Amlan Das

Ecriture par laser ultrarapide dans le silicium : dépendances aux caractéristiques temporelles des impulsions appliquées: Il existe un intérêt croissant pour le développement de technologies d'écriture laser tridimensionnelle applicables au silicium (Si) pour relever certains défis associés à la microfabrication en microélectronique. Ce travail se concentre sur les investigations des paramètres de contrôle dans le domaine temporel, c'est-à-dire les caractéristiques temporelles des impulsions lasers. Il révèle la possibilité d'écrire des micro-modifications dans le volume de Si pour des durées d'impulsion dans le domaine picoseconde alors que l'écriture en régime femtoseconde reste impossible avec des configurations classiques de focalisation. Dans une démarche d'optimisation, des stratégies d'accumulation avec des trains d'impulsions à des fréquences de répétition THz sont également mises en œuvre et étudiées. Par rapport à l'irradiation avec des impulsions uniques, une amélioration systématique est obtenue. L’approche permet des démonstrations avec une fiabilité d’écriture améliorée. En complément de ces analyses, une étude de la dépendance du problème au contraste temporel des impulsions femtosecondes est proposée. Elle repose sur une approche à double-impulsion avec le développement d'un dispositif d'irradiation avec des impulsions femtosecondes synchronisées avec des impulsions picosecondes ou nanosecondes. Une forte dépendance au contraste est révélée. Ceci encourage à la mise en œuvre d'irradiations contrôlées sur plusieurs échelles de temps pour de futures optimisations. Dans sa globalité, l'étude donne une vision générale sur les caractéristiques temporelles des impulsions à considérer pour les applications d’écriture laser à l’intérieur des semiconducteurs.

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 creation of localized bulk modification using femtosecond pulses inside semiconductors like silicon (Si) is quite challenging, whereas it is not difficult to achieve it for dielectric materials like fused silica (FS). This report addresses the fundamental origin of this issue. By taking a simple numerical approach, it has been found that in FS we can deliver stronger fluence due to self-focusing at higher power levels compared to Si. The origin for the above lies in the spatio-temporal pulse-splitting behavior, which is dominant in the case of FS at the focus, whereas, for Si, it is only effective after focus. We have also considered the influence of plasma and Kerr terms to elucidate the reason behind these nonlinearities. For the FS case, omission of Kerr term dominates, whereas, for Si, the influence of each term does not significantly create self-focusing like FS under a similar focusing condition. This study could provide an important guideline for researchers to understand the complexity of laser-matter interaction in transparent materials specifically being studied by many laser-processing industries.

Highly reproducible and precisely controlled gradual variation in optical reflectivity or electrical resistance between amorphous and crystalline phases of phase change (PC) material is a key requirement for multilevel programming. Here we report high-contrast multilevel set and reset operations through accumulative switching in growth-dominated AgInSbTe PC material using a nanosecond laser-based pump-probe technique. The precise tuning of fractions of crystallized or re-amorphized region is achieved by means of controlling the number of irradiated laser pulses enabling six stable multilevels with high-reflectivity contrast of 2% between any two states. Furthermore, Raman spectra of irradiated spots validate the structural changes involved during multilevel switching between amorphous and crystalline phases.

Light-matter interaction in nanoscale regime have unprecedented and accelerating demand in optoelectronics, valley electronics and device applications. Such interaction in 1-dimention (1D) metal chalcogenides has emerged as an important research topic because of its possibility to custom design optical properties, implying enormous application including optical computers, communications, bioimaging, and so on. However, understanding of nonlinear optical response of these nanostructures is still lacking, although it constitutes an interesting problem on the light-matter interaction. Here, we have presented the nonlinear optical response in Sb2S3 nanorod using Z-scan technique. Our experimental findings show a strong saturable absorption (SA). In this context, we have numerically simulated the experimental result using two level rate equation. The solutions of these two-level rate equation for a Gaussian shaped pulse exactly replicated the experimental data. From the best numerical fit, we found excited state decay time (τ ≈ 0.15ns) and saturation intensity (I_S ≈ 0.01 GW/cm²). Additionally, we have calculated number of career density (N ≈ 5.31 × 10^-17 cm⁻³), ground state absorption cross section (σ₁ ≈ 1.63 × 10^-17 cm²). Our experimental finding indicates that they can be employed as saturable absorbers.

In this manuscript, we demonstrate the strong resonant two photon absorption coefficient ≈71 ± 5 cm/GW at 532 nm in α-NiMoO₄ nanoparticles prepared by a facile hydrothermal method. Strikingly, we have obtained the optical limiting onset threshold fluence (F_ON) of 36 mJ/cm² for the linear transmittance of 0.64 with an excellent two photon absorption cross section (38 × 10⁻⁴⁵ cm⁴ s), which suggests that they can be utilized as passive optical limiters. To explain the observed effects, we present a two-level rate equation model and numerically simulated the Z-scan peak shape, which is in good agreement with the experimental data. Further, we also show the normalized population density of the carriers in excited and ground states.

Development of a practical stereo vision sensor for real-world applications must account for the variability of high-volume production processes and the impact of unknown environmental conditions during its operation. One critical factor of stereo depth estimation performance is the relative alignment of the stereo camera pair. While imperfectly aligned stereo cameras may be rectified in the image domain, there are some errors introduced by both the calibration recovery and image rectification processes. Finally, additional uncalibrated misalignments, for example due to thermal or mechanical deformation in a harsh automotive environment, may occur which will further deteriorate stereo depth estimation. This paper describes an experimental framework for determining these limits using image processing algorithms, operating on graphically synthesized imagery, with performance envelope validation on real stereo image data.