Alexandre Mermillod-Blondin

Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction.

Laser writing of longitudinal waveguides in bulk transparent materials degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface. Using adaptive spatial tailoring of ultrashort laser pulses, we show that spherical aberrations can be dynamically compensated in optical glasses, in synchronization with the writing procedure. Aberration-free structures can thus be induced at different depths, showing higher flexibility for 3D processing. This enables optimal writing of homogeneous longitudinal waveguides over more significant lengths. The corrective process becomes increasingly important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive refractive index change.

Spatial distribution of refractive index variations induced in bulk fused silica by single ultrashort and short laser pulses

We correlate phase-contrast microscopy of modification tracks induced by tightly focused single ultrashort and short laser pulses inside fused silica with numerical simulations of nonlinear laser excitation footprints. Different pulse durations on the femtosecond and picosecond range are compared in order to validate the experimental and theoretical observations on the subsequent refractive index variations in a regime where linear and nonlinear contributions play a comparable role. The nature of the laser-induced structural changes depends essentially on the characteristics of pulse propagation in different regions of the irradiated zone. Numerical simulations of laser pulse propagation in the excited region show that accumulation of excess energy and swift nonlinear absorption contribute to the formation of either positive or negative phase-shift regions within the same single-pulse-induced damage trace. The decrease in the refractive index can be unambiguously correlated with the regions of maximum energy deposition during prolonged exposure times.

Dynamics of femtosecond laser induced voidlike structures in fused silica

Focused ultrafast laser irradiation of fused silica usually induces a spatially modulated refractive index variation in the bulk material. Strong energy concentration leads to the localized formation of a lower-density cavitylike depressed structure surrounded by compacted matter. We report on applying time-resolved phase contrast microscopy to investigate the timescale of the void formation. We indicate a temporal behavior consistent with shock wave generation and subsequent rarefaction.

Low-loss waveguides fabricated in BK7 glass by high repetition rate femtosecond fiber laser

For the first time femtosecond-laser writing has inscribed low-loss optical waveguides in Schott BK7 glass, a commercially important type of borosilicate widely used in optical applications. The use of a variable repetition rate laser enabled the identification of a narrow processing window at 1 MHz repetition rate with optimal waveguides exhibiting propagation losses of 0.3 dB/cm and efficient mode matching to standard optical fibers at a 1550 nm wavelength. The waveguides were characterized by complementary phase contrast and optical transmission microscopy, identifying a micrometer-sized guiding region within a larger complex structure of both positive and negative refractive index variations.

Analysis of the effects of spherical aberration on ultrafast laser-induced refractive index variation in glass

We propose a comprehensive analysis of the effects that spherical aberration may have on the process of ultrafast laser photowriting in bulk transparent materials and discuss the consequences for the generated refractive index changes. Practical aspects for a longitudinal photowriting configuration are emphasized. Laser-induced index variation in BK7 optical glass and fused silica (a-SiO(2)) affected by spherical aberration are characterized experimentally using phase-contrast optical microscopy. Experimental data are matched by analytical equations describing light propagation through dielectric interfaces. Corrective solutions are proposed with a particular focus on the spatial resolution achievable and on the conditions to obtain homogeneously photo-induced waveguides in a longitudinal writing configuration.

Time-resolved imaging of laser-induced refractive index changes in transparent media.

We describe a method to visualize ultrafast laser-induced refractive index changes in transparent materials with a 310 fs impulse response and a submicrometer spatial resolution. The temporal profile of the laser excitation sequence can be arbitrarily set on the subpicosecond and picosecond time scales with a pulse shaping unit, allowing for complex laser excitation. Time-resolved phase contrast microscopy reveals the real part of the refractive index change and complementary time-resolved optical transmission microscopy measurements give access to the imaginary part of the refractive index in the irradiated region. A femtosecond laser source probes the complex refractive index changes from the excitation time up to 1 ns, and a frequency-doubled Nd:YAG laser emitting 1 ns duration pulses is employed for collecting data at longer time delays, when the evolution is slow. We demonstrate the performance of our setup by studying the energy relaxation in a fused silica sample after irradiation with a double pulse sequence. The excitation pulses are separated by 3 ps. Our results show two dimensional refractive index maps at different times from 200 fs to 100 μs after the laser excitation. On the subpicosecond time scale we have access to the spatial characteristics of the energy deposition into the sample. At longer times (800 ps), time-resolved phase contrast microscopy shows the appearance of a strong compression wave emitted from the excited region. On the microsecond time scale, we observe energy transfer outside the irradiated region.

Size correction in ultrafast laser processing of fused silica by temporal pulse shaping

Laser structuring of bulk dielectric materials involves focusing through air-dielectric interfaces, which is prone to induce spherical aberrations. This elongates the energy deposition volume and restricts the processing accuracy at large focusing depths. Although spatial phase corrections are commonly applied to correct wavefront distortions, we show that temporal forming of ultrafast laser pulses achieves comparable counteracting function, limiting the spatial extent while creating a dominant region of positive index change. We indicate the role of reduced nonlinearity in plasma formation as a control factor which couples the spatial and temporal responses of the material.

Nanosize structural modifications with polarization functions in ultrafast laser irradiated bulk fused silica

Laser-induced self-organization of regular nanoscale layered patterns in fused silica is investigated using spectroscopy and microscopy methods, revealing a high presence of stable broken oxygen bonds. Longitudinal traces are then generated by replicating static irradiation structures where the nanoscale modulation can cover partially or completely the photoinscribed traces. The resulting birefringence, the observed anisotropic light scattering properties, and the capacity to write and erase modulated patterns can be used in designing bulk polarization sensitive devices. Various laser-induced structures with optical properties combining guiding, scattering, and polarization sensitivity are reported. The attached polarization functions were evaluated as a function of the fill factor of the nanostructured domains. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.

Structural modifications of binary lithium silicate glasses upon femtosecond laser pulse irradiation probed by micro-Raman spectroscopy

The effects of single femtosecond laser pulse irradiation (130 fs pulse duration, 800 nm center wavelength) on the structure of binary lithium silicate glasses of varying chemical compositions were investigated by micro-Raman spectroscopy. Permanent modifications were generated at the surface of the glass samples with varying laser fluences in the ablative regime and evaluated for changes in the corresponding Raman band positions and bandwidths. For increasing laser fluences, the position of certain Raman bands changed, indicating an increase in the mass density of the glass inside the irradiated area. Simultaneously, the widths of all investigated bands increased, indicating a higher degree of disorder in the glass structure with respect to bond-angle and bond-length variations.

Time-resolved microscopy with random lasers

We demonstrate that random lasers provide an outstanding strobe light source for time-resolved microscopy. Utilizing a random laser to illuminate a commercially available microscope enables single exposure, speckle-free time-resolved imaging. Aside from conventional optical transmission microscopy, we also perform time-resolved investigations in phase contrast mode. We apply this method to the monitoring of fs-laser-induced microdot formation in bulk a-SIO(2). Time-resolved investigations show that microdot formation lasts over several microseconds after laser excitation.