Extremely Low Ablation Rate of Metals Using XeCl Excimer Laser

Ablation thresholds for the metallic elements Ti, Mo, Pt, Au, and Al were investigated using a XeCl excimer laser in the fluence range of 0.1–20 J/cm2 . The ablation thresholds were estimated from the dependence of the ablation rate and the diameter of the crater produced by the laser irradiation on laser fluence. Two ablation thresholds obtained from the ablation rate dependence on fluence agreed well with those obtained from the diameter dependence. The lower ablation threshold is related to the threshold calculated using a one-dimensional thermal diffusion model. The results suggest that even at such extremely low ablation rates, melting plays an essential role in excimer laser ablation

Jitter-free 40-fs 375-keV electron pulses directly accelerated by an intense laser beam and their application to direct observation of laser pulse propagation in a vacuum

ジッターフリーな超短パルスな電子ビームを実証 --超短パルス電子による超高速な電磁場観察--. 京都大学プレスリリース. 2020-12-04.We report the generation of ultrashort bright electron pulses directly driven by irradiating a solid target with intense femtosecond laser pulses. The duration of electron pulses after compression by a phase rotator composed of permanent magnets was measured as 89 fs via the ponderomotive scattering of electron and laser pulses, which were almost at the compression limit due to the dispersion of the electron optics. The electron pulse compression system consisting of permanent magnets enabled extremely high timing stability between the laser pulse and electron pulse. The long-term RMS arrival time drift was below 14 fs in 4 h, which was limited by the resolution of the current setup. Because there was no time-varying field to generate jitter, the timing jitter was essentially reduced to zero. To demonstrate the capability of the ultrafast electron pulses, we used them to directly visualize laser pulse propagation in a vacuum and perform 2D mapping of the electric fields generated by low-density plasma in real time

Search for sub-eV scalar and pseudoscalar resonances via four-wave mixing with a laser collider

The quasi-parallel photon-photon scattering by combining two-color laser fields is an approach to produce resonant states of low-mass fields in laboratory. In this system resonances can be probed via the four-wave mixing process in the vacuum. A search for scalar and pseudoscalar fields was performed by combining 9.3 $\mu$J/0.9 ps Ti-Sapphire laser and 100 $\mu$J/9 ns Nd:YAG laser. No significant signal of four-wave mixing was observed. We provide the upper limits on the coupling-mass relation for scalar and pseudoscalar fields, respectively, at a 95\% confidence level in the mass region below 0.15~eV.Comment: Accepted by Prog. Theor. Exp. Phy

Ion emission from a metal surface through a multiphoton process and optical field ionization

In order to investigate the physics of ion emission under an intense optical field, the ions emitted from a laser-irradiated copper surface were studied by time-of-flight energy spectroscopy. The lowest laser fluence at which ions are emitted, F_{th,L}, is 0.028 J/cm[2], and two higher emission thresholds were identified at fluences of F_{th,M}=0.195 J/cm[2] and F_{th,H}=0.470 J/cm[2]. The relation between the number of emitted ions per pulse N_{i} and the laser fluence F was in good agreement with N_{i}∝F[4] for F_{th,L}−F_{th,M}, N_{i}∝F[3] for F_{th,M}−F_{th,H}, and N_{i}∝F[2] for ≥F_{th,H}. The dependence of ion production on laser energy fluence is explained well by multiphoton absorption and optical field ionization. Even at a low laser fluence such as 0.136 J/cm[2], the emitted ions have an energy of 30 eV, and the ion energy depends on the laser fluence (790 eV at 14.4 J/cm[2]). The laser fluence dependence of ion energy is reasonably well related to those of the interspaces of gratings that are self-organized on a metal surface by femtosecond laser pulses

Laser-induced fine structures on silicon exposed to THz-FEL

We found the irradiation of focused linearly polarized terahertz (THz)-waves emitted from THz free-electron laser (THz-FEL) engraved fine periodic stripe structures on the surfaces of single-crystal Si wafers. The experiments were performed at several wavelengths ranging from 50 to 82 μm with a macro-pulse fluence up to 32 J/cm2. The engraved structures are considered equivalent to the laser-induced periodic surface structures (LIPSS) produced by the irradiation of a femtosecond (fs)-pulsed laser in the near-infrared (NIR) region. However, the minimum period of ∼1/25 of the wavelength in the present case of THz-FEL is surely much smaller than those reported so far by use of fs-lasers and no more explicable by the so far proposed mechanisms. The finer LIPSS confirmed by longer-wavelength laser excitation by means of THz-FEL motivates investigation into the universal mechanism of LIPSS formation, which has been under a hot debate for decades

Creation of NV centers over a millimeter-sized region by intense single-shot ultrashort laser irradiation

一つの超短レーザーパルスでダイヤモンド量子センサ源を広領域で作製 --超短時間でダイヤモンドを超高感度量子センサに--. 京都大学プレスリリース. 2023-03-15.Recently, ultrashort laser processing has attracted attention for creating nitrogen-vacancy (NV) centers because this method can create single NV centers in spatially-controlled positions, which is an advantage for quantum information devices. On the other hand, creating high-density NV centers in a wide region is also important for quantum sensing because the sensitivity is directly enhanced by increasing the number of NV centers. A recent study demonstrated the creation of high-density NV centers by irradiating femtosecond laser pulses, but the created region was limited to micrometer size, and this technique required many laser pulses to avoid graphitization of diamond. Here, we demonstrate the creation of NV centers in a wide region using only an intense single femtosecond laser pulse irradiation. We irradiated a diamond sample with a femtosecond laser with a focal spot size of 41 µm and a laser fluence of up to 54 J/cm², which is much higher than the typical graphitization threshold in multi-pulse processing. We found that single-pulse irradiation created NV centers without post-annealing for a laser fluence higher than 1.8 J/cm², and the region containing NV centers expanded with increasing laser fluence. The diameter of the area was larger than the focal spot size and reached over 100 µm at a fluence of 54 J/cm². Furthermore, we demonstrated the NV centers' creation in a millimeter-sized region by a single-shot defocused laser pulse over 1100 µm with a fluence of 33 J/cm². The demonstrated technique will bring interest in the fundamentals and applications of fabricating ultrahigh-sensitivity quantum sensors

Directional linearly polarized terahertz emission from argon clusters irradiated by noncollinear double-pulse beams

It has been demonstrated that the interaction between argon clusters and intense femtosecond double laser pulses with appropriate intervals in time and space provides important properties for terahertz electromagnetic wave generation, namely, high forward directivity, power enhancement, and linear polarization with a variable direction. Irradiating argon clusters with double pulses (1 and 3 mJ, 40 fs, 810 nm) in 133-ps and 40-μm intervals results in terahertz wave emission in the forward direction that is 10 times greater than that for a single pulse. The polarization direction of terahertz electromagnetic waves can be varied by changing the relative focal positions of the first and second pulses

Demonstration of periodic nanostructure formation with less ablation by double-pulse laser irradiation on titanium

By pairing femtosecond laser pulses (duration ∼40 fs and central wavelength ∼810 nm) at an appropriate time interval, a laser-induced periodic surface structure (LIPSS) is formed with much less ablation than one formed with a single pulse. On a titanium plate, a pair of laser pulses with fluences of 70 and 140 mJ/cm² and a rather large time interval (>10 ps) creates a LIPSS with an interspace of 600 nm, the same as that formed by a single pulse of 210 mJ/cm², while the double pulse ablates only 4 nm, a quarter of the ablation depth of a single pulse

Terahertz Radiation from Combined Metallic Slit Arrays

We report an approach to efficiently generate terahertz radiation from a combined periodic structure. The proposed configuration is composed of two metallic slit arrays deliberately designed with different periodic length, slit width and depth. We found that the combination of the two slit arrays could provide special electromagnetic modes, which exhibit nonradiative property above the surface of one slit array and radiative property inside the other one. An electron beam holding proper energy could resonate with those modes to generate strong and directional electromagnetic radiations in the terahertz regime, indicating that the approach has the potential in developing high-performance terahertz radiation sources