Deep Ultraviolet Spectral Region Research Articles

Deep-trap ultraviolet persistent phosphor for advanced optical storage application in bright environments

Extensive research has been conducted on visible-light and longer-wavelength infrared-light storage phosphors, which are utilized as promising rewritable memory media for optical information storage applications in dark environments. However, storage phosphors emitting in the deep ultraviolet spectral region (200–300 nm) are relatively lacking. Here, we report an appealing deep-trap ultraviolet storage phosphor, ScBO3:Bi3+, which exhibits an ultra-narrowband light emission centered at 299 nm with a full width at half maximum (FWHM) of 0.21 eV and excellent X-ray energy storage capabilities. When persistently stimulated by longer-wavelength white/NIR light or heated at elevated temperatures, ScBO3:Bi3+ phosphor exhibits intense and long-lasting ultraviolet luminescence due to the interplay between defect levels and external stimulus, while the natural decay in the dark at room temperature is extremely weak after X-ray irradiation. The impact of the spectral distribution and illuminance of ambient light and ambient temperature on ultraviolet light emission has been studied by comprehensive experimental and theoretical investigations, which elucidate that both O vacancy and Sc interstitial serve as deep electron traps for enhanced and prolonged ultraviolet luminescence upon continuous optical or thermal stimulation. Based on the unique spectral features and trap distribution in ScBO3:Bi3+ phosphor, controllable optical information read-out is demonstrated via external light or heat manipulation, highlighting the great potential of ScBO3:Bi3+ phosphor for advanced optical storage application in bright environments.

Poly(cyclohexylsilsesquioxane)-Based Hydrophilic Thermoset-Resistant Deep-Ultraviolet-Transparent Glasses with Low Melting Temperatures.

Silsesquioxane (SQ)-based glasses with low melting temperatures were prepared by the cosolvent-free (solventless) hydrolytic polycondensation of organotrimethoxysilanes with cyclopentyl (c-Pe) and cyclohexyl (c-Hx) groups. Copolymers consisting of phenylsilsesquioxane (Ph-SQ) units and c-Pe-SQ units [poly(Ph-co-c-Pe-SQ)] or c-Hx-SQ units [poly(Ph-co-c-Hx-SQ)] were melted at 140 °C and formed clear glasses. The glasses prepared by this method contained many residual SiOH groups and exhibited high adhesive strength to microscope glass plates, metals, and several polymers. The glass-transition temperature of poly(c-Hx-SQ) was lower than that of poly(Ph-SQ) by only a small margin, whereas that of poly(c-Pe-SQ) was much lower. The poly(c-Hx-SQ)-based glasses were stiff at room temperature and transparent in the deep-ultraviolet spectral region (≲300 nm). They formed fragile melts with kinetic fragility parameters as high as ∼0.8. The melts of poly(c-Hx-SQ) and poly(c-Hx-co-Et-SQ) exhibited better resistance to thermal curing than that of poly(Ph-SQ) and maintained thermoplasticity even after heat treatment at 200 °C for 6 h.

A Demonstration of Broadband Cavity-Enhanced Absorption Spectroscopy at Deep-Ultraviolet Wavelengths: Application to Sensitive Real-Time Detection of the Aromatic Pollutants Benzene, Toluene, and Xylene.

Benzene, toluene, and xylene (BTX) are serious air pollutants emitted by the chemical industry. Real-time monitoring of these air pollutants would be a valuable tool to regulate emissions of these compounds and reduce the harm they cause to human health. Here, we demonstrate the first detection of BTX using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). The instrument was operated in the deep-ultraviolet spectral region between 252 and 286 nm, where aromatic compounds have intense π → π* absorption bands. The mirror reflectivity was calibrated by two methods and exceeded 99.63% at 266 nm. At an integration time of 60 s, the 1σ measurement sensitivities were estimated to be 7.2 ppbv for benzene, 21.9 ppbv for toluene, 10.2 ppbv for m-xylene, and 4.8 ppbv for p-xylene, respectively. The absorption cross sections of BTX were measured in this work with an uncertainty of 10.0% at a resolution of 0.74 nm. The absorption cross sections reported in this work were in good agreement with those from earlier studies after accounting for differences in spectral resolution. To demonstrate the ability of the instrument to quantify complex mixtures, the concentrations of m-xylene and p-xylene have been retrieved under five different mixing ratios. Instrumental improvements and measurements strategies for use in different applications are discussed.

Pushing KTiOPO4-like Nonlinear Optical Sulfates into the Deep-Ultraviolet Spectral Region.

Accurately designing and synthesizing new deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals that are limited by the so-called "200 nm wall" on their transparency windows remain challenging. On the basis of a bandgap-directed computer-aided material design approach, two new NLO sulfates, KMgSO4F and KZnSO4F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO4 (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO4F is significantly blue-shifted to <190 nm from 350 nm for KTP. The microscopic origin of this significant transparent window blue shift is illustrated well by first-principles calculations. This work pushes the transparency windows of KTP-like NLO sulfates into the deep-UV spectral region for the first time and will pave a prospective way to the accurate design and synthesis of new deep-UV NLO materials.

A self-powered solar-blind photodetector based on polyaniline/α-Ga2O3 p–n heterojunction

In this work, we demonstrated the self-powered solar-blind photodetector based on a polyaniline/α-Ga2O3 hybrid heterojunction. The resultant device exhibited distinct self-power characteristics with a peak photoresponsivity (R) of 8.2 mA/W, a UVC (UV light of wavelength range at 200–280 nm)/UVA (UV light of wavelength range at 320–400 nm) rejection ratio (R220 nm/R400 nm) of 2.97 × 104, and a response decay time (τdec) of 176 μs at zero bias. With an elevated bias to 5 V, the dark current remained in an ultralow level of 0.21 pA, while the rejection ratio and τdec were improved to be 7.13 × 104 and 153 μs, respectively, together with the corresponding external quantum efficiency of 38.4% and a detectivity of 6.63 × 1013 Jones. Thanks to the dual functions of bandpass transmission in the deep-ultraviolet spectral region and the hole spreading transport of the polyaniline layer, the responsivity to the visible light is suppressed with the negligible internal photoemission effect, thus leading to the improved rejection ratio. Furthermore, weak interface interactions in such polyaniline/α-Ga2O3 organic–inorganic hybrid systems avoid the introduction of interfacial trapping centers by lattice mismatch and the stabilization of negatively charged anions (O2−) by (−NH2)-+species in polyaniline deactivate oxygen vacancies at the α-Ga2O3 surface, both of which lead to the negligible persistent photoconductivity effect. As a result, in the aid of the interfacial built-in field, the constructed hybrid heterojunction exhibited a self-powered detecting characteristic and a fast response speed. These findings verify the feasibility of delivering high performance photodetectors by implementing the inorganic/organic hybrid bipolar device design to overcome the difficulty in p-type Ga2O3.

Generation and compression of 10-fs deep ultraviolet pulses at high repetition rate using standard optics.

The generation and characterization of ultrashort laser pulses in the deep ultraviolet spectral region is challenging, especially at high pulse repetition rates and low pulse energies. Here, we combine achromatic second harmonic generation and adaptive pulse compression for the efficient generation of sub-10 fs deep ultraviolet laser pulses at a laser repetition rate of 200 kHz. Furthermore, we simplify the pulse compression scheme and reach pulse durations of ≈10 fs without the use of adaptive optics. We demonstrate straight-forward tuning from 250 to 320 nm, broad pulse spectra of up to 63 nm width, excellent stability and a high robustness against misalignment. These features make the approach appealing for numerous spectroscopy and imaging applications.

Lattice and electronic structures of BAlN in the deep ultraviolet spectral region

BAlN exhibit intriguing properties for both fundamental research and potential application in optoelectronic devices operating in the deep ultraviolet spectral ranges. For BxAl1-xN, the crystal lattice constant derived for the whole range of the alloy composition closely follows Vegard's law. The mechanical stability is represented by elastic constants. The fundamental bandgap is converted from direct in AlN to indirect in BN. The orbital projected band structure of BAlN is obtained to gain insight into the electronic structure and the optical anisotropy with favorable light polarization is understood in terms of the crystal field split-off energy. The band structure parameters including the band gap, the effective masses of electrons and holes are presented and the corresponding electron mobility of BAlN with the direct band gap are predicted to be larger than that of AlN. It is worth noting that BAlN with the direct band gap are quite promising for application in electronic and optoelectronic devices operating in the UVC range.

Investigation of the elimination kinetics of low and middle molecular weight uremic markers during hemodialysis treatment with the optoelectronic multispectral sensor

The elimination kinetics of conventional low-molecular weight uremic markers and advanced glycation end products (AGE products), which are considered as a possible middle molecular weight marker of uraemia in hemodialysis (HD) patients, was investigated by direct ultraviolet (UV) absorption spectroscopy of waste dialysis fluid. Light emitting diodes (LED) were exploited as quasi-monochromatic light sources in the deep UV spectral region for measuring optical transmission at the wavelengths 285 nm (the maximum of uric acid UV absorption) and 365 nm (the maximum of AGE absorption band). Optical absorption of waste dialysate was monitored during multiple HD sessions for a group of patients; the double-pool model of elimination kinetics was used for the curve fitting and approximation. It was revealed that kinetic behaviour of AGE products is almost not different comparing with uric acid and other conventional clinically proved uremic markers.

A Nonlinear Optical Switchable Sulfate of Ultrawide Bandgap

Nonlinear optical (NLO) switchable materials have attracted intense attention because of their promising applications in optoelectronic devices. However, previous studies are mainly limited to mole...

Photoionization of KCs Molecule: Origin of the Structured Continuum?

We report the experimental observation of photoionization bands of the KCs molecule in the deep ultraviolet spectral region between 200 and 420 nm. We discuss the origin of observed photoionization bands as stemming from the absorption from the ground state of the KCs molecule to the excited states of KCs+ molecule for which we used existing potential curves of the KCs+ molecule. An alternative explanation relies on the absorption from the ground state of the KCs molecule to the doubly excited states of the KCs** molecule, situated above the lowest molecular state of KCs+. The relevant potential curves of KCs** are not known yet, but all those KCs** potential curves are certainly autoionizing. However, these two photoionization pathways may interfere resulting in a special interference structured continuum, which is observed as complex bands.

Characterization of photoelastic materials by combined Mach-Zehnder and conoscopic interferometry: Application to tetragonal lithium tetraborate crystals

Mach-Zehnder and conoscopic interferometry are used to explore photoelastic properties of anisotropic crystal materials. In a number of cases an application of both techniques significantly improves an accuracy of piezooptic and photoelastic measurements. The performance of such combined approach is demonstrated on tetragonal lithium tetraborate (LTB) single crystals, as an example. Special attention is paid to methodological and metrological aspects, such as measurement accuracy and the quantitative error analysis of the resulting measurements. Performing the interferometric measurements for different geometries of piezooptic coupling the full sets of piezooptic and photoelastic tensor constants of LTB crystals have been determined. The acoustooptic efficiency, on the other hand, has been evaluated using the magnitudes of photoelastic constants derived from the piezooptical measurements. For the geometries with strong photoelastic coupling LTB demonstrates quite large acoustooptic performance with figure of merit value, М2, achieving 2.12 × 10–15 s3/kg. It is several times larger than that of strontium borate crystals, nowadays the best acoustooptic material in deep-ultraviolet spectral region.

Pushing the limits of deep-ultraviolet scanning near-field optical microscopy

Deep-ultraviolet (DUV) microscopy and microspectroscopy have received much attention in label-free live-cell imaging, selective molecular analysis, and optical characterizations of ultrawide bandgap materials. Far-field optics approaches usually suffer from the diffraction limit of light. Meanwhile, near-field optics technology is immature in the DUV spectral region. Herein, we develop a DUV scanning near-field optical microscope (SNOM) with an excitation wavelength of 210 nm. The fourth harmonic generation of a continuous-wave Ti:sapphire laser is the excitation source. Two negative feedback control systems stabilize the power and pointing, allowing scanning microscopy to be carried out over a long period of time even in the DUV spectral region. A reflective objective couples the excitation beam onto an optical fiber probe. The optical fiber probe is solarization-resistant and is not coated by metallic materials. The reflective-based objective optical design does not introduce chromatic aberrations in the detection signals. Our DUV-SNOM is used to measure the photoluminescence (PL) spectra of the quantum well structure of aluminum gallium nitride, which is an ultrawide bandgap material. The PL mapping images indicate that emissive localization centers can be individually visualized via our DUV-SNOM with a lateral resolution exceeding 150 nm.

MOVPE Growth of Smooth and Homogeneous Al0.8Ga0.2N:Si Superlattices as UVC Laser Cladding Layers

Smooth, uniform, and conductive AlxGa1−xN layers with high aluminum mole fractions of x &gt; 0.7 are required as cladding layers for laser diodes emitting in the deep ultraviolet spectral region. In this paper, the growth of silicon‐doped AlGaN/AlGaN superlattices by metal‐organic vapor phase epitaxy is investigated and compared to bulk AlGaN layers. It is found that the superlattice approach enables the growth of AlGaN layers with improved lateral uniformity of composition and strain state. In order to reduce the surface roughness, growth interruptions between the superlattice layers are investigated. With increasing growth interruption time, the average aluminum content is increasing and the superlattice period thickness is decreasing. Scanning transmission electron microscopy investigations show that the growth interruptions lead to more abrupt interfaces and to the formation of one to two monolayer thin aluminum‐rich AlGaN layers at each interface. This also leads to a significantly smoother surface morphology. Low resistivities of 0.025 Ω cm are obtained for AlGaN:Si superlattices with average aluminum content of x = 0.8. The influence of the surface morphology of the AlGaN cladding layers on optically pumped laser heterostructures is investigated. By using a smooth AlGaN super­lattice, the lasing threshold decreases by a factor of 2 compared to lasers with bulk AlGaN cladding layers.

High-performance solar-blind flexible deep-UV photodetectors based on quantum dots synthesized by femtosecond-laser ablation

High-performance deep ultraviolet (DUV) photodetectors operating at ambient conditions with < 280 nm detection wavelengths are in high demand because of their potential applications in diverse fields. We demonstrate for the first time, high-performance flexible DUV photodetectors operating at ambient conditions based on quantum dots (QDs) synthesized by femtosecond-laser ablation in liquid (FLAL) technique. Our method is facile without complex chemical procedures, which allows large-scale cost-effective devices. This synthesis method is demonstrated to produce highly stable and reproducible ZnO QDs from zinc nitride target (Zn3N2) without any material degradation due to water and oxygen molecule species, allowing photodetectors operate at ambient conditions. Carbon-doped ZnO QD-based photodetector is capable of detecting efficiently in the DUV spectral region, down to 224 nm, and exhibits high photo-responsivity and stability. As fast-response of DUV photodetector remains significant parameter for high-speed communication; we show fast-response QD-based DUV photodetector. Such surfactant-free synthesis by FLAL can lead to commercially available high-performance low-cost optoelectronic devices based on nanostructures for large scale applications.

A novel Raman optical activity instrument operating in the deep ultraviolet spectral region

Raman optical activity (ROA) has been exclusively observed in the visible (VIS) and near-infrared (NIR) spectral regions to date. During the last few years, we have designed, constructed and tested the first ROA instrument, operating in the deep-ultraviolet (DUV) spectral region employing 244-nm excitation. This novel DUV ROA instrument is based on a backscattering geometry and incident circular polarization modulation (ICP); it makes use of a fast DUV imaging lens-based spectrograph and specially designed DUV grade polarization optics. The performance of this instrument has been evaluated by analysing measured non-resonant DUV ROA spectra of non-absorbing enantiomeric liquid samples and by comparing these with corresponding ROA spectra recorded in the visible spectral region. Copyright © 2015 John Wiley & Sons, Ltd.

Enhancement of TE polarized light extraction efficiency in nanoscale (AlN)m /(GaN)n (m>n) superlattice substitution for Al-rich AlGaN disorder alloy: ultra-thin GaN layer modulation

The problem of achieving high light extraction efficiency in Al-rich AlxGaN is of paramount importance for the realization of AlGaN-based deep ultraviolet (DUV) optoelectronic devices. To solve this problem, we investigate the microscopic mechanism of valence band inversion and light polarization, a crucial factor for enhancing light extraction efficiency, in Al-rich AlxGaN alloy using the Heyd–Scuseria–Ernzerhof hybrid functional, local-density approximation with 1/2 occupation, and the Perdew–Burke–Ernzerhof functional, in which the spin–orbit coupling effect is included. We find that the microscopic Ga-atom distribution can effectively modulate the valence band structure of Al-rich AlxGaN. Moreover, we prove that the valence band arrangement in the decreasing order of heavy hole, light hole, and crystal-field split-off hole can be realized by using nanoscale (AlN)m/(GaN)n (m>n) superlattice (SL) substituting for Al-rich AlxGaN disorder alloy as the active layer of optoelectronic devices due to the ultra-thin GaN layer modulation. The valence band maximum, i.e., the heavy hole band, has px- and py-like characteristics and is highly localized in the SL structure, which leads to the desired transverse electric (TE) polarized (E⊥c) light emission with improved light extraction efficiency in the DUV spectral region. Some important band-structure parameters and electron/hole effective masses are also given. The physical origin for the valence band inversion and TE polarization in (AlN)m/(GaN)n SL is analyzed in depth.

Optical properties of UV-transparent aluminum oxide / aluminum fluoride mixture films, prepared by plasma-ion assisted evaporation and ion beam sputtering

Electron beam evaporation (without and with plasma assistance) as well as ion beam sputtering are used to prepare optical mixture coatings for applications in the ultraviolet spectral range. It is demonstrated that intermixing aluminum oxide/ aluminum fluoride materials by these physical vapor deposition techniques results in optical coatings with flexible refractive indices varying between 1.40 and 1.75 in the deep ultraviolet spectral region. At the same time, extinction coefficients vary between less than 1x10−4 and 2x10−3. For evaporated layers, at certain mixture ratios, mechanical stress appears to be close to zero.

Extremely high internal quantum efficiencies from AlGaN/AlN quantum wells emitting in the deep ultraviolet spectral region

Internal quantum efficiencies (IQEs) as high as 69% were realized at room temperature from AlGaN/AlN quantum wells (QWs) emitting at 247 nm grown by metalorganic vapor phase epitaxy. The extremely high IQEs were achieved by examining the source–supply sequence. QWs fabricated by a continuous source–supply method have longer emission wavelengths (λ) and higher IQEs compared to QWs fabricated by modified migration enhanced epitaxy (MMEE). MMEE is an alternating source–supply method where the NH3 interruption promotes Ga evaporation. Thus, to obtain the same λ, MMEE requires a lower growth temperature than the continuous method, compromising the quality of the AlN and AlGaN layers as well as the IQE of QWs.

Epitaxial Growth of Hexagonal Boron Nitride and the Ultraviolet Luminescence Properties

Hexagonal BN (h-BN) is graphite-like sp2-bonded crystal phase, which is the most stable among crystalline BN phases, and has a potential for optical device applications in the deep ultraviolet spectral region. Crystalline phase of BN film grown on a foreign substrate is turbostratic BN (t-BN) or mixed phase with t-BN, h-BN and cubic BN due to the large lattice mismatch and its various crystalline BN phases. Therefore, control of the crystalline phase in the BN film is challenging for single-phase BN growth. Here, we demonstrate that single-phase h-BN epitaxial films can be grown on nearly lattice-matched substrates by optimizing growth conditions during metalorganic vapor phase epitaxy and molecular beam epitaxy (MBE). We also investigate the ultraviolet luminescence properties of near band-gap emission in the h-BN epitaxial films grown by MBE.

Hexagonal boron nitride grown by MOVPE

Hexagonal boron nitride (h-BN) has a potential for optical device applications in the deep ultraviolet spectral region. For several decades, only amorphous and turbostratic boron nitride (BN) films had been grown by chemical vapor deposition and metalorganic vapor phase epitaxy. By introducing flow-rate modulation epitaxy (FME), which enables us to reduce parasitic reactions and lower the optimal growth temperature, we have succeeded in growing single-phase h-BN epitaxial films on nearly lattice-matched (1 1 1) Ni substrates. The h-BN epitaxial films exhibit near-band-gap ultraviolet luminescence at a wavelength of 227 nm in cathodoluminescence at room temperature. The combination of FME and the lattice-matched substrate paves the way for the epitaxial growth of high-quality h-BN.