Ultraviolet Wavelength Region Research Articles

Effective Thermal Management in Ultraviolet Light-Emitting Diodes With Micro-LED Arrays

We report on the use of micro-LED arrays, consisting of a matrix of interconnected micrometer-size light-emitting diodes (LEDs), to ensure uniform current injection, reduced series resistance, and improved heat extraction in LEDs emitting in the ultraviolet wavelength region. With the help of experiments and simulations, we show that, in both 380- and 300- to 325-nm LEDs, a greater than twofold decrease in the thermal resistance of the device, as compared with a conventional large area contact, is possible with the use of micro-LED arrays. The thermal resistance was found to linearly decrease with the size of the individual micro-LED, indicating that the improved heat dissipation is mainly due to the division of the heat source from one large area for the square contact into smaller pixels, thus allowing a sufficient area for the dissipation of the generated heat.

Rough eyes of the Northeast-Asian Wood White Leptidea amurensis

The northeast-Asian wood white, Leptidea amurensis (Lepidoptera, Pieridae), belongs to the Dismorphiinae, a subfamily of the family Pieridae. We studied the structure of the compound eye in this species through a combination of anatomy, molecular biology and intracellular electrophysiology, with a particular focus on the evolution of butterfly eyes. We found that their eyes consist of three types of ommatidia, with a basic set of one short-, one middle- and one long-wavelength-absorbing visual pigment. The spectral sensitivities of the photoreceptors are rather simple, and peak in the ultraviolet, blue and green wavelength regions. The ommatidia have neither perirhabdomal nor fluorescent pigments, which modulate photoreceptor spectral sensitivities in a number of other butterfly species. These features are primitive, but the eyes of Leptidea exhibit another unique feature: the rough appearance of the ventral two-thirds of the eye. The roughness is due to the irregular distribution of facets of two distinct sizes. As this phenomenon exists only in males, it may represent a newly evolved sex-related feature.

Synchronization of FEL and high-order harmonics of ultrashort-pulsed laser for generating intense full-coherent EUV light pulses

Seeding of free-electron laser by the 13th harmonic of ultrashort-pulsed 800 nm laser light was achieved using a synchronization technique of a spectral decoded EO-sampling, and intense full-coherent light pulses in the extreme ultraviolet wavelength region were generated.

Triple Resonant Four-Wave Mixing Boosts the Yield of Continuous Coherent Vacuum Ultraviolet Generation

Efficient continuous-wave four-wave mixing by using three different fundamental wavelengths with individual detunings to resonances of the nonlinear medium is shown. Up to 6 μW of vacuum ultraviolet light at 121 nm can be generated, which corresponds to an increase of three orders of magnitude in efficiency. This opens the field of quantum information processing by Rydberg entanglement of trapped ions.

Spectrally selective ultra-high temperature ceramic absorbers for high-temperature solar plants

Ultra-high temperature ceramics are the ideal materials for extreme conditions owing to their very high melting points and good thermo-mechanical properties at high temperatures. For these reasons, they are widely known as materials for aerospace applications. This paper presents a comparative spectral characterization of zirconium, hafnium, and tantalum carbides ultra-high temperature ceramics for concentrating solar power applications. Room-temperature reflectance spectra have been measured from the ultraviolet wavelength region to the mid-infrared band. Using these spectral properties, the ceramics were evaluated as sunlight absorbers in receivers for high-temperature thermodynamic solar plants.

Initial investigation of the wavelength dependence of optical properties measured with a new multi-pass Aerosol Extinction Differential Optical Absorption Spectrometer (AE-DOAS)

Abstract. Atmospheric aerosols directly affect climate by scattering and absorbing radiation. The magnitude of the impact is dependent upon the wavelength of light, but is often estimated near 550 nm. When light scattering and absorption by aerosols is approximated, the wavelength dependence of the refractive index for specific components is lost. As a result, climate models would have inherent uncertainties for aerosol contributions to radiative forcing when considering the entire solar spectrum. An aerosol extinction differential optical absorption spectrometer has been developed to directly measure aerosol extinction at mid-ultraviolet to near infrared wavelengths. The instrument consists of a spectrometer coupled to a closed White-type multi-pass gas cell with an adjustable path length of up to approximately 20 m. Laboratory measurements of various gases are compared with known absorption cross sections. Additionally, the extinction of monodisperse samples of polystyrene latex spheres are measured and compared to Mie theory generated with refractive index values from the literature to validate the new instrument. The polystyrene experiments also emphasize the ability of the new instrument to retrieve the wavelength dependent refractive index, especially in the ultraviolet wavelength regions where variability is expected. The spectrometer will be a significant advancement for determining wavelength dependent complex refractive indices in future laboratory studies as well as provide the ability to monitor ambient aerosol light extinction.

High-resolution nanopatterning by achromatic spatial frequency multiplication with electroplated grating structures

The authors demonstrate generation of high-resolution nanostructures using achromatic spatial frequency multiplication in the extreme ultraviolet wavelength region. The technique based on the achromatic Talbot effect is used for periodic transmission gratings under wideband illumination, enabling one- and two-dimensional nanopatterns with sub-20 nm feature sizes. The transmission masks with desired properties are fabricated with electron-beam lithography followed by electroplating of gold. Features sizes down to 12 nm are obtained. The presented technique provides high-throughput and large-area nanopatterning with great flexibility in tuning pattern parameters such as linewidth and dot size.

Crystalline perfection, Raman, UV-VIS-NIR and prism coupler investigations on Cz-grown pure and Zn-doped LiNbO3 single crystals

Pure and Zn-doped (1 mol%) LiNbO3 single crystals have been grown by the Czochralski technique. From powder X-ray diffraction analysis, the crystal structure and the lattice strains have been evaluated. The crystalline perfection analysis by high resolution X-ray diffraction revealed that the pure crystal is free from structural grain boundaries and contains predominantly vacancy defects whereas the doped crystal contains low angle structural grain boundaries. The Raman spectral studies indicate no variation in lattice vibrations with slight changes in the peak width and intensity. The Zn doping in crystals leads to an enhancement in optical transparency over the entire visible and near-infrared wavelength range and optical band gap. The prism coupler, refractive indices and birefringence measurements infer that the doped crystal is suitable for photorefractive applications in the ultraviolet wavelength region. The evaluated Li concentration by Raman and UV, as well as birefringence depicts that the grown crystals are congruent in nature with a slight deviation from stoichiometry.

Observation of Free-Electron-Laser-Induced Collective Spontaneous Emission (Superfluorescence)

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1s(2) → 1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ are observed, corresponding to a photon number conversion efficiency of up to 10%. We observe the peak intensity to scale as ρ(2) and the emitted pulse width and delay to scale as ρ(-1), where ρ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75,000 times faster than the spontaneous 1s3p → 1s2s decay rate. To our knowledge, this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of extreme ultraviolet and x-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.

The Talbot effect of plasmonic nanolenses

The Talbot effect of an Ag nanolens with five periodic concentric rings that are illuminated by the radially polarized light was numerically studied by means of rigorous finite-difference and time-domain (FDTD) algorithm. It was found that the Talbot effect occurs only when the incident wavelength is at the scale of less than half of period of the grating structures of the nanolenses. Specifically, in this work, the nanolenses with a 500 nm period grating structures has five focal points due to Talbot effect for the incident wavelength of λ = 248 nm. The diameter of the first focal spot after the exit plane in free space is 100 nm. In contrast, we analyzed the corresponding focal points on the basis of Talbot self-imaging by scalar diffraction theory. It was found that the scalar Talbot effect cannot interpret the Talbot effect phenomenon for the metallic nanolenses. It may attribute to the paraxial approximation applied in the Talbot effect theory in far-field region. However, the approximation does not hold in our nanolenses structures during the light propagation. In addition, the Talbot effect appears at the short-wavelength regime only, especially in the ultraviolet wavelength region.

EUV emission spectra of iron ions following charge exchange collisions with He

The emission spectra in collisions of Feq+ (q=8–16) with He were measured in the extreme ultraviolet wavelength region at a collision energy of 20×q keV. The contribution of the double electron capture process cannot be ignored, at least in Feq+ (q=9–13) collisions. By comparing the emission spectra between charge exchange spectroscopy and an electron beam ion trap (EBIT) experiment, the resonance lines can be distinguished from other emission lines in the charge exchange spectra, and the charge state distribution of trapped ions can be estimated from the observed EBIT spectrum.

Development of Wide Ion Beam Profile Measurement Method for a Period-by-Period Extreme Ultraviolet Multilayer Milling System

To develop an ion milling system for multilayer mirrors in the extreme ultraviolet (EUV) wavelength region, the current density profile of a φ100-mm-ion beam has been measured by plural methods. The profile has been measured indirectly via milling thickness by an optical surface profiler (WYKO TOPO-2D) and an X-ray diffractometer. Direct current detection by a commercial Faraday cup and a newly developed array electrode sensor was also performed. These methods were compared and evaluated in terms of their experimental results. It was demonstrated that our array electrode sensor is useful for observing a wide ion beam profile with reasonable accuracy.

Nonlinear Fourier-transform spectroscopy ofD2using high-order harmonic radiation

High-order harmonic radiation is available to investigate ultrafast nonlinear optical phenomena, such as two-photon absorption process, in the vacuum and extreme ultraviolet wavelength region. We have determined the sequential two-photon dissociative ionization pathways of D2 with the simultaneous irradiation of multiple order harmonics of a Ti:sapphire laser in the visible-vacuum ultraviolet region with the aid of both an interferometric autocorrelation measurement and a Fourier-transform spectroscopy. By analyzing the interferometric autocorrelation signals appearing in the measured momentum distribution of D + , we were able to decompose the measured momentum distribution into three momentum distribution images representing the three distinct dissociation pathways, which are sequentially two-photon excitation processes of D2. \({\mathrm{D}}_{2}^{+}\) is prepared in the electronic ground state of \({\mathrm{D}}_{2}^{+}\), and then excited to the first excited state, leading to the dissociation into D + and D.

Effect of the electric field on the primary scintillation from CF 4

The effect of an electric field on the primary scintillation from CF 4 in the ultraviolet and visible wavelength regions (200–800 nm) is reported. The study was performed in the pressure range from 1 to 5 bar and at electric fields of up to 2 kV/cm. Photon emission spectra, fully corrected for the detection response, and the absolute photon yields in the ultraviolet and visible regions are reported. The CF 4 emission spectra and the photon fluxes show no variations with the field at low pressures ( ∼ 1 bar ) , while at higher pressures the effect of the field on the scintillation is strong: the ultraviolet emission intensity increases and the visible intensity decreases with the field strength. Time spectra of the primary scintillation for several applied electric field strengths are also reported for the two wavelength regions of light emission.

Reflectance of polytetrafluoroethylene for xenon scintillation light

Gaseous and liquid xenon particle detectors are being used in a number of applications including dark matter search and neutrino-less double beta decay experiments. Polytetrafluoroethylene (PTFE) is often used in these detectors both as electrical insulator and as a light reflector to improve the efficiency of detection of scintillation photons. However, xenon emits in the vacuum ultraviolet (VUV) wavelength region (λ≃175 nm) where the reflecting properties of PTFE are not sufficiently known. In this work, we report on measurements of PTFE reflectance, including its angular distribution, for the xenon scintillation light. Various samples of PTFE, manufactured by different processes (extruded, expanded, skived, and pressed) have been studied. The data were interpreted with a physical model comprising both specular and diffuse reflections. The reflectance obtained for these samples ranges from about 47% to 66% for VUV light. Other fluoropolymers, namely, ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), and perfluoro-alkoxyalkane (PFA) were also measured.

Crystal growth and scintillation properties of NdF3 single crystal

Abstract NdF 3 single crystal was efficiently grown from the melt by the micro-pulling-down (μ-PD) method. The grown crystal had a single-phase confirmed by XRD and showed a highly symmetric peak by XRC. Then optical properties and α-ray excited scintillation properties in Vacuum Ultraviolet (VUV) wavelength region were investigated. It showed 60–80% transmittance down to 170 nm, and broad luminescence centered at 175 nm. The light yield and decay time of it were also evaluated under 5.5 MeV α-ray excitation ( 241 Am). The light yield was evacuated to be 60 ± 6 [Ph/5.5 MeV-α], and the decay time was also evacuated to be 6.9 ns under 5.5 MeV α-ray excitation.

Experimental determination of optical constants of MgF 2 and AlF 3 thin films in the vacuum ultra-violet wavelength region (60–124 nm), and its application to optical designs

The investigation of some solar radiations of interest for astrophysicists requires optics in the 80–130 nm vacuum ultra-violet spectral range (VUV). In this domain, where both transmittance and reflectance of most materials are very low, the measurement of optical constants is specifically difficult, and optical data are consequently often either inexistent or uncertain. Reliable modelling of optical components for VUV, like polarizing multi-layered mirrors, necessitates prior measurement of complex indices of the thin films involved in the coating. Fluorides like MgF 2 or AlF 3 are among the rare materials capable to contribute to multi-layer mirrors in the VUV. We have determined optical constants of thin films of these two materials by using a two media reflectance method at normal incidence and a graphical determination particularly suited to this VUV region, which we presented in a previous paper. Optical constants are determined in the range 60–124 nm with 2 nm step, and are compared to existing data. On the basis of these measured indices, polarizing mirrors for λ 1 = 121.6 nm or λ 2 = 103.2 nm have been modelled and fabricated. Their reflectance measured versus incidence angle by using monochromatized synchrotron radiation at the above wavelengths is found in agreement with the calculated predictions.

Interferometry of Attosecond Pulse Trains in the Extreme Ultraviolet Wavelength Region

The temporal coherence of an attosecond optical field in the extreme ultraviolet wavelength region can be defined in terms of the extent of interference in time domain. We successfully measured this phenomenon both with and without spectral decomposition. We also report the results of using this approach to directly observe both symmetry and symmetry breaking of interference fringes in an attosecond pulse train.

Divertor Impurity-Influx Monitor for ITER: Spectral Throughput Measurement on an Optical Prototype for the Upper Port and Optimization of Viewing Chords based on Computerized Tomography

We are developing a spectroscopic diagnostics system in ultraviolet and visible wavelength regions for monitoring ITER divertor plasmas. An equivalent-size prototype of the optical components for viewing upper port fanarray chords has been assembled as a system to measure spectral throughput, i.e., etendu. Collisional-radiative models for HeI and CIV are used to estimate the emission line intensities of helium ash and carbon impurity ions in a divertor region of a burning plasma. The estimated line intensity of CIV, λ772.6 nm, satisfies ITER requirements for the time resolution of measurement of Ti. A numerical simulation of the computerized tomographic technique for various pairs of viewing fan arrays has been applied to the divertor plasma region to reconstruct a two-dimensional distribution. The optimized pair of viewing fan arrays resolves a model distribution with a reasonable spatial resolution. We measure the reflectance of surfaces of carbon-fiber-composite and tungsten blocks, which make up the plasma-facing divertor target plates and the divertor dome. The reflectance of the surface of the tungsten divertor block is 23% at Hα (λ656.3 nm). A sandblast-processed tungsten surface effectively reduces direct reflectance; the resultant reflectance is less than 0.7%.

极紫外多层膜-离子束刻蚀矩形光栅的制作及衍射效率测量

We fabricate and measure a multilayer-coated ion-beam-etched laminar grating for the extreme ultraviolet wavelength region. The fabrication process is carefully controlled so that the grating groove and multilayer-coating parameters well meet the design targets. At an incident angle of 10\degree, the peak diffraction efficiency of the +1st order is 20.6% at 14.56 nm and that of the -1st order is 22.1% at 14.65 nm, both close to the theoretical limits.