Nanosecond Pulsed Laser Excitation Research Articles

Nanobelt and nanoplatelet structured molybdenum disulfide thin film as a saturable absorber under nanopulsed green laser excitation

Demonstration of saturable absorption (SA) in MoS2 thin films with varying NLO coefficients as a function of reaction time of hydrothermal method is made using the open aperture Z-scan technique. Under nanosecond pulsed laser excitation (532 ​nm, 9 ns, 10 HZ), saturable absorption occurred involving the overtone of electron - hole pair state and band edge state of MoS2. Further higher ground state cross-section than excited state cross-section favors SA process in MoS2 thin films. The strong saturable absorption behaviors of nanobelt like structured MoS2 film could be attributed to the coexistence of 1T and 2H crystal phases and lower dimension morphology that allows more photons to transit to the higher energy state. Earlier the fabrication of MoS2 thin film on a FTO substrate with a varying reaction time of the hydrothermal technique such as 24, 36, 48 and 60 ​h was made. Raman spectrum has two sets of characteristics vibration representing the metallic 1T – octahedral coordination Oh phase and the semiconducting 2H – trigonal prismatic D3h phase. XRD and Raman studies confirm the formation of mixed phases of MoS2 with 1T phase dominating over 2H phase in MoS2 thin films. Here the absorbance and band gap of the MoS2 thin film increased with increase in reaction time. The luminescence spectra exhibit prominent peaks at 594 ​nm and 621 ​nm resulting from a direct bandgap transition and they are mostly due to the VBM splitting at the K-point. All the films exhibit maximum absorbance in the UV region (291 ​nm) due to the excitonic features of MoS2 and notable absorbance in the visible region (680 ​nm) due to the overtones of MoS2 ‘s electron-hole pair. SEM images confirm that nanoplatelet and nanobelt structured MoS2 was formed with varying size with respect to reaction time. Furthermore, while the nanoplatelet structure faded, nanobelts were grown for a prolonged reaction time. Thus, MoS2 nanostructure coated thin film would be useful for the design of saturable absorbers for fabricating optical switches and mode locking devices.

Influence of silicon particle morphology on laser-induced plasma properties

In this work, Si particles of varying sizes and a bulk Si wafer are exposed to high-energy nanosecond-pulsed laser excitation. The subsequent laser-induced plasma is characterized using a suite of diagnostic techniques, including spatiotemporal species imaging via high-speed videography and high-resolution gated spectroscopy. The intensity and location of various species in the plasma (e.g., Si I and Si II) are tracked as a function of time over two time regimes, 2–10 μs and 20–80 μs, and show differences in impurities and peak intensities between the various particle sizes and compared to the Si wafer. Electron densities under these conditions decrease from ≈1.8 to 0.8 × 1017 cm−3 from 2 to 8 μs, while plasma excitation temperatures determined from the Boltzmann method range from 9200 to 8000 K over the same time window. The powdered Si samples have characteristically larger and more intense plasmas than the wafer sample. The laser-induced air-shock from energetic materials (LASEM) method was employed on these materials, in concert with their acoustic response, to establish their relative microsecond energy releases. While the powders have similar laser-induced shock velocities, they appear to have differences in their acoustic response which roughly scales as a function of plasma carbon content. The proposed reasons for these shock and acoustic trends are discussed, which include particle size and morphology, surface impurities, and dispersion and flow within the plasma.

Ultrasonication assisted exfoliation of MoSe2 nanosheets for optical and optical power limiting applications

We report direct, economic, and facile preparation of two-dimensional transition metal dichalcogenide material molybdenum diselenide nanosheets through the ultrasonication assisted liquid exfoliation method. The optical absorption spectra confirm the formation of MoSe2 nanosheets. The Kramers–Kronig expression was used to estimate the refractive index and extinction coefficient values of the prepared nanosheets. The emission properties of the prepared samples were done using photoluminescence spectroscopy. The nonlinear optical studies were performed using an open aperture Z-scan method under nanosecond pulsed laser excitation of a second harmonic generation wavelength of 532 nm. The obtained results demonstrate the strong optical power limiting (OPL) properties of the MoSe2 nanosheets. This OPL is due to two-photon absorption behavior of MoSe2 nanosheets.

Ultra-Broadband Random Laser and White-Light Emissive Carbon Dots/Crystal In-Situ Hybrids.

The continuous white-light emission of carbon dots (CDs) can be applied to producing multicolor laser emissions by one single medium. Meanwhile, the solid-state emission greatly contributes to its practical application. In this work, a strategy to realize the in-situ hybridization of silane-functionalized CDs (SiCDs) and 1,3,5-benzenetricarboxylic acid trimethyl ester (Et3BTC) by a one-pot solvothermal method is reported. Significantly, the SiCDs/Et3BTC hybrid crystals exhibit ultra-broadband random laser emission over the near ultraviolet-visible region under 265nm nanosecond pulsed laser excitation. The wavelength region of laser emission is achieved from 315 to 600nm within an emission band of CDs-based materials. It is worth noting that the wavelength range of the laser is wider than the previously reported works. It is proposed that the continuous white-light emission of SiCDs caused by multiple fluorescence centers mainly gives rise to the broadband random laser emission. Moreover, the crystals are conducive to forming resonance and realizing solid-state laser emission. This in-situ method is expected to enable a more convenient, cheaper, and greener approach to prepare luminescent hybrids for application in multicolor laser displays, multi-level laser anti-counterfeiting, supercontinuum light sources, and so on.

Is orange carotenoid protein photoactivation a single-photon process?

In all published photoactivation mechanisms of orange carotenoid protein (OCP), absorption of a single photon by the orange dark state starts a cascade of red-shifted OCP ground-state intermediates that subsequently decay within hundreds of milliseconds, resulting in the formation of the final red form OCPR, which is the biologically active form that plays a key role in cyanobacteria photoprotection. A major challenge in deducing the photoactivation mechanism is to create a uniform description explaining both single-pulse excitation experiments, involving single-photon absorption, and continuous light irradiation experiments, where the red-shifted OCP intermediate species may undergo re-excitation. We thus investigated photoactivation of Synechocystis OCP using stationary irradiation light with a biologically relevant photon flux density coupled with nanosecond laser pulse excitation. The kinetics of photoactivation upon continuous and nanosecond pulse irradiation light show that the OCPR formation quantum yield increases with photon flux density; thus, a simple single-photon model cannot describe the data recorded for OCP in vitro. The results strongly suggest a consecutive absorption of two photons involving a red intermediate with ≈100 millisecond lifetime. This intermediate is required in the photoactivation mechanism and formation of the red active form OCPR.

All-optical laser ultrasonic technique for imaging of subsurface defects in carbon fiber reinforced polymer (CFRP) using an optical microphone

Defects, such as delamination and debonding, are critical to the performance of carbon fiber-reinforced polymer (CFRP) composites. In recent years, non-destructive testing techniques have been improved for the inspection of these defects among CFRPs. In this study, an all-optical and non-destructive laser ultrasonic technique with an optical microphone detection module has been presented to detect the artificial subsurface defects among the CFRP composites. A finite element simulation based on the thermo-mechanical coupling model was used to study the process of nanosecond pulsed laser excitation of the CFRP laminate to produce ultrasound and the propagation behavior of ultrasound among the CFRP laminate. A series of non-contact laser ultrasonic testing experiments were carried out to study the flat bottom holes of different sizes via a laser ultrasonic detection system. The artificial subsurface defects were reliably identified by the presented all-optical laser ultrasonic system imbedded in the optical microphone using four feature images.

Silver Nanoparticles under Nanosecond Pulsed Laser Excitation as an Intensity Sensitive Saturable Absorption to Reverse Saturable Absorption Switching Material

Optical nonlinearity involved switching draws an important consideration in nonlinear optical studies. Based on that, we explored nonlinear absorption processes in silver nanoparticles synthesized by liquid phase laser ablation technique employing a second harmonic wavelength (532 nm) of Q switched Nd:YAG laser pulses with 7 ns pulse width and 10 Hz repetition rates. The typical surface plasmon resonance induced absorption (~418 nm) confirmed the formation of Ag NPs. The Z-scan technique was used to study the nonlinear optical processes, employing the same laser system used for ablation. Our study reveals that there is an occurrence of a saturable to reverse saturable absorption switching activity in the Ag nanoparticles, which is strongly on-axis input intensity dependent as well. The closed aperture Z-scan analysis revealed the self-defocusing nature of the sample.

Chromophore Orientation-Dependent Photophysical Properties of Pyrene-Naphthalimide Compact Electron Donor-Acceptor Dyads: Electron Transfer and Intersystem Crossing.

In order to study the effect of mutual orientation of the chromophores in compact electron donor-acceptor dyads on the spin-orbit charge transfer intersystem crossing (SOCT-ISC), we prepared naphthalimide (NI)-pyrene (Py) compact electron donor-acceptor dyads, in which pyrene acts as an electron donor and NI is an electron acceptor. The connection of the two units is at the 4-C and 3-C positions of the NI unit and the 1-position of the pyrene moiety for dyads NI-Py-1 and NI-Py-2, respectively. A charge transfer absorption band was observed for both dyads in the UV-vis absorption spectra. Upon nanosecond pulsed laser excitation, long-lived triplet states (lifetime is 220 μs) were observed and the triplet state was confined to the pyrene moiety. The ISC efficiency is moderate to high in nonpolar to polar solvents (singlet oxygen quantum yield: ΦΔ = 14-52%). Ultrafast charge separation (ca. 0.81 ps) and charge recombination-induced ISC (∼3.0 ns) were observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism; interestingly, the observed electron spin polarization pattern of the triplet state is chromophore orientation-dependent; and the population rates of the triplet sublevels of NI-Py-1 (Px:Py:Pz = 0.2:0.8:0) are drastically different from those of NI-Py-2 (Px:Py:Pz = 0:0:1).

Spontaneous and Stimulated Emission in Thin Films of Cu(In1 –xGax)(SySe1 –y)2 Solid Solutions in the Сomposition of Solar Cells

The emission spectra of thin nanocrystalline films of Cu(In1 – xGax)(SySe1 – y)2 (CIGSSe) direct-gap solid solutions in the structure of solar cells, recorded upon continuous-wave laser excitation (~0.5 W/cm2) and nanosecond pulsed laser excitation with a power density in the range of 0.1–53 kW/cm2 at temperatures ranging from 10 to 300 K, are analyzed. It is found that stimulated emission (SE) occurs in thin CIGSSe films at temperatures from 10 to 90 K in the spectral range hν = 1.062–1.081 eV, with a minimum threshold-pump level of ~1 kW/cm2. It is shown that the spontaneous emission band is blue-shifted with an increase in the excitation intensity. It is also established that, with an increase in temperature, the photoluminescence (PL) band (upon weak excitation) and the stimulated-emission band are blue-shifted, whereas the PL band is red-shifted upon strong excitation. The possible reasons and mechanisms for the influence of temperature and excitation intensity on the spectral positions of the spontaneous and stimulated emission bands for the solid-solution films are discussed.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4'-R-2,2':6',2″-terpyridine (4'-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured 1π,π* absorption bands at <400 nm and broad charge transfer (1CT)/1π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the 1CT/1π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the 3π,π*/3CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Physicochemical properties of cesium tetroxalate dihydrate single crystal: An efficient material for nonlinear optical applications

Herein we report the tunable optical limiting property of a metal–organic cesium tetroxalate dihydrate (CTD) complex. Temperature and pH were optimized to be 40°C and 3.5 respectively to grow CTD single crystal using the solvent evaporation method. The compound formation was confirmed by its response to single crystal and powder x-ray diffraction analysis. Thermal, mechanical, dielectric, linear optical, photoconductivity and photoluminescence properties of title crystal are reported. Nonlinear coupling between delocalized π-electrons of metal–ligand interaction and the oscillating electric field of a laser beam enhanced the nonlinear optical parameters. The third order nonlinear refractive index (n2), absorption coefficient (β) and susceptibility (χ(3)) of CTD complex under continuous-wave laser excitation are found to be (3.26±0.033)×10−8 cm2W−1, (10.33±0.41)×10−4 cmW−1 and (8.98±0.35)×10−5 esu respectively. Variation in two-photon absorption (2PA) coefficient measured under nanosecond pulsed laser excitation of different intensities confirmed the presence of sequential 2PA in CTD complex. The optical limiting threshold was investigated under different intensities were found to vary from (1.09±0.05)×1012 Wm−2 to (2.49±0.05)×1012 Wm−2 showing the intensity tunable optical limiting property of CTD material.

Facile synthesis, characterization and intensity-dependent nonlinear absorption of Ni-doped (γ and β)-BaB2O4 nanostructures

Intensity-dependent nonlinear optical absorption and optical limiting behavior of Ni2+-doped (γ and β)-BaB2O4 nanostructures were examined by open-aperture Z-scan technique under nanosecond pulsed green laser excitation. Observation of reverse saturable absorption (RSA) with variation in nonlinear absorption coefficient as function of on-axis peak intensity ascertains the presence of sequential 2PA process (1PA + ESA). Due to the introduced near-resonant energy state through incorporation of Ni2+ ions, the material exhibits excited state absorption (ESA). Here, the observed sequential 2PA in Ni2+-doped γ-BaB2O4 involves the 1T1g(G) states of 3d8–3d8 and 1T1g(D) states of Ni2+, while Ni-doped β-BaB2O4 undergoes the electronic transition involving intraionic 3d8–3d8 transition of Ni2+ and self-trapped excitonic state of BBO. Interestingly, as the dopant concentration and on-axis intensity increased, 2PA coefficient was found to be increased. 0.05 M Ni2+-doped β-BaB2O4 nanostructures possess higher 2PA coefficient (2.31 × 10−10 m/W) and lower onset limiting threshold (0.79 × 1012 W/m2), which makes it a promising candidate for optical limiting applications. The result suggests that band structure tunability to induce excited state absorption with enhanced nonlinear absorption coefficient is possible through Ni doping in β-BaB2O4 nanostructures.

Спонтанное и стимулированное излучение в тонких пленках твердых растворов Cu(In-=SUB=-1-x-=/SUB=-Ga-=SUB=-x-=/SUB=-)(S-=SUB=-y-=/SUB=-Se-=SUB=-1-y-=/SUB=-)-=SUB=-2-=/SUB=- в составе солнечных элементов

The results of a study of the emission spectra of thin nanocrystalline films of Cu(In1-xGax)(SySe1-y)2 (CIGSSe) direct-band-gap solid solutions in the structure of solar cells at ~ 0.5 W/cm2 continuous wave and nanosecond pulsed laser excitation in the range of excitation power density 0.1 - 53 kW/cm2 and temperatures of 10-300 K are presented. It was found that stimulated emission (SE) occurs in thin CIGSSe films in the temperature range from 10 K to 90 K in the spectral region h = 1.062 - 1.081 eV with a minimum threshold pump level of about 1 kW/cm2. It was shown, that, with increasing intensity of the exciting emission, the spontaneous emission bands shift toward higher energies. It was found that the photoluminescence bands at low excitation levels and the SE bands shift with increasing temperature toward higher energies, and the PL bands at high excitation levels shift toward low energies. Possible causes and mechanisms of the influence of temperature and excitation intensity on the spectral positions of spontaneous and SE of the films of solid solutions are discussed.

Damping of Cu-Associated Photoluminescence and Formation of Induced Absorption in Heavily Cu-Doped CdSe Quantum Dots

We studied photoluminescence (PL) properties and nonlinear absorption characteristics of heavily Cu-doped CdSe colloidal quantum dots (QDs) under nanosecond laser pulse excitation. A strong dependence of basic exciton transition behavior and Cu dopant-associated PL on the concentration of copper ions and pump intensities was revealed for the first time. The saturation of the copper-associated PL signal was found to happen much faster than the saturation of nonlinear absorption. The induced absorption and saturation of high energy transitions due to the exciton–exciton interaction were investigated in Cu-doped CdSe QDs. The obtained results could be useful for the development of optoelectronic devices operating at room temperature on the basis of Cu-doped colloidal QDs.

Photoluminescence and nonlinear transmission of Cu-doped CdSe quantum dots

In this paper we investigate the photoluminescence (PL) properties and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under a nanosecond laser pulse excitation. We find a strong difference of the pump intensity dependent behavior between the basic exciton transition and Cu dopants associated PL. We also study the saturation of absorption in Cu-doped CdSe QDs. The role of a strong electron-phonon interaction in the PL properties and nonlinear transmission of colloidal Cu-doped CdSe QDs is explained. The presence of biexciton PL exciton-phonon interaction and its strong influence on the basic exciton PL and non-linear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity.

Relationship of alcohol and/or cannabis use in the last year and the severity of the Internet gaming disorder symptoms

In this paper we investigate the photoluminescence (PL) properties and nonlinear transmission of Cu-doped CdSe colloidal quantum dots (QDs) under a nanosecond laser pulse excitation. We find a strong difference of the pump intensity dependent behavior between the basic exciton transition and Cu dopants associated PL. We also study the saturation of absorption in Cu-doped CdSe QDs. The role of a strong electron-phonon interaction in the PL properties and nonlinear transmission of colloidal Cu-doped CdSe QDs is explained. The presence of biexciton PL exciton-phonon interaction and its strong influence on the basic exciton PL and non-linear properties of Cu-doped colloidal CdSe QDs are proved by the simultaneous linear growth of basic excitons PL, the growth of the Stokes shift and significant decrease of absorption at the basic exciton transition wavelength with the increase of pump intensity.

Scattering of Ginzburg–Frank and Cherenkov Types Under Self-Focusing of Nanosecond Laser Pulses in Liquids

We study the dynamics of nonlinear optical processes such as self-focusing, self-phase modulation, and stimulated Raman scattering in Kerr-liquids under the nanosecond laser pulse excitation. The results prove the existence of the transition Ginzburg–Frank-type effect, which promotes the appearance of new spectral components of the laser radiation at the medium boundary. The generation of extended anti-Stokes frequency-angular bands of stimulated Raman scattering is explained. When the velocity of a self-focusing focal spot matches the phase velocity of the non-linear polarization at the anti-Stokes Raman frequency and the phase velocity of the scattered axial radiation, the most intense frequency-angular bands appear. They are described by the equations typical of the Cherenkov radiation.

Cavitation and shock waves emission on the rigid boundary of water under mid-IR nanosecond laser pulse excitation

The processes of conversion of light energy into mechanical energy under mid-IR nanosecond laser excitation on a rigid boundary of water are investigated. Strong water absorption of Q-switched Cr:Yb:Ho:YSGG (2.85 µm, 6 mJ, 45 ns) laser radiation provides rapid energy deposition of ~8 kJ cm−3 accompanied with strong mechanical transients. The evolution of shock waves and cavitation bubbles is studied using the technique of shadowgraphy and acoustic measurements, and the conversion efficiency into these energy channels for various laser fluence (0.75–2.0 J cm−2) is calculated. For 6 mJ laser pulse with fluence of 2.0 J cm−2, the conversion into shock wave energy reaches 67%. The major part of the shock wave energy (92%) is dissipated when the shock front travels the first 250 µm, and the remaining 8% is transferred to the acoustic far field. The calculated pressure in the vicinity of water-silicon interface is 0.9 GPa. Cavitation efficiency is significantly less and reaches up to 5% of the light energy. The results of the current study could be used in laser parameters optimization for micromachining and biological tissue ablation.

Nanosecond time‐resolved Raman spectroscopy for solving some Raman problems such as luminescence or thermal emission

AbstractRaman spectroscopy is experimentally widely available and relatively simple to perform at room temperature. Some difficulties nevertheless remain, as for instance separating some various contributions from the Raman spectrum, such as thermal emission for high temperature measurements or luminescence. Here, an optimized time‐resolved Raman spectroscopy system based on a gated detection and a nanosecond pulsed laser excitation (30 ns width, 532 nm wavelength) is described. The system allows contactless Raman measurements, at high temperatures, of some materials such as zirconia and yttria. An example of Raman spectra collected on yttria at very high temperature up to 2100 °C is provided. This optimized system may also be used to discriminate between Raman scattering and luminescence as demonstrated in zirconia material.

Simulation Studies of Photoacoustic Response from Gold-Silica Core-Shell Nanoparticles

Metal nanoparticles especially of noble metals are used as an exogenous contrast agent for biomedical photoacoustic (PA) imaging in the tissue transmission window extending from visible to near infrared 700–1100 nm band. Different geometrical configurations of gold and silver nanoparticles like spherical core-shell, nanorod, and nanocages are promising candidates for thermoplasmonics, photothermal therapy, photothermal imaging, and photoacoustic imaging. In the current study, we simulated the photoacoustic response of gold and silica core-shell nanoparticle in water medium. Finite element simulations were carried out to study the spectral absorption response and effect of nanosecond laser pulse excitation on the spatial/temporal temperature as well as photoacoustic pressure variations of different core-shell geometry of nanoparticle. We have optimized the dimensions of gold nanosphere, gold-silica, and silica-gold core-shell geometries for optimum photoacoustic conversion efficiency. Further, the effect of shell thickness on the pulse photoacoustic signals for core-shell gold-silica and silica-gold nanoparticle has been studied. We concluded that silica-gold core-shell nanoparticles possess better photoacoustic conversion efficiency in comparison to gold nanosphere and gold-silica core-shell geometries. The prime aim of this study is to design efficient nano-probes for photoacoustic imaging, photoacoustic tomography, photothermal therapy, and drug delivery.