Hoang Minh Ngo

Controlled coupling of a single nanoparticle in polymeric microstructure by low one-photon absorption—based direct laser writing technique

We investigated the coupling of a single nanoparticle (NP) into a polymer-based photonic structure (PS). The low one-photon absorption microscopy with a two-step technique allowed us first to accurately determine the location of a NP and then to embed it as desired into an arbitrary PS. The coupling of a gold NP and a polymer-based PS was experimentally investigated showing a six-fold photon collection enhancement as compared to that of a NP in unpatterned film. The simulation results based on finite-difference time-domain calculation method confirmed this observation and showed a 2.86-fold enhancement in extraction efficiency thanks to the NP/PS coupling.

Non-linear optical, electrochemical and spectroelectrochemical properties of amphiphilic inner salt porphyrinic systems

Three zwitterionic meso-substituted A3B- and AB2C-porphyrins containing one sulfonato alkylpyridinium substituent and three or two alkoxy-substituted phenyl groups were synthesized in good yield and fully characterized as to their physicochemical properties by a variety of techniques. This new series of inner salt donor-acceptor meso-substituted porphyrin derivatives were prepared for possible application as amphiphilic probes for membrane insertion in the area of combined second-harmonic and two-photon fluorescence cellular microscopy. To this end, the linear and nonlinear optical properties of the compounds were characterized, together with their electrochemical and spectroelectrochemical properties in non-aqueous media. The neutral design of such molecules enabled us to determine their second order non-linear properties, both by Electric Field Induced Second Harmonic Generation and Hyper–Rayleigh Scattering. Two-photon absorption cross sections of these dyes were also measured by the two-photon induced fluorescence method. The zwitterionic nature of the inner salt results in very specific solvent-dependent redox-properties, which could be rationalized in terms of solvent-dependent ion-pairing. The overall data electrochemical and photophysical data indicates that these new porphyrinic systems should be good probes for membrane potential sensing.

Deterministic embedding of a single gold nanoparticle into polymeric microstructures by direct laser writing technique

We have precisely positioned and embedded a single gold nanoparticle (Au NP) into a desired polymeric photonic structure (PS) using a simple and low-cost technique called low one-photon absorption direct laser writing (LOPA DLW), with a two-step process: identification and fabrication. First, the position of the Au NP was identified with a precision of 20 nm by using DLW technique with ultralow excitation laser power (μW). This power did not induce the polymerization of the photoresist (SU8) due to its low absorption at the excitation wavelength (532 nm). Then, the structure containing the NP was fabricated by using the same DLW system with high excitation power (mW). Different 2D photonic structures have been fabricated, which contain a single Au NP at desired position. In particular, we obtained a microsphere instead of a micropillar at the position of the Au NP. The formation of such microsphere was explained by the thermal effect of the Au NP at the wavelength of 532 nm, which induced thermal polymerization of surrounding photoresist. The effect of the post-exposure bake on the quality of structures was taken into account, revealing a more efficient fabrication way by exploiting the local thermal effect of the laser. We studied further the influence of the NP size on the NP/PS coupling by investigating the fabrication and fluorescence measurement of Au NPs of different sizes: 10, 30, 50, 80, and 100 nm. The photon collection enhancements in each case were 12.9 ± 2.5, 12.6 ± 5.6, 3.9 ± 2.7, 5.9 ± 4.4, and 6.6 ± 5.1 times, respectively. The gain in fluorescence could reach up to 36.6 times for 10-nm gold NPs.

Second harmonic generation from silver nanoparticles in aqueous solution with different protective agents

Nanometer-sized metallic colloidal particles with plasmonic resonances in the visible range are widely investigated for their attractive optical properties as sensors, for imaging and cancer treatment. Their second-order nonlinear optical properties are remarkably high. In this work, silver colloidal solutions have been synthesized by a simple and quick method in aqueous solutions with different protective agents (PVA, PVP). The first hyperpolarizability β values of Ag per atom and per particle for nanospheres at 1064 nm have been measured. Silver nanoparticles, which possess intense visible region surface plasmon absorption bands, prove to be excellent nonlinear scatterers.