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.

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.

A thorough study of the femtosecond photo-inscription of optical waveguides in fused silica is presented. Quantitative phase microscopy was used to study the variation of the index contrast of the waveguides as a function of the writing conditions. It is revealed that waveguides based exclusively on Type I refractive index modifications are difficult to form for pulses longer than 300 fs. We show that this limitation can be circumvented by scanning the laser beam multiple times at low pulse energy. We also demonstrate that by equally multiplying the scan speed and the number of passes, the index contrast can be increased, which was not expected for the low-repetition-rate regime. Based on the nonlinear ionization memory, we propose an explanation for this phenomenon. For shorter pulses, multiple passes of the beam allowed for the formation of waveguides with an enhanced index contrast while preserving the morphology and uniformity of Type I modifications. Index contrasts up to 9 × 10⁻³ in Heraeus F300 fused silica are reported. Using this method, waveguides that exhibits single mode operation at wavelengths of 405, 633, 980 and 1550 nm were successfully inscribed.

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>0.1typically obtained using the standard astigmatic beam technique, or>0.7with 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 byin situlaser-induced plasma emission imaging is also discussed.

From the perspective of developing robust mid-infrared (mid-IR) integrated photonic devices, barium-gallium-germanium (BGG) oxide glasses are strong candidates among other mid-IR glasses. Indeed, compared to fluoride, tellurite or chalcogenide glasses, BGG glasses present the highest thermal and chemical stabilities, while transmitting light up to 6 µm. In parallel to this, technological advances in ultrafast direct laser writing (UDLW)-based devices are driving the development of novel photonic glasses. Specifically, there is a need to identify the most efficient mid-infrared transmitting BGG glass compositions for sustaining the UDLW process. In this article, we thoroughly investigate the BGG physicochemical properties through absorption and Raman spectroscopies, refractive index, density, and glass transition temperature measurements in two relevant glass series: one via a Ga³⁺/Ge⁴⁺ ratio fixed to 1 and a barium content varying from 25 to 40 cationic percent, the other via a 2Ba²⁺/Ga³⁺ ratio fixed to 1 and a germanium content varying from 20 to 80 cationic percent. In the meantime, we explore the photosensitivity of these glasses under UDLW. Our findings reveal the valuable role of both barium and gallium ions, notably through their concentration, structural stabilization sites and viscosity influence. Finally, we demonstrate the fabrication of an 8.2 cm-long UDLW-induced waveguide with propagation losses of < 0.3 dB.cm^-1 at 1550 nm.