Frederic Chaput

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about t...

Near-field optical patterning on azo-hybrid sol–gel films

We report on the near-field optical patterning of photochromic sol–gel films with subwavelength resolution. The sample containing functionalized azobenzene species is locally illuminated in the visible absorption band of these photochromes through the aperture of a metallized tapered optical fiber. The surface topography imaged by in situ shear-force microscopy reveals that, due to repeated photoisomerization cycles of the azobenzene molecules, photoinduced matter migration occurs under the tip leading to the formation of a surface relief. The shape of this structure is characteristic of the electromagnetic field distribution and strongly depends on the tip-to-sample distance. In near-field illumination conditions, protrusions of lateral dimension as small as 60 nm (≈λ/10) are currently produced. When repeating this process, compact arrays of nanodots are optically inscribed.

Second-order optical nonlinearities of azo chromophores covalently attached to a sol-gel matrix

Optically nonlinear disperse red 1 (DR1) was covalently bound into a silica gel network by the coupling of DR1 and 3-isocyanatopropyltriethoxysilane. Organic-inorganic hybrid materials present a large resonant second-order susceptibility (d33 = 55 pm/V for a fundamental wavelength of 1.06 μm) with an excellent room-temperature stability.

Stable photorefractive memory effect in sol-gel materials

We report on the synthesis and the optical characterization of a photorefractive sol-gel material which contains nonlinear azo chromophore and carbazole charge transporting molecules. Both of these functional groups are covalently attached to the silica based backbone. Thin sol-gel films exhibit stable optically nonlinear properties: the electro-optic coefficient r13 measured at 633 nm is found to be 17 pm/V one day after corona poling and 15 pm/V after three weeks and over a period of several months. Photorefractivity is demonstrated by two beam coupling experiments without external applied electric field. Sol-gel films present a stable photorefractive memory effect with a net internal gain of 200 cm−1.

Reverse saturable absorption in palladium and zinc tetraphenyltetrabenzoporphyrin doped xerogels

Abstract Solid-state inorganic matrices (xerogels) doped with tetraphenyltetrabenzoporphyrins of Pd and Zn (PdTPTBP and ZnTPTBP) have been prepared using the sol-gel process. Nonlinear reverse saturable absorption was observed for both materials when they were illuminated with nanosecond laser pulses at 532 nm. PdTPTBP doped xerogels exhibit nonlinear activation thresholds of about 10 mJ/cm 2 , which is much lower than the value of 80 mJ/cm 2 obtained under similar conditions for Al phthalocyanine chloride. The experimental results are discussed using classical 4 energy level diagrams.

Novel photorefractive sol-gel materials

We have developed new photorefractive media based on hybrid organic-inorganic materials containing a charge transporting (CT) molecule either as side-chain or main-chain substituents on the silica backbone. Second order nonlinear optical (NLO) chromophores were introduced either as side chain or as guest units. These materials were prepared by the sol-gel process in the form of thin films of a few μm-thick. NLO and photorefractive properties have been evaluated using electro-optic measurements, two beam coupling experiments and photoconductivity measurements.

Sol-gel hosts doped with porphyrin derivatives. Part I. Spectroscopy, hole-burning and spectral diffusion

Abstract Pure inorganic sol-gel matrices as well as hybrid organic/inorganic xerogels have been doped with porphyrins derivatives and studied using line narrowing techniques. The role of residual hydroxyl groups is investigated. Free-base porphyrins are protonated in pure inorganic hosts, but the matrix acidity is reduced in hybrid matrices or when fluorinated porphyrins derivatives are used. The linear electron-phonon coupling can be controlled with the choice of the organic group in organic/inorganic matrices. Persistent spectral hole widths increase with temperature according a glass-like T n dependence and evidence of spectral diffusion is shown in one of these systems.

All-optical gel memory

We have studied the behavior of organic molecules (Rhodamine 640) that are encaged in a matrix synthesized by the sol-gel process. By applying a strong optical-polarized electric field, we can control the alignment of the molecules within the sample and locally create a birefringent effect. Using this sample in a Kerr experiment, we obtained a transmission ratio of 6%. Since the birefringent effect lasts at least several days, this type of sample could be used in an optical process for storage of information. Moreover, we show that such information can be deleted and rewritten repetitively

High temperature spectral hole burning on porphyrins grafted to sol-gel matrices

Protoporphyrin IX is grafted by two chemical bonds to a rigid Si-O-Si network prepared by using the sol-gel route. Efficient hole burning is observed in hybrid organic-inorganic matrices. Persistent spectral holes are still burnt at 120 K.

Perylene, Pyrromethene and Grafted Rhodamine Doped Xerogels for Tunable Solid State Laser

Organic dye molecules have been widely used in solution as amplifying media in tunable lasers. The possibility of trapping these molecules in a solid state matrix may lead to a new field of application outside the laboratory. Many types of matrices can be used. We present results with new hybrid organic/inorganic xerogels prepared by hydrolysis-condensation of the methyl- or vinyl-triethoxysilane precursor under acid-catalyzed hydrolysis and basic-catalyzed condensation. Organic methyl or vinyl groups attached to the silica network provide a favorable environment to dye fluorescence. We have studied the effect induced by using different basic catalysts: N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and 3- aminopropyltriethoxysilane are found to lead to the best matrix characteristics. Perylenes and pyrromethenes were chosen because of their thermal- and photo-stability. After gelation, 10 mm thick samples, polished to a surface roughness of about 4 nm, are placed in a plano- concave cavity as gain media and pumped by a frequency doubled nanosecond Q:switched Nd:YAG laser. Efficiency is the greatest with a pyrromethene 597 doped sample: more than 6 mJ output energy for a 10 mJ pump energy (2 J/cm2 fluence) leading to a slope efficiency of 63%. The lifetime of the laser emission when the same point of the sample is sequentially used is also an important characteristic. Best results are obtained with perylene red dye: for a pump energy of 0.7 mJ at 1 Hz repetition rate, the output energy is still 50% of the initial value (0.1 mJ) after 80,000 shots. For a pyrromethene 597 doped MTEOS sample, it appears that the measured lifetimes are independent of the operating frequency in the 0.2 - 20 Hz range, indicating a reasonable heat conductivity for this type of matrix. We have also studied tunability by adding a grating or prism to the cavity: a typical tunability of over 50 nm or more is obtained with many of these dyes. Finally we report laser activity obtained with rhodamine B doped xerogels in which the dye molecules were covalently bonded to the silica network in order to stabilize the organic molecule structure.