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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.

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.

In this Letter, an efficient tunable upconversion module is demonstrated and characterized. The module combines high conversion efficiency and low noise with broad continuous tuning, covering the spectroscopically important range from 1.9 to 5.5 µm. A fully computer-controlled, compact, portable system is presented and characterized in terms of efficiency, spectral coverage, and bandwidth, using simple globar illumination. The upconverted signal is in the 700–900 nm range, ideal for Si-based detection systems. The output from the upconversion module is fiber coupled, enabling flexible connection to commercial NIR detectors or NIR spectrometers. In order to cover the spectral range of interest using periodically poled LiNbO3 as the nonlinear material, poling periods ranging from 15 to 23.5 µm are needed. The full spectral coverage is reached using a stack of four fanned poled crystals, enabling maximal upconversion efficiency of any spectral signature of interest in the 1.9 to 5.5 µm range.

Journal of luminescence 251, 119237 (2022). doi:10.1016/j.jlumin.2022.119237, Published by Elsevier, New York, NY [u.a.]

Degradation of perovskite solar cells (PSCs) is often found to be partially or fully reversible when the cells are allowed to recover in the dark. Unlike the dynamics of degradation, knowledge about the dynamics of PSC cell recovery is very limited. Here, we demonstrate that the PSC recovery strongly depends on the electrical bias conditions during the light-induced degradation and that it can be manipulated by applying an external electrical bias during the recovery phase. Investigation of the recovery dynamics allows us to analyze the degradation mechanisms in detail. More specifically, we aged a mixed-cation mixed-halide PSC with a n-i-p structure under illumination in open-circuit (OC) or short-circuit (SC) conditions, and periodically measured their characteristics during the recovery. PSCs aged in SC degrade faster and fully recover after the light is switched off, while the performance of the cells aged in OC does not recover but instead further decreases after the light is switched off (“drop-in-dark” effect). With the use of transient photoluminescence, secondary ion mass spectrometry, and drift-diffusion-based simulations, we hypothesize that extrinsic ion migration causes the drop-in-dark effect, by forming an electron extraction barrier at the metal oxide electron transport layer. The applied bias alleviates this effect. Our results are relevant for gaining a deeper understanding of the multiple degradation mechanisms present in perovskite solar cells, and for finding a practical way to assist their recovery.

The introduction of radiotherapy beam delivery with high spatial modulation, including spot-scanning proton therapy, calls for development of 3D dosimeters for patient-specific treatment verifications. Inherently reusable methods like optically stimulated luminescence (OSL) pose a potential way to introduce a clinically viable 3D-dosimetry solution. Here we present a characterization of the OSL properties of the well-known thermoluminescence material LiF:Mg,Cu,P (MCP). The characterization was performed by monitoring the wavelength-dispersed emission of continuous-wave stimulated MCP pellets. We show that OSL emission is observed under blue (460 nm) and green (532 nm) laser stimulation, while no OSL signal is seen in MCP under red (664 nm) laser stimulation. Long-wavelength fluorescence including accumulating radiophotoluminescence is very intense under blue stimulation and must be suppressed for continuous use of a future 3D-dosimetry system. An evaluation of the number of emitted photons is used to estimate a measurable dose range of 0.1–10 Gy, which prompts further research in the use of MCP for 3D OSL-based dosimetry.

Organic photovoltaics (OPVs) has recently reached power conversion efficiencies of 17.3%, making it a green technology that not only offers short energy payback times and diverse photovoltaic integration schemes, but also can deliver competitive power outputs. OPVs typically employs electron selective contact layers made from low work function n-type metal oxide semiconductors, such as titanium oxide (TiO2) or zinc oxide (ZnO), developed from a variety of deposition techniques. However, in the case of TiO2 interlayers, the appearance of unwanted s-shape characteristics has been reported extensively in the literature in the past, for a variety of different deposition methods used. It has been shown that the s-shape arises from negatively charged chemisorbed oxygen, and that it can be deactivated by UV light illumination, which, however, is hardly compatible with real-life OPV application. In this work, we introduce sputtered crystalline titanium oxide layers as efficient s-shape-free electron selective extraction layers in organic solar-cell devices. We demonstrate that the onset of crystallization takes place at substrate growth temperatures of approximately 100 °C for the TiOx thin films, and that the crystallization onset temperature correlates well with a strong increase in device performance and the removal of any s-shape characteristics. Optical, structural, compositional, and electronic energy-level characterizations of the TiOx layers are shown in the present work, and point to the formation of an oxide with a low surface-defect density, developed from the sputter-crystallization process. Importantly, well-functioning s-shape-free PTB7:PC70BM devices are demonstrated for TiOx growth temperatures of 155 °C.

We present an experimental setup capable of time-resolved photoluminescence spectroscopy for photon energies in the range of 0.51 to 0.56 eV with an instrument time response of 75 ps. The detection system is based on optical parametric three-wave mixing, operates at room temperature, has spectral resolving power, and is shown to be well suited for investigating dynamical processes in germanium-tin alloys. In particular, the carrier lifetime of a direct-bandgap Ge 1 − x Sn x film with concentration x = 12.5 % and biaxial strain − 0.55 % is determined to be 217 ± 15 ps at a temperature of 20 K. A room-temperature investigation indicates that the variation in this lifetime with temperature is very modest. The characteristics of the photoluminescence as a function of pump fluence are discussed.

Upconversion of sunlight with energy below the band gap of a solar cell is a promising technique for enhancing the cell efficiency, simply by utilizing a larger part of the solar spectrum. The present topical review addresses this concept and discusses the material properties needed for an efficient upconversion process with focus on both silicon and organic solar cells. To design efficient upconverters, insight into topics such as quantum-optics, nano-optics, numerical modeling, optimization, material fabrication, and material characterization is paramount, and the necessary concepts are introduced throughout the review. Upconversion modeling is done using rate equations, while optical modeling is done by solving Maxwell's equations using the finite element method. Topology optimization is introduced and used to generate geometries of gold nanoparticles capable of greatly enhancing the upconversion yield. Fabrication and experimental characterization methods are discussed. Some recent results are presented and finally the possibility of designing upconverting materials capable of increasing the short-circuit current in a solar cell is discussed.

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.

With the introduction of highly conformal treatment modalities, dose verification in 3D is becoming more important than ever for patient-specific quality assurance of radiotherapy. Reusability of 3D dosimeters may be the path to cope with the cost-benefit issues caused by batch-to-batch fluctuations and intense calibration protocols in existing 3D systems. We present the idea of an envisioned (optically stimulated luminescence) OSL-based 3D readout system, which exploits the inherently reusable dosimetry properties of OSL. We provide the emission spectra of the OSL active material LiF:Mg, Cu, P (MCP) for three stimulation wavelengths (460 nm, 532 nm, and 664 nm), and summarize recently published optical characterization results to highlight the requirements of a readout system for an MCP-based dosimeter.

A comparative study is performed of the properties of SiC films, grown in the presence and absence of a Si1-xGex interfacial layer on top of a Si(111) substrate with x = 0.23. The SiC film growth is carried out at 960 degrees C by carbonization of the substrates with a solid carbon source using molecular beam epitaxy, leading to void formation at the SiC/substrate interface. The film properties have been investigated by transmission electron microscopy, x-ray diffraction, Rutherford backscattering spectrometry, and atomic force microscopy. The results show that the presence of the Si1-xGex layer improves the crystal quality and surface smoothness.

Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10 −3 Ω cm 2. Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.

Unfortunately, the first author name was incorrectly published in the original publication.

Tin-containing Group IV alloys show great promise for a number of next-generation CMOS-compatible devices. Not least of those are optoelectronic devices such as lasers and light-emitting diodes. To obtain reliable operation, a high control over the doping in such materials is needed at all stages of device processing. In this paper, we report tin-based donors in silicon, which appear after heat treatment of a silicon-tin alloy at temperatures between 650 °C and 900 °C. Two stages of the donor are observed, called SD I and SD II, which are formed subsequently. A broad long-lifetime infrared photoluminescence is also observed during the first stages of donor formation. We discuss evolving tin clusters as the origin of both the observed donors and the photoluminescence, in analogy to the oxygen-based thermal donors in silicon and germanium.Tin-containing Group IV alloys show great promise for a number of next-generation CMOS-compatible devices. Not least of those are optoelectronic devices such as lasers and light-emitting diodes. To obtain reliable operation, a high control over the doping in such materials is needed at all stages of device processing. In this paper, we report tin-based donors in silicon, which appear after heat treatment of a silicon-tin alloy at temperatures between 650 °C and 900 °C. Two stages of the donor are observed, called SD I and SD II, which are formed subsequently. A broad long-lifetime infrared photoluminescence is also observed during the first stages of donor formation. We discuss evolving tin clusters as the origin of both the observed donors and the photoluminescence, in analogy to the oxygen-based thermal donors in silicon and germanium.

Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500 to 980 nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO$_2$ thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500 nm for both s- and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913(51) is observed at an excitation intensity of 1.7 Wcm$^{-2}$. We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion., Comment: 9 pages (main text), 4 figures, 1 supporting information of 15 pages

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er3+-doped NaYF4 core as a function of the thickness of a NaLuF4 shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er3+ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCN...

Thin SiC films, grown on Si by substitution of C into Si on Si substrates with and without a SiGe buffer layer, have been investigated with optical techniques. The formation of SiC domains leads to strong green and blue photoluminescence from stacking faults and surface oxides. Introduction of a 10-nm-thick SiGe buffer layer leads to improved crystallinity as evidenced by X-ray diffraction and optical second-harmonic generation (SHG). Nonlinear optical azimuthal rotational spectra demonstrate the presence of cubic SiC in the film. Furthermore, angle-of-incidence scans are consistent with simulations based on a film with cubic symmetry which demonstrates that the cubic phase dominates the SiC film. Growth on vicinal Si(111) leads to a SiC film with the same c1v symmetry as the substrate, demonstrating that the lattice planes of the SiC film follow those of the Si substrate. Spatially resolved SHG scans show structures that are related to the underlying structure of the Si interface resulting from the growth process.

This paper describes a systematic design study of periodic gold-nanostrip arrays placed on a thin film aimed at enhancing the electric field inside the film when irradiated by light. Based on the study, a "selection rule" is proposed, which provides optimization-based design methods with an a priori choice between field-enhancement dominated by coupling to guided modes, by plasmonic near-field enhancement or by a mix hereof. An appropriate choice of wavelength and grating period is shown to selectively suppress or include waveguiding effects for the optimized designs. The validity of the selection rule is demonstrated through a numerical topology optimization study in which gold nanostrips are optimized for electric-field enhancement in an erbium-doped TiO2 thin film, targeting increased spectral upconversion in the erbium ions. The obtained designs exhibit waveguide excitation within the predicted intervals and, for light polarized perpendicularly to the strips, plasmonic response outside.

Spectral conversion of the sunlight has been proposed as a method for enhancing the efficiency of photovoltaicvdevices, which are limited in current production by the mismatch between the solar spectrum and the wavelength range for efficient carrier generation. For example, the photo current can be increased by conversion of two low-energy photons (below the band gap of the absorber) to one higher-energy photon (i.e. upconversion). In this paper, we will review our ongoing activities aimed at enhancing such spectral-conversion processes by employing appropriately designed plasmonic nanoparticles. The nanoparticles serve as light-concentrating elements in order to enhance the non-linear upconversion process. From the theoretical side, we approach the optimization of nanoparticles by finite-element modelling of the plasmonic near fields in combination with topological optimization of the particle geometries. Experimentally, the nanostructures are formed by electronbeam lithography on thin films of Er3+-containing transparent materials, foremost TiO2 made by radio-frequency magnetron sputtering, and layers of chemically synthesized NaYF4 nanoparticles. The properties of theupconverter are measured using a variety of optical methods, including time-resolved luminescence spectroscopy on erbium transitions and spectrally resolved upconversion-yield measurements at ∼1500-nm-light excitation.The calculated near-field enhancements are validated using a technique of near-field-enhanced ablation by tunable, ultrashort laser pulses.

Using monochromated electron energy-loss spectroscopy (EELS), we are able to map different allotropes in Sn-nanocrystals embedded in Si. It is demonstrated that α-Sn and β-Sn, as well as an interface related plasmon, can be distinguished in embedded Sn-nanostructures. The EELS data is interpreted by standard non-negative matrix factorization followed by a manual Lorentzian decomposition. The decomposition allows for a more physical understanding of the EELS mapping without reducing the level of information. Extending the analysis from a reference system to smaller nanocrystals demonstrates that allotrope determination in nanoscale systems down below 5 nm is possible. Such local information proves the use of monochromated EELS mapping as a powerful technique to study nanoscale systems. This possibility enables investigation of small nanostructures that cannot be investigated through other means, allowing for a better understanding and thus leading to realizations that can result in nanomaterials with improved properties.

Quantitative measurements of the electric near-field distribution of star-shaped gold nanoparticles have been performed by femtosecond laser ablation. Measurements were carried out on and off the plasmon resonance. A detailed comparison with numerical simulations of the electric fields is presented. Semi-quantitative agreement is found, with slight systematic differences between experimentally observed and simulated near-field patterns close to strong electric-field gradients. The deviations are attributed to carrier transport preceding ablation.

We propose a simplified rate-equation model for the 1500 nm to 980 nm upconversion in Er$^{3+}$. The simplifications, based on typical experimental conditions as well as on conclusions based on previously published more advanced models, enable an analytical solution of the rate equations, which reproduces known properties of upconversion. We have compared the model predictions with intensity-dependent measurements on four samples with different optical properties, such as upconversion-luminescence yield and the characteristic lifetime of the $^4I_{13/2}$ state. The saturation of the upconversion is in all cases well-described by the model over several orders of magnitude in excitation intensities. Finally, the model provides a new measure for the quality of upconverter systems based on Er$^{3+}$ -- the saturation intensity. This parameter provides valuable information on upconversion parameters such as the rates of energy-transfer upconversion and cross-relaxation. In the present investigation, we used the saturation intensity to conclude that the differences in upconversion performance of the investigated samples are mainly due to differences in the non-radiative relaxation rates., Comment: 8 pages, 6 figures, to be submitted to PRB

The short-circuit behavior of power devices is highly relevant for converter design and fault protection. In this paper, the degradation during short circuit of a 10-kV 10-A 4H-SiC MOSFET is investigated at 6 kV dc-link voltage. The study aims to present the behavior of the device during short-circuit transients as it sustains increasing short-circuit pulses during its lifetime. As the short-circuit pulse length increases, degradation of the device can be observed in periodically performed characterizations. The initial degradation seems to be associated with the channel region, and continuous stressing leads to an overall increase in device on-state resistance at the end of the degradation study. Thermal simulation shows that the surface aluminum metalization reached its melting temperature and the top part of the device reaches temperatures above the rated junction temperature. Scanning electron microscope investigation shows aluminum reconstruction and cavities at the contact interface between the aluminum surface metalization and source contacts.

A two-particle model is proposed which enables the assessment of particle-particle interactions in large, sparse arrays of randomly distributed plasmonic metal nanoparticles of arbitrary geometry in inhomogeneous environments. The two-particle model predicts experimentally observed peak splittings in the extinction cross section spectrum for randomly distributed gold nanocones on a TiO2:Er3+ thin film with average center-to-center spacings of 3-5 diameters. The main physical mechanism responsible is found to be interference between the incident field and the far-field component of the single-particle scattered field which is guided along the film.

The use of interfacial layers in organic solar cells has been investigated intensively over the past years, as it has a strong impact on both the power conversion efficiency and stability of the devices. Among the systems investigated are for example alkali salts, ionic liquids, neutral polymers, conjugated polyelectrolytes and various metal-oxides, which have been demonstrated to result in promising device characteristics1. In this context, n-type metal-oxides such as titanium oxide (TiOx) have drawn a lot of attention as electron-transport and exciton-blocking layers, mainly due to its high optical transparency and favorable energy-level alignment with many commonly used electron-acceptor materials. There are several methods available for fabricating compact TiOx thin-films for use in organic solar cells, including sol-gel solution processing, spray pyrolysis and atomic-layer deposition; however, these methods either require sintering at high temperatures, or result in the existence of trapped sites in the films, which lead to the presence of the well-known s-shape in the J-V curves of the devices. In order to remove the observed s-shape, and thus improve the fill factor of the cells, light soaking is typically performed, which however brings along several unwanted side-effects in terms of energy losses, material degradation, batch-to-batch variations, etc.2In this work, we demonstrate the development of TiOx based light-soaking-free inverted polymer solar cells with power conversion efficiency of around 7%, by incorporating sputter deposited TiOx thin-films as electron-transport and exciton-blocking layers. In the work, we report on the effect of different TiOx deposition temperatures and thicknesses on the organic-solar-cell device performance. Besides optical characterization, AFM and XRD analyses are performed to characterize the morphology and crystal structure of the films, and external quantum efficiency measurements are employed to shed further light on the device performance. Our study presents a novel method for implementation of TiOx thin-films as electron-transport layer in organic solar cells that eliminates the need for light soaking and still allows for integration on flexible plastic substrates, which is beneficial for roll-to-roll mass production of flexible organic solar cells. 1. Steim, R.; Kogler, F. R.; Brabec, C. J., Interface materials for organic solar cells. Journal of Materials Chemistry 2010, 20 (13), 2499-2512.2. Kim, J.; Kim, G.; Choi, Y.; Lee, J.; Park, S. H.; Lee, K., Light-soaking issue in polymer solar cells: Photoinduced energy level alignment at the sol-gel processed metal oxide and indium tin oxide interface. Journal of Applied Physics 2012, 111 (11), 114511.

The large energy-density enhancements, associated with the near-field of plasmonic metal nanoparticles (MNPs), can potentially be utilized to increase the efficiency of nonlinear processes such as upconversion (UC). A drawback of employing metallic structures for UC applications is luminescence quenching, i.e. the transfer of energy from the upconverter material to the metal, where it is dissipated as heat. In this study, a rate-equation model is applied to study the interplay between near-field enhancement and luminescence quenching for a range of different geometries. It is found that while shapes that incorporate pointy features and/or narrow gaps support stronger near-field enhancements, they also suffer more severely from luminescence quenching. Due to the strong correlation between the two effects, the predicted enhancement in UC luminescence is similar across all considered geometries ranging from 1 to 3. Our results indicate that the near-field of plasmonic MNPs might not be suitable for increasing UC efficiency.

The present work reports on a distinct and very reproducible bistable-like behavior of two defects at around E C - 0.5 eV in MOCVD-grown GaN. The kinetics of the thermally activated transformation between the two states are analyzed in an Arrhenius model, yielding an energy barrier of 0.4 ± 0.1 eV, and a frequency factor of 10 6±1 s -1 . Depth profiles suggest that the charge state of the defects determines the observed amplitude variation. Relevant models for the observed behavior, and their shortcomings are discussed: (i) passivating properties of hydrogen, and (ii) bistable defect component(s). A proper explanation of the experimental observations represents, however, a further challenge.

International audience; This work investigates light emission from sili- con with embedded Sn nanocrystals. The composition of the nanocrystals is determined to be pure Sn by atom probe tomog- raphy, and the light emission is strongest when the nanocrystal structure is closest to the host Si lattice diamond structure.

Upconversion enhancement of 1500 nm light has been achieved using geometrically optimized Au nano-discs placed in close vicinity of Er+3 ions in a TiO2 matrix. A finite element modeling is compared to the experimental findings.

This work presents a study of the luminescence properties of Er3+ when included into two different TiO2 hosts: a polycrystalline and an amorphous host. The two host environments were produced by depositing two thin films with different Er3+ concentration using radio-frequency magnetron sputtering. Structural analysis revealed the presence of the rutile and anatase phases in the polycrystalline film. Time-resolved and steady-state photoluminescence measurements evidenced the presence of two distinct local Er3+ environments in the polycrystalline host. For the amorphous TiO2 host, only one Er3+ environment was observed, which differed from the two environments in the polycrystalline host. A method for extracting a fast and slow time-resolved emission spectrum from the two observed local environments in the polycrystalline host is also presented. (C) 2016 Optical Society of America

Up-conversion luminescence of rare-earth ion-doped materials has been demonstrated in the TiO2:Er3+ system when Au nanoparticles are in close vicinity to the Er3+ ions. From the combination of photoluminescence and photospectro-metry studies it is concluded that the up-conversion is enhanced as a result of the excitation of localized surface plamon resonances in the Au nanoparticles. Indications are found that the dominant source of the enhancement is a resonant local-field enhancement of the excitation process.

The optical properties of radio-frequency magnetron sputtered Ti O 2 thin films doped with E r 3 + are strongly influenced by the deposition and post-annealing temperatures. This has an impact on the applications of the material for upconversion, i.e., the merging of two low-energy photons to one photon of higher energy. Maximum upconversion luminescence (UCL) yield is obtained using a deposition temperature of 350 °C without post-annealing. Motivated by the possibilities that become available by sequential depositions (several layers), the effect of post-annealing is systematically investigated. In general, post-annealing treatments reduce the UCL; however, for the lowest deposition temperatures, post-annealing has a positive impact on the UCL provided that the samples are not exposed to ambient air prior to the annealing step. These observations are further analyzed using time-resolved photoluminescence spectroscopy for determining the characteristic decay times of the E r 3 + energy levels in the different samples. It is found that the UCL yield scales to a good approximation linearly with the product of the decay times of the two lowest-lying E r 3 + excited energy levels ( 4 I 11 / 2 and 4 I 13 / 2). The combined data provide strong evidence that the reduction in UCL is due to the opening of non-radiative decay channels from the E r 3 + excited levels. Structural measurements show no change of the amorphous samples upon annealing, so these decay channels are most likely related to energy transfer between E r 3 + and defect states in the Ti O 2 bandgap. The non-radiative decay could possibly be related to the loss of hydrogen termination of dangling bonds or related to the oxygen vacancies in Ti O 2.The optical properties of radio-frequency magnetron sputtered Ti O 2 thin films doped with E r 3 + are strongly influenced by the deposition and post-annealing temperatures. This has an impact on the applications of the material for upconversion, i.e., the merging of two low-energy photons to one photon of higher energy. Maximum upconversion luminescence (UCL) yield is obtained using a deposition temperature of 350 °C without post-annealing. Motivated by the possibilities that become available by sequential depositions (several layers), the effect of post-annealing is systematically investigated. In general, post-annealing treatments reduce the UCL; however, for the lowest deposition temperatures, post-annealing has a positive impact on the UCL provided that the samples are not exposed to ambient air prior to the annealing step. These observations are further analyzed using time-resolved photoluminescence spectroscopy for determining the characteristic decay times of the E r 3 + energy l...

A setup for time-resolved photoluminescence spectroscopy, based on parametric three-wave mixing in a periodically poled lithium niobate crystal, is characterized. Special attention is given to adjusting the phase matching condition by angle tuning of the luminescent light relative to a strong, continuous-wave laser beam within the crystal. The detection system is capable of operating at room temperature and in a wavelength range from 1.55 to 2.20 mu m. Its sensitivity is compared to a commercial photomultiplier, and its capability of nanosecond time resolution is demonstrated. (C) 2018 Optical Society of America

Tin nanocrystals embedded in a SiSn layer grown by molecular beam epitaxy on n-type Si are investigated by means of deep level transient spectroscopy. Two Sn related deep traps are observed, depending on the annealing temperature of the samples. A deep level at [Formula: see text] (Sn1) is observed for annealing temperatures up to [Formula: see text]C, whereas a level at [Formula: see text] (Sn2) appears for annealing temperatures above [Formula: see text]C. Scanning transmission electron microscopy shows the formation of Sn nanocrystals at [Formula: see text]C, which coincides with the appearance of Sn2. Sn1 is tentatively assigned to a Sn related precursor defect, which transforms upon annealing into either Sn nanocrystals or an interface defect located at the nanocrystal surface.

Correlations in photodetection signals from quantum light sources are conventionally calculated by application of the source master equation and the quantum regression theorem. In this article we show how the conditioned dynamics, associated with the quantum theory of measurements, allows calculations and offers interpretations of the behaviour of the same quantities. Our theory is illustrated for photon counting and field-amplitude measurements, and we show, in particular, how transient correlations between field-amplitude measurements and later photon counting events can be accounted for by a recently developed theory of past quantum states of a monitored quantum system., Comment: 15 pages, 5 illustrations

A quantum system can be monitored through repeated interactions with meter systems. The state of the system at time $t$, represented by the density matrix $\ensuremath{\rho}(t)$, then becomes conditioned on the information obtained by the meters until that time. More insight in the state of the system at any time $t$ is provided, however, by taking into account the full detection of all meters interacting with the system both in the past and in the future of $t$. We present experiments that use near-resonant atomic probes to monitor the evolution of the quantized field in a cavity. The application of the forward-backward smoothing method to this quantum problem, justified by the past quantum state formalism [S. Gammelmark et al., Phys. Rev. Lett. 111, 160401 (2013)], allows us to resolve a posteriori dynamics, which is not reflected by the usual quantum state $\ensuremath{\rho}(t)$.

We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of $10^{10}$ NV centers in a diamond crystal coupled to a superconducting LC resonator. The energy of the signal, retrieved $100\, \mu \mathrm{s}$ later by spin-echo techniques, reaches $0.3\%$ of the energy absorbed by the spins, and this storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits., Comment: 6 pages

We report pulsed electron-spin resonance (ESR) measurements on an ensemble of Bismuth donors in Silicon cooled at 10mK in a dilution refrigerator. Using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance., Main text : 10 pages, 4 figures. Supplementary text : 16 pages, 8 figures

This article reviews efforts to build a new type of quantum device, which combines an ensemble of electronic spins with long coherence times, and a small-scale superconducting quantum processor. The goal is to store over long times arbitrary qubit states in orthogonal collective modes of the spin-ensemble, and to retrieve them on-demand. We first present the protocol devised for such a multi-mode quantum memory. We then describe a series of experimental results using NV center spins in diamond, which demonstrate its main building blocks: the transfer of arbitrary quantum states from a qubit into the spin ensemble, and the multi-mode retrieval of classical microwave pulses down to the single-photon level with a Hahn-echo like sequence. A reset of the spin memory is implemented in-between two successive sequences using optical repumping of the spins., Comment: 27 pages, 7 figures

Ohmic contact formation to n-type GaN often involves high temperature steps, for example sintering at about 800 °C in the case of Ti-based contacts. Such processing steps might cause changes in the distribution, concentration, and properties of the defects. The present work aims at contributing to the knowledge about defect evolution in GaN upon processing at different temperatures. The processing temperatures are selected according to fabrication procedures for commonly used ohmic contacts to n-GaN: 300 °C (In-based), 550 °C (Ta-based), and 800 °C (Ti-based). Properties and concentration of the defects are studied by the means of deep level transient spectroscopy (DLTS). Changes in carrier capture kinetics are monitored with varying filling pulse duration.