Optical Bleaching Research Articles

OSL and TL retrospective dosimetry with leucite glass-based dental ceramics

Dental ceramics are radiation sensitive materials and exhibit luminescent properties. Their samples can be potentially used for individual dose assessment in situations where retrospective dosimetry is required. In this work, we investigated dosimetry properties of two leucite glass-based dental ceramics using Optically Stimulated Luminescence (OSL) and Thermoluminescence (TL). We concentrated on characteristics as reproducibility, fading, optical bleaching, minimum detectable dose (MDD), dose response and photon energy response of TL and OSL signals. Both the dental ceramics exhibited very good reproducibility (<5% at 2σ level) when irradiated and measured repeatedly. Their luminescence response was found to be linear for a wide range of doses up to 8.5 Gy. The MDD values for the samples investigated were a few mGy. The ceramics are not tissue equivalent materials, and their luminescent signals measured were strongly dependent on incident photon energy. Because of fading and optical bleaching the OSL and TL signals were not stable in time. Especially optical bleaching represents a main limiting factor for dose reconstruction. However, one of the ceramics provided a relatively resistant high temperature TL peak for dosimetry.

Study of the effect of optical bleaching at selected photon energies on the optical absorption and thermoluminescence of LiF: Mg, Ti (TLD-100)

A new interpretation of the optical absorption energy spectrum of LiF:Mg,Ti (TLD-100) is presented in which the 4.0 eV band is composed of two-sub-bands at 3.84 eV and 4.3 eV. The new interpretation is of central importance in the understanding of the geminate recombination mechanisms giving rise to glow peak 4 and composite glow peak 5 in the thermoluminescence (TL) of TLD-100. Photon bleaches at energies of 3.6 eV and 4.2 eV on samples of gamma irradiated TLD-100 were carried out in order to investigate the correlation of the 4.3 eV absorption band with the electron-hole configuration giving rise to glow peak 5a and the 3.84 eV absorption band with the e-only configuration giving rise to glow peak 5. The increase in peak 4 (correlated with the hole-only configuration) induced by the 4.2 eV bleach is enhanced by 90% over the increase induced by the 3.6 eV bleach. The decrease in peaks 5a and 5 induced by the 4.2 eV bleach are enhanced by 400% and 50% respectively over the 3.6 eV bleach. The results are consistent with the suggested structures giving rise to the 4.3 eV and 3.84 eV bands observed in the optical absorption energy spectrum.

Optically stimulated luminescence radiation response of Au/Al2O3 phosphors

Crystal defects such as the doping of a variety of various elements are known to enhance the optically stimulated luminescent dosimeter (OSLD) performance of Al2O3. Moreover, the use of Au nanoparticles (NPs) has attracted considerable attention because of their photoluminescence properties. In this paper, a facile method is proposed to prepare and disperse Au NPs on an Al2O3 surface. The effects of Au nanoparticles on linearity between light emission and dose, reusability, and angular dependence of our proposed Au/Al2O3 OSLD are examined and discussed. We found that the linearity between light emission and dose with a wider dose range from 5Gy to 30Gy can be obtained. The sensitivities were tuneable by adjusting the Au/Al2O3 ratio. The result also shows a comparable angular dependence properties with commercially-available Al2O3:C dosimeters. In addition, the optical bleaching durations of Au/Al2O3 and Al2O3:C phosphors were also studied, in which the required bleaching time for our proposed Au/Al2O3 was 2h shorter than for Al2O3:C. In other words, this research also helps to reduce the bleaching time before re-use.

The effect of optical bleaching at selected photon energies on the thermoluminescence of LiF:Mg,Ti

A new interpretation of the optical absorption energy spectrum of LiF:Mg,Ti (TLD-100) is presented in which the 4.0 eV band is composed of two-sub-bands at 3.8 eV and 4.3 eV. The new interpretation is of central importance in the understanding of the geminate recombination mechanisms giving rise to composite glow peak 5 in the thermoluminescence (TL) of TLD-100. Photon bleaches at energies of 3.6 eV and 4.3 eV on samples of gamma irradiated TLD-100 were carried out in order to investigate the correlation of the 4.3 eV absorption band with the electron-hole configuration giving rise to glow peak 5a and the 3.8 eV absorption band with the e-only configuration giving rise to glow peak 5. The 4.3 eV bleach decreases peak 5a dramatically and increases peak 4 (a hole-only configuration). No such effect is observed following the 3.6 eV bleach. The results strongly support the suggested structures giving rise to the 4.3 eV and 3.8 eV bands observed in the optical absorption energy spectrum.

Effects of electron-irradiation darkening and its posterior bleaching by light in novel Cr–Mg–YAS fiber

Two remarkable effects for the recently invented chromium–magnesium (Cr–Mg) co-doped yttria-alumino-silicate fiber are reported: (i) strong and spectrally peculiar darkening under the action of energetic (6 MeV) β-electrons with dosage up to 1.0 × 1015 cm−2 and (ii) posterior optical bleaching of the darkening loss at exposure to low-power (of a mW-range) 633 nm light. Both phenomena are revealed to be conspecific to co-doping the fiber with Mg and to the presence of versatile valence forms of Cr ions. The reported results seem to be impactful for exploiting fiber of such type for dosimetry and in space technology.

Power-Dependent Transient Gain Study on Direct Gap GeSn Crystallized on Amorphous Layers

Group IV semiconductors attract intensive interests for potential monolithically integrated photonics on the Si platform. The realization of Group IV-based light emitters is challenging yet highly desirable. Ge-on-Si lasing from the direct gap transition has been demonstrated at room temperature by employing n-type doping and tensile strain to reduce the energy difference between direct bandgap and indirect bandgap[1-2]. An alternative method is incorporating Sn into Ge, which shrinks the direct bandgap much faster than the indirect bandgap, thereby leading to direct bandgap GeSn alloys. We found that the incorporation of Sn into Ge can also enhance the crystallization of GeSn on amorphous substrate, which is beneficial for 3D photonic integration on Si by moving all the photonic components to the metal/dielectric interconnect level well above the CMOS layer using back-end-of-line (BEOL) processing. In our previous work, high crystallinity GeSn substitutional alloy thin films with up to 9 at.% Sn are directly grown on amorphous SiO2 layers at low crystallization temperatures of 370~470 °C, eliminating the thick buffer layer commonly used in epitaxial GeSn on Si.[3] Recently, epitaxial GeSn-based lasing is reported in the near mid-infrared wavelength range, but only observed at low temperatures [4]. A detailed understanding of the ultrafast carrier dynamics is necessary to design and optimize high-performance GeSn lasers operating at room temperature. In this study, we investigate the power-dependent femtosecond transient gain of direct-gap GeSn with 9% Sn thin films crystallized on amorphous SiO2 layers in the wavelength range of 1950~2350 nm. It is found at carrier injection density Δn<1×1017 cm-3 , the transmission of GeSn is same as that measured by Fourier Transfer Infrared spectroscopy (FTIR), i.e. no optical bleaching. When the injection density is increased to Δn~8×1018 cm-3 , the GeSn film becomes transparent at λ~2000 nm. At higher injection of Δn~2×1019 cm-3 , net transient gain is observed at λ=1900-2150 nm, with a peak gain of 6300 cm-1 at λ=2000 nm . Further increasing the carrier injection to Δn~1×1020 cm-3, the peak of transient gain red-shifted to 2100 nm due to the bandgap renormalization, and the peak gain further increases to ~8000 cm-1. The transient gain of GeSn with 9% Sn is similar to III-V direct bandgap semiconductors at similar injection levels. These results confirm that the GeSn thin film crystalized on amorphous SiO2 is a good optical gain medium. Further studies will reveal more detailed carrier dynamics between direct and indirect conduction valleys. [1] R. E. Camacho-Aguilera, Y. Cai, N. Patel, J. T. Bessette, M. Romagnoli, L. C. Kimerling, and J. Michel, Opt. Express. 20, 11316 (2012). [2] R. Koerner, M. Oehme, M. Gollhofer, et al, Opt. express, 23, 14815 (2015). [3] H. Li, J. Brouillet, A. Salas, X. Wang and J. Liu, Opt. Mater. Express 3, 1385 (2013). [4] S.Wirths, R. Geiger, N.von den Diresch, et al, Nature Photonics, 9, 88 (2015).

Power-Dependent Transient Gain Study on Direct Gap GeSn Crystallized on Amorphous Layers

In this study we investigate the power-dependent femtosecond transient gain of direct-gap Ge0.91Sn0.09 thin films crystallized on amorphous SiO2 layers. At carrier injection density Δn<1×1017 cm-3, there is no optical bleaching. When Δn increases to ~8×1018 cm-3, the Ge0.91Sn0.09 film becomes transparent at λ~2000 nm. At higher injection of Δn~2×1019 cm-3, net transient gain is observed at λ=1900-2150 nm, with a peak gain of 6300 cm-1 at λ=2000 nm. Further increasing Δn to ~1×1020 cm-3, the peak of transient gain red-shifted to 2100 nm due to the bandgap renormalization, and the peak gain further increases to ~8000 cm-1. The transient gain coefficient of Ge0.91Sn0.09 is similar to III-V direct bandgap semiconductors at similar injection levels. The gain lifetime was measured to be ~0.1 ns at λ=2100 nm and Δn~6×1018 cm-3. These results confirm that direct-gap GeSn crystalized on amorphous SiO2 is a good optical gain medium towards 3D photonic integration.

Layer-by-layer hyaluronic acid/chitosan polyelectrolyte coated mesoporous silica nanoparticles as pH-responsive nanocontainers for optical bleaching of cellulose fabrics

Polyelectrolyte multilayer (PEM) films composed of two natural polysaccharides (hyaluronic acid and chitosan) were deposited through layer-by-layer (LbL) assembly on top of biocompatible mesoporous silica nanoparticles (MSNPs). Polyelectrolyte multilayer coated mesoporous silica nanoparticles (PEM-MSNPs) were characterized by using attenuated total reflection–fourier transform infrared spectroscopy (ATR–FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption-desorption isotherms, UV–vis and fluorescence spectrophotometer. A commercially available and industrially used optical brightener, 4,4′-distyrylbiphenyl sulfonate sodium salt (CBSX) was loaded into nanocontainers (CBSX@PEM-MSNPs) to evaluate their use for the pH-sensitive release. The controlled release of CBSX from nanocontainers in response to pH is monitored by using UV–vis and fluorescence spectrophotometer in water and the almost 100% of encapsulated CBSX is released within 2h at pH 7. The increase in both whiteness and total color change at pH 7 on a cellulose fabric demonstrates the great potential of nanocontainers as pH-sensitive whitening agents for optical bleaching of cellulose fabrics.

Dosimetry characteristics of NaLi2PO4:Ce3+ OSLD phosphor

High Sensitivity NaLi2PO4:Ce3+ optically stimulated luminescence dosimetry (OSLD) phosphor material was prepared by a simple solid state diffusion method. The material was characterized by XRD to confirm its formation in a single phase. Comparison of the experimental data with that of the data available in the literature (JCPDF # 80-2110) shows formation of a single orthorhombic phase, with the space group Pmnb (62). Study on the particle size effect shows that particle size in the range 50–150μm is more suitable for the continuous wave-optically stimulated luminescence (CW-OSL) dosimetry. Dosimetric properties of the phosphor material (particle size 50–150μm) using CW-OSL technique for blue light stimulation (λ≈470nm) show that it is highly sensitive (~6 times more sensitive than the commercially available Al2O3:C phosphor) in the linear dose range. The phosphor has found to have all the other good dosimetric characteristics, such as, easy method of synthesis, tissue equivalence, i.e., low-Z material (Zeff. ~10.8), low fading (~ 25% in 20 days), wide range of dose response (3mGy–500Gy), easy optical bleaching for its reuse, excellent reusability, etc., which make it useful for OSL dosimetry of high-energy radiations. Another advantage of this OSLD phosphor is that its dosimetric TL glow peak does not fade much on taking OSL and thus it could also be used as TLD phosphor for re-estimating doses in case of any doubts.

Opto-ultrasound imaging in vivo in deep tissue

It is of keen importance of deep tissue imaging with high resolution in vivo. Here we present an opto-ultrasound imaging method which utilizes an ultrasound to confine the laser pulse in a very tiny spot as a guide star. The results show that the imaging depth is 2mm with a resolution of 10um. Meanwhile, the excitation power we used is less than 2mW, which indicates that our methods can be applied in vivo without optical toxicity and optical bleaching due to the excitation power.

Encapsulation and pH-responsive release of an optical brightener (CBUS) from chitosan microcontainers for optical bleaching of cellulosic fabrics

Encapsulation of an industrially used optical brightener, namely disodium 4,4′-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]-stilbene-2,2′-disulfonate (CBUS), with a biodegradable natural chitosan biopolymer is studied. CBUS-loaded chitosan microcontainers were produced by the simple coacervation/precipitation method. Their morphologies in dry state were investigated by scanning electron microscopy. Controlled release of CBUS from the microcontainers in response to pH was monitored by using ultraviolet–visible (UV–Vis) and fluorescence spectrophotometry in water, with almost 100 % of the encapsulated CBUS being released within the first 90 min at pH 2. Fluorescent microscopy measurements indicated that such encapsulation of CBUS with natural chitosan biopolymer is an effective way to protect and improve light stability. The increase in both whiteness and total color change at pH 2 for the cellulosic fabric demonstrates the great potential of CBUS-loaded chitosan microcontainers as smart pH-responsive whitening agents for optical bleaching of cellulosic fabrics.

Ge and GeSn Light Emitters on Si

The heteroepitaxial growth of GeSn and Ge crystals on Si substrates are investigated for Si-based photonic applications. Light Emitting Diodes with emission wavelengths from 2,100 to 1,550 nm could be demonstrated with active intrinsic GeSn light emitting layers between Ge barriers. A clear shift of the direct band gap toward the infrared beyond 2 μm is measured. Emission intensity is increased compared to Ge Light Emitting Diodes. Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices are demonstrated. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Pérot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1,660 nm and 1,700 nm.

On the effect of optical and isothermal treatments on luminescence signals from feldspars

During luminescence dosimetry and luminescence dating applications it is often necessary to precondition the geological samples by applying a thermal or optical treatment before measuring the luminescence signal. In luminescence applications using apatites or feldspars, measurement of continuous-wave infrared or optically stimulated signals (CW-IRSL and CW-OSL) are customarily preceded by either an isothermal heating of the samples at a fixed temperature for a short time interval, or alternatively by optically bleaching the samples using light from LEDs with the appropriate wavelength. This paper presents new analytical equations which can be used to describe these commonly employed double experimental procedures. The equations are based on a recently published model which assumes that tunneling processes are taking place in random distributions of donor–acceptor pairs. The concentration of charge carriers during the CW-IRSL or CW-OSL experiment is expressed in terms of the parameters of the preceding thermal or optical bleaching procedure, and depends also on the distribution of distances between electron and hole pairs. The analytical equations in this paper are compared with experimental data from a feldspar sample which undergoes an isothermal procedure followed by measurement of the CW-IRSL signal. Additional comparisons with experiment are provided using a feldspar sample which undergoes an infrared bleaching process, followed by measurement of the CW-OSL signal. These results and conditions under which the equations can be used are discussed within the framework of the model.

UV laser induced changes to morphological, optical and electrical properties of conductive nanocrystalline diamond films

The possibility of laser induced variation of optical and electrical properties of conductive nanocrystalline diamond (CNCD) films has been demonstrated. The films were produced by microwave plasma chemical vapor deposition (MPCVD) from CH4:H2:N2 gas mixtures. The films were irradiated in air with 20ns pulses of an ArF excimer laser (λ=193nm). It was found that low laser pulse intensity (~0.05J/cm2), well below film surface graphitization (~0.3J/cm2) and nanoablation (~0.08J/cm2) thresholds, induces changes of the film properties. The effect requires multiple pulsed irradiation and results in a decrease of the film electrical conductivity, which is accompanied by optical bleaching of the diamond film absorption.

Investigating the components of the optically stimulated luminescence signals of quartz grains from sand dunes in China

The optically stimulated luminescence (OSL) signals from quartz consist of several physically distinct components, which are commonly referred to as fast, medium and slow components. In this study, the OSL components of quartz from the Taklimakan Desert and the Hunshandake sandy land in north China are investigated. Our results show that the relative contributions of OSL components to the bulk OSL signal can be significantly different among quartz grains from both deserts. Laboratory dosing, optical bleaching and heating experiments are used to test their effects on the relative contributions of quartz OSL components. It is found that cycles of dosing and optical bleaching have insignificant impact on the relative contributions of quartz OSL components, while heating to high temperature (500 °C) can significantly enhance the contribution of the fast component to the bulk OSL signals, especially for quartz samples from the Taklimakan Desert. Such results suggest that the different heating history of natural quartz grains plays an important role in controlling OSL components. Additionally, the quartz grains from the Hunshandake sandy land can easily be distinguished from those of the Taklimakan Desert, by using a ternary plot of fast-medium-slow components. The quartz grains from the Hunshandake sandy land exhibit a much stronger fast component than those from the Taklimakan Desert. This can be explained by that the quartz grains from the Hunshandake sandy land are mainly of igneous origin, while most of the quartz grains from the Taklimakan Desert are of low grade metamorphic origin.

Electrically pumped lasing from Ge Fabry-Perot resonators on Si.

Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices has been observed. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Perot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1660 nm and 1700 nm. This fits the theoretically predicted behavior for the n-type Ge material system. With further pulsed electrical injection of 500 kA/cm2 it was possible to reach the lasing threshold for such edge emitters. Different lengths and widths of devices have been investigated in order to maintain best gain-absorption ratios.

User Guide for Luminescence Sampling in Archaeological and Geological Contexts

AbstractLuminescence dating provides a direct age estimate of the time of last exposure of quartz or feldspar minerals to light or heat and has been successfully applied to deposits, rock surfaces, and fired materials in a number of archaeological and geological settings. Sampling strategies are diverse and can be customized depending on local circumstances, although all sediment samples need to include a light-safe sample and material for dose-rate determination. The accuracy and precision of luminescence dating results are directly related to the type and quality of the material sampled and sample collection methods in the field. Selection of target material for dating should include considerations of adequacy of resetting of the luminescence signal (optical and thermal bleaching), the ability to characterize the radioactive environment surrounding the sample (dose rate), and the lack of evidence for post-depositional mixing (bioturbation in soils and sediment). Sample strategies for collection of samples from sedimentary settings and fired materials are discussed. This paper should be used as a guide for luminescence sampling and is meant to provide essential background information on how to properly collect samples and on the types of materials suitable for luminescence dating.

Evaluation of ESR residual dose in quartz modern samples, an investigation on environmental dependence

Luminescence and ESR dating methods of quartz sediment are based on the natural resetting of the signal by light exposure (optical bleaching). When the bleaching is incomplete, a residual dose (DeR) is added to the post-depositional dose accumulated since the deposit and hence the age is overestimated.Insufficient bleaching is usually linked to the environment and conditions of transport/deposition of the quartz grains affecting the light exposure duration. Indeed, each transportation mode – fluvial, marine or aeolian – is associated to specific conditions of light exposure, depending mainly to the location of grains in the transport agent during the transport phase, the opacity of the transport environment and the velocity of the transport.The present study attempts to discriminate the modes of transport/deposition providing a satisfying reset of the ESR signals of quartz grains. For this purpose, we investigated bleaching rates and ESR residual doses of aluminum centers from “present-day” aeolian, fluvial and marine sediments sampled in various sedimentary environments. The bleaching efficiency evaluation in these different environments may help for a better understanding of the resetting phenomenon for quartz signals which represents presently the main difficulty for ESR dating.The results show that the residual doses are small enough to allow an ESR dating of the main part of the sediment transported in almost all the context examined in this study. The smallest residual doses are obtained from quartz grains within the range of 100–200 μm and transported in clear water. Some limits for the application of optically bleached quartz ESR dating appears nevertheless, mainly when the residual dose and the dose accumulated after the deposit are quite similar, i.e. for Upper Pleistocene samples.

Photo-induced optical bleaching in Ge12Sb25S63 amorphous chalcogenide thin films: effect of 532 nm laser illumination

The photo-induced effects of Ge12Sb25S63 films illuminated with 532 nm laser light are investigated from transmission spectra measured by FTIR spectroscopy. The material exhibits photo-bleaching (PB) when exposed to band gap light for a prolonged time in a vacuum. The PB is ascribed to structural changes inside the film as well as surface photo-oxidation. The amorphous nature of thin films was detected by x-ray diffraction. The chemical composition of the deposited thin films was examined by energy dispersive x-ray analysis (EDAX). The refractive indices of the films were obtained from the transmission spectra based on an inverse synthesis method and the optical band gaps were derived from optical absorption spectra using the Tauc plot. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. It was found that the mechanism of the optical absorption follows the rule of the allowed non-direct transition. Raman and x-ray photoelectron spectra (XPS) were measured and decomposed into several peaks that correspond to the different structural units which support the optical changes.

Optically stimulated luminescence (OSL) response of Al2O3:C, BaFCl:Eu and K2Ca2(SO4)3:Eu phosphors.

This paper investigates the optically stimulated luminescence (OSL) response of BaFCl:Eu and K2Ca2(SO4)3:Eu phosphors for different doses and bleaching durations. The results have also been compared with the commercially available Landauer Al2O3:C (Luxel®) dosemeter. Nanocrystalline K2Ca2(SO4)3:Eu is known to be a sensitive thermoluminescent phosphor, but its OSL response is hardly reported. At first, pellets of nanocrystalline K2Ca2(SO4)3:Eu powder were prepared by adding Teflon as a binder. Their OSL signal was compared with that of the material in pure form, i.e. without adding the binder (in 100:1 ratio). It was observed that adding the binder does not appreciably affect the OSL intensity. On comparison with the commercially available Al2O3:C from Landauer, it was found that K2Ca2(SO4)3:Eu is around 15 times less sensitive than Al2O3:C. 'Homemade' BaFCl:Eu phosphor has also been studied. The intensity of BaFCl:Eu was ∼20 times more than the standard Al2O3:C dosemeter and ∼200 times more sensitive than K2Ca2(SO4)3:Eu in the dose range of 13-200 cGy. OSL dosemeters are believed to give luminescence signal even if they are read before, i.e. multiple reading may be possible under suitable conditions after single exposure. This was also checked for all the prepared dosemeters and it was found that Al2O3:C showed the least decrease of <2 %, followed by BaFCl:Eu of 15 % and K2Ca2(SO4)3:Eu with 20 %. Finally, Al2O3:C and BaFCl:Eu phosphors were also studied for their optical bleaching durations to which the respective signals get completely removed so that the phosphor can be re-used. It was observed that BaFCl:Eu is bleached faster and more easily than Al2O3:C.