Femtosecond Z-scan Technique Research Articles

Nonlinear optical studies of Bismuth-doped Titanium di-oxide colloids achieved by femtosecond Z-Scan technique

In this paper, we explored the nonlinear optical (NLO) properties of Bismuth (Bi) doped titanium dioxide (TiO2) nanoparticle (NP) colloids in ethanol, with laser pulses of duration ∼ 150 femtoseconds (fs). The Bi-doped TiO2 NPs were synthesized via the chemical route, sol-gel method. The increased photo response of TiO2 upon doping it with metals has become a burning issue to extend the applications of metal-doped TiO2 NPs as integral parts in solar cells, catalysts, phototherapy, etc. The bandgap of anatase TiO2 NPs was observed to be reduced to 2.89 eV upon doping Bi. The modified band structure of the Bi-doped TiO2 NPs demonstrates novel chemical, mechanical, and optical properties. NLO studies were conducted on Bismuth-doped TiO2 NPs submerged in ethanol employing femtosecond laser input pulses with incoming wavelengths 700, 750, 800, 850, 900, and 950 nm. It has been detected that open aperture (OA) studies performed at an input peak intensity of 100 MW/cm2 on Bi-doped TiO2 NPs in ethanol were exhibiting complex NLO behavior, i.e., reverse saturable absorption (RSA) in saturable absorption (SA) and SA in RSA with mostly effective two-photon absorption (1 + 1) coefficients. Particularly, at ∼800 nm, the behavior observed was so complex, i.e., RSA in SA in RSA in SA, with an effective 3 PA (2 + 1) co-efficient (γ) ∼6.5 × 10−24 m3/W2. The closed aperture (CA) studies at ∼37 MW/cm2 exhibited self-defocusing effects, i.e., an intensity-dependent refractive index (n2) with a negative signature.

Synthesis, structural, spectroscopic and theoretical insights into a new malononitrile derivative: A candidate for nonlinear optical applications

Functionalized alkylidene malononitriles play a crucial role as donor-acceptor molecules, exhibiting notable nonlinear optical (NLO) properties. Allylidene malononitriles, an analogue of alkylidene malononitriles, have not been explored before for their optical properties. The present work deals with the synthesis of (Z)-2-(3-(4-bromophenyl)-3-chloroallylidene) malononitrile using Knoevenagel condensation reaction of Z-3-(4-bromophenyl)-3-chloroacrylaldehyde and malononitrile. Structure of the compound was elucidated by single-crystal X-ray crystallography and related spectroscopic studies like FT-IR, NMR (1H, 13C), and UV–Vis spectroscopy, which were further supported with computational studies using Density Functional Theory (DFT) approach. The photoluminescence spectra showed that the compound exhibited an emission peak at 425 nm with an excitation wavelength of 352 nm. The single crystal study reveals that the compound belongs to a monoclinic system with space group P21/c. The thermal characteristics of the compound was analysed using TGA-DTA/DSC. Investigation of the third-order nonlinear susceptibility was done by femtosecond Z-scan technique and demonstrated the potential of phenyl-substituted allylidene malononitriles for its nonlinear optical applications. The observed positive lensing and strong reverse saturable absorption (RSA) at both visible (532 nm) and Near IR (800 nm) regions make this compound a promising candidate for optoelectronic applications.

An insight into structural, spectroscopic and nonlinear optical Application of a promising malononitrile derivative: (Z)-2-(3-chloro-3-(4-chlorophenyl)allylidene)malononitrile

Conjugated organic single crystals play a pivotal role as donor-acceptor molecules, showcasing nonlinear optical properties due to their unique molecular structure. In the present study, a novel functionalised allylidene malononitrile derivative, (Z)-2-(3-chloro-3-(4-chlorophenyl)allylidene)malononitrile, was synthesised and subjected to various spectroscopic and structural analyses for its optical and structural characterisation. This compound serves as an essential building block in developing photonic and optoelectronic materials. We synthesised the compound through a Vilsmeier-Haack reaction followed by a Knoevenagel condensation reaction and crystallised it using a slow evaporation method. In addition to SCXRD, the structural analysis of the compound was performed using NMR (1H, 13C), mass spectroscopy and FT-IR spectroscopy. The Intermolecular interactions were studied using Hirshfeld surfaces. The optical properties were analysed using UV–visible and photoluminescence spectroscopy. The third-order nonlinearity of the grown crystal was estimated using the NLO coefficients via the widely recognised femtosecond Z-scan technique, equipped with Ti:sapphire-based laser oscillator using laser pulses of 90 fs and a wavelength of 500 nm at various incident powers. It was observed that with increasing laser power, the nonlinear absorption coefficient of the sample also increased. These findings, combined with the calculation of the nonlinear refractive index, conclusively establish the potential of (Z)-2-(3-chloro-3-(4-chlorophenyl)allylidene)malononitrile for optoelectronic applications.

Preparation, structure, and nonlinear optical properties of Al-CdTe films

Metal-doped semiconductors have attracted a lot of attention due to their favorable properties that can be used in the optical devices field. In this paper, Al-CdTe (ACE) films were prepared by dual-target co-sputtering. By controlling the sputtering power of the DC target to change the content of the Al dopant, it was found that the morphology of the films was slightly changed, the band gap became larger, and the crystalline quality became better. The nonlinear coefficients were measured at 800 nm using the femtosecond (fs) Z-scan technique, and the results were that the nonlinear optical (NLO) properties of the samples were controlled by Al content, and there was a significant enhancement of the nonlinear coefficients of the ACE films. The results of this study show that ACE films have a promising application in both optoelectronic devices and photodetectors.

Study on interface engineering and chemical bonding of the ReS2@ZnO heterointerface for efficient charge transfer and nonlinear optical conversion efficiency.

Rhenium sulfide (ReS2) has emerged as a promising two-dimensional material, demonstrating broad-spectrum visible absorption properties that make it highly relevant for diverse optoelectronic applications. Manipulating and optimizing the pathway of photogenerated carriers play a pivotal role in enhancing the efficiency of charge separation and transfer in novel semiconductor composites. This study focuses on the strategic construction of a semiconductor heterostructure by synthesizing ZnO on vacancy-containing ReS2 (VRe-ReS2) through chemical bonding processes. The ingeniously engineered built-in electric field within the heterostructure effectively suppresses the recombination of photogenerated electron-hole pairs. A direct and well-established interfacial connection between VRe-ReS2 and ZnO is achieved through a robust Zn-S bond. This distinctive bond configuration leads to enhanced nonlinear optical conversion efficiency, attributed to shortened carrier migration distances and accelerated charge transfer rates. Furthermore, theoretical calculations unveil the superior chemical interactions between Re vacancies and sulfide moieties, facilitating the formation of Zn-S bonds. The photoluminescence (PL) intensity is increased by the formation of VRe-ReS2 and ZnO heterostructure and the PL quantum yield of VRe-ReS2 is improved. The intricate impact of the Zn-S bond on the nonlinear absorption behavior of the VRe-ReS2@ZnO heterostructure is systematically investigated using femtosecond Z-scan techniques. The charge transfer from ZnO to ReS2 defect levels induces a transition from saturable absorption to reverse saturable absorption in the VRe-ReS2@ZnO heterostructure. Transient absorption measurements further confirm the presence of the Zn-S bond between the interfaces, as evidenced by the prolonged relaxation time (τ3) in the VRe-ReS2@ZnO heterostructure. This study offers valuable insights into the rational construction of heterojunctions through tailored interfacial bonding and surface/interface charge transfer pathways. These endeavors facilitate the modulation of electron transfer dynamics, ultimately yielding superior nonlinear optical conversion efficiency and effective charge regulation in optoelectronic functional materials.

Strong Two and Three-Photon Absorption Coefficients of Gold-Doped Titanium Di-Oxide Nanoparticles Achieved by Femtosecond Z-Scan Technique

Strong Two and Three-Photon Absorption Coefficients of Gold-Doped Titanium Di-Oxide Nanoparticles Achieved by Femtosecond Z-Scan Technique

Nonlinear optical properties of methyl green investigated by the femtosecond Z-scan technique

The third-order optical nonlinearities of methyl green in aqueous solution are studied by the Z-scan technique using a femtosecond laser at 800 nm. The valley shape in the open aperture measurements reveals the presence of reverse saturable absorption effect, which is ascribed to the three-photon absorption process. The closed aperture Z-scan traces demonstrate a negative sign for the nonlinear refractive coefficient, indicating the self-defocusing optical nonlinearity. Furthermore, the magnitudes of the nonlinear refractive index n 2, the three-photon absorption coefficient α 3, and the real part of the third-order nonlinear optical susceptibility Re χ (3) are determined and found to be linearly proportional to the dye concentration within the investigated range. Our results suggest that methyl green may be a hopeful candidate for applications in nonlinear optics.

Influence of peripherical modifications in linear and nonlinear photophysical properties of imidazo[4,5-b]pyridine derivatives

This study reports a wide study on linear and nonlinear optical properties of six imidazo[4,5-b]pyridines derivates. The compounds are coupled with a series of peripheral groups forming different dipolar structures (D-π-A and A-π-A). Photophysical parameters such as the anisotropy coefficient, transition dipole moment, and permanent dipole moment difference were determined using spectroscopic techniques. The two-photon absorption (2PA) cross-section spectra were obtained by the tunable femtosecond Z-scan technique. In addition, quantum chemical calculations and the Sum Over States (SOS) model for two energy levels were applied to support the experimental data. A single 2PA band, with 2PA cross-section varying from 70 to 110 GM, was observed in the 2PA cross-section spectra in the molecules with only acceptor groups. In addition, the presence of a second band around 660 nm and an increase in 2PA cross-section values (around 110 to 130 GM in the principal band) was observed for the molecules with a combination of the methoxy radical (OMe) with halogen groups (D-π-A structure). These results indicate that the non-centrosymetric A-Imp-D structures facilitate the polarizability and internal charge transfer processes, resulting in photophysical and two-photon absorption properties enhancement compared to A-Imp-A structures.

Ultrafast nonlinear optical properties of Ag-CdTe thin films by co-sputtering in the near-infrared region

Metal-doped CdTe has become a heated topic in recent years due to its ultrafast nonlinear optical properties. Herein, Ag-CdTe thin films with different doping amount (1W, 3W and 5W) and pure CdTe thin films were fabricated on quartz substrates by magnetron sputtering. The effects of Ag doping on the morphology, optical band gap and nonlinear optical properties of the thin films were systematically investigated. The nonlinear optical properties were investigated by femtosecond(fs) Z-scan technique under 800 nm. The results indicated that the Ag-CdTe thin films exhibit good nonlinear optical properties in the near-infrared region due to free carrier absorption and two-photon absorption. The results showed that the dopant content and the variation of the incident laser energy can control the RSA intensity, the nonlinear absorption coefficient, and the nonlinear refractive index of Ag-CdTe thin films. These results can provide new support for their applications in the field of nonlinear optoelectronic devices.

Ultrafast dynamic processes and nonlinear optical properties of Platinum(II) fluoren-acetylides

Two N-(4-ethylnylphenyl)-9H-fluoren-9-imine (EFT)arylacetylides (1, 2) and three EFT based Platinum(II) arylacetylides (3, 4, 5) were synthesized. The ultrafast dynamic processes after excitation and the nonlinear optical properties for picosecond lasers were investigatedusing femtosecond transient absorption and Z-scan techniques. The influence of conjugated length and Pt(II) content percentage on the properties ofthese moleculeswas discussed in detail. For compounds 1 and 2, two dynamic decay processes are observed after excitationby the pump laser beams. The first process is an ultrafast singlet (first singlet excited state, S1S*) → singlet state (ground state, GS), followed by a geometric twisting charge transfer state (CTS*) of the fluoren units relative to the central arylacetylide plane. For 3, 4, and 5, three dynamic decay processes are observed. The first process is the same as those of 1 and 2. However,the geometric twisting of 3, 4, and 5 is easier than 1 and 2 due to the presence of Pt atoms in the molecular skeleton of 3, 4, and 5, which makes a fast S1S* → geometric twisting state (TS*) occurred before the charge transfer occurring, and then a subsequent geometric twisting state (TS*) to geometric twisting charge transfer state (CTS*) occurs. The nonlinear optical properties of these compounds, studied using the open aperture Z-scan technique (λ = 532 nm, pulse width 23 ps), show that the values of the title compounds are around tens of cm/GW, which are comparable to newly reported porphyrin-based MOF materials and much better than Ru(II) terpyridine complexes..The mechanism of the nonlinear optical properties is attributed to the excited state absorption based on S1S*, TS*, and CTS* states, according to the femtosecond transient absorption and Z-scan results. The increase of Pt content percentage does not benefit the nonlinear absorption of Pt-containing compounds for the picosecond pulse laser. This is because the increase of the intersystem crossing rate weakly contributes to the nonlinear optical properties.

Observation of the two-photon transition enhanced first hyperpolarizability spectra in cinnamaldehyde derivatives: A femtosecond regime study.

The application of nonlinear optical effects in optoelectronic devices is still scarce because the irradiance threshold necessary to induce a specific effect is very high. In this context, knowing the frequency-resolved first order molecular hyperpolarizability (β) is essential to identifying regions where this response is intense enough to allow for applications in commercial devices. Thus, herein, we have determined the β spectral dependence of five new push-pull cinnamylidene acetophenone derivatives using femtosecond laser-induced Hyper-Rayleigh Scattering (HRS). A considerable increase in β values was observed in molecules. We found remarkable β values in regions near the two-photon resonance, which are mediated by electron withdrawing and donating groups. This effect was mapped using wavelength-tunable femtosecond Z-scan technique. Furthermore, it was modeled in light of the sum-over-states approach for the second- and third-order nonlinearities. Finally, our outcomes suggest a strategy to obtain large β values mediated by the 2PA transition.

Zn_{1-x}Ni_xTe semiconductor nanocrystals in transparent glass for optoelectronic device applications

Doping glass with semiconductors, particularly with nanostructured semiconductors, has attracted attention due to the large optical absorption cross-sections of the latter. Based on this property, Ni^{2+} (5 wt%) doped phosphate glass and Zn_{1-x}Ni_xTe (x = 0.5, 1.0, 5.0 and 10.0 wt% of Ni^{2+}) nanocrystals (NCs) doped phosphate glasses (GCs) were prepared by fusion method and subsequent heat treatment. Influence of Ni^{2+} on structural, thermo-optical and third-order nonlinear optical properties have been analysed through various spectroscopic characterizations. The XRD pattern of the glass (G) exhibits the amorphous nature of the host material while GCs exhibit not only amorphous halo but also the presence of quantum dots (QDs) or nanocrystals (NCs) phases. TEM analysis of the studied GCs samples confirm the presence of quantum dots (QDs) and bulk NCs with an average diameter of approximately 4.2 {pm } 0.3 nm and 13.4 {pm } 0.2 nm, respectively. Several phosphate groups were observed and reported from Raman and FTIR-ATR spectra. The absorption band positions confirmed that Ni^{2+} ions resemble to the octahedral symmetry. The intensity of absorption band around 1352 nm (^3T{_1}(F) rightarrow^3A{_2}(F)) increased with the increase of Ni^{2+} in GCs which is an indicative of the ^{[6]}Ni^{2+} coordination. The emission properties such as emission cross-sections ({sigma }_{emi}) full width at half maxima (FWHM) for the ^1T{_2}(D) rightarrow^3T{_2}(F) (visible) and ^3A{_2}(F) rightarrow^3T{_1}(F) (near-infrared) emission transitions were reported. Among the glass-containing semiconductor nanocrystals (GCs), the emission cross-sections in GC4 sample (x = 10% of Ni^{2+}) are the largest for both the visible (11.88 times 10^{-18} cm^2) and infrared (0.98 times 10^{-20} cm^2) transitions. Thermal diffusivity (D), thermal conductivity (K) and temperature dependent optical path length change (ds/dT) were obtained through time-resolved thermal lens (TL) and thermal relaxation (TR) methods. The D and K parameters do not change significantly with increase of Ni^{2+} ions (0.5–5%) in GCs. Nonlinear-refractive index and nonlinear absorption of the studied samples were also obtained using femtosecond Z-scan technique. The increase of nonlinear absorption coefficient (beta) is observed from GC2 (2.53 {times } 10^{-10} cm/W) to GC4 (7.98 {times } 10^{-10} cm/W). The GC4, sample with 10 wt% of Ni^{2+}, showed the lowest ds/dT (1.22 times 10^{-6} K^{-1}) with good lasing (FOM and emission cross-sections) and nonlinear absorption properties suggesting that it can be a good candidate for visible-red emission light conversion in LED technology.

AgScP2S6 van der Waals Layered Crystal: A Material with a Unique Combination of Extreme Nonlinear Optical Properties

Research in two-dimensional layered materials (2DLMs) has exploded over the past several years for a variety of applications in photonics and optoelectronics. The 2D nature of these materials allows for a very local electronic probe of material as well as flexible integration with other functional components. Herein, using the femtosecond Z-scan technique, we report a giant two photon absorption (TPA) process and its saturation in the van der Waals gapped silver scandium thiophosphate (AgScP2S6) crystal. We have found a TPA coefficient of the order of 104 cm/GW which is orders of magnitude larger compared to many existing semiconductors and nonlinear crystals. Furthermore, we found a TPA cross-section of 103 GM and characterized the optical limiting (OL) response (0.2 mJ/cm2) and the multipulse laser damage threshold (1.09 ± 0.19 J/cm2). The combination of giant TPA, extremely low OL, and very high damage threshold suggests that this material could be extremely useful in applications like optical limiters or switches.

Linear and nonlinear absorption properties of hemicyanine dyes containing 5-(4H)-oxazolone as heterocyclic ring

A series of dyes containing electron-donating (D) and electron-accepting (A) groups connected through a π-conjugated linker were designed and their linear and nonlinear absorption properties were studied. The dyes combined 1,3-oxazol-5(4H)-one core with methoxy, methyl, phenyl, cyano, and nitro group in the para-position of a phenyl ring. Spectrally resolved two-photon absorption cross-section measurements carried out by the femtosecond Z-scan technique showed wide wavelength regions of nonlinear optical absorption. The studied compound containing the phenyl ring exhibits relatively high effective two-photon absorption cross-section equal to 772 ± 100 GM at 600 nm. The experimental optical characterization is supported by the results of electronic-structure calculations.

Saturable absorption and reverse saturable absorption of CdGa2Se4 nanoparticles determined by bond strength

The nonlinear optical (NLO) properties of selenide gallium-cadmium (CdGa2Se4) nanoparticles (NPs) are investigated by femtosecond open-aperture Z-scan technique. The results show that they exhibit saturable absorption at 400 nm while reverse saturable absorption at 800 nm, which is ascribed to single-photon absorption and two-photon absorption, respectively. The NLO response conversion can be attributed to its band gap determined by the moderate bond strength of gallium and selenium atoms. An empirical equation for band gap and proton number of CdGa2Se4's belonging class of compounds is proposed. It can be used to estimate the band gap values of this class of compounds and predict their nonlinear optical responses at different waveband.

Third-order optical nonlinearity of CsPb(Br/I)3 metal halide perovskites nano-crystals embedded chalcogenide glass.

The nonlinear optical properties of emerging metal halide perovskites (MHP) materials are sufficiently intriguing that this topic has become the hotspots in the realm of material science. Hence, we investigate the third-order nonlinear optical properties of CsPbBrx/I3-x (x = 1, 2, 3) MHP nano-crystals (NCs) embedded chalcogenide glass (ChG) within a GeS2-Sb2S3 pseudo-binary system, by monitoring the composition, excitation wavelength and intensity dependencies via femtosecond Z-scan technique. We have found that the intrinsic large optical nonlinearity of ChG can be further enhanced because of the incorporation of MHP NCs, and that the optical nonlinearity of MHP-ChG containing pristine Br NCs is more pronounced compared to its counterparts with mixed Br/I NCs, due to a combination of multiple factors.

Layer-dependent nonlinear optical properties of two-dimensional InSe and its applications in waveguide lasers.

The thickness-dependent third-order nonlinear optical properties of two-dimensional β-InSe and its potential applications as a saturable absorber in pulsed laser generation are investigated. InSe sheets with different layers are prepared by the chemical vapor deposition. Using open-aperture femtosecond Z-scan technique at 1030 nm, the modulation depth and nonlinear absorption coefficient are obtained to be 36% and -1.6 × 104 cm·GW-1, respectively. The intrinsic mechanism of the layer-dependent energy band structure evolution is analyzed based on density functional theory, and the theoretical analysis is consistent with the experimental results. Based on a waveguide cavity, a Q-switched mode-locked laser at 1 µm with a repetition frequency of 8.51 GHz and a pulse duration of 28 ps is achieved by utilizing the layered InSe as a saturable absorber. This work provides an in-depth understanding of layer-dependent properties of InSe and extends its applications in laser technology for compact light devices.

Boosting optical nonlinearities of graphene oxide films by laser direct writing

In this work, we study the boosted optical nonlinearities of graphene oxide (GO) films by laser direct writing with femtosecond laser (532 nm, 330fs) excitation. We firstly prepare the uniform GO films by vacuum filtration method, which are then partly reduced by commercial carbon dioxide laser machine. The morphology, structure and luminescence traits of both the pristine and reduced GO are further characterized by SEM, XRD, Raman spectra, XPS, PL and UV–Vis absorption spectrum. Moreover, their third-order optical nonlinearities (TONs) are measured by the femtosecond laser Z-scan technique. The experimental results show that the reduced GO films take on stronger TONs and the related degree of enhancement depends on the spot size of laser processing. In particular, when the processed diameter is around 400 μm, the optical nonlinear absorption and refraction coefficients of the reduced GO are −2.9 × 10−7 m/W and 2.5 × 10−15 m2/W, respectively, which are in turn 4 and 2 times higher than that of GO film. Our results provide powerful backing for the GO-based applications, such as ultra-fast optical devices, super-resolution imaging technology and photodetector.

Investigating the influence of ITO thin film thickness on the optical Kerr nonlinearity using ultrashort laser pulses

In this paper, we present results from the measurements of the optical Kerr nonlinearity of indium tin oxide (ITO) thin films of different thicknesses using the femtosecond (fs) Z-scan technique. ITO thin films were prepared by radio frequency magnetron sputtering on a glass substrate at room temperature. The coated ITO thin films were subsequently characterized by UV–visible absorption spectroscopy, x-ray diffraction, and scanning electron microscopy. Using ∼ 100 f s pulses at 80 MHz repetition rate, the optical Kerr nonlinearity in ITO with different thicknesses was investigated at different excitation wavelengths and incident pulse energies. The optical Kerr nonlinearity was found to be dependent on excitation wavelength, incident power, and ITO thickness, with a maximum value of ∼ 9 × 1 0 − 12 c m 2 / W at a wavelength of 820 nm, power of 1 W, and 170 nm ITO thickness. These results suggest that Kerr nonlinearity in ITO can be tailored by varying the film thickness, which would be ideal for ultrafast all-optical switching in future optoelectronic devices.

Effect of peripherical groups on the two-photon brightness of (E)-3-benzyl-6-bromo-2-styryl-3H-imidazo[4,5-b]pyridine derivatives

Here, a study of the effect of several electron-withdrawing (F, Br, CF3, Cl) and electron-donating (OMe, OTBDMS, CH3, S) on the two-photon brightness (2 PB) of nine imidazo[4,5-b]pyridine derivatives was developed. The two-photon absorption cross-section (2PACS) spectra were acquired by the tunable femtosecond Z-scan technique from 470 nm up to 700 nm, and hence the 2 PB was evaluated at the 2PACS peak (around 540 nm) and 700 nm. Transition dipole moment, the difference of state dipole moments, anisotropy coefficient, and fluorescence lifetime were figured out as a result of the Gaussian decomposition of one-photon absorption spectra, solvatochromism, fluorescence anisotropy, and Strickler Berg equation, respectively. Besides the photophysical analysis, the photophysical parameters were used as input parameters to phenomenologically model the 2PACS applying the Sum-Over-States approach regarding a three-level energy system. Results have shown a 2PACS peak of around 130 GM for all imidazo[4,5-b]pyridine derivatives, and values of 2PACS at 700 nm ranged from 4 up to 14 GM. Results show that for Br-π-electron withdrawing (Br-π-EW) molecular arrangement, higher values of 2 PB can be reached, mainly through the control of ϕ, which demonstrated to be more sensible than the 2PACS to the addition of different peripherical groups.