We report on high-field terahertz transients with 0.9-mJ pulse energy produced in a 400  mm² partitioned organic crystal by optical rectification of a 30-mJ laser pulse centered at 1.25 μm wavelength. The phase-locked single-cycle terahertz pulses cover the hard-to-access low-frequency range between 0.1 and 5 THz and carry peak fields of more than 42 MV/cm and 14 Tesla with the potential to reach over 80 MV/cm by choosing appropriate focusing optics. The scheme based on a Cr:Mg₂SiO₄ laser offers a high conversion efficiency of 3% using uncooled organic crystal. The collimated pump laser configuration provides excellent terahertz focusing conditions.

We investigated Terahertz generation in organic crystals DSTMS, DAST and OH1 directly pumped by a Cr:forsterite laser at central wavelength of 1.25 μm. This pump laser technology provides a laser-to-THz energy conversion efficiency higher than 3 percent. Phase-matching is demonstrated over a broad 0.1-8 THz frequency range. In our simple setup we achieved hundred μJ pulses in tight focus resulting in electric and magnetic field larger than 10 MV/cm and 3 Tesla.

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176Femtosecond (τ L ~ 100 fs) laser pulses incident on a metallic target are absorbed by conduction electrons in the skin layer with a depth of 10-20 nm. Owing to heat conduction, the energy absorbed by the electrons propagates into the target in the form of an electron heat wave [1][2][3][4][5]. Electronic energy is simultaneously transferred to the lattice through electron-phonon heat transfer. As a result, a heated layer with thickness d T ≈ 120 nm is formed in aluminum within the elec tron-ion relaxation time t eq = 2-3 ps. The lattice is melted when the energy flux of the femtosecond laser pulse exceeds the melting threshold for the given material. For aluminum, the absorbed energy at the melting threshold is estimated at ~15 mJ/cm 2 .The pressure in the heated layer increases strongly owing to the supersonic heat conduction for which the acoustic time t s = d T /c s ≈ 20 ps is much longer than the heating time t eq [1, 2, 5]. Acoustic decay of the heated and pressurized layer (d'Alembert's solution) and sub sequent reflection of acoustic waves at the vacuum interface are accompanied by generation of thermo mechanical tensile stresses [1,2,[5][6][7], where the amplitude of the tensile stresses increases with the energy density of the laser pulse F. Such motion results in deformation of the surface layer d T . When F exceeds the ablation threshold F abl , spallation (thermome chanical ablation) of part of the melted layer occurs as a result of the tensile stress exceeding the tensile strength of the liquid metal, and a crater forms on the surface. Our experiments and calculations indicate that, when the energy flux is slightly higher than the ablation threshold, the heated layer first expands to a certain value and then returns back with some residual deformation (see, e.g., Fig. 1).In the described experiments, the deformation of the surface of the target in the heated region was stud ied using femtosecond interference microscopy [8]. To analyze the structure of the surface layer after irradia tion near the ablation threshold, transmission electron microscopy was used.To heat and probe the surface of the target, 100 fs pulses generated by a femtosecond chromium-for sterite laser system were used. The surface of the target was heated by pulses having a fundamental wavelength of 1240 nm at an angle of incidence of 45°. The target was probed by the 620 nm second harmonic pulses with the measured time delay with respect to the heat ing pulse. The spatial distribution of the energy density in the focal spot had the Gaussian formIt has been revealed experimentally that nanocavities remain inside a surface layer of aluminum after action of a femtosecond laser pulse. This result is in agreement with numerical simulation. A detailed picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt has been reconstructed through atomistic simulation. It has been shown that the bubbles do not fully collapse but remain as frozen disk shaped nanocavities upon recrystallization ...

The theory and experiments concerned with the electron-ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature equation of state, many-body interatomic potential, the electron-ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid-liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump-probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are 1% and ∼ 1 nm , respectively. It is found that, firstly, the electron-electron collisions make a minor contribution to a light absorbtion in solid Al at moderate intensities; secondly, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during 0 < t < 4 ps, where t is time delay between the pump and probe pulses measured from the maximum of the pump; thirdly the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of d-electrons.Key words: femtosecond laser ablation, pump-probe, optics of hot Al and Au PACS: 52.38.Mf, 52.25.Os, 02.70.Ns Supersonic heating and meltingFigures 1,2 show diagrams of processes in pump femtosecond laser pulse (fsLP) action on metal. The three time slices "ei", m 1 m 2 , and c 1 c 2 in Fig. 1 correspond to the following non-equilibrium processes: (e-i) the electron-ion thermal relaxation, (m) the melting of an overheated crystal lattice, and (c) the cavitation decay of a metastable state. Duration of fsLP τ L ∼ 40 − 100 fs is shorter than characteristic times of these three processes. They have very various time scales from subpicoseconds to nanoseconds. The electron overheating (T e ≫ T i ) starts from ei 1 when a fsLP arrives [1,2,3,4,5,6,7,8,9] and disappears at ei 2 when temperatures T e , T i equilibrate (t eq = t ei2 = 3 − 6 ps for Al at our intensities). The time is reckoned from the maximum of pump fsLP in Fig. 1. Since arriving of the pump to a target the conductivity electrons become much hotter than the ions. Two-temperature (2T) matter with hot electrons transits to a peculiar state with thermodynamic and optical characteristics different from one-temperature (1T) case. In 2T there are appearance of excesses of electron energy and pressure above equilibrium 1T ones. Also there are changes in elastic moduli and band structure. In semiconductor lattice the binding forces become weaker with increase of T e , while in metals situation is opposite. Large changes in optics of Au at high T e result from * +7- 495-7029317, Russian Federation, 142432, Chernogolovka Email address: nailinogamov@googlemail.com excitation of d-electrons. On account of the ion heat capacity C i (thermal "inertia" of a lattice) the beginning of melting t m1 ∼ C i T m /αT e is de...

We report on the experimental observation of high-power terahertz-radiation-induced damage in a thin aluminum film with a thickness less than a terahertz skin depth. Damage in a thin metal film produced by a single terahertz pulse is observed for the first time. The damage mechanism induced by a single terahertz pulse could be attributed to thermal expansion of the film causing debonding of the film from the substrate, film cracking, and ablation. The damage pattern induced by multiple terahertz pulses at fluences below the damage threshold is quite different from that observed in single-pulse experiments. The observed damage pattern resembles an array of microcracks elongated perpendicular to the in-plane field direction. A mechanism related to microcracks' generation and based on a new phenomenon of electrostriction in thin metal films is proposed.

Second Harmonic Generation induced by the electric field of a strong nearly single-cycle terahertz pulse with the peak amplitude of 300 kV/cm is studied in a classical inorganic ferroelectric thin film of (Ba0.8Sr0.2)TiO3. The dependences of the SHG intensity on the polarization of the incoming light is revealed and interpreted in terms of electric polarization induced in the plane of the film. As the THz pulse pumps the medium in the range of phononic excitations, the induced polarization is explained as a dynamical change of the ferrolectric order parameter. It is estimated that under action of the THz pulse the ferroelectric order parameter acquires an in-plane component up to 6% of the net polarization.