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

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

A passively Q-switched compact Er:Lu2O3 ceramics laser at 2.8 μm with a graphene saturable absorber

We have demonstrated a passively Q-switched Er:Lu2O3 ceramics laser using a monolayer graphene saturable absorber (SA). Stable pulsed operation with watt-level average power was achieved by a compact linear cavity without focusing on the SA. This is the first demonstration of a passively Q-switched mid-IR Er:Lu2O3 laser using a graphene SA. A maximum pulse energy of 9.4 μJ and a peak power of 33 W were achieved with a 247 ns pulse duration. To our knowledge, this is the shortest pulse duration, highest pulse energy, and highest peak power obtained with a graphene SA in the 3 μm wavelength region

High-efficiency, continuous-wave Fe:ZnSe mid-IR laser end pumped by an Er:YAP laser

Fe:ZnSe lasers operating in the mid-IR spectral region have gained widespread attention due to their numerous potential applications. This study presents a high-efficiency, continuous-wave Fe:ZnSe laser end pumped by an Er:YAP laser at 2920 nm. The Er:YAP laser was home-constructed and generated an output power of 3.6 W and an average slope efficiency of 36.6% with a good beam quality (M2 ≤ 2). The Fe:ZnSe laser produced a maximum output power of 1 W at 4.06 µm for 2.1 W of absorbed pump power, corresponding to an average slope efficiency of 48%. Theoretical modeling of the continuous-wave Fe:ZnSe laser is presented to determine the prospects for further power scaling

Power scalable 30-W mid-infrared fluoride fiber amplifier

A fluoride-fiber-based master oscillator power amplifier (MOPA) for 30-W class continuous-wave (cw) operation at 2.8-μm wavelength has been demonstrated. To overcome the low durability of ZBLAN fibers, various novel technologies for using fluoride glass with a ZBLAN-fiber-based side-pump combiner have been adopted in the system. A maximum cw output power of 33 W and stable operation under 23-W output have been demonstrated. We suggest that such fiber MOPA systems will open up advanced fluoride fiber technology for next-generation high-power mid-IR lasers

Magnetized Fast Isochoric Laser Heating for Efficient Creation of Ultra-High-Energy-Density States

The quest for the inertial confinement fusion (ICF) ignition is a grand challenge, as exemplified by extraordinary large laser facilities. Fast isochoric heating of a pre-compressed plasma core with a high-intensity short-pulse laser is an attractive and alternative approach to create ultra-high-energy-density states like those found in ICF ignition sparks. This avoids the ignition quench caused by the hot spark mixing with the surrounding cold fuel, which is the crucial problem of the currently pursued ignition scheme. High-intensity lasers efficiently produce relativistic electron beams (REB). A part of the REB kinetic energy is deposited in the core, and then the heated region becomes the hot spark to trigger the ignition. However, only a small portion of the REB collides with the core because of its large divergence. Here we have demonstrated enhanced laser-to-core energy coupling with the magnetized fast isochoric heating. The method employs a kilo-tesla-level magnetic field that is applied to the transport region from the REB generation point to the core which results in guiding the REB along the magnetic field lines to the core. 7.7 $\pm$ 1.3 % of the maximum coupling was achieved even with a relatively small radial area density core ($\rho R$ $\sim$ 0.1 g/cm$^2$). The guided REB transport was clearly visualized in a pre-compressed core by using Cu-$K_\alpha$ imaging technique. A simplified model coupled with the comprehensive diagnostics yields 6.2\% of the coupling that agrees fairly with the measured coupling. This model also reveals that an ignition-scale areal density core ($\rho R$ $\sim$ 0.4 g/cm$^2$) leads to much higher laser-to-core coupling ($>$ 15%), this is much higher than that achieved by the current scheme

Hot electron and ion spectra on blow-off plasma free target in GXII-LFEX direct fast ignition experiment

Polystyrene deuteride shell targets with two holes were imploded by the Gekko XII laser and additionally heated by the LFEX laser in a direct fast ignition experiment. In general, when an ultra-intense laser is injected into a blow-off plasma created by the imploding laser, electrons are generated far from the target core and the energies of electrons increase because the electron acceleration distance has been extended. The blow-off plasma moves not only to the vertical direction but to the lateral direction against the target surface. In a shell target with holes, a lower effective electron temperature can be realized by reducing the inflow of the implosion plasma onto the LFEX path, and high coupling efficiency can be expected. The energies of hot electrons and ions absorbed into the target core were calculated from the energy spectra using three electron energy spectrometers and a neutron time-of-flight measurement system, Mandala. The ions have a large contribution of 74% (electron heating of 4.9 J and ion heating of 14.1 J) to target heating in direct fast ignition