Laser-Induced Vortex Shock Waves

• Laser-induced vortex shock waves are high-energy, vortex-structured phenomena generated by CNT absorption in nanoengineered media, enabling precise ablation and deposition.
• Experimental setups use laser irradiation of CNT/methanol solutions on fiber tips to achieve shock velocities over 5700 m/s and pressures up to 6.7 GPa.
• Applications include creating fiber-integrated photonic devices and sensors through controlled CNT deposition and Fibonacci-patterned ablation, advancing nanoengineering.

Laser-induced vortex shock waves denote a class of high-energy, vortex-structured shock phenomena generated by the absorption of pulsed or continuous-wave laser energy in nanoengineered media. In particular, the recent demonstration of carbon nanotube (CNT)-induced vortex shock waves during the laser drilling of single-mode optical fibers establishes a new paradigm for structuring material interfaces using optoacoustic and nanofluidic phenomena confined within mesoscale acoustic cavities. Shock velocities surpassing 5700 m/s and pressures exceeding 6.7 GPa have been observed, with ablation patterns that trace Fibonacci-structured helical vortices and enable controlled deposition of nanoscale CNT–silica composite layers (Silva et al., 9 Dec 2025).

1. Experimental Foundations and Methodology

The experimental realization involves the irradiation of pre-cleaned standard single-mode fiber (SMF) tips (core radius $R_c = 4.1\,\mu\mathrm{m}$) with a high-power 980 nm continuous-wave laser (maximum tip power ≈161 mW) after immersion in a CNT/methanol solution. The solution, comprising single-walled CNTs (diameter ∼0.84 nm, purity ≥95%) dispersed in methanol (purity ≥99.8%) at 0.15% vol. concentration (~2.5 mg/mL), is contained in a polypropylene syringe (internal radius $R=5\,\mathrm{mm}$, length $H=20.5\,\mathrm{mm}$), forming a high aspect ratio acoustic cavity ($H/R \approx 4$). Fibers are subjected to exposure durations ($t_{\mathrm{ON}}$) of 5, 10, and 20 minutes.

Critical diagnostics incorporate SEM and EDS for post-mortem compositional and morphological analysis, Raman spectroscopy for CNT confirmation, and stylus profilometry for precision depth and profile mapping of ablated structures. Visible acoustic emissions are monitored optically through the syringe wall, while XYZ micrometer alignment optimizes fiber centering.

2. Mechanisms Underpinning Vortex Shock Generation

Initiation of vortex shock waves proceeds through photon absorption by CNT bundles at the fiber tip, triggering rapid local thermoelastic expansion and launching piston-like acoustic waves into the surrounding fluid. This expansion is supplemented by concurrent vaporization-induced pressure waves from the heated methanol. The flat fiber tip diffracts these disturbances: compressive plane-wavefronts emanate axially, while toroidal tensile waves originate from the periphery.

The cylindrical geometry of the syringe promotes confinement and overlapping of these toroidal waves, aiding the emergence of a self-organized acoustic vortex. Within this rotating flow, cavitation bubbles nucleate at tensile antinodes; subsequent bubble collapses eject CNTs as high-velocity jets along helical streamlines toward the fiber axis. Accumulating at the CNT–silica boundary, these jets impulsively impact pre-existing CNT layers, generating strong shocks internal to the CNT aggregates. Shock wave partial reflection at the mismatched CNT–silica boundary yields tensile phases; when the induced stress exceeds the silica tensile strength ($\sim5$), spallation and ablation ensue, manifesting as longitudinally-structured, helical cavities.

3. Quantitative Characterization of Shock Physics

The effective density and viscosity of the CNT/methanol suspension are given by

$$\rho_{\rm eff}=(1-n)\rho_F + n\rho_{CNT} \approx 785.8\,\mathrm{kg/m^3}, \quad \mu_{\rm eff} \approx 0.5412\,\mathrm{mPa\cdot s}$$

where $\rho_{CNT}=1610\,\mathrm{kg/m^3}$, $\rho_F=784.6\,\mathrm{kg/m^3}$, and $n=0.0015$.

The vortex-induced jet particle velocity at the fiber axis is estimated via confined swirl models, yielding

$R=5\,\mathrm{mm}$0

The shock velocity in the CNT bundle is governed by

$R=5\,\mathrm{mm}$1

resulting in

$R=5\,\mathrm{mm}$2

The Rankine–Hugoniot relations predict shock pressures

$R=5\,\mathrm{mm}$3

These pressure levels exceed the tensile strength of silica, explaining the efficient ablation and structuring of the fiber tip (Silva et al., 9 Dec 2025).

4. Geometrical and Analytical Modelling of Vortex Ablation

Empirical profiling, supported by stylus profilometry and SEM imaging, reveals ablated vortices with depths up to 5 μm and concentric, annular rings whose peak thickness increases linearly with exposure duration: $R=5\,\mathrm{mm}$4 The spatial patterning of ablated rings is consistent with Fibonacci spiral geometries. The cross-sectional loci of ablation are accurately described by parametric equations for Fibonacci spirals: $R=5\,\mathrm{mm}$5

$R=5\,\mathrm{mm}$6

where $R=5\,\mathrm{mm}$7 and $R=5\,\mathrm{mm}$8 for the three helical families observed. For 20 min exposures, ring radii attain $R=5\,\mathrm{mm}$9. Overlay of analytical trajectories on SEM images confirms relevance within ±0.5 μm in both radius and phase, with larger deviations at high radii attributed to fluid instabilities and geometric tilt.

5. CNT Deposition, Silica Ablation, and Composite Material Formation

Initial deposition of CNTs at the silica interface seeds an acoustic impedance mismatch, facilitating partial reflection and enhancing the tensile phase of subsequent shock waves. At shock pressures near 6–7 GPa, tensile spallation fractures the silica, dictating the topography of cavities and rings. During and after ablation, cavitation-driven CNT jets fill and reinforce emerging cavities, yielding wall structures with composite CNT–silica composition.

The resulting nanoengineered walls exhibit chain-buckled CNTs subjected to reversible mechanical compression, which enhances mechanical strength, and display modified optical absorption properties linked to spatial phase structuring.

6. Applications and Outlook

Laser-induced CNT vortex shock waves permit precisely structured ablation and CNT deposition at fiber tips, producing sub-micron to ~10 μm phase structures tailored for integrated photonic devices. Confirmed and prospective applications include:

• All-fiber spatial phase modulators generating optical OAM modes;
• High-power pulsed fiber lasers with increased damage thresholds;
• Fiber-tip photoacoustic transmitters for targeted ultrasonic neurostimulation;
• Fiber-end chemical/biological sensors exploiting surface-bound CNT functionalization.

The demonstrated method fuses nanomaterial engineering, optoacoustic shock wave generation, and advanced topological patterning at the fiber–solution interface. A plausible implication is the future extension to more complex 2D and 3D patterning and to diverse nanomaterial matrices and host geometries, broadening the reach of vortex-induced structured machining (Silva et al., 9 Dec 2025).