Laser-Inscribed Stress-Induced Birefringence of Sapphire

Synergistic engineering of ultraviolet metal-free crystals with exceptional birefringence via pyridine-derived dimers

Crystals with giant birefringence are essential for practical applications in lasers, optical communication, and polarimetry, where precise control of polarized light is critical. Coplanar six-membered ring (6-MR) primitives with large polarizability anisotropy are particularly effective in enhancing birefringence. This study successfully combines different pyridine derivatives into two novel metal-free crystals: supramolecular one-dimensional [(4-HP)(4-H2P)][3-pySO3] (1; HP = hydroxypyridine; py = pyridine) and two-dimensional [4-AP][3-pySO3] (2; AP = aminopyridne), synthesized via a facile aqueous solution method. Compound 1 features unique [4-HP/4-H2P]+ cationic dimer pairs linked by hydrogen bonds, in contrast to the single monovalent [4-AP]+ cations in compound 2. This structural distinction leads to optimized anionic [3-pySO3]- arrangements, a reduced dimensionality of linkage, and denser spatial distribution of cationic pseudo-layers, significantly enhancing birefringence. Compound 1 exhibits a superior birefringence value of 0.443 at 546 nm, compared to 0.296 for compound 2, representing the highest birefringence among sulfate derivatives incorporating an additional birefringence-active group in the ultraviolet (UV) region. It also surpasses all reported metal-free compounds with single 6-MRs in the short-wave UV range. Theoretical calculations confirm the synergistic effects between cationic and anionic pyridine derivatives, further elucidating their contributions to enhanced birefringence. In addition, compound 1 demonstrates a short UV cut-off edge at 279 nm and favorable growth characteristics, making it a promising candidate for UV birefringent applications. This research offers new insights into designing and optimizing birefringence by exploring the relationship between the composition and arrangement of organic cations and their optical properties in UV metal-free systems through synergistic effects.

Research advances of femtosecond laser-induced nanogratings for transparent materials

Femtosecond laser-induced nanogratings generation inside transparent materials has been the focus of research in the field of femtosecond laser precision processing. Due to the advantages of optical birefringence phenomenon, periodicity, thermal stability, controllability of delay value and optical axis direction, and re-writability, nanogratings are widely used in research fields such as optical storage and optical devices. This mini-review explores the existing mechanism of femtosecond laser-induced formation of nanogratings and the progress of inducing nanogratings in different types of glass and sapphire crystals. It also describes the prospects of nanogratings for a wide range of applications in optical components and optical devices.

Nanohydrodynamic Local Compaction and Nanoplasmonic Form-Birefringence Inscription by Ultrashort Laser Pulses in Nanoporous Fused Silica

The inscription regimes and formation mechanisms of form-birefringent microstructures inside nano-porous fused silica by tightly focused 1030- and 515-nm ultrashort laser pulses of variable energy levels and pulsewidths in the sub-filamentary regime were explored. Energy-dispersion X-ray micro-spectroscopy and 3D scanning confocal Raman micro-spectroscopy revealed the micro-tracks compacted by the multi-shot laser exposure with the nanopores hydrodynamically driven on a microscale to their periphery. Nearly homogeneous polarimetrically acquired subwavelength-scale form-birefringence (refractive index modulation ~10^-3) was simultaneously produced as birefringent nanogratings inside the microtracks of wavelength-, energy- and pulsewidth-dependent lengths, enabling the scaling of their total retardance for perspective phase-modulation nanophotonic applications. The observed form-birefringence was related to the hierarchical multi-scale structure of the microtracks, envisioned by cross-sectional atomic-force microscopy and numerical modeling.

Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., S_a parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average S_a value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

Monolithic 3D phase profile formation in glass for spatial and temporal control of optical waves

Optical manufacturing technologies play a central role in modern science and engineering. Progress on both subtractive and additive fabrications is transforming the implementation of optical technologies. Despite the recent advances, modern fabrication still faces challenges in the accuracy, dimension, durability, intensity, and wavelength range. Here we present a direct monolithic 3D phase profile formation in glass and demonstrate its versatile applications for high-accuracy spatial and temporal control of optical waves in the extreme wavelength and intensity domains, direct fabrication of microlenses, and in situ aberration correction for refractive components. These advances and flexibilities will provide a new dimension for high-performance optical design and manufacture and enable novel applications in a broad range of disciplines.

Single-mode sapphire fiber Bragg grating

Sapphire optical fiber has the ability to withstand ultrahigh temperatures and high radiation, but it is multimoded which prevents its use in many sensing applications. Problematically, Bragg gratings in such fiber exhibit multiple reflection peaks with a fluctuating power distribution. In this work, we write single-mode waveguides with Bragg gratings in sapphire using a novel multi-layer depressed cladding design in the 1550 nm telecommunications waveband. The Bragg gratings have a narrow bandwidth (<0.5 nm) and have survived annealing at 1000°C. The structures are inscribed with femtosecond laser direct writing, using adaptive beam shaping with a non-immersion objective. A single-mode sapphire fiber Bragg grating is created by writing a waveguide with a Bragg grating within a 425 µm diameter sapphire optical fiber, providing significant potential for accurate remote sensing in ultra-extreme environments.

Ultrafast laser-inscribed nanogratings in sapphire for geometric phase elements

We systematically studied femtosecond laser-inscribed self-organized nanogratings and geometric phase elements such as a polarization diffraction focusing lens and Q-plate in sapphire crystal. Besides the void structures observed in the focus, nanogratings with periods of 150~300 nm were observed, depending on a nanoslit that took the role of a seeding effect by localized light field enhancement. The non-polarized refractive index change and birefringence were measured with values around 1∼2×10^-3 and 6×10^-4, respectively. Based on the laser-inscribed form birefringence, a geometric phase lens and Q-plate were successfully demonstrated in sapphire with high imaging and a focusing effect. We expect that our findings may promote the understanding of laser-induced nanogratings in bulk and potential applications in geometric phase elements.

Tilted black-Si: ∼0.45 form-birefringence from sub-wavelength needles

The self-organised conical needles produced by plasma etching of silicon (Si), known as black silicon (b-Si), create a form-birefringent surface texture when etching of Si orientated at angles of θ_i < 50 - 70^° (angle between the Si surface and vertical plasma E-field). The height of the needles in the form-birefringent region following 15 min etching was d ∼ 200 nm and had a 100 μm width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si ∼λ/4 for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is proposed to directly measure the retardance Δn × d/λ ≈ 0.15 of the region with tilted b-Si needles. The quantified form birefringence of Δn = -0.45 over λ = 400 - 700 nm spectral window was obtained. Such high values of Δn at visible wavelengths can only be observed in the most birefringence calcite or barium borate as well as in liquid crystals. The replication of b-Si into Ni-shim with high fidelity was also demonstrated and can be used for imprinting of the b-Si nanopattern into other materials.