Femtosecond Open-aperture Z-scan Research Articles

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.

Ultrafast Nonlinear Optical Response and Carrier Dynamics in Layered Gallium Sulfide (GaS) Single-Crystalline Thin Films

Gallium sulfide (GaS) is a layered metal monochalcogenide semiconductor that has recently garnered considerable attention in various fields. In this study, we investigated the nonlinear absorption characteristics of multilayer β-GaS thin films on sapphire substrate by using femtosecond open-aperture Z-scan method. The β-GaS films exhibit saturable absorption behavior at 532 nm while nonlinear absorption appears under 650 nm excitation. The nonlinear absorption coefficient of β-GaS was determined to be −1.8 × 10–8 m/W and 4.9 × 10–8 m/W at 532 and 650 nm, respectively. The carrier dynamics of β-GaS films was studied via femtosecond transient absorption (TA) measurements. The TA results demonstrated that β-GaS films have broad photo-induced absorption in the visible regime and sub-nanosecond lifetime. Our results indicate that gallium sulfide has large nonlinear optical response and long carrier lifetime, which could be applied in future photonic devices.

Colloidal Au–CsPbBr3 for nonlinear saturable absorption effects

We demonstrate the nonlinear saturable absorption effects of colloidal Au–CsPbBr3 inorganic nanocrystals (INCs) that may useful as a saturable absorber in passively mode-locked pulse lasers. Femtosecond open-aperture Z-scan technique is responsible for measuring the nonlinear saturable absorption effects at 800 nm with a repetition rate of 76 MHz and a laser pulse duration of 130 fs. Compared to colloidal pure-CsPbBr3 INCs, the nonlinear saturable absorption coefficient β ~ 2.34 × 10–10 cm/W and the second-order absorption cross section σ2 ~ 2.13 × 104 GM of colloidal Au–CsPbBr3 INCs can be improved by up to one order of magnitude at the Au-doping concentration of 3.5 mmol/L, thanks to an obviously cocked tail absorption from visible to near-infrared spectral region through the localized surface plasmon-induced charge transition at Au–CsPbBr3 INCs interface rather than the light scattering of the deposited Au nanoparticles. And the deposited Au nanoparticles have not changed the crystalline structure, stability, and morphology of colloidal pure-CsPbBr3 INCs. As a result, colloidal Au–CsPbBr3 INCs are expected to become a potential saturable absorption candidate material used in passively mode-locked pulse lasers.

Ultrafast broadband nonlinear optical properties and excited-state dynamics of two bis-chalcone derivatives.

The development of organic nonlinear optical (NLO) chromophores is vital for various fields such as two-photon biomedical imaging, optical limiting, etc. In this work, two bis-chalcone molecules 1,4-bis[3-(2,4-dimethoxyphenyl)-2-acryloyl]benzene (C1) and 4,4′-bis[3-(2,4-bimethoxy phenyl)-2-acryloyl]biphenyl (C2) were synthesized and characterized. The excited-state dynamics of these two chromophores were studied using femtosecond transient absorption (TA) measurements. And their broadband nonlinear absorption properties and optical limiting (OL) response were investigated by femtosecond open-aperture Z-scan and intensity-dependent transmittance measurements in the wavelength range from 515 nm to 800 nm, respectively. The TA results demonstrate that C2 has strong excited-state absorption behavior and longer lifetime. In addition, the nonlinear absorption response of C2 was found to be superior to that of C1 in the visible range after 500 nm, which is attributed to a two-photon-absorption induced excited-state absorption mechanism. These results indicate that the nonlinear optical response and excited-state dynamics in bis-chalcone compounds could be enhanced via intramolecular charge-transfer.

Impact of the Subunit Arrangement on the Nonlinear Absorption Properties of Organometallic Complexes with Ruthenium(II) σ-Acetylide and Benzothiadiazole as Building Units §

In this paper, the nonlinear absorption properties of two complexes consisting of Ru(C≡CPh)(C≡C)(dppe)2 (dppe = Ph2PCH2CH2PPh2) as electron donor (D) and 4,7-di(2-thienyl)benzo[c][1,2,5]thiadiazole as electron acceptor (A) units in two different arrangement, i.e., A–D–A and D–A–D, are presented. They were measured in solution by the femtosecond open-aperture Z-scan method. The complexes show moderate two-photon absorption cross-sections σ(2) of several hundred to one thousand GM (here 1 GM = 10−50 cm4 s molecule−1 photon−1). Although they are formed by the same building units, it was found that the two-photon absorption values of the D–A–D arrangement are six times higher than that of the A–D–A one. This difference can be explained by the number of metal cores (one or two ruthenium centers), the geometrical configurations of the complexes (more or less planar), and the resonance enhancement by lowering the intermediate state.

Two-photon absorption cross-section results of three tri-branched derivatives: A comparison between open-aperture Z-scan and two-photon excited fluorescence method

Femtosecond open-aperture Z-scan and two-photon excited fluorescence method, were performed to study the two-photon absorption cross-section of several tri-branched derivatives. The two-photon absorption cross-section values measured by the two different methods were compared. Our results indicate that, the absolute values of the two-photon absorption cross-section measured by different technique are obviously different. The absorption cross-sections measured by two-photon excited fluorescence method exhibited 2.1–2.3 times larger than that measured by open-aperture Z-scan method. And the difference is most likely caused by the system error of different measurements. Our results may provide scientific responsible reference as well as some useful information for the comparison of the two-photon absorption cross-section values measured by different methods.

Two-photon absorption and upconversion luminescence of colloidal CsPbX3 quantum dots

The nonlinear optical and the upconversion luminescence (UCL) properties of CsPbX3 (X = Br or its binary mixtures with Cl, I) quantum dots (QDs) are investigated by femtosecond open-aperture (OA) Z-scan and time-resolved luminescence techniques in nonresonant spectral region. The OA Z-scan results show that CsPbX3 QDs have strong reverse saturable absorption (RSA), which is ascribed to two-photon absorption. Partially changing halide composition from Cl to Br, to I, two-photon absorption cross sections become larger at the same laser excitation intensity. The composition-tunable nonlinear absorption should be attributed to the gradual decrease of the lowest direct band gaps with the halide substitute. Moreover, the strong UCL can be observed under near infrared femtosecond laser excitation. Halide composition-tunable UCL dynamics of CsPbX3 QDs is analyzed by use of two-exponential fitting with deconvolution. When CsPbX3 QDs have similar sizes (10–13 nm), with partially changing halide composition from Cl to Br, to I, the average UCL lifetime becomes longer due to the variation of Kane energy. Our findings suggest all-inorganic perovskite QDs can be used as excellent gain medium for high-performance frequency-upconversion lasers and provide reference to engineer such QDs toward practical optoelectronic applications.

Ultraviolet saturable absorption and ultrafast carrier dynamics in ultrasmall black phosphorus quantum dots.

Understanding the photoexcited carrier-relaxation actions in ultrasmall black phosphorus quantum dots (BPQDs) will play a crucial role in the fields of electronics and optoelectronics. Herein, we report the ultraviolet (UV) saturable absorption and ultrafast photoexcited carrier-relaxation dynamics of BPQDs. The ultrasmall BPQDs are synthesized using a facile liquid-exfoliation method and possess a diameter of 3.8 ± 0.6 nm and a thickness of 1.5 ± 0.4 nm. Femtosecond open-aperture (OA) Z-scan measurements showed typical saturable absorption properties in the UV band. A negative nonlinear optical (NLO) absorption coefficient of -(1.4 ± 0.3) × 10-3 cm GW-1 and a saturable intensity of 6.6 ± 1.3 GW cm-2 were determined. Using a degenerate pump-probe technique, an ultrafast photoexcited carrier-recombination time was observed in the range of 216-305 fs, which was 3 orders of magnitude faster than that of BP nanosheets. Such an ultrafast relaxation component may be attributable to the edge- and step-mediated recombination and was confirmed by our density functional theory (DFT) calculations. This work provides fundamental insight into the underlying mechanism of the photoexcited carrier relaxation dynamic action in BPQDs which can enable UV photonic devices.

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.

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.

Polarization-induced fluorescence modulation in a self-assembled coordination cage-shaped complex

Polarization-induced fluorescence modulation behavior of a self-assembled coordination cage-shaped complex was investigated using femtosecond laser pulses. The variations of the total two-photon-induced fluorescence (TPF) intensity were found to be strongly modulated by different polarized incident lights and tightly dependent on the linearly polarized component of the excited light. The polarization-induced modulation efficiency of the TPF underwent intensity-dependent decrease, which could be attributed to the two-photon-induced excited-state absorption. The nonlinear absorption behavior of the complex was also studied by performing both femtosecond open aperture Z-scan and nonlinear transmission measurements, which help to better understand the intrinsic optical properties of the molecule and portend its practical applications.

Study of the Reverse Saturate Absorption in Copper Phthalocyanine Solution

The nonlinear absorption (NLA) properties of copper phthalocyanine (CuPc) acetone solution were investigated by femtosecond open-aperture (OA) Z-scan technique. The reverse saturable absorption (RSA) was observed, and it was found that the RSA effect was origin from three-photon absorption. Moreover, the influences of concentration and excitation laser power on the NLA of CuPc solution are studied. Results show that the RSA effect increases with the increase of sample concentration and excitation laser power. It indicates that the CuPc could be a promising candidate for optical limiting material.

Three-photon absorption of copper phthalocyanine solution by femtosecond Z-scan technique

The nonlinear absorption (NLA) properties of copper phthalocyanine (CuPc) solution in acetone were investigated by the femtosecond open-aperture (OA) Z-scan technique. The reverse saturable absorption (RSA) phenomenon was observed. And the physical mechanism of RSA effect was revealed, which was originated from three-photon absorption. Moreover, the influences of concentration and excitation laser power on the NLA of CuPc solution were studied respectively. Results showed that the RSA effect increased with the increase of sample concentration and excitation laser power. It indicated that the CuPc could be a promising candidate for optical limiting material.

Crystal structure, nonlinear optical and photophysical properties of a novel chromophore constructed with terpyridine, triphenylamine and ethyl cyanocaetate functional moieties

A novel chromophore (Z)-ethyl-3-(4-((4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl) (phenyl) amino)phenyl)-2-cyanoacrylate(3), constructed with triphenylamine moiety as the electron donor (D), 2, 2: 6, 2-terpyridine moiety as an electron acceptor (A), and ethyl cyanocaetate group as an auxiliary electron acceptor (A′), has been designed and synthesized. The crystal structures of 3 and its mediator 2 (4″-(4′-(4-(Diphenylamino) phenyl) aldehyde)-2,2′:6′,2″-Terpyridine), have been determined by single crystal X-ray diffraction analysis. The linear and nonlinear spectra of these chromophores were investigated on the basis of experimental and calculation methods. The two-photon absorption (TPA) cross-sections of 1–3 were determined by a femtosecond open-aperture Z-scan technique. The maximum value of the TPA cross-section (σ) for 3 is 7938.3 GM in DMF solution. However, much weaker two-photon absorption responses were observed at the same condition for chromophore 1 and 2, respectively. The results of the work indicate that, the photophysical properties of the D–A configuration group are influenced largely by the auxiliary moiety (A′) attached.

Two-Photon Absorption Properties of Two-Dimensional π-Conjugated Chromophores: Combined Experimental and Theoretical Study

The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.

Analysis on the origin of the ultrafast optical nonlinearity of carbon disulfide around 800 nm

The femtosecond open-aperture Z-scan of standard sample CS(2) at 800 and 780 nm is present here by using thinner sample length and an integrating sphere. The open-aperture Z-scan signal is verified to arise mainly from nonlinear scattering, not from two- or three-photon absorption as reported in literature. And the two- and three-photon absorption coefficients of CS(2) are negligible. Therefore, around 800 nm the femtosecond open-aperture Z-scan of CS(2) cannot be used to calibrate Z-scan measurement system, but the closed-aperture Z-scan is capable.

Investigation of Two-Photon Absorption Induced Excited State Absorption in a Fluorenyl-Based Chromophore

Two-photon absorption induced excited state absorption in the solution of a new fluorenyl-based chromophore is investigated by a time-resolved pump-probe technique using femtosecond pulses. With the help of an additional femtosecond open-aperture Z-scan technique, numerical simulations based on a three-energy level model are used to interpret the experimental results, and we determine the nonlinear optical parameters of this new chromophore uniquely. Large two-photon absorption cross section and excited state absorption cross section for singlet excited state are obtained, indicating a good candidate for optical limiting devices. Moreover, the influence of two-beam coupling induced energy transfer in neat N,N'-dimethylformamide solvent is also considered, although this effect is strongly restrained by the instantaneous two-photon absorption.

Nonlinear optical switching behavior of Au nanocubes and nano-octahedra investigated by femtosecond Z-scan measurements

Two types of gold nanoparticles, nanocubes and nano-octahedra were prepared and their nonlinear absorption properties were investigated by using femtosecond open aperture Z-scan experiments. Both nanocubes and nano-octahedra exhibited a shift from saturable absorption (SA) to reverse saturable absorption (RSA) at higher excitation intensities. The SA behavior was ascribed to ground state plasmon band bleaching and RSA was ascribed to excited state absorption and two-photon absorption of these materials. The competition of different processes is responsible for the observed switching behavior.

Cationic Two-Photon Absorption Chromophores with Double- and Triple-Bond Cores in Symmetric/Asymmetric Arrangements

Two-photon absorption (TPA) properties of a series of cationic, linear π-conjugated systems with donor(D)/acceptor(A) substitution have been examined systematically in terms of the type and length of the π-conjugation core and symmetric and asymmetric D/A-arrangement. The TPA spectra of the chromophores measured by the femtosecond open-aperture Z-scan method reveal that the double-bond core results in much more intense TPA than the triple-bond one, with a significant red shift of the TPA peak while the TPA becomes more intense as the chain is extended for the triple-bond core compounds with a little red shift. It is also found that the difference in the D/A-arrangement gives a small difference in the amplitude of the TPA spectrum although the spectral shapes differ considerably.

TWO-PHOTON ABSORPTION IN QUANTUM DOTS OFCdSxSe1-xUSING OPEN APERTURE Z-SCAN AND FEMTOSECOND LASER

Two-photon optical nonlinear absorption has been studied in quantum dots of CdSxSe1-xgrown in borosilicate glass matrix by two-step annealing technique. Femtosecond laser and open aperture Z-scan technique has been used for measuring the third-order nonlinear two-photon absorption coefficient (β(3)). Only the third-order and not the fifth-order nonlinear effects are observed at low intensities (1.6–3.2 GW cm-2) of laser used in the present experiment. At such low intensities, the variation of β(3)is found to be almost intensity-independent. For a given annealing duration of the quantum dots, the value of β(3)is found to be higher for sulfur-rich samples as compared to that for selenium-rich samples. This is attributed to the presence of shallow traps formed due to sulfur vacancies in the sulfur-rich samples. Further, the value of β(3)increases with the increase in the size of quantum dots and the rate of increase of β(3)with the increase of average radius is found to be higher for sulfur-rich samples.