A simple technique to overcome self-focusing, filamentation, supercontinuum generation, aberrations, depth dependence and waveguide interface roughness using fs laser processing

Enhancing Evanescent Wave Coupling of Near-Surface Waveguides with Plasmonic Nanoparticles

Evanescent field excitation is a powerful means to achieve a high surface-to-bulk signal ratio for bioimaging and sensing applications. However, standard evanescent wave techniques such as TIRF and SNOM require complex microscopy setups. Additionally, the precise positioning of the source relative to the analytes of interest is required, as the evanescent wave is critically distance-dependent. In this work, we present a detailed investigation of evanescent field excitation of near-surface waveguides written using femtosecond laser in glass. We studied the waveguide-to-surface distance and refractive index change to attain a high coupling efficiency between evanescent waves and organic fluorophores. First, our study demonstrated a reduction in sensing efficiency for waveguides written at their minimum distance to the surface without ablation as the refractive index contrast of the waveguide increased. While this result was anticipated, it had not been previously demonstrated in the literature. Moreover, we found that fluorescence excitation by waveguides can be enhanced using plasmonic silver nanoparticles. The nanoparticles were also organized in linear assemblies, perpendicular to the waveguide, with a wrinkled PDMS stamp technique, which resulted in an excitation enhancement of over 20 times compared to the setup without nanoparticles.

One-Stage Femtosecond Laser-Assisted Deposition of Gold Micropatterns on Dielectric Substrate

In this study, a simple one-stage laser-assisted metallization technique based on laser-induced backside wet etching and laser-induced chemical liquid-phase deposition is proposed. It allows for the fabrication of gold micropatterns inside the laser-written trace on a glass substrate. The reduction and deposition of gold inside and outside the laser-ablated channel were confirmed. The presence of Au nanoparticles on the surface of the laser-written micropattern is revealed by atomic force microscopy. The specific resistivity of the gold trace formed by ultrafast light-assisted metal micropatterning on a dielectric glass substrate is estimated as 0.04 ± 0.02 mΩ·cm. The obtained results empower the method of the selective laser-assisted deposition of metals on dielectrics and are of interest for the development of microelectronic components and catalysts, heaters, and sensors for lab-on-a-chip devices.

Modified astigmatic beam technique for laser writing

The ultrafast laser writing of optical waveguides and devices is increasingly ubiquitous among the photonics community, mostly for its flexibility and three-dimensional fabrication capability. The well-known astigmatic beam technique is the simplest method to inscribe near-circular cross-section waveguides. In this paper, we report on a significant enhancement to the widely used astigmatic beam technique that makes it more flexible and yields a more circular waveguide cross section. By simply superposing a long-focus lens before the laser inscription objective lens, we demonstrate that the normalized squared radial deviation from a perfectly circular waveguide cross section can be reduced to <4×10^-4, which is a significant improvement compared to >0.1 typically obtained using the standard astigmatic beam technique, or >0.7 with a Gaussian beam. The modified technique also makes it easy to use the full power delivered by the laser, which is not usually the case with the standard technique. A technique to optimize the waveguide shape prior to the inscription by in situ laser-induced plasma emission imaging is also discussed.

From near-UV to long-wave infrared waveguides inscribed in barium fluoride using a femtosecond laser

Depressed-cladding waveguides (DCWs) of various sizes were inscribed in barium fluoride, allowing single-mode operation in the entirety of its transmission window (λ=0.2-12µm). Using femtosecond laser pulses at 515 nm, type I laser modified tracks were overlapped to form circular waveguides, whose cross-sectional geometry and numerical aperture were tailored to accommodate 0.405, 2.85, and 10.6 µm light. The mode profile, propagation loss, refractive index profile, and numerical aperture of the optimized waveguides were analyzed and compared with theory. We particularly demonstrate the challenging inscription of a large DCW for single-mode operation at 10.6 µm with propagation loss of <0.63dB/cm.

Smartphone Screen Integrated Optical Breathalyzer

One third of fatal car accidents and so many tragedies are due to alcohol abuse. These sad numbers could be mitigated if everyone had access to a breathalyzer anytime and anywhere. Having a breathalyzer built into a phone or wearable technology could be the way to get around reluctance to carry a separate device. With this goal, we propose an inexpensive breathalyzer that could be integrated in the screens of mobile devices. Our technology is based on the evaporation rate of the fog produced by the breath on the phone screen, which increases with increasing breath alcohol content. The device simply uses a photodiode placed on the side of the screen to measure the signature of the scattered light intensity from the phone display that is guided through the stress layer of the Gorilla glass screen. A part of the display light is coupled to the stress layer via the evanescent field induced at the edge of the breath microdroplets. We demonstrate that the intensity signature measured at the detector can be linked to blood alcohol content. We fabricated a prototype in a smartphone case powered by the phone’s battery, controlled by an application installed on the smartphone, and tested it in real-world environments. Limitations and future work toward a fully operational device are discussed.

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.

Nonlinear increase, invisibility, and sign inversion of a localized fs-laser-induced refractive index change in crystals and glasses

Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers-Kronig relations. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications.

Influence of the spatial confinement on the self-focusing of ultrashort pulses in hollow-core fibers

The collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value. The complex self-focusing dynamics observed in the region below the critical power depends on the temporal evolution of the pulse and, also, on the interaction between the different spatial modes of the hollow-core fiber.