Mai Hoang Luong

Fabrication and Characterization of Large-Area Unpatterned and Patterned Plasmonic Gold Nanostructures

We report fabrication of Au nanoisland films on different substrates by thermally annealing a sputtered Au nanolayer and investigation of their structure, morphology, and optical properties. It was found that high-temperature annealing leads to transformation of the initial, continuous film into the forms of hillock and isolated island film. The final nanoisland films exhibit remarkably enhanced and localized plasmon resonance spectra with respect to the original sputtered film. The strong dependence of the resonance band spectra of the resulting structures on the annealing temperature and supporting substrate is presented and analyzed, suggesting that both of these factors could be used to tune the optical spectroscopic properties of such structures. Moreover, we propose and demonstrate a novel and effective approach for fabrication of patterned Au structures by thermally annealing the Au layer deposited onto modulated-surface substrates. The experimental results indicate that this method could become a promising approach for manufacturing plasmonic array structures, which have been extensively investigated and widely applied in many fields.

Direct laser writing of polymeric nanostructures via optically induced local thermal effect

We demonstrate the fabrication of desired structures with feature size below the diffraction limit by use of a positive photoresist. The direct laser writing technique employing a continuous-wave laser was used to optically induce a local thermal effect in a positive photoresist, which then allowed the formation of solid nanostructures. This technique enabled us to realize multi-dimensional sub-microstructures by use of a positive photoresist, with a feature size down to 57 nm. This mechanism acting on positive photoresists opens a simple and low-cost way for nanofabrication.

Study of all-polymer-based waveguide resonant gratings and their applications for optimization of second-harmonic generation

We investigated theoretically and experimentally the optical properties of all-polymer-based one-dimensional waveguide resonant gratings (WRGs) and their important applications for the optimization of second-harmonic generation (SHG). We first studied the basic theory of the resonant modes of a simple grating-coupled waveguide realized on a material possessing a low refractive index contrast. The optical properties of any WRG were numerically simulated by using the finite-difference time domain method, performed by commercial Lumerical software. The polymer-based surface relief gratings were fabricated on azopolymer Disperse Red 1-Poly-Methyl-Methacrylate (DR1–PMMA) thin films by using the two-beam interference method and mass transport effect. Their experimental reflection spectra measured as a function of incident light wavelength are in good agreement with the theoretical predictions. We then demonstrated a first application of such a polymer-based WRG for nonlinear optics. Thanks to the strong local electrical field in the WRG, due to a guided-mode resonance condition, the SHG signal of an infrared light beam was strongly enhanced by a factor of 25 as compared to the result obtained in a sample without a grating.

Realization of Desired Plasmonic Structures via a Direct Laser Writing Technique

We present a recent investigation of fabrication of desired plasmonic structures. First, the polymeric templates were realized by a simple and low-cost fabrication technique based on direct laser writing with a continuous-wave laser source. The plasmonic structures have been then realized by two methods, namely, a combination of gold evaporation and lift-off techniques, and a combination of gold sputtering and thermal annealing techniques. Each method presents its own advantages. Numerous metallic submicro- and nano-structures have been realized, which should be very interesting for different applications, such as high-transmission bandpass filters, plasmonic data storage, and plasmonic photonic devices.

Micro and nanostructuration of polymer materials and applications

Polymer materials offer unique opportunities in nanophotonics and nanobiosystems since both top-down and bottom-up strategies can be pursued and combined towards the nanoscale. Besides, polymer materials can be, with simple methods, functionalized with nonlinear optical or fluorescent materials (organic, inorganic, or metal). The ensemble can be optically structured in a flexible way to obtain a polymer-based photonic nanostructure (host) containing active materials (guest), which may provide an enhancement of the guest optical response, leading to attractive applications. We have developed two important fabrication techniques, namely interference and one-photon absorption direct laser writing, which present different advantages and allow both to obtain desired micro and nanometric 2D and 3D structures. These polymer-based structures are promising for many potential applications, for example, laser, nonlinear optics, and plasmonics.

Direct Laser Writing of Gold Nanostructures: Application to Data Storage and Color Nanoprinting

We have demonstrated a simple way to fabricate gold islands by using the laser direct writing method. The gold nano-islands were formed by the optically induced local thermal effect, resulting from the excitation of a continuous-wave laser beam on gold layer. By moving the laser focusing spot, it was possible to write desired plasmonic texts and images on demand at a nanoscale. By changing the exposure dose, the size of gold nano-islands was controlled, resulted in different colors. This demonstration paves the way for many interesting applications, such as plasmonic data storage and nano-scaled color printing.

Direct laser coding of plasmonic nanostructures for data storage applications

We demonstrate the realization of desired gold plasmonic nanostructures by direct laser writing (DLW) method employing a continuous-wave 532 nm laser. By moving the laser focusing spot of the DLW system, the gold nanoparticles (NPs) are formed following the laser moving path. The plasmonic patterns composing of gold NPs enable a lot of applications, such as direct text writing and inverse text writing at micro scale, bar code and QR code to store the messages, binary recordings as compared to standard CDs and DVDs. Furthermore, by controlling the laser intensity and/or the exposure time, the NPs size was changed resulting in different plasmonic colors, and offering a new method to nanoprint with color.