Long-lived defect states in large-band-gap materials can potentially store energy from ionising radiation allowing the materials to be used as dosimeters. The population in many of these dosimetric traps can be read out with optically stimulated luminescence (OSL). OSL is associated with a cross section for the excitation probability, and the wavelength dependence of the cross section provides insight into the physical characteristics of the traps and allows for an optimised readout of OSL-dosimeters. This paper suggests a novel approach to measuring the optical cross section using a pulsed tunable laser. A model of the evolution of the trap population is used to analyse data taken pulse-by-pulse to determine the cross sections in Y2SiO5:Ce. The cross sections are compared to theoretical models in order to determine the trap depth and phonon contribution. The wavelength-dependent cross section increases monotonically as the wavelength decreases, varying by more than two orders of magnitude from 10-24 at the longest wavelengths (800 nm) to more than 10-22 m2 in the green and blue part of the spectrum. Depending on the theoretical model used, the main trap depth is within the range 2.1-2.6 eV with a characteristic value of 2.3 eV, aligning with published results. Additionally, a notable phonon contribution is found with a magnitude that is consistent with previously reported phonon energies.

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.

Ultrathin solar cell devices based on amorphous silicon offer significant advantages in terms of cost and stability, provided they are effectively integrated with light-trapping strategies. However, integrating these devices with photonic nanotextures is challenging due to the high defect concentrations that may result from the deposition of ultrathin material layers on textured substrates. This study utilizes a cost-effective, scalable approach using quasiperiodic nanowrinkles as textured substrates for ultrathin amorphous silicon solar cells fabricated in ap-i-nconfiguration, with a 100 nm absorber layer. To enhance the performance on the nanowrinkles, a dualp-layer architecture, comprising a thin hydrogenated amorphous silicon protective layer combined with a nanocrystallinep-type layer is employed. These nanowrinkle solar cell devices show significant improvements, up to ∼33%, in power conversion efficiency compared to their flat substrate counterparts. The dualp-layer approach is effective in mitigating the adverse effect of defects, demonstrating a maximum of ∼33% increase in short-circuit photocurrent densities compared to single-p-layer configuration in the highest efficiency device. Simulation studies are conducted to analyze the electrical characteristics and charge transport phenomenon of the device layers, and the improved performance of the final device.

Commercially feasible self-assembly nanofabrication methods are used to create quasiperiodic nanotexture designs for thin-film solar cells to improve effective light-trapping. In this work, experimental and theoretical methods were used to study the structure, light-trapping capability, and device performance in amorphous silicon solar cells deposited on four distinct quasiperiodic nanowrinkle substrates. The best device showed 9.46 % power conversion efficiency with similar to 36 % improvement in short-circuit photocurrent density compared to its flat counterpart. Confocal micro-Raman spectroscopy was used to ascertain the degree of light-trapping obtainable from a particular nanotexture. Further, device modeling studies elucidate the effects of variations in layer properties arising from surface irregularities, showing that crowding nanofeatures leads to low fill factors from the increased defects, affecting the apparent doping. The correspondence obtained between the fill factor, autocorrelation length, and recombination rate helps to determine the quality of the material that grows on a particular nanowrinkle surface.

We investigate the nonlinear optical response of bulk ZnO under intense short-wave infrared excitation, focusing on the interplay between high-harmonic generation (HHG) and photoluminescence (PL). While HHG exhibits non-perturbative intensity scaling and a spectral blueshift consistent with plasma-induced refractive index changes, the PL signal shows a pronounced superlinear increase and a redshift, attributed to a combination of exciton-exciton scattering and phonon-assisted exciton recombination emission. A similar PL response under above-bandgap excitation supports its intrinsic origin. Spectral analysis of the HHG emission reveals an intensity-driven transition in the characteristics of the fifth harmonic, indicating a change in the underlying generation mechanism. These findings establish PL and spectral HHG analysis as complementary probes of strong-field and many-body effects in wide-bandgap semiconductors.

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.

The deposition of ultrathin amorphous silicon (a-Si:H) solar cells on textured substrates introduces challenges in maintaining optimal device performance due to defect-prone interfaces. This work aims at identifying and addressing the issues associated with the fabrication of ultrathin a-Si:H (absorber layer of 100 nm) solar cells on quasiperiodic textured surfaces. Batches of solar-cell devices were fabricated on four distinct designs of quasiperiodic nanowrinkle textures, and their optical and electrical responses were obtained experimentally. The best device reported similar to 8 % efficiency with nearly 15 mA cm(-2) short-circuit current density. Using the measured characteristics of their surface morphologies as a guide, mathematical models of the growth profile of device layers on the four nanowrinkle designs were formulated, which were integrated into the optical and electrical modeling of the batches of ultrathin devices. By examining the relation between the layer characteristics and the device performance of ultrathin solar-cell devices with two window-layer types, the study offers critical insights into the selection of optimal textures and strategies for device design. A device architecture with a nanocrystalline p-layer overlaying a thin protective amorphous p-layer offers minimal electrical losses and enhances power conversion efficiencies when deposited on the nanowrinkle textures.

The formation of dense plasmas inside dielectric materials by ultrashort laser pulses has many applications ranging from refractive-index modifications to the formation of channels and voids. Furthermore, such plasmas enable the fundamental investigation of ultrafast non-equilibrium dynamics in highly excited materials. The present paper provides an overview of current experimental approaches to investigating such plasmas. Much information about the plasma relaxation is obtained by measuring the spatial and temporal evolution of the dielectric properties of the excited material through time-resolved absorption and phase-shift measurements. In order to investigate and resolve the individual stages of plasma formation, experimental approaches with a temporal resolution beyond the capabilities of traditional optical pump-probe studies are required. Recent examples for schemes that may enable the investigation of the plasma formation with sub-cycle time resolution are thus reviewed. These include recent results from time-resolved high-harmonic generation as well as the two-color pump-probe analysis of non-perturbative low-order wave mixing for the tracking of strong-field excitation dynamics. Alternative approaches employ attosecond transient absorption spectroscopy, attosecond polarization spectroscopy and nonlinear photoconductive sampling for resolving the temporal evolution of the carrier dynamics down to sub-optical-cycle timescales. In solid dielectrics, the formation of an electron-hole plasma produced by irradiation with an intense, ultrashort electromagnetic field plays a central role in such diverse areas as high-harmonic generation, laser processing, and optical damage. In this paper, experimental techniques are reviewed for investigating the formation and relaxation of such a confined, microscopic plasma in bulk transparent solids. image

Accurate surface-dose measurements in proton therapy are challenging and often of poor spatial resolution for most dosimeter types. However, optically stimulated luminescence (OSL) based 2D dosimeters could provide the required spatial dose resolution. The aim of this study was to investigate the dosimetric precision and energy dependence of an in-house made reusable dosimeter composed of a silicone-film containing OSL-active nanoparticles. The dosimeter was irradiated with a clinical proton therapy field and the readout dose was compared to the results from a commercial dosimeter. The pixelwise noise-to-signal ratio for the OSL dosimeter remained below 2% for doses above 1 Gy, and the energy dependence was negligible in the investigated energy range.

Ultrashort-pulse-laser ablation of dielectric thin films is strongly affected by the interference of the exciting laser pulse with itself, which causes the deposited energy to be confined to narrow regions equidistantly spaced along the propagation direction of the laser. We investigate how this affects the ablation mechanism of freestanding Al_2O_3 thin films by analyzing the laser-generated structures with several post-mortem imaging and spectroscopic techniques. Close to the ablation threshold, the laser-irradiated region exhibits surface blistering. Higher intensities cause layers of material to be removed corresponding to ejection of material initiated from the regions of high excitation. Significantly above the ablation threshold, the laser-generated structure is remarkably stable and consists of a membrane of thickness corresponding to the distance between neighboring interference maxima, which is uniquely determined by the central wavelength of the laser pulse and the refractive index of the film. The electronic excitation as a function of depth is simulated using a multiple-rate-equation model in combination with finite-difference-time-domain propagation of the laser field. The simulations confirm the strongly localized excitation and thus correlate well with the observed laser-generated structures.

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

Fiducial markers are necessary for some tumor sites to ensure safe and accurate delivery of radiotherapy including proton therapy (PT). However, the high metal content of the markers may lead to shadowing of the proton beam. The aim of this study was to investigate the dose degradations around three fiducial marker types implanted in different configurations into a 3D dosimeter. The dosimeters were irradiated with a clinically relevant prostate cancer PT plan. Gamma comparisons (2%-2mm) between the control (no marker) and marker dosimeters resulted in a pass rate of 97%, and no significant differences were observed in the isodoses, indicating that markers did not affect dose coverage to the target. However, due to optical artifacts, the 3D dosimeter was unable to resolve the regions within a few millimeters from the markers.

Understanding the fundamentals of laser-matter interactions is crucial for developing and optimizing ultrafast laser processing strategies. In optically transparent solids, the key event by which energy is deposited in the material is through the generation of an electron–hole plasma via nonlinear excitation mechanisms. As the energy stored in the plasma relaxes, local distortions of the lattice may occur, such as point defects. These defects give rise to new discrete energy states located in the bandgap. In this study, we investigate how the presence of these energy states influences the transmission of ultrashort near-infrared laser pulses in fused silica. Experimental results of laser pulse transmission and photoluminescence from defects are correlated with optical microscopy of the irradiated spots, allowing us to identify different nonlinear interaction regimes. Numerical simulations indicate that photo-induced defects influence the nonlinear losses of ultrashort laser pulses and explain why a non-destructive damage regime with detectable excitation is only observed for a narrow intensity range in multipulse experiments.

Background and purpose:The impact of intrafractional motion and deformations on clinical radiotherapy delivery has so far only been investigated by simulations as well as point and planar dose measurements. The aim of this study was to combine anthropomorphic 3D dosimetry with a deformable abdominal phantom to measure the influence of intra-fractional motion and gating in photon radiotherapy and evaluate the applicability in proton therapy. Material and methods:An abdominal phantom was modified to hold a deformable anthropomorphic 3D dosimeter shaped as a human liver. A liver-specific photon radiotherapy and a proton pencil beam scanning therapy plan were delivered to the phantom without motion as well as with 12 mm sinusoidal motion while using either no respiratory gating or respiratory gating. Results:Using the stationary irradiation as reference the local 3 %/2 mm 3D gamma index pass rate of the motion experiments in the planning target volume (PTV) was above 97 % (photon) and 78 % (proton) with gating whereas it was below 74 % (photon) and 45 % (proton) without gating. Conclusions:For the first time a high-resolution deformable anthropomorphic 3D dosimeter embedded in a deformable abdominal phantom was applied for experimental validation of both photon and proton treatments of targets exhibiting respiratory motion. It was experimentally shown that gating improves dose coverage and the geometrical accuracy for both photon radiotherapy and proton therapy.

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.

The dose response of materials used in radiochromic three-dimensional (3D) dosimeters is often characterised via spectrophotometry of small cuvette samples as it is less time-consuming and cheaper. However, spatially-resolved dose measurements for clinical purposes are obtained using 3D optical computed tomography (CT). Hence, the link between the method of characterisation and the method for extracting 3D dose information must be established. The aim of this study was to investigate if spectrophotometry- based dose-response characterisation using cuvette-sized samples is valid for calibration of 3D optical CT readout systems using a silicone-based radiochromic dosimeter. Additionally, the uniformity of 3D optical CT dose-response readout of larger samples was investigated. We found the dose response of cuvette-sized samples read out by spectrophotometry to agree with the averaged dose response across cuvette-sized samples read out by optical CT and with the response near the edges of larger samples scanned using the same apparatus. Thus, cuvette-based dose-response spectrophotometry can be used to calibrate 3D optical CT readout using silicone-based radiochromic dosimeters. However, caution should be taken when considering larger samples as they exhibit a gradual radial increase in dose response from the centre and outwards.

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.