Pulsed Regime Research Articles

Effect of silver nanoparticles size on the ultrafast optical nonlinear and optical limiting properties of Nd3+ doped antimony borate glasses

Nd3+ ions containing borate glasses embedded with silver (Ag) nanoparticles (NPs) were prepared by melting and quenching process. Thermal treatment at 450 °C for different holding durations has been performed to tune the size of the NPs embedded in the glass. The influence of growth of NPs on nonlinear optical (NLO) and optical limiting (OL) properties of glasses has been evaluated in the near–infrared spectral provenience under femtosecond pulse regime. The NLO and OL attributes evaluated using Z–scan configuration disclose the enhanced trend of two–photon absorption and nonlinear refraction coefficients and a decrease behaviour of OL threshold with the augment of thermal holding time. This enhancement in NLO coefficients and OL performance with thermal annealing duration is ascribed to the enhancement of the local field with the size of the NPs. The outcomes acknowledge the Nd3+ ions bearing borate glasses embedded with bigger Ag NPs are preferable for nanophotonic applications and optoelectronic functionalities.

Water Dose influence to the ALD hafnium oxide process: Simulation and experiment

In this work, we report a two-dimensional gas-dynamic model for the formation of hafnium oxide from TDMAH and water during the atomic layer deposition (ALD) process. Application of the model is demonstrated by the example of water cycle, in which the monolayer of functional groups –OH is formed. The simulation results show that the duration of the water pulse affects the dynamics of surface coverage with the -OH groups. At short pulse durations, the resulting coverage of the film surface with -OH groups becomes incomplete and has heterogeneous distribution over the film surface, which is due to the design of the reaction chamber and regimes of gas supply. The amount of residual water vapor depends on the duration of the water pulse and pumping regimes. Experimental data show the influence of water pulse duration on the refractive index, which is related to the structure of the deposited layer (composition, morphology, etc.). Based on experimental data and modeling results, we explain the effect of water pulse duration on the refractive index by the change in reaction mechanisms depending on water amount in the chamber. With a lack of water, the incomplete monolayer of –OH groups is formed and a {HfO2-y} layer grows as a result of the monomolecular decomposition of the precursor. With an excess of water, we expect the implementation of multilayer adsorption of water, provoking the reaction of the precursor in the volume of the water film, leading to the formation of the product {HfO2+y}.

Iterative suppression of Kerr-induced instabilities in Bessel beams using on-axis intensity shaping

Kerr-induced instabilities in zeroth-order Bessel beams with low focusing angle prevent the formation of longitudinally uniform plasma rods in the filamentation regime. These instabilities lead to the oscillation of the beam on-axis intensity via the generation of new spatial frequencies by a first stage of spectral broadening followed by a second stage of four-wave mixing. Here, we numerically demonstrate an efficient approach to drastically reduce the instabilities due to the second stage. It is based on shaping the longitudinal intensity profile with spatio-spectral components in opposition of phase to the Kerr-generated ones via an iterative approach. Zeroth-order Bessel beams with a longitudinal flat intensity plateau can be generated in a few iterations in the nonlinear regime. This is performed in both monochromatic and pulsed femtosecond regimes.

Silver molybdate: an excellent optical limiting material under nanoregime for photonic device application

There is a mounting demand for nonlinear optical materials with superior optical limiting performance which has a noticeable impact on protecting the delicate optical components from laser-induced damage. Transition metal molybdates have garnered attention in the nonlinear optics field due to their outstanding optical and luminescent properties, which give rise to widespread applications in next-generation optoelectronics devices. The structural confirmation of the as prepared silver molybdate nanoparticles were made by XRD and Raman spectroscopy analysis. The linear optical properties and the band gap of the synthesized material were studied using UV–Visible and photoluminescence spectroscopy. SEM analysis revealed the pebble like morphology of the silver molybdate nanostructures. The nonlinear responses of the samples were studied using open aperture z-scan approach with Nd:YAG pulsed laser (532 nm, 9 ns, 10 Hz). The sample exhibits reverse saturable absorption pattern attributed to the two photon absorption (2PA) mechanism. The obtained OL threshold value is in the order of 1012 which is suitable for fabricating optical limiters in nano second pulsed laser regime.

A nanoscale study of hafnium oxide resistive memory switching dynamics

This paper deals with the set and reset time measurements of a resistive memory consisting of a Ti/TiN/HfO2 layer stack contacted with the tip of a conductive atomic force microscope in ultra high vacuum. We present measurements of the set and reset switching times in voltage pulse regime for different voltages and compliance currents. The experimental results are well reproduced by simulation. We derive analytical expressions for the set and reset times as function of experimental conditions. The effect of voltage and current on reset and set switching times is then discussed with the help of their analytical expressions, which are also applied on standard devices characteristics.

816 nm All-Fiber Mode-Locked Tm-Doped Laser in Noise-Like Pulse Regime

816 nm All-Fiber Mode-Locked Tm-Doped Laser in Noise-Like Pulse Regime

A web of sticky strands: how localized stress controls spatio-temporal fluctuations in viscoelastic flows through a lattice of obstacles

Recent microfluidic experiments have evidenced complex spatio-temporal fluctuations in low-Reynolds-number flows of polymer solutions through lattices of obstacles. However, understanding the nonlinear physics of such systems remains a challenge. Here, we use high performance simulations to study viscoelastic flows through a hexagonal lattice of cylindrical obstacles. We find that structures of localized polymer stress – in particular birefringent strands – control the stability and the dynamics. We first show that, at steady state, strands act as a web of sticky flow barriers that induce channelization, multistability and hysteresis. We then demonstrate that a spontaneous destabilization of the strands drives the transition to unsteady flow with regimes of self-sustained oscillations, travelling waves and strand pulsations. We further show that these pulsations, which result from the destabilization of envelope patterns of stress with strands wrapped around multiple obstacles, are integral to the transition towards elastic turbulence in our two-dimensional simulations. Our study provides a new perspective on the role of birefringent strands and a framework for understanding experimental observations. We anticipate that it is an important step towards unifying existing interpretations of the nonlinear physics of viscoelastic flows through complex structures.

Switching and transformation of multi-state solitons in a slightly normal dispersion mode-locked fiber laser

Passively mode locked fiber lasers are excellent platforms for soliton generation. Here, multi-state switchable behaviors in a slightly normal dispersion mode-locked fiber laser are experimentally demonstrated with an identical configuration. A saturable absorber based on single-wall carbon nanotube is served as mode-locking device and an intrinsic birefringence spectral filtering is employed as a tunable inline filter. Consequently, with properly modifying pumping powers and orientations of polarization controller paddles, the operation regimes of the proposed laser could be reversibly switched between dissipative solitons and amplifier similaritons, and the characteristics of intermediate states are recorded. Tunable dissipative solitons with a tuning range of 38 nm and noise-like pulses could also alternatively be generated. This multi-state behavior switching could be attributed to the strong filtering caused by the limited gain bandwidth under strong pump strength. The experimental observations of these multi-state switchable transitions may offer insight into the pulse regime dynamics in the near-zero dispersion region and offer flexibility in multi-functional designs of the laser configuration, which would enable innovations in nonlinear optics and its applications.

A kinetic model of a quantum-dot microlaser

The results of a qualitative analysis of a semiclassical model of light generation in low-dimensional solidstate lasers, including quantum-dot microlasers and, on its basis, a numerical modelling of the regular pulsation regime that occurs under conditions of nonlinear shift and broadening of the resonant spectral gain line due to the influence of dipoledipoles interaction and absorption in quasi-resonant transitions on the dielectric susceptibility of the active medium are herein presented. The modelling of lasing was carried out for the parameters of semiconductor quantum-dot structures.

Express-analysis of quasi-periodic oxidation-reduction potential pulses in Baikal Coastal Groundwater during real-time monitoring (February 07–14, 2024)

Real-time groundwater monitoring on a shore of Lake Baikal reveals peculiar effects in the form of quasi-periodic negative ORP pulses with varying time intervals and a decrease in ORP relative to the background during the week of February 2024. We perform an express analysis of the observed effects and pay attention to their occurrence during increasing air temperature. At the beginning of the observation interval, we determine the regime of initial ORP pulses, which occur in conditions of a slightly increasing air temperature, then the middle one in a conventionally-optimal regime in conditions of significant warming, and the final regime of retrograde evolution in cooling conditions. The revealed regularities in changes of pauses between pulses and amplitudes of ORP reductions relative to the background form the basis for the analysis of ongoing effects.

Spatio-temporal study of single and multi-wavelength quasi-continuous-wave generation regimes in an all-normal dispersion ring cavity

In this paper, we report a comprehensive experimental study of quasi-continuous-wave operation in an all-normal-dispersion ring cavity laser with nonlinear polarization rotation based saturable absorber action. Using the temporal mapping technique, we analyze in real time spiking dynamics, characterized by the existence of ephemeral spikes that continually emerge and decay amidst the background radiation. In particular, the effects of the pumping level and its direction of variation on the laser emission dynamics were analyzed. The overall behavior, in particular the well-defined pattern of spike evolution between their emergence until their vanishing, is found to be qualitatively invariant over a broad range of pump power, from 70 mW to 550 mW, albeit the values of spiking parameters vary widely over this range. The spiking phenomenon appears from low pumping powers, just above the laser threshold. A phenomenon of hysteresis was also discovered, as some emission parameters, in particular the optical spectrum, depend on whether pump power is being raised or decreased. 2D autocorrelation analysis also proved insightful, showing that each spike appears at random, having no connection with other spikes. Finally, high pump power settings and birefringence adjustments enabled the discovery of more complex spiking dynamics where emission at various wavelengths occur, some of the lines presenting additional slow dynamics at the millisecond scale. These results indicate that deterministic complexity is, together with stochasticity, an essential ingredient to take into account for a better understanding of the operational modes in fiber lasers far from the pulsed regime.

Numerical modelling of multi-component mass transfer regimes in four-component gas systems

For argon and carbon dioxide, which are part of the tetra-component gas mixture He + Ar + CO2 – N2 and are the heaviest compared to other components, graphs of the behavior of the concentration of these components at different points of the diffusion channel and time intervals are presented. To simulate convective flows in the four-component mixture under consideration, the Flow Simulation computer package included in the SolidWorks engineering design system was used. The equations are solved by the finite volume method using the standard k – ε turbulence model and with initial and boundary conditions. Indicated, that obtained distributions change nonlinearly both in time and along the length of the diffusion channel. In this case, there is a change in the diffusion process to a convective one, which is due to the nonlinearity in the distribution of the components concentration, which is connected with the imbalance of mechanical equilibrium. It was found that the most significant change in the behavior of the concentrations of heavy-weight components occurs within 120 s. This time interval coincides with the appearance of two contrasting areas on the graphs, namely, the formation of a developed convective flow. An enhancement in the time of the numerical experiment showed that the concentration of the component with the maximal molar mass remains practically unchanged. This behavior of the concentration of heavy-weight components is characteristic of the effect when the mixture is enriched in the heavy-weight component. An analysis of the streamlines on the plots of concentration distributions showed that vortices of various scales interacting with each other and leading to a pulsating mixing regime are formed along the length of the channel. It has been established that the presence of large-scale vortices determines the preferential migration of the components with the maximal molar mass.

Performance of brushite plaster as kidney stone phantoms for laser lithotripsy.

Artificial phantoms used in photothermal near-infrared laser lithotripsy research generally fail to mimic both the chemical and the physical properties of human stones. Though high-energy, 1J pulses are capable of fracturing hard human stones into several large fragments along natural boundaries, similar behavior has not been observed in commonly used gypsum plasters like BegoStone. We developed a new brushite-based plaster formulation composed of ≈90% brushite that undergoes rapid fracture in the manner of human stones under fragmentation pulse regimes. Single-pulse (1J) ablation crater volumes for phantoms were not significantly different from those of pure brushite stones. Control over crater volumes was demonstrated by varying phosphorous acid concentration in the plaster formulation. Fragmentation of cylindrical brushite phantoms was filmed using a high-speed camera which demonstrated rapid fragmentation in < 100µs during the bubble expansion phase of a short pulse from a high-powered Ho:YAG laser (Lumenis Pulse 120H). The rapid nature of observed fracture suggests increasing laser pulse energy by increasing laser pulse duration will not improve fragmentation performance of laser lithotripters. Brushite plaster phantoms are a superior alternative to gypsum plasters for laser lithotripsy research due to their better mimicry of stone composition, controllable single-pulse crater volumes, and fragmentation behavior.

Multimodal optical clearing to minimize light attenuation in biological tissues

The biggest obstacle to optical imaging is light attenuation in biological tissues. Conventional clearing techniques, such as agent-based clearing, improve light penetration depth by reducing scattering, but they are hampered by drawbacks including toxicity, low efficiency, slowness, and superficial performance, which prevent them from resolving the attenuation problem on their own. Therefore, quick, safe, and effective procedures have been developed. One of them involves using standing ultrasonic waves to build light waveguides that function effectively in the tissue depth while minimizing scattering. Temporal optical clearing is another agent-free strategy that we introduced in our previous article. Whereas not deep, this technique minimizes both light absorption and scattering by pulse width variation in ultra-short pulse regime. Consequently, it can be a complementary method for ultrasonic optical clearing. In this work, we enhanced the light penetration depth in chicken breast tissue by 10 times (0.67–6.7 cm), setting a record in literature by integrating three clearing methods: agent-based, ultrasound-based, and temporal. Here, optical coherence tomography, Bear–Lambert, and fluorescence tests have been used to study the light penetration depth and optical clearing efficiency. Presented work is an essential step in development of diagnostic techniques for human body, from cells to organs.

Study on Electrically Modulated Quasi-Continuous Wave Fe: ZnSe Solid-State Laser with Hundred-Hertz.

Iron-doped binary chalcogenide crystals are very promising for tunable solid-state lasers operating over the 3~5 μm spectral range. Fe: ZnSe is one of the most important gain crystals with the obvious advantages of material characteristics and conversion efficiency. By adjusting the output mode of the pump source, an Fe: ZnSe laser can operate in two modes at liquid nitrogen temperatures: continuous wave (CW) and pulse output. In terms of CW output, the Fe: ZnSe laser obtained a maximum 2.63 W continuous power output which was confined to the power of the pump source. An optical-to-optical efficiency of 47.05% was acquired. Direct electrical modulation was applied to the pump source. The highest average power of the quasi-CW laser, whose central wavelength is 4.02 μm, has a value of 253 mW with an optical-to-optical efficiency of 42.88% and a full width at half maximum (FWHM) of 23 nm when the pulse frequency is 100 Hz of 10% duty factor. The output waveform is consistent with the modulation waveform applied to the pump source. We report to the first of our knowledge an electrically modulated quasi-CW Fe: ZnSe laser in the pulse regime, equipped with features of compactness in structure, ignoring additional modulators, convenience in control, high efficiency, and sustainable operation, of great interest for solving numerous scientific and applied problems.

Self-starting mode-locking in an all-PM Yb-doped fiber laser oscillator enabled by a 3x3 nonlinear optical lossy loop mirror

Ultrafast fiber oscillators based on loop mirror saturable absorbers often suffer from a problematic self-starting mode-locking operation. It is usually necessary to provide an additional phase shift in the loop, guaranteeing repeatable initiation of the pulsed regime. 3x3 even splitting ratio fiber couplers have recently introduced a required phase shift in an all-fiber loop mirror architecture. Until now, mode-locking induced by a 3x3 fiber coupler has been associated only with nonlinear amplifying loop mirrors. Here, we present a self-starting ultrafast dispersion-managed all-polarization-maintaining Yb-doped oscillator that utilizes a nonlinear optical lossy loop mirror instead. We show three ways of inserting asymmetric losses in a loop via a variable optical attenuator, a fiber coupler, and a very simple lossy splice. Complete characterization of all output ports of the oscillator proves significant spectral and temporal breathing of the pulse when circulating through the net normal dispersion cavity, which can deliver nJ-level pulse energy. The system guarantees excellent stability, low noise and performance comparable to nonlinear amplifying loop mirrors while being simpler, cheaper, and providing more usable output ports with different pulse characteristics.

Active Region Overheating in Pulsed Quantum Cascade Lasers: Effects of Nonequilibrium Heat Dissipation on Laser Performance

Mid IR Quantum cascade lasers are of high interest for the scientific community due to their unique applications. However, the QCL designs require careful engineering to overcome some crucial disadvantages. One of them is active region (ARn) overheating, which significantly affects laser characteristics, even in the pulsed mode. In this work, we consider the effects related to the nonequilibrium temperature distribution when thermal resistance formalism is irrelevant. We employ the heat equation and discuss the possible limitations and structural features stemming from the chemical composition of the ARn. We show that the presence of solid solutions in the ARn structure fundamentally limits the heat dissipation in pulsed and CW regimes due to their low thermal conductivity compared with binary compounds. Also, the QCL postgrowths affect the thermal properties of a device closer to CW mode, while it is by far less important in the short-pulsed mode.

Optimal design for wavelength conversion with a configuration of walk-off compensation in free space in the nanosecond pulsed regime

On the basis of standard wavelength conversion by the use of angular phase matching of nonlinear optical crystals arranged in free space, applicable to a wide range of wavelengths and laser intensities, we both numerically and experimentally present an optimal design for achieving near-full energy conversion while maintaining good TEM00 mode property of fundamental laser radiation in the nanosecond regime.

Laser-Induced Graphene Formation on Polyimide Using UV to Mid-Infrared Laser Radiation

Our study presents laser-assisted methods to produce conductive graphene layers on the polymer surface. Specimens were treated using two different lasers at ambient and nitrogen atmospheres. A solid-state picosecond laser generating 355 nm, 532 nm, or 1064 nm wavelengths and a CO2 laser generating mid-infrared 10.6 µm wavelength radiation operating in a pulsed regime were used in experiments. Sheet resistance measurements and microscopic analysis of treated sample surfaces were made. The chemical structure of laser-treated surfaces was investigated using Raman spectroscopy, and it showed the formation of high-quality few-layer graphene structures on the PI surface. The intensity ratios I(2D)/I(G) and I(D)/I(G) of samples treated with 1064 nm wavelength in nitrogen atmosphere were 0.81 and 0.46, respectively. After laser treatment, a conductive laser-induced graphene layer with a sheet resistance as low as 5 Ω was formed. Further, copper layers with a thickness of 3–10 µm were deposited on laser-formed graphene using a galvanic plating. The techniques of forming a conductive graphene layer on a polymer surface have a great perspective in many fields, especially in advanced electronic applications to fabricate copper tracks on 3D materials.

Effects of repeated sediment bypassing on stream ecosystems in a Japanese mountainous river

Sediment bypassing connects the upstream and downstream regions of dam impoundments through tunnels and is expected to partly restore natural processes (flood pulse and sediment regime) in dam‐altered river ecosystems. However, few reports examined the effects of restoration on stream biota. We examined the effectiveness of repeated sediment bypassing (12 times) over a period of 5 years in a Japanese dammed river using a before‐after‐reference‐impact design. We compared the abiotic parameters (water quality and substrate environment), biotic assemblages (algae, macroinvertebrates, and fish), and similarities between the dam downstream and undammed reference reaches. The reference sites exhibited high temporal variability in abiotic environments and species composition over the 5‐year period. Most abiotic and biotic parameters downstream of the dam improved and approached those of the changing reference states. However, these indices returned to pretrial conditions after the last trial. Biotic similarities between the downstream and reference reaches gradually increased throughout successive sediment‐bypassing trials for macroinvertebrates and algae; however, these similarities diminished after the last trial. Conversely, the abundance‐based composition of fish assemblages in the restored reach consistently diverged from that of the reference conditions throughout the study period. Our results show that the success and failure of restoration by sediment bypassing may be affected by the amount of bedload in the flushing flows and other regional processes such as migratory connectivity. Sediment bypassing can partially re‐establish natural processes and is a useful tool for improving sediment‐associated environments and biota.