Open-aperture Z-scan Method Research Articles

Enhanced Two-Photon Absorption of Chromophores Confined in Two-Dimensional Nanospace

We report the enhancement of molecular two-photon absorption (2PA) of a dicationic 2PA chromophore (MPPBT) by confining it in a two-dimensional (2D) nanospace formed in nanolayered silicate clay by the cation exchange method in a water−dimethyl sulfoxide mixture. The 2PA property of the MPPBT/silicate nanocomposite prepared from the mixture by casting is characterized by the femtosecond open-aperture Z-scan method. We found that the 2PA cross-section of the chromophore in the nanocomposite is enhanced by a few times that of the dimethylsulfoxide solution at all wavelengths employed. This enhancement is accompanied by a bathochromic shift of the 2PA peak. The origin of this enhancement is discussed in terms of low dimensionality, suppressed internal twisting motion of the chromophore in the 2D confined space, and ionic interactions with the silicate surfaces.

Syntheses and nonlinear absorption properties of conjugated porphyrin supramolecules

Self-assemblies consisting of acetylene-linked bisporphyrins with a 4-nitrophenylethynyl substituent and with a phenylethynyl substituent were synthesized. The Q-band of the phenylethynyl-substituted compound appears at 762 nm whereas that of the 4-nitrophenylethynyl-substituted one was red-shifted to 784 nm and intensified. This may be attributed to the participation of the nitro group in the π-electronic communication system. HOMOs and LUMOs were calculated for these compounds using an INDO/S-CI method. The effective two-photon absorption cross-section values of these self-assemblies were measured by using a nanosecond open aperture Z-scan method. The maximum effective two-photon absorption cross-section values of 4-nitrophenylethynyl- and phenylethynyl-substituted bisporphyrins were obtained as 1.2 × 105 GM and 8.1 × 104 GM , respectively, at 890 nm. A 1.5 enhancement factor was obtained by introducing nitro groups, which was similar to the value previously reported for ferrocene- and fullerene-connected supramolecular porphyrin.

One‐ and Two‐Photon Singlet Oxygen Generation with New Fluorene‐Based Photosensitizers

The spectral properties of new fluorene-based photosensitizers for efficient singlet oxygen production are investigated at room temperature and 77 K. Two-photon absorption (2PA) cross-sections of the fluorene derivatives are measured by the open aperture Z-scan method. The quantum yields of singlet oxygen generation under one- and two-photon excitation (phi(delta) and 2PAphi(delta), respectively), are determined by the direct measurement of singlet oxygen luminescence at approximately 1270 nm. The values of phi(delta) are independent of excitation wavelength, ranging from 0.6-0.9. The singlet oxygen quantum yields under two-photon excitation are 2PAphi(delta) approximately 1/2 phi(delta), indicating that the two processes exhibit the same mechanism of singlet oxygen production, independent of the mechanism of photon absorption.

Poly( N-vinyl carbazole)-grafted multiwalled carbon nanotubes: Synthesis via direct free radical reaction and optical limiting properties

In this work, samples of poly( N-vinyl carbazole) (PVK)-grafted multiwalled carbon nanotubes (MWCNTs) were synthesized via free radical reaction. The ready-made PVK was allowed to react directly with MWCNTs at 70 °C in the presence of azo-bis-isobutyronitrile (AIBN). The purified deep grey products, which can dissolve in common organic solvents such as chloroform and 1,2-dichlorobenzene (DCB), were then characterized by FTIR spectra, TEM, TGA, elemental analysis, XPS, UV–vis spectra and Raman spectra. It was confirmed that PVK chains were grafted onto the surface of the carbon nanotubes (CNTs). The optical limiting properties of these PVK-grafted MWCNTs samples were investigated by open-aperture z-scan method. All of the samples of PVK-modified carbon nanotubes in chloroform showed optical limiting behavior better than that of C 60 in toluene.

22π Smaragdyrin Molecular Conjugates with Aromatic Phenylacetylenes and Ferrocenes: Syntheses, Electrochemical, and Photonic Properties

Syntheses, spectroscopic, electrochemical, and third-order nonlinear optical susceptibilities of a series of 22pi smaragdyrins and their corresponding Rh(I) derivatives bearing phenylacetylene substituents and ferrocene-containing substituents are reported. The synthetic strategy involved a [3 + 2] acid-catalyzed oxidative coupling reaction of the appropriate dipyrromethane and oxatripyrrane. The desired meso substituents, such as phenylacetylenylphenyl and the ferrocenes, were incorporated to the dipyrromethane unit prior to the oxidative coupling reaction. The optical absorption, emission characteristics, and the quantum yield of the smaragdyrin conjugates depends on the nature of the substituent, nature of linker group, and the spacer length. Theoretical studies at the DFT level suggest high delocalization of electrons confined to only four of the five available heterocyclic rings for the free bases. However, upon Rh(I) metalation, the pi-electron delocalization is extended to all the heterocyclic rings. The two-photon absorption cross section (TPA) values sigma(2) measured through the open aperture Z-scan method, increases linearly with enhanced pi-electron delocalization for the smaragdyrins containing phenylacetylene substituents. The meta branching of substituents decreases sigma(2) values. Introduction of Rh(I) to the smaragdyrin cavity enhances the sigma(2) values by about 3-10 orders of magnitude, attributed to the increased aromatic character upon Rh(I) insertion. The calculated molecular electrostatic potential (MESP) and harmonic oscillator model of aromaticity (HOMA) for the free bases and the Rh(I) derivatives justifies such a conclusion. A linear correlation observed for the second oxidation potential of Rh(I) derivatives and corresponding free bases also support the increased aromaticity upon Rh(I) insertion. The electrochemical data for ferrocene-containing smaragdyrins reveal easier ring oxidation by about 50-130 mV and harder ferrocene oxidation by 40-180 mV suggesting electron-donating nature of the ferrocene upon linking with the smaragdyrin system. The TPA cross section value of 88782 GM observed for 5g represents one of the highest values known for a metalloexpanded porphyrin derivative.

Experimental study of nonlinear absorption in polymethine dye solutions by the z-scan method

The nonlinear absorption coefficients β and the ratios k of absorption cross sections in the ground and excited states are measured by the open-aperture z-scan method for three polymethine dyes excited by nanosecond 532-nm second-harmonic pulses of a Nd:YAG laser. The values of k measured for the 789, 842, and 823 dyes are 10, 11, and 11, respectively, and correlate with the limiting properties of these dyes.

Two-photon absorption properties of conjugated supramolecular porphyrins with electron donor and acceptor

Two-photon absorption (2PA) cross-section values of supramolecular porphyrin tetramer consisting of two monoacetylene-linked bisporphyrins with and without ferrocene/C 60 groups as electron donor/acceptor through the coordination of pyridyl to zinc were measured using a nanosecond open aperture Z-scan method. The maximum effective cross-section value was observed for the compound having asymmetric terminals of ferrocene and C 60 as 2.0 × 10 5 GM. The values obtained for compounds having two C 60's, two ferrocene's, no donor/acceptor, and stacked isomer were 1.7 × 10 5 GM, 1.5 × 10 5 GM, 1.1 × 10 5 GM, and 1.3 × 10 5 GM, respectively. These results indicate that the asymmetric donor–π–acceptor structure is the best for 2PA enhancement for this series of molecules. These values are extremely large compared to those of unit dimer 2D (190 GM at 780 nm) and compounds reported hitherto in literatures.

Ce3+-Doped LiCaAlF6 Crystals as a Solid-State Ultraviolet Saturable Absorber and Role of Excited State Absorption

Nonlinear absorption properties of Ce3+-doped LiCaAlF6 (Ce:LiCAF) crystals at wavelength of 266 nm are studied using open-aperture Z-scan method and a Q-switch Nd:YAG laser. Saturable absorption of solid-state materials in ultraviolet region is demonstrated for the first time.

Two-Photon Absorption Properties of Self-Assemblies of Butadiyne-Linked Bis(Imidazolylporphyrin)

Supramolecular porphyrin self-assemblies have been prepared from butadiyne-linked bis(imidazolylporphyrin) by complementary coordination of imidazole to zinc, and their two-photon absorption (2PA) and higher-order nonlinear absorption properties were investigated over femtosecond time scales using an open-aperture Z-scan method. The self-assembled porphyrin dimer of the conjugated monozinc bisporphyrin 7D was shown to have a large 2PA cross section (7.6 x 10(3) GM, where 1 GM = 10(-50) cm(4) s molecule(-1) photon(-1)) at 887 nm. By comparison of this result with that for a meso-meso-linked porphyrin array without the butadiyne connection (3.7 x 10(2) GM at 964 nm), it was demonstrated that the predominant factor in this significant enhancement of the cross section was the expansion of porphyrin-porphyrin pi-conjugation. Self-coordination and monozinc metalation were also found to be contributing factors. Furthermore, a novel self-assembled porphyrin polymer 8P consisting of a biszinc complex with a mean molecular weight of M(n) = 1.5 x 10(5) Da was shown to exhibit an extraordinarily large two-photon absorption cross section (4.4 x 10(5) GM at 873 nm). Nanosecond Z-scan experiments for 7D and 8P were also undertaken and resulted in the measurement of large effective 2PA cross sections, including the excited-state absorption (2.1 x 10(5) GM for 7D and 2.2 x 10(7) GM for 8P, respectively). Finally, three-photon absorption was observed by femtosecond Z-scan experiments at 1188 nm (7.1 x 10(-89) m(6) s(2)) and 1282 nm (1.8 x 10(-89) m(6) s(2)), an observation which is the first of its kind in porphyrin chemistry.

Substituent effect on two-photon absorption properties of conjugated porphyrins

Two-photon absorption (2PA) cross section values of bisacetylene-connected bisporphyrins were measured by using a nanosecond open aperture Z-scan method. The maximum effective cross section values of alkyl and aryl meso-substituted bisporphyrins were 14,000 GM at 890 nm and 7,700 GM at 860 nm, respectively, and the value of alkyl-substituted bisporphyrin was almost twice of that of aryl-substituted one. In one-photon absorption spectra, Q-bands of alkyl-substituted bisporphyrin were red-shifted at 635 nm and 685 nm, and intensified compared with those of aryl-substituted one at 625 nm and 678 nm, respectively, indicating higher π-electron density of the former compound compared to that of the later. The large enhancement observed for alkyl-substituted bisporphyrin is attributed to the increase in π- electron density compared with aryl-substituted one. It was found that the alkyl group is better to obtain large 2PA cross section values than the aryl group as the meso-substituent of the porphyrin.

Core-Modified Expanded Porphyrins with Large Third-Order Nonlinear Optical Response

The third-order nonlinear optical response through measurement of two-photon absorption cross-sections (TPACS) for aromatic core-modified expanded porphyrin analogues by a femtosecond open aperture Z-scan method is reported. The values reported here are among the largest known so far in the literature for any organic molecules.

Characterization and nonlinear optical property of a multi-walled carbon nanotube/silica xerogel composite

A multi-walled carbon nanotube (MWNT)/silica xerogel composite was prepared by sol–gel technique and characterized by Raman spectroscopy and transmission electron microscopy. The nonlinear optical property of MWNTs in liquid suspension and solid state matrix were investigated separately by open aperture Z-scan method, using 532 and 1064 nm nanosecond laser pulses. The results show that both MWNT composite and suspension are nonlinear efficient at 1064 and 532 nm, while MWNT composite may be more suitable for practical use because of their better nonlinear effect and possibly nondestructive nonlinear absorption mechanism.

Measurement of the two-photon absorption cross sections of dicyanomethylene-pyrans by the z-scan method

The two-photon absorption cross sections of three dicyanomethylene-pyrane DCM dyes excited by a femtosecond pulsed from a Ti:sapphire laser are measured by the open-aperture z-scan method. The experimental transmission curves are approximated by the corresponding model curves to give the two-photon absorption cross sections equal to 5.1×103 GM, 5.7×103 GM, and 8.4×103 GM for DCM, DCM-17, and DCM-doa, respectively (1 GM = 10-50 cm4 s phot-1 mol-1).

Development of Photochromic Two-Photon Absorption Dyes

ABSTRACT We synthesized three photochromic two-photon absorptive diarylethene derivatives with styrylbenzene unit and measured two-photon absorption cross-sections at 800 nm by using an open aperture Z-scan method with 120 femtosecond pulses. It was found that two of the dyes have large values (δ = 52 GM) equivalent to AF-50.

Saturable absorption and reverse saturable absorption in platinum nanoparticles

The optical nonlinear absorption of the aqueous solution of platinum nanospheres protected by poly (N-vinyl-2-pyrrolidone) was investigated using open aperture Z-scan method with nanosecond pulse laser at the wavelength of 532nm. A shift from saturable absorption to reverse saturable absorption was observed at higher input pump intensities. The transition process was analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity.