Generation of extremely high-angle Bessel beams.

We present a setup to generate tightly focused Bessel beams that is composed of a half-ball lens coupled with a relay lens. The system is simple and compact compared to conventional imaging of axicons based on microscope objectives. We experimentally demonstrate the generation of a Bessel beam with a 42° cone angle at 980 nm in air with a typical beam length of 500µm and a central core radius of about 550 nm. We numerically studied the effects of the misalignment of the different optical elements and the range of tilt and shift that are acceptable to obtain a regular Bessel beam.

Femtosecond laser direct writing multilayer chiral waveplates with minimal linear birefringence.

Chirality transfer from femtosecond laser direct writing in achiral transparent materials mainly originates from the interplay between anisotropic nanogratings and mechanical stress with non-parallel and non-perpendicular (oblique) neutral axes. Yet, the laser fabrication simultaneously induces non-negligible linear birefringence. For precise manipulation of circular polarization properties, as well as to unlock the full functionality, we report here a geometry-inspired multilayer method for direct writing of chiral waveplates with minimal linear birefringence. We perform a theoretical analysis of both circular and linear properties response for different multilayer configurations and achieve strong circular birefringence of up to -2.25 rad with an extinction ratio of circular birefringence to total linear birefringence of up to 5.5 dB at 550 nm. Our strategy enables the precise control of circular properties and provides a facile platform for chiral device exploration with almost no linear property existence.

Caustics of the axially symmetric vortex beams: analysis and engineering.

We demonstrate that our theoretical scheme developed in the previous study on the caustics of the abruptly autofocusing vortex beams [Xiao et al., Opt. Express29, 19975 (2021)10.1364/OE.430497] is universal for all the axially symmetric vortex beams. Further analyses based on this method show the complex compositions of the vortex caustics in real space. Fine features of the global caustics are well reproduced, including their deviations from the trajectories of the host beams. Besides, we also show the possibility of tailoring the vortex caustics in paraxial optics based on our theory. The excellent agreements of our theoretical results with both numerical and experimental results confirm the validity of this scheme.

High Aspect Ratio Structuring of Glass with Ultrafast Bessel Beams.

Controlling the formation of high aspect ratio void channels inside glass is important for applications like the high-speed dicing of glass. Here, we investigate void formation using ultrafast Bessel beams in the single shot illumination regime. We characterize the morphology of the damages as a function of pulse energy, pulse duration, and position of the beam inside fused silica, Corning Eagle XG, and Corning Gorilla glass. While a large set of parameters allow for void formation inside fused silica, the operating window is much more restricted for Eagle XG and Gorilla glass. The transient formation of a molten layer around voids enables us interpreting the evolution of the morphology with pulse energy and duration.

Caustic Interpretation of the Abruptly Autofocusing Vortex beams.

We propose an effective scheme to interpret the abruptly autofocusing vortex beam. In our scheme, a set of analytical formulae are deduced to well predict not only the global caustic, before and after the focal plane, but also the focusing properties of the abruptly autofocusing vortex beam, including the axial position as well as the diameter of focal ring. Our analytical results are in excellent agreement with both numerical simulation and experimental results. Besides, we apply our analytical technique to the fine manipulation of the focusing properties with a scaling factor. This set of methods would be beneficial to a broad range of applications such as particle trapping and micromachinings.

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging.

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump-probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump-probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump-probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale.

Generation of a Bessel beam in FDTD using a cylindrical antenna.

Bessel beams are becoming a very useful tool in many areas of optics and photonics, because of the invariance of their intensity profile over an extended propagation range. Finite-Difference-Time-Domain (FDTD) approach is widely used for the modeling of the beam interaction with nanostructures. However, the generation of the Bessel beam in this approach is a computationally challenging problem. In this work, we report an approach for the generation of the infinite Bessel beams in three-dimensional FDTD. It is based on the injection of the Bessel solutions of Maxwell's equations from a cylindrical hollow annulus. This configuration is compatible with Particle In Cell simulations of laser plasma interactions. This configuration allows using a smaller computation box and is therefore computationally more efficient than the creation of a Bessel-Gauss beam from a wall and models more precisely the analytical infinite Bessel beam. Zeroth and higher-order Bessel beams with different cone angles are successfully produced. We investigate the effects of the injector parameters on the error with respect to the analytical solution. In all cases, the relative deviation is in the range of 0.01-7.0 percent.

Advancing Fourier: space-time concepts in ultrafast optics, imaging, and photonic neural networks.

The concepts of Fourier optics were established in France in the 1940s by Pierre-Michel Duffieux, and laid the foundations of an extensive series of activities in the French research community that have touched on nearly every aspect of contemporary optics and photonics. In this paper, we review a selection of results where applications of the Fourier transform and transfer functions in optics have been applied to yield significant advances in unexpected areas of optics, including the spatial shaping of complex laser beams in amplitude and in phase, real-time ultrafast measurements, novel ghost imaging techniques, and the development of parallel processing methodologies for photonic artificial intelligence.

Generation of high conical angle Bessel-Gauss beams with reflective axicons.

We report the generation of Bessel-Gauss beams of high conical angle, up to 35 deg, using reflective off-axis axicons and a magnification optical system. We experimentally characterize the beams with three-dimensional scans. The high precision of fabrication of the axicons in the vicinity of the axicon singularity allows us to generate a beam with intensity distribution close to analytical description.

Scaling the abruptly autofocusing beams in the direct-space.

We propose a simple technique to scale the abruptly autofocusing beams in the direct space by introducing a scaling factor in the phase. Analytical formulas are deduced based on optical caustics, explicitly revealing how the scaling factor controls location, peak intensity, and size of the focal spot. We demonstrate that the multiplication of a scaling factor on the phase is equivalent to the axial-scaling transformation under the paraxial approximation. Further numerical and experimental results confirm theoretical predictions. In addition, amplitude modulation using phase-only holograms is used to maintain the peak intensity level of the focal spots.

Controlling nonlinear instabilities in Bessel beams through longitudinal intensity shaping.

During ultrafast laser pulse propagation in dielectrics, the nonlinear generation of new spatial frequencies can be deleterious to reach high intensities and to generate uniform plasma channels. In this context, diffraction-free Bessel beams have attracted major recent interest because of their enhanced stability when compared to conventional Gaussian beams. However, Bessel beams can still suffer from significant modulation instability arising from noise-induced nonlinear four-wave mixing (FWM). In this Letter we report control of the nonlinear instability growth by shaping the longitudinal intensity profile of the incident field. Our results show that tailored longitudinal intensity shaping of a nondiffracting Bessel beam can strongly reduce FWM-induced oscillations and stabilize nonlinear propagation at ablation-level intensities.

Arbitrary shaping of on-axis amplitude of femtosecond Bessel beams with a single phase-only spatial light modulator.

Arbitrary shaping of the on-axis intensity of Bessel beams requires spatial modulation of both amplitude and phase. We develop a non-iterative direct space beam shaping method to generate Bessel beams with high energy throughput from direct space with a single phase-only spatial light modulator. For this purpose, we generalize the approach of Bolduc et al. to non-uniform input beams. We point out the physical limitations imposed on the on-axis intensity profile for unidirectional beams. Analytical, numerical and experimental results are provided.

Light trajectory in Bessel-Gauss vortex beams.

We investigate the early stage of propagation of Bessel-Gauss vortex beams where a transition regime shows a progressive lateral expansion of the main intensity ring before reaching a diffraction-free regime. The eikonal equation is used to characterize the beam structure. The beam is featured by a family of hyperboloids with variable waists, generating a tapered tubular caustic. Our analytical results are in excellent agreement with numerical and experimental results. We show the transition regime can be well eliminated by using hollow input beams.

Tubular filamentation for laser material processing.

An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 10(13) W/cm(2) has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.

Resolution of strongly competitive product channels with optimal dynamic discrimination: application to flavins.

Fundamental molecular selectivity limits are probed by exploiting laser-controlled quantum interferences for the creation of distinct spectral signatures in two flavin molecules, erstwhile nearly indistinguishable via steady-state methods. Optimal dynamic discrimination (ODD) uses optimally shaped laser fields to transiently amplify minute molecular variations that would otherwise go unnoticed with linear absorption and fluorescence techniques. ODD is experimentally demonstrated by combining an optimally shaped UV pump pulse with a time-delayed, fluorescence-depleting IR pulse for discrimination amongst riboflavin and flavin mononucleotide in aqueous solution, which are structurally and spectroscopically very similar. Closed-loop, adaptive pulse shaping discovers a set of UV pulses that induce disparate responses from the two flavins and allows for concomitant flavin discrimination of ∼16σ. Additionally, attainment of ODD permits quantitative, analytical detection of the individual constituents in a flavin mixture. The successful implementation of ODD on quantum systems of such high complexity bodes well for the future development of the field and the use of ODD techniques in a variety of demanding practical applications.

Quantum control of tightly competitive product channels.

Fundamental selectivity limits of quantum control are pushed by introducing laser driven optimal dynamic discrimination to create distinguishing excitations on two nearly identical flavin molecules. Even with modest spectral resources, significant specificity is achieved with optimal pulse shapes, which amplify small molecular differences to create distinct, identifying signals. Rather than being a hindrance, system complexity appears to aid the control process and augments control field capability, which bodes well for implementation of quantum control in a variety of demanding applications.

Nano-FROG: Frequency resolved optical gating by a nanometric object.

We present a technique to characterize ultrashort pulses at the focal plane of a high numerical aperture objective with unprecedented spatial resolution, by performing a FROG measurement with a single nanocrystal as nonlinear medium. This approach can be extended to develop novel phase-sensitive techniques in laser scanning microscopy, probing the microscopic environment by monitoring phase and amplitude distortions of femtosecond laser pulses.

Identification of biological microparticles using ultrafast depletion spectroscopy.

We show how an ultrafast pump-pump excitation induces strong fluorescence depletion in biological samples, such as bacteria-containing droplets, in contrast with fluorescent interferents, such as polycyclic aromatic compounds, despite similar spectroscopic properties. Application to the optical remote discrimination of biotic versus non-biotic particles is proposed. Further improvement is required to allow the discrimination of one pathogenic among other non-pathogenic micro-organisms. This improved selectivity may be reached with optimal coherent control experiments, as discussed in the paper.

Plasma formation dynamics within a water microdroplet on femtosecond time scales.

We demonstrate that a highly confined plasma inclusion is formed inside a micrometric water droplet that is excited by a femtosecond laser. An 800-nm laser pulse generates a plasma inclusion by laser-induced breakdown that is subsequently probed by the elastic scattering of a second (400-nm) time-delayed ultrashort pulse. For a 25-microm-radius droplet and an incident intensity of 7 x 10(12) W/cm2 the radius of the highly localized plasma is 1.9 +/- 0.4 microm. We probed the plasma formation dynamics on femtosecond time scales by varying the delay between the pump and the probe pulses. Good agreement with numerical simulations of the process was found.