Laser nano-filament explosion for enabling open-grating sensing in optical fibre.

Embedding strong photonic stopbands into traditional optical fibre that can directly access and sense the outside environment is challenging, relying on tedious nano-processing steps that result in fragile thinned fibre. Ultrashort-pulsed laser filaments have recently provided a non-contact means of opening high-aspect ratio nano-holes inside of bulk transparent glasses. This method has been extended here to optical fibre, resulting in high density arrays of laser filamented holes penetrating transversely through the silica cladding and guiding core to provide high refractive index contrast Bragg gratings in the telecommunication band. The point-by-point fabrication was combined with post-chemical etching to engineer strong photonic stopbands directly inside of the compact and flexible fibre. Fibre Bragg gratings with sharply resolved π-shifts are presented for high resolution refractive index sensing from [Formula: see text] = 1 to 1.67 as the nano-holes were readily wetted and filled with various solvents and oils through an intact fibre cladding.

Inhibition and enhancement of linear and nonlinear optical effects by conical phase front shaping for femtosecond laser material processing.

The emergence of high-powered femtosecond lasers presents the opportunity for large volume processing inside of transparent materials, wherein a myriad of nonlinear optical and aberration effects typically convolves to distort the focused beam shape. In this paper, convex and concave conical phase fronts were imposed on femtosecond laser beams and focussed into wide-bandgap glass to generate a vortex beam with tuneable Gaussian-Bessel features offset from the focal plane. The influence of Kerr lensing, plasma defocussing, and surface aberration on the conical phase front shaping were examined over low to high pulse energy delivery and for shallow to deep processing tested to 2.5 mm focussing depth. By isolating the underlying processes, the results demonstrate how conical beams can systematically manipulate the degree of nonlinear interaction and surface aberration to facilitate a controllable inhibition or enhancement of Kerr lensing, plasma defocussing, and surface aberration effects. In this way, long and uniform filament tracks have been generated over shallow to deep focussing by harnessing surface aberration and conical beam shaping without the destabilizing Kerr lensing and plasma defocussing effects. A facile means for compressing and stretching of the focal interaction volume is presented for controlling the three-dimensional micro- and nano-structuring of transparent materials.

Ultrafast laser burst-train filamentation for non-contact scribing of optical glasses.

A systematic study of glass scribing is presented on the benefits of ultrafast laser burst trains in generating filamentation tracks to guide cleaving of glass substrates. The interplay of Kerr self-focusing, plasma defocusing, and burst-train accumulation effects in filament formation was characterized by time-resolved in-situ microscopic imaging. Various filament-track scribing geometries were compared with and without assistance from burst-train pulse delivery or surface V-groove ablation. The cleaving guidance and reproducibility were examined together with the breaking force, facet morphology and flexural strength of cleaved substrates to assess the overall scribing and cleaving quality. The reported results attest to the benefits and flexibility of burst-mode ultrafast laser interactions to assist cleaving of optically transparent materials along well formed filament arrays.

Femtosecond laser filaments for rapid and flexible writing of fiber Bragg grating.

A new beam delivery method is introduced for controlling filament formation in optical fiber that enables point-by-point writing of 1st order fiber Bragg gratings (FBGs) with single femtosecond laser pulses. Uniform filament tracks with azimuthal symmetry were formed fully through the 9.3 µm core waveguide by a modified immersion focusing method to eliminate astigmatism by the cylindrical fiber shape. Filament arrays were precisely assembled inside of single-mode fiber, generating strong FBG resonances in the telecommunication band. Laser exposure control within this unique thin-grating geometry were key to manipulating the relative strength of the Bragg and cladding mode resonances while also independently tailoring their spectral resolution and features. This filament-by-filament writing rapidly forms gratings with highly flexible pattern control to tune wavelength, or introduce optical defects, demonstrated by a π-shifted FBG having a sharp 25 pm resonance embedded within a broader Bragg peak.

Surface plasmon resonance sensing properties of a 3D nanostructure consisting of aligned nanohole and nanocone arrays.

Molecular surface plasmon resonance (SPR) sensing is one of the most common applications of an array of periodic nanoholes in a metal film. However, metallic nanohole arrays (NHAs) with low-hole count have lower resolution and SPR sensing performance compared to NHAs with high-hole count. In this paper, we present a compact three-dimensional (3D) plasmonic nanostructure with extraordinary optical transmission properties benefiting from surface plasmon matching and enhanced localized surface plasmon coupling. The 3D nanostructure consisted of a gold film containing a NHA with an underlying cavity and a gold nanocone array (NCA) at the bottom of the cavity. Each nanocone was aligned with the nanohole above and the truncated apex of each nanocone was in close proximity (100 nm) to the gold film. The NHA-NCA structures outperformed conventional NHA structures in terms of bulk sensitivity and Figure of Merit (FOM). Furthermore, the NHA-NCA structure with 525 nm periodicity was capable of sensing streptavidin down to 2 nM exhibiting a 10-fold increase in streptavidin sensitivity compared to conventional NHA structures. The sensitivity and performance of the 3D nanostructure can be further improved by exploiting multiplexing methods in combination with stable light sources and detection systems.

Large area periodic, systematically changing, multishape nanostructures by laser interference lithography and cell response to these topographies.

The fabrication details to form large area systematically changing multishape nanoscale structures on a chip by laser interference lithography (LIL) are described. The feasibility of fabricating different geometries including dots, ellipses, holes, and elliptical holes in both x- and y- directions on a single substrate is shown by implementing a Lloyd's interferometer. The fabricated structures at different substrate positions with respect to exposure time, exposure angle and associated light intensity profile are analyzed. Experimental details related to the fabrication of symmetric and biaxial periodic nanostructures on photoresist, silicon surfaces, and ion milled glass substrates are presented. Primary rat calvarial osteoblasts were grown on ion-milled glass substrates with nanotopography with a periodicity of 1200 nm. Fluorescent microscopy revealed that cells formed adhesions sites coincident with the nanotopography after 24 h of growth on the substrates. The results suggest that laser LIL is an easy and inexpensive method to fabricate systematically changing nanostructures for cell adhesion studies. The effect of the different periodicities and transition structures can be studied on a single substrate to reduce the number of samples significantly.