Gold nanoparticle-decorated graphene as a nonlinear optical material in the visible and near-infrared spectral range.

There have been numerous reports of boosting nonlinear optical phenomena in plasmonic nanostructures through local-field enhancement as well as through intrinsic metal nonlinearities. Here, we study the possibility of plasmonic enhancement of graphene's nonlinear absorption by creating a composite material: gold nanoparticle-decorated graphene dispersed in water. To evaluate the additive effect of combining the two materials on the saturable absorption efficiency we performed a series of f-scan (modified Z-scan) measurements using femtosecond laser pulses in the broad spectral range from 530 to 1600 nm. These studies are supplemented by TEM, UV-vis, ATR and Raman spectroscopy, revealing the mechanisms behind the formation of the composite material.

Remote-control of the enantiomeric supramolecular recognition mediated by chiral azobenzenes bound to human serum albumin.

Three novel tyrosine-conjugated azobenzene molecules were designed and their ability to target a natural chiral host matrix (human serum albumin, HSA) was investigated. We found that the interplay between the spatial configuration of the chiral substituents and the change in local symmetry resulting from the photoisomerization process strongly affects the optical activity of the bound photochromes. In particular, the different signal amplification obtained upon binding of the photoswitches to the biopolymer enables obtaining a chirooptical system tunable over a wide range of wavelengths.

Spectrally-resolved third-harmonic generation and the fundamental role of O-HCl hydrogen bonding in Oh, Td-cobalt(ii) tetraphenylmethane-based coordination polymers.

The reaction of a phosphonate-diester tetraphenylmethane-based tecton, tetrakis[4-(diethoxyphosphoryl)phenyl]methane, (L) with cobalt(ii) chloride afforded a centrosymmetric coordination polymer (CP), [L·2Co(H2O)42+·2CoCl42-]n, 2-Cl, possessing simultaneously octahedral (Oh) and tetrahedral (Td) metal centers. This material served as a model compound for the demonstration of factors influencing the spectral dependence of one of the nonlinear optical (NLO) phenomena, the third-harmonic generation (THG). The spectrally-resolved THG (SR-THG) measurements in the range from 1125 to 1750 nm revealed that a maximum of THG response is obtained when the fundamental beam is around 1300 nm. The SR-THG study was combined with an analysis of the self-absorption effects of pumping and of third-harmonic radiation; based on these results, we put forward a hypothesis that the THG action spectrum is influenced more by the ability of the material to self-absorb the third harmonic rather than by the extent of self-absorption of the pumping radiation. Apart from investigations of NLO properties, we have explored coordination and particularly the supramolecular interactions that build up the 2-Cl CP. Despite the tetrahedral, spatial shape of the ligand L, CP 2-Cl has a two-dimensional net. The structure was found to be strongly supported by O-HCl hydrogen bonds, since each CoCl42- complex anion is an acceptor of eight of such interactions within a distorted square grid layer of cobalt(ii) ions. While coordination and hydrogen-bonded nets are both featuring the sql topology when treated separately, the consideration of both of them as topological paths yields a trinodal 4,4,6-connected net, described by the point symbol (42·84)(45·6)2(46·66·83)2. SR-THG and structural studies of 2-Cl have been also supported by far- and mid-infrared spectroscopy, UV-Vis-NIR solid state absorption analysis, thermogravimetry and preliminary magnetic characterization.

Effective control of the intrinsic DNA morphology by photosensitive polyamines.

Non-viral vectors for gene therapy such as DNA-cationic probe complexes offer important bio-safety advantages over viral approaches, due to their reduced pathogenicity, immunogenicity and cytotoxicity. In the present study we examine two polycationic water-soluble azobenzene derivatives (bis-Azo-2N and bis-Azo-3N) containing different linear unsubstituted polyamine moieties and we demonstrate the ability of such photochromes to destabilize the intrinsic B-DNA secondary structure in a concentration-dependent manner. Furthermore, through a detailed series of biophysical experiments, varying the photochrome conformation, temperature, salt and DNA concentration, we provide a detailed insight into the azobenzene-DNA binding pathway (Ka: bis-Azo-2N(trans)-DNA = 5.3 ± 0.3 × 104 M-1, Ka: bis-Azo-2N(cis)-DNA = 2.6 ± 0.2 × 104 M-1, Ka: bis-Azo-3N(trans)-DNA = 7.1 ± 0.4 × 104 M-1 and Ka: bis-Azo-3N(cis)-DNA = 5.1 ± 0.4 × 104 M-1) establishing the versatility of such materials as promising candidates for use in non-viral gene delivery systems.

Polymeric nanocapsules with up-converting nanocrystals cargo make ideal fluorescent bioprobes.

An innovative approach for up-converting nanoparticles adaptation for bio-related and theranostic applications is presented. We have successfully encapsulated multiple, ~8 nm in size NaYF4 nanoparticles inside the polymeric nanocarriers with average size of ~150 nm. The initial coating of nanoparticles surfaces was preserved due to the hydrophobic environment inside the nanocapsules, and thus no single nanoparticle surface functionalization was necessary. The selection of biodegradable and sugar-based polyelectrolyte shells ensured biocompatibility of the nanostructures, while the choice of Tm(3+) and Yb(3+) NaYF4 nanoparticles co-doping allowed for near-infrared to near-infrared bioimaging of healthy and cancerous cell lines. The protective role of organic shell resulted in not only preserved high up-converted emission intensity and long luminescence lifetimes, without quenching from water environment, but also ensured low cytotoxicity and high cellular uptake of the engineered nanocapsules. The multifunctionality of the proposed nanocarriers is a consequence of both the organic exterior part that is accessible for conjugation with biologically important molecules, and the hydrophobic interior, which in future application may be used as a container for co-encapsulation of inorganic nanoparticles and anticancer drug cargo.

A Fluorescent Polymer Probe with High Selectivity toward Vascular Endothelial Cells for and beyond Noninvasive Two-Photon Intravital Imaging of Brain Vasculature.

A chromophore-engineering strategy that relies on the introduction of a ground-state distortion in a quadrupolar chromophore was used to obtain a quasi-quadrupolar chromophore with red emission and large two-photon absorption (2PA) cross-section in polar solvents. This molecule was functionalized with water-solubilizing polymer chains. It constitutes not only a remarkable contrast agent for intravital two-photon microscopy of the functional cerebral vasculature in a minimally invasive configuration but presents intriguing endothelial staining ability that makes it a valuable probe for premortem histological staining.

End-to-end self-assembly of gold nanorods in isopropanol solution: experimental and theoretical studies.

ABSTRACT: We describe here a modification of properties of colloidal gold nanorods (NRs) resulting from the chemical treatment used to carry out their transfer into isopropanol (IPA) solution. The NRs acquire a tendency to attach one to another by their ends (end-to-end assembly). We focus on the investigation of the change in position and shape of the longitudinal surface plasmon (l-SPR) band after self-assembly. The experimental results are supported by a theoretical calculation, which rationalizes the dramatic change in optical properties when the NRs are positioned end-to-end at short distances. The detailed spectroscopic characterization performed at the consecutive stages of transfer of the NRs from water into IPA solution revealed the features of the interaction between the polymers used as ligands and their contribution to the final stage, when the NRs were dispersed in IPA solution. The efficient method of aligning the NRs detailed here may facilitate applications of the self-assembled NRs as building blocks for optical materials and biological sensing.

Photophysical, amplified spontaneous emission and charge transport properties of oligofluorene derivatives in thin films.

We investigate the photophysical and amplified spontaneous emission properties of a series of monodisperse solution-processable oligofluorenes functionalized with hexyl chains at the C9 position of each fluorene unit. Thin films of these oligofluorenes are then used in organic field-effect transistors and their charge transport properties are examined. We have particularly focused our attention on the influence of oligofluorene length on the absorption and steady-state fluorescence spectra, on the HOMO/LUMO energy levels, on the photoluminescence lifetime and quantum yield as well as on the amplified spontaneous emission properties and the charge carrier mobilities. Differential scanning calorimetry and X-ray diffraction measurements demonstrate that, among all oligofluorene derivatives used in this study, only the structure and morphology of the pentafluorene film is significantly modified by a thermal treatment above the glass transition temperature, resulting in a 9 nm blue-shift of the fluorescence spectrum without significant changes in the photoluminescence quantum yield and in the amplified spontaneous emission threshold. In parallel, hole field-effect mobility is significantly increased from 8.6 × 10(-7) to 3.8 × 10(-5) cm(2) V(-1) s(-1) upon thermal treatment, due to an increase of crystallinity. This study provides useful insights into the morphological control of oligofluorene thin films and how it affects their photophysical and charge transport properties. Moreover, we provide evidence that, because of the low threshold, the tunability of the amplified spontaneous emission and the photostability of the films, these oligofluorenes are promising candidates for organic solid-state laser applications.

Optimisation of ligand exchange towards stable water suspensions of crystalline NaYF4: Er3+, Yb3+ nanoluminophors.

NaYF4: 2%Er3+, 20%Yb3+ nanoparticles were synthesized through a wet chemical route. In order to transfer the nanoparticles from chloroform to aqueous solution an oleic acid to 3-mercaptopropionic acid ligand exchange process has been performed and optimized. The influence of temperature and atmosphere onto the NaYF4:2%Er3+, 20%Yb3+ nanoparticles water suspensions dispersibility and stability after replacing oleic acid with 3-mercaptopropionic acid ligands has been investigated. The results revealed an improvement of nanoparticles water suspension transparency and luminescence yield after ligand exchange process performed under inert atmosphere and at elevated temperature.

Giant enhancement of upconversion in ultra-small Er³âº/Yb³âº:NaYF4 nanoparticles via laser annealing.

Most of the synthesis routes of lanthanide-doped phosphors involve thermal processing which results in nanocrystallite growth, stabilization of the crystal structure and augmentation of luminescence intensity. It is of great interest to be able to transform the sample in a spatially localized manner, which may lead to many applications like 2D and 3D data storage, anti-counterfeiting protection, novel design bio-sensors and, potentially, to fabrication of metamaterials, 3D photonic crystals or plasmonic devices. Here we demonstrate irreversible spatially confined infrared-laser-induced annealing (LIA) achieved in a thin layer of dried colloidal solution of ultra-small ∼8 nm NaYF4 nanocrystals (NCs) co-doped with 2% Er³âº and 20% Yb³âº ions under a localized tightly focused beam from a continuous wave 976 nm medium power laser diode excitation. The LIA results from self-heating due to non-radiative relaxation accompanying the NIR laser energy upconversion in lanthanide ions. We notice that localized LIA appears at optical power densities as low as 15.5 kW cm⁻² (∼354 ± 29 mW) threshold in spots of 54 ± 3 µm diameter obtained with a 10 × microscope objective. In the course of detailed studies, a complete recrystallization to different phases and giant 2-3 order enhancement in luminescence yield is found. Our results are highly encouraging and let us conclude that the upconverting ultra-small lanthanide-doped nanophosphors are particularly promising for direct laser writing applications.

Higher order mode conversion via focused ion beam milled Bragg gratings in Silicon-on-Insulator waveguides.

We report the first Bragg gratings fabricated by Focused Ion Beam milling in optical waveguides. We observe striking features in the optical transmission spectra of surface relief gratings in silicon-on-insulator waveguides and achieve good agreement with theoretical results obtained using a novel adaptation of the beam propagation method and coupled mode theory. We demonstrate that leaky Higher Order Modes (HOM), often present in large numbers (although normally not observed) even in nominally single mode rib waveguides, can dramatically affect the Bragg grating optical transmission spectra. We investigate the dependence of the grating spectrum on grating dimensions and etch depth, and show that our results have significant implications for designing narrow spectral width gratings in high index waveguides, either for minimizing HOM effects for conventional WDM filters, or potentially for designing devices to capitalize on very efficient HOM conversion.

Synthesis, structure, and optical-limiting properties of heterobimetallic [M(3)CuS(4)] cuboidal clusters (M = Mo or W) with terminal phosphine ligands.

Cubane-type clusters of formula [Mo(3)CuS(4)Cl(4)(dmpe)(3)](PF(6)) (4), [Mo(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (5), and [W(3)CuS(4)Br(4)(dmpe)(3)](PF(6)) (6) have been prepared by reacting the incomplete cuboidal trimers [Mo(3)S(4)Cl(3)(dmpe)(3)](PF(6)) (1), [Mo(3)S(4)Br(3)(dmpe)(3)](PF(6)) (2), and [W(3)S(4)Br(3)(dmpe)(3)](PF(6)) (3), respectively, with CuX (X = Cl or Br) or the mononuclear copper complex [Cu(CH(3)CN)(4)](+) in THF. The reaction takes place without global changes in the metal oxidation states, and compounds 4-6 with a [M(3)CuS(4)](5+) core possess 16 e(-) for metal-metal bonding. X-ray structural analysis of 4-6 revealed an effective C(3v) symmetry for the M(3)Cu unit with the M-M distances being statistically the same for M = Mo or W. However, the M-Cu distance is 0.04 and 0.1 A longer than the M-M bond length for Mo and W, respectively. There is no significant structural rearrangement of the ligand-metal bonding in proceeding from [M(3)S(4)X(3)(dmpe)(3)](+) to [M(3)CuS(4)X(4)(dmpe)(3)](+). The cyclic voltammograms of the [Mo(3)CuS(4)] cubane clusters show one quasi-reversible reduction process at E(1/2) = -0.31 V for 4 and at E(1/2) = -0.23 V for 5 and one irreversible reduction at -0.69 and -0.58 V for 4 and 5, respectively. The tungsten cluster 6 shows a unique quasi-reversible reduction wave at E(1/2) = -0.71 V. The incorporation of copper into the incomplete [M(3)S(4)] cuboidal complexes produces a decrease of the reduction potential for both molybdenum and tungsten. Absorption spectra of 1-6 are broadly similar; replacing Mo by W in proceeding from 2 to 3 or from 5 to 6 and replacing Br by Cl in proceeding from 2 to 1 or from 5 to 4 results in a blue shift of the three UV-visible absorption bands. All six clusters exhibit optical limiting, as measured by the Z-scan technique at 523 nm using 40 ns pulses. The power-limiting mechanism remains obscure, but under the conditions employed, threshold-limiting fluence decreases on replacing W by Mo on proceeding from 3 to 2 or 6 to 5 and on proceeding from tetranuclear cluster (4-6) to trinuclear precursor (1-3, respectively). For all six clusters, values of the excited-state cross section sigma(eff) are larger than those of the corresponding ground-state cross section sigma(0); i.e., all clusters are efficient optical limiters.

Dynamics of light-induced reflectivity switching in gallium films deposited on silica by pulsed laser ablation.

We present what is to our knowledge the first experimental study of light-induced reflectivity changes at an alpha-Ga/Si interface irradiated by femtosecond and picosecond laser pulses. After exposure, the reflectivity can increase from R?0.55 , which is typical for alpha-Ga , to R?0.8 , which is close to that of liquid Ga. The initial step in the reflectivity change of 2-4 ps is resolved with 150-fs laser pulses. The light-induced reflectivity change relaxes during 100ns-10 mus , depending strongly on the background temperature of the Ga mirror and the laser fluence.

Saturable absorption in poly(indenofluorene): a picket-fence polymer.

Poly(indenofluorene) shows a strong degenerate four-wave mixing (DFWM) response when it is excited with 100-fs pulses at 800 nm. The DFWM signal scales with the 1.5 power of the input intensity, which we interpret as being due to absorption saturation phenomena. The saturation was studied by open-aperture Z scan in dilute solutions of poly(indenofluorene) in chloroform. The changes in the absorption coefficient alpha are described by the formula alpha = alpha(0)/[1 + (I/I(sat))(1/2)], where I(sat) is the saturation intensity, which is found to be of the order of 100>MW/cm(2).

Tuning the properties of poly(p-phenylenevinylene) for use in all-optical switching.

We show that, as the conjugation length of poly(p-phenylenevinylene) increases, the ratio of its two-photon absorption coeff icient to third-order nonlinearity at 800 nm decreases, rendering the material suitable for alloptical switching.