Ludovic Rapp

Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties.

Generation of high energy density by fs-laser-induced confined microexplosion

Confined microexplosion produced by a tightly focused fs-laser pulse inside transparent material proved to be an efficient and inexpensive method for achieving high energy density up to several MJ per cm3 in the laboratory table-top experiments. First studies already confirmed the generation of TPa-range pressure, the formation of novel super-dense material phases and revealed an unexpected phenomenon of spatial separation of ions with different masses in hot non-equilibrium plasma of confined microexplosion. In this paper, we show that the intense focused pulse propagation accompanied by a gradual increase of ionization nonlinearity changes the profile and spectrum of the pulse. We demonstrate that the motion of the ionization front in the direction opposite to the pulse propagation reduces the absorbed energy density. The voids in our experiments with fused silica produced by the microexplosion-generated pressure above Youngs modulus indicate, however, that laser fluence up to 50 times above the ionization threshold is effectively absorbed in the bulk of the material. The analysis shows that the ion separation is enhanced in the non-ideal plasma of microexplosion. These findings open new avenues for the studies of high-pressure material transformations and warm dense matter conditions by confined microexplosion produced by intense fs-laser.

Ultrafast Laser Induced Confined Microexplosion: A New Route to Form Super-Dense Material Phases

Laser 3d Nanolithography.- A Decade of Advances in Femtosecond Laser Fabrication: Mechanisms and Applications.- Photophysics of Nanostructured Metal and Metal-Contained Composite Films.- Light Scattering by Small Particles and Their Light Heating: New Aspects of the Old Problems.- Ultrafast Laser-Induced Confined Microexplosion: A New Route to Form Super-Dense Material Phases.- Phase-Structure Changes in Optical Materials under Laser Action.- Fs Laser Induced Reversible and Irreversible Processes in the Material Bulk.- Laser Decontamination: Problems and Prospects.- Atomistic Modeling of Generation of Crystal Defects and Microstructure Development in Short-Pulse Laser Processing of Metals. Optical Breakdown in Ambient Gas and its Role in Material Processing by Short-Pulsed Lasers.- Laser -Induced Local Oxidation :Theory, Experiment and Applications.- Laser Nanostructuring of Polymers.- Selective Ablation of Thin Films by Short-Laser Pulses.- Laser Crystallization of Metals.

Photoluminescence from voids created by femtosecond-laser pulses inside cubic-BN.

Photoluminescence (PL) from femtosecond-laser-modified regions inside cubic-boron nitride (c-BN) was measured under UV and visible light excitation. Bright PL at the red spectral range was observed, with a typical excited state lifetime of ∼4  ns. Sharp emission lines are consistent with PL of intrinsic vibronic defects linked to the nitrogen vacancy formation (via Frenkel pair) observed earlier in high-energy electron-irradiated and ion-implanted c-BN. These, formerly known as the radiation centers, RC1, RC2, and RC3, have been identified at the locus of the voids formed by a single femtosecond-laser pulse. The method is promising to engineer color centers in c-BN for photonic applications.

Ultrafast laser-induced microexplosion: material modification tool

Femtosecond Bessel pulses with a needle-like intensity distribution were focused inside sapphire crystal to create voids and the shock-wave affected volume which is by more than two orders of magnitude larger as compared with that made by the Gaussian pulse.

Experimental observation for new polymorphs of silicon formed through ultrafast-laser-induced microexplosion

Summary form only given. Intense ultrafast laser pulses tightly focused in the bulk of transparent material produce plasma in the extreme conditions similar to those in the cores of planets. The plasma generates strong shock waves in such confined geometry, thus inducing a laser-ignited microexplosion. This new method of compression of matter by ultra-short laser induced micro-explosion generates pressures in excess of Terapascals, leaving all the pressure/temperature-affected material confined inside the bulk of pristine crystal for the further investigations. In contrast to dynamic (shock wave) and static (diamond-anvil cell) methods, the initial materials in a microexplosion are transformed into the high entropy state of extreme dense plasma where the memory of the initial state is completely lost. This state is similar to “a primeval soup” at the early stages of the Universe evolution. The randomised material swiftly cools down isochorically to ambient in a short, nanosecond-scale time. For example, it was demonstrated that a sapphire crystal converted by a fs-laser pulse to plasma returns to the ambient state as a mixture of nano-crystallites of the previously unobserved form of bcc-aluminium. In this presentation the new experimental results evidencing the formation of novel structures in laserinduced confined micro-explosion in silicon will be highlighted. Electron diffraction pattern of the shock wave isochorically affected areas reveals the presence of a mixture of silicon phases with a number of previously unidentified diffraction spots. Indexation of the diffraction patterns from various microexplosion sites demonstrate close correlation between the numerically predicted phases and the observed diffracted spots. The observation of a new Si phase is a confirmation that ultrafast laser-induced microexplosion in confine geometry is a unique method for dynamic generation of transient states of matter by fast quenching from the laser-induced plasma where the new phases are reserved for further studies.

Evidence of new high-pressure silicon phases in Fs-laser induced confined microexplosion

We report on formation of high-pressure polymorphs of Si in confined microexplosion experiments. The results show that Si has undergone pressure-induced transitions into the realm of the metallic phases conventionally formed above 11 GPa.

Selective localised modifications of silicon crystal by ultrafast laser induced micro-explosion

Femtosecond (fs) laser pulses focused and confined inside the bulk of a material can deposit a volume energy density up to several MJ/cm3 in a sub-micron volume. This creates highly non-equilibrium, hot, dense and short-lived plasmas with conditions favorable for arrangement of atoms into unusual material phases. Singlecrystal silicon was exposed to strong shock waves induced by laser micro-explosion in confined geometry. The conditions of confinement were realized by focusing 170-fs pulses, with the energy up to 2.5 μJ, on a Si surface buried under a 10-μm thick SiO2-layer formed by oxidation of a Si-wafer. The generated intensity was 1015 W/cm2, well above the threshold for optical breakdown and plasma formation. The shock wave modified areas of the Si crystal were sectioned using a focused-ion beam and characterized with scanning electron microscopy. A void surrounded by a shock-wave-modified Si was observed at the Si/SiO2 boundary. The results demonstrate that confined micro-explosion opens up new perspectives for studies of high-pressure materials at the laboratory table-top expanding the laser-induced micro-explosion capabilities into the domain of non-transparent materials.