Femtosecond Z-scan Technique Research Articles

The photoluminescence, ultrafast nonlinear optical properties and carrier dynamics of 1D In-doped ZnO nanostructures: Experiment and mechanism

1D In-doped ZnO (IZO) nanostructures with different morphologies, namely nanowires, nanoseries(serial), nanorods and nanoprisms, have been successfully synthesized by chemical vapour deposition method. The structural, morphology and optical properties of IZO nanowires have been characterized. The ultrafast nonlinear optical and carrier dynamics characteristics of IZO were studied using femtosecond Z-scan, time-resolved photoluminescence (TRPL) and two-color pump-probe (TCPP) technique, respectively. The TRPL showed that the luminescence decay lifetimes of about several 10 ps and nanoseconds time response scale can be attributed to the shallow trap (ST) state, and deep trap (DT) state emissions, respectively. The mechanisms of the TRPL behavior in the different emission band were analyzed. The result of TCPP experiment showed that the bleach recovery with lifetimes on the fast (100–200 ps), and slow (>1 ns) time scale, caused by the shallow donor mixed with donor bound recombination, and donor−acceptor pair recombination (DAP), respectively. The optical transition process in the IZO nanostructures has demonstrated that the energy transfer process from the host electron-hole pairs can efficiently excite the metal ions. Comparing with the results obtained from the TRPL experiment, we found the fast decay process was resulted from the exciton migration and the slow decay to the recombination process of the relaxed excitons. Our results show that IZO nanostructures are a promising candidate in further optoelectronic device applications.

Enhanced nonlinear optical properties of reduced graphene oxide decorated with silver nanoparticles

Reduced graphene oxide (rGO) decorated with silver nanoparticles (Ag NPs) is synthesized by femtosecond laser ablation in solution method. The nonlinear optical properties of both rGO and Ag NPs/rGO are measured using a femtosecond laser Z-scan technique. The results reveal that both the nonlinear absorption and nonlinear refraction in the hybrid are enhanced due to the interaction of the energy state of Ag NPs. The composite shows a saturation intensity 18.5 MW/cm2 and boosts the nonlinear refraction as large as 2~3 times of that of rGO reaching to −1.1 × 10−12 m2/W. The enhancement of the saturable absorption might be caused by the further bleaching the valence band of rGO due to the transfer of the light excited carriers from graphene to the metal state of the NPs. The slow relaxation of the excited carriers to the ground state of rGO will also cause the increase of the carrier density and thereby result in the enhancement of the nonlinear refractive index of the material.

Photoluminescence and enhanced third order nonlinear optical properties of InZnO thin films

We proposed the photoluminescence and third order nonlinear optical properties of indium doped zinc oxide (InZnO) thin films with various dopant concentrations. InZnO thin films were grown by simultaneous RF/DC magnetron sputtering technique with different DC power 2 W, 6 W, 8 W and 10 W respectively. The structural characterization and surface morphology of obtained thin films are characterized by XRD and SEM techniques. XRD results revealed that thin film quality is enhanced after doping indium in ZnO. The observed enhanced crystallite size from SEM images agreed with XRD results. In linear optical properties analysis, all ZnO thin films possess high optical transmittance in visible region. The resulted room temperature photoluminescence (RTPL) provided the deeply understanding of optical properties and defect states characteristics. Third order nonlinear optical properties of ZnO thin films were detected using femtosecond Z-scan technique. The domination of oxygen defects (Vo) from RTPL results prohibited direct photon transition in ZnO thin films. But, it leads to observed indirect two photon transition via intermediate defect levels. The self focusing to self defocusing transition when dopant concentration heavily increased also could be interesting result of indium heavily doped ZnO thin film. However, the magnitude of third order nonlinear optical absorption as well as refraction of all ZnO thin films is found to be relatively enhanced with dopant concentration.

Characterization of the third-order optical nonlinearity spectrum of barium borate glasses

Borate glasses have proven to be an important material for applications ranging from radiation dosimetry to nonlinear optics. In particular, B2O3-BaO based glasses are attractive to frequency generation since their barium metaborate phase (β-BaB2O4 or β-BBO) may be crystallized under proper heat treatment. Despite the vast literature covering their linear and second-order optical nonlinear properties, their third-order nonlinearities remain overlooked. This paper thus reports a study on the nonlinear refraction (n2) of BBO and BBS-DyEu glasses through femtosecond Z-scan technique. The results were modeled using the BGO approach, which showed that oxygen ions are playing a role in the nonlinear optical properties of the glasses studied here. In addition, the barium borate glasses containing rare-earths ions were found to exhibit larger nonlinearities, which is in agreement with previous studies.

Experimental and computational studies on second-and third-order nonlinear optical properties of a novel D-π-A type chalcone derivative: 3-(4-methoxyphenyl)-1-(4-nitrophenyl) prop-2-en-1-one

The present study reports the theoretical and experimental investigations of linear and nonlinear optical (static and dynamic) properties of 3-(4-methoxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one (MNC). The crystal structure was confirmed from powder X-ray diffraction (PXRD) analysis and evaluated the crystalline quality using high-resolution X-ray diffraction (HRXRD). Linear absorption and fluorescence spectra were recorded and the optical band gap was determined by Tauc’s relation. Third-order nonlinear optical (NLO) characteristics along with optical limiting behavior were explored using the femtosecond (fs) Z-scan technique at 800 and 900nm wavelengths (Ti: sapphire laser, 150fs, 80MHz). The experimental results are supported by theoretical calculations obtained from the density functional theory (DFT). The optimized geometry, linear optical absorption, HOMO-LUMO energy gap, molecular electrostatic potential (MEP), dipole moments and global chemical reactivity descriptors (GCRD) were computed by employing B3LYP/6-311+G(d) level of theory. The static and dynamic linear polarizability (α), first hyperpolarizability (β) and second hyperpolarizability (γ) components were calculated using time-dependent Hartree–Fock (TDHF) method. The computed first hyperpolarizability β(−2ω;ω,ω) at 1064nm wavelength was found to be 55 times greater than that of urea standard. The experimental and calculated dynamic molecular second hyperpolarizabilities γ(−3ω;ω,ω,ω) are in good accordance at 800 and 900nm wavelengths.

Key functions analysis of a novel nonlinear optical D-π-A bridge type (2E)-3-(4-Methylphenyl)-1-(3-nitrophenyl) prop-2-en-1-one chalcone: An experimental and theoretical approach

In the current work a new third-order nonlinear optical organic single crystal of (2E)-3-(4-Methylphenyl)-1-(3-nitrophenyl) prop-2-en-1-one (ML3NC) has been grown with well-defined morphology using the slow evaporation solution growth technique. X-ray diffraction technique was used to confirm the crystal system. The presence of functional groups in the molecular structure was identified by robust FT-IR and FT-Raman spectra by experimental and theoretical analysis. The ultraviolet-visible-near infrared and photoluminescence studies shows that the grown crystals possess excellent transparency window and green emission band (∼560 nm) confirms their use in green OLEDs. The third-order nonlinear and optical limiting studies have been performed using femtosecond (fs) Z-scan technique. The third-order nonlinear optical susceptibility (χ(3)), second-order hyperpolarizability (γ), nonlinear refractive index (n2) and limiting threshold values are found to be 4.03 × 10−12 esu, 14.2 × 10−32 esu, −4.33 × 10−14 cm2/W and 2.41 mJ/cm2, respectively. Furthermore, the quantum chemical studies were carried out to achieve the ground state molecular geometry and correlate with experimental results. The experimental value of absorption wavelength (λabs = 328 nm) is found to be in excellent accord with the theoretical value (λabs = 328 nm) at TD-DFT/B3LYP/6-31G* level of theory. To understand the static and dynamic NLO behavior, the polarizability (α) and second hyperpolarizability (γ) values were determined using TD-HF method. The computed second hyperpolarizability γ(-3ω; ω,ω,ω) at 800 nm wavelength was found to be 0.499 × 10−32 esu which is in good agreement with experimental value at the same wavelength. These results confirms the applied nature of title molecule in optoelectronic and nonlinear optical devices.

Construction and nonlinear optical characterization of CuO quantum dots doped Na2O–CaO–B2O3–SiO2 bulk glass

The spherical shape copper oxide (CuO) quantum dots (QDs) were successfully fabricated via copper basic calcium sodium borosilicate (Na2O–CaO–B2O3–SiO2) precursor obtained with a facile sol–gel technique. The microstructural analysis of doped QDs are systemically characterized, such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photo-electron spectroscopy. And the results reveal that the CuO QDs with the small size are well dispersed doped in sodium calcium borosilicate glass. Remarkably, the CuO glass materials exhibit the good third-order optical nonlinear susceptibility χ(3) (1.379 × 10−12 esu), which was investigated by femto-second Z-scan technique at the wavelength of 1550 nm, pulse duration of 50 fs, repetition rate of 50 MHz. The glass hybrids displayed a reverse saturable absorption and self-focusing refraction performance. And the mechanism to explain the third-order nonlinearity of CuO glass may be predominantly originated from the surface plasmon resonance effect, the quantum confinement effect and partly from the thermal effect. Besides, it is interesting that the glass hybrids have significant nonlinear absorption effects that endow the material to the potential value of the application of optical limiting device.

Femtosecond Two-Photon Absorption Spectroscopy of Poly(fluorene) Derivatives Containing Benzoselenadiazole and Benzothiadiazole.

We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.

Synthesis and femtosecond third order nonlinear optical properties of push-pull trans-A2B-corroles

Six novel push-pull trans-A2B free base corroles (1–6) were synthesized and characterized by various spectroscopic techniques. The charge-transfer ability of these trans-A2B-corroles was reflected from the broadness of the UV–visible absorption spectral bands. Corroles 1 and 2 exhibited absorption at lower wavelengths, higher quantum yield and longer singlet state lifetime as compared to 3–6. The ultrafast nonlinear optical (NLO) properties of these corroles were evaluated in visible range (at 800 nm) using the femtosecond Z-scan technique. The investigated corroles possess negative type of nonlinearity and self-defocusing behavior with the magnitude of nonlinear refractive index (n2) in the range of 0.36–2.84 × 10−11 cm2/W, while the two-photon absorption (β) coefficients were in the range of 6.5–68 cm/GW with |χ(3)| in the range of 10−9 e.s.u. The resultant dipole moments of these corroles were calculated theoretically and found to be 1.47–7.26 D depending on the nature of push-pull groups on the meso-phenyl substituents. 1–6 demonstrated excellent NLO activity due to their ‘push-pull’ design. Among the investigated corroles, 1 and 2 possess strong two-photon absorption coefficient and large values of χ(3) suggesting that the nonlinearity of corrole molecules highly depends on the electronic nature and number of meso-substituents. Corroles 1 and 2 possessing two acceptor and one donor moieties depicted superior NLO behavior as compared to corroles (3–6) with one acceptor and two donors due to strong electron withdrawing nature of pentafluorophenyl groups as well as higher singlet excited state lifetime and quantum yield.

Effect of –OH functionalization, C2 methylation, and high radiation fields on the non-linear optical response of imidazolium ionic liquids

Considering the impending applications of room temperature ionic liquids (RTILs) in various areas involving high optical and radiation fields, it is pertinent to probe the structure–property correlation of these solvents exposed to such conditions. Herein, femtosecond Z-scan technique (at high pulse repetition rate, 80 MHz) was employed to investigate the non-linear optical response of imidazolium RTILs in 3 scenarios: (1) -OH functionalization, (2) C2 methylation, and (3) influence of high radiation fields. Large negative non-linear refractive values (n 2) were observed in all the RTIL samples and have been attributed predominantly due to the thermal effects. In order to isolate and determine the contribution of electronic Kerr effect, the Z-scan experiments were also carried out at low pulse repetition rate (i.e. 500 Hz) by means of a mechanical chopper. The closed aperture transmittance profile showed the valley-peak pattern, which signifies positive non-linearity. Nonetheless, the variation in the n2 values of the RTILs follows the same trend in low pulse repetition rate as was observed in case of high pulse repetition rate. The trend in the n 2 values clearly showed the decrease in the non-linearity in the first two cases and has been attributed to the weakening of the ion-pair formation, which adversely affects the charge transfer between the ionic moieties via C2 position. However, an increase in the n 2 values was observed in case of ILs irradiated to high radiation doses. This enhancement in the non-linearity has been assigned to the formation of double bond order radiolytic products. These results clearly indicate a strong correlation between the non-linearity and the strength of cation–anion interaction amongst them. Therefore, such information about these solvents may significantly contribute to the fundamental understanding of their structure–property relationships.

Improved nonlinear optical properties of chalcogenide glasses in Ge-Sn-Se ternary system by thermal treatment

In this work, a series of chalcogenide glass ceramics were prepared by thermal treating a precursor glass in molar composition of Ge20Sn5Se75 with different duration. X-ray powder diffraction (XRD) measurements showed the precipitation of nano-crystals belonging to GeSe2 and SnSe2 phases in the heat-treated glasses, which was confirmed by Raman spectra as well. The crystallization in glass matrix caused the variation of optical properties (i.e. Optical band gap, Urbach energy) due to localized field effect of the nano-crystals, while the infrared transmittance was kept unchanged. Third-order optical nonlinear properties of the precursor glass and glass ceramic samples were investigated by femtosecond Z-scan technique at telecom wavelength of 1550 nm. The maximum nonlinear refraction (γ) of the Ge-Sn-Se glass ceramics reached 5.319 × 10−16 m2/W, which is almost one order higher than that of the precursor glass, demonstrating the significant influence of nano-crystals on optical nonlinear property of the chalcogenide glasses.

Nonlinear Optical Properties of D-π-A-π-D Type Oligomers with Different Conjugation Length

The conjugation length-dependent nonlinear optical properties of fluorenone-based linear conjugated oligomers have been investigated by experimental and theoretical methods. Infrared spectra and the steady-state absorption spectra show that the increase of conjugated unit could enhance the stretching vibration peaks of C=C and lead to a red-shift of the absorption peaks. Meanwhile, the two-photon fluorescence (TPF) intensity is gradually enhanced with the increase of excitation energy, and the TPF efficiency is obviously higher after the introduction of fluorene-ethylene units. The sum-over-states approach was used to model the two-photon absorption (TPA) cross-sections of oligomers, and the theoretical values agree well with the experimental data obtained from the femtosecond open-aperture z-scan technique. The results exhibit that the extension of conjugated system indeed plays a role in the improvement of TPA behavior of oligomers.

Third-order optical nonlinearities of chalcogenide glasses within Ge-Sn-Se ternary system at a mid-infrared window

Experimental results of third-order optical nonlinearities of chalcogenide glasses (ChGs) within Ge–Sn–Se ternary system at a mid-infrared window of 3 and 3.5 μm are presented in this study. Using femtosecond Z-scan technique, positive nonlinear refractive indexes (n2) of the ChGs were obtained at the mid-infrared wavelengths, with no nonlinear absorption observed. The n2 values at 3 μm were lower than those at 3.5 μm because of the presence of hydroxyl (–OH) group. The maximum n2 was obtained from the Ge–Sn–Se ChGs with highest Sn content, reaching 2.43 × 10−17 m2/W at 3.5 μm, near two times that of As2Se3 glass. Thus, Ge–Sn–Se ChGs can be regarded as a good candidate for photonic devices that operated at mid-infrared window.

Nonlinear properties of silver nanoparticles explored by a femtosecond Z-scan technique

In this report we present results of linear and nonlinear optical properties of colloidal material consisting of triangle silver nanoparticles in distilled water. The nonlinear optical properties of the material were investigated by a Z-scan technique using femtosecond laser pulses with tunable wavelength. Nanoparticle suspension showed distinct spectra with absorption lines, emerging due to the plasmonic properties of the silver nanoparticles. Surface plasmon resonance peak change over a wide range of wavelengths from 400 to almost 1100 nm was observed when the size of silver nanoparticles varied from 20 to 150 nm. In the samples different nonlinear effects such as saturable absorption, two photon absorption and self-focusing were observed when the femtosecond pulse intensity was changed from 1 up to 100 GW/cm2.

Optical nonlinearities and ultrafast all-optical switching of m-plane GaN in the near-infrared

We reported a systematic investigation on the three-photon absorption (3PA) spectra and wavelength dispersion of Kerr refraction of bulk m-plane GaN crystal with both polarization E⊥c and E//c by femtosecond Z-scan technique in the near-infrared region from 760 to 1030 nm. Both 3PA spectra and Kerr refraction dispersion were in good agreement with two-band models. The calculated nonlinear figure of merit and measured ultrafast nonlinear refraction dynamics via femtosecond pump-probe with phase object method revealed that m-plane GaN would be a promising candidate for ultrafast all-optical switching and autocorrelation applications at telecommunication wavelengths.

Mid-infrared optical nonlinearities of chalcogenide glasses in Ge-Sb-Se ternary system.

Optical nonlinearities of chalcogenide glasses in the Ge-Sb-Se ternary system are investigated at mid-infrared wavelengths (2000 and 2500 nm) with femtosecond Z-scan technique. Strong nonlinear refraction in the chalcogenide glasses is observed at 2000 nm, due to three-photon absorption resonance. In addition, the variation in the nonlinear refraction (γ) of the Ge-Sb-Se glasses shows two jumpoints as mean co-ordination number (MCN) approaches 2.4 or 2.7, consistent with the theoretical predication.

Understanding the two-photon absorption spectrum of PE2 platinum acetylide complex.

Herein, we report on the two-absorption cross-section spectrum of trans-Pt(PBu3)2 (C≡C-C6H4-C≡C-C6H5)2 (PE2) platinum acetylide complex employing the femtosecond wavelength-tunable Z-scan technique. The PE2 complex can be visualized as two branches containing two phenylacetylene units, each one linked by a platinum center, completely transparent in the visible region. Because of this structure, large delocalization of π-electrons allied to the strong intramolecular interaction between the branches is expected. The 2PA absorption spectrum was measured using the femtosecond wavelength-tunable Z-scan technique with low repetition rate (1 kHz), in order to obtain the 2PA spectrum without excited-state contributions. Our results reveal that PE2 in dichloromethane solution presents two 2PA allowed bands located at 570 and 710 nm, with cross section of about 320 and 45 GM, respectively. The first one is related to the strong intramolecular interaction between the molecule's branches due to the presence of platinum atom, while the second one is associated with the breaking of symmetry of the chromophore in solution due, most probably to a large twisting angle of the ligand's phenyl rings relative to the Pt core.

Enhancement of Two-Photon Absorption Cross Section in CdSe Quantum Rods

Nonlinear optical properties of semiconducting CdSe quantum rods (QRs), with three various aspect ratios, were examined in a wide wavelength range using femtosecond Z-scan technique. The two-photon absorption cross section σ2 was found to be as large as 164 000 GM at the wavelength of 750 nm: about 4 times larger than that expected for CdSe quantum dots of the same mass. On the basis of the obtained dispersion of the two-photon absorption cross section, we have selected wavelength ranges for optimal excitation of two-photon-induced emission. We have also studied the luminescence kinetics using degenerate pump–probe and time-correlated single-photon counting techniques. A strong influence of semiconducting CdSe rods morphology on their steady-state and time-resolved optical properties was found.

Spectral dependence of third-order nonlinear optical properties in InN

We report on the nonlinear optical properties of InN measured in a wide near-infrared spectral range with the femtosecond Z-scan technique. The above-bandgap nonlinear absorption in InN is found to originate from the saturation of absorption by the band-state-filling and its cross-section increases drastically near the bandgap energy. With below-bandgap excitation, the nonlinear absorption undergoes a transition from saturation absorption (SA) to reverse-SA (RSA), attributed to the competition between SA of band-tail states and two-photon-related RSA. The measured large nonlinear refractive index of the order of 10−10 cm2/W indicates InN as a potential material for all-optical switching and related applications.

Molecular Structure – Optical Property Relationships for a Series of Non-Centrosymmetric Two-photon Absorbing Push-Pull Triarylamine Molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.