Nanoscale reshaping of resonant dielectric microstructures by light-driven explosions

The spatial limitations of light-based material processing, set by the diffraction limit (λ/2n), hinder the creation of deeply subwavelength features. While nonlinear optical processes like multiphoton absorption offer potential improvements, their effectiveness remains debated. Femtosecond lasers have enabled sub-micron precision laser ablation and nanoparticle formation, primarily on planar surfaces. However, the resulting nano-patterns are limited. Plasmonic nanostructures offer a route to overcome the diffraction limit by funneling light into subwavelength hotspots, but all-dielectric nanoresonators provide an alternative platform for boosting nonlinear light-matter interactions. This research leverages the concept of hotspot-assisted subwavelength material modification using all-dielectric structures in the optical domain, aiming for high-throughput nanoscale 'machining' of solid materials to enhance current nanofabrication methods. The study focuses on achieving deeply subwavelength laser-induced modification of all-dielectric microstructures via tailored explosions of the constituent material (silicon) using resonantly tuned mid-infrared laser pulses. The goal is to control the modification of micron-scale prefabricated resonators with femtosecond laser pulses, creating nanoscale features like narrow, high aspect ratio trenches. The efficiency of light coupling into the photo-generated electron-hole plasma is a key factor, and the use of a three-dimensional dielectric microstructure is proposed to circumvent limitations encountered in planar surface modification. This approach is also distinct from near-field ablation enabled by sub-wavelength plasmonic nanoparticles, as it leverages the highly nonlinear nature of all-dielectric resonators to achieve superior localization. The researchers aim to demonstrate the controllability of the nanotrench formation through manipulation of laser pulse parameters.

The paper reviews previous research on light-based material processing, focusing on limitations imposed by the diffraction limit and the potential of nonlinear optical processes like multiphoton absorption. It discusses the advancements in femtosecond laser-assisted material restructuring, including various mechanisms of surface modification. The literature review also highlights the use of plasmonic and all-dielectric nanoresonators for enhancing nonlinear light-matter interactions and achieving subwavelength features. It acknowledges the potential of resonant all-dielectric structures for high-throughput nanoscale material modification, positioning the current research within the context of existing advancements and limitations.

The study involved the fabrication of arrays of M-shaped silicon microresonators on a silicon-on-sapphire wafer. The fabrication process utilized electron beam lithography (EBL) to define the pattern in a PMMA resist layer, followed by chromium deposition and reactive ion etching (RIE) to transfer the pattern to the silicon layer. The resonators exhibited an optical resonance around 4 μm, confirmed by Fourier transform infrared (FTIR) spectroscopy. The samples were then irradiated with trains of mid-infrared (MIR) femtosecond laser pulses (λp = 3.9 μm, tp = 200 fs) from an optical parametric amplifier (OPA). The number of pulses (N ≤ 100), peak fluence, and polarization angle (θ) were varied to control the nanotrench formation. The resulting structures were characterized using scanning electron microscopy (SEM). To understand the mechanism, particle-in-cell (PIC) simulations using the EPOCH code were performed, modeling the interaction between the laser pulse and the resonator, including photoionization and energy transfer processes. The simulations incorporated a homebuilt refractive index and dynamic Keldysh photoionization modules. The simulations focused on the evolution of electronic plasma density and lattice ion temperature, aiming to elucidate the nanoscale ablation process. A separate set of experiments were conducted using a longer wavelength (λ = 7 μm) to demonstrate scalability. The experimental setup for the 7μm experiments was similar, using a Ti:sapphire pump laser and an OPA system to generate the laser pulses. The resulting structures were again characterized by SEM.

The researchers successfully fabricated high-aspect-ratio (>10:1) nanotrenches in silicon microresonators using mid-infrared femtosecond laser pulses. The narrowest trenches achieved were approximately λ1/80 (around 50 nm). The formation of these deeply subwavelength features was demonstrated to be controllable by varying the number of laser pulses, pulse intensity, and polarization. The average lateral trench propagation speed was estimated to be approximately 30 nm per pulse. The orientation of the nanotrench was shown to be directly controlled by the polarization of the incident laser beam. The process was demonstrated to be scalable, with similar results observed at a longer wavelength (7 μm) using proportionally scaled resonators. Particle-in-cell simulations revealed localized heating of the silicon beyond its boiling point, resulting in a phase explosion consistent with the observed trench formation. The simulations confirmed that the trench formation is driven by a highly localized process, initiating at the apex of the M-shaped resonator, and spreading through the entire thickness of the resonator. The simulations also supported the experimental findings on polarization sensitivity. The threshold fluence for nanotrench formation was approximately 0.1 J cm−2, significantly lower than that typically required for multiphoton laser patterning of flat silicon surfaces. The analysis suggests that the material removal is attributed to highly localized Coulomb or phase explosions, driven by the resonant enhancement of the optical field within the microresonator. The study confirms the feasibility of achieving deeply subwavelength features using a resonantly-enhanced laser-driven explosion. The process is consistent across different wavelengths, suggesting scalability. This scalability is important for the possible implementation of this method to different fabrication technologies. The experiments conducted with a longer wavelength further supported this finding.

The findings directly address the research question by demonstrating the creation of deeply subwavelength features using resonantly enhanced mid-infrared laser pulses. The controllability of the process through the manipulation of laser parameters opens exciting possibilities for high-throughput nanofabrication. The achieved feature sizes and aspect ratios significantly surpass limitations imposed by the diffraction limit. The combination of experimental results and particle-in-cell simulations provides a comprehensive understanding of the underlying mechanism. The low threshold fluence required for nanotrench formation makes the process energy-efficient. This new technique, FLANEM, presents a significant advancement in nanofabrication, offering high precision and scalability with potentially wide applicability in various fields. The successful demonstration of polarization-controlled trench orientation adds further controllability. The use of mid-infrared radiation is justified by the enhanced multiphoton ionization and tunneling, along with the larger size of the resonant structures, enabling reliable fabrication.

This research successfully demonstrated a novel method for nanoscale reshaping of silicon microresonators using light-driven explosions. The technique enables the creation of deeply subwavelength features with high aspect ratios, offering precise control over the resulting nanostructures. The findings expand the nanofabrication toolbox and show promise for high-throughput optical methods of nanoscale structuring. Future research should explore the impact of resonator geometry and material choice on the process. Investigation of different materials and resonator designs will further expand the potential applications of FLANEM. The development of more sophisticated simulation models could also refine the understanding of the underlying physical mechanisms and potentially enable even more precise control over the nanostructuring process.

While the study demonstrates the potential of FLANEM, further research is needed to fully understand the influence of the microresonator's minimum feature size on the nanostructuring capacity. The current simulations primarily focus on direct inter-band excitation, neglecting indirect transitions which might play a minor role. The precise mechanism responsible for the polarization-dependent asymmetry in ablation requires further investigation. The study focuses on silicon; exploring other materials would broaden the applicability of this approach. While the researchers note that time-of-flight mass spectrometry could confirm certain aspects of the process, this was not performed in the current study.