Open-aperture Z-scan Measurements Research Articles

Using Femtosecond Laser Pulses to Explore the Nonlinear Optical Properties of Ag/Au Alloy Nanoparticles Synthesized by Pulsed Laser Ablation in a Liquid.

Metallic nanoparticles have gained attention in technological fields, particularly photonics. The creation of silver/gold (Ag/Au) alloy NPs upon laser exposure of an assembly of these NPs was described. First, using the Nd: YAG pulsed laser ablation's second harmonic at the same average power and exposure time, Ag and Au NPs in distilled water were created individually. Next, the assembly of Ag and Au NP colloids was exposed again to the pulsed laser, and the effects were examined at different average powers and exposure times. Furthermore, Ag/Au alloy nanoparticles were synthesized with by raising the average power and exposure time. The absorption spectrum, average size, and shape of alloy NPs were obtained by using an ultraviolet-visible (UV-Vis) spectrophotometer and transmission electron microscope instrument. Ag/Au alloy NPs have been obtained in the limit of quantum dots (<10 nm). The optical band gap energies of the Ag/Au alloy colloidal solutions were assessed for different Ag/Au alloy NP concentrations and NP sizes as a function of the exposure time and average power. The experimental data showed a trend toward an increasing bandgap with decreasing nanoparticle size. The nonlinear optical characteristics of Ag/Au NPs were evaluated and measured by the Z-scan technique using high repetition rate (80 MHz), femtosecond (100 fs), and near-infrared (NIR) (750-850 nm) laser pulses. In open aperture (OA) Z-scan measurements, Ag, Au, and Ag/AuNPs present reverse saturation absorption (RSA) behavior, indicating a positive nonlinear absorption (NLA) coefficient. In the close-aperture (CA) measurements, the nonlinear refractive (NLR) indices (n2) of the Ag, Au, and Ag/Au NP samples were ascribed to the self-defocusing effect, indicating an effective negative nonlinearity for the nanoparticles. The NLA and NLR characteristics of the Ag/Au NPs colloids were found to be influenced by the incident power and excitation wavelength. The optical limiting (OL) effects of the Ag/Au alloy solution at various excitation wavelengths were studied. The OL effect of alloy NPs is greater than that of monometallic NPs. The Ag/Au bimetallic nanoparticles were found to be more suitable for optical-limiting applications.

Visible-light optical limiting of vanadia–polyvinylpyrrolidone nanofibers

In this work, vanadium pentoxide (V2O5) nanoparticles-filled electrospun polyvinylpyrrolidone (PVP) nanofibers were investigated systematically at various nanofiller weight percentages (8 and 10 wt%) and input intensities to reveal the effective optical limiting feature in the visible spectrum. XRD analysis demonstrated the purity of the produced V2O5 nanoparticles. According to SEM findings, V2O5 nanoparticles were effectively integrated into the PVP nanofibers. Two distinct absorption bands were observed at around 400 and 217 nm. These bands were related to PVP and V2O5 nanoparticles in linear absorption measurements, respectively. Moreover, an increased Urbach energy value was obtained with an increase in V2O5 nanofiller content within PVP. Open-aperture Z-scan measurements were taken at 532 nm considering the band gap energy of the V2O5 nanofillers in PVP composite nanofibers. In 8 wt% V2O5 nanofilled PVP nanofibers, one-photon absorption (OPA) was the main nonlinear absorption (NA) mechanism, and the defect states of the V2O5 nanoparticles had no contribution to NA. On the other hand, sequential two-photon absorption was the main NA mechanism, and the defect states of the nanoparticles caused more efficient NA behavior in 10 wt% V2O5 nanofilled PVP nanofibers. The effective optical limiting behavior was obtained for this composite nanofiber with lower limiting threshold as 1.49 × 10–5 J/cm2. The V2O5 nanofilled PVP nanofibers presented strong potential optical limiters in the visible wavelength region. This was attributed to their high linear transmittance at low input intensities and their robust NA behavior at higher input intensities.

Three-photon and four-photon absorption in lithium niobate measured by the Z-scan technique

Open-aperture Z-scan measurements have been carried out to investigate the three-photon (3 PA) and four-photon absorption (4 PA) coefficients at 800 nm and 1030 nm wavelengths, respectively in congruent and stoichiometric lithium niobate (cLN, sLN) with different concentrations of Mg doping. The laser pulse duration at the two wavelengths were 40 and 190 fs. The peak intensity inside the crystals varied between approximately 110 and 550 GW/cm2. The 3 PA and 4 PA coefficients were evaluated using a theoretical model and the results suggest that their minima are at or around the Mg doping level corresponding to the threshold for suppressing photo-refraction for both cLN and sLN. This result can be attributed to the contribution of crystal defects to the 3 PA and 4 PA processes. Furthermore, the 4 PA at 1030 nm exhibited greater nonlinear absorption than the 3 PA at 800 nm under the same intensity level. Possible reasons for this unexpected behavior are discussed. Overall, comparing the 3 PA and 4 PA values of these crystals will enable for selection of the optimum composition of LN crystal for efficient THz generation and for other nonlinear optical processes requiring high pump intensities.

O,S-Chelated bis(pentafluorophenyl)boron and diphenylboron-β-thioketonates: synthesis, photophysical, electrochemical and NLO properties.

Boron-β-diketonates are classical emissive materials that have been utilized in various fields, however, boron monothio-β-thioketonates, where one oxygen atom is exchanged for a sulphur atom, have not been explored in detail. To gain a better understanding of this class of materials, we synthesised various aryl substituted monothio-β-diketonate boron complexes with two different aryl substitutions on the boron center and studied their structural, optical and electrochemical properties. Single crystal X-ray analysis revealed that there is considerable deviation in B-O and B-S bond lengths for bis(pentafluorophenyl)boron complexes against diphenyl boron complexes. The bis(pentafluorophenyl)boron complexes have a relatively high absorption coefficient over diphenyl boron complexes. More importantly, a striking difference was observed for the emission behaviour of these compounds. The bis(pentafluorophenyl)boron complexes exhibit weak emission in the solution as well as in the solid state, whereas diphenyl boron complexes do not show any emission in either solution or the solid state. Further, the electrochemical study reveals that diphenyl boron complexes show a reduction potential that is more negative compared to the bis(pentafluorophenyl)boron complexes. The high absorption coefficient of the compounds pointed towards the possibility of high first order hyperpolarizability upon optical excitation, which motivated us to ascertain the nonlinear optical coefficients in the near infrared range, towards applicability of such compounds in optical limiting and switching. The open aperture Z-scan measurements at ultrashort time scales elucidated a few critical features of such compounds towards optical limiting applications.

Ultrafast laser pulse repetition rate dependent switching of nonlinearity in water

Tailoring the nonlinear optical properties of materials through extrinsic parameter controls is advantageous for many applications. In this article, ultrafast laser pulse repetition rate induced switching of reverse saturable absorption to saturable absorption of water is reported. For the investigations, a conventional open aperture z-scan measurement system equipped with a variable repetition rate ultrafast laser source operating at the wavelength of 1030 nm was employed. A combination of weak saturable absorption and strong reverse saturable absorption induced by three-photon absorption is observed at low and moderate repetition rates. At repetition rates above 5 kHz, there is a significant variation in nonlinear absorption, where the reverse saturable absorption starts to diminish and saturable absorption increases. For repetition rates beyond 100 kHz, the nonlinearity gets reversed, where water tends to behave more like a saturable absorber with a weak three-photon absorption component.

Broadly tunable continuous wave 2.3-µm Tm3+:tellurite bulk glass laser.

We report, for the first time to the best of our knowledge, a continuous wave trivalent thulium ion (Tm3+)-doped bulk glass near 2.3 µm. In the experiments, a bulk Tm3+-doped tellurite glass with the stoichiometric composition of 74TeO2-12ZnO-4La2O3-10Na2O (Tm3+:TZLN) was used. Lasing operation was achieved by using an x-fold cavity at the free-running wavelength of 2303 nm. The maximum slope efficiency of 6.2% was obtained with respect to the absorbed pump power with a 1% transmitting output coupler. In this case, as high as 100-mW output power was generated with 2.2 W of absorbed pump power. Continuous, broad tuning was achieved from 2233 nm to 2400 nm. The excitation spectrum of the laser was also investigated and 2.3-µm lasing was obtained by varying the pump wavelength over the 773-809-nm range. The absorption cross section was determined to be 4.4 × 10-21 cm2, based on open-aperture z-scan measurement. By using the laser efficiency data, the emission cross section of the Tm3+:TZLN glass was further determined to be 1.3 × 10-20 cm2 at 2.3 µm.

Two-Photon Absorption Response of Functionalized BODIPY Dyes in Near-IR Region by Tuning Conjugation Length and Meso-Substituents.

BODIPY dyes substituted by phenol or -COOMe units at the meso-position (C8) with and without a distyryl group including a methoxy moiety at the -C3 and -C5 positions of the BODIPY have been synthesized to analyze the photophysical properties. To clarify the ground-state interaction, absorption and emission features were investigated in the THF environment. Extending the π-conjugation with the methoxy moiety at -C3 and -C5 positions of BODIPY leads to a spectral shifting of the absorption maxima toward red by 120 nm. In addition, attaching the -COOMe unit at the meso-position of the BODIPY structure increases nonradiative molecular relaxation as compared to compounds possessing phenol substituents at the same position. We have investigated the effect of phenol and a -COOMe group and π-extended conjugation length with a methoxy moiety on the properties of two-photon absorption (TPA) and electron transfer dynamics by performing open-aperture (OA) Z-scan and femtosecond transient absorption spectroscopy measurements, respectively. The synthesized BODIPY compounds with the distyryl group including the methoxy unit show TPA character due to the longer conjugation length and therefore intramolecular charge transfer ability. Based on the OA Z-scan experiments upon photoexcitation with 800 nm pulsed laser light, TPA cross-section values were obtained as 74 and 81 GM for the compounds possessing phenol and -COOMe units at the meso-position of BODIPY treated by distyryl group with methoxy moieties, respectively. Additionally, optical and electronic properties were calculated theoretically by using the DFT method.

Red-Emitting Tetraphenylethylene-Based Boron Thioketonates: Effect of Substitution and Study of Nonlinear Optical Properties

We present tetraphenylethylene (TPE) and N,N-dimethylaniline (DMA)-substituted monothio-β-diketonate boron compounds. All O,S-chelated boron compounds showed red emission in solution as well as in the solid state. A comparison of O,O-chelated vs O,S-chelated compound reveals that the latter showed a pronounced red shift in both absorption and emission. Furthermore, the O,S-chelated compound showed the reduction potential at a less-negative position over the O,O-chelated compound. The marked red shift in O,S-chelated compounds indicates that there is a discernible enhancement in linear polarizability upon optical excitation, and consequently, we explore the nonlinear optical properties of the compounds at infrared wavelengths. The open-aperture Z-scan measurements using ultrashort pulses reveal the optical-limiting behavior of these compounds along with a greater nonlinear absorption for those variants, which exhibits a greater red shift in absorption and emission spectrum.

A Cut-to-Link Strategy for Cubane-Based Heterometallic Sulfide Clusters with Giant Third-Order Nonlinear Optical Response

Although the synthesis of low-dimensional metal sulfides by assembling cluster-based units is expected to promote the development of optical materials and models of enzyme active centers such as dinitrogenase, it is faced with limited assembly methodology. Herein we present a cut-to-link strategy to generate high-nuclearity assemblies, inspired by the formation of a Z-type dimer of the W-S-Cu analogues of PN cluster through in situ release of active linkers. Four new compounds with structures based on the same {Tp*WS3Cu3} incomplete cubane-like units were obtained using varied combinations of mild reagents. Open-aperture Z-scan measurements demonstrated the highest-nuclearity complex has the largest nonlinear optical absorption coefficient among discrete cluster-based materials reported to date. This approach enables building high-nuclearity metal sulfide clusters through cluster-based building blocks and opens a way to the design and exploration of materials based on well-identified building blocks.

Nonlinear optical absorption and two-photon luminescent properties of FAPbBr3 nanocrystals

Typical lead-based halide perovskite has been extensively explored as a light-harvesting and emission material for solar cell and light-emitting diodes in the past ten years. But it remains challenging to improve the optoelectronic characteristics and clarify the photophysical mechanism with the interaction of laser. Herein, the nonlinear optical absorption and two-photon luminescent properties of FAPbBr3 (FA=CH(NH2)2+) nanocrystals (NCs) are investigated by means of open-aperture Z-scan, steady-state/time-resolved photoluminescence (PL), and transient absorption (TA) techniques. When the laser excitation wavelength varies from 475 to 532 nm in open-aperture Z-scan measurement, strong saturated absorption can be found and its saturation intensity increases. Compared with normal luminescence, the PL emission peak has a red shift of 6 nm and increased PL lifetime under 800 nm femtosecond laser excitation, and the linewidth changes smaller considerably from 114 to 94 meV. This difference is mainly attributed to the different relaxation pathways. The TA spectra show the PL peak positions and full width at half maximum different features under 400 or 800 nm femtosecond excitation on the time scale. Due to different selection rules, the 400 or 800 nm femtosecond excitations can induce electrons populated different states and then relax to the same lowest excited state. Hence, we can estimate that different states induce the difference in steady-state PL, TRPL, and TA decays. These results may contribute to the application of perovskite NCs for multifunctional photonic sources and nanolaser devices under strong laser field.

Investigation of Two Photon Absorption of Ligand-Modified CsPbBr3 Quantum Dots.

The characteristics and application of nonlinear absorption from CsPbBr3 QDs film with the ligand-modified strategy have been investigated in this work. By means of a near-infrared fs Ti:sapphire laser as a light source, the up-conversion emission of CsPbBr3 QDs film of around 518 nm revealed a quadratic increase with the pump intensity. Through the temperature-dependent up-conversion emission, we obtained the binding energy and longitudinal optical (LO) phonon energy of CsPbBr3 QDs film of around 58.1 and 61.2 meV, respectively. Due to more active thermal coupling between the excited electron or hot phonon effect, the photon decay trace under two-photon excitation was prolonged at higher temperatures. The ligand-modified CsPbBr3 QDs film exhibits a relatively large TPA coefficient of around 28.6 cm/GW by the open aperture Z-scan measurement, and it has been demonstrated as a promising nonlinear medium to obtain the pulsewidth of ultrafast lasers.

Third-order nonlinear optical manifestations in an intramolecular proton transfer fluorophore due to Tamm-plasmon based broadband optical absorbers

The imidazole ring has proven to be a very promising candidate for realizing molecular systems that facilitate efficient generation of broadband optical radiation using an excited-state intramolecular proton-transfer (ESIPT) mechanism. To obtain a targeted RGB-color composition, the conventional route is to create optimum mixtures of dyads and triads of non-interacting ESIPT molecules that cover the entire visible spectral band as well as restrict any energy transfer between wide bandgap donors and narrow bandgap acceptors. However, there is a need for alternate routes that can facilitate dynamic control over color composition during operation. The nonlinear optical (NLO) response of ESIPT fluorophores and configurations derived out of them could be explored from this perspective. In the present work, we design a T i O 2 / S i O 2 based distributed-Bragg-reflector (DBR) configuration with a terminating layer consisting of a film of an ESIPT fluorophore, namely, 2-(4,5-diphenyl-1-(4-(trifluoromethyl)phenyl)-1H-imidazol-2yl)phenol (DTIP). In the presence of thin Ag-film, the DTIP-DBR multilayer geometry supports a Tamm-plasmon-polariton (TPP) mode within the photonic bandgap of DBR, which exhibits an unusual spectral bandwidth greater than 50 nm. The open-aperture and closed-aperture Z-scan measurements reveal the strong TPP based broadband absorption that leads to reversal of third-order NLO characteristics, i.e., the multilayer geometry exhibits saturation absorption behavior at wavelengths where the bare DTIP fluorophore (in solution and film) shows a two-photon absorption signature.

Interfacial charge transfers and carrier regulation characteristics of narrow/wide band gap TMDs@Ga2O3 n-n heterojunction film

The application of transition metal dichalcogenides (TMDs) in many optical devices has been seriously limited by its narrow band-gap and high electron-hole recombination rate. Therefore, more studied are desired for exploring the novel TMDs/Ga2O3 heterostructures with excellent photoexcitation properties and interfacial dynamics. Herein, the WSe2/Ga2O3 and MoS2/Ga2O3 heterostructures were successfully prepared by a two-step radio frequency magnetron sputtering method. The growth mechanism and interfacial charge transfer of the wide and narrow TMDs/Ga2O3 heterostructures are considered. The X-ray diffraction results show that the (002) orientation of the WSe2 and MoS2 was suppressed, which can be attributed to interfacial interaction. Compared to pure TMDs, the A1 g was blue-shifted in the Raman spectrum for the proposed heterostructures, which is related to the decrease of the electron density. The photoluminescence emission of TMDs/Ga2O3 heterostructures was enhanced mainly due to the transition from trions recombination to exciton recombination. Meanwhile, the transmission properties of electrons under the built-in electric field are further confirmed by the transient absorption spectroscopy. Effective charge separation not only reduces radiation emission rate but also significantly enhances the nonlinear optical response of TMDs. An open-aperture Z-scan measurement was carried out to confirm that Ga2O3 is a promising candidate for enhancing the saturable absorption or reverse saturable absorption of TMDs.

A comparative analysis of structural, optical, and electrical characteristics of c-plane and a-plane ZnO:Al thin films fabricated by a pulsed laser ablation technique

The effective growth of the non-polar a-plane ZnO:Al thin films with a large grain size using the pulsed laser ablation technique under controlled conditions (in an oxygen-specific environment) over the easily grown polar c-plane films (under ambient conditions) function well as heterojunction diodes possessing excellent rectification behaviour. The absence of a built-in electric field has resulted in enhanced electrical transport properties making the a-plane films better than the c-plane films. Both the thin films, homogeneous and highly oriented along the respective crystal planes as observed in the structural studies and thickness maps, further exhibit defect mediated variations in the transmittance spectra. The values of Drude damping and the imaginary part of the dielectric constant suggest a diminished optical loss in the a-plane ZnO:Al thin film, whereas the c-plane film showed a greater optical loss. An enhanced near-band edge emission in the non-polar a-plane film is inferred from photoluminescence. The open-aperture z-scan measurements indicate an exceptional transition from saturable absorption to reverse saturable absorption in both the a-plane and c-plane oriented films. The closed-aperture measurements exhibit a rare phenomenon of self-focusing mechanism in both the thin films. The detailed structural, optical and electrical studies reveal that the a-plane ZnO:Al thin films are an alternative to the c-plane ZnO:Al thin films for nonlinear and optoelectronic applications.

Nonlinear optical properties and passively Q-switched laser application of a layered molybdenum carbide at 639 nm.

Molybdenum carbide (Mo2C) exhibits enormous potential applications in various optoelectronic and photonic fields due to its remarkably electrical and optical characteristics. Here, we fabricate a high-quality Mo2C film by the radio frequency magnetron sputtering deposition method. The nonlinear optical response and ultrafast dynamics are thoroughly studied based on open-aperture Z-scan and nondegenerate pump-probe experimental measurements. The open-aperture Z-scan experimental result exhibits a modulation depth of 8.5% and a saturation fluence of 0.28 mJ/cm2. Simultaneously, the relaxation time constant is fitted by a biexponential decay function, showing an ultrafast intraband carrier recovery time of 0.58 ps at 530 nm. Consequently, by employing the Mo2C film as a saturable absorber (SA), stable Q-switched Pr:YLF laser pulses with the shortest pulse width of 160 ns are generated at 639 nm. Our experimental results demonstrate excellent nonlinear optical properties of the layered Mo2C in the visible region and will further advance their potential applications in visible nonlinear optics.

Atomically thin gallium telluride nanosheets: A new 2D material for efficient broadband nonlinear optical devices

Here, we experimentally demonstrate the nonlinear optical properties of atomically thin two-dimensional layered gallium telluride (GaTe). We performed open aperture and close aperture Z-scan measurements in the femtosecond regime for the spectral range of 520–700 nm to study the broadband nonlinear absorption and nonlinear refraction. Interestingly, exfoliated GaTe displays strong saturable absorption and high negative Kerr nonlinearity in this spectral range. We observed a high nonlinear refraction coefficient, n2I, of −(7.61±0.07)×10−1 cm2/GW with near band edge excitation at 700 nm. Additionally, it shows a significant nonlinear absorption coefficient, β, of −(18.02±0.20)×104 cm/GW at an intensity of 40 GW/cm2 with high modulation depth and low saturation intensity. Transient absorption spectroscopy measurement is also performed to investigate the carrier dynamics of exfoliated GaTe and to identify the physical mechanisms responsible for the optical nonlinearity, such as Pauli blocking. The measured nonlinear optical data of the exfoliated GaTe presented in this Letter will pave the way for potential application in ultrafast photonic devices like optical switching, passive Q-switching, and mode-locking, due to high Kerr nonlinearity and saturable absorption in the femtosecond regime.

Third- and fifth-order optical nonlinearities of norbixin

In this work, we investigated the nonlinear response of norbixin (C24H28O4) in acetone. The third- and fifth-order optical nonlinearities were measured using the conventional Z-scan technique, with 357 fs laser pulses at 1040 nm and low-repetition rate (1 kHz). It is shown that, at intensities lower than 99GW/cm2, the nonlinear refraction process was determined by the only third-order process. However, at higher intensities, the third- and fifth-order refractive nonlinearities coexist with opposite signs: n2 = (4.43 ± 0.44) × 10−16 cm2/W and n4 = (−4.65 ± 0.46) × 10−27 cm4/W2. On the other hand, the open-aperture Z-scan measurements show an effective three-photon absorption (3PA) with 9.19±0.92×10-23 cm4/W2. The physical mechanisms of the observed nonlinearity are also discussed.

Saturation and reverse saturation of nonlinear absorption in laser ablated gold nanoparticles

The optoelectronic application of gold nanoparticles is majorly influenced by certain unique features they exhibit, like surface plasmon resonance, ready functionalization and ease of conjugation with bio-analytes etc. Production of gold nanoparticles through laser ablation is the simplest method with the advantage of obtaining high purity particles. In this work, the saturation and reverse saturation behaviour during ultrafast absorption in laser ablated gold nanoparticles is studied using the Z-scan technique. The Z-scan experiment is conducted using 800 nm, 84 MHz and 120 fs intense laser pulses at different excitation intensities from 6.23 GW/cm2 to 23.37 GW/cm2. The open aperture Z-scan measurements show distinct positive region till an excitation of 17 GW/cm2 after which there is a clear cross-over into negative trend. The strength of this negative wing increases with the excitation intensity, which is featured by the nonlinear absorption due to the two-photon and three-photon absorption contributions with their respective coefficients in the range of 10-3. The optical limiting nature of these gold nanoparticles is demonstrated. An understanding of these nonlinear optical properties provides better insight in their application towards biomedical diagnostics and optical switching and limiting.

Investigation of third order nonlinearity of Ethidium bromide doped deoxyribonucleic acid) DNA)

The concentrations-dependent refractive index n 2 and the nonlinear absorption coefficient β of Ethidium bromide dye-doped deoxyribonucleic acid (biological polymer DNA) solutions in the SDL regime at 532 nm are reported. The Z-scan technique was performed in two ways and two different wavelengths, 532nm and 473nm, the open aperture technique and the closed aperture technique. From open aperture Z-scan measurements it is found that the Ethidium bromide doped deoxyribonucleic acid films exhibited reverse saturable absorption. The coefficient of nonlinear refraction and nonlinear absorption coefficient at 473nm wavelength is greater than at 532nm.

Third-order nonlinear optical responses of CuO nanosheets for ultrafast pulse generation

As a representative transition metal oxide, cupric oxide (CuO) has attracted a lot of interest in miscellaneous fields. In the present work, the prominent third-order nonlinear optical (NLO) features of CuO nanosheets were demonstrated via the typical Z-scan technique at 1 μm. The open-aperture (OA) Z-scan measurements at different intensities clearly showed that CuO nanosheets possessed the saturable absorption (SA), reverse saturable absorption (RSA) and optical limiting behaviors. The closed-aperture (CA) Z-scan technology was implanted to investigate the nonlinear refractive index and the third-order nonlinear susceptibility. In virtue of the strong nonlinear optical absorption properties of CuO nanosheets, a stable all-solid-state mode-locked laser with the as-prepared CuO nanosheets as saturable absorber at 1.06 μm was realized for the first time. Our work confirmed that CuO nanosheets exhibited outstanding NLO properties, which might stimulate the development in ultrafast photonics and nonlinear optics.