Kock Khuen Seet

Two-photon lithography of nanorods in SU-8 photoresist

Studies on two-photon lithography in negative SU-8 photoresist demonstrate the possibility of obtaining mechanically stable, stress-free, extended nanorods having lateral sizes of about 30 nm (corresponding to λ/25 resolution). The high resolution achievable with the given combination of materials and fabrication techniques demonstrates its potential for the fabrication of large-scale nanostructures, such as photonic crystals with photonic stop gaps at visible wavelengths.

Feature-size reduction of photopolymerized structures by femtosecond optical curing of SU-8

The universal scaling of a width of photopolymerized line on the exposure dose was observed in polymerization by direct laser writing using tightly focused femtosecond pulses in SU-8 resist. This scaling can be explained as the photopolymerization by a blackbody-type emission. A spectrally broad thermal emission of electrons heated up to temperatures approximately Te∼103K coincides with an IR-absorption band of SU-8 centered at the 2.9μm and had caused polymerization by a cumulative direct absorption. Three-dimensional photonic crystal templates with approximately twice reduced feature size were fabricated with stop band at the fiber communication wavelength of 1.3μm.

Three-dimensional woodpile photonic crystal templates for the infrared spectral range

High-quality templates of three-dimensional woodpile photonic crystals are fabricated in photoresist SU-8 by use of femtosecond laser lithography. The samples have smooth surfaces, are mechanically stable, and are resistant to degradation under environmental and chemical influences. Fundamental and higher-order photonic stopgaps are identified in the wavelength range 2.0-8.0 microm. These templates can be used for subsequent infiltration by optically active or high-refractive-index materials.

Three-dimensional horizontal circular spiral photonic crystals with stop gaps below 1μm

Three-dimensional photonic crystals with a circular spiral architecture were fabricated by direct laser writing (DLW) in a photoresist. DLW was performed with a laser beam having the direction of propagation and elongation of the ellipsoidal focal region perpendicular to the spirals’ orientation. This allowed the reduction of the turning period of the spirals while avoiding the overlap between their adjacent turns. Consequently, optical transmission and reflection spectra of the fabricated samples revealed multiple photonic stop gaps whose shortest wavelength was 0.88μm, the lowest observed so far in spiral structures. These results were qualitatively reproduced by finite-difference time-domain simulations. This fabrication scheme can be useful for DLW of structures where photonic stop gaps along particular directions, rather than complete photonic band gaps, are required.

Mechanical properties and tuning of three-dimensional polymeric photonic crystals

Mechanical properties of photopolymerized photonic crystal (PhC) structures having woodpile and spiral three-dimensional architectures were examined using flat-punch indentation. The structures were found to exhibit a foamlike response with a bend-dominated elastic deformation regime observed at strain levels up to 10%. Numerical simulations of optical properties of these PhC structures demonstrate the possibility of achieving a substantial and reversible spectral tuning of the photonic stop gap wavelength by applying a mechanical load to the PhC.

Templating and Replication of Spiral Photonic Crystals for Silicon Photonics

This paper describes femtosecond laser lithography of 3-D photonic crystal templates in commercial photoresist SU-8 and replication of these templates with silicon. Using this approach, silicon-based photonic crystals having 3-D square spiral architecture and exhibiting photonic stop gaps near the 2.5- mum wavelength were fabricated. Possibilities to use a multiple-beam interference technique for two-photon absorption templating of photonic crystals are explored.

Nanofabrication by direct laser writing and holography (Invited Paper)

The physical principles underlying a three-dimensional (3D) laser microstructuring technique are outlined, its applications for the fabrication of 3D (nano)micro-structured materials are presented. The direct laser writing and holographic recording in SU-8 photoresist are described. The limits of the lateral and axial light localization of a Gaussian pulse/beam at the focus are derived for the multi-photon absorption taking into account the threshold of photomodification. Prospective holographic patterns formed by interference of circularly and linearly polarized beams are discussed.

Three-dimensional recording inside dielectrics for photonic applications

Three-dimensional (3D) structuring of polymers by tightly focused femtosecond laser pulses and interference is demonstrated. Photo-structuring by the tightly focused sub-1 ps laser pulses realizes a direct laser writing inside dielectric materials without self-focusing. Photo-polymerization of 3D photonic crystal templates with photonic stop bands in IR-spectral region has been achieved. Holographic photo-polymerization of photonic crystal templates with counter-propagating beams was numerically modeled. Three-dimensional structures with an axial period smaller than the lateral can be recorded using the counter-propagating beam alignment. Photo-polymerization of large-area polymeric structures with a high surface-to-volume ratio is demonstrated.

Femtosecond laser microfabrication of 3D photonic structures

The physical principles underlying the three-dimensional (3D) laser microstructuring technique are outlined, its applications for the fabrication of 3D photonic crystals are described. Structures recorded by direct laser writing and holographic recording techniques in SU-8 photoresist are presented along with their structural and optical characterization.

Laser microfabrication of three-dimensional photonic crystal templates in polymers

Three-dimensional woodpile photonic crystal structures were recorded in films of polymeric photoresists polymethylcrylate (PMMA) and SU-8 films by femtosecond laser microfabrication technique. The fabricated structures consist of periodic arrays of extended elliptical void-channels in PMMA, and of solid elliptical rods of cross-linked polymer in SU-8. Their lattice periods of several micrometers and typical sub-micrometric feature sizes allowed to confirm presence of pronounced photonic stop-bands at infrared wavelengths around 2.0 - 8.0 μm.