, X-ray specular and diffusive reflectivity, and X-ray diffraction investigations. The results confirmed that nanocrystalline films could withstand high irradiation fluences without degrading their crystalline structure, while the Si substrate was completely amorphized. Grazing incidence XRD investigations found that there is grain growth in ZrC films as an effect of ion irradiation, while a decrease of the grain size was observed under similar irradiation conditions for ZrN. A decrease of the carriers scattering rates was observed from IR optical reflectivity measurements for both films. The results are compared to those reported on polycrystalline or single crystal materials.

Photoresponsive Layered Double Hydroxides Thin Films Containing Organic Dyes

We report here fabrication of optically transparent thin films with photoluminescent properties based on co-intercalation of two organic compounds (curcumin and coumarin-343) into the galleries of Mg-Al (Mg/Al molar ratio of 2.5) layered double hydroxides thin films opened a way to novel nanostructured materials with designed optical properties. Curcumin is a well-known natural compound with antioxidant properties, while coumarin is a chemical compound found in plants.

Pulsed laser deposition (PLD) and Matrix Assisted Pulsed Laser Evaporation (MAPLE) was employed for the deposition of hybrid LDH-curcumin and LDH-coumarin thin films.

Different characterizations techniques have been used for powder and thin films (XRD, scanning electron microscopy, atomic force microscopy, FT-IR spectroscopy and photoluminescence measurements).

Hybrid Amplification – an Advanced Approach for Multi-PW Femtosecond Laser Systems Development

In the last years, PW-class femtosecond lasers have been demonstrated by chirped pulse amplification (CPA) in Ti:sapphire crystals. For many fundamental and applicative research works, a very high laser intensity of more than 10²² W/cm² in the focused beam of multi-PW femtosecond lasers is expected. If a laser intensity of about 10¹¹ W/cm² is reached on the target before the main laser pulse, a pre-plasma which disturbs the experiment can be created. An ultra-high intensity contrast is required in case of experiments involving tightly focused multi-PW laser beams of 10²² - 10 ²³ W/cm² peak intensity.

Due to the amplified spontaneous emission, reaching 10¹² intensity contrast in the picosecond range represents a challenging task in high-power Ti:sapphire femtosecond laser systems. On the other hand, in all-Ti:sapphire CPA laser systems, the pulse spectral bandwidth is restricted by the gain narrowing and red-shifting, giving rise to re-compressed amplified pulses of down-limited pulse duration.

Optical parametric chirped pulsed amplification (OPCPA) provides ultra-broad spectral phase-matching bandwidths able to support the amplification of less than ten-femtosecond duration laser pulses. In parametric amplifiers, out of the temporal window where the seed and pump laser pulses are overlapped, the intensity contrast is improved by the amplification factor. In multi-PW laser systems based on high energy OPCPA, serious technical difficulties are encountered to build the required single beam high energy picosecond-nanosecond pump lasers.

Hybrid femtosecond lasers combine the CPA in broad gain bandwidth laser media with OPCPA in nonlinear crystals (figure 1). A key feature of these systems consists in the adaptation of the parametric amplification phase-matching bandwidth of nonlinear crystals to the spectral gain bandwidth of laser amplifying media. Picosecond OPCPA in BBO crystals up to ten-mJ energy level in the laser Front-End, followed by CPA in Ti:sapphire crystals up to ten/hundred Joules, represents an appropriate solution for a high intensity contrast PW-class femtosecond laser system. Configuration, technical solutions, and expected output beam characteristics of worldwide developed hybrid amplification PW-class femtosecond laser systems are shortly described.

Raising the Multifunctionality of Laser Processed Perovskite Thin Films/Nanostructures through Chemical Pressure and Epitaxial Strain

Being considered by many scientists the main advantage over other material’s types, the multifunctionality of perovskite materials is, however, hard to achieve. Lead-free materials with perovskitic structure have become very attractive lately for a broad range of applications such as photovoltaic, photocatalytic, electronics and so on. Exhibiting a wide range of functional properties, ranging from normal ferroelectrics up to relaxor ferroelectrics by varying the amount of A-site and/or B-site substitutions in the perovskite systems, bulk lead free perovskites are intensively studied. This is the case of perovskite materials with small band gap values such as bismuth ferrite (BiFeO3-BFO) which have become very attractive for photovoltaic and photocatalytic applications. BFO exhibit both ferroelectric and ferromagnetic properties with a high remnant ferroelectric polarization (95 μC/cm²) and Curie temperature (Tc~1103 K). The band gap value of BiFeO₃ (Eg~2.71 eV) corresponding to maximum absorptivity at visible wavelengths and, if doped with Yttrium or Lanthanum, the band gap value can be decreased even lower. BiFeO₃ is by far the most promising and investigated multiferroic material but it has two major drawbacks: low dielectric susceptibility and high dielectric loss. Recently, we have underlined how a critically important material for eco-friendly (Pb-free) multiferroic multifunctional devices can be tailored, by joining doping and epitaxial strain engineering to create a nanoscale stripe structure, in order to overcome its major drawback namely the low dielectric response. By high resolution transmission electron microscopy (HR-TEM) and geometric phase analysis (GPA) we have evidenced nanostripe domains with alternating compressive and tensile strain in the Y-doped BiFeO3 epitaxial thin films. Dielectric constant behavior for undoped samples is presented in the same frequency and temperature conditions and it is clearly lower than for doped samples, which points to the joined role of doping and epitaxial strain for the enhancement of dielectric constant, due to easy response of nanodomains to electrical stimulus.

Another example of how the strain engineering in thin films approach can be applied for growing thin films with enhanced the dielectric and piezoelectric responses, is for the case of (Ba_1-xCa_x)(Zr_yTi_1-y)O₃(BCZT) materials. Our recent studies have demonstrated the possibility to obtain lead-free BCZT thin films with very high dielectric permittivity and piezoelectric coefficients. Epitaxial thin films of BCTZ have been deposited on single-crystalline substrates with different lattice parameters (SrTiO₃, SrLaAlO₄, LaAlO₃, GdScO₃) substrates by pulsed laser deposition. The high dielectric permittivity of BCZT thin films was attributed, besides to their high structural quality, to the enhanced susceptibility of the nanoscale domain configuration to a small external perturbation. The enhanced switching of such nanodomain configuration was probed by piezoforce microscopy, and values up to 230 pm/V has been obtained for d33 piezoelectric coefficients.

Linear Optical Constants’ Dependency on Thickness of Zinc Selenide Thin Films

Materials of sub-wavelength dimensionality are characterized by linear optical constants that can significantly vary from those corresponding to their bulk form. In order to tailor thin film designs with dissimilar attributes, the accurate influence of thin film thickness on the refractive index and on the absorption coefficient for a given spectral domain must be known, substrate influence having also to be taken into account.

The linear optical constants of zinc selenide (ZnSe) thin films obtained by thermal evaporation in vacuum technique are determined from transmittance experimental data only. ZnSe thin films with thicknesses ranging from 200 nm to 800 nm, deposited on transparent quartz substrates, are considered. The thin film linear optical constants, refractive index and absorption coefficient, are computed over a broad wavelength spectrum (300 nm-2500 nm), considering the contribution of the quartz substrate to the overall experimental transmittance. For the determination of the refractive index dispersion curve, the coefficients of the Sellmeier equation are computed, whilst for the absorption coefficient values, a wavelength-to-wavelength step-like approach is used to determine the wavelength dependence over the whole spectral domain. The derived algorithms employed to determine the linear optical constants’ dependency on thickness for the experimentally obtained ZnSe thin films are also used to extrapolate the results for practically any thickness in the range considering the same deposition method.

Whereas different deposition methods conduct to distinct optical constants’ dependencies for the same material and same invariant thin film thickness, the analytical approach keeps its validity. The work raises the importance and solves the issue of a good knowledge on linear optical constants’ dependency on thin film thickness for structure designs with customizable features for linear and nonlinear photonics applications.

Experimental Investigation of the Ultrafast Optical Nonlinearity by Third Harmonic Generation

The nonlinear (NL) optical response of materials at high light intensities is important for understanding the light – matter interaction processes as well as for the design of photonic functionalities. Various methods to determine the NL optical parameters, as Z-scan, wave mixing, nonlinear interferometric techniques, harmonic generation, have been developed. As the NL response can be produced by more than a single NL process, the discrimination between their contributions (magnitude, time scale) to the overall response is important but usually is not an easy task.

We present our recent results in the experimental study, by third harmonic generation (THG), of the ultrafast third-order NL response excited in the As₂S₃ semiconductor material by high-repetition-rate fs laser pulses at the wavelength λ=1550 nm, both the material and the wavelength being important for applications in optical communications.

When the NL optical response is excited by high-repetition rate fs laser pulses, the change of the refractive index of the sample by thermo-optic effect arising from the sample heating by cumulative thermal effects can mask the ultrafast one induced by the electronic nonlinearity. However, in the case of the THG, which is inherently an ultrafast electronic process, with a response time 1 fs, the thermo-optic effect cannot contribute to the NL frequency conversion. So, the ultrafast effect of pure electronic origin can be investigated by THG, in spite of the use of high-repetition-rate fs laser pulses.

In THG, a strong fundamental laser beam with the frequency wFH is incident on a NL optical material, exciting inside it a NL optical polarization, which is the source for the generation of the TH beam, at frequency w_TH = 3w _FH. The THG process, characterized by the third-order NL optical susceptibility, χ⁽³⁾, is possible in materials with any symmetry. From the dependence of the TH intensity on the FH intensity, the χ⁽³⁾ and the corresponding nonlinear refractive index, n₂, can be determined, as it will be discussed.

In our study the FH beam is generated by an Er-doped fiber laser (Toptica, ~ 150 fs pulse duration, 80 MHz repetition rate, λ=1550 nm). The very low average power (~pW) of the TH beam (λ=517 nm) generated in non-phase-matching conditions was measured using a common camera as an ultra-sensitive powermeter, a method recently introduced by us [6]. The principle, the camera calibration procedure, and the advantages of this method will be briefly presented. The experimentally derived dependence of the TH intensity on the FH intensity allowed the computing of optical NL parameters, χ⁽³⁾ and n₂, of As₂S₃ semiconductor material at the considered wavelength. These parameters are important for applications of this material in ultrafast photonic functionalities.

CO₂ Laser-Based Photoacoustic System for Trace Gas Analysis in Life Sciences

Infrared gas spectroscopy is becoming most widely used in many life science applications. In this paper we present a type of trace gas detection system based on a continuous wave (cw) CO₂ laser in combination with photoacoustic spectroscopy. Examples are included to expose the suitability of CO₂ laser system (over the last few years) to monitor in real time gases emission from various dynamic processes in: fruits, plants and human respiration as well as from a plasma generator.

Recent applications on a laser-based spectrometer using a cw CO₂ laser (preferably a watt level) are presented and a comparison is made to an Optical Emission Spectroscopy (OES) investigation for the determination of the dissociated CO₂ gas from plasma generator. Relationships between the photoacoustic signal and gas pressure, laser power and gas concentration were measured and discussed in detail, respectively.

The combination of photoacoustic spectroscopy and the CO₂ laser has resulted in simple, robust and easy to maintain designs which are giving photoacoustic spectroscopy a competitive advantage over other sensitive techniques.

Applications from different field of life sciences demonstrate their potential for laboratory and field experiments, respectively. For detecting a single species, the CO₂ laser remains a powerful source especially in combination with photoacoustic spectroscopy.

Industrial Laser Applications – State of the Art

An overview of recent advances in different fields of industrial laser applications is presented. Basic principles as well as latest processing techniques are put into discussion. The examples are given by the most important laser companies as well as by some own experience. Recent results in fs laser microprocessing, plastic welding, laser cladding, laser cleaning and other high power laser processing applications are presented. Opportunities offered to the young scientist to join the field of laser applications are emphasized.

Laser Ablation Applications on Contemporary Polychrome Artworks

One of the most important aspects in restoration of the polychrome art objects represents the color preservation and, as the art domain evolves, new materials and painting techniques emerge, opening a whole new area for investigations and analyses for their preventive conservation. The chemical composition of the pigments and the binding media is subjective to each time frame or art trend, holding a distinctive fingerprint that should be documented and treated accordingly.

Traditional cleaning applications on contemporary polychrome artworks on/with plaster (such as a seco, a fresco, graffiti, sgraffito etc.) can be challenging due to their high sensitivity and laser cleaning is chosen based on its versatility and elevated control. Laser cleaning allows us to tune in specific parameters (fluency, wavelength, pulse duration) in order to achieve a precise and a safe removal of the encrustation without affecting the object (substrate). The selective removal of impurity layers is regulated by the ablation threshold of the fluency values for the substances to be removed or to be untouched.

A series of pigments used in contemporary paintings were subjected to laser cleaning using a Q-switched Nd:YAG laser. Preliminary investigations were made in order to select the proper cleaning parameters and the ablation thresholds, mostly using the 1064 nm and 532 nm wavelengths, with a pulse duration of 6-8 ns. There were some cases where the UV radiation was used, and some that require a shorter pulse duration - subject of another study.

The artworks were characterized before and after cleaning using colorimetry, optical microscopy, FTIR and hyperspectral imaging, depending on the casuistic of each surface. The pigments and the paint layers were analyzed using elemental and molecular spectroscopic techniques - such as XRF, LIBS, FTIR, ATR - in order to determine their composition and create a data base dedicated to contemporary pigments and binding materials.

Generation and Identification of Antimicrobial Species from Medicines Exposed to Laser Radiation in view of Fighting Multiple Drug Resistance Acquired by Bacteria

One of the most important current drawbacks that have to be addressed in fighting infections with multiple drug resistant bacteria is the ineffectiveness of existing antibiotics to destroy them and the lack of new antibiotic molecules and of new treatment schemes. At the same time, a fast and cheap approach for drug development is needed, such as exposure of existing drugs to laser beams.

One direction for shortening the path of potential new antimicrobial agents to clinical applications is repurposing current drugs, photo-generate molecules derived from them and test the new products for the desired activity. An example is the phenothiazine class, whose numerous derivatives exhibit significant pharmacological activities, such as: insecticidal, antifungal, antibacterial and anthelmintic.

The aim of this study was to investigate the efficacy of 266 nm laser irradiation in generating from some available medicines compounds prepared in water solutions, new compounds in the same solutions, with enhanced antimicrobial properties. The laser exposed solutions were evaluated by UV-Vis-NIR absorption, Thin Layer Chromatography, Laser Induced Fluorescence, FTIR spectroscopy, minimal biofilm eradication concentration, minimal inhibitory concentration, and flow cytometry.

The antimicrobial and antibiofilm activity assays suggested that during exposure at 266 nm laser beam antimicrobial and antibiofilm species are generated. The spectroscopic studies showed the formation of photoproducts that depend on laser exposure time intervals and on photoreactions that occur during irradiation.

In conclusion, this method could constitute a fast approach in developing new antimicrobial agents and could be considered a new kind of study in chemistry, biology, and pharmacology.