Ultraviolet Beam Research Articles

A Closer Look at Heritage Systems from Medieval Colors to Modern and Contemporary Artworks

This microreview, conducted by interdisciplinary teams, examines complex heritage material systems, such as medieval colors and modern and contemporary artworks. Our multi-analytical approach, a significant aspect of our research, is a means to this end. The conservation of works of art is our shared goal, as it ensures their accessibility and the transfer of cultural heritage to future generations. We seek to interpret the damage, usefulness, and innovation of the experimental design in this context. As Jan Wouters rightly points out, “The terminology used nowadays to describe the potential damage to objects caused by analysis should be refined beyond the destructiveness/non-invasiveness polarization. A terminology should include at least degree level intervention (low, medium, high), usefulness, and innovation”. Complementing micro- or sub-micro-sampling with the appropriate analytical methods is crucial, as exemplified in medieval, modern, and contemporary collections studies. Finally, a novel perspective for exploring the information contained in the multiscale heterogeneity of organic historical materials is envisaged, and it includes UV/Visible photoluminescence spectral imaging using a low-intensity ultraviolet synchrotron beam.

Efficient ultraviolet beam generation at 294.5 nm by frequency-quadrupling LD-pumped actively Qswitched Nd:YAG/SrWO4 Raman laser

Efficient ultraviolet beam generation at 294.5 nm by frequency-quadrupling LD-pumped actively Qswitched Nd:YAG/SrWO4 Raman laser

Comparison of wavefront sensing and ablation imprinting for FEL focus diagnostics at FLASH2

Extreme ultraviolet (EUV) photon beam characterization techniques, Hartmann wavefront sensing and single shot ablation imprinting, were compared along the caustic of a tightly focused free-electron laser (FEL) beam at beamline FL24 of FLASH2, the Free-electron LASer in Hamburg at DESY. The transverse coherence of the EUV FEL was determined by a Young’s double pinhole experiment and used in a back-propagation algorithm which includes partial coherence to calculate the beam intensity profiles along the caustic from the wavefront measurements. A very good agreement of the profile structure and size is observed for different wavelengths between the back-propagated profiles, an indirect technique, and ablation imprints. As a result, the Hartmann wavefront sensor including its software MrBeam is a very useful, single shot pulse resolved and fast tool for non-invasive determination of focal spot size and shape and also for beam profiles along the caustic.

High-resolution ptychographic imaging at a seeded free-electron laser source using OAM beams

Electromagnetic waves possessing orbital angular momentum (OAM) are powerful tools for applications in optical communications, quantum technologies, and optical tweezers. Recently, they have attracted growing interest since they can be harnessed to detect peculiar helical dichroic effects in chiral molecular media and in magnetic nanostructures. In this work, we perform single-shot per position ptychography on a nanostructured object at a seeded free-electron laser, using extreme ultraviolet OAM beams of different topological charge orders ℓ generated with spiral zone plates. By controlling ℓ, we demonstrate how the structural features of OAM beam profiles determine an improvement of about 30% in image resolution with respect to conventional Gaussian beam illumination. This result extends the capabilities of coherent diffraction imaging techniques, and paves the way for achieving time-resolved high-resolution (below 100 nm) microscopy on large area samples.

Optimization of extreme ultraviolet vortex beam based on high harmonic generation

In high harmonic generation (HHG), Laguerre–Gaussian (LG) beams are used to generate extreme ultraviolet (XUV) vortices with well-defined orbital angular momentum (OAM), which have potential applications in fields such as microscopy and spectroscopy. An experimental study on the HHG driven by vortex and Gaussian beams is conducted in this work. It is found that the intensity of vortex harmonics is positively correlated with the laser energy and gas pressure. The structure and intensity distribution of the vortex harmonics exhibit significant dependence on the relative position between the gas jet and the laser focus. The ring-like structures observed in the vortex harmonics, and the interference of quantum paths provide an explanation for the distinct structural characteristics. Moreover, by adjusting the relative position between the jet and laser focus, it is possible to discern the contributions from different quantum paths. The optimization of the HH vortex field is applicable to the XUV, which opens up a new way for exploiting the potential in optical spin or manipulating electrons by using the photon with tunable orbital angular momentum.

UV-ultrashort pulsed laser ablation of fused silica

The authors report on ultraviolet ultrashort pulsed laser ablation of fused silica and compare the achievable micromachining results to those obtained by using the fundamental emission wavelength in infrared. Ablation in ultraviolet reveals a stable efficiency for increasing fluences, whereas using an infrared beam exhibits a decreasing trend of the ablation efficiency at higher and increasing fluences. In addition, a significant improvement in the surface quality is found by using an ultraviolet wavelength in a fluence range up to 20 J/cm2 compared to infrared, e.g., revealing an Ra of down to 0.45 μm on using the ultraviolet wavelength compared to Ra = 0.56 μm on using infrared at fluences up 15 J/cm2. Moreover, taking advantage of the high available pulse energy, the authors compare the achievable ablation efficiency and surface roughness using a conventionally focused ultraviolet beam and a defocused ultraviolet beam, finding that the defocused ultraviolet beam possesses a processing quality comparable to that of the focused beam. Finally, the authors exemplify the potential of ultraviolet ultrashort pulsed laser ablation by using a Tesla mixer for microfluidic integration of fused silica.

Mechanisms of acid generation from ionic photoacid generators for extreme ultraviolet and electron beam lithography.

Photoacid generators (PAGs) are important components of chemically amplified resists. The properties of PAGs directly affect the sensitivity of photoresists, line edge roughness, and resolution. Understanding the photoacid generation process in extreme ultraviolet (EUV) and electron beam (EB) lithography is helpful for photoresist design. However, the microscopic mechanisms remain largely unclear and the large variety in the molecular structure of PAGs presents a challenge to overcome. In this work, we investigate the microscopic processes of photoacid production of ionic PAGs for EUV and EB lithography. The PAG dissociation pathway is found to depend on the molecular structure and conformations. The processes of photoacid production and by-product generation are also revealed. The results contribute to a better understanding of the photochemical reactions in EUV and EB lithography, providing insights into the molecular design of novel PAGs and photoresists.

Integrated Cross-Scale Manipulation and Modulable Encapsulation of Cell-Laden Hydrogel for Constructing Tissue-Mimicking Microstructures.

Engineered microstructures that mimic invivo tissues have demonstrated great potential for applications in regenerative medicine, drug screening, and cell behavior exploration. However, current methods for engineering microstructures that mimic the multi-extracellular matrix and multicellular features of natural tissues to realize tissue-mimicking microstructures invitro remain insufficient. Here, we propose a versatile method for constructing tissue-mimicking heterogeneous microstructures by orderly integration of macroscopic hydrogel exchange, microscopic cell manipulation, and encapsulation modulation. First, various cell-laden hydrogel droplets are manipulated at the millimeter scale using electrowetting on dielectric to achieve efficient hydrogel exchange. Second, the cells are manipulated at the micrometer scale using dielectrophoresis to adjust their density and arrangement within the hydrogel droplets. Third, the photopolymerization of these hydrogel droplets is triggered in designated regions by dynamically modulating the shape and position of the excitation ultraviolet beam. Thus, heterogeneous microstructures with different extracellular matrix geometries and components were constructed, including specific cell densities and patterns. The resulting heterogeneous microstructure supported long-term culture of hepatocytes and fibroblasts with high cell viability (over 90%). Moreover, the density and distribution of the 2 cell types had significant effects on the cell proliferation and urea secretion. We propose that our method can lead to the construction of additional biomimetic heterogeneous microstructures with unprecedented potential for use in future tissue engineering applications.

The utility of ultraviolet beam in advanced oxidation-reduction processes: a review on the mechanism of processes and possible production free radicals.

Advanced oxidation processes (AOPs) and advanced reduction processes (ARPs) are a set of chemical treatment procedures designed to eliminate organic (sometimes inorganic) contamination in water and wastewater by producing free reactive radicals (FRR). UV irradiation is one of the factors that are effectively used in oxidation-reduction processes. Not only does the UV beam cause the photolysis of contamination, but it also leads to the product of FRR by affecting oxidants-reductant, and the pollutant decomposition occurs by FRR. UV rays produce active radical species indirectly in an advanced redox process by affecting an oxidant (O3, H2O2), persulfate (PS), or reducer (dithionite, sulfite, sulfide, iodide, ferrous). Produced FRR with high redox potential (including oxidized or reduced radicals) causes detoxification and degradation of target contaminants by attacking them. In this review, it was found that ultraviolet radiation is one of the important and practical parameters in redox processes, which can be used to control a wide range of impurities.

Generation of Complex Vector and Vortex Extreme Ultraviolet Beams Using the S-Waveplate and Spiral Phase Plate during High-Order Harmonics Generation in Argon

Generation of Complex Vector and Vortex Extreme Ultraviolet Beams Using the S-Waveplate and Spiral Phase Plate during High-Order Harmonics Generation in Argon

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet (XUV) light beam carrying orbital angular momentum (OAM) can be produced via high-order harmonic generation (HHG) due to the interaction of an intense vortex infrared laser and a gas medium. Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node (from 0 to 2) in the driving laser consisting of two mixed Laguerre–Gaussian (LG) beams. We find that due to the change in spatial profile of HHG, the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node. We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes. Through the map of coherence length and the evolution of harmonic field in the medium, we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node. We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

Dynamic ultraviolet harmonic beam pattern control by programmable spatial wavefront modulation of near-infrared fundamental beam

Abstract The ultraviolet (UV) wavelength regime is attracting increasing attention because of its growing demand in semiconductor lithography, spectroscopy, and imaging applications owing to its high spatial resolution and high photon energy. However, beam shape control, beam delivery, and wavefront manipulation of UV laser beams usually require highly dedicated optics because of the strong UV absorption of most optical materials and the high surface precision required for tailoring short wavelengths, thus limiting a broader application of UV wavelengths. Here, we demonstrate a novel dynamic UV harmonic beam pattern control by manipulating the near-infrared (NIR) wavefront of the fundamental wavelength of a femtosecond pulse laser. The temporal and spatial coherences in an optical harmonic generation are known to be well preserved. Therefore, the spatial beam distribution of UV harmonic beams (λ = 400 and 266 nm for second and third harmonics, respectively) could be readily controlled by tailoring the wavefront of the driving infrared (IR) beam, and this approach can be expanded to higher-order harmonics in the vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) regimes. Moreover, this enables fast polarization-sensitive UV beam switching at a speed of 6.7 frames/s in a depth-resolving manner. To efficiently separate the UV beam from the strong fundamental IR background beam, a non-collinear harmonic generation configuration is introduced. This facile dynamic UV beam control technique enables arbitrary wavefront control of UV laser beams for high-precision laser patterning, polarization-sensitive encryption, and 3D holograms.

Structural investigation of zinc‐based photoresists with different substituents for high‐resolution lithography

The fineness of the photolithographic pattern depends largely on the composition and structure of the photoresist, and different structures endow metal‐based photoresists with different photon‐absorbing capabilities. Here, we synthesized zinc‐based photoresists with different electron‐effect substituents (amine and bromine) in the ligands and investigated the influence of the ligand groups on the sensitivity and resolution at deep ultraviolet and electron beam. The developed resist patterns exhibit that the sensitivity of the Zn‐based photoresists containing bromine groups (Zn‐PBBA) is nearly seven times higher than the Zn‐based photoresists containing amine groups (Zn‐PABA) at the same resolution under ultraviolet irradiation, which is supported by the density functional theory (DFT) calculations where a smaller HOMO–LUMO gap for Zn‐PBBA is obtained. Analogously, the Zn‐PBBA also showed better sensitivity and resolution than Zn‐PABA at EBL, with a resolution of 70 nm at a dose of 800 mJ cm−2 by characterization with AFM. This work provides an understanding of the efficient structural design of photoresists and future guidance for the development of better‐performing materials.

Universal Behavior of Highly Confined Heat Flow in Semiconductor Nanosystems: From Nanomeshes to Metalattices

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first-principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalattice with deep nanoscale feature size and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeability component and an intrinsic viscous contribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using experiments and atomistic simulations, we show that our theory applies to a general set of highly confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires, to nanowire networks, of great interest for next-generation energy-efficient devices.

High-resolution wavefront sensing and aberration analysis of multi-spectral extreme ultraviolet beams

Coherent multi-spectral extreme ultraviolet beams have great potential for providing high spatial and temporal resolution for microscopy and spectroscopy applications. But due to the limitations of short-wavelength optics and the broad bandwidth, it remains a challenge to perform quantitative, high-resolution beam characterization. Here we present a wavefront sensing solution based on multiplexed ptychography, with which we show spectrally resolved, high-resolution beam reconstructions. Furthermore, using these high-fidelity quantitative wavefront measurements, we investigate aberration transfer mechanisms in the high-harmonic-generation process, where we present and explain harmonic-order-dependent astigmatism inheritance from the fundamental wavefront. This ptychographic wavefront sensing concept thus enables detailed studies of the high-harmonic-generation process, such as spatiotemporal effects in attosecond pulse formation.

Chromatic aberrations correction of attosecond high-order harmonic beams by flat-top spatial shaping of the fundamental beam

Attosecond pulses created by high-order harmonic generation in gases often exhibit strong chromatic aberrations, arising from the broad bandwidth and wavelength-dependent nonlinear light–matter interaction. When the driving laser intensity varies spatially, as for Gaussian driving beams, the apparent source position of the harmonics differs significantly from one order to the next, thus affecting the achievable intensity and duration of the attosecond pulses when they are focused on a target. We show that these chromatic aberrations can be reduced by spatially shaping the fundamental beam to generate high-order harmonics with a driver having a flat-top profile inside the gas medium. By measuring both the intensity profile and wavefront for each harmonic in a plane, we access the extreme ultra-violet (XUV) beam properties and investigate these properties near focus. We observe that controlling chromatic aberrations by flat-top spatial shaping strongly reduces the variation of the XUV spectrum on the beam axis during propagation and, in return, the longitudinal sensitivity of both the temporal profiles and the temporal shifts of the focused attosecond pulses.

Canny Algorithm Enabling Precise Offline Line Edge Roughness Acquisition in High-Resolution Lithography.

The line edge roughness (LER) is one of the most critical indicators of photoresist imaging performance, and its measurement using a reliable method is of great significance for lithography. However, most studies only investigate photoresist resolution and sensitivity because LER measurements require an expensive and not widely available critical dimension scanning electron microscopy (SEM) technology; thus, the imaging performance of photoresist has not been adequately evaluated. Here, we report an image processing software developed for offline calculation of LER that can analyze lithographic patterns with resolutions up to ∼15 nm. This software can effectively process all graphic files obtained from commonly used SEM machines by utilizing the adjustable double threshold. To realize the effective detection of high-resolution patterns in advanced lithography, we used SEM images generated from extreme ultraviolet and electron beam lithography to develop and validate the software's graphic recognition algorithm. This image processing software can process typical SEM images and produce reliable LER in an efficient and user-friendly manner, constituting a powerful tool for promoting the development of high-performance photoresist materials.

5.32 W ultraviolet laser generation at 266 nm using sum-frequency method with CsB3O5 crystal.

Ultraviolet (UV) beam generation at 266 nm using the sum-frequency (SFG) method with CsB3O5 (CBO) crystals was first suggested in 1997 [Opt. Lett.22, 1840 (1997).10.1364/OL.22.001840]; however, there has been no further research in the past 25 years. Herein, by sum-frequency mixing in CBO crystals, we obtained a high conversion efficiency picosecond (ps) and a high-power nanosecond (ns) 266 nm UV beam output. First, a ps laser device with simultaneously radiated wavelengths of 1064 and 355 nm and repetition frequency of 10 Hz was used as the fundamental laser source, and the conversion efficiency from 1064 + 355 nm to 266 nm reached 20.35%. We then used a 1064 nm ns laser with a high output power and repetition frequency of 10 kHz as the pump source. We accurately modified the optimal phase matching direction of the CBO crystal, and the achieved output power at 266 nm reached 5.32 W.

High power and conversion efficiency intracavity ultraviolet laser at 355 nm

In this study, we investigated and provided a compact and efficient diode-pumped-solid-state ultraviolet laser at 355 nm with high commercial value. The 355 nm laser was coupled out with an uncoated Brewster-cut LBO crystal in order to prolong its lifetime. A maximum output power of 12.2 W was generated when pump power was 74.5 W and the pulse repetition rate was 40 kHz. The conversion efficiency from pump laser to ultraviolet laser was 16.4 %, which we believe to be the highest among the reported papers in which the pump source was 887 nm. The stability of 355 nm laser power was less than ± 1 % at 12 W over 20 h. Output characteristic of ultraviolet beam varying with the temperature of two LBO crystals was also investigated to located the exact optimal operating temperature.

Dinitriles and nitriles are common intermediates of pyrrole pyrolysis

Pyrrole is a typical fuel nitrogen model compound of coal and it is the precursor of fuel-NOx. The mechanism and kinetics of pyrrole pyrolysis were studied in a jet-stirred reactor coupled with a synchrotron vacuum ultraviolet molecular beam mass spectrometer and a gas chromometer over the temperature range of 940–1240 K at atmospheric pressure. Two sets of new data of pyrrole pyrolysis were provided. Based on the photoionization efficiency analysis, nitrogenous products are mainly produced in the form of nitriles and dinitriles. Of the pyrrole pyrolysis products, six nitriles were newly identified including methyl isocyanide (C2H3N), cyanogen (NCCN), isocyanogen (CNCN), propionitrile (C3H5N), propanedinitrile (C3H2N2), butanedinitrile (C4H4N2). Mole fractions of pyridine (C5H5N), C3H2N2, cyanovinylacetylene (C5H3N), and benzonitrile (C7H5N) were also quantified. HCN is the dominant nitrogenous product of pyrrole pyrolysis. C2H2 and CH4 are the major carbonaceous products. An updated pyrrole kinetic model was proposed comprising 196 species and 2939 reactions. The production of pyridine and several nitriles during pyrrole pyrolysis could be reasonably predicted using the current model. C3H2N2 is mainly produced by the reaction between CH2CN and other nitriles, and it is converted into CH2CN and CH3CN. The reaction between CH3 and C3H2N2 is the most sensitive reaction for promoting the consumption of C3H2N2. Results of this work confirm the importance of dinitriles and nitriles during the pyrolysis of pyrrole as an important fuel-NOx model compound of coal and other nitrogen-containing fuels.