An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.

An overview of pulsed laser-assisted methods for nanofabrication, which are currently developed in our Institute (LP3), is presented. The methods compass a variety of possibilities for material nanostructuring offered by laser–matter interactions and imply either the nanostructuring of the laser-illuminated surface itself, as in cases of direct laser ablation or laser plasma-assisted treatment of semiconductors to form light-absorbing and light-emitting nano-architectures, as well as periodic nanoarrays, or laser-assisted production of nanoclusters and their controlled growth in gaseous or liquid medium to form nanostructured films or colloidal nanoparticles. Nanomaterials synthesized by laser-assisted methods have a variety of unique properties, not reproducible by any other route, and are of importance for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics.

In a femtosecond pump-probe experiment the pump pulse injects a modest density of free carriers by multiphoton absorption. Measuring the nonlinear absorption of the probe pulse we observe exciton-seeded multiphoton ionization. The excitons self-trap in SiO 2 in Ͻ300 fs following free-carrier injection and decay biexponentially with lifetimes of 34Ϯ 8 and 338Ϯ 67 ps at room temperature. The extent of the probe-pulse absorption provides a model-independent demonstration that avalanche ionization plays a significant role in free-carrier generation by laser pulses as short as 45 fs. Free-carriers injected into a dielectric from extremeultraviolet ͑XUV͒ sources created by high harmonic or attosecond pulse generation and then avalanched with a perfectly synchronized infrared ͑fundamental͒ probe pulse, can provide a route to nanoscale laser machining.

Femtosecond laser ablation of Ti, Zr and Hf has been investigated by means of in-situ plasma diagnostics. Fast plasma imaging with the aid of an intensified charged coupled device (ICCD) camera was used to characterise the plasma plume expansion on a nanosecond time scale. Time-and spaceresolved optical emission spectroscopy was employed to perform time-of-flight measurements of ions and neutral atoms. It is shown that two plasma components with different expansion velocities are generated by the ultra-short laser ablation process. The expansion behaviour of these two components has been analysed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ultra-short laser ablation.

The metrology of laser-induced damage usually finds a single transition from 0% to 100% damage probability when progressively increasing the laser energy in experiments. We observe that picosecond pulses at 2-µm wavelength focused inside silicon provide a response that strongly deviates from this. Supported by nonlinear propagation simulations and energy flow analyses, we reveal an increased light delocalization for near critical power conditions. This leads to a nonmonotonic evolution of the peak delivered fluence as a function of the incoming pulse of the energy, a situation more complex than the clamping of the intensity until now observed in ultrafast regimes. Compared to femtosecond lasers, our measurements show that picosecond sources lead to reduced thresholds for three-dimensional (3D) writing inside silicon that is highly desirable. However, strong interplays between nonlinear effects persist and should not be ignored for the performance of future technological developments. We illustrate this aspect by carefully retrieving from the study the conditions for a demonstration of 3D data inscription inside a silicon wafer.

International audienceTwo-photon ionization by focused femtosecond laser pulses initiates the development of micrometer-scale plasmas in the bulk of silicon. Using pump-and-probe transmission microscopy with infrared light, we investigate the space-time characteristics of these plasmas for laser intensities up to 10(12) W/cm(2). The measurements reveal a self-limitation of the excitation at a maximum free-carrier density of congruent to 10(19) cm(-3), which is more than one order of magnitude below the threshold for permanent modification. The plasmas remain unchanged in the similar to 100 ps timescale revealing slow carrier kinetics. The results underline the limits in local control of silicon dielectric permittivity, which are inherent to the use of single near-infrared ultrashort Gaussian pulses. (C) 2014 AIP Publishing LLC

International audienceUsing various band-gap materials and tightly focused femtosecond laser pulses with wavelengths in the range 1200-2200 nm, we show that nonlinear absorption is independent of the wavelength except for narrow gap semiconductor materials. This observation corresponds to a transition between multiphoton ionization and tunnel ionization for an adiabaticity parameter of about 3, which compares favorably with Keldysh predictions. Our results indicate that long wavelengths must open up an alternative to pulse shortening for ultraprecision optical breakdown in dielectrics

Direct three-dimensional (3D) laser writing of waveguides is highly advanced in a wide range of bandgap materials, but has no equivalent in silicon so far. We show that nanosecond laser single-pass irradiation is capable of producing channel micro-modifications deep into crystalline silicon. With an appropriate shot overlap, a relative change of the refractive index exceeding 10^-3 is obtained without apparent nonuniformity at the micrometer scale. Despite the remaining challenge of propagation losses, we show that the created structures form, to the best of our knowledge, the first laser-written waveguides in the bulk of monolithic silicon samples. This paves the way toward the capability of producing 3D architectures for the rapidly growing field of silicon photonics.