Dam Thuy Trang Nguyen

One-step fabrication of submicrostructures by low one-photon absorption direct laser writing technique with local thermal effect

In this work, local thermal effect induced by a continuous-wave laser has been investigated and exploited to optimize the low one-photon absorption (LOPA) direct laser writing (DLW) technique for fabrication of polymer-based microstructures. It was demonstrated that the temperature of excited SU8 photoresist at the focusing area increases to above 100 °C due to high excitation intensity and becomes stable at that temperature thanks to the use of a continuous-wave laser at 532 nm-wavelength. This optically induced thermal effect immediately completes the crosslinking process at the photopolymerized region, allowing obtain desired structures without using the conventional post-exposure bake (PEB) step, which is usually realized after the exposure. Theoretical calculation of the temperature distribution induced by local optical excitation using finite element method confirmed the experimental results. LOPA-based DLW technique combined with optically induced thermal effect (local PEB) shows great advantages o...

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.

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.

LOPA-based direct laser writing of multi-dimensional and multi-functional photonic submicrostructures

We have recently developed a simple fabrication technique, called low one-photon absorption (LOPA) direct laser writing (DLW), to realize multi-dimensional and multi-functional polymer-based photonic submicrostructures. This technique employs a continuous-wave laser at 532 nm-wavelength with only few milliwatts and a simple optical setup, allowing to decrease the cost of the fabrication system by a factor of ten as compared to a commercial DLW system. In this report, we present various photonic structures, such as 2D and 3D micro- resonators, photonic and magnetic submicrostructures, and nonlinear optical structures fabricated by this LOPA- based DLW method. We also discuss about potential applications of those fabricated multi-dimensional and multi-functional photonic submicrostructures in opto-electronics, bio, as well as in opto-mechanics.

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.

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.