We present a study on the micro-drilling process by means of a picosecond Bessel–Gauss beam, and the achievements obtained on a 200-µm-thick AF32 glass sample in different laser fabrication regimes. ...In particular, we compare the results and morphology of the holes generated with a high-repetition-rate pulsed laser, respectively, in the single-pulse mode and in the burst mode machining regimes. We highlight the advantages or drawbacks of these two types of microfabrication for the generation of through-holes. For a given pulse density, the burst mode turns out to be advantageous with respect to the single-pulse mode fabrication in terms of lower energy per pulse needed and higher speed of drilling, even if the stronger thermal effects can more easily lead to surface cracks. On the other hand, by adjusting the pulse density below a critical level, it can be shown that the single-pulse regime can be adopted for the generation of more regular through-holes and cleaner apertures, even if multiple pass operation is likely to be needed.
An artificial skylight has been developed which aims at creating an impression of a daylight opening in the ceiling by means of an artificial sunspot and a blue sky with perceived infinite depth. It ...is the goal of this work to compare and quantify the perceived room atmosphere created by this artificial system with that created by a conventional lighting system. A total of 100 subjects participated in a randomized controlled study within which the short-term effects on room lighting- and room atmosphere-appearance were determined. Both the artificial skylight and the resulting room atmosphere were perceived as very natural and attractive. Thus, the first strong evidence for the expected positive effects of this artificial skylight was determined.
The formation of long stationary filaments resulting in uniform high density plasma strings in air using short pulse UV laser Bessel beams is shown. The length and the electron density of the plasma ...strings can be easily tuned by adjusting the conical Bessel wavefront angle. It is shown that in this regime the length of the plasma string can be extended over meter-long scales without any compromise in the string uniformity or any temporal evolution of the filamented laser pulse.
We use a Spatial Light Modulator (SLM) to produce arrays of Bessel beams by using multiple axicon phase-masks on the SLM. This approach utilises the whole of the SLM, rather than just a thin annular ...region (which is the case if the SLM is in the far-field of the generated Bessel beams). Using the whole SLM rather than just an annular region means that the required intensity on the SLM is an order of magnitude lower for a given power in the Bessel beams. Spreading the power over the whole SLM is important for high-power applications such as laser micromachining. We allow the axicons to overlap and interfere in the hologram, so the axial length of the Bessel beam core is maintained as we add more beams to the array.
Focus Serial: Frontiers of Nonlinear Optics We investigate ultrashort laser pulse filamentation within the framework of spontaneous X Wave formation. After a brief overview of the filamentation ...process we study the case of an intense filament co-propagating with a weaker seed pulse. The filament is shown to induce strong Cross-Phase-Modulation (XPM) effects on the weak seed pulse: driven by the pump, the seed pulse undergoes pulse splitting with the daughter pulses slaved to their pump counterparts. They undergo strong spatio-temporal reshaping and are transformed into XWaves traveling at the same group velocities as the pump split-off pulses. In the presence of a gain mechanism such as Four-Wave-Mixing or Stimulated Raman Scattering, energy is then transferred from the pump filament leading to amplification of the seed X Wave and formation of a temporally compressed intensity peak.
We demonstrate the ultranarrow temporal correlation (6 fs full width half maximum) of twin beams generated by parametric down-conversion by using its reverse process, i.e., sum-frequency generation. ...The result relies on an achromatic imaging of a huge bandwidth of twin beams and on a careful control of their spatial degrees of freedom. The detrimental effects of spatial filtering and imperfect imaging are shown, along with the theoretical model used to describe the results.
The advantages of using Bessel beams for the generation of graphitic structures in diamond bulk are presented. We show that by irradiating the sample with a pulsed Bessel beam whose non-diffracting ...zone is of the same order of the sample thickness, it is possible to produce without any sample translation straight graphitic through-microstructures, whose size depends on the input pulse energy. The microstructure growth is investigated as a function of the number of irradiating pulses, and the femtosecond and picosecond regimes are contrasted.
Spatial Light Modulators (SLMs) are a powerful tool in many optics laboratories, but due to the technology required for their fabrication, they are usually very expensive. Recently some inexpensive ...devices have been produced, however their phase shift range is less than 2π, leading to a loss of diffraction efficiency for the SLM. We show how to improve the first order diffraction efficiency of such an SLM by adjusting the blazing function, and obtain a 1.5 times increase in first order diffracted power. Even a perfect SLM with 2π phase throw can produce undesired effects in some situations; for example in holographic optical tweezers it is common to find unwanted “ghost spots” near to the array of first-order spots. Modulating the amplitude, by spatially modulating the blazing function, allows us to suppress the ghost spots. This increases the contrast between desired and unwanted spots by more than an order of magnitude.
Considering the problem of parametric nonlinear interaction, we report the experimental observation of electromagnetic waves characterized by an X-shaped spatiotemporal coherence; i.e., coherence is ...neither spatial nor temporal, but skewed along specific spatiotemporal trajectories. The application of the usual, purely spatial or temporal, measures of coherence would erroneously lead to the conclusion that the field is fully incoherent. Such hidden coherence has been identified owing to an innovative diagnostic technique based on simultaneous analysis of both the spatial and temporal spectra.
We numerically investigate the possibility to generate freely accelerating or decelerating pulses. In particular it is shown that acceleration along the propagation direction z may be obtained by a ...purely spatial modulation of an input Gaussian pulse in the form of finite-energy Bessel pulses with a cone angle that varies along the radial coordinate.We discuss simple practical implementations of such accelerating Bessel beams.
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