Space-time coupling in the up-conversion of broadband down-converted light

We investigate the up-conversion process of broadband light from parametric down-conversion (PDC), focusing on the spatio-temporal spectral properties of the sum-frequency generated (SFG) radiation. We demonstrate that the incoherent component of the SFG spectrum is characterized by a skewed geometry in space-time, which originates from a compensation between the group-velocity mismatch and the spatial walk-off of the fundamental and the SFG fields. The results are illustrated both by a theoretical modeling of the optical system and by experimental measurements.Comment: 11 pages, 9 figures. arXiv admin note: text overlap with arXiv:1308.236

Spatio-temporal entanglement of twin photons: an intuitive picture

We draw an intuitive picture of the spatio-temporal properties of the entangled state of twin photons, where they are described as classical wave-packets. This picture predicts a precise relation between their temporal and transverse spatial separations at the crystal output. The space-time coupling described by classical arguments turns out to determine in a precise way the spatio-temporal structure of the quantum entanglement, analysed by means of the biphotonic correlation and of the Schmidt dimensionality of the entanglement.Comment: 12 pages, 3 figure

Study of Through-Hole Micro-Drilling in Sapphire by Means of Pulsed Bessel Beams

Ultrashort Bessel beams have been used in this work to study the response of a 430-m-thick monocrystalline sapphire sample to laser–matter interaction when injecting the beam orthogonally through the whole sample thickness. We show that with a 12 Bessel beam cone angle, we are able to internally modify the material and generate tailorable elongated microstructures while preventing the formation of surface cracks, even in the picosecond regime, contrary to what was previously reported in the literature. On the other hand, by means of Bessel beam machining combined with a trepanning technique where very high energy pulses are needed, we were able to generate 100 m diameter through-holes, eventually with negligible cracks and very low taper angles thanks to an optimization achieved by using a 60-m-thick layer of Kapton Polyimide removable tape

Burst mode versus single-pulse machining for Bessel beam micro-drilling of thin glass: study and comparison

We present a study on the micro-drilling process by means of a picosecond Bessel-Gauss beam, and the achievements obtained on a 200-mu m-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

Coherence properties of high-gain twin beams generated in pump-depletion regime

Twin-beam coherence properties are analyzed both in the spatial and spectral domains at high-gain regime including pump depletion. The increase of the size of intensity auto- and cross-correlation areas at increasing pump power is replaced by a decrease in the pump depletion regime. This effect is interpreted as a progressive loss in the mode selection occurring at high-gain amplification. The experimental determination of the number of spatio-spectral modes from $g^{(2)}$-function measurements confirms this explanation.Comment: 7 pages, 7 figure

Micro-Hole Generation by High-Energy Pulsed Bessel Beams in Different Transparent Materials

Micro-drilling transparent dielectric materials by using non-diffracting beams impinging orthogonally to the sample can be performed without scanning the beam position along the sample thickness. In this work, the laser micromachining process, based on the combination of picosecond pulsed Bessel beams with the trepanning technique, is applied to different transparent materials. We show the possibility to create through-apertures with diameter on the order of tens of micrometers, on dielectric samples with different thermal and mechanical characteristics as well as different thicknesses ranging from two hundred to five hundred micrometers. Advantages and drawbacks of the application of this technique to different materials such as glass, polymer, or diamond are highlighted by analyzing the features, the morphology, and the aspect-ratio of the through-holes generated. Alternative Bessel beam drilling configurations, and the possibility of optimization of the quality of the aperture at the output sample/air interface is also discussed in the case of glass

Fabrication of conductive micro electrodes in diamond bulk using pulsed Bessel beams

High-quality, in-bulk conductive graphitic microelectrodes are fabricated perpendicular to the surface of a 500 μm thick monocrystalline CVD diamond sample using pulsed Bessel beams. With a 12o cone angle beam, different pulse parameters are explored to optimize the graphitic wires which are written without sample translation. The quality of the electrodes and their electrical and structural properties have been analysed through currentvoltage characterization and micro-Raman spectroscopy. We have found that higher pulse duration favours better conductivity while pulse energy has an optimum value for the same. This trend is confirmed by the presence and the different amounts of graphitic-like sp2 bonded carbon revealed by the micro-Raman spectra in different configurations. Using suitable writing parameters, we are able to create electrodes with the resistivity of 0.04 Ω cm, which, to the best of our knowledge, is one of the lowest values ever reported in literature in the case of graphitic-like wires fabricated through laser micromachining

Evaluation of microscale crystallinity modification induced by laser writing on Mn3O4 thin films

Defining microstructures and managing local crystallinity allow the implementation of several functionalities in thin film technology. The use of ultrashort Bessel beams for bulk crystallinity modification has garnered considerable attention as a versatile technique for semiconductor materials, dielectrics, or metal oxide substrates. The aim of this work is the quantitative evaluation of the crystalline changes induced by ultrafast laser micromachining on manganese oxide thin films using micro-Raman spectroscopy. Pulsed Bessel beams featured by a 1 micrometer-sized central core are used to define structures with high spatial precision. The dispersion relation of Mn3O4 optical phonons is determined by considering the conjunction between X-ray diffraction characterization and the phonon localization model. The asymmetries in Raman spectra indicate phonon localization and enable a quantitative tool to determine the crystallite size at micrometer resolution. The results indicate that laser-writing is effective in modifying the low-crystallinity films locally, increasing crystallite sizes from ~8 nm up to 12 nm, and thus highlighting an interesting approach to evaluate laser-induced structural modifications on metal oxide thin films.Comment: 27 page

Diamond photonics platform enabled by femtosecond laser writing

We demonstrate the first buried optical waveguides in diamond using focused femtosecond laser pulses. The properties of nitrogen vacancy centers are preserved in the waveguides, making them promising for diamond-based magnetometers or quantum information systems.Comment: 24 pages, 6 figure