Fluorescent Aluminum Oxide Crystals for Volumetric Optical Data Storage and Imaging Applications

Wavelength stabilization and spectra narrowing of a 405 nm external-cavity semiconductor laser based on a volume Bragg grating

We introduce a 405 nm external-cavity semiconductor laser using a volume Bragg grating (VBG) as the feedback element. By decreasing the length of the external cavity and reducing the wavelength difference between the output wavelength of the laser diode during free running and Bragg wavelength of the VBG, the emission wavelength of the semiconductor laser is stably locked at 405.1 nm with a spectral linewidth of 0.08 nm. The output power reaches 292 mW, and the wavelength drift with temperature reduces to 0.0006 nm/°C. These results are helping for the spectroscopy applications of a blue-violet laser diode. In contrast to traditional external-cavity semiconductor lasers, this laser is less expensive and more compact, in addition to having a narrow linewidth and good wavelength stability. These advantages would facilitate the development of associated areas of research, including optical data storage, laser display, and laser medicine.

Electron and hole capture processes in Cu-doped glass exhibiting radiophotoluminescence

Radiophotoluminescence (RPL) is a radiation effect, and materials exhibiting RPL can be used in dosimeters. In this study, we observed remarkable RPL in Cu-doped aluminoborosilicate and silica glasses upon their exposure to⁶⁰Coγ-rays. The RPL intensity increased proportionally with the irradiation dose up to several hundreds of grays and then saturated beyond a certain dose level. An equation was derived theoretically to express the relationship between the RPL intensity and irradiation dose based on the RPL mechanism, in which copper ions, Cu²⁺and Cu⁺, capture electrons and holes, generated by the irradiation, respectively, resulting in a change in the valence. The equation fitted well with the experimental results, providing two parameters for the equation. These parameters are associated with the saturation dose level and sensitivity, which are important for the application of materials to dosimeters. These parameters were discussed based on electron and hole capture processes in the RPL mechanism.

Two-dimensional real-time quality assurance dosimetry system using μ-Al2O3:C,Mg radioluminescence films

BACKGROUND AND PURPOSE

There is a continual need for more accurate and effective dosimetric systems for quality assurance (QA) as radiotherapy evolves in complexity. The purpose of this project was to introduce a new system that minimally perturbs the main beam, while assessing its real time 2D dose-rate and field shapes. The system combined reusability, linear dose-rate response, and high spatial and time resolution in a single radiation detection technology that can be applied to surface dose estimation and QA.

MATERIALS AND METHODS

We developed a 2D prototype system consisting of a camera, focusing lenses and short pass filter, placed on the head of a linear accelerator, facing an Al2O3:C,Mg radioluminescent film. To check the appropriateness of multi-leaf collimator, stability/reproducibility QA tests were prepared using the treatment planning system: including the routinely used alternating leaves, chair and pyramid checks.

RESULTS

The Al2O3:C,Mg film did not perturb the dose vs. depth dose curves determined with a point detector (-0.5% difference). Our results showed a dose-rate linear film response (R2 = 0.999), from 5 to 600 MU/min. Measured output factors agreed with reference data within ~1%, indicating a potential for small field dosimetry. Both chair and pyramid measured profiles were comparable with those obtained with the treatment planning system within 1%. The alternating leaves test showed an average discrepancy in the valleys of 14%.

CONCLUSIONS

The prototype demonstrated promising results. It obviated the need for corrections regarding the relative position of the camera, confirming accurate dose-rate delivery and detection of radiation fields.

The effect of pre-dose on thermally and optically stimulated luminescence from α-Al2O3:C,Mg and α-Al2O3:C

We report the effect of pre-dose on the thermoluminescence (TL) and optically stimulated luminescence (OSL) dose response of α-Al₂O₃:C,Mg and α-Al₂O₃:C. Before any luminescence measurement, the samples were irradiated with different doses, namely 100, 500 and 1000 Gy to populate the deep electron traps. This is the pre-dose. The results from TL and OSL studies are compared with results from samples used without any pre-measurement dose. The TL glow curves and OSL decay curves of α-Al₂O₃:C,Mg recorded after pre-doses of 100, 500 and 1000 Gy are identical to those from a sample used without any pre-dose. Further, the TL and OSL dose response of all α-Al₂O₃:C,Mg samples are similar regardless of pre-dose. In comparison, the TL glow curves and OSL decay curves of α-Al₂O₃:C are influenced by pre-dose. We conclude that the differences in the TL and OSL dose response of various pre-dosed samples of α-Al₂O₃:C are due to the concentration of charge in the deep traps. On the other hand, owing to the lower concentration of such deep traps in α-Al₂O₃:C,Mg, the TL or OSL dose responses are not affected by pre-dose in this material.

Alpha particle spectroscopy using FNTD and SIM super-resolution microscopy

Structured illumination microscopy (SIM) for the imaging of alpha particle tracks in fluorescent nuclear track detectors (FNTD) was evaluated and compared to confocal laser scanning microscopy (CLSM). FNTDs were irradiated with an external alpha source and imaged using both methodologies. SIM imaging resulted in improved resolution, without increase in scan time. Alpha particle energy estimation based on the track length, direction and intensity produced results in good agreement with the expected alpha particle energy distribution. A pronounced difference was seen in the spatial scattering of alpha particles in the detectors, where SIM showed an almost 50% reduction compared to CLSM. The improved resolution of SIM allows for more detailed studies of the tracks induced by ionising particles. The combination of SIM and FNTDs for alpha radiation paves the way for affordable and fast alpha spectroscopy and dosimetry.

A COMPARATIVE STUDY OF THE DOSIMETRIC FEATURES OF α-Al2O3:C,Mg AND α-Al2O3:C

A comparative study of the dosimetric features of α-Al2O3:C,Mg and α-Al2O3:C relevant to thermoluminescence dosimetry is reported. A glow curve of α-Al2O3:C,Mg measured at 1°C/s after beta irradiation to 1 Gy shows two subsidiary peaks at 42°C (labelled as I) and 72°C (II) and the main peak at 161°C (III) whereas a glow curve of α-Al2O3:C measured under the same conditions shows the main peak at 178°C (II') and a lower intensity peak at 48°C (I'). Apart from these ones, there are several other peaks at temperatures beyond that of the main peak in both α-Al2O3:C,Mg and α-Al2O3:C. However, the latter are not included in this study. We report a comparative quantitative analysis of dose response and fading of peaks I, II and III of α-Al2O3:C,Mg and peaks I' and II' of α-Al2O3:C. Analysis shows that the dose response of peaks I and III is sublinear within 1-10 Gy whereas that of peak II is superlinear within 1-4 Gy followed by a sublinear region within 4-10 Gy. In comparison, the dose response of peak I' is superlinear within 1-4 Gy followed by a sublinear region within 4-10 Gy whereas that of peak II' is sublinear within 1-4 Gy followed by a superlinear region within 4-10 Gy. As regards to fading corresponding to 1 Gy, peak I is very unstable and fades within 300 s, peak II is more stable and takes up to 43200 s to fade. In comparison, peak III fades down to 30% of its initial intensity within 2400 s. Interestingly, between 2400 and 800 s, the intensity fades by 17% only. Regarding fading in α-Al2O3:C, peak I' fades within 600 s whereas peak II' shows an inverse fading behaviour up to 64800 s. The rate of fading for peaks I, II and III in α-Al2O3:C,Mg was found to decrease with increase in dose. However, no such behaviour was observed in α-Al2O3:C. The fading in both samples is discussed on the basis of a charge hopping mechanism.

Laser-assisted fabrication of gold nanoparticle-composed structures embedded in borosilicate glass

We present results on laser-assisted formation of two- and three-dimensional structures comprised of gold nanoparticles in glass. The sample material was gold-ion-doped borosilicate glass prepared by conventional melt quenching. The nanoparticle growth technique consisted of two steps - laser-induced defect formation and annealing. The first step was realized by irradiating the glass by nanosecond and femtosecond laser pulses over a wide range of fluences and number of applied pulses. The irradiation by nanosecond laser pulses (emitted by a Nd:YAG laser system) induced defect formation, expressed by brown coloration of the glass sample, only at a wavelength of 266 nm. At 355, 532 and 1064 nm, no coloration of the sample was observed. The femtosecond laser irradiation at 800 nm also induced defects, again observed as brown coloration. The absorbance spectra indicated that this coloration was related to the formation of oxygen deficiency defects. After annealing, the color of the irradiated areas changed to pink, with a corresponding well-defined peak in the absorbance spectrum. We relate this effect to the formation of gold nanoparticles with optical properties defined by plasmon excitation. Their presence was confirmed by high-resolution TEM analysis. No nanoparticle formation was observed in the samples irradiated by nanosecond pulses at 355, 532 and 1064 nm. The optical properties of the irradiated areas were found to depend on the laser processing parameters; these properties were studied based on Mie theory, which was also used to correlate the experimental optical spectra and the characteristics of the nanoparticles formed. We also discuss the influence of the processing conditions on the characteristics of the particles formed and the mechanism of their formation and demonstrate the fabrication of structures composed of nanoparticles inside the glass sample. This technique can be used for the preparation of 3D nanoparticle systems embedded in transparent materials with potential applications in the design of new optical components, such as metamaterials and in plasmonics.

Nanoscale measurements of proton tracks using fluorescent nuclear track detectors

PURPOSE

The authors describe a method in which fluorescence nuclear track detectors (FNTDs), novel track detectors with nanoscale spatial resolution, are used to determine the linear energy transfer (LET) of individual proton tracks from proton therapy beams by allowing visualization and 3D reconstruction of such tracks.

METHODS

FNTDs were exposed to proton therapy beams with nominal energies ranging from 100 to 250 MeV. Proton track images were then recorded by confocal microscopy of the FNTDs. Proton tracks in the FNTD images were fit by using a Gaussian function to extract fluorescence amplitudes. Histograms of fluorescence amplitudes were then compared with LET spectra.

RESULTS

The authors successfully used FNTDs to register individual proton tracks from high-energy proton therapy beams, allowing reconstruction of 3D images of proton tracks along with delta rays. The track amplitudes from FNTDs could be used to parameterize LET spectra, allowing the LET of individual proton tracks from therapeutic proton beams to be determined.

CONCLUSIONS

FNTDs can be used to directly visualize proton tracks and their delta rays at the nanoscale level. Because the track intensities in the FNTDs correlate with LET, they could be used further to measure LET of individual proton tracks. This method may be useful for measuring nanoscale radiation quantities and for measuring the LET of individual proton tracks in radiation biology experiments.

Two-Step Cycle for Producing Multiple Anodic Aluminum Oxide (AAO) Films with Increasing Long-Range Order

Nanoporous anodic aluminum oxide (AAO) membranes are being used for an increasing number of applications. However, the original two-step anodization method in which the first anodization is sacrificial to pre-pattern the second is still widely used to produce them. This method provides relatively low throughput and material utilization as half of the films are discarded. An alternative scheme that relies on alternating anodization and cathodic delamination is demonstrated that allows for the fabrication of several AAO films with only one sacrificial layer thus greatly improving total aluminum to alumina yield. The thickness for which the cathodic delamination performs best to yield full, unbroken AAO sheets is around 85 μm. Additionally, an image analysis method is used to quantify the degree of long-range ordering of the unit cells in the AAO films which was found to increase with each successive iteration of the fabrication cycle.

Structural Properties of Al₂O₃:C:Mg Thin Films by RF Magnetron Sputtering

Carbon doped aluminum oxide co-doped with magnesium (Al2O3:C:Mg) thin films were deposited using radio frequency magnetron sputtering method on Si (100) substrates. The deposition chamber temperature was manipulated to allow control over the crystalline phase. The crystalline phase of deposited thin films were determined by x-ray diffraction (XRD) technique. Slight change in crystallite size was observed with respect to the increasing deposition chamber temperature. Fourier transform infrared (FTIR) analyses indicated a negligible interfacial SiO2 growth during deposition. Transmission spectra of FTIR showed the bond and functional group of deposited thin films.

Photoluminescence-based detection of particle contamination on extreme ultraviolet reticles

Here, we propose a comparison-free inspection technique to detect particle contamination on the reticle of extreme ultraviolet (EUV) lithography systems, based on the photoluminescence spectral characteristics of the contaminant particles and their elemental composition. We have analyzed the spectra from different particles found on reticles in EUV lithographic systems and have determined the minimum detectable particle size: 25 nm for organic particles and 100 nm for Al particles. Stainless steel coatings (50 nm thick and 50 × 50 μm(2) in area) exhibit detectable photoluminescence, and the estimated minimum detectable particle is 2 μm.

Absorbed dose in ion beams: comparison of ionisation- and fluence-based measurements

A direct comparison measurement of fluorescent nuclear track detectors (FNTDs) and a thimble ionisation chamber is presented. Irradiations were performed using monoenergetic protons (142.66 MeV, ϕ=3×10(6) cm(-2)) and carbon ions (270.55 MeV u(-1), ϕ=3 × 10(6) cm(-2)). It was found that absorbed dose to water values as determined by fluence measurements using FNTDs are, in case of protons, in good agreement (2.4 %) with ionisation chamber measurements, if slower protons and Helium secondaries were accounted for by an effective stopping power. For carbon, however, a significant discrepancy of 4.5 % was seen, which could not be explained by fragmentation, uncertainties or experimental design. The results rather suggest a W-value of 32.10 eV ± 2.6 %. Additionally, the abundance of secondary protons expected from Monte-Carlo transport simulation was not observed.

Engineering cell-fluorescent ion track hybrid detectors

BACKGROUND

The lack of sensitive biocompatible particle track detectors has so far limited parallel detection of physical energy deposition and biological response. Fluorescent nuclear track detectors (FNTDs) based on Al₂O₃:C,Mg single crystals combined with confocal laser scanning microscopy (CLSM) provide 3D information on ion tracks with a resolution limited by light diffraction. Here we report the development of next generation cell-fluorescent ion track hybrid detectors (Cell-Fit-HD).

METHODS

The biocompatibility of FNTDs was tested using six different cell lines, i.e. human non-small cell lung carcinoma (A549), glioblastoma (U87), androgen independent prostate cancer (PC3), epidermoid cancer (A431) and murine (VmDk) glioma SMA-560. To evaluate cell adherence, viability and conformal coverage of the crystals different seeding densities and alternative coating with extracellular matrix (fibronectin) was tested. Carbon irradiation was performed in Bragg peak (initial 270.55 MeV u⁻¹). A series of cell compartment specific fluorescence stains including nuclear (HOECHST), membrane (Glut-1), cytoplasm (Calcein AM, CM-DiI) were tested on Cell-Fit-HDs and a single CLSM was employed to co-detect the physical (crystal) as well as the biological (cell layer) information.

RESULTS

The FNTD provides a biocompatible surface. Among the cells tested, A549 cells formed the most uniform, viable, tightly packed epithelial like monolayer. The ion track information was not compromised in Cell-Fit-HD as compared to the FNTD alone. Neither cell coating and culturing, nor additional staining procedures affected the properties of the FNTD surface to detect ion tracks. Standard immunofluorescence and live staining procedures could be employed to co-register cell biology and ion track information.

CONCLUSIONS

The Cell-Fit-Hybrid Detector system is a promising platform for a multitude of studies linking biological response to energy deposition at high level of optical microscopy resolution.

New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging

Optical and dosimetric properties of a new radiophotoluminescent material based on aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) and having aggregate oxygen vacancy defects are presented. The Al2O3:C,Mg crystals are characterized by several new optical absorption and emission bands. It is suggested that the main optical properties of this material are due to the formation of aggregate defects composed of two oxygen vacancies and two Mg-impurity atoms. Radiation-induced optical absorption bands are centered at 335 and 620 nm and produce fluorescent emission at 750 nm with a 75 +/- 5 ns lifetime. The dose measurements are performed by illumination of the Al2O3:C,Mg crystal with 335 nm or 650 nm light and by measuring the intensity of the 750 nm fluorescence. The detector material is insensitive to room light before and after the irradiation and the traps are stable up to 600 degrees C. A dose measurement range between 5 mGy and 200 Gy, suitable for therapeutic radiology applications, was demonstrated. The short luminescent lifetime and nondestructive readout is favorable for imaging applications.

Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals

A completely optical, non-destructive imaging of tracks in a fluorescent crystal provides a new way to detect and to assess doses from heavy charged particles and neutrons. The technique combines confocal fluorescent microscopy with a new radiation-sensitive, luminescent material based on aluminium oxide single crystals doped with carbon, magnesium and having aggregate oxygen vacancy defects (Al2O3:C,Mg). Radiation-induced colour centres in the new material have an absorption band at 620 nm and produce fluorescence at 750 nm with a high quantum yield and a short, 75 +/- 5 ns, fluorescence lifetime. Three-dimensional spatial distribution of fluorescent intensity allows one to obtain depth-dose distributions and to discriminate between high- and low-linear energy transfer radiations. Images of single tracks produced by different types of radiation have been obtained. Irradiations with a calibrated 241Am alpha source showed high efficiency for track detection. Thermal neutrons were detected using a nuclear reaction with a 6LiF radiator and production of alpha particles and tritium ions. Fast neutrons were detected using recoil protons produced in a polyethylene radiator installed in front of the crystalline detector. Three-dimensional reconstruction of a recoil proton propagating through the crystal was demonstrated.