Vacuum Ultraviolet Transmission Research Articles

Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials.

Thorium-doped vacuum ultraviolet (VUV) transparent crystals is a promising candidate for establishing a solid-state nuclear clock. Here, we report the research results on high-concentration doping of 232Th:CaF2 single crystals. The structures, defects, and VUV transmittance performances of highly doped Th:CaF2 crystals are investigated by theoretical and experimental methods. The defect configurations formed by Th and the charge compensation mechanism (Ca vacancy or interstitial F atoms) located at its first nearest neighbor position are mainly considered and studied. The preferred defect configuration is identified according to the doping concentration dependence of structural changes caused by the defects and the formation energies of the defects at different Ca or F chemical potentials. The cultivated Th:CaF2 crystals maintain considerable high VUV transmittance levels while accommodating high doping concentrations, showcasing an exceptional comprehensive performance. The transmittances of 1-mm-thick samples with doping concentrations of 1.91 × 1020 and 2.76 × 1020 cm-3 can reach ∼62% and 53% at 150 nm, respectively. The VUV transmittance exhibits a weak negative doping concentration dependence. The system factors that may cause distortion and additional deterioration of the VUV transmittance are discussed. Balancing and controlling the impacts of various factors will be of great significance for fully exploiting the advantages of Th:CaF2 and other Th-doped crystals for a solid-state nuclear optical clock.

Ultrathin Silica Integration for Enhancing Reliability of Microfluidic Photoionization Detectors.

Microfluidic photoionization detectors (μPIDs) based on silicon chips can rapidly and sensitively detect volatile compounds. However, the applications of μPID are limited by the manual assembly process using glue, which may outgas and clog the fluidic channel, and by the short lifetime of the vacuum ultraviolet (VUV) lamps (especially, argon lamps). Here, we developed a gold-gold cold welding-based microfabrication process to integrate ultrathin (10 nm) silica into μPID. The silica coating enables direct bonding of the VUV window to silicon under amicable conditions and works as a moisture and plasma exposure barrier for VUV windows that are susceptible to hygroscopicity and solarization. Detailed characterization of the silica coating was conducted, showing that the 10 nm silica coating allows 40-80% VUV transmission from 8.5 to 11.5 eV. It is further shown that the silica-protected μPID maintained 90% of its original sensitivity after 2200 h of exposure to ambient (dew point = 8.0 ± 1.8 °C), compared to 39% without silica. Furthermore, argon plasma inside an argon VUV lamp was identified as the dominant degradation source for the LiF window with color centers formation in UV-vis and VUV transmission spectra. Ultrathin silica was then also demonstrated effective in protecting the LiF from argon plasma exposure. Lastly, thermal annealing was found to bleach the color centers and restore VUV transmission of degraded LiF windows effectively, which will lead to future development of a new type of VUV lamp and the corresponding μPID (and PID in general) that can be mass produced with a high yield, a longer lifetime, and better regenerability.

Atmospheric Photoionization Detector with Improved Photon Efficiency: A Proof of Concept for Application of a Nanolayer Thin-Film Electrode.

Atmospheric photoionization is a widely applied soft ionization mechanism in gas sensing devices for the detection of volatile organic compounds in ambient air. Photoionization is typically induced by low-pressure Vacuum Ultraviolet (VUV) lamps with MgF2 or LiF lamp surface windows depending on the gas fill and the required wavelength transmission window. These lamps are known to exhibit gradually reduced VUV transmission due to hydrocarbon contamination. LiF surface windows are known to be especially problematic due to their hygroscopic nature, reducing VUV lamp lifetime to a mere 100 h, approximately. Here, we present a new design for the electrode of a photoionization detector based on thin-film technology. By replacing the commonplace metal grid electrode’s VUV lamp surface window with a chromium/gold thin film we obtain a doubling of photon efficiency for photoionization. Replacing the hygroscopic LiF lamp window surface with a metallic layer additionally offers the possibility to vastly increase operational lifetime of low-pressure Argon VUV lamps.

Construction of a vacuum ultraviolet transmission spectrometer

Construction of a vacuum ultraviolet transmission spectrometer

Subsurface investigation of the surface modification of polydimethylsiloxane by 172‐nm vacuum ultraviolet irradiation using ToF‐SIMS and VUV spectrometry

We investigate the mechanism of polydimethylsiloxane (PDMS) surface modification by 172‐nm vacuum ultraviolet (VUV) light. Time‐of‐flight secondary ion mass spectrometry and optical spectrometry are used to measure the chemical composition and VUV transmittance of the PDMS before and after surface modification, respectively. For modified samples of bulk PDMS, the VUV transmittance and the depth of the modified region increased with increasing VUV dose. This can be explained by the following self‐reinforcing cycle of (1) modification of PDMS by VUV light to a more silica‐like composition, (2) improvement of the VUV light transparency, and (3) deeper modification. For thin‐film samples of PDMS formed on sapphire substrates, the transmittance at 172 nm also increased with increasing VUV dose and exceeded that of sapphire in the region from 172 to 300 nm. Finally, thin‐film samples of PDMS formed on silicon substrates, which function as a VUV reflector, were also investigated. For these samples, the secondary ion depth profiles for several chemical species in the PDMS were oscillatory, probably due to the interference of the incident and reflected VUV light. These results strongly suggest that the photon energy of the VUV light plays an important role in modifying PDMS.

Vacuum-Ultraviolet Photovoltaic Detector.

Over the past two decades, solar- and astrophysicists and material scientists have been researching and developing new-generation semiconductor-based vacuum ultraviolet (VUV) detectors with low power consumption and small size for replacing traditional heavy and high-energy-consuming microchannel-detection systems, to study the formation and evolution of stars. However, the most desirable semiconductor-based VUV photovoltaic detector capable of achieving zero power consumption has not yet been achieved. With high-crystallinity multistep epitaxial grown AlN as a VUV-absorbing layer for photogenerated carriers and p-type graphene (with unexpected VUV transmittance >96%) as a transparent electrode to collect excited holes, we constructed a heterojunction device with photovoltaic detection for VUV light. The device exhibits an encouraging VUV photoresponse, high external quantum efficiency (EQE) and extremely fast tempera response (80 ns, 104-106 times faster than that of the currently reported VUV photoconductive devices). This work has provided an idea for developing zero power consumption and integrated VUV photovoltaic detectors with ultrafast and high-sensitivity VUV detection capability, which not only allows future spacecraft to operate with longer service time and lower launching cost but also ensures an ultrafast evolution of interstellar objects.

Design of an ultrashort optical transmission cell for vacuum ultraviolet spectroscopy of supercritical fluids.

We present the design and characteristics of an ultrathin flow cell optimized for vacuum ultraviolet transmission spectroscopy experiments on supercritical fluids. The cell operates satisfactorily at pressures up to 300 bar and temperatures up to 390 °C. The variable path length concept of the cell allows for optical transmission studies of analytes ranging from dense condensed-phase systems to gas-phase systems. The path length of the cell can be adjusted from hundreds of nanometers to hundreds of micrometers by an exchange of a variable thickness spacer sandwiched between two sapphire windows. In the path length range from nanometers to single micrometers, metal vapor deposited on one or both of the two sandwiched optical windows constitute the spacer. Spacers with thicknesses of 2 μm and greater can be constructed from simple commercially available metal foils. The cell has been used to measure the lowest-lying absorption band of water in both the vapor and condensed phases from room temperature up to and above the critical point. It has also found application in the studies of aqueous ions and nonaqueous liquids including various common organic solvents and carbon dioxide.

Study on the vacuum ultraviolet transmittance of barium fluoride crystals at different temperature

Two VUV-grade BaF2 windows with 0.5 mm-thick and 1 mm-thick respectively were selected to study the transmittance variety with the temperature. The results show that the cutoff wavelength of BaF2 crystals will shift towards the long wave with the increase in temperature. In a certain temperature range, BaF2 crystals can depress 130.4 nm radiation well, and also has a high transmittance at 135.6 nm. Compared with the reported method in which SrF2 crystals can be applied to suppress 130.4 nm stray light by heating, BaF2 crystal can inhibit the 130. 4 nm emission line completely, and thus reduce the power consumption of the device at the same time. This indicates that BaF2 crystals can play an important role in the ionosphere optical remote sensing detection.

Sol–Gel Strategy for Self-Induced Fluorination and Dehydration of Silica with Extended Vacuum Ultraviolet Transmittance and Radiation Hardness

Fluorinated silica glasses with improved vacuum ultraviolet (VUV) transmittance have been synthesized following a sol–gel route starting from fluorinated alkoxide precursors. Matrix dehydration and fluorination have been investigated by Raman, IR, and refractive index measurements. The results show that, during the densification step, the residual hydroxyl and fluorine content can be controlled by governing the kinetic equilibrium between matrix collapse and fluorine evolution. This highlights the pivotal role of in situ reactions of fluorine-containing molecular precursors in the networking mechanism of a sol–gel material after thermal activation. Superior VUV transmittance and radiation hardness with respect to pure commercial silica and undoped sol–gel silica have been confirmed by synchrotron light absorption measurements before and after X-ray exposure. These properties make the material very attractive for applications in VUV photolithography technologies and optical fibers, especially for use in en...

A differential pumping system to deliver windowless VUV photons at atmospheric pressure

In order to deliver VUV (vacuum ultraviolet) photons under atmospheric pressure conditions, a differential pumping system has been built on the DISCO beamline at the SOLEIL synchrotron radiation facility. The system is made of four stages and is 840 mm long. The conductance-limiting body has been designed to allow practicable optical alignment. VUV transmission of the system was tested under air, nitrogen, argon and neon, and photons could be delivered down to 60 nm (20 eV).

Fluorine laser-induced silicon hydride Si–H groups in silica

Formation and destruction of silicon hydride (Si–H) groups in silica by F 2 laser irradiation and their vacuum ultraviolet (VUV) optical absorption was examined by infrared (IR) and VUV spectroscopy. Photoinduced creation of Si–H groups in H 2-impregnated oxygen deficient silica is accompanied by a growth of infrared absorption band at 2250 cm −1 and by a strong increase of VUV transmission at 7.9 eV. Photolysis of Si–H groups by 7.9 eV photons in this glass was not detected when the irradiation was performed at temperature 80 K. However, a slight destruction of Si–H groups under 7.9 eV irradiation was observed at the room temperature. This finding gives a tentative estimate of VUV absorption cross section of Si–H groups at 7.9 eV as 4 × 10 −21 cm 2, showing that Si–H groups do not strongly contribute to the absorption at the VUV fundamental absorption edge of silica glass.

Vacuum Ultraviolet Spectroscopic System Using a Laser-Produced Plasma

We have developed a vacuum ultraviolet (VUV) spectroscopic system using a laser-produced plasma. Laser-produced rare gas plasma was initiated by a 10 Hz Q-switched YAG laser at the focused intensity of 1011 W/cm2. Among three different rare gas plasma emissions (He, Ne, and Ar), argon plasma was characterized by broad continuum emission with an integrated emission power of 8.4 kW in the VUV spectral region. The pulse width of the VUV emission of 62 ns (Full width at half maximum), which was governed by recombination processes in the plasma, determined the temporal resolution. Using this emission source, we have measured VUV transmission spectra of Nd3+:LaF3 crystal, which was a potential laser medium in the VUV. The transmission peak of the crystal coincided with the reported laser oscillation wavelength at 174 nm. The absorption coefficient at the wavelength was measured to be 0.7 cm-1.

Induced Absorption Phenomena, Thermoluminescence and Colour Centres in KMgF3, BaLiF3 and LiCaAlF6 Complex Fluorides

KMgF3, BaLiF3 and LiCaAlF6 (LiCAF) single crystals were grown by the Czochralski method under CF4 atmosphere. X-ray irradiation was used to carry out a comparative study of induced optical absorption phenomena and colour centre creation in the vacuum-ultra-violet (VUV), ultra-violet and visible spectral regions. In the case of LiCAF, the X-ray induced absorption peak at 262 nm can be ascribed to F centre. The integral of the induced absorption spectra is significantly lower in LiCAF with respect to the other studied materials; moreover, LiCAF shows stable VUV transmission characteristics and the most resistant surface toward what is regarded as the damage due to reaction with air atmosphere.

Dependence of luminescence processes and transmission in vacuum-ultraviolet region on surface condition in CaF2 single crystals

We investigated the effect of the surface condition on luminescence processes in a surface layer of single CaF2 crystals under the excitation by vacuum-ultraviolet (VUV) light (hν≤13.5 eV). It has been shown that the intensity of the excitonic emission at 4.4 eV in polished crystals essentially decreases with respect to that in freshly cleaved crystals. The decrease at the interband excitation is larger than that at the direct optical creation of excitons. A new luminescence band peaking at 2.55 eV has been discovered and ascribed to the oxygen-related centres situated in a surface layer of CaF2. The main excitation band of this band is in strict correlation with the excitonic absorption band at 11.2 eV. It is almost absent in polished crystals. The ageing of the cleaved surface of CaF2 crystals causes decrease in the intensities of the 4.4 and 2.55 eV emissions with different rates. The etching of a polished surface restores both emission bands. The correlation has been established between the evolution of these luminescence phenomena and the changes in VUV transmission spectra at 7.5–10.2 eV near the fundamental absorption edge of CaF2, induced by the cleavage, polishing or etching of the sample.

Effects of gap width on vacuum-ultraviolet transmission through submicrometer-period, freestanding transmission gratings

The effects of gap width on the transmission coefficient of vacuum-ultraviolet light through submicrometer-period, freestanding transmission gratings are reported. Results from computations and an analytical waveguide model are shown to be consistent with experimental measurements. These results show that thin gratings with narrow gaps and thick gratings with wide gaps are equally effective at eliminating 121.6-nm radiation. The thin gratings with the narrow gaps have the advantage of better attenuation of shorter-wavelength radiation than the thick gratings with the larger gaps.

Variability in the vacuum-ultraviolet transmittance of magnesium fluoride windows

In the course of the development of a domed magnesium fluoride detector window for the Space Telescope Imaging Spectrograph, slated to be a second-generation instrument aboard the Hubble Space Telescope, sample window materials from various commercial sources displayed a wide variability in vacuum ultraviolet transmittance. As a result a test program was undertaken in cooperation with the supplier of a prototype domed window to maximize transmittance. Results of the program have provided clues to the causes of the variations experienced, and they point to careful selection of raw materials and strict process control to achieve optimization.

An improved window for ring imaging Cerenkov detectors

Calcium fluoride crystal is used as a window in RICH (ring imaging Cerenkov) detectors because of its vacuum ultraviolet transmission down to 125.7 nm. This permits triethylamine (TEA) photosensitive gas to be used in the detector. The crystal would be more useful if larger, single crystals could be achieved. A crystal growing furnace was designed, based upon the concepts of a vertical slab geometry, to produce large, single crystals. These concepts are embodied in a furnace which produces calcium fluoride crystals 35 cm square by 7.5 cm thick. These ingots are an optimal geometry for the fabrication of RICH detector windows. Surface strengthening experiments indicate that large-area calcium fluoride windows can be made much more reliable than historic, nominal stress-at-failure values would predict. It is noted that maximum transmission of Cerenkov radiation in the vacuum ultraviolet spectral region can be ensured with special purification technology. Very pure calcium fluoride crystals show increased resistance to forming color centers after exposure to several megarads of ionizing radiation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Proton-induced degradation of VUV transmission of LiF and MgF_2

Proton-induced degradation of vacuum ultraviolet (VUV) transmittance of LiF and MgF(2) was measured for 85- and 600-MeV protons for a fluence up to 2.8 x 10(13)p/cm(2). Transmittances were measured from 105 to 210 nm. When the irradiation level for a given material is expressed in terms of absorbed energy per unit of volume of crystal, 85- and 600-MeV protons produce the same degradation. MgF(2) is substantially more radiation resistant than LiF in the VUV. Irradiation of LiF with 1.8 x 10(13)p/cm(2) at 85 MeV changed the transmittance of the hydrogen Lyman-alpha line at 121.6 nm from 55 to 23%. The corresponding change for MgF(2) was from 52 to 42% for 2.8 x 10(13)p/cm(2).

Vacuum-ultraviolet absorption spectra and optical constants of poly (N -vinyl carbazole) films

The results of a detailed study of the vacuum-ultraviolet transmittance of poly-N-vinyl carbazole (PVK) and its subsequent conversion to optical constants are reported. Optical data for PVK films, 0.01 μm, cover the energy range from 3 to 11.8 ev. Calculated spectra of absorption coefficient, real and imaginary parts of the complex refractive index, and the reflectance are displayed. Good agreement is found between the calculated and measured reflectance.

Optical absorption spectra of some potentially interesting gases for Cherenkov counters

For gas Cherenkov counters employing wavelength shifters coupled to normal-glass phototubes, the vacuum ultraviolet transmission of the filler gas is of great interest. We have measured transmission to 1100 Å for an optical path of one atmosphere-meter of methane, ethane, Freon 14, propane, butane, isobutane, nitrogen, sulfur hexafluoride, Freon 22, and methyl chloride. Aging results on wavelength shifters are reported.