Ultrafast third harmonic generation imaging of microplasma at the threshold of laser-induced plasma formation in solids

Smetanina

et al. 2020

the development of high power mid-iR laser applications requires a study on laser induced damage threshold (LIDT) in the mid-IR. In this paper we have measured the wavelength dependence of the plasma formation threshold (PFT) that is a LIDT precursor. In order to interpret the observed trends numerically, a model describing the laser induced electron dynamics, based on multiple rate equations, has been developed. We show both theoretically and experimentally that PFT at mid-IR wavelengths is controlled by a transition from weak-to strong-field regime of free carrier absorption. in the case of Mgf 2 this transition occurs around 3-4 µ m corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 µ m and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF 2 and Sio 2. The PFT observed in considered materials exhibits a universal dependence on the excitation wavelength in dielectrics. Thus, the presented results pave the route towards efficient and controllable laser-induced material modifications and should be of direct interest to laser researchers and application engineers for prevention of laser-induced damage of optical components in high-intensity mid-IR laser systems. Recent advances in the development of ultra-short laser sources in mid-and far-infrared (IR) regions 1-8 lead to a vast number of new applications. However, a primary technological challenge still relevant for researches is the manufacturing of optical components with high laser-induced damage threshold and desired properties (for instance, high transparency or reflectivity in a wide wavelength range, group delay, etc.). This implies a need for experimental investigation of material damage threshold in the IR spectral region 9,10. Thin-films made of multi-layer stacks of low (fluorides, SiO 2) and high (silicon, germanium) refractive index materials are a key component of high-performance coatings in the mid-IR. Fluorides are also popular host matrices for mid-IR solid-state 11 and fiber lasers 12. The knowledge on damage threshold is also important for a new fascinating mid-IR application of dielectric laser accelerators (DLAs) 13,14. In the mid-IR spectral region, promising materials for on-chip DLA technology are dielectric materials, which have a higher material damage threshold compared to silicon 14,15. Well-developed applications such as ultrafast laser materials processing 16 and direct laser writing 17 may also have advantages in switching from visible and near-IR to mid-IR. A promising way for controlling the energy deposition inside transparent dielectric materials is to use a two-color ultrafast excitation 18-24. In this scheme a long wavelength heating pulse is more advantageous due to favorable scaling of avalanche ionization (AI) rate and free electron absorption. However, a lack of knowledge of the laser-induced damage threshold (LIDT) dependence on the wavelength prevents the optimization of such a concept.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR

Migal

Smetanina

et al. 2020

“…In addition to post-mortem microscopy, we also use in-situ diagnostics of micromodification formation based on the third harmonic (~ 1.5 μm) generation. Under tight focusing, the third harmonic is generated on the medium inhomogeneities 27 . Thus, the creation of micromodification will lead to the appearance of the third harmonic signal, which serves as an indicator of successful bulk micromachining (see Fig.…”

Section: Resultsmentioning

confidence: 99%

“…2 b). We scanned the created micromodifications using the third harmonic imaging technique 27 . When the sample with micromodification to be inscribed in bulk is shifted along the direction perpendicular to the optical axis, a peak in the third harmonic signal corresponds to the center of the micromodification.…”

Section: Resultsmentioning

confidence: 99%

Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams

Pushkin

Migal

et al. 2022

Being the second most abundant element on earth after oxygen, silicon remains the working horse for key technologies for the years. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. Mid-IR (beyond 4 µm) ultrafast lasers provide the required control for all these parameters not only on the surface (as in the case of the lithographic techniques), but also inside the bulk of the semiconductor, paving the way to an unprecedented variety of properties that can be encoded via such an excitation. We estimated the deposited energy density as 6 kJ cm−3 inside silicon under tight focusing of mid-IR femtosecond laser radiation, which exceeds the threshold value determined by the specific heat of fusion (~ 4 kJ cm−3). In such a regime, we successfully performed single-pulse silicon microstructuring. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a “ring” with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 × 1021 cm−3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon.

“…The TH signal was registered by a photomultiplier tube (Hamamatsu H10722-113). As we have shown in our publications earlier, this technique is much more effective than the traditional one, within which the radiation is recorded at the fundamental wavelength 29 , 36 . A detailed description of the experimental setup can be found elsewhere 29 , 36 , 37 .…”

Section: Methodsmentioning

confidence: 90%

Dynamics of ultrafast phase transitions in MgF2 triggered by laser-induced THz coherent phonons

Potemkin

2022

The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump–probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF2 in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3 ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5–10 ps, and the lifetime of each phase is on the order of 40–60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.