Open-aperture Z-scan Method Research Articles

Polymer Monoliths Containing Two-Photon Absorbing Phenylenevinylene Platinum(II) Acetylide Chromophores for Optical Power Limiting.

A series of platinum(II) acetylide complexes containing p-phenylenevinylene and moieties end-capped with triphenylamine groups have been incorporated into poly(methyl methacrylate) (PMMA) monoliths for optical power limiting applications. The one- and two-photon photophysical properties were investigated and compared to the photophysical properties in THF. The absolute two-photon absorption cross-section values for the monolith samples were measured and are comparable to the values obtained in solution. In the PMMA monoliths, the complexes retained the important two-photon absorption and reverse saturable absorption properties necessary for optical power limiting via dual mode mechanism, and their strong nonlinear absorption property was demonstrated by the open-aperture Z-scan method. Photostability studies of the p-phenylenevinylene platinum(II) acetylide complexes showed two photodegradation processes: a trans-to-cis isomerization and a singlet-oxygen sensitized self-oxidative cleavage. The photostability of the least photostable complex TPV0 was increased upon incorporation into a PMMA matrix.

Ultrafast saturable absorption in topological insulator Bi₂SeTe₂ nanosheets.

Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.

Nonlinear optical response of gold/silicon nanocomposite prepared by consecutive laser ablation

A gold/silicon nanocomposite has been prepared by a consecutive laser ablation method and characterized by UV–visible absorption spectrometry, Fourier transform infrared spectrum, x-ray diffraction pattern and transmission electron microscopy methods. The Au/Si nanocomposite was formed by nanosecond pulsed laser irradiation on a gold plate in a silicon nanoparticle colloidal solution which has been prepared by laser ablation of Si plate ethanol. The UV–visible absorption spectrum of nanocomposite colloidal solution shows a 40 nm red shift of the surface plasmon peak compared with an Au nanoparticle in ethanol. The presence of Si nanoparticles in the solution was confirmed by the Fourier transform infrared spectrum. X-ray diffraction pattern of Au/Si nanocompsite powder only contains gold structures, and thus Si nanoparticles are amorphous. The diameter of Si nanoparticles and Au/Si nanocomposites are determined by transmission electron microscopy images about 13 and 30 nm, respectively. The nonlinear absorption coefficient of colloidal Au/Si was studied by the open aperture Z-scan method. The value of the nonlinear absorption is measured 5.8 × 10−3 cm W−1 with a positive sign which illustrates the two photon absorption phenomena.

THz saturable absorption in turbostratic multilayer graphene on silicon carbide.

We investigated the room-temperature Terahertz (THz) response as saturable absorber of turbostratic multilayer graphene grown on the carbon-face of silicon carbide. By employing an open-aperture z-scan method and a 2.9 THz quantum cascade laser as source, a 10% enhancement of transparency is observed. The saturation intensity is several W/cm2, mostly attributed to the Pauli blocking effect in the intrinsic graphene layers. A visible increase of the modulation depth as a function of the number of graphene sheets was recorded as consequence of the low nonsaturable losses. The latter in turn revealed that crystalline disorder is the main limitation to larger modulations, demonstrating that the THz nonlinear absorption properties of turbostratic graphene can be engineered via a proper control of the crystalline disorder and the layers number.

Nonlinear optical absorption tuning in Bi3.15Nd0.85Ti3O12 ferroelectric thin films by thickness

The tunability of nonlinear optical (NLO) absorption in Bi3.15Nd0.85Ti3O12 (BNT) ferroelectric thin films was investigated through the open aperture Z-scan method with femtosecond laser pulses at the wavelength of 800 nm. NLO absorption responses of the BNT films were observed to be highly sensitive to the film thickness. As the film thickness increases from 106.8 to 139.7 nm, the NLO absorption changes from saturable absorption (SA) to reverse saturable absorption (RSA). When the film thickness further increases to 312.9 nm, the RSA effect is enhanced. A band-gap-related competition between the ground-state excitation and the two-photon absorption is responsible for the absorption switching behavior. Such a tunable NLO absorption can widen the photonic application of the BNT thin films.

Steady-state and femtosecond transient absorption spectroscopy of new two-photon absorbing fluorene-containing quinolizinium cation membrane probes.

The synthesis, linear photophysical characterization, and nonlinear optical properties of two new symmetrical fluorene-containing quinolizinium derivatives, 2,8-bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)vinyl)quinolizinium hexafluorophosphate (1) and 2,8-bis((E)-2-(7-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)ethynyl)-9,9-dihexyl-9H-fluoren-2yl)vinyl)quinolizinium hexafluorophosphate (2), are reported. The nature of the dual-band steady-state fluorescence emission of 1 and 2 was determined, and violation of Kasha's rule along with a strong dependence on solvent polarity were shown. A relatively complex structure of two-photon absorption (2PA) spectra of 1 and 2, with maximum cross sections of ∼400-600 GM, was determined using the open aperture Z-scan method. Different types of fast relaxation processes with characteristic times of 0.3-0.5 ps and 1.5-2 ps were observed in the excited states of the new compounds via femtosecond transient absorption pump-probe spectroscopy. To better understand the photophysical behavior of 1 and 2, a quantum-mechanical study was undertaken using TD-DFT and ZINDO/S methods. Simulated linear absorption spectra were found to be in good agreement with experimental data, while 2PA cross sections were overestimated. Although the new dyes were highly fluorescent in nonpolar solvents, they were essentially nonfluorescent in polar media. Significantly, the quinolizinium dyes exhibited fluorescence turn-on behavior upon binding to bovine serum album (BSA) protein, exhibiting over 4-fold fluorescence enhancement, which was a finding that was leveraged to demonstrate cell membrane fluorescence imaging of HeLa cells.

Nonlinear absorption and scattering properties of copper sulfide nanocrystals

We investigate the nonlinear optical properties of Cu9S5 nanocrystals using the open aperture Z-scan method with 20ps and 4ns laser pulses at 532nm. We found that optical nonlinearity arises from free carrier absorption in the former case, whereas in the latter it occurs mostly due to nonlinear scattering. Parameters characterizing free carrier absorption and nonlinear scattering were determined. The dynamics of the nonlinear processes was confirmed by pump-probe measurements with 20ps pulses. Observation of optical limiting at both excitation time regimes indicates the potential of Cu9S5 nanocrystals in laser safety devices.

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

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

Preparation, optical and non-linear optical power limiting properties of Cu, CuNi nanowires

Metallic nanowires show excellent Plasmon absorption which is tunable based on its aspect ratio and alloying nature. We prepared Cu and CuNi metallic nanowires and studied its optical and nonlinear optical behavior. Optical properties of nanowires are theoretically explained using Gans theory. Nonlinear optical behavior is studied using a single beam open aperture z-scan method with the use of 5 ns Nd: YAG laser. Optical limiting is found to arise from two-photon absorption.

Wavelength selective optical limiting effect on MoS2 solution

Using discrete method, N,N-Dimethylformamide (DMF) and tetrahydrofuran(THF) are adopted as solvent to fabricate MoS2 suspension solution, and its nonlinear optical properties in the visible and near infrared region are studied by open aperture Z-scan method. The results show that under the intense laser, in MoS2 (in DMF) suspension solution, saturable absorption property can be detected in the visible waveband (530 nm), showing that its transmittance is 1.54 times of ordinary, and reverse saturable absorption can be observed in the near infrared region (790 nm), indicating that its transmittance is 0.6 time of ordinary one and very good wavelength selection optical limiting effect. As a comparison, in the MoS2 (in DMF) suspension solution there does not appear the wavelength selection feature, but the reverse saturable absorption is present in all band. This phenomena may be produced through the two mechanisms: saturableabsorption and thermally-induced self-diffraction.

Synthesis, two-photon absorption properties and bioimaging applications of mono-, di- and hexa-branched pyrimidine derivatives

A series of mono-, di- and hexa-branched pyrimidine derivatives possessing two-photon absorption have been synthesized. The spectral features of the dyes showed solvatochromic with significant bathochromic shifting of the emission spectra and large Stokes shifts in more polar solvents mainly due to the intramolecular charge-transfer. The two-photon absorption cross-section values were measured by two-photon excited fluorescence and open aperture Z-scan methods. The results revealed that the two-photon absorption cross-section values were enhanced with increasing electron-donating strength of the end group and branch number. The photophysical properties of the dyes were further investigated with the aid of theoretical calculations. In addition, the di-branched dyes has the largest values of two-photon absorption cross-section per molecular weight, low cytotoxicity and can be internalized by human liver cancer cell line and accumulates in the cytoplasm, allowing for live cell two-photon fluorescence imaging, so it is promising for in vitro and vivo cellular imaging applications.

Superfluorescent Squaraine with Efficient Two‐Photon Absorption and High Photostability

The synthesis, linear photophysical, two-photon absorption (2PA), femtosecond transient absorption, and superfluorescence properties of a new symmetrical squaraine derivative (1) are reported. Steady-state linear spectral and photochemical properties, fluorescence lifetimes, and excitation anisotropy of 1 were investigated in various organic solvents. High fluorescence quantum yields (≈0.7) and very high photostability (photodecomposition quantum yields ≈10(-6)-10(-8)) were observed. An open-aperture Z-scan method was used to obtain 2PA spectra of 1 over a broad spectral range (maximum 2PA cross section ≈1000 GM). Excited-state absorption (ESA) and gain was observed by femtosecond transient absorption spectroscopy, in which both reached a maximum at approximately 500 fs. Squaraine 1 exhibits efficient superfluorescence. The quantum chemical study of 1 revealed the simulated vibronic nature of the 1PA and 2PA spectra were in good agreement with experimental data; this may provide the ability to predict potential advanced photonic materials.

New Fluorene-Based Fluorescent Probe with Efficient Two-Photon Absorption

The synthesis, linear photophysical characterization, and two-photon absorption (2PA) properties of new fluorene derivative 3,30-(pyridine-2,6-diyl)bis(1-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)propane-1,3-dione) (1) have been presented. The steady-state absorption, fluorescence and excitation anisotropy spectra along with the fluorescence decay kinetics of 1 are obtained in the solvents of different polarities at room temperature with respect to its potential application in bioimaging. The analysis of linear photophysical properties revealed a complicated nature of the main one-photon absorption band of 1, and the strong solvatochromic effect in steady-state fluorescence spectra is observed. The degenerate 2PA spectrum of 1 is measured in the spectral range 570–970 nm with the use of the open aperture Z-scan method under the 1-kHz femtosecond excitation, and the maximum values of two-photon action cross sections ~(100–130) GM are obtained. The nature of the linear absorption and the 2PA bands is analyzed by quantum chemical methods using a Gaussian program package.

Nonlinear optical properties of boron doped single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) exhibit excellent nonlinear optical (NLO) properties due to the delocalized π electron states present along their tube axis. Using the open aperture Z-scan method in tandem with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, we demonstrate the simultaneous tailoring of both electronic and NLO properties of SWCNTs, from ultrafast (femtosecond) to relatively slow (nanosecond) timescales, by doping with a single substituent, viz., boron. SWCNTs were doped via a wet chemical method using B2O3, and the boron content and bonding configurations were identified using XPS. While in the ns excitation regime, the nonlinear absorption was found to increase with increasing boron concentration in the SWCNTs (due to the increasing disorder and enhanced metallicity of the SWCNTs), the saturation intensity in the fs excitation regime decreased. We attribute this counter-intuitive behavior to excited state absorption on ns timescales, and saturable absorption combined with weak two-photon transitions on fs timescales between van Hove singularities.

Femtosecond nonlinear absorption of Ag nanoparticles at surface plasmon resonance

Nonlinear absorption of Ag nanoparticles was investigated using open aperture Z-scan method with femtosecond laser pulses at 400nm. It was found that, at low intensities, Ag nanoparticles behave as saturable absorbers but at higher intensities a shift from saturable absorption to reverse saturable absorption occurs. And open aperture Z-scan curves become asymmetric when the repetition rate of pulse laser is high. The transition process and asymmetry were analyzed in terms of plasmon bleaching, free carrier absorption and migration of Ag nanoparticles at surface plasmon resonance.

Near-infrared-emitting squaraine dyes with high 2PA cross-sections for multiphoton fluorescence imaging.

Designed to achieve high two-photon absorptivity, new near-infrared (NIR) emitting squaraine dyes, (E)-2-(1-(2-(2-methoxyethoxy)ethyl)-5-(3,4,5-trimethoxystyryl)-1H-pyrrol-2-yl)-4-(1-(2-(2-methoxyethoxy)ethyl)-5-(3,4,5-trimethoxystyryl)-2H-pyrrolium-2-ylidene)-3-oxocyclobut-1-enolate (1) and (Z)-2-(4-(dibutylamino)-2-hydroxyphenyl)-4-(4-(dibutyliminio)-2-hydroxycyclohexa-2,5-dienylidene)-3-oxocyclobut-1-enolate (2), were synthesized and characterized. Their linear photophysical properties were investigated via UV-visible absorption spectroscopy and fluorescence spectroscopy in various solvents, while their nonlinear photophysical properties were investigated using a combination of two-photon induced fluorescence and open aperture z-scan methods. Squaraine 1 exhibited a high two-photon absorption (2PA) cross-section (δ2PA), ∼20 000 GM at 800 nm, and high photostability with the photochemical decomposition quantum yield one order of magnitude lower than Cy 5, a commercially available pentamethine cyanine NIR dye. The cytotoxicity of the squaraine dyes were evaluated in HCT 116 and COS 7 cell lines to assess the potential of these probes for biomedical imaging. The viability of both cell lines was maintained above 80% at dye concentrations up to 30 μM, indicating good biocompatibility of the probes. Finally, one-photon fluorescence microscopy (1PFM) and two-photon fluorescence microscopy (2PFM) imaging was accomplished after incubation of micelle-encapsulated squaraine probes with HCT 116 and COS 7 cells, demonstrating their potential in 2PFM bioimaging.

Synthesis, characterization and two-photon absorption based broadband optical limiting in diarylideneacetone derivative

Abstract A donor-π-acceptor-π-donor (D-π-A-π-D) type Bis-Chalcone, (1E,4E)-1,5-di-2-thienylpenta-1,4-dien-3-one (D2TDO) was synthesized by Claisen–Schmidt reaction. Structural characterization was done with the help of UV, FT-IR, 1H NMR, 13C NMR and Mass Spectroscopic techniques. Through gravity settling method, a homogenous film of D2TDO with 65 μm thickness was made in poly(methylmethacrylate) matrix. The third-order absorptive nonlinearity and resultant optical limiting behavior of the D2TDO film were studied by employing open aperture z-scan method in both nanosecond and femtosecond excitation regimes. Through numerical simulations, the mechanism of nonlinear absorption was found to be a two-photon absorption process with broadband response. The optical limiting performance of the molecule has its origin from the molecule's peculiar D-π-A-π-D structure which allows for a greater extent of π electron delocalization due to a strong π-electron donor like thiophene.

Synthesis and Properties of Hybrid Porphyrin Tapes

Hybrid porphyrin tapes 3 and 4, consisting of a mixture of 3,5-di-tert-butylphenyl-substituted donor-type Zn(II)-porphyrins and pentafluorophenyl-substituted acceptor-type Zn(II)-porphyrins, were prepared by a synthetic route involving cross-condensation reaction of a Ni(II)-porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring-closure reaction. The Ni(II)-substituted porphyrin tapes 5 (Ni-Zn-Ni) and 6 (Ni-H(2)-Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor-type Zn(II)-porphyrins. The solid-state and crystal packing structures of 3, 4, and 5 were elucidated by single-crystal X-ray diffraction analysis. Singly meso-meso-linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two-photon absorption (TPA) values of 1-6 were measured by using a wavelength-scanning open aperture Z-scan method and found to be 1900, 21,000, 2200, 27,000, 24,000, and 26,000 GM, respectively. These results illustrate an important effect of elongation of π-electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.

Tunable nonlinear absorption of hydrogenated nanocrystalline silicon

Nonlinear absorption (NLA) of hydrogenated nanocrystalline silicon (nc-Si:H) has been investigated through the open aperture Z-scan method for the photon energy of the incident irradiance slightly less than the bandgap of the sample. NLA responses have been observed to be highly sensitive to the wavelength and intensity of the incident irradiance as well as to the bandgap of the sample, indicating greatly tunable NLA of nc-Si:H. The band tail of nc-Si:H appears to play a crucial role in such NLA responses.

Efficient Photochromic Transformation of a New Fluorenyl Diarylethene: One- and Two-Photon Absorption Spectroscopy

Efficient reversible phototransformation of a new diarylethene-fluorene derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9H-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (1), was demonstrated in organic media under low-intensity laser excitation. Linear photophysical characterization of 1 was performed at room temperature in solvents of different polarity and viscosity. Significantly, close to unity quantum yield for the cyclization reaction of 1 was shown in nonpolar solutions. The lifetimes of the excited states of the open (OF) and closed (CF) forms of 1 were measured by a femtosecond transient absorption technique, and corresponding values of ∼0.7 and ∼0.9 ps were shown in dichloromethane (DCM), respectively. Degenerate two-photon absorption (2PA) spectra of the OF and CF of 1 were obtained over a broad spectral range by the open aperture Z-scan method under 1 kHz femtosecond excitation. The values of 2PA cross sections of the OF in DCM (∼50-70 GM) were found to increase up to 1 order of magnitude (∼600 GM) after cyclization to the CF. The nature of cyclization and cylcoreversion processes were investigated by quantum chemistry with employment of DFT-based methods implemented in the Gaussian'09 program. The potential of 1 for application in optical data storage was shown using poly(methyl methacrylate)-doped films and two-photon fluorescence microscopy readout.