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

Abstract

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.

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

“…The deposited energy density reveals power dependence on the laser fluence with a degree of 2.8, corresponding to the probability of impact ionization α = 0.01 fs −1 . It agrees with the value proposed in some works 16 .

Figure 5 ( a ) Dependence of deposited energy density (DED) for the different probability of impact ionization α on fluence.

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“…In particular, they heat the lattice 19 and induce GPa pressures 20 . The amount of transferred energy strongly depends not only on the plasma electron density but also on the electron temperature 16 . It also can be described in the framework of the deposited energy density (DED) 21 .…”

“…Shifting central wavelength of a driving pulse into the mid-IR can make the process of photoionization 16 dominant in semiconductors, avoid two- and three-photon absorption, and create single-shot femtosecond micromodification in a bulk Si for a broad range of energies. In this paper, we demonstrate that the tightly focused mid-IR femtosecond pulses are capable of micromodification creation due to overcoming deposited energy density threshold, determined by the latent heat of fusion (about 4 kJ cm −3 ).…”

High-Power Solid-State Near- and Mid-IR Ultrafast Laser Sources for Strong-Field Science

“…The possibility of using non-oxide crystals provides access to longer wavelengths. Additionally, such sources are preferred for other applications, such as studying energy transfer processes [52] or free carrier absorption in dielectrics [53]. At present, Cr:Forsterite laser sources with ~1 mJ pulse energy are commercially available.…”

“…Another way to localize laser energy inside the volume is to move to the mid-IR wavelength range. In this case (in comparison with the near-IR), the balance between tunneling, multiphoton and avalanche ionization shifts, and the heating in the laser pulse fields, begins to play an important role [53]. In the case of semiconductors, considering the relatively high multiphoton order, plasma delivery in the pre-focal region is weaker.…”