Vladimir Yu. Fedorov

The nonlinear interaction of near-infrared ultrashort laser pulses with bulk fused silica at intensities below the catastrophic damage threshold is the birthplace of local electronic excitation events that may relax as point defects. The accumulation of defect centers creates additional sources of electronic excitation, gradually lowering the ionization threshold upon multi-pulse irradiation. However, the use of the near single-cycle sub-4 fs pulses erases in a surprising manner the contribution of defects to the damage threshold. This is the result of increasing the strong intensity clamping that limits energy density and therefore carrier collisional multiplication. Thus, the role of additional defect sources in the overall carrier population becomes secondary. In addition, the significant broadening of the near single-cycle pulse may enable impulsive depletion of defect levels, providing a possible additional pathway contributing to the suppression of nonlinear ionization memory.

Sky-high optical nonlinearities make semiconductors ideal platforms for multifunctional photonic devices. The fabrication of such complex devices could greatly benefit from in-volume ultrafast laser writing for monolithic and contactless integration. Ironically, as exemplified for Si, nonlinearities act as an efficient immune system that self-protects the material from internal permanent modifications. Predicting high-intensity ultrashort-pulse propagation beyond Si is further limited by incomplete descriptions of carrier dynamics in narrow-gap materials. Here, we demonstrate that filamentation universally dictates ultrashort laser pulse propagation in various semiconductors. The effective key nonlinear parameters extracted differ markedly from past measurements with low-intensity pulses, while temporal scaling laws for these parameters are also derived. Based on these findings, appropriate temporal-spectral shaping is proposed for tailored energy deposition inside semiconductors. The effective parameters also provide predictive inputs for semiconductor backside processing, microelectronics security, and high-harmonic, supercontinuum and terahertz wave generation.

We describe the nonlinear evolution of ultrafast circular and elliptical non-diffractive light pulses designed by asymmetric spatial Fourier filtering of zeroth-order Bessel–Gauss beams in fused silica glass, in a parameter range close to plasma generation for laser material processing. We numerically demonstrate a genuine resilience of the core shape, a consequence of a self-regulated action of the Kerr nonlinearity and of the carrier plasma for preserving and even reinforcing beam ellipticity. Overcoming symmetrization trends, the asymmetric core remains constantly elliptical along its propagation for a range of input fluence close to the threshold of permanent damage. These findings, showing the emergence of a beam with a section stable against nonlinear distortions, aim to assist current ultrafast laser dicing strategies.

Ultrafast laser three-dimensional writing has made breakthroughs in manufacturing technologies. However, it remains rarely adopted for semiconductor technologies due to in-chip propagation nonlinearities causing a lack of controllability for intense infrared light. To solve this problem, plasma-optics concepts are promising since ultrashort laser pulses, even if inappropriate for direct writing, can readily inject high-density free-carriers inside semiconductors. To achieve highly localized and reliable processing, we create plasma seeds with tightly focused pre-ionizing femtosecond pulses. We show how critical density conditions can be used for extremely confined energy deposition with a synchronized writing irradiation and create ~ 1-µm-sized isotropic modifications inside silicon. Drastic improvement is also found on the material change controllability leading to unique demonstrations including rewritable optical memories (>100 writing/erasure cycles) and graded-index functionalities. By solving its controllability issues with critical plasma seeds, we show the potential of ultrafast laser writing for flexible fabrication of reconfigurable monolithic silicon-based optical devices.

The ability to create surface structures with precisely controlled chirality remains a major challenge in laser‐matter interaction experiments. In this work, the interaction of vortex laser beams, characterized by spiral polarization patterns and twisted wavefronts, with rough metallic surfaces in order to create surface patterns with chirality is theoretically studied. Using numerical simulations based on the finite‐difference time‐domain method, how spin and orbital angular momenta influence the inhomogeneous energy absorption at the surface is investigated and twisted optical forces that can drive topographic reorganization are generated. How different structured light fields can create intricate patterns with chiral features on a material surface is shown. The crucial role of polarization and spatial inhomogeneity of the light field in the generation of asymmetric torque forces that directly affect the surface dynamics is emphasized. The electromagnetic simulations show how vortex beams can be used to create chiral surface structures, expanding the knowledge of laser‐generated periodic surface structures and opening up new possibilities for chiral surface engineering.

The ability to create surface structures with precisely controlled chirality remains a major challenge in laser-matter interaction experiments. In this work, we theoretically study the interaction of vortex laser beams, characterized by spiral polarization patterns and twisted wavefronts, with rough metallic surfaces in order to create surface patterns with chirality. Using numerical simulations based on the finite-difference time-domain method, we investigate how spin and orbital angular momenta influence the inhomogeneous energy absorption at the surface and generate twisted optical forces that can drive topographic reorganization. We show how different structured light fields can create intricate patterns with chiral features on a material surface. We emphasize the crucial role of polarization and spatial inhomogeneity of the light field in the generation of asymmetric torque forces that directly affect the surface dynamics. Our electromagnetic simulations show how vortex beams can be used to create chiral surface structures, expanding our knowledge of laser-generated periodic surface structures and opening up new possibilities for chiral surface engineering.

Ultrafast laser three-dimensional (3D) writing has made breakthrough in manufacturing processes with for instance the advent of super-resolution 3D printing and perennial optical memory technologies. However, similar precision applications were inaccessible for silicon and other semiconductor technologies due to a lack of in-chip controllability of intense infrared beams caused by strong propagation nonlinearities. To solve this problem, plasma-optics concepts are promising since a single ultrashort laser pulse, even if inappropriate for direct writing, can readily inject high-density free-carriers inside semiconductors. To achieve highly localized and reliable processing, we create plasma seeds with tightly focused pre-ionizing femtosecond pulses. We show how critical density conditions can be found under damage threshold inside silicon. This is used for extremely confined energy deposition with a synchronized writing irradiation creating isotropic modifications with sub-diffraction-limit size for the first time. Drastic improvement is also found on the material change controllability leading to unique demonstrations including rewritable optical memories (>100 writing/erasure cycles) and graded-index functionalities. By solving its controllability issues with critical plasma seeds, ultrafast laser writing is poised to offer its full potential for flexible fabrication of reconfigurable monolithic silicon-based devices.

We use ab-initio electromagnetic simulations to investigate light absorption by rough surfaces in the context of the formation of laser-induced periodic surface structures. Our approach involves modeling a realistic rough surface using a statistical description of its continuous height distribution via a corresponding correlation function. We study the influence of incident light polarization and various statistical properties of surface roughness, such as root-mean-square height and correlation length, on the distribution of absorbed laser energy. By analyzing light absorption in different layers of the surface selvedge, we elucidate how different features of surface morphology influence the shape of the resulting periodic surface structures. We show that circularly polarized laser pulses are highly sensitive to initial or progressively developing asymmetries in surface roughness.

We experimentally demonstrate that 2D Airy wave packets can produce intense curved two-color filaments that emit terahertz (THz) radiation with unique characteristics. Due to the curvature of the plasma channel, THz waves, emitted from different longitudinal regions of the plasma, propagate in different directions resulting in non-concentric THz cones in the far-field. These cones have different cone angles and polarization which we attribute to the way the two-color 2D Airy driving fields are produced in the nonlinear crystal and then propagate to form the curved plasma filament.