Submicron-scale structures with high-aspect ratios, particularly tunable nano-holes, are pivotal for photonic devices^[1,2], optical fiber sensing^[3–5], and biomedicine^[6]. Their functions critically depend on flexible control of axial morphology — a challenge for conventional fabrication techniques that struggle to simultaneously achieve high spatial resolution, sufficient depth penetration, and geometrical shape control^[7]. Ultrafast Bessel beams have emerged as a promising solution due to their non-diffracting propagation and extended focal depth^[8–11]. These features have facilitated breakthroughs in high-aspect-ratio processing, including volumetric structuring and deep-channel drilling^[12,13].

Fabricating micro-/nano-structures using laser ablation is fundamentally influenced by the spatial energy distribution of the focused beam^[14]. A conventional axicon-based Bessel beam exhibits an axial energy intensity profile characterized by gradual axial enhancement followed by progressive attenuation, resulting in corresponding material modification or ablation removed profiles that generally conform to the Bessel beam’s energy gradient^[10,15]. This inherent energy distribution limits the achievable diversity of nano-hole shapes, necessitating further tailoring of the beam’s axial energy profile.

This work introduces a novel approach to nano-hole fabrication on fused silica using a ring-lens-tailored Bessel beam with a longitudinally tunable energy distribution, enabling precise control over the nano-hole geometry. By varying the ring-lens radius (R), as shown in Fig. 1(a), the axial energy profile of the Bessel beam can transition from a steep energy peak to a more uniform longitudinal distribution. This results in a gradual reduction in energy concentration at the beam’s front, accompanied by progressive homogenization along the propagation axis. Notably, when R ranges from 1.25 to 2.50 mm, the tailored beam exhibits a significantly higher initial peak intensity compared to conventional axicon-based beams. The steep energy peak of the Bessel beam induces localized ablation near the surface, creating a conical entrance that transitions smoothly into a uniform-diameter channel. This results in a well-defined structure with a tapered top segment (h1), where the entrance diameter dout gradually narrows to din, followed by a straight channel segment h2. The experimental control of conical-to-straight transition through beam tailoring suggests potential applications in nano-structure engineering. This previously unexplored feature is utilized here to fabricate nano-holes with tailored tapers at the glass samples’ top surface [Figs. 1(b) and 1(c)]. Such structures are relevant to applications like nanophotonic devices ^[16] and microchannel plates^[17], where the taper geometry can influence optical or detection performance.

The Bessel beam generation method proposed in this work is illustrated in Fig. 2. When a Gaussian beam passes through a ring-lens, it is transformed into an annular beam due to the ring-lens’s unique circular focusing property^[18]. As the annular beam propagates after the lens, a Bessel beam is formed at the lens focus, referred to as a “ring-lens-tailored Bessel beam”. Unlike conventional axicon-based Bessel beams, whose energy distribution depends on the conical angle and the input Gaussian beam’s diameter^[19], the energy distribution and focal properties of the ring-lens-tailored Bessel beam are primarily determined by the spatial characteristics of the input annular beam. For instance, adjusting the annular beam’s radius can influence the Bessel beam’s waist size and non-diffracting region.

To enable flexible modulation of R, a spatial light modulator (SLM) was employed to implement the phase delay of the ring-lens transmittance function, which can be described as T(r)=exp[i2πλ(f−f2+(r−R)2)],(1)where λ is the laser wavelength, and f and R are the focal length and the annular beam radius determined by the physical design of the ring-lens, as illustrated in the inset in Fig. 1(a). The r represents the radial coordinate. By modulating the phase mask on the SLM, the size of the annular beam (thereby determining the spatial energy distribution of the Bessel beam) can be precisely controlled.

The experimental setup employed an ultrafast laser system (Pharos, Light Conversion) operating at a 1030 nm wavelength with a pulse duration of 1 ps and a single pulse energy of ∼5.2μJ. The beam was modulated using a reflective, phase-only spatial light modulator (X15213-16, Hamamatsu), with a pixel pitch of 12.5 µm and 8-bit input (256 gray levels), programmed with the phase diagram of the ring-lens. The modulated beam was transformed into the ring-lens-tailored Bessel beam by a lens (f=500mm) placed 500 mm after the SLM. Subsequently, a 4f system composed of a lens (f=500mm) and a 50× objective lens (NA=0.65) demagnified and focused the Bessel beam into fused silica glass (JGS1) samples. Precise sample positioning was achieved using a 3D x–y–z displacement platform (XMS100-S, Newport). The laser fluence was adjusted to keep the side-lobe intensity below the material damage threshold, effectively suppressing ablation caused by Bessel beam side lobes.

To minimize potential optical aberrations in the system, aspherical lenses were used to suppress spherical distortions. The SLM was calibrated using the factory-provided look-up table (LUT) to compensate for the nonlinear voltage-to-phase response. In addition, the beam profile was monitored using a beam profiling camera (BeamGage LT665, Ophir Spiricon) during alignment to ensure the quality and symmetry of the ring-shaped beam.

To ensure structure stability and surface quality, post-processing steps after laser ablation were applied: 1) thermal annealing: samples were annealed at 1100°C for 3 h in a tube furnace (GSL-1700X, Kejing Star Technology Company) to relieve random stress concentrations and prevent crack formation; 2) chemical etching: a hot alkaline etching process was performed in an 8 mol/L KOH aqueous solution at 85°C for approximately 30 min to remove debris. The detailed procedure can be found in Ref. [10].

The spatial energy distribution of the ring-lens-tailored Bessel beam was investigated through simulations and experiments, as shown in Figs. 3 and 4. The position and intensity of the energy peak play a critical role in adjusting the taper geometry of nano-holes, as indicated in Fig. 1(c). Therefore, understanding how the energy distribution evolves with varying the radius (R) of the ring-lens is important.

Figure 3(a) presents the phase diagrams of ring-lenses with different radii (R=1.25, 2.50, 3.75, and 5.00 mm). The corresponding simulated x–y plane energy distributions of ring-shaped beams focused by the ring-lens [Fig. 3(b)] and the experimental measurements show that as R increases, the hollow region of the annular beam becomes larger [Fig. 3(c)]. The annular beam is subsequently focused by a lens (f=800mm), forming a Bessel beam, as depicted in Fig. 4. Interestingly, along the z-axis, the energy peak at the front shifts forward with increasing R, accompanied by a reduction in peak intensity and a smoother axial energy distribution. This shifting characteristic of the Bessel beam’s energy peak is advantageous for adjusting the nano-hole taper, as it directly influences the ablation intensity near the material’s surface. The observed deviation [Fig. 4(b)] between the experimental and simulated axial intensity profiles may stem from experimental uncertainties such as slight optical misalignment, imperfections in the beam-shaping components, and the finite pixel resolution of the SLM. In contrast, the simulation assumes ideal beam input and perfect phase modulation, which may not fully capture real experimental conditions.

The capability to control the taper geometry of nano-holes using ring-lens-tailored Bessel beams was systematically investigated, as illustrated in Fig. 5. To quantify the fabricated structures, we defined three geometric parameters: the outer diameter (dout), inner diameter (din), and taper depth (h1), as shown schematically in Fig. 5(b).

By varying the ring-lens radius (R) from 1.25 to 2.50 mm, the Bessel beam’s energy peak shifts downstream while its intensity gradually decreases [Fig. 5(a)], enabling continuous modulation of the axial energy profile. This shift affects the ablation strength near the surface of the fused silica, leading to controllable taper formation.

As shown in Figs. 5(c)–5(e), increasing R results in a gradual decrease in dout and a slight increase in h1 for R<2.50mm. This is due to the reduced energy concentration near the surface, which suppresses surface ablation while shifting the modification region deeper along the propagation axis. When R exceeds 2.50 mm, the taper depth drops sharply, indicating the disappearance of the taper structure. This is attributed to the increasingly uniform axial energy distribution and weakened peak intensity, which together fail to induce localized ablation near the surface [Figs. 5(b) and 5(c)]. For comparison, nano-holes fabricated using a conventional axicon-based Bessel beam—reported in our previous work^[10]—also exhibit a cylindrical shape without taper, consistent with the results obtained here at large R [Fig. 5(c)].

This morphology evolution can be attributed to the axial shift of the beam’s peak intensity, which governs where nonlinear absorption and plasma formation occur. When the intensity peak is located near the surface (small R, 1.25–2.5 mm), strong surface ablation leads to wider entries and larger taper angles. As the peak intensity moves deeper into the bulk and the axial energy distribution becomes more uniform (large R, >2.5mm), energy deposition near the surface decreases, resulting in weaker surface ablation and straighter nano-holes.

These results highlight the ability to tune the taper geometry by adjusting R. The taper angle (θ) can be modulated from 0° to 52°, offering flexible control over the nano-hole morphology [Figs. 5(c) and 5(f)]. Furthermore, as shown in Fig. 6, the fabricated nano-hole arrays demonstrate high uniformity and reproducibility, confirming the robustness and consistency of the proposed method.

In conclusion, we have developed a ring-lens-tailored Bessel beam approach for fabricating nano-holes with tunable taper geometries on fused silica glass. By adjusting the ring-lens radius (R), this method generates a steep spatial energy peak compared to conventional axicon-based methods. The Bessel beam’s energy peak is reduced by varying R between 1.25 and 2.50 mm, allowing the hole’s taper angle to be tuned within 52°, with the taper effect diminishing beyond R=2.50mm. The good uniformity and reproducibility of processed nano-holes make this single-pulse ablation technique particularly promising for large-scale nano-manufacturing. Our future work will focus on extending this technique to a broader range of materials and realizing more complex morphologies, such as axially varying geometries along the hole depth.