High-resolution Optical Absorption Research Articles

Crystal Phase, Electronic Structure, and Surface Band Bending of (InxGa1–x)2O3 Alloy Wide-Band-Gap Semiconductors

Ga2O3 is emerging as a promising wide-band-gap semiconductor for high-power electronics and deep ultraviolet optoelectronics. Alloying Ga2O3 with In2O3 leads to the modulation of the band gaps and possible carrier confinement achieved at the heterointerface. In this work, we report a systematic study on the crystallographic phase, electronic structure, and surface band bending of (InxGa1–x)2O3 thin films over the whole composition range of 0 ≤ x ≤ 1 grown on α-Al2O3 (0001) substrates by pulsed laser deposition. It was found that with In content x < 0.2, a monoclinic β-phase of an (InxGa1–x)2O3 film is epitaxially grown on Al2O3 (0001), while a bixbyite phase of the (InxGa1–x)2O3 film grows when x ≥ 0.8. When 0.2 ≤ x < 0.8, a mixed β-phase and bixbyite phase coexist. The evolution of electronic structures of the (InxGa1–x)2O3 films was examined by high-resolution X-ray photoemission spectroscopy (XPS) and optical absorption spectroscopy. For the β-phase films, the optical band gaps decrease from 4.96 eV for Ga2O3 to 4.43 eV for x = 0.4, while for bixbyite (InxGa1–x)2O3, the optical band gaps slightly increase from 3.57 eV for In2O3 to 3.70 eV for x = 0.8. Detailed electronic structure study indicates that the decrease of band gaps of (InxGa1–x)2O3 with an increase of In content mainly results from the upper movement of the valence band edge because the shallow In 4d orbitals introduce a hybridized state with O 2p at the top of the valence band. In addition, the presence of surface electron accumulation (i.e., downward band bending) was identified at the surface region of the (InxGa1–x)2O3 films with x ≥ 0.6, which would provide an opportunity to modulate the surface electronic properties for device applications.

Comparison of formaldehyde measurements by Hantzsch, CRDS and DOAS in the SAPHIR chamber

Abstract. Three instruments that use different techniques to measure gaseous formaldehyde (HCHO) concentrations were compared in experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. One instrument (AL4021, Aero-Laser GmbH) detects HCHO using the wet-chemical Hantzsch reaction (for efficient gas-phase stripping), chemical conversion and fluorescence measurement. An internal HCHO permeation source allows for daily calibrations. This instrument was characterized by sulfuric acid titration (overall accuracy 8.6 %) and yields measurements with a time resolution of 90 s and a limit of detection (3σ) of 0.3 ppbv. In addition, a new commercial instrument that makes use of cavity ring-down spectroscopy (CRDS) determined the concentrations of HCHO, water vapour, and methane (G2307, Picarro, Inc.). Its limit of detection (3σ) is specified as 0.3 ppbv for an integration time of 300 s, and its accuracy is limited by the drift of the zero signal (manufacturer specification 1.5 ppbv). A custom-built high-resolution laser differential optical absorption spectroscopy (DOAS) instrument provided HCHO measurements with a limit of detection (3σ) of 0.9 ppbv and an accuracy of 7 %​​​​​​​ using an optical multiple reflection cell. The measurements were conducted from June to December 2019 in experiments in which either ambient air flowed through the chamber or the photochemical degradation of organic compounds in synthetic air was investigated. Measured HCHO concentrations were up to 8 ppbv. Various mixtures of organic compounds, water vapour, nitrogen oxides and ozone were present in these experiments. Results demonstrate the need to correct the baseline in measurements performed by the Hantzsch instrument to compensate for drifting background signals. Corrections were equivalent to HCHO mixing ratios in the range of 0.5–1.5 ppbv. The baseline of the CRDS instrument showed a linear dependence on the water vapour mixing ratio with a slope of (-11.20±1.60) ppbv %−1 below and (-0.72±0.08) ppbv %−1 above a water vapour mixing ratio of 0.2 %. In addition, the intercepts of these linear relationships drifted within the specification of the instrument (1.5 ppbv) over time but appeared to be equal for all water mixing ratios. Regular zero measurements are needed to account for the changes in the instrument zero. After correcting for the baselines of measurements by the Hantzsch and the CRDS instruments, linear regression analysis of measurements from all three instruments in experiments with ambient air indicated good agreement, with slopes of between 0.98 and 1.08 and negligible intercepts (linear correlation coefficients R2&gt;0.96). The new small CRDS instrument measures HCHO with good precision and is accurate if the instrument zero is taken into account. Therefore, it can provide measurements with similar accuracy to the DOAS instrument but with slightly reduced precision compared to the Hantzsch instrument.

Synthesis and optical absorption properties of nanocrystalline rare earth hexaborides Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders

Nanocrystalline rare earth hexaborides Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders are successfully synthesized by the simple solid-state reaction in vacuum condition for the first time. The effect of Eu doping on the crystal structure, grain morphology, microstructure and optical absorption properties of nanocrystalline NdB&lt;sub&gt;6&lt;/sub&gt; are investigated by X-ray diffraction, scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM) and optical absorption measurements. The results show that all the synthesized samples have a single-phase CsCl-type cubic structure with space group of &lt;i&gt;Pm-&lt;/i&gt;3&lt;i&gt;m&lt;/i&gt;. The SEM results show that the average grain size of the synthesized Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders is 50 nm. The HRTEM results show that nanocrystalline Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; has good crystallinity. The results of optical absorption show that the absorption valley of nanocrystalline Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; is redshifted from 629 nm to higher than 1000 nm with the increase of Eu doping, indicating that the transparency of NdB&lt;sub&gt;6&lt;/sub&gt; is tunable. Additionally, the X-ray absorption near-edge structure spectra &lt;i&gt;μ&lt;/i&gt;(&lt;i&gt;E&lt;/i&gt;) around the Nd and Eu &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; edges for nanocrystalline NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; show that total valence of Nd ion is estimated at +3 in nanocrystalline NdB&lt;sub&gt;6&lt;/sub&gt; and total valence of Eu ion in nanocrystalline EuB&lt;sub&gt;6&lt;/sub&gt; is +2. Therefore, the Eu-doping into NdB&lt;sub&gt;6&lt;/sub&gt; effectively reduces the electron conduction number and it leads the plasma resonance frequency energy to decrease. In order to further qualitatively explain the influence of Eu doping on the optical absorption mechanism, the first principle calculations are used to calculate the band structure, density of states, dielectric function and plasma resonance frequency energy. The calculation results show that the electron band of NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; cross the Fermi energy, indicating that they are typical conductors. In addition, the plasmon resonance frequency can be described in the electron energy loss function. The plasmon resonance frequency energy of NdB&lt;sub&gt;6&lt;/sub&gt; and EuB&lt;sub&gt;6&lt;/sub&gt; are 1.98 and 1.04 eV, which are corresponding to the absorption valley of 626.26 and 1192.31 nm, respectively. This confirms that the first principle calculation results are in good consistence with the experimental optical absorption valley. Therefore, as an efficient optical absorption material, nanocrystalline Nd&lt;sub&gt;1–&lt;/sub&gt;&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;Eu&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt;B&lt;sub&gt;6&lt;/sub&gt; powders can expand the optical application scope of rare earth hexaborides.

Large substitutional impurity isotope shift in infrared spectra of boron-doped diamond

Isotopic enrichment offers cutting-edge properties of materials opening exciting research and development opportunities. In semiconductors, reached progress of ultimate control in growth and doping techniques follows nowadays the high level isotopic purification. This requires deep understanding of isotopic disorder effects and techniques of their effective determination. Isotopic content of both crystal lattice and impurity centers cause the effects, which can be examined by different optical techniques. While disorder in the host lattice can be straight forward evaluated by inelastic light scattering or by SIMS measurements, determination of isotopic contributions of many orders less presented impurities remains challenging and usually observed in high-resolution photoluminescence or optical absorption spectra. Boron-doped diamonds exhibit complex infrared absorption spectra while boron-related luminescence remains unobserved. Boron, as a most light element acting as a hydrogen-like dopant in elemental semiconductors, has a largest relative difference in its isotope masses, and by this, cause the largest isotopic disorder in semiconductors, including diamond, an elemental semiconductor with the lightest atomic mass of a host lattice. This enables an access to the isotopic constitution of boron in diamond by infrared absorption spectroscopy. By comparison of low-temperature absorption spectra of a natural (20% of 10B and 80% of 11B isotopes) and 11B enriched (up to 99%) doped diamonds we differentiate the intracenter transitions related to 10B and to 11B isotopes. We have found that the isotopic spectral lines of the same boron intracenter transition are separated with the energy of about 0.7 meV. This is the largest impurity isotopic shift ever observed in semiconductors doped by hydrogen-like impurity centers.

Luminescence investigation of a cerium-doped yttrium vanadate phosphor.

Cerium (Ce3+ )-doped (1, 3, and 7mol%) yttrium vanadate phosphors were prepared using a co-precipitation technique. The structural and optical properties of the synthesized samples were studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), optical absorption, and photoluminescence (PL) spectroscopy techniques. The tetragonal structure and the formation of the nanosized crystallites in the YVO4 :Ce phosphor were confirmed using XRD analysis. HR-TEM morphology showed rod-like nanoparticles of different sizes. Optical absorption spectra demonstrated strong absorption bands at 268 and 276nm. PL spectra showed strong peaks at 546, 574, and 691nm following excitation at 300nm. The calculated CIE chromaticity coordinates demonstrated that YVO4 :Ce could be used as a novel phosphor for the development of light-emitting diode lamps.

Structural and optical characteristics investigations in oxygen ion implanted GaN epitaxial layers

This paper presents the lattice disorder-induced effect by oxygen ion implantation in GaN epitaxial layers. Oxygen ion implantation in GaN epitaxial layers is done with fluencies 5 × 1014 cm−2 and 5 × 1015 cm−2 at 120 keV energy on two different pieces of a GaN sample. Unimplanted and implanted GaN layers are investigated by High-resolution X-ray diffraction (HRXRD), Optical absorption and Raman spectroscopy. The in-plane and out of plane lattice parameters (i.e. a and c) of the distorted and undistorted region in the GaN samples are determined from the respective XRD peak positions. Due to the incorporation of oxygen ions in GaN layers up to implantation depth, lattice parameters are extended, as a result, the samples are under hydrostatic strain. Raman spectroscopic studies on the unimplanted and oxygen ion-implanted GaN samples are done by green and ultra-violet (UV) lasers. Raman spectroscopy results indicate that the broadening of the E2 (High) peak and intensity of A1 (LO) peak increases with increasing fluence. A doublet in A1 (LO) peak is found in UV excited Raman spectra in unimplanted and implanted GaN samples. The occurrence of the A1 (LO) doublet is due to surface defects and phonon-plasmon coupled-mode generated due to nonequilibrium photoexcited carriers. In this article, the origin of doublet has been studied and reported.

Growth of high-quality GaN on (1 0 0) Ga2O3 substrates by facet-controlled MOVPE

We report a comprehensive study of the growth of GaN layers on (1 0 0)-oriented Ga2O3 substrates by metalorganic vapour phase epitaxy (MOVPE), especially the improvement on using an intermediate facet-control layer between the low-temperature buffer and the high-temperature GaN layer compared to the usual two-step growth process. We examined in detail the effects of substrate nitridation, carrier gas, and variation of the thickness of the low-temperature buffer GaN layer, and the thickness and growth temperature of the facet layer. This optimization allowed us to obtain GaN layers on (1 0 0)-oriented Ga2O3 with high structural and optical quality, as shown by high-resolution X-ray diffraction, electron microscopy, optical absorption, and luminescence measurements.

Highly Effective Disinfection of E. coli Using the Nanohybrids Ti1−xNixO2/CNTs

The nanohybrids Ti1−xNixO2/CNTs (x = 0; 0.01; 0.03; 0.06; 0.09) were synthesized by hydrolysis. The samples were characterized by high-resolution transmission electron microscopy (HR-TEM), x-ray diffraction (XRD), and optical absorption spectra (UV–Vis). The XRD analysis shows that the Ni-doped TiO2 exists mainly in the form of the anatase phase. The average size of TiO2 nanoparticles attached on the surface of the carbon nanotubes (CNTs) was found to be around 8 nm (based on the results from the XRD and HR-TEM images). The UV–Vis spectra of the Ti1−xNixO2/CNTs reveal not only a redshift due to the Ni-doping, but also an increase in the absorbance in the visible range due to nanohybridization of CNTs. The photocatalytic activity of the samples was tested by antibacterial activity against Escherichia coli. The results show that almost 100% of E. coli was destroyed, indicating a significantly much higher activity than that of pure TiO2.

Direct observation of hyperfine level anticrossings in the optical spectra of a LiYF47:Ho3+ single crystal

We have investigated the hyperfine structure in high-resolution broadband optical absorption spectra of a strongly diluted paramagnet $^{7}\mathrm{LiYF}_{4}:{\mathrm{Ho}}^{3+}$ (0.1 at. %) in external magnetic fields parallel to the tetragonal symmetry axis of the crystal. Anticrossings in the hyperfine spectral pattern were observed. It is the first direct observation of the hyperfine level anticrossings in optical spectra. Analysis of the spectral envelopes corresponding to transitions between the electron-nuclear sublevels of crystal-field non-Kramers doublets and singlets of the ${\mathrm{Ho}}^{3+}$ ions, based on the microscopic model of the electronic $4{f}^{10}$ configuration, allowed us to retrieve information on the hyperfine structure of electronic singlets, nuclear quadroupole interactions, and random lattice strains.

In Vivo 3D Imaging of Retinal Neovascularization Using Multimodal Photoacoustic Microscopy and Optical Coherence Tomography Imaging.

The pathological process of neovascularization of the retina plays a critical role in causing vision loss in several diseases, including diabetes, retinal vein occlusion, and sickle cell disease. Retinal neovascularization can lead to vitreous hemorrhage and retinal detachment, yet the pathological process of neovascularization is a complex phenomenon under active investigation. Understanding and monitoring retinal neovascularization is critically important in clinical ophthalmology. This study describes a novel multimodal ocular imaging system which combines photoacoustic microscopy (PAM) and a spectral domain optical coherence tomography (SD-OCT) to improve the visualization of retinal neovascularization (RNV), their depth, and the surrounding anatomy in living rabbits. RNV was induced in New Zealand rabbits by intravitreal injection of vascular endothelial growth factor (VEGF). The retinal vasculature before and after injection at various times was monitored and evaluated using multimodal imaging including color fundus photography, fluorescein angiography (FA), OCT, and PAM. In vivo experiments demonstrate that PAM imaging distinctly characterized the location as well as the morphology of individual RNV with high contrast at a safe laser energy of 80 nJ. SD-OCT was used to identify a cross-sectional structure of RNV. In addition, dynamic changes in the retinal morphology and retinal neovascularization were observed at day 4, 5, 6, 7, 9, 11, 14, 28, and day 35 after VEGF injection. PAM demonstrated high-resolution optical absorption of hemoglobin and vascular imaging of the retina and choroid with increased depth of penetration. With the current multimodal imaging system, RNV can be easily visualized in both 2D and 3D angiography. This multimodal ocular imaging system provides improved characterization of the microvasculature in a safe manner in larger rabbit eyes.

Extraction of Linear Carbon Chains Unravels the Role of the Carbon Nanotube Host.

Linear carbon chains (LCCs) have been shown to grow inside double-walled carbon nanotubes (DWCNTs), but isolating them from this hosting material represents one of the most challenging tasks toward applications. Herein we report the extraction and separation of LCCs inside single-walled carbon nanotubes (LCCs@SWCNTs) extracted from a double-walled host LCCs@DWCNTs by applying a combined tip-ultrasonic and density gradient ultracentrifugation (DGU) process. High-resolution transmission electron microscopy, optical absorption, and Raman spectroscopy show that not only short LCCs but clearly long LCCs (LLCCs) can be extracted and separated from the host. Moreover, the LLCCs can even be condensed by DGU. The Raman spectral frequency of LCCs remains almost unchanged regardless of the presence of the outer tube of the DWCNTs. This suggests that the major importance of the outer tubes is making the whole synthesis viable. We have also been able to observe the interaction between the LCCs and the inner tubes of DWCNTs, playing a major role in modifying the optical properties of LCCs. Our extraction method suggests the possibility toward the complete isolation of LCCs from CNTs.

Ex vivo validation of photo-magnetic imaging.

We recently introduced a new high-resolution diffuse optical imaging technique termed photo-magnetic imaging (PMI), which utilizes magnetic resonance thermometry (MRT) to monitor the 3D temperature distribution induced in a medium illuminated with a near-infrared light. The spatiotemporal temperature distribution due to light absorption can be accurately estimated using a combined photon propagation and heat diffusion model. High-resolution optical absorption images are then obtained by iteratively minimizing the error between the measured and modeled temperature distributions. We have previously demonstrated the feasibility of PMI with experimental studies using tissue simulating agarose phantoms. In this Letter, we present the preliminary ex vivo PMI results obtained with a chicken breast sample. Similarly to the results obtained on phantoms, the reconstructed images reveal that PMI can quantitatively resolve an inclusion with a 3mm diameter embedded deep in a biological tissue sample with only 10% error. These encouraging results demonstrate the high performance of PMI in ex vivo biological tissue and its potential for in vivo imaging.

Optical Access Schemes for High Speed and Spatial Resolution Optical Absorption Tomography in Energy Engineering

Optical diagnostic techniques play an important role in the engineering of modern energy processes. Optical absorption tomography as a diagnostic technique allows imaging of chemical species in flows with spatio-temporal resolution. The optical access scheme employed to acquire the tomographic projections impacts the limiting spatial resolution and imaging speed of the instrument. Our review of conventional optical access schemes indicates that there currently exist no practical schemes that can achieve simultaneous high-speed, high-spatial resolution imaging. Here, we show that advanced solid-state beam deflectors can be used to realize such a system. We evaluated a state-of-the-art electro-optic deflector combining a multi-pass scheme in a space-charged crystal and found that it can achieve a full deflection angle of 216 mrad (12.4°) at 90-kHz scan rate. We present how optical access schemes based on electro-optic deflectors can be arranged, estimating the increased bandwidth requirement for the data acquisition system. Using an existing tomography system and an image reconstruction algorithm, we show by simulation that the spatial resolution under non-optimum conditions can be improved by 38%. We describe in detail our implementation of the spatial resolution quantification algorithm. Our results demonstrate how advances in other disciplines can be exploited to further improve the performance of an optical tomography instrument. We anticipate our assay to motivate further development of optical access schemes as well as optimized image reconstruction algorithms.

Optical response from terahertz to visible light of electronuclear transitions inLiYF4:Ho3+

Because of its role as a model system with tunable quantum fluctuations and quenched disorder, and the desire for optical control and readout of its states, we have used high-resolution optical absorption spectroscopy to measure the crystal-field excitations for Ho^(3+) ions in LiHo_xY_(1−x)F_4 from the terahertz to visible regimes. We show that many of the excitations yield very narrow lines visibly split even by the nuclear hyperfine interaction, making Ho^(3+) in LiHo_xY_(1−x)F_4 a candidate host for optically addressable electronuclear qubits with quality factors as high as Q = 4.7 × 10^5, where the higher-lying levels are electronic singlets. Optical transitions in the easily accessible near- and mid-infrared are narrow enough to allow readout of the ground-state electronuclear qubits responsible for the interesting magnetism of LiHo_xY_(1−x)F_4. While many of the higher-lying states have been observed previously, we also report here detailed spectra of terahertz excitations. The strengths of the electric and magnetic dipole crystal-field transition lines of five of the lowest excited spin-orbit manifolds of dilute LiYF_4:Ho^(3+) were calculated and compared with measurement. The magnitude of the nuclear hyperfine coupling was used to assign the correct upper and lower states to transition lines.

An accelerated photo-magnetic imaging reconstruction algorithm based on an analytical forward solution and a fast Jacobian assembly method

We previously introduced photo-magnetic imaging (PMI), an imaging technique that illuminates the medium under investigation with near-infrared light and measures the induced temperature increase using magnetic resonance thermometry (MRT). Using a multiphysics solver combining photon migration and heat diffusion, PMI models the spatiotemporal distribution of temperature variation and recovers high resolution optical absorption images using these temperature maps. In this paper, we present a new fast non-iterative reconstruction algorithm for PMI. This new algorithm uses analytic methods during the resolution of the forward problem and the assembly of the sensitivity matrix. We validate our new analytic-based algorithm with the first generation finite element method (FEM) based reconstruction algorithm previously developed by our team. The validation is performed using, first synthetic data and afterwards, real MRT measured temperature maps. Our new method accelerates the reconstruction process 30-fold when compared to a single iteration of the FEM-based algorithm.

Compositional dependence of optical properties of Sm3+-doped Y3ScxAl5-xO12 polycrystalline ceramics

Disordered Sm3+ doped Y3ScxAl5-xO12 (Sm:YSxAG) polycrystalline ceramics with compositional parameter x=(0.05, 0.5, 1, 2) and different Sm3+ ions concentrations were obtained by solid state reaction method. XRD investigations show the linear increase of the lattice constant with compositional parameters x. New spectroscopic data obtained from high resolution optical absorption and emission spectra of Sm3+ doped YSxAG ceramics reveal composition effects on Sm3+ optical spectra: modification of the shapes and shifts of the lines, presence of the additional satellites and unresolved multicenter structure, etc. The analysis of these data allowed us to establish a partial structure of Sm3+ energy levels in Sm:YSxAG (x = 2) ceramics. The emission kinetics of the 4G5/2 level for different Sm3+ concentrations and Sc content in Sm:YSxAG were measured and analyzed.

Synthesis, X-ray crystal structure, and optical properties of novel 9,9-diethyl-1,2-diaryl-1,9-dihydrofluoreno[2,3-d]imidazoles

A series of 9,9-diethyl-1,2-diaryl-1,9-dihydrofluoreno[2,3-d]imidazole derivatives were conveniently synthesized by condensation of the key intermediate N 2-aryl-9,9-diethyl-9H-fluorene-2,3-diamine with aldehydes under very mild conditions with good yields. The entire target compounds were characterized using 1H NMR spectroscopy, 13C NMR spectroscopy, high resolution MS, optical absorption, and emission spectra. The crystal structure of 2-(4-bromophenyl)-9,9-diethyl-1-phenyl-1,9-dihydrofluoreno[2,3-d]imidazole was determined as monoclinic, space group P2 1 /c type, using single X-ray crystallography. Most compounds possess medium fluorescence-emitting ability with Φ FL values in the region of 0.40–0.84 and displayed different emission within 379–425 nm in CH2Cl2 depending on the nature of the whole molecule.

Experimental validation of a high-resolution diffuse optical imaging modality: photomagnetic imaging.

We present experimental results that validate our imaging technique termed photomagnetic imaging (PMI). PMI illuminates the medium under investigation with a near-infrared light and measures the induced temperature increase using magnetic resonance imaging. A multiphysics solver combining light and heat propagation is used to model spatiotemporal distribution of temperature increase. Furthermore, a dedicated PMI reconstruction algorithm has been developed to reveal high-resolution optical absorption maps from temperature measurements. Being able to perform measurements at any point within the medium, PMI overcomes the limitations of conventional diffuse optical imaging. We present experimental results obtained on agarose phantoms mimicking biological tissue with inclusions having either different sizes or absorption contrasts, located at various depths. The reconstructed images show that PMI can successfully resolve these inclusions with high resolution and recover their absorption coefficient with high-quantitative accuracy. Even a 1-mm inclusion located 6-mm deep is recovered successfully and its absorption coefficient is underestimated by only 32%. The improved PMI system presented here successfully operates under the maximum skin exposure limits defined by the American National Standards Institute, which opens up the exciting possibility of its future clinical use for diagnostic purposes.

Thermal annealing and UV irradiation effects on structure, morphology, photoluminescence and optical absorption spectra of EDTA-capped ZnS nanoparticles

Monodispersed ZnS nanoparticles (NPs) were prepared by the chemical precipitation method. Thermally induced structural, morphological and optical changes have been investigated using x-ray diffraction, high-resolution transmission electron microscopy, optical absorption, photoluminescence (PL), and Fourier transform infrared and Raman spectroscopy. It was found that D increases with increasing annealing temperature (Ta). The onset of the ZnS phase transition from cubic to hexagonal structure takes place at 400 °C, while cubic ZnS transforms into hexagonal ZnO via thermal oxidation in air at 600 °C. It is also noted that increasing Ta results in the red shift of the optical band gap () and the thermal bleaching of exciton absorption. The PL spectrum of as-prepared ZnS nanopowder shows UV emission bands at 363 and 395 nm and blue and green emission at 438 and 515 nm, respectively. With increasing Ta up to 500 °C, these bands were quenched and red-shifted. In addition, the UV irradiation effects on colloidal ZnS NPs were investigated. UV irradiation at a dose <13 J cm−2 leads to a decrease in D, the blue shift of and the enhancement of PL intensity. This behavior was explained in terms of surface modification by photopolymerization, the formation of a ZnSO4 passivation layer, as well as the reduction of D by photocorrosion. At a UV irradiation dose <13 J cm−2 both and D did not change and PL intensity was quenched, which were caused by the creation of nonradiative surface states by the photodegradation of the capping agent and photopassivated layer. The mechanism of the PL emission process in ZnS NPs was discussed and an energy band diagram was proposed.

Formation of Au4Zr nanocrystals in yttria stabilized zirconia in the course of implantation of gold ions

High-resolution transmission microscopy and optical absorption spectroscopy have been used to examine how metallic nanocrystals (NCs) are formed in the course of implantation of Au ions into thin films of yttria stabilized zirconia (YSZ) and subsequent thermal annealing. It was found that, under certain implantation conditions, Au4Zr NCs are formed in addition to Au NCs in the YSZ matrix. The conditions in which Au4Zr NCs are formed in the course of implantation and decompose under the subsequent annealing were determined.