Nonlinear Absorption Mechanism Research Articles

Experimental evidence of two-photon absorption and its saturation in malachite green oxalate: a femtosecond Z-scan study

Third-order nonlinear optical (NLO) properties of malachite green oxalate (MGO) dye were systematically studied at different input intensities and in a wide range of wavelengths by the Z-scan method using ∼150  fs, MHz femtosecond pulses. The sample transmittance results are explained by fitting the data with phenomenological models of linear and multi-photon absorption and their saturations. Intensity-dependent measurements demonstrated the influence of saturation on the absorption process. The data obtained suggests that by tuning the input intensity single or combination of nonlinear phenomena can be achieved in MGO. Wavelength-based measurements reveal a strong correlation between linear and NLO responses. The combinations of two- and three-photon absorption and saturation of two-photon absorption occur simultaneously in a wide wavelength range (750–900 nm). Multi-photon absorption cross sections have been calculated using the NLO coefficients and the concentration data. Two- and three-photon absorptions (including an excited state) are determined as the excitation mechanisms of nonlinear absorption in MGO.

Nonlinear absorption properties and excited-state charge-transfer dynamics of Er doped ZnO films

This work reports on the nonlinear optical properties of Er doped ZnO (EZO) films, measured by the Z-scan technique with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism of EZO films is analyzed by a singlet state three and four-level model, respectively. The effect of the direct current sputtering power of EZO films on the nonlinear optical properties was studied. From the results, the samples show the self-focusing effect and two photon absorption (TPA) induced excited state reverse saturable absorption (RSA) behavior. Under the different laser parameter excitation, it has been determined that the TPA induced saturable absorption (SA) or RSA properties for EZO films with 8 W dc sputtering power. Moreover, these findings indicate that free carrier density increases with Er doping and this behavior leads to excited state absorption. In addition, the ultrafast dynamics of EZO films have been investigated using the two-color pump-probe (TCPP) technique with 325, 380 and 400 nm wavelength excitation and probing over a broad range in the visible region. The decay of the positive signal is found to be biexponential, which we have assigned to the pump-induced deep-level (DL) or excited state absorption. We also present experimental results that the few decade picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state ( 1.2 ns). Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widens. Our results show that EZO films are a promising candidate in further optoelectronic device applications.

High-performance all-optical limiting based on nonlinear refraction of metal-doped PbS/PVA freestanding nanocomposite films

A modified Z-scan technique was used to define the nature of nonlinear refraction and absorption mechanisms with a single measurement. Undoped and metal (Zn, Mg, Cd)-doped PbS/PVA freestanding nanocomposite films were prepared using a simple chemical method with different doping concentrations. X-ray diffraction, scanning electron microscopy, and optical absorption spectroscopy were performed to characterize the prepared films. The crystallinity and optical band gap of the prepared metal-doped PbS/PVA were affected by Zn, Mg, and Cd doping concentration. All prepared samples showed a blue shift in the absorption edge, and the optical band gap increased with increasing doping concentration. The nanocomposite films displayed self-defocusing nonlinearity under the experimental conditions. Increasing the laser input power of the Zn-PbS/PVA sample yielded a higher optical nonlinearity as compared with the other prepared samples under a doping concentration of 0.008 M. The all-optical limiting performance of the metal-doped PbS/PVA prepared under different Zn, Mg, and Cd doping concentrations was investigated. The PbS/PVA sample doped with 0.008 M Zn realized all-optical limiting at a limiting threshold of 3 mW with an optical damage of 21 mW for a high dynamic range of 7.

Solvent specific and nonspecific interactions' effects on nonlinear optical responses of Cresyl Violet dye

In this study, solvents polarity effects on nonlinear properties of Crysel violet acetate (CVA) dye were investigated by Z-scan method. Nonlinear parameters (absorption coefficient and refractive index) for CVA dye in solvent media were obtained using experimental setup. The obtained experimental results demonstrate that the mechanism of nonlinear absorption depend highly on the properties of surrounding molecular media and the character of intermolecular interactions. Contributions of intermolecular interactions were determined by exploiting these results with Kamlet-Taft (KAT) polarity parameters. The refractive index in different solvents shows negative sign (coefficient's order is 10−7). Also, nonlinear absorptions in different solvents show positive sign and the obtained coefficient's order is 10−4.

Precision structuring and functionalization of ceramics with ultra-short laser pulses

High-performance ceramics have been firmly established for the manufacturing of tools and components in the modern electronics industry and mechatronics. Various components such as circuit boards, bearings, and sensors benefit from their specific characteristics, such as wear resistance, stiffness, and electrical neutrality. Apart from these advantages, the brittleness and hardness of ceramics turn the mechanical processing into a challenging and difficult task. Against this background, modern laser technologies have already been used to process ceramics for many years, enabling a contactless and wear-free machining. However, regarding high precision applications, for instance, the drilling of micro-holes or the fabrication of well-defined cavities and three-dimensional structures, conventional laser processes reach their limits. Especially due to thermal influences of the laser radiation, brittle edges, stresses, and redeposited layers emerge. Ultrashort pulse lasers enable completely new processing qualities in these fields. The extremely short pulse durations within the pico- and femtosecond range lead to nonlinear absorption mechanisms and an almost athermal material removal. Thereby, dielectric materials can be processed precisely and gently. In the course of a comprehensive process study, the beam–material interactions of ultrashort pulses with ceramics have been investigated. Besides the material properties, the ablation process is influenced by a multitude of laser parameters, such as wavelength, pulse overlap, and fluence. In order to reveal the most important variables, the experiments have been conducted by applying modern statistical methods. Using alumina (Al2O3) as an example, it is shown how different parameter regimes lead to disparate process qualities and efficiencies. The generated models have been used to optimize industrially interesting applications, from the separation of ceramic printed circuit boards to the realization of precise design structures.

Absorption modification by laser irradiation in DKDP crystals

Laser-induced modification at 355 nm of deuterated potassium dihydrogen phosphate (DKDP) crystals following exposure to nanosecond (ns) and sub-ns laser irradiation is investigated in order to probe the absorption mechanism in damage initiation. Laser damage resistance is greatly improved by sub-ns laser conditioning, whereas only a little improvement occurred after ns laser conditioning at the same laser fluence. Moreover, scattering and transmittance variations after the two types of laser conditioning indicate similar reduction of linear absorption. However, by contrast, large differences on nonlinear absorption modification are discovered using Z-scan measurement. This characteristic absorption modification by laser irradiation provides evidence that a nonlinear absorption mechanism plays a key role in damage initiation at 355 nm.

Linear and nonlinear optical properties of Tellurium Vanadate (Te2V2O9)

We report here the structure property correlation of Tellurium Vanadate (Te2V2O9) through various optical and vibrational spectroscopic investigations. Pure phase polycrystalline powder of Te2V2O9 was prepared by solid state reaction technique. Phase purity of the sample was confirmed by Powder X-Ray diffraction and the microstructural investigation was analyzed using Scanning Electron Microscopy. Raman microscopy was employed to validate the molecular structure. Diffused Reflectance and Photoluminescence spectroscopy were employed to study the optical properties. Because of non-centrosymmetry, we also observed second harmonic generation in tellurium vanadate. Subsequently, third order nonlinear optical response of Te2V2O9 was probed using open-aperture Z-scan technique estimating the nonlinear absorption coefficient to be 1e-10 mW−1. The mechanism of nonlinear absorption was deduced to be a two-photon absorption process. This was ascertained through existence of excited states predicted from electronic structure of Te2V2O9 using Density Functional Theory. It is also noteworthy to highlight that Te2V2O9 possess higher nonlinear optical coefficient than other vanadate compounds reported in literature.

Smart Nanostructures for Cargo Delivery: Uncaging and Activating by Light.

Nanotechnology has begun to play a remarkable role in various fields of science and technology. In biomedical applications, nanoparticles have opened new horizons, especially for biosensing, targeted delivery of therapeutics, and so forth. Among drug delivery systems (DDSs), smart nanocarriers that respond to specific stimuli in their environment represent a growing field. Nanoplatforms that can be activated by an external application of light can be used for a wide variety of photoactivated therapies, especially light-triggered DDSs, relying on photoisomerization, photo-cross-linking/un-cross-linking, photoreduction, and so forth. In addition, light activation has potential in photodynamic therapy, photothermal therapy, radiotherapy, protected delivery of bioactive moieties, anticancer drug delivery systems, and theranostics (i.e., real-time monitoring and tracking combined with a therapeutic action to different diseases sites and organs). Combinations of these approaches can lead to enhanced and synergistic therapies, employing light as a trigger or for activation. Nonlinear light absorption mechanisms such as two-photon absorption and photon upconversion have been employed in the design of light-responsive DDSs. The integration of a light stimulus into dual/multiresponsive nanocarriers can provide spatiotemporal controlled delivery and release of therapeutic agents, targeted and controlled nanosystems, combined delivery of two or more agents, their on-demand release under specific conditions, and so forth. Overall, light-activated nanomedicines and DDSs are expected to provide more effective therapies against serious diseases such as cancers, inflammation, infections, and cardiovascular disease with reduced side effects and will open new doors toward the treatment of patients worldwide.

Environment effects on the nonlinear absorption properties of Methylene blue under different power of excitation beam

Abstract In this paper, the nonlinear optical absorption properties of Methylene blue solutions were studied by open aperture Z-scan technique under a continuous-wave laser irradiation with various input powers. The experimental results show that the mechanism of nonlinear absorption and the calculated coefficients depend highly on the molecular surrounding media properties and the nature of intermolecular interactions. From the experimental results, the contributions of various intermolecular interactions were determined as well. Exploiting these results, it is possible to determine the effective media polarity parameters on the nonlinear responses of optical materials. According to the experimental results, by increasing of excitation beam power, the strong hydrogen bonds with high contributions increase the third order nonlinear responses of Methylene blue molecules.

Femtosecond laser-induced hard X-ray generation in air from a solution flow of Au nano-sphere suspension using an automatic positioning system.

Femtosecond laser-induced hard X-ray generation in air from a 100-µm-thick solution film of distilled water or Au nano-sphere suspension was carried out by using a newly-developed automatic positioning system with 1-µm precision. By positioning the solution film for the highest X-ray intensity, the optimum position shifted upstream as the laser power increased due to breakdown. Optimized positioning allowed us to control X-ray intensity with high fidelity. X-ray generation from Au nano-sphere suspension and distilled water showed different power scaling. Linear and nonlinear absorption mechanism are analyzed together with numerical modeling of light delivery.

Study of nonlinear optical absorption properties of V2O5 nanoparticles in the femtosecond excitation regime

In this work, we report for the first time, the nonlinear optical absorption properties of vanadium pentoxide (V2O5) nanoparticles in the femtosecond excitation regime. V2O5 nanoparticles were synthesized through solution combustion technique. The as-synthesized samples were further characterized using XRD, FESEM, EDAX, TEM and UV–visible spectroscopy. X-ray diffraction results revealed the crystalline nature of the nanoparticles. Electron microscopy studies showed the size of the nanoparticles to be ~200 nm. Open-aperture z-scan technique was employed to study the nonlinear optical absorption behavior of the synthesized samples using a 100-fs laser pulses at 800 nm from a regeneratively amplified Ti: sapphire laser. The mechanism of nonlinear absorption was found to be a three-photon absorption process which was explained using the density of states of V2O5 obtained using density functional theory. These nanoparticles exhibit strong intensity-dependent nonlinear optical absorption and hence could be considered for optical-power-limiting applications.

Nonlinear absorption mechanisms during femtosecond laser surface ablation of silica glass

Spatial profiles of single-shot microcraters produced by tightly focused femtosecond laser pulses with variable pulse energies are measured by means of a laser confocal microscope. Dependences of crater depth on laser intensity at different pulse energies appear as overlapping saturating curves with the same threshold, indicating the presence of nonlinear absorption and absence of nonlocal ablation effects. A monotonic twofold increase in absorption nonlinearity is related to the transition from minor defect-band absorption to fundamental band-to-band absorption.

Light absorption in perovskite solar cell: Fundamentals and plasmonic enhancement of infrared band absorption

Perovskite solar cells have attracted great attention in recent years due to its advantageous features including low production cost, ease of fabrication and rapidly-improving device efficiencies. Research activities have been mainly devoted to the development of new manufacturing processes, materials, device structures and better stability. There is little research on the fundamentals of solar-spectrum absorption in the device layers and almost no attempts have been made to optically improve the weak absorption of perovskite materials in infrared (IR) bands. Using a full-wave simulation approach, we report for the first time the contributions of each device layers in light absorptions across the whole solar spectrum. It is found that perovskite layer dominants the absorption in UV and visible bands, while the electrode layers dominants the IR bands. In order to optically enhance the light absorption in perovskite layer in IR band, we propose to use closely-spaced plasmonic nanoparticle array to achieve considerable optical absorption in the IR band, which haven’t been investigated in the literature. The concept is to use plasmonics to create hot spots in active layers, which can considerably enhance the localized light absorption in perovskite material via light-flow-circulating and nonlinear absorption mechanisms. Under optimized conditions, a 58.2% IR-band absorption enhancement has been achieved in this study. This work provides a new path towards achieving higher efficiency perovskite solar cells.

Nonlinear optical studies of inorganic nanoparticles–polymer nanocomposite coatings fabricated by electron beam curing

The optical nonlinearity of metal nanoparticles in dielectrics is of special interest because of their high polarizability and ultrafast response that can be utilized in potential device applications. In this study nanocomposite thin films containing in situ generated Ag nanoparticles dispersed in an aliphatic urethane acrylate (AUA) matrix were synthesized using electron beam curing technique, in presence of an optimized concentration of diluent Trimethylolpropanetriacrylate (TMPTA). The metal nanocomposite films were characterized using UV–visible spectrophotometry, transmission electron microscope (TEM) and field emission scanning electron microscope (FE-SEM) techniques. Ag nanoparticle impregnated films demonstrated an absorption peak at ∼420nm whose intensity increased with increase in the Ag concentration. The optical limiting property of the coatings was tested using a nanosecond Nd-YAG laser operated at third harmonic wavelength of 355nm. For a 25ns pulse and 10Hz cycle, Ag-polymer coatings showed good optical limiting property and the threshold fluence for optical limiting was found to be ∼3.8×10–2J/cm2 while the transmission decreased to 82%. The nonlinear optical coefficients were also determined using the standard Z-scan technique with picosecond (∼2ps, 1kHz) and femtosecond (∼150fs, 100MHz) pulses. Open aperture Z-scan data clearly suggested two-photon absorption as the dominant nonlinear absorption mechanism. Our detailed studies suggest these composites are potential candidates for optical limiting applications.

Resonant plasmon-stimulated nonlinear absorption in three-level systems.

Mechanisms of resonant nonlinear absorption in the hybrid exciton-plasmon systems based on gold nanoparticles and macroheterocyclical compounds are presented. We used novel substituted subphthalocyanines (SubPc) with 14 π-electronic system as powerful excitonic 3 – level systems, and 50 nm gold nanoparticles (AuNp) as plasmonic structures. We succeeded to define mechanisms of resonant nonlinearities and to show that local field factor (LFF) and Purcell factor (PF) lead to dramatic enhancement in SubPc nonlinearity. We suggest a theoretical model which describes exciton-plasmon interaction of porphyrin/phthalocyanine related compounds with gold nanoparticles in the resonant high power laser field.

Laser technologies for processing wide band-gap semiconductors and insulators: Nonlinear absorption mechanisms

The results of the theoretical study of damage and nonlinear light absorption mechanisms in transparent materials, i.e., wide band-gap semiconductors and insulators, are presented. It is shown that ablation processes in transparent materials exposed to laser pulses with intensity of the order of tens of TW/cm2 and pulse duration of the order of hundreds femtoseconds are efficient for various surface treatment technologies. The mechanism of tunneling nonlinear light absorption is studied. Ablation thresholds of GaN and other transparent materials such as sapphire (Al2O3), vitreous SiO2, and the same SiO2 with Ge impurity are determined. It is found that the ablation threshold depends on the band gap (absorption band edge) Eg as Eg3, which is in good agreement with experiment.

The effect of external magnetic field on the bremsstrahlung nonlinear absorption mechanism in the interaction of high intensity short laser pulse with collisional underdense plasma

Laser absorption in the interaction between ultra-intense femtosecond laser and solid density plasma is studied theoretically here in the intensity range Iλ2≃1014−1016Wcm−2μm2. The collisional effect is found to be significant when the incident laser intensity is less than 1016Wcm−2μm2. In the current work, the propagation of a high frequency electromagnetic wave, for underdense collisional plasma in the presence of an external magnetic field is investigated. It is shown that, by considering the effect of the ponderomotive force in collisional magnetized plasmas, the increase of laser pulse intensity leads to steepening of the electron density profile and the electron bunches of plasma makes narrower. Moreover, it is found that the wavelength of electric and magnetic fields oscillations increases by increasing the external magnetic field and the density distribution of electrons also grows in comparison with the unmagnetized collisional plasma. Furthermore, the spatial damping rate of laser energy and the nonlinear bremsstrahlung absorption coefficient are obtained in the collisional regime of magnetized plasma. The other remarkable result is that by increasing the external magnetic field in this case, the absorption coefficient increases strongly.

Petawatt laser absorption bounded.

The interaction of petawatt (1015 W) lasers with solid matter forms the basis for advanced scientific applications such as table-top particle accelerators, ultrafast imaging systems and laser fusion. Key metrics for these applications relate to absorption, yet conditions in this regime are so nonlinear that it is often impossible to know the fraction of absorbed light f, and even the range of f is unknown. Here using a relativistic Rankine-Hugoniot-like analysis, we show for the first time that f exhibits a theoretical maximum and minimum. These bounds constrain nonlinear absorption mechanisms across the petawatt regime, forbidding high absorption values at low laser power and low absorption values at high laser power. For applications needing to circumvent the absorption bounds, these results will accelerate a shift from solid targets, towards structured and multilayer targets, and lead the development of new materials.

Second- and third-order nonlinear optical properties of bis-chalcone derivatives

• 2,5-Bis(4-methoxy-benzylidene)-cyclopentanone (CP-POME) showed very high second harmonic generation (SHG) efficiency (5.3 times than that of urea). • All the compounds showed good third order NLO property. 2,5-Bis(cinnamylidene)-cyclopentanone (CP-CAL) exhibited good NLA activity with β coefficient of 1 × 10 −10 m/W. • The NLA observed in the present study is of two photon absorption (2PA) and optical limiting type. • The compounds can have potential applications in sensor and eye protection. Bis-chalcone derivatives: 2,5-bis(benzylidene)-cyclopentanone (CP-DBA), 2,5-bis(4-methoxy-benzylidene)-cyclopentanone (CP-POME), 2,5-bis(4-isopropyl-benzylidene)-cyclopentanone (CP-ISO) and 2,5-bis(cinnamylidene)-cyclopentanone (CP-CAL) are synthesized by Claisen–Schmidt reaction and characterized using, FT-IR, 1 H NMR, 13 C NMR, and mass spectroscopic techniques. The second- and third-order nonlinear optical properties of these samples are studied using second harmonic generation and nanosecond open-aperture Z-scan methods. CP-POME has shown a very high second harmonic generation (SHG) efficiency (5.3 times that of urea). These molecules reveal a strong third-order nonlinear absorption (NLA) that is of optical limiting type, due to their peculiar D–π–A–π–D structure that leads to high polarization of the delocalized electron cloud. CP-CAL exhibits highest NLA activity with a β coefficient of 1 × 10 −10 m/W. Through numerical simulations, the mechanism of NLA is found to be a two-photon absorption process in the case of all the samples.

Polarization Dependent Nonlinear Absorption in Au Nanowire Array

The polarization dependent nonlinear absorption was observed in ordered Au nanowire array. The linear and nonlinear polarized absorption at 780 nm is investigated by Z-scan. When the angle between the polarization direction of input laser and the long axis of Au nanowires is 90°(perpendicular), the linear absorption reaches the maximum and the nonlinear absorption reaches the minimum. The nonlinear absorption coefficient β decreases from positive to negative as the angle increase from 0° to 90°. There may be two nonlinear absorption mechanisms resulting in this phenomenon: two-photon absorption and saturable absorption in nanoscale.