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.

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.

Direct-laser-written integrated mid-IR directional couplers in a BGG glass.

The development of coherent sources and other optical components for the mid-infrared has been hampered by the lack of sturdy materials that can withstand high power radiation or exposition to harsh environment. BGG glasses are robust materials transmitting over the 2.5-5 µm region. We report here the direct femtosecond laser fabrication of efficient directional couplers integrated in a BGG glass chip. The photonic components are characterized from 2.1 to 4.2 µm and compared to similar structures inscribed in silica glass samples. At 2.85 µm, a 99% relative cross transmission is reported in BGG glass. The experimental measurements are in good agreement with the coupled mode theory for wavelengths up to 3.5 µm.

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.

Direct Inscription of on-surface waveguides in polymers using a mid-ir fiber laser.

A detailed study of photo-inscribed optical waveguides in PMMA and polycarbonate using a mid-IR laser is presented. The wavelength of the laser is tuned near the absorption peaks of stretching C-H molecular bonds and the focused beam is scanned onto the surface of planar polymer samples. For the first time, we report the formation of optical waveguides in both polymers through resonant absorption of the laser beam. The optical properties of the waveguides were thoroughly assessed. An elliptic Gaussian mode is guided at the surface of both polymers. Insertion losses of 3.1 dB for a 30 mm long on-surface waveguide inscribed in PMMA were recorded. Such waveguides can interact with the external medium through evanescent coupling. As a proof of concept, the surface waveguides are used as highly sensitive refractometric sensors. An attenuation dynamical range of 35 dB was obtained for a liquid that matches the index of the PMMA substrate. Our results pave the way for large scale manufacturing of low cost biocompatible photonic devices.

Femtosecond laser writing of near-surface waveguides for refractive-index sensing.

Femtosecond laser writing of optical waveguides and components in glasses has been a remarkably growing research field during the last two decades. However, such laser- inscribed optical components were mostly written within the volume of the glass due to the unavoidable ablation that arises when the focal spot is approaching the glass surface. This has generally limited the interaction of light with the surrounding medium thus preventing sensing functionality. In this paper, we present the inscription of surface and near-surface silver based waveguides in a silver containing glass with no need for additional processing as it is the case for standard type I waveguides. In addition, an ultra-sensitive refractive index sensor in a 1 cm glass chip is obtained based on near-surface waveguides interacting with liquid droplets acting as top-layer on the glass surface. Remarkably, the device exhibits a novel double-wing feature that sharpens the response and enhances its sensitivity. Our results highlight the advantages of silver based waveguides paving the way towards further surface based sensors in fibers.

Femtosecond laser inscription of depressed cladding single-mode mid-infrared waveguides in sapphire.

Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515 nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850 nm and propagation loss of <0.37 dB/cm are reported for a 33 mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn=∼2.5×10-3) up to 1400°C.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides.

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

Direct laser writing of a new type of waveguides in silver containing glasses.

Direct laser writing in glasses is a growing field of research in photonics since it provides a robust and efficient way to directly address 3D material structuring. Generally, direct laser writing in glasses induces physical modifications such as refractive index changes that have been classified under three different types (Type I, II & III). In a silver-containing zinc phosphate glass, direct laser writing additionally proceeds via the formation of silver clusters at the periphery of the interaction voxel. In this paper, we introduce a novel type of refractive index modification based on the creation of the photo-induced silver clusters allowing the inscription of a new type of optical waveguides. Various waveguides as well as a 50-50 beam splitter were written inside bulk glasses and characterized. The waveguiding properties observed in the bulk of such silver-containing glass samples were further transposed to ribbon shaped fibers made of the same material. Our results pave the way for the fabrication of 3D integrated circuits and fiber sensors with original fluorescent, nonlinear optical and plasmonic properties. The universality of these new findings should further extend in any silver-containing glasses that show similar laser-induced behavior in terms of silver cluster production.

Glass interposer for short reach optical connectivity.

We propose a glass interposer containing femtosecond laser-scribed waveguides to interconnect silicon photonic chips. The glass interposer has an insertion loss of about 1.5 dB/cm, and simplifies alignment of silicon photonic chips. Our experiment shows that the insertion loss for the grating coupler/inscribed glass interface was only 0.5 dB higher than the estimated coupling loss of grating coupler to SMF. The 3 dB coupling degradation occurs after 5 µm of in-plane displacement between the laser-inscribed waveguide and the grating coupler.

Femtosecond laser direct inscription of surface skimming waveguides in bulk glass.

We present a detailed study of waveguide inscription near the surface of bulk glass using a femtosecond laser. Three silicate glasses used extensively as hosts for photo-induced photonic devices were examined. Our results show that near-surface waveguides generally present a low-index contrast, as the pulse energy damage threshold decreases sharply at close proximity to the surface. We devised a novel method to allow the formation of optical waveguides that exhibit a high-index contrast up to the surface of any transparent material. As a proof of concept, the inscription of near-surface single-mode waveguides operating at a wavelength of 405 nm is demonstrated.

Refractive index and dispersion control of ultrafast laser inscribed waveguides in gallium lanthanum sulphide for near and mid-infrared applications.

The powerful ultrafast laser inscription technique is used to fabricate optical waveguides in gallium lanthanum sulphide substrates. For the first time the refractive index profile and the dispersion of such ultrafast laser inscribed waveguides are experimentally measured. In addition the Zero Dispersion Wavelength of both the waveguides and bulk substrate is experimentally determined. The Zero Dispersion Wavelength was determined to be between 3.66 and 3.71 µm for the waveguides and about 3.61 µm for the bulk. This work paves the way for realizing ultrafast laser inscribed waveguide devices in gallium lanthanum sulphide glasses for near and mid-IR applications.

Refractive index-modified structures in glass written by 266nm fs laser pulses.

We demonstrate the inscription of embedded waveguides, anti-waveguides and Bragg gratings by use of intense femtosecond (fs) UV laser pulses at 266nm in pure fused silica, and for the first time, in bulk fused quartz and ZBLAN glasses. The magnitude of induced index changes, depends, besides pulse energy and translation speed, largely on writing depth and varies from ~10(-4) for smooth modifications to ~10(-3) for damaged structures. The obtained results are promising as they present the feasibility of fabrication of short (< 0.2µm) period first-order fiber Bragg gratings (FBGs) for applications such as in realization of all-fiber lasers operating at short wavelengths.

Self and forced periodic arrangement of multiple filaments in glass.

The formation of permanent periodic structural changes in fused silica induced by the multifilamentation process was investigated. A cylindrical lens was used to focus 800 nm 50 fs pulses with 0.5 - 3 mJ energy down to a line, resulting in a quasi-periodic linear self-arrangement of multiple filaments (MF). The quasi-period of multiple filaments is shown to be uniquely defined by the critical power of the material and the peak intensity on the sample entrance surface. A novel technique to control this spatial self-arrangement of MF is demonstrated based on the use of a binary phase mask. This technique allowed us to decrease the relative variation of spacing between the adjacent tracks of refractive index modifications by a factor of 4 as compared with the case without the phase mask. 3D + time numerical simulations qualitatively reproduce the main features of multiple filament formation obtained in the experiment.

Long-term stable device for tuning fiber Bragg gratings.

It is demonstrated that with a proper choice of embedding material, the composite beam bending method constitutes an effective and reliable approach for tuning fiber Bragg gratings. A long-term stable device is presented with a dynamic range of 80 nm, which exhibits insertion losses smaller than 0.28 dB and small variations of the full width at half-maximum.