Gain materials constitute the fundamental and crucial medium within the realm of solid-state lasers^[1–3], playing a vital and expansive role in various fields such as laser precision machining^[4–6], environmental monitoring, and laser-based medical applications^[7–9]. Yttrium aluminum garnet (YAG) is frequently employed in laser systems as host material due to its uniform crystal lattice structure, high thermal conductivity, and broad transparency range in the visible and infrared spectra^[10–13]. The growth of high-quality YAG crystals doped with rare-earth elements presents challenges in uniformity, purity, structural integrity, growth rate, liquid transparency, internal stress, crack management, and melt preparation, leading to high cost to achieve high-quality single crystals that meet the performance requirements of lasers^[14]. However, conventional crystal slicing methods, such as diamond wire-saw or cutting wheel, incur substantial material wastage during the cutting process (the cutting width exceeds 300 µm), resulting in considerable inefficiency.

Laser slicing technology, as an emerging technique, has been employed in the slicing of hard and brittle materials such as silicon carbide as well as materials that are prone to decomposition and vaporization^[15–19]. Laser slicing technology typically incurs losses less than ∼50μm, markedly enhancing material utilization and concurrently diminishing processing costs. Tanaka et al. employed a subnanosecond laser for slicing GaN, facilitating crack propagation through the expansion of nitrogen bubbles when heated to 350°C^[20]. Kim et al. performed exfoliation of a 4H-SiC (0001) single crystal by the femtosecond laser^[15]. The SiC undergoes decomposition to form amorphous silicon and amorphous carbon within the modified region, inducing compressive stress and crack formation along the laser-modified layer. Moreover, Si, Al2O3, and glass can be separated along crystal planes by laser-assisted slicing^[21]. Recently, with the growing demand for high-power and high-energy lasers, thin-disk lasers have garnered widespread attention due to their exceptional output power and good beam quality^[22–27]. However, the exceptional thermal stability, high hardness, tensile strength, and isotropic properties inherent in gain materials make it challenging to obtain thin slices of crystals using traditional laser slicing methods. Up to now, there have been no reports on laser slicing techniques for gain materials.

In this work, a laser-assisted Yb:YAG single-crystal slicing strategy is first reported. Utilizing multipulse laser processing to induce cavity formation within the crystal interior, thereby promoting crack initiation, and employing external rapid cooling to generate stress for driving crack propagation, successful crystal slicing is achieved. By employing aberration correction techniques, the optical field distribution of the laser within the crystal is refined, achieving a compression of the focused laser spot length to within 10 µm. This optimization not only promoted the initiation and expansion of cracks but also concurrently minimized laser processing losses. After slicing, the thin slices of crystals maintained a high-quality crystalline state, with a surface roughness of less than 2.5 µm. We believe that this technology has the potential to improve the utilization efficiency of single-crystal gain materials, reduce slicing costs, and advance the development of thin-disk lasers. Furthermore, it holds promise for application in the slicing of other gain materials in the future.

The laser employed is a Pharos PH1-20-0400-10-30, sourced from Light Conversion Ltd., featuring a laser beam quality (M2) of ≤1.2. This laser provides a fundamental Gaussian mode characterized by a central wavelength of 1030 nm and a pulse width of 290 fs. The laser system achieves a maximum single-pulse energy of 420 µJ and sustains an average power output of 20 W. The YAG crystal is secured in a fixture on an XY displacement platform, with the z-axis movement used to control the height of the objective lens.

The simulation utilized electromagnetic waves in the frequency domain module, with a Gaussian beam as the incident light. The beam radius was set to half of the objective focusing spot radius (0.75 µm), the wavenumber to 6.1×106m−1, Rayleigh distance to 1.71×10−6m, far-field divergence angle to 0.437 rad, calculation domain length to 200 times the Rayleigh distance (3.43×10−4m), and width was calculated from the far-field divergence angle and length (1.60×10−4m). The mesh was partitioned into 1.2 million elements. The simulation was performed using COMSOL Multiphysics 6.1.

The experimental configuration comprises an ultrafast laser integrated with a three-axis motion stage. The laser employed is a femtosecond laser providing a fundamental Gaussian mode characterized by a central wavelength of 1030 nm and a pulse width of 290 fs. It is noteworthy that the pulse width, repetition rate, and single-pulse energy are dynamically adjustable through computer control. The processing utilizes a 50× objective lens with a high numerical aperture of 0.65 (LMH-50X-1064, Thorlabs Inc.). The crystals employed in this experiment are of high quality, being single crystals of Yb:YAG. XPS testing reveals a Yb doping concentration of 2%, as illustrated in Fig. 1(a). In the laser processing procedure, ultrafast laser beams are concentrated through the objective lens, inducing elevated temperature and high pressure within the crystal. This multipulse laser processing leads to the creation of voids and the emergence of microcracks, as illustrated in Fig. 1(b). By manipulating the XY motion platform, multiple laser processing iterations are applied within the horizontal modified layer, inducing the formation of voids and resulting in continuous microcracks. Ultimately, this process facilitates the slicing of the crystals [Figs. 1(c)–1(e)].

During the laser processing procedure, the laser pulse width plays a crucial role in the thermal effects^[28–32]. When the laser pulse width is shorter than the electron relaxation time, the thermal impact generated during processing is negligible^[31]. As the laser pulse width increases, the thermal effects induced by laser action propagate to the lattice, leading to thermal shock during processing and the formation of microcracks, as depicted in Fig. 2(a). When the pulse width surpasses 700 fs, single-pulse processing progressively induces microcracks within the crystal lattice. In addition, during the propagation of ultrafast laser within a crystal, the crystal exhibits a response to the laser field characterized by both linear and nonlinear polarizations. The alteration in refractive index is contingent upon not only the frequency of the laser field but also its intensity across both temporal and spatial dimensions^[32,33], n=n0+n2I(r,t),(1)where n0 represents the linear refractive index of the crystal, n2 is associated with the nonlinear refractive index induced by the Kerr effect, and I denotes the laser intensity. For Yb:YAG, n2 is a positive value (∼3.8×10−20m2/W)^[34]. When the light beam has peak intensity along the central axis, the refractive index change is maximized at the center of the beam. As the distance from the axis increases, the light intensity decreases, leading to a reduction in refractive index. In such a scenario, the medium behaves akin to a convex lens, causing the beam to refocus, as illustrated in Fig. 2(b). The variation in refractive index depends on laser intensity, but the self-focusing effect is contingent solely upon the peak power of the laser pulse. When the peak power surpasses the critical power for self-focusing, irreversible damage occurs within the crystal^[35]. The critical power for self-focusing, denoted as Pcr, is defined as follows: Pcr=3.77λ28πn0n2.(2)

During the slicing processing, when the single-pulse energy surpasses 14 µJ, the laser experiences refocusing within the crystal, resulting in secondary crystal damage [Fig. 2(c)]. However, with a single-pulse energy of 6 µJ, no secondary crystal damage is detected. During the crystal processing, the occurrence of microcracks generated by single-pulse processing is notably scarce, posing challenges in realizing crystal slicing. Utilizing a low-pulse-energy laser (0.5 µJ@50 kHz) in a multipulse processing mode, 750 pulses are applied at a single point. The transient high temperature generated by a high peak power laser (1.6 GW) induced the formation of voids within Yb:YAG, and the stress resulting from high pressure led to the development of cracks in the modified region [Figs. 2(d) and 2(e)]. Nevertheless, in contrast to crystal materials such as SiC, the observed crack formation did not exhibit a distinct preferred orientation when examined from the crystal surface^[21].

As the laser beam is focused from a medium with a refractive index of n1 to a sample with a refractive index of n2, the refraction occurring at the interface introduces variations in optical path lengths among beams entering at different angles. This discrepancy results in aberrations at the focal point^[36,37]. As shown in Fig. 3(a), due to the high refractive indices of Yb:YAG (1.82), the focused spot is notably elongated during the laser machining process. We conducted simulations to calculate the optical field distribution within the crystal. The simulation of internal electric field distribution within the crystal was carried out. As the processing depth increases from 0.17 to 0.36 mm, the laser focal depth is significantly elongated, and simultaneously, the laser power decreases [Figs. 3(b) and 3(c)]. In the course of the experiments, as the processing depth increases from 0.3 to 0.66 and 1 mm, the focal depth expands from approximately 15 to 75 µm [Fig. 3(d)]. However, with a depth of 1 mm, the laser energy experiences attenuation, leading to a reduction in the focal depth to around 43 µm. These experimental findings are consistent with the outcomes predicted by simulations.

The extension of laser focal depth has implications for the reduction in slicing precision and is unfavorable for crack generation and propagation. To enhance the accuracy of crystal slicing, aberration correction methods are essential to compress the focal spot depth. We employed an adjustment of the spacing within the lens group of the objective lens to compensate for and optimize the wavefront phase, achieving high-quality focusing within the Yb:YAG crystal. Zemax simulations [Figs. 3(e)–3(h)] demonstrated a significant compression of the focal spot depth after aberration correction. At a defocus of ±50μm, the disparity in spot size increased, providing evidence that aberration correction can enhance the quality of laser focusing and compress the focal depth. Figures 3(i) and 3(j) depict that, prior to aberration correction, the focal depth measures ∼50μm, whereas after aberration correction, it is compressed to ∼10μm. The energy consumed in generating a new surface through laser processing is given by the formula ES=AS×λS.(3)

In this context, ES represents the energy consumed for generating a new surface (J·m2), AS is the surface area of the newly generated surface (m2), and λS denotes the total surface energy of the material (J·m2). Before aberration correction, the modified area in the processing region results in a larger surface area, consuming more energy and reducing stress, leading to fewer microcracks. Aberration correction, on the other hand, compresses the focal depth, reducing the energy consumption for generating the new surface and increasing stress, resulting in more cracks. When the point spacing is 5 µm, cracks penetrate each focal spot in the modified area, as depicted in Figs. 3(g)–3(i).

Leveraging optimized laser parameters, a Yb:YAG crystal measuring 5mm×5mm×4mm underwent precision laser slicing, as depicted in Fig. 4(a). Upon comparison with the original crystal, it is evident that the modified region presents as a light-colored translucent layer. Figure 4(b) illustrates a homogeneous distribution of the modified region, accompanied by partial penetration of cracks. Specifically, the lateral perspective depicted in Fig. 4(c) reveals that cracks within the modified layer do not propagate into the interior of the crystal on either side of the modified layer.

Following laser processing, as the crystal did not cleave directly on both sides of the modified layer, external force is applied to facilitate the sustained extension of cracks along the modified layer, achieving the complete cleaving of the crystal. The process of cleaving is depicted in Fig. 4(d); the crystal is affixed to an aluminum metal block using epoxy resin. Following complete curing of the epoxy resin, the metal block is immersed in liquid nitrogen. During the cooling process, the linear thermal expansion model for cooling is expressed as ΔL=L0×α×ΔT,(4)where ΔL denotes the length change of the metal, L0 is its initial length, α is the linear expansion coefficient, and ΔT is the temperature change. Aluminum, with a thermal expansion coefficient of 22×10−6°C−1, higher than common metals like copper and iron, is chosen for Yb:YAG crystal cleaving. During liquid nitrogen cooling, the metal’s contraction induces compressive stress at one end of the crystal, causing nonuniform stress distribution inside the crystal and promoting the extension of cracks. The process can be described by Griffith’s theory of linear elastic fracture mechanics, σ=E·γπ·α,(5)where σ represents the stress at the crack tip, E is the elastic modulus of the material, γ is the surface energy of the glass, and α is the crack length at the crack tip. The condition for crack extension is met when the stress at the crack tip reaches the tensile strength of the material. The initiation of crack propagation occurs when the stress at the crack tip surpasses the tensile strength of Yb:YAG crystal (200–400 MPa), as defined by the mentioned formula. Ultimately, the propagation of cracks leading to crystal cleavage is driven by applied external stress [Fig. 4(e)]. The slicing mechanism involving the peeling of the modified layer from the substrate for Yb:YAG differs significantly from the mechanisms observed in gas expansion or crack propagation along specific crystallographic axis directions, as observed in GaN or SiC^[19,20]. The thickness of the sliced Yb:YAG is ∼700μm. The X-ray rocking curve measurement reveals that post laser slicing, the crystal wafer demonstrates a full width at half-maximum of 50.4′′, aligning with the high-quality single crystals reported previously^[38]. This observation implies that laser slicing had no discernible impact on the crystal structure of the YAG crystal. Following laser slicing, the crystal surface displays a periodic undulating structure [Figs. 4(g) and 4(h)]. Confocal microscopy testing reveals a surface roughness of 2.5 µm [Figs. 4(i) and 4(j)].

During the laser slicing processing, the crystal material absorbs multipulse laser energy, conducting part of the heat to the lattice, resulting in elevated temperatures and the formation of voids within the crystal [Fig. 5(a)]. This phenomenon induces internal stress, leading to the development of cracks in the proximity of the laser processing area. SEM images reveal an elevated oxygen content and reduced Al and Y element content in the laser processing region, indicative of partial material decomposition during the laser processing [Figs. 5(b) and 5(c)]. XPS analysis reveals a marked decrease in lattice oxygen content and a notable increase in defect-associated oxygen content following laser processing [Figs. 5(d) and 5(e)]. This implies structural disruption in the YAG lattice due to laser-induced effects.

Furthermore, numerical simulations are conducted to investigate the temperature and stress evolution within the crystal after laser processing, as illustrated in Figs. 5(f) and 5(g). The pulsed laser induces transient high temperatures within the crystal, leading to internal stresses of up to 250 MPa. These stresses trigger the formation of cracks within the crystal, which propagate within the modified layer under the influence of stress induced by external temperature gradients [Fig. 5(h)]^[39]. The process loss of the laser slicing method is reduced to ∼10μm, representing a substantial decrease in loss compared to traditional mechanical slicing methods (∼300μm). Consequently, the modified layer separates the upper and lower portions of the crystal [Figs. 5(i)–5(l)], leading to the successful production of thin slices of Yb:YAG (Φ12 mm).

To conclude, we have achieved laser slicing of a large Yb:YAG crystal by employing a multipulse laser processing approach. Specifically, by adjusting the laser pulse width and optimizing the thermal deposition process during laser modification, controlled crack propagation is achieved in the modified layer. Furthermore, the use of aberration correction methods optimized the energy distribution within the crystal, compressing the focal spot length of the laser in high refractive index crystals, reducing the thickness of the modified layer to ∼10μm. Moreover, thermal shock is applied to introduce external stress, driving the continuous expansion of cracks in the modified layer and ultimately realizing crystal slicing. The surface roughness of the crystal after laser slicing has been reduced to as low as 2.5 µm. This study provides an easy route for gain material slicing and presents a promising avenue towards achieving low-loss and cost-effective fabrication of large-size thin slices of crystals.