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

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.

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

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.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front.

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to ß-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

Control of spectral shift, broadening, and pulse compression during mid-IR self-guiding in high-pressure gases and their mixtures.

Precise control of the nonlinear optical phenomena is the limiting factor for the spectral broadening and pulse compression techniques for high-power laser systems. Here we demonstrate that generation of the blue and red components under filamentation of 4.55-µm mid-IR pulses can be easily adjusted independently through the use of inert and molecular gases, while uniform broadening up to 1-µm bandwidth at the 1/e2 level relies on the proper choice of gas mixture and its compounds partial pressure. Such synthesized media provide a feasible route for the free of damage control of pulse spectral broadening and compression for gigawatt peak power laser systems operating in the mid-IR. Additional management of a generated spectrum can be realized through the adjustment of focusing conditions. The resulted pulse is compressed by a factor of 2.6 down to 62 fs pulse duration (4.1 optical cycles) with additional dispersion compensation. Controllable nonlinear compression down to four optical cycles keeping the millijoule energy level of a mid-IR laser pulse provides direct access to extreme nonlinear optics.

High-gain broadband laser amplification of mid-IR pulses in Fe:CdSe crystal at 5†µm with millijoule output energy and multigigawatt peak power.

We report on a first of its kind, to our knowledge broadband amplification in a Fe:CdSe single crystal in the mid-IR beyond 5 µm. The experimentally measured gain properties demonstrate saturation fluence close to 13 mJ/cm2 and support the bandwidth up to 320 nm (full width at half maximum). Such properties allow the energy of the seeding mid-IR laser pulse, generated by an optical parametric amplifier, to be pushed up to more than 1 mJ. Dispersion management with bulk stretcher and prism compressor enables 5-µm laser pulses of 134-fs duration, providing access to multigigawatt peak power. Ultrafast laser amplifiers based on a family of Fe-doped chalcogenides open the route for wavelength tuning together with energy scaling of mid-IR laser pulses that are strongly demanded for the areas of spectroscopy, laser-matter interaction, and attoscience.

Hybrid x-ray laser-plasma/laser-synchrotron facility for pump-probe studies of the extreme state of matter at NRC "Kurchatov Institute".

We developed a hybrid optical pump-x-ray probe facility based on the "Kurchatov's synchrotron radiation source" and terawatt (TW) femtosecond laser. The bright x-ray photon source is based on either synchrotron radiation [up to 6 × 1014 photons/(s mm2 mrad2 0.1% bandwidth)] or laser-plasma generators (up to 108 photons/sr/pulse). The terawatt (TW) femtosecond laser pulse initiated phase transitions and a non-stationary "extreme" state of matter, while the delayed x-ray pulse acts as a probe. The synchronization between synchrotron radiation and laser pulses is achieved at 60.3 MHz using an intelligent field-programmable gate array-based phased locked loop. The timing jitter of the system is less than 30 ps. In laser-plasma sources, the x-ray and laser pulses are automatically synchronized because they are produced by using the same laser source (TW laser system). We have reached an x-ray yield of about 106 photons/sr/pulse with 6-mJ sub-ps laser pulses and using helium as a local gas medium. Under vacuum conditions, the laser energy increase up to 40 mJ leads to the enhancement of the x-ray yield of up to 108 photons/sr/pulse. The developed hybrid facility paves the way for a new class of time-resolved x-ray optical pump-probe experiments in the time interval from femtoseconds to microseconds and the energy spectrum from 3 to 30 keV.

Optical Diagnostics of Supercritical CO2 and CO2-Ethanol Mixture in the Widom Delta.

The supercritical CO2 (scCO2) is widely used as solvent and transport media in different technologies. The technological aspects of scCO2 fluid applications strongly depend on spatial-temporal fluctuations of its thermodynamic parameters. The region of these parameters' maximal fluctuations on the p-T (pressure-temperature) diagram is called Widom delta. It has significant practical and fundamental interest. We offer an approach that combines optical measurements and molecular dynamics simulation in a wide range of pressures and temperatures. We studied the microstructure of supercritical CO2 fluid and its binary mixture with ethanol in a wide range of temperatures and pressures using molecular dynamics (MD) simulation. MD is used to retrieve a set of optical characteristics such as Raman spectra, refractive indexes and molecular refraction and was verified by appropriate experimental measurements. We demonstrated that in the Widom delta the monotonic dependence of the optical properties on the CO2 density is violated. It is caused by the rapid increase of density fluctuations and medium-sized (20-30 molecules) cluster formation. We identified the correlation between cluster parameters and optical properties of the media; in particular, it is established that the clusters in the Widom delta acts as a seed for clustering in molecular jets. MD demonstrates that the cluster formation is stronger in the supercritical CO2-ethanol mixture, where the extended binary clusters are formed; that is, the nonlinear refractive index significantly increased. The influence of the supercritical state in the cell on the formation of supersonic cluster jets is studied using the Mie scattering technique.

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.

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[Formula: see text] this transition occurs around 3-4 [Formula: see text]m corresponding to the region of the lowermost PFT. The region of the uppermost PFT is reached around 1 [Formula: see text]m and is governed by an interplay of photoionization and weak-field free carrier absorption which manifests itself in both MgF[Formula: see text] and SiO[Formula: see text]. 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.

40 kHz, 20 ns acousto-optically Q-switched 4 µm Fe:ZnSe laser pumped by a fluoride fiber laser.

An actively Q-switched mid-infrared Fe:ZnSe laser pumped by a continuous wave fluoride fiber laser has been developed. Stable operation with a pulse duration of 20 ns and a repetition rate of 40 kHz at 4 µm was achieved. The maximum peak power was 1.1 kW. The high-repetition rate, high-peak power nanosecond pulsed laser, which has been created for the first time, to the best of our knowledge, in an actively Q-switched Fe:ZnSe laser, should prove a suitable light source for laser processing and molecular sensing.

3.5-mJ 150-fs Fe:ZnSe hybrid mid-IR femtosecond laser at 4.4 µm for driving extreme nonlinear optics.

We report on entering a new era of mid-IR femtosecond lasers based on amplification in a relatively new gain chalcogenide medium, Fe:ZnSe. Our hybrid all-solid-state laser system is based on direct pulse amplification of femtosecond seed from three-stage AGS-based-optical parametric amplification (OPA) in a Fe:ZnSe laser crystal optically pumped by a Cr:Yb:Ho:YSGG Q-switched nanosecond laser. The development of the pump source with output energy up to 90 mJ operating at a 10 Hz repetition rate regime and highly efficient grating compressor (80%) provides 3.5-mJ 150-fs femtosecond pulses centered at 4.4 µm. Diode-pumped Er:YAG/Er:YLF lasers make it possible to increase the beam quality and repetition rate of the proposed laser system up to 100 Hz. Focusing such a laser radiation into the ∼3λ beam diameter allows us to reach a focus laser intensity up to 1016 W/cm2 which is only an order of magnitude lower than a relativistic intensity of 1017 W/cm2 and enough to drive strong nonlinear optics in mid-IR. We show as a proof-of-principle experiment the generation of four-octave spanning (from 350 nm up to 5.5 µm) supercontinuum in xenon.

Anomalous behavior of nonlinear refractive indexes of CO2 and Xe in supercritical states.

Direct measurement of pressure dependent nonlinear refractive index of CO2 and Xe in subcritical and supercritical states are reported. In the vicinity of the ridge (or the Widom line), corresponding to the maximum density fluctuations, the nonlinear refractive index reaches a maximum value (up to 4.8*10-20m2/W in CO2 and 3.5*10-20m2/W in Xe). Anomalous behavior of the nonlinear refractive index in the vicinity of a ridge is caused by the cluster formation. That corresponds to the results of our theoretical assumption based on the modified Langevin theory.

Generation of an adjustable multi-octave supercontinuum under near-IR filamentation in gaseous, supercritical, and liquid carbon dioxide.

A new supercontinuum (SC) source from high-pressure gas, liquid, and supercritical fluid CO2 aggregate states that covers more than two octaves (from 400 nm up to 2 µm) is successively reported by using a 200 fs pulsed pump at 1.24 µm under femtosecond filamentation. The key features of the proposed source are the highly adjustable nonlinear properties (comparable with condensed matter) of the medium. This allows an easy-to-achieve filamentation process, even at microjoule laser pulse energies, giving the ultrabright and broadband SC. The molecular vibrations significantly modify the SC spectrum; as a result, a bright peak in a 1.4-1.9 µm is generated. Its position could be finely tuned by the pressure and temperature. We report that the generation of the SC in the monofilamentation regime (unlike multifilamentation) is more stable and promising for seeded optical parametric amplifiers, and the most efficient SC generation is achieved in the liquid phase of CO2.