Three-dimensional femtosecond laser nanolithography of crystals (1904.08264v1)

Abstract: Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.

Three-Dimensional Laser Nanolithography of Laser Crystals

The paper by Ródenas et al. introduces a novel approach in the domain of laser nanolithography, specifically targeting the volume nanostructuring of hard optical crystals. Historically, attempts to nanostructure optical crystals have been limited to surface-level modifications, primarily due to the challenges posed by stress-induced crack formation which has rendered precision volume processes ineffective. This paper posits a transformation in nanostructuring capabilities, utilizing Three-Dimensional Laser Writing (3DLW) paired with chemical etching to achieve internal modifications without fracture.

Overview and Methods

The research emphasizes enhancing the inner chemical etching reactivity of a crystal by up to five orders of magnitude through direct laser writing. Utilizing yttrium aluminum garnet (YAG) and sapphire, the paper successfully demonstrates the creation of cm-scale three-dimensional structures with 100 nm feature sizes inside crystals. This method notably circumvents the typical brittleness associated with such processes, allowing for the fabrication of complex photonic devices, including sub-wavelength diffraction gratings and optical waveguides.

The methodology employed involves multiphoton absorption induced by infrared femtosecond laser pulses. This technique, pivotal in achieving 3D structuring, modifies the crystal volume to alter its wet etch rate selectively. The paper reports an etching selectivity surpassing $10^5$, a significant enhancement over previously recorded rates for photo-irradiated materials. The authors established this high selectivity using standard phosphoric acid solutions and also explored methods to structure sapphire with high precision.

Numerical Results and Practical Applications

Numerical results highlight an etching rate of modified YAG nanopores at 129 ± 6.8 µm/h, with pristine YAG etching rates under 1 nm/h, demonstrating significant efficiency and precision in the fabrication process. The research successfully produces void millimeter-scale nanopores, evidencing the capability to create long, continuous structures by implementing auxiliary vertical etching pores for extended lengths. The documented nanopore dimensions range from 368 x 726 nm to 3.1 mm in length, achieved through strategic etching from both ends.

The potential applications are twofold: the paper propels advancements in both photonic crystal fabrication and nonlinear optics, offering a pathway to integrate complex photonic circuits directly within solid-state laser media. Furthermore, the prospect of embedding crucial laser components like dispersion control elements and cooling channels directly into the laser gain medium could revolutionize the design and efficiency of compact lasers.

Theoretical Implications and Future Directions

The paper carries significant theoretical implications, suggesting new directions in crystal optics and solid-state laser engineering. The possibility of creating 3D photonic band-gap lattices within solid laser crystals can drastically alter the manipulation of light within these materials, ushering in prospects for ultra-fast nonlinear supercontinuum generation using standard pulsed lasers.

Looking ahead, this technique could facilitate improvements in both the robustness and operational efficiency of laser systems. It is conceivable that further optimization of laser writing parameters and etching conditions could refine the technique, enhancing the spatial resolution and scalability of nanostructures. As the process becomes more adaptable to different crystal types, broader applications in various domains of photonics and optoelectronics are expected.

Overall, this paper represents a significant stride in laser crystal nanolithography, offering new methods to achieve high-precision internal modifications while maintaining structural integrity. The implications for developing advanced photonic devices are profound and invite further exploration.