Spatial Beam Profile Research Articles

Direct Measurement of the Electron Beam Spatial Intensity Profile via Carbon Nanotube Tomography.

Electron microscopes are ubiquitous across the scientific landscape and have been improved to achieve ever smaller beam spots, a key parameter that determines the instrument's resolution. However, the traditional techniques to characterize the electron beam have limited effectiveness for today's instruments. Consequently, there is an ongoing need to develop detection technologies that can potentially measure the smallest electron beam, which is valuable for the continual advancement of microscope performance. We report on a new electron beam detector based on a single-wall carbon nanotube. The nanotubes are atomically smooth, have a well-defined diameter that is similar in size to the finest electron probes, and can be used to directly measure the beam profile. Additionally, by rotating the nanotube in a plane perpendicular to the beam path and scanning the beam at different angles, we can apply tomographic reconstruction techniques to determine the spatial intensity profile of an electron beam accurately.

Xcalib: a focal spot calibrator for intense X-ray free-electron laser pulses based on the charge state distributions of light atoms.

The xcalib toolkit has been developed to calibrate the beam profile of an X-ray free-electron laser (XFEL) at the focal spot based on the experimental charge state distributions (CSDs) of light atoms. Characterization of the fluence distribution at the focal spot is essential to perform the volume integrations of physical quantities for a quantitative comparison between theoretical and experimental results, especially for fluence-dependent quantities. The use of the CSDs of light atoms is advantageous because CSDs directly reflect experimental conditions at the focal spot, and the properties of light atoms have been well established in both theory and experiment. Theoretical CSDs are obtained using xatom, a toolkit to calculate atomic electronic structure and to simulate ionization dynamics of atoms exposed to intense XFEL pulses, which involves highly excited multiple core-hole states. Employing a simple function with a few parameters, the spatial profile of an XFEL beam is determined by minimizing the difference between theoretical and experimental results. The optimization procedure employing the reinforcement learning technique can automatize and organize calibration procedures which, before, had been performed manually. xcalib has high flexibility, simultaneously combining different optimization methods, sets of charge states, and a wide range of parameter space. Hence, in combination with xatom, xcalib serves as a comprehensive tool to calibrate the fluence profile of a tightly focused XFEL beam in the interaction region.

Surface plasmon resonance tuning of Ag nanoisland films using a CO2 laser

We demonstrate that the surface plasmon resonance of Ag nanoislands (NIs) can be conveniently tuned through the laser power and irradiation time. For instance, the resonance wavelength of Ag NIs can be tuned from 548 nm to 452 nm under the 3 W laser power simply by varying the irradiation time from 5 s to 2 min. Interestingly, the position-dependent optical absorption spectra and SEM analysis reveals that the Ag NIs gradually disappear from the irradiation center even at 3 W if the irradiation time is longer. This means that the macroscopic diffusion of Ag atoms on the substrate takes place during the CO2 laser irradiation, and its driving force would be the temperature gradient along the radial direction with respect to the irradiation center due to the spatial beam profile of the laser.

КОГЕРЕНТНОЕ РАСПРОСТРАНЕНИЕ ЛАЗЕРНЫХ ПУЧКОВ В МАЛОРАЗМЕРНОЙ СИСТЕМЕ СЛАБОСВЯЗАННЫХ СВЕТОВОДОВ

The successful development of fiber-optic technologies in recent decades has stimulated research on the replacement of components of solid-state laser systems with fiber components, which can drastically change the attractiveness of the corresponding applied developments. Yielding on the energy characteristics of solid-state systems, fiber lasers and nonlinear optical devices have high efficiency of conversion of pump energy to radiation energy associated with waveguide geometry, high quality of the spatial profile of the laser beam, as well as low cost, compactness, lack of alignment in work process. Note that the maximum achievable radiation power in a single fiber is limited primarily by the process of self-focusing, which leads to fiber damage. The use of a multi-core fiber (MCF), consisting of identical equidistant weakly coupled optical fibers, makes it possible to realize initially coherent propagation of laser radiation with a total power noticeably higher than it can be transmitted in a single optical fiber. However, as theoretical and experimental studies have shown, such systems have its own critical power (Balakin et al., 2016) whereby the self-focusing of the quasihomogeneous distribution of the wave field and its separation into a set of incoherent structures occurs. Therefore, we have considered a small-sized optical system of 2N identical weakly coupled optical fibers arranged in a ring (Balakin et. al., 2018). In such systems, it is possible to find stable distributions of intense wave beams, which allow coherent radiation transport over long distances. The total radiation power in the found distributions can significantly (up to 2N times) exceed the critical self-focusing power in continuous media. This manifests itself most clearly for the distribution of un ~ (-1)n with antiphase fields in neighboring waveguides, which is stable at an arbitrary wave beam power. Direct numerical simulation of a nonlinear wave equation confirms the stability of the field distributions found. The research was supported by the RAS Presidium Program «Nonlinear dynamics: fundamental problems and applications».

Photon diagnostics at the FLASH THz beamline.

The THz beamline at FLASH, DESY, provides both tunable (1-300 THz) narrow-bandwidth (∼10%) and broad-bandwidth intense (up to 150 uJ) THz pulses delivered in 1 MHz bursts and naturally synchronized with free-electron laser X-ray pulses. Combination of these pulses, along with the auxiliary NIR and VIS ultrashort lasers, supports a plethora of dynamic investigations in physics, material science and biology. The unique features of the FLASH THz pulses and the accelerator source, however, bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles. Here, these challenges are discussed and the pulse diagnostic tools developed at FLASH are presented. In particular, a radiometric power measurement is presented that enables the derivation of the average pulse energy within a pulse burst across the spectral range, jitter-corrected electro-optical sampling for the full spectro-temporal pulse characterization, spatial beam profiling along the beam transport line and at the sample, and a lamellar grating based Fourier transform infrared spectrometer for the on-line assessment of the average THz pulse spectra. Corresponding measurement results provide a comprehensive insight into the THz beamline capabilities.

Chirp-controlled filamentation and formation of light bullets in the mid-IR.

Formation of light bullets-tightly localized in space and time light packets, retaining their spatiotemporal shape during propagation-is, for the first time, experimentally observed and investigated in a new regime of mid-infrared filamentation in ambient air. It is suggested that the light bullets generated in ambient air by multi-mJ, positively chirped 3.9-μm pulses originate from a dynamic interplay between the anomalous dispersion in the vicinity of CO2 resonance and positive chirp, both intrinsic, carried by the driver pulse, and accumulated, originating from nonlinear propagation in air. By adjusting the initial chirp of the driving pulses, one can control the spatial beam profile, energy losses, and spectral-temporal dynamics of filamenting pulses and deliver sub-3-cycle mid-IR pulses in high-quality beam on a remote target.

Neutron beam spatial profile at n_TOF using CR-39 nuclear track detectors

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Low-energy/pulse response and high-resolution-CMOS camera for spatiotemporal femtosecond laser pulses characterization @ 1.55 μm.

In this work, we present a commercial CMOS (Complementary Metal Oxide Semiconductor) Raspberry Pi camera implemented as a Near-Infrared detector for both spatial and temporal characterization of femtosecond pulses delivered from a femtosecond Erbium Doped Fiber laser (fs-EDFL) @ 1.55 µm, based on the Two Photon Absorption (TPA) process. The capacity of the device was assessed by measuring the spatial beam profile of the fs-EDFL and comparing the experimental results with the theoretical Fresnel diffraction pattern. We also demonstrate the potential of the CMOS Raspberry Pi camera as a wavefront sensor through its a nonlinear response in a Shack-Hartmann array and for the temporal characterization of the femtosecond pulses delivered from the fs-EDFL through TPA Intensity autocorrelation measurements. The direct pulse detection and measurement, through the nonlinear response with a CMOS, is proposed as a novel and affordable high-resolution and high-sensitivity alternative to costly detectors such as CCDs, wavefront sensors and beam profilers @ 1.55 µm. The measured fluence threshold, down to 17.5 µJ/cm2, and pJ/pulse energy response represents the lowest reported values applied as a beam profiler and a TPA Shack-Hartmann wavefront sensor, to our knowledge.

Determination of the Neutron Beam Spatial Profile at n_TOF EAR-2 using the CR-39 Track Detectors

Within the present work the neutron beam spatial profile was determined at the sample position in EAR-2 at the n_TOF facility at CERN. The CR-39 detectors were coupled with a 2mm PE foil serving as neutron-to-proton converter. Two irradiations were performed in the 10x5 cm surface of CR-39 detectors. Proton tracks were revealed in the CR-39 detectors resulting from the elastic scattering of fast neutrons on the hydrogen atoms in the PE converter. Afterwards, the CR-39 detectors were chemically etched in aqueous NaOH solution and latent tracks were considerably enlarged to become visible under an optical microscope. After the scanning of the detectors surface, the acquired images were analyzed using the ImageJ program. In the present work, the experimental setup and procedure will be presented along with the results concerning the characterization of the neutron beam spatial profile at the sample position in the n_TOF EAR-2.

Metalized Poly-methacrylate Off-Axis Parabolic Mirrors for Terahertz Imaging Fabricated by Additive Manufacturing

Terahertz radiation sources are currently one of the most widely used non-ionizing illumination mechanisms for security applications and also find increasing utilization in quality control of commercial products. Presently, a majority of these applications rely on scanning rather than direct imaging and implicitly suffer from temporal latency due to post processing. The monetary and temporal cost associated with procuring commercially manufactured optics that are suitable for imaging leads to fundamental limitations in the ability to rapidly develop application-specific imaging modalities using terahertz sources. Herein, we show a novel method for the rapid prototyping of metallic coated poly-methacrylate parabolic reflectors fabricated by stereolithographic 3D printing. Images comparing the performance of a commercially available off-axis parabolic reflector to our metalized poly-methacrylate prototype, which was designed to be identical to the commercially available mirror, are subsequently presented. The images show that at 530 GHz it is possible to produce a metalized poly-methacrylate off-axis paraboloid whose spatial beam profile is nearly identical to that of a commercially available equivalent.

Selective suppression of CARS signal with two competing stimulated Raman scattering processes.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy have been used in studying the structure and dynamics of a wide range of chemical and biological systems. However, the spatial resolution of CARS microscopy is still limited by the diffraction barrier, and hence a suitable scheme to selectively switch off the CARS imaging signal is essential for super-resolution CARS microscopy. Here, we present theoretical descriptions about three different ways to selectively suppress the pump-Stokes-pump two-beam CARS signal by employing three-beam double stimulated Raman scattering (SRS) schemes. Using a semiclassical theory for the interaction of radiation with the Raman-active molecule, we obtain coupled differential equations for the intensities of the pump, Stokes, depletion, and the generated CARS signal fields. We find approximate solutions of these coupled differential equations. They are then used to show that the pump-Stokes-pump CARS signal can be selectively suppressed by increasing the added depletion beam intensity, when the three injected beam frequencies are tuned in such a way that they can induce two SRS processes simultaneously. To show that these switching-off methods can be used to develop super-resolution CARS imaging techniques, we numerically calculate the full-width-at-half-maximum of the CARS imaging point spread function assuming that the spatial profiles of the pump and Stokes beams are Gaussian functions and that the spatial profile of the depletion beam is doughnut-shaped. We anticipate that the proposed selective CARS suppression schemes will be of use in developing super-resolution, label-free CARS microscopy.

Design and fabrication of 1D and 2D optical scanner for electron beams using color centre generation

The present paper focuses on the design and fabrication of 1D and 2D optical scanners to characterize the spatial profile of electron beams. The basis of the two systems is the generation of color centers in glass plates by irradiation, followed by the scanning of the plates, illuminated by a fixed power source (LED) with a maximum power of 3 W in the spectral range of 520–560 nm, with a TCS3200 sensor. In both systems, 6 mm thick glass blades (SiO2:76%) are used instead of expensive dosimetric films. The experimental results section first focuses on the validation and calibration accuracy of the effective absorbance of the irradiated glass in the LED spectral range with a radiation dose of up to 20 kGy. Then, it describes how the 1D optical scanner was used to scan the spatial profile obtained under the scanning horn of a Rhodotron accelerator with a maximum radiation dose of 7.8 kGy. The experimental results obtained from this method are entirely consistent with the results of other research studies, while this method is more cost-efficient and accurate compared to the previously reported methods. The 2D optical scanner was also used to accurately measure the spatial profile of the Rhodotron electron beam in static mode by scanning the color centers generated by irradiation.

Effect of non-linear amplification of phase and amplitude modulations on laser-induced damage of thick fused silica optics with large beams at 351 nm

Nonlinear propagation of high power, large aperture laser beams leads to the amplification of the spatial pre-existing small scale modulations, and therefore locally increases the laser energy density as the beam propagates through the optical component. The measurement of the energy density distribution on the exit face of the tested component is therefore more challenging, as the impact of the propagation must be taken into account. It is for this reason that the damage densities of thick fused silica components have never been precisely measured. To this end, a dedicated imaging setup has been developed to accurately measure the energy density distribution on the exit face of 34-mm thick fused silica optical components during laser damage experiments with a centimeter-scale laser beam. The impact of nonlinear propagation on the beam spatial profile has been studied experimentally and confronted to numerical simulations that solve the nonlinear Schrödinger equation for the experimental spatial and phase profiles. The results clearly indicate that the imaging setup can accurately measure the amplification of the beam spatial modulations. The beam energy density on the exit face is therefore correctly measured and, for the first time, the damage density of thick optics can be accurately determined.

Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases

Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-μJ input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25–50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular.

3.1–5.2 μm Coherent MIR Frequency Comb Based on Yb-Doped Fiber Laser

An offset-free, wavelength-tunable, and midinfrared frequency comb working at 184 MHz based on difference frequency generation (DFG) in a PPMgSLT crystal was demonstrated. A coherent Raman soliton pulse and Yb-fiber laser output were used as the seed pulses of DFG, and wavelength tunability in the range 3.1–5.2 μ m was realized. A quantum cascade laser stabilized by the absorption line of N2O was used to measure the beat signal with an MIR comb. The MIR comb had a good spatial beam profile, and a high signal-to-noise ratio (SNR) of 70 dB at a wavelength of 4.52 μ m was confirmed. A stable and sharp beat signal was observed in the MIR region with a linewidth of 160 kHz, and the SNR was larger than 20 dB.

Experimental progress in positronium laser physics

The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.Graphical abstract

Spatial characterization of light beams analyzed by cylindrical-grating slit-less spectrometers.

In this work, we theoretically analyze the spatial information provided by cylindrical-grating slit-less spectrometers. We raise attention on the often not considered property that the spatial features acquired using these spectrometers are different from what can be obtained using a spectrometer with an entrance slit. In relation to this, we also highlight that they do not provide information directly on the real spatial beam profile. It is important to consider this fact in spatio-spectral analysis of extreme ultraviolet radiation, often carried out using cylindrical-grating slit-less spectrometers. Since the models used are based on the Fresnel diffraction integral and ideal optical systems, the results are valid also for other spectral regions.

Laser-driven ion acceleration via target normal sheath acceleration in the relativistic transparency regime

We present an experimental study investigating laser-driven proton acceleration via target normal sheath acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant regime (∼1 μm) down to below the onset of relativistic laser-transparency (<40 nm). This is done with a single target material in the form of freely adjustable films of liquid crystals along with high contrast (via plasma mirror) laser interaction (∼2.65 J, 30 fs, W cm−2). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ∼40 nm indicate the influence of relativistic transparency on TNSA, observed via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed the fielding of numerous diagnostics to determine the interaction quality and details: ion energy and spatial distribution was measured along the laser axis and both front and rear target normal directions; these along with reflected and transmitted light measurements on-shot verify TNSA as dominant during high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-in-cell simulations qualitatively support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

Development and validation of an inverse method for identification of thermal characteristics of a short laser pulse

The paper presents development and validation of an inverse method for the identification of thermal characteristics of a short single laser pulse which stroke in a metal sample. The inverse method was applied to find unknown power of the laser pulse, the dimensionless shape parameter of the super-Gaussian function which describes the beam spatial profile as well as beginning and end times of the exposition of the metal sample to the laser pulse. The proposed inverse algorithm was based on the Levenberg-Marquardt technique as well as on temporal and spatial distributions of temperature on the rear surface of the sample, i.e., the opposite to the irradiated one, measured using the experimental stand. The performed investigations showed that the problem was ill-posed but good accuracy was obtained. The low sensitivity of registered temperature to changes in both power and duration of the pulse affected the retrieving accuracy most significantly. Moreover, the dependence of solution of the inverse problem on the initial guess was observed. The accuracy was also affected by low temporal resolution (500 Hz, with the exposure time from 0.2 to 1 ms) of the IR camera. This resolution affected the temporal sampling of measured temperatures. Despite these problems, the method was able to retrieve unknown pulse parameters with 20-25% accuracy.

Neutron imager with micro channel plates (MCP) in electrostatic mirror configuration: Experimental test with radiation source

The design of a new high-transparency device based on a Micro Channel Plate (MCP) detector was recently proposed for monitoring the flux and beam spatial profile of neutron beams. The proposed device consists of a very thin aluminum (Al) foil (with a [Formula: see text]Li deposit) placed in the neutron beam and an MCP detector equipped with a phosphor-screen readout linked to a charge-coupled device (CCD) camera outside the neutron beam. A critical feature of this device is that it uses an electrostatic mirror to minimize the perturbation of the neutron beam (i.e., absorption and scattering). It can be used at existing neutron time-of-flight (n_TOF) facilities (in particular at the n_TOF facility at CERN) for monitoring the flux and spatial profile of neutron beams in the thermal and epithermal region. The experimental tests conducted for this study using a radioactive source to determine the behavior of the electrostatic mirror behavior will be presented and discussed in this paper.