Ben McMillen

Over the past several years, non-ablative femtosecond laser exposure with spatially overlapping (i.e., spatially cumulative) pulses has emerged as a key process in three-dimensional writing of patterns in bulk of dielectric substrates. When temporally non-cumulative and combined with post-processing steps, this process defines a novel manufacturing technique in fused silica, finding uses in a broad number of applications, including—but not limited to—micromechanics, integrated optics, microelectronics, microfluidics, information storage, and combinations of these fields for novel integrated sensing applications. For fused silica, evidence has shown that there is a pulse-length duration threshold around 200 fs, marking the boundary between two radically different characteristic material modification regimes, each leading to a specific application. Pulse widths below 200 fs lead to localized densification, enabling the direct-write of optical waveguides, while pulse widths above this value produce self-organized nanostructures causing a localized volume expansion and enhanced etching susceptibility to various chemicals. Here, we focus our attention on the regime below 200 fs, using low repetition rates and temporally non-cumulative pulses. In particular, we use very short pulses, i.e., in the range of 30 fs—a regime as yet unexplored from the viewpoint of spatially cumulative modifications. Our goal is to understand how structural modifications obtained by overlapping pulses evolve with varying pulse overlap, and how shorter pulse duration may correlate with higher material densification. This knowledge is particularly important for the next generation of photonics devices, where increasing the level of laser-induced densification is a key factor for high-density photonic integration.

Summary form only given. Exposure of silica glass to focused femtosecond laser radiation produces a variety of structural modifications [1] that are accompanied by a localized change in volume [2, 3]. This volume change can be used to apply displacements in a controlled manner, allowing the generation of intense, localized stress fields [4] whose magnitude, orientation, and shape are easily manipulated through tuning of the writing parameters [4-6]. This concept of tailored stress fields provides a new toolset for device engineering, which when combined with the optical and mechanical properties of silica glass, allows for a versatile range of implementations. To illustrate this further, we have recently demonstrated a waveplate based on stress-induced birefringence [7], which is shown in Fig. 1.

The use of femtosecond lasers to introduce controlled stress states has recently been demonstrated in silica glass. We use this technique, in combination with chemical etching, to generate and control stress-induced birefringence over a well-defined region of interest, demonstrating direct-write wave plates with precisely tailored retardance levels. This tailoring enables the fabrication of laser-written polarization optics that can be tuned to any wavelength for which silica is transparent, and with a clear aperture free of any laser modifications. Using this approach, we achieve sufficient retardance to act as a quarter-wave plate. The stress distribution within the clear aperture is analyzed and modeled, providing a generic template that can be used as a set of design rules for laser-machined polarization devices.

In recent years, micro manufacturing with femtosecond lasers has received considerable attention as an efficient technique for producing three-dimensional devices, combining multiple functionalities in a single monolithic substrate. In this manufacturing process, stress-anisotropy resulting from non-ablative laser exposure can have both positive and negative effects on the process outcome. In this work, we present a simple method for visualizing stress anisotropy, combining highly symmetric laser-written patterns with polarization microscopy, as a tool for identifying the various anisotropic contributions to the laser fabrication process.

Femtosecond laser exposure in the non-ablative regime induces a variety of bulk structural modifications, in which anisotropy may depend on the polarization of the writing beam. In this work, we investigate the correlation between polarization state and stress anisotropy. In particular, we introduce a methodology that allows for rapid analysis and visualization of laser-induced stress anisotropy in glasses and crystals. Using radial and azimuthal polarization, we also demonstrate stress states that are nearly isotropic.

Bragg waveguides are fundamental components in photonic integrated circuits and are particularly interesting for mid-IR applications in high index, highly nonlinear materials. In this work, we present Bragg waveguides fabricated in bulk chalcogenide glass using an ultrafast laser. Waveguides with near circularly symmetric cross sections and low propagation loss are obtained through spatial and temporal beam shaping. Using a single-pass technique, the waveguide and Bragg structure are formed at the same time. First through sixth order gratings with strengths of up to 25 dB are realized, and performance is evaluated based on the modulation duty cycle of the writing beam.

We report on the use of femtosecond laser for fabricating, loading and in-situ measuring using third-harmonic signal generation a micro-tensile tester for characterizing silica and its polymorphic phases.

We present work on the fabrication of Bragg waveguides in gallium-lanthanum-sulfide chalcogenide glass using an ultrafast laser. Waveguides were written with a single pass while modulating the writing beam. The spatial and temporal profile of the writing beam was controlled during waveguide fabrication in order to control the shape and size of the waveguide cross-section. Over the last decade, ultrafast lasers have become the staple research tool of many research institutions. The applications of these lasers have moved beyond basic materials studies toward ever more complex systems, such as micro-fluidic devices [1-2], micromechanical actuators [3], and integrated monolithic waveguide lasers [4-5]. The materials used in these studies are often silica-based glasses, however considerable effort has been put toward the study of highly non-linear optical materials such as chalcogenide glass [6]. Waveguide fabrication in highly nonlinear, high-index materials presents significant challenges for ultrafast laser processing. High nonlinearities contribute to temporal distortion of focused femtosecond pulses, depleting pulse energy and distorting the focal volume, while a high index contrast further contributes to spatial distortions due to index mismatch induced spherical aberration. Building on previous work [7], we present the application of both spatial beam shaping and temporal pulse tailoring for the fabrication of Bragg waveguides in gallium-lanthanum-sulfide (GLS) chalcogenide glass. The waveguides presented here were fabricated using a Ti:sapphire system, delivering 150 pulses at 800 nm and a repetition rate of 250 kHz. The GLS sample was mounted to a motion stage system, incorporating a fixed focusing objective with a numerical aperture of 0.75. Distortions due to index-mismatch induced spherical aberrations were corrected at the focal volume with a cylindrical de-magnifying telescope, providing a demagnification factor of 3 in the y direction (perpendicular to the writing direction). Finally, the pulse-compressor of the laser was adjusted to provide a pulse duration of ~1.5 picoseconds, which was has been shown to give the lowest loss waveguides in GLS chalcogenide [7].