Verification of suitable phonon modes. Zirconia is an ionic material with a large phonon anharmonicity and it has been shown that zirconia doped with about 3 mol% Y2O3 consists mainly of a meta-stable tetragonal phase at room temperature 23,24. In this work, we used commercial partially stabilized zirconia ceramic plates (3.5 mol% Y2O3) with a high volume-fraction of t-ZrO2. t-ZrO2 becomes cubic (c-ZrO2) at higher temperatures and can become monoclinic (m-ZrO2) due to mechanical strain 23,24,25. Figure 1a shows the crystal structures of ZrO2 (different projections are shown in Fig. S1 of the Supplementary Information). The skeleton of the m-ZrO2 lattice exhibits a shear distortion of 9° parallel to the basal plane of the unit cell of t-ZrO2 23. Since the tetragonal to monoclinic transformation is accompanied by a volume expansion, the microcracks that are normally caused by the deformation of a polycrystalline material, are suppressed via this stress-induced transformation of t-ZrO2 23,24,25,26. Figure 1b shows the infrared reflection and Raman spectra, indicating that the t-ZrO2 has phonons near 4 THz 27. This mode was excited resonantly by the THz-FEL. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) data of our sample are provided in Fig. S2 of the Supplementary Information.
Irradiation of the sample with a single THz macropulse. For this experiment, a partially stabilized zirconia ceramic plate was irradiated with a single macropulse as shown in Fig. 2a. Each photograph in Figs. 2b–2e shows a scar on the surface of the t-ZrO2 plate generated by a different macropulse energy. The photograph of the whole plate is shown in Fig. S3 of the Supplementary Information. Irradiation scars were already clearly visible above macropulse energies of 2 mJ. Figure 2g shows that, when a macropulse energy of 29 mJ is used, the height of the centre of the irradiation spot lies below the initial level while the outer regions lie above. These features indicate the occurrence of melting; we interpret that the t-ZrO2 was melted in the centre by thermal heating and a strong elastic wave propagated towards the outer region and the rebound of the elastic wave results in the higher region around the depressed centre area. On the other hand, at a macropulse energy of 7.5 mJ as shown in Fig. 2f, the height of the centre of the irradiation spot of t-ZrO2 rises. We confirmed that this rough surface does not consist of individual small particles that can be simply removed by cleaning the surface.
In order to investigate the THz-induced crystal-structure change, we performed XRD measurements. Figure 3a shows three XRD patterns recorded at sample areas without irradiation (black curve), with weak single-shot irradiation (red curve; no melting), and with strong single-shot irradiation (blue curve; melting occurred). The photograph of the plate used in this experiment and the XRD data of the whole data range are shown in Fig. S4 of the Supplementary Information. The peaks at 30.16°, 34.60°, and 35.14° are characteristic of t-ZrO2 28. We also can confirm two peaks at 28.16° and 31.36°, which are the same 2θ-values as those of peaks observed for m-ZrO2 29. However, the intensity ratio of these two peaks in the spectrum of the area without irradiation is not that observed for typical m-ZrO2. It has been reported that t-ZrO2 sometimes has planar defects with the same structure as m-ZrO2 for the crystal twining 23,24. This is also supported by the broad linewidth of the XRD peak at 30.16° in Fig. 3a and the transmission electron microscope (TEM) image in Fig. S5a of the Supplementary Information. By the irradiation of the sample with the weak THz pulse, the XRD peak at 28.16° decreases and that at 31.36° increases. These relative intensities are consistent with bulk monoclinic zirconia, suggesting that the weak THz pulses induce a martensitic transformation to the monoclinic phase.
To selectively characterize the phase at the centre of an individual irradiation scar, we performed Raman spectroscopy using a confocal microscope. Figure 3c shows the Raman spectra obtained at the centres of irradiation scars generated by different macropulse energies. For the region without irradiation, Raman peaks characteristic of t-ZrO2 appear 27,30. On the other hand, for the irradiation scar generated by a macropulse with an energy of 7.5 mJ (Fig. 3c; red curve), new Raman peaks appear at 170, 184, and 369 cm-1. These peaks are characteristic of m-ZrO2 27,30, suggesting that the phase transformation contributed to the overall height increase in Fig. 2f. Above excitation pulse energies of 20 mJ (Fig. 3c; blue curve), the Raman peaks that are characteristic of m-ZrO2, are less pronounced and all Raman peaks become broad due to crystal melting.
To clarify the contribution of the phase transformation to the deformation of the zirconia surface as seen in Fig. 2f, we also investigated the irradiation scar on an 8-mol% Y2O3-stabilized zirconia sintered plate. It is intrinsically cubic and does not exhibit a phase transformation under pressure or exposure to high temperatures 24. The photographs, height profiles, and Raman spectra of irradiation scars for different macropulse energies are shown in the Extended Data Fig. 1. The Raman spectroscopy data clarifies that a phase transformation has not occurred in this sample. Innumerable peeling structures on the surface suggesting a thermal volume expansion and its rebound without melting.
Temporal evolution of the surface roughness. The changes in the surface condition of the irradiation spot were investigated in the time domain by detecting the intensity of a visible laser reflected from the irradiation spot. Figure 4a shows the experimental setup. In this experiment, we reduced the macropulse width to 1 ms to improve the temporal resolution. Fifty consecutive shots of a weak macropulse (repetition rate 5 Hz) were used and we measured the temporal evolution of the change in the reflected intensity, , for each macropulse. Figure 4b shows the photographs of the three irradiation scars generated by the irradiation with fifty macropulses at three different pulse energies (0.99, 1.7, and 2.4 mJ). Note that irradiation scars appeared even below the threshold macropulse energy of 2 mJ for the single-shot excitation, implying a partial phase transformation even by low-energy macropulses. Figure 4c shows the three time profiles of the changes in observed after the first macropulse for 0.99, 1.7, and 2.4 mJ, respectively is the initial reflected-beam intensity). Here, decreases sharply within the macropulse width, indicating that the THz pulse irradiation causes a rapid volume expansion. The reflected intensity decreases, because the surface roughness increases as shown in Fig. 2f. In all three curves, this reduction of continues up to 10 ms after excitation. This is the time required by all modes, including high-frequency phonons, to reach thermal equilibrium. The inset plots the macropulse energy dependence of measured at a time of 10 ms after the excitation.
These observed changes of in the early time region were mainly reversible and they decayed within several milliseconds. We extracted the magnitude of the THz-induced irreversible change of each macropulse by measuring the that occurs immediately before irradiation with the next macropulse about 200 ms later. Figure 4d plots the dependences of the irreversible component on the number of macropulse shots. For pulse energies larger than 2 mJ, the irreversible component is positive after the first pulse and it becomes negative in subsequent shots. For 1.7 mJ, a gradual increase of the irreversible component with the shot number can be confirmed. A plausible interpretation is that a partial phase transformation compensated the voids in the ceramic 24.
Irradiation with a near-infrared pulse
Figures 5a and 5b show the irradiation scars generated by irradiation with near-infrared (NIR) pulses using excitation intensities of 10 μJ and 200 μJ, respectively. Clear traces of ablation due to electron heating and evaporation are observed. The black spots are characteristic of oxygen-deficient zirconia 31. These results reproduce the results of a previous report, in which laser ablation of zirconia was investigated from the aspect of laser machining and laser pulse deposition techniques 32. In spite of the large bandgap energy (>5 eV) 31, laser-induced plasma is generated via multi-photon absorption, which accelerates the desorption of oxygen atoms.
Figure 5c shows a Raman spectrum at the centre position of the irradiation scar generated by using a NIR optical pulse. Tiny Raman peaks that are characteristic of the monoclinic phase can be confirmed, which indicates a phase transformation due to stress induced by rapid heating and/or cooling near the ablation trace. However, the comparison between the purple and red curves in Fig. 5c shows that the peak intensity of the monoclinic phase induced by the NIR optical pulse is much weaker than that induced by the THz pulse. We found that the irradiation scar generated by the NIR pulse is quite different from that generated by the THz pulse. We consider that this is a result of the selective excitation of the lowest infrared-active phonon by the THz pulse, while the NIR pulse can only populate the phonon modes via electron–phonon equilibration.