UV-A Spectral Range Research Articles

Novel LED-based radiation source and its application in diffuse reflectometry and polarization measurements

An LED sphere radiator (LED-SR) was constructed to improve the accuracy in spectral radiance factor measurements performed with the robot-based gonioreflectometer at PTB. Its properties with respect to the spectral range and coverage, the temporal stability, and the homogeneity of the radiation field are presented. Two types of matte ceramic reflection standards were used for spectral radiance factor validation measurements comparing the standardly used halogen sphere radiator (Halogen-SR) and the LED-SR. Due to its designed spectral range at the border between the visible and the UV-A spectral range, the LED-SR is well suited for many applications in diffuse reflectometry. Its use for absolute radiance factor measurements and investigations of the fluorescence properties of diffuse reflecting samples is shown. Reliable polarization-resolved measurements at wavelengths below 430 nm could be carried out with PTB’s gonioreflectometer for the first time due to the beneficial signal-to-noise ratio of the LED-SR.

Review of microbial touchscreen contamination for the determination of reasonable ultraviolet disinfection doses.

Background: Touchscreens are usually microbially contaminated and can therefore act as fomites inside and outside healthcare environments. Due to the increasing use of such touchscreens and the growing awareness of infection risks, approaches that allow safe and automatic disinfection are desired. Ultraviolet (UV) irradiation, with its known antimicrobial efficacy, could achieve this goal, but should be executed with limited touchscreen degradation, disinfection duration, and energy consumption. It should also pose as little harm as possible to humans even in case of failure. Materials and methods: A literature search was performed first to identify the microorganisms most commonly found on touchscreens. Then, the 90% reduction doses (D90 doses) for the different relevant microorganisms and UV spectral ranges were determined from the literature, and irradiation doses are suggested that should reduce most of these important microorganisms by 5 log-levels. Results: The most frequent microorganisms are staphylococci, bacilli, micrococci, enterococci, pseudomonads and E. coli with small differences between hospital and community environments, if antibiotic resistance properties are ignored. The determined irradiation doses for a 5 log-reduction of the most frequent microorganisms are about 40 mJ/cm2, 80 J/cm2, 500 J/cm2 and 50 mJ/cm2 for the UV spectral ranges UVC, UVB, UVA and far-UVC, respectively. These doses are also sufficient to inactivate all nosocomial ESKAPE pathogens on touchscreens by at least 99.999%.Conclusion: Disinfection is achievable in all UV spectral ranges, with UVC being the most effective, enabling automatic disinfection within a minute or less. The much higher doses required in the UVB and UVA spectral range result in much longer disinfection durations, with the advantage of a reduced risk to humans. For all kinds of UV irradiation, the doses should be limited to reasonable values to avoid irradiating an already more or less sterile surface and to prevent degradation of touchscreen devices.

(Invited) Development of Ultraviolet Vertical-Cavity Surface-Emitting Light Sources

Nitride-based vertical-cavity surface-emitting lasers (VCSELs) at wavelengths below 400 nm have drawn tremendous interest as they are potentially utilized in various applications as highly-efficient and temperature-insensitive optical sources. One example is for a compact, energy efficient, portable ytterbium-ion-based atomic clock that requires a high-stability CW optical pump at near ultraviolet (UV) wavelengths. In this work, we have used metalorganic chemical vapor deposition (MOCVD) to grow optically pumped UV VCSEL epitaxial structures and UV resonant-cavity light-emitting diodes (RCLEDs) to demonstrate several design concepts targeting the ~360-370nm UVA spectral range. The UV VCSELs are designed using a proprietary III-N device simulation tool developed at Georgia Institute of Technology. Previously, we have demonstrated a variety of optically pumped III-N VCSELs lasing at ~375 nm that showed the feasibility of building the near-UV coherent light source on GaN materials platform. The test structures consisted of a five-pair InGaN/Al0.15Ga0.85N multiple-quantum-well (MQW) active region embedded in Al0.1Ga0.9N spacers with the total optical cavity thickness of 3λ. The lowest threshold incident power density for optically pumped VCSELs was estimated to be ~270 kW/cm2 under pulsed conditions at the room temperature. In further work, we implemented a hybrid distributed Bragg reflectors (DBR) combination to demonstrate vertical resonant cavity light emitting diodes (RCLEDs) targeting at 370-nm peak emission wavelengths. The hybrid mirrors use a dielectric DBR with twelve pairs of ¼-wavelength HfO2/SiO2 layers as the topside mirror, and a semiconductor-based mirror composed of five periods of ¾λ air-gap/Al0.05Ga0.95N DBR at the bottom side of the vertical cavity. The RCLED structure comprises three distinct regions: (1) bottom DBR mirror of five-pair 3λ/4-air-gap/ Al0.045Ga0.95fN, (2) a 6λ-cavity LED active region of n-Al0.08Ga0.92N (n~8x1018cm-3) spacer/InGaN-AlGaN MQW/p-AlGaN electron blocking/p-Al0.08Ga0.92N (p~3x1017cm-3) spacer/graded p +-AlGaN, and (3) a 12-pair λ/4 HfO2/SiO2 dielectric DBR as the top-side mirror. The device fabrication employed a nitrogen ion-implantation isolation to form a current confinement aperture. The mesa-type devices have the anode and the cathode electrodes on the topside of the wafer. The air-gap DBRs were formed using optical lithography and subsequent inductively coupled plasma dry etching to create a deep trench, followed by laterally removing sacrificial layers using a conductivity-selective electrochemical etching process. Once formed, this airgap DBR provides a high reflectivity of ~99.9% covering the spectral range from 360 to 400 nm while the dielectric DBR has a reflectivity of >99% covering the range from 350 to 405 nm. The fabricated RCLEDs can be operated at a current density >50 kA/cm2 in the pulsed current mode with a peak emission wavelength of ~375 nm at the room temperature. Details of MOCVD growth, device fabrication, and characterization will be presented.

New view on the variation of forward conduction mechanisms derived from electrical stress in UV-A light emitting diodes

Abstract The electrical stress induced variation of forward conduction mechanism in ultraviolet light emitting diodes in UV-A spectral range is usually attributed to the increase of trap density. The influence from trap energy level, which plays an equally significant role in the trap-assisted tunneling (TAT) theory, is nevertheless neglected in previous studies. In this work, the effect of electrical stress on the forward conduction mechanism of UV-A light emitting diodes has been studied using temperature-dependent current-voltage measurement and the variation of trap physical parameters has been monitored using deep level transient spectroscopy (DLTS) measurement. For the first time, it is demonstrated that, the rate of TAT increases after electrical stress, and has a much stronger correlation with trap energy level as compared to the trap density.

Influence of constant current stress on the conduction mechanisms of reverse leakage current in UV-A light emitting diodes

The influence of constant current stress on the conduction mechanism of reverse leakage current in ultraviolet (UV) light emitting diodes (LEDs) in the UV-A spectral range has been investigated for the first time using temperature-dependent current-voltage measurement from 370 K to 55 K. Below 220 K, variable range hopping mechanism dominates in UV-A LEDs. While, above 220 K, the leakage current is attributed to Poole-Frenkel emission mechanism within the bias range of −3 ∼ −6 V. With the increasing of the reverse bias, the conduction mechanism transforms from Poole-Frenkel emission to space-charge-limited conduction mechanism. In particular, applying electrical stress yields an alteration of the transition voltage from −7.5 V to −6.5 V. We propose that stress could lead to a reduction of the thermal activation energy, and therefore alters the transition voltage.

Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes

We discuss the engineering of p-AlGaN cladding layers for achieving efficient tunnel-injected III-Nitride ultraviolet light emitting diodes (UV LEDs) in the UV-A spectral range. We show that the capacitance-voltage measurements can be used to estimate the compensation and doping in the p-AlGaN layers located between the multi-quantum well region and the tunnel junction layer. By increasing the p-type doping concentration to overcome the background compensation, on-wafer external quantum efficiency and wall-plug efficiency of 3.37% and 1.62%, respectively, were achieved for the tunnel-injected UV LEDs emitting at 325 nm. We also show that interband tunneling hole injection can be used to realize UV LEDs without any acceptor doping. The work discussed here provides new understanding of hole doping and transport in AlGaN-based UV LEDs and demonstrates the excellent performance of tunnel-injected LEDs for the UV-A wavelength range.

Construction of UV-A radiometer for irradiance measurements

This work presents preliminary results aiming on providing Inmetro with a radiometric transfer standard in the UV-A spectral range. A broadband UV radiometer head was constructed using an UV photodiode, a commercially available UV-A optical filter and a precision aperture. These components have been characterized in the calibration and measurement facilities available at Inmetro. The preliminary characterization of the assembled UV broadband radiometer was carried out and the results are presented and discussed in this text.

Exposure of vitamins to UVB and UVA radiation generates singlet oxygen

Deleterious effects of UV radiation in tissue are usually attributed to different mechanisms. Absorption of UVB radiation in cell constituents like DNA causes photochemical reactions. Absorption of UVA radiation in endogenous photosensitizers like vitamins generates singlet oxygen via photosensitized reactions. We investigated two further mechanisms that might be involved in UV mediated cell tissue damage. Firstly, UVB radiation and vitamins also generate singlet oxygen. Secondly, UVB radiation may change the chemical structure of vitamins that may change the role of such endogenous photosensitizers in UVA mediated mechanisms. Vitamins were irradiated in solution using monochromatic UVB (308 nm) or UVA (330, 355, or 370 nm) radiation. Singlet oxygen was directly detected and quantified by its luminescence at 1270 nm. All investigated molecules generated singlet oxygen with a quantum yield ranging from 0.007 (vitamin D3) to 0.64 (nicotinamide) independent of the excitation wavelength. Moreover, pre-irradiation of vitamins with UVB changed their absorption in the UVB and UVA spectral range. Subsequently, molecules such as vitamin E and vitamin K1, which normally exhibit no singlet oxygen generation in the UVA, now produce singlet oxygen when exposed to UVA at 355 nm. This interplay of different UV sources is inevitable when applying serial or parallel irradiation with UVA and UVB in experiments in vitro. These results should be of particular importance for parallel irradiation with UVA and UVB in vivo, e.g. when exposing the skin to solar radiation.

The amino acids surrounding the flavin 7a-methyl group determine the UVA spectral features of a LOV protein

Flavin-binding light, oxygen, and voltage (LOV) domains are UVA/blue-light-sensing protein units that form a reversible flavin mononucleotide-cysteine adduct upon light induction. In their dark-adapted state, LOV domains exhibit the typical spectral features of fully oxidized riboflavin derivatives. A survey on the absorption spectra of various LOV domains revealed that the UVA spectral range is the most variable region (whereas the absorption band at 450 nm is virtually unchanged), showing essentially two distinct patterns found in plant phototropin LOV1 and LOV2 domains, respectively. In this work, we have identified a residue directly interacting with the isoalloxazine methyl group at C(7a) as the major UVA spectral tuner. In YtvA from Bacillus subtilis, this amino acid is threonine 30, and its mutation into apolar residues converts the LOV2-like spectrum of native YtvA into a LOV1-like pattern. Mutation T30A also accelerates the photocycle ca. 4-fold. Together with control mutations at different positions, our results experimentally confirm the previously calculated direction of the transition dipole moment for the UVA ππ* state and identify the mechanisms underlying spectral tuning in the LOV domains.

Tryptophan oxidation photosensitized by pterin

Pterins are normal components of cells and they have been previously identified as good photosensitizers under UV-A irradiation, inducing DNA damage and oxidation of nucleotides. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of another class of biomolecules, amino acids, using tryptophan (Trp) as a model compound. Irradiation of Ptr in the UV-A spectral range (350nm) in aerated aqueous solutions containing Trp led to the consumption of the latter, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H2O2) was produced. Although Ptr is a singlet oxygen (1O2) sensitizer, the degradation of Trp was inhibited in O2-saturated solutions, indicating that a 1O2-mediated process (type II oxidation) was not an important pathway leading to Trp oxidation. By combining different analytical techniques, we could establish that a type I photooxidation was the prevailing mechanism, initiated by an electron transfer from the Trp molecule to the Ptr triplet excited state, yielding the corresponding radical ions (Trp+/Trp(-H) and Ptr−). The Trp reaction products that could be identified by UPLC-mass spectrometry are in agreement with this conclusion.

Ultraviolet photodetector fabricated from atomic-layer-deposited ZnO films

Zinc oxide (ZnO) films have been prepared on glass substrate in layer-by-layer mode using an atomic-layer deposition (ALD) technique, and a metal-semiconductor-metal structured photodetector has been fabricated on the ZnO films employing interdigital Au as metal contacts. The photodetector shows a cutoff wavelength at around 390nm and has an obvious responsivity in the whole UVA spectral range. Because the response of the ZnO photodetector covers the whole UV solar irradiation that can reach the earth, the photodetector promises to be useful in monitoring UV solar irradiation to protect people from harm caused by the solar irradiation. Furthermore, the capability of preparing large-area uniform ZnO films of ALD makes it favorable for possible mass production of this kind of photodetector.

Sol–gel processing and UVA patterning of epoxy-based hybrid organic–inorganic thin films

Sol–gel processed photosensitive hybrid organic–inorganic films, deposited from 2-(3,4-epoxycyclohexylethyltrimethoxysilane) (EETMOS), have been studied. EETMOS sols have been optimized with respect to several criteria that guarantee a good quality, photosensitivity, and reproducibility of derived films. Photo-sensitivity of EETMOS-based films has been assessed by UVA exposure experiments. Optimized films have been photo-patterned using a mercury lamp or a He–Cd laser source, both emitting in the UVA spectral range. Promising micronic size motives have been laser patterned. The quality of derived motives is discussed with respect to photopolymerization mechanisms and photo-patterning parameters.

The UV-A and visible solar irradiance spectrum: inter-comparison of absolutely calibrated, spectrally medium resolution solar irradiance spectra from balloon- and satellite-borne measurements

Abstract. Within the framework of the ENVISAT/-SCIAMACHY satellite validation, solar irradiance spectra are absolutely measured at moderate resolution in the UV/visible spectral range (in the UV from 316.7-418 nm and the visible from 400-652 nm at a full width half maximum resolution of 0.55 nm and 1.48 nm, respectively) from aboard the azimuth-controlled LPMA/DOAS balloon gondola at around 32 km balloon float altitude. After accounting for the atmospheric extinction due to Rayleigh scattering and gaseous absorption (O3 and NO2), the measured solar spectra are compared with previous observations. Our solar irradiance spectrum perfectly agrees within +0.03% with the re-calibrated Kurucz et al. (1984) solar spectrum (Fontenla et al., 1999, called MODTRAN 3.7) in the visible spectral range (415-650 nm), but it is +2.1% larger in the (370-415 nm) wavelength interval, and -4% smaller in the UV-A spectral range (316.7-370 nm), when the Kurucz spectrum is convolved to the spectral resolution of our instrument. Similar comparisons of the SOLSPEC (Thuillier et al., 1997, 1998a, b) and SORCE/SIM (Harder et al., 2000) solar spectra with MODTRAN 3.7 confirms our findings with the values being -0.5%, +2%, and -1.4% for SOLSPEC -0.33%, -0.47%, and -6.2% for SORCE/SIM, respectively. Comparison of the SCIAMACHY solar spectrum from channels 1 to 4 (- re-calibrated by the University of Bremen -) with MODTRAN 3.7 indicates an agreement within -0.4% in the visible spectral range (415-585 nm), -1.6% within the 370-415 nm, and -5.7% within 325-370 nm wavelength interval, in agreement with the results of the other sensors. In agreement with findings of Skupin et al. (2002) our study emphasizes that the present ESA SCIAMACHY level 1 calibration is systematically +15% larger in the considered wavelength intervals when compared to all available other solar irradiance measurements.

The effect of UV absorbing sunscreens on the reflectance and the consequent protection of skin.

The in vivo reflectance spectra of Caucasian skin, coated with preparations containing sunscreen vehicle, vehicle with olive oil and vehicle with the UVB and UVA absorbers 2-ethylhexyl-4-methoxycinnamate and 4-t-butyl-4'-methoxydibenzoylmethane were determined. All preparations reduced the reflectance of skin throughout the UVA spectral range (320 to 400 nm), with the sunscreen preparations containing the UVB and UVB plus UVA absorbers reducing the reflectance more than the sunscreen vehicle alone. This phenomenon, which facilitates the penetration of UV radiation to the lower epidermis and dermal layers of skin and therefore lessens sunscreen efficacy, is attributed to optical coupling mediated by refractive index matching of the sunscreen to the upper epidermis. The greater reduction in skin diffuse reflectance caused by sunscreens containing methoxycinnamate is associated with this compound's high refractive index. Also, by determining the excitation spectra of the autofluorescence originating from the dermal layer of skin, the transmission spectra of the various components of sunscreen on skin were established, and these were in good general agreement with previously published spectra.

Experimental investigation of variability of ultraviolet fluxes at the earth's surface

Some results of two years of regular measurements of UV radiation at the earth's surface in Siberia (56.5o N, 85o E) are given. It is shown that the presence of snow cover increases the UV irradiation of the earth by scattered radiation by 9–15%. In cloudless but windy weather, quasi-periodic fluctuations of UV irradiation with periods of 5–15 min and with fluctuation swings of 1–10% are observed. Low-level, heavy cloudiness in summer considerably reduces (by an average of 30–35%) UV irradiation, especially when the sun is low (down to 30o). Mid-and upper-level clouds increase the UV irradiation of the earth by scattered radiation. When the sun is high (30–50o) and there is heavy cloudiness, this increase can reach 28% in the UV-A spectral range.

Radiative properties of hardwood leaves to ultraviolet irradiation

Spectral reflectance and transmittance of leaves to ultraviolet irradiation were determined under laboratory conditions for seven species of hardwood trees, namely red oak (Quercus rubra, L), black oak (Q. velutina, Lamarch), white oak (Q. alba, L.), sugar maple (Acer saccharum), Norway maple (A. plantanoides), hickory (Carya tomemtosa), sweetgum (Liquidambar styraciflua), and black oak litter. The experimental system consisted of a solar simulator, an integrating sphere, and a spectroradiometer. Experiments were repeated three to five times for both adaxial and abaxial surfaces of fresh leaves chosen at randomly. The spectral distributions and simple averages of the radiative properties in the wavelength ranges of ultraviolet-B (UV-B, 280–320 nm) and ultraviolet-A (UV-A, 320–400 nm) were determined. The spectral distributions of reflectance were similar between adaxial and abaxial surfaces, although the magnitude varied among tree species. Leaf reflectance was very low for the ultraviolet spectrum in general and varied among species and between adaxial and abaxial surfaces. It was generally higher over the UV-A waveband compared to UV-B, and higher on the abaxial than adaxial surface. The broadband reflectance in the UV-A range (over all species) was 5.0 and 3.9% for abaxial and adaxial surface, respectively, compared to 3.5 and 2.8% in UV-B. The transmittance through leaves was extremely small in the UV-B (<0.1%) and nearly zero in the UV-A spectral range. Consequently, the absorptance of ultraviolet radiation by leaves, as determined from the measured reflectance and transmittance, was quite high, being more than 90% for all the combinations of species and wavebands examined. The reported results are useful for studies requiring spectral radiative properties of the examined leaves with respect to ultraviolet irradiation.