Atomic motion in solids is conventionally driven by elastic collisions between ionizing particles and atoms, which transfer momentum and induce lattice displacements. In this work, we demonstrate a different mechanism for atomic displacement based on optical excitation of scintillating ionic crystals. Ionic crystals are unique systems because of the closed-shell electronic configuration of their constituent ions. In these materials, excitation above the band gap generates a hole that strongly distorts the lattice, resulting in the formation of a self-trapped hole (STH). The STH is Coulomb-attracted to the electron, thereby forming a self-trapped exciton (STE). Here, we demonstrate that in BaF2 - one of the fastest scintillators - the STE structure promotes the formation of long-lived electron and hole traps that persist in the lattice at room temperature. Such trapped electron-hole pairs occupy vacancy-interstitial fluorine pair positions, and can be created indiscernibly using optical or ionizing radiation excitation, as long as the STH is formed. Further, we demonstrate that it is possible to control the defect evolution with light. Selective optical stimulation of the trapped electrons or holes enables the regeneration of the STE at later times. This light-controlled defect engineering allows us to increase the yield of the STE signal appearing as optically stimulated luminescence (OSL) and to image the spatial distribution of the initial energy deposition, holding strong potential for ionizing-radiation detection. These findings provide a common framework underlying scintillation and OSL in ionic crystals of the fluorite structure, allowing for optical manipulation of atomic vacancies-interstitial pairs in similar systems.

The conversion of gamma particles into optical photons in state-of-the-art scintillator materials is limited to maximum 10 emitted photons per MeV of energy deposited per picosecond, when the material is excited at room-temperature. Breaking this limit has both fundamental and applied importance, and motivates the search for fast and efficient optical emitters at excitation densities relevant for particle detection, up to 1020 electron-hole pairs (eh) per cm3. In this work, we address this challenge by probing the optical response of a promising nanomaterial, CdSe/CdS core/crown nanoplatelets (NPLs), in the shape of drop cast films using intense femtosecond laser pulses. The study finds that the NPL films exhibit a bright optical response at low to medium excitation densities but suffer from high levels of nonlinear quenching, dominated by exciton-exciton annihilation (EEA), at densities exceeding 1017 eh/cm3. The experimental data and theoretical calculations suggest that EEA is enhanced in the drop cast film by the close packing of NPLs which allows excitons to migrate between NPLs in the film. Despite this, light yield estimations based on a simulated distribution of excitation densities predict values upwards of 2000 ph/MeV, while showing ample room for improvement and the future potential of surpassing the 10 ph/MeV/ps benchmark.

Nonlinear transmission measurements of silicon are used to determine the two- and three-photon absorption coefficients for photon energies above the indirect band gap of Si in the range of 1.14-2.07 eV. By careful characterization of the temporal and spatial profiles of the ultrashort optical pulses, the nonlinear transmission as a function of peak optical intensity can be determined. Two- and three-photon absorption coefficients are acquired by fitting both an analytical and numerical model to the transmission traces, from which two distinct spectral regimes emerge with a clear threshold at 1.7 eV-this corresponds to half the direct band gap in Si. At photon energies above the threshold, the nonlinear transmission traces are dominated by two-photon absorption, whereas below the threshold three-photon absorption is dominant with a nondetectable contribution from indirect two-photon absorption. This result is at variance with the current understanding of above-band-gap nonlinear absorption in Si, which is exclusively attributed to two-photon absorption processes.

Transient reflectivity measurements are used to probe the strain waves induced by ultrashort laser pulses in bulk [100] germanium. The measurement signals are compared to purely analytical model functions based on the known material parameters for germanium. The modeling includes (i) a derivation of analytical solutions to the wave equation for strain waves coupled to the diffusion equation for heat and charge carriers and (ii) an expression for the impact on reflection coefficients that are caused by perturbations to the dielectric function but extended to cover a non-isotropic, uniaxial dielectric tensorial form. The model is held up against transient reflectivity measurements with an s- and a p-polarized probe and with a probe wavelength in the range of 502–710 nm. Excellent agreement is found when comparing the oscillatory shape of the measurement signals to the models. As for the magnitude of the oscillations, the models reproduce the overall trends of the experiment when using the previously published values for the elasto-optical tensor measured under static strain.

A new tissue-equivalent, reusable material for dosimetric verification of radiotherapy is developed and characterized. The material comprises a hydrogel matrix containing copper-doped lithium fluoride nanoparticles that exhibit optically stimulated luminescence (OSL). Using a laser light sheet and a sensitive camera equipped with appropriate filters, the OSL signal is read out layer by layer thus constructing the 3D dose distribution. The good tissue equivalence allows the new material to register the dose from a prescribed treatment plan as if delivered to a patient. Cubic dosimeters measuring 50 x 50 x 50 mm3 are developed and used to measure 3D dose distributions (from voxels measuring 0.8 x 0.8 x 1.0 mm3) with a statistical dose precision of 5% at 100 Gy dose levels. The spatial precision of offsets in dose gradients between planned and measured dose distributions is below the voxel size. A 3% / 3 mm gamma analysis between a prescribed treatment plan and the retrieved and corrected readout yields a pass rate of 89.5%. Light scattering within the dosimetric volume affects the dosimeter performance, indicating the potential for future improvements of spatial and dose precision. In conclusion, the dosimeter system shows high promise for future clinical validation of radiotherapy. This paper demonstrates 3D validation of radiotherapy using a novel material and readout method. The dose readout is based on optically stimulated luminescence from copper-doped lithium fluoride nanoparticles embedded in a hydrogel matrix. The material is reusable and shows good tissue equivalence, and the dose and spatial precisions show promise for future clinical use. image

BACKGROUND The continued development of new radiotherapy techniques requires dosimetry systems that satisfy increasingly rigorous requirements, such as high sensitivity, wide dose range, and high spatial resolution. An emerging requirement is the ability to read out doses in three dimensions (3D) with high precision and spatial resolution. A few dosimetry systems with 3D capabilities are available, but their application in a clinical workflow is limited for various reasons, primarily originating from their chemical nature. The search for a 3D dosimetry system with potential for clinical implementation is thus ongoing. PURPOSE To demonstrate the capabilities of a novel optically-stimulated-luminescence (OSL)-based 3D dosimetry system capable of measuring radiation doses in clinically relevant volumes. METHODS A laser-based readout system was used to measure dose distributions delivered by both photons and protons, utilizing the OSL from a 50 × 50 × 50 $50\times 50\times 50$  mm3 YSO:Ce crystal. A homogeneous treatment plan consisting of two opposing photon fields was used to establish an inhomogeneity correction map of the crystal response and demonstrated the accuracy and precision of the system. The crystal was additionally irradiated with a photon treatment plan consisting of three overlapping 10 × 10 mm2 fields delivered from different angles, and a proton treatment plan consisting of four pencil beams with energies 90 MeV (× 2), 115 MeV, and 140 MeV. The system abilities were quantified by comparing the 3D-resolved measurements to Monte Carlo simulations. RESULTS The dose map reproducibility of the system was found to be within 2% including both statistical and systematic errors. The measurements yielded integrated doses from a volume of 50 × 50 × 40 $50\times 50\times 40$  mm3 with voxel volumes of just 0.28 × 0.28 × 0.50 $0.28\times 0.28\times 0.50$  mm3 . An excellent agreement between the 3D-resolved measurements and the simulations was found for both photon- and proton-irradiation. CONCLUSIONS The capabilities of the devised system for measuring clinically relevant fields of photons and proton pencil beams within a clinically relevant volume were demonstrated. The system poses as a promising candidate for clinical applications, and enables future research in the field of OSL-based tissue-equivalent 3D dosimetry.

Charge-carrier transport is investigated theoretically in a high-band-gap dielectric material excited by intense ultrashort laser pulses. Simulations of fused silica reveal that the photo-Dember effect causes a 10(7) Vm(-1 )electric field to build up inside the material. It is shown that this electric field can induce a significant transient optical anisotropy in the dielectric, which may help reconcile earlier reports of an ultrafast onset of birefringence in highly excited fused silica. The time-dependent electric field will also lead to emission of a broadband THz pulse propagating out from and along the dielectric surface.

Reusable high-resolution 2D dosimeters have a great potential for radiotherapy verification and quality assurance, not least due to the reduced cost and simpler calibration procedures implied by reusability. We have performed proof-of-concept measurements in both photon and proton beams with a reusable 2D dosimeter (5×7 cm 2 ; 1 mm thickness) based on optically stimulated luminescence, using a readout setup scalable to 3D. Our data indicated a strong linear-energy-transfer quenching for this type of dosimeter.

Nanoparticles displaying optically stimulated luminescence embedded in a transparent polymer matrix have been proposed as a reusable high-spatial resolution 3D dosimeter. We measured the refractive indices of LiF:Cu nanoparticles and silicone and found a mismatch of 0.03-0.05 for the relevant wavelengths, explaining the transmission loss through 1 cm-sized nanocomposite dosimeters. We propose to bridge the refractive index gap by coating the LiF nanoparticles with a shell of SiO 2 . Initial studies show successful SiO 2 growth, although more work is required to produce core-shell nanoparticles with an optimized refractive index.

Integrating dosimeters are used in fields like personal dosimetry and radiotherapy quality assurance. Nanoparticles of LiBaF3 have been synthesized using a low-cost, high-throughput flow-synthesis setup and are found to be crystallographically phase-pure. Their radioluminescence (RL) emission shows three peaks corresponding to core-valence-band luminescence, self-trapped-exciton luminescence, and possibly impurity-related violet emission. Optically- and thermally stimulated luminescence (OSL and TL, respectively) overlap spectrally and show only the low-energy violet band, indicating a common luminescence pathway. The rapidly read-out OSL signal is linear in dose up to ∼1 kGy. The main TL glow peak, found around 120 ∘C, is depleted by optical stimulation, and similarly the OSL signal disappears following a TL readout. The thermal trap depth is found by the peak position method to be ∼1 eV. The high OSL light yield and reasonable fading of the nanoparticles qualifies them as an excellent candidate for OSL dosimetry.

LiF:Mg,Cu,P (MCP) is well-known for its high thermoluminescence (TL) sensitivity and is widely used for TL dosimetry. Following recent studies, highlighting its optically stimulated luminescence (OSL), the properties of MCP have been studied for OSL-based 2D and 3D dosimetry. Here, we have measured the spectrally resolved OSL emission as a function of stimulation time, identifying the peak of the emission band to be at ti 355 nm during the early stimulation period before shifting to peaking at ti 382 nm during the remaining part of the experiment. By assuming the spectra to originate from radiative recombination centers with Gaussian emission bands, we identified two recombination centers with emission centered at similar to 355 nm (RC1) and similar to 394 nm (RC2), respectively. From the OSL decay curves, we have identified two dominating types of OSL traps (fast and slowly decaying OSL traps). We provide indicative values for the photoexcitation cross section of these OSL traps. The fast-decaying OSL traps were found to predominantly emit OSL through RC1. By depleting the fast-decaying OSL traps before TL measurements, we have identified these traps to be associated with a previously undetected TL peak at similar to 87 degrees C (1 degrees C/s heating rate), located between the TL peaks usually denoted as peaks one and two. We further found that the features related to the fast-decaying OSL traps were not observable for powder samples without a heat treatment at similar to 560-700 degrees C. Pellet samples show these features without the heat treatment, but could potentially have undergone a similar treatment during sintering.

In this contribution, we extend the current understanding of energy-trapping mechanisms in wide-band-gap inorganic crystals via the formation and decay of the self-trapped exciton (STE) using pure and copper-doped lithium fluoride (LiF) nanoparticles. The study aims at filling in some of the knowledge gaps underpinning the search for enhanced dosimetric performance, enabling novel 3D imaging techniques based on transparent materials exhibiting intrinsic scintillation phenomena. Here, LiF stands out as the lightest and highest-band-gap dielectric. We use a combination of luminescence-spectroscopy techniques at low and elevated temperatures to confirm the role of an STE triplet state as the radiative recombination center driving ionizing-radiation-induced luminescence phenomena centered at 325 nm, namely radioluminescence and optically stimulated luminescence (OSL). We demonstrate that the role of copper doping as a luminescence enhancer is based on the trapping of the self-trapped hole at room temperature, which would otherwise become mobile allowing for hopping of electronic excitation in the lattice at temperatures above 150 K. Finally, we show results from optimization studies aiming at establishing the best synthesis parameters in terms of OSL yield and discuss in more detail the role of the nanoparticle surface, possible energy trapping, and recombination pathways.

Bandgap tunable quantum dots are very interesting for a variety of electronic applications, e.g. solar cells where the tunability of the bandgap can potentially be exploited to produce high-efficiency multi-junction solar cells. One interesting candidate from the group-IV semiconductors is Ge 1-x Sn x , where the bandgap is highly dependent on the Sn concentration. An increasing amount of Sn will lower the direct bandgap faster than the indirect bandgap, resulting in a direct bandgap material at approximately 8 % Sn in unstrained bulk Ge 1-x Sn x [1] –[3]. Combining this property of the alloy with the quantum confinement effect, which will increase the bandgap as the nanoparticles get smaller, will potentially allow for a very specific tunability of the bandgap if the composition and size can be controlled.

Defect states in nitrogen‐containing float‐zone silicon are investigated in both n‐ and p‐type materials using both deep‐level transient spectroscopy (DLTS) and minority‐carrier transient spectroscopy (MCTS). This enables a mapping of the defect landscape in the entire electronic bandgap and an investigation of whether the properties of the defects depend on the semiconductor type. Two defects, the E1/E2 pair and the E4/E6 pair, are investigated, and no evidence is found for the defect properties to depend on the semiconductor type.

The search for a reusable 3D dosimeter is ongoing and motivated by the impact it would have on development and verification of complex modalities in radiotherapy. We present a proof-of-concept 3D measurement of a proton-irradiated LYSO:Ce scintillator, using the resettable photon-emission mechanism known as optically stimulated luminescence and a novel optical readout system. Through this demonstration, we show that LYSO:Ce, in addition to being capable of real-time beam imaging, can be employed as a reusable post-irradiation 3D dosimeter with high spatial resolution.

Ultrafast dynamics in dielectric materials has now been investigated for three decades! The studies have continuously been motivated both by the quest to understand the material dynamics in still greater detail and by the various applications that were developed concurrently, e.g. for micro- and nanostructuring or local modification of the refractive index. The combination of detailed experimental studies and different modelling approaches has led to a general picture of the qualitative phenomena [1], and in some cases, models have even demonstrated a reasonable quantitative agreement with experiments [2], [3].

Radiotherapy is a well-established and important treatment for cancer tumors, and advanced technologies can deliver doses in complex three-dimensional geometries tailored to each patient's specific anatomy. A 3D dosimeter, based on optically stimulated luminescence (OSL), could provide a high accuracy and reusable tool for verifying such dose delivery. Nanoparticles of an OSL material embedded in a transparent matrix have previously been proposed as an inexpensive dosimeter, which can be read out using laser-based methods. Here, we show that Cu-doped LiF nanocubes (nano-LiF:Cu) are excellent candidates for 3D OSL dosimetry owing to their high sensitivity, dose linearity, and stability at ambient conditions. We demonstrate a scalable synthesis technique producing a material with the attractive properties of a single dosimetric trap and a single near-ultraviolet emission line well separated from visible-light stimulation sources. The observed transparency and light yield of silicone sheets with embedded nanocubes hold promise for future 3D OSL-based dosimetry.

n-type float-zone silicon grown in a nitrogen atmosphere contains defects which are activated by temperatures between 450 and 700 °C. We use deep level transient spectroscopy (DLTS) to study the nature of these defects and the impact of the nitrogen content and the polysilicon feed stock type. We find four dominant DLTS peaks with activation energies of Ena = 0.16 eV (E1), Ena = 0.21 eV (E2), Ena = 0.34 eV (E4), and Ena = 0.64 eV (E6). We tentatively assign the two DLTS peaks E1 and E2 to single acceptor and single donor levels of the same defect, a complex of nitrogen with an impurity. Furthermore, we tentatively assign the two DLTS peaks labeled E4 and E6 to two levels of the off-center substitutional nitrogen. Based on the apparent electron capture cross sections and an analysis of the electric field effect on the emission rates, we propose them to be double and single acceptor levels, respectively. Due to its position at midgap and the competing electron and hole emission, the apparent concentration of E6 is reduced to one fifth of the total defect concentration. Correcting for these processes, we find the activation energies for electron and hole emission to be En = 0.50 eV and Ep = 0.68 eV.

For 3D optically stimulated luminescence (OSL) based dosimeters to be clinically applicable, certain standards must be met. Among these are low detectable doses with high accuracy and precision, ideally comparable with those of point-like detectors. By investigating a model of the central part of an OSL readout-system, we present an estimate of ∼ 4 ⋅ 10 7 photons Gy ⋅ mm 3 as the minimum required signal from OSL active materials embedded in a transparent matrix to allow measuring doses of 0.1 Gy with an accuracy and precision of 2 %. Further, 2D spatially resolved measurements of OSL emission from commercially available LiF:Mg, Cu, P pellets are presented and discussed.

Advanced radiotherapy techniques, which plan and deliver a treatment in complicated 3D geometries with steep dose gradients, push 3D dosimetry with correspondingly high spatial resolution to the top of scientific and clinical agendas. This paper presents the first steps taken towards an inexpensive and reusable material for 3D dosimetry based on optically stimulated luminescence (OSL). Carbon-doped alumina (Al2O3:C) nanoparticles were synthesized using supercritical flow synthesis, in which product properties can be finely controlled. The particles were characterized using electron microscopy and powder x-ray diffraction. C-doping did not alter the crystallographic structure appreciably, and a high elemental signal from C could be measured. Nanoparticles of amorphous γ-Al2O3:C were achieved, however calcining these to produce the OSL-relevant α-phase yielded microparticles. Future work will aim at producing phase-pure α-Al2O3:C nanoparticles with a narrow size distribution below 10 nm, and controllable C-concentration and O-deficiency.