Universal threshold for femtosecond laser ablation with oblique illumination

We quantify the dependence of the single-shot ablation threshold on the angle of incidence and polarization of a femtosecond laser beam, for three dissimilar solid-state materials: a metal, a dielectric and a semiconductor. Using the constant, linear value of the index of refraction, we calculate the laser fluence transmitted through the air-material interface at the point of ablation threshold. We show that, in spite of the highly nonlinear ionization dynamics involved in the ablation process, the so defined transmitted threshold fluence is universally independent of the angle of incidence and polarization of the laser beam for all three material types. We suggest that angular dependence of ablation threshold can be utilized for profiling fluence distributions in ultra-intense femtosecond laser beams.Comment: 4 pages, 5 figure

Measurements of fluence profiles in femtosecond laser filaments in air

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our ap- proach is based on the dependence of single-shot laser abla- tion threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser fila- ments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the ex- ternal beam focusing. The limiting value of the peak inten- sity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f -number above ∼500), is about 55 TW∕cm2

Endoscopic findings and psychometric abnormalities: what is the relationship in upper endoscopic outpatients?

Background. Psychological disorders are often associated with diseases of the upper digestive tract. Although emotions can influence gastrointestinal function in healthy individuals, psychological setting in upper gastrointestinal patients are unclear. We evaluate the psychological alterations prevalence in outpatients submitted to upper endoscopy. Materials and Methods. A total of 130 patients (50 males and 80 females; mean age 54±17 years) submitted to upper gastrointestinal endoscopy, were enrolled over the period May 2009 - September 2010. Subjects were asked to complete questionnaires before endoscopic examination. Alexithymia, anxiety, depression and coping style were assessed using the Toronto Alexithymia Scale, Spielberger Trait Anxiety Inventory, Beck Depression Inventory and Coping Inventory for Stressful Situations, respectively. Results. Coping impairment, Alexithymia, Anxiety and Depression were found respectively in 80.3%, 25.4%, 24.6% and 17.2%, often in association. Task-oriented, emotion-oriented and avoidance-oriented alterations were found in 41.8%, 40% and 30.6%, respectively. No correlations were demonstrated between diagnosis of upper gastrointestinal disease and psychometric results. Conclusions. In our study, a high prevalence of psychometric alterations in gastrointestinal outpatients was unconnected with endoscopic findings, especially considering coping style alterations. This aspect should be taken into account in patients management and a long-term follow-up should clarify a possible role of these factors in patients prognosis and compliance

Selection rules for the orbital angular momentum of optically-produced THz radiation

In this work we theoretically study the transduction ofthe Orbital Angular Momentum (OAM)lfor infraredpump lasers into the THz domain. In the case of opti-cal rectification, the transduction of OAM occurs onlythrough a spin-orbit interaction, with the selection ruleon the OAMl=0valid for any kind of polarizationof the pump, which means that there is no transfer ofOAM along the propagation axis. In the difference fre-quency generation the selection rule for the difference∆lbetween the OAM of the pump fields with linear orcircular polarization isl=∆l, whereaslranges from∆l−2to∆l+2in both the cases of radial and azimuthalpolarization. Moreover, for THz generation in the lat-ter case, high diffraction obtained with tightly focusedpumps yieldsltending to∆l±2, whileltends to zeroin the opposite case of large pump beam

Intensity and phase retrieval of IR laser pulse by THz-based measurement and THz waveform modulation

Abstract THz radiation is of great interest for a variety of applications. Simultaneously with the demonstration of high-intensity THz sources the idea to use this radiation for particle acceleration started to be investigated. THz accelerating gradients up to GV/m have been demonstrated in laboratory. THz radiation can be generated through the optical rectification process induced in non-linear crystals by a pump laser. The temporal shape of the pump laser and in general its characteristics are important aspects to be known in order to produce THz radiation via optical rectification in a controlled way. Here we present a technique that can be used to retrieve the temporal profile characteristics (envelope and phase) of the pump laser, starting from the detection of the THz waveform/spectrum and the knowledge of the physical/optical properties of the crystal used to produce it. This work also shows that the THz field can be shaped by properly acting on the pump laser phase. The possibility to opportunely shape the THz field is of great importance for many applications. Therefore this work paves the way to the possibility to coherently and dynamically control the THz field shape

Intense terahertz pulses from SPARC-LAB coherent radiation source

The linac-based Terahertz source at the SPARC_LAB test facility is able to gene rate highly intense Terahertz broadband pulses via coherent transition radiation (CTR) from high brightness electron beams. The THz pulse duration is typically down to 100 fs RMS and can be tuned through the electron bunch duration and shaping. The measured stored energy in a single THz pulse has reached 40 μ J, which corresponds to a peak electric field of 1.6 MV/cm at the THz focus. Here we present the main features, in particular spatial and sp ectral distributions and energy characterizations of the SPARC_LAB THz source, which is very competitive for investigations in Condensed Matter, as well as a valid tool for electron beam longitudinal diagnostics

Performance Evaluation of a THz Pulsed Imaging System: Point Spread Function, Broadband THz Beam Visualization and Image Reconstruction

Terahertz (THz) technology is a promising research field for various applications in basic science and technology. In particular, THz imaging is a new field in imaging science, where theories, mathematical models and techniques for describing and assessing THz images have not completely matured yet. In this work, we investigate the performances of a broadband pulsed THz imaging system (0.2–2.5 THz). We characterize our broadband THz beam, emitted from a photoconductive antenna (PCA), and estimate its point spread function (PSF) and the corresponding spatial resolution. We provide the first, to our knowledge, 3D beam profile of THz radiation emitted from a PCA, along its propagation axis, without the using of THz cameras or profilers, showing the beam spatial intensity distribution. Finally, we evaluate the THz image formation on a test-sample composed by a regular linen natural pattern

Secondary Structures of MERS-CoV, SARS-CoV, and SARS-CoV-2 Spike Proteins Revealed by Infrared Vibrational Spectroscopy

All coronaviruses are characterized by spike glycoproteins whose S1 subunits contain the receptor binding domain (RBD). The RBD anchors the virus to the host cellular membrane to regulate the virus transmissibility and infectious process. Although the protein/receptor interaction mainly depends on the spike’s conformation, particularly on its S1 unit, their secondary structures are poorly known. In this paper, the S1 conformation was investigated for MERS-CoV, SARS-CoV, and SARS-CoV-2 at serological pH by measuring their Amide I infrared absorption bands. The SARS-CoV-2 S1 secondary structure revealed a strong difference compared to those of MERS-CoV and SARS-CoV, with a significant presence of extended β-sheets. Furthermore, the conformation of the SARS-CoV-2 S1 showed a significant change by moving from serological pH to mild acidic and alkaline pH conditions. Both results suggest the capability of infrared spectroscopy to follow the secondary structure adaptation of the SARS-CoV-2 S1 to different environments