High-energy Optical Transitions Research Articles

Transparent and radiation shielding effective Na2O–CrO3 borate glasses via AgI additives

The current paper investigates the impact of silver iodide (AgI) on the structure, optical, and radiation shielding parameters of borate glasses embedded with low amounts of CrO3, high levels of Na2O, and various quantities of AgI (from 1 to 5 mol%). These glasses (named BNaCrAg-glasses) were synthesized and their properties were investigated. Fourier transform infrared (FTIR) analyses of BNaCrAg-glasses demonstrated that the ratio of BO4/BO3 units increased with increasing AgI. The existence of Cr cations, with their Cr6+ state, was recognized from the absorption bands at 384–392 nm and 334–338 nm. Furthermore, the absorption bands at 580–589 nm at 409–440 nm through the visible region confirmed the existence of Cr-ions as Cr3+ in the octahedral symmetry. The crystal field splitting (10Dq) was calculated from the 580–589 nm bands. The values of 10Dq decreased with AgI doping due to replacing oxygen with iodide anions. The transmittance at 700 nm increased from 24 to 80%, while AgI addition increased from 0 to 5 mol%. The glass sample with the highest AgI content had the lowest half-value layer and maximum linear attenuation coefficient. Thus, the new BNaCrAg-glasses combine good transparency with good attenuation of low energy radiation and can be used to protect the radiation workers. Furthermore, these glasses offer high-energy optical transitions suitable for UV optics, such as ultraviolet digital photography and insect light traps.

Reevaluation of Cr6+ optical transitions through Gd2O3 doping of chromium-borate glasses

Abstract Chromium-doped borate glasses are appealing for potential optical applications as they feature distinct optical transitions in the visible and ultraviolet spectral regimes. However, such intense UV transitions are often screened in borate glasses due to their proximity to the bulk absorption edge. Here, we utilize variable concentrations of the wide band gap Gd2O3 dopant to detach these near-edge optical transitions. The amorphous nature of all Gd2O3-doped borate glass samples, containing a fixed Cr amount, was confirmed by X-ray diffraction. Gd2O3 additives were found to enforce the compactness of the glassy network as inferred from the increased experimental density and calculated average boron-boron separation. Analysis of the infrared spectra revealed the essential vibration modes of structural units, the non-bridge oxygen, and the fraction of tetrahedrally-coordinated boron atoms ratios. These ratios together with a noticeable BO3 to BO4 conversion suggest an open glass structure with enlarged band gap energy. The latter is confirmed by recording the optical absorption spectra for all samples, which exhibited clear blue shifts of the absorption edge and an estimated wide band gap of size ~3.38 eV. In addition to Cr3+ transitions manifesting in the visible region, two well-resolved ultra-violet transitions characteristic for Cr6+ were unambiguously identified down to ~339 nm. These transitions and other relevant quantities, such as crystal field strength and Racah parameters, were accurately determined from a deconvolution process applied to all optical spectra. The present Gd2O3-doped glasses offer wide band gap materials with high energy optical transitions suitable for UV optics, such as ultraviolet digital photography and insect light traps.

Very high oscillator strength in the band-edge light absorption of zincblende, chalcopyrite, kesterite, and hybrid perovskite solar cell materials

In semiconducting solar-cell absorbers, high absorption coefficient (alpha) near the band-edge region is critical to maximize the photocurrent generation and collection. Nevertheless, despite the importance of the band-edge absorption characteristics, the quantitative analysis of the band-edge optical transitions has not been performed. In this study, we have implemented systematic density functional theory (DFT) calculation, focusing on the band-edge oscillator strength of seven practical solar cell absorbers (GaAs, InP, CdTe, CuInSe2, CuGaSe2, Cu2ZnSnSe4, and Cu2ZnSnS4) with zincblende, chalcopyrite and kesterite structures. We find that all these crystals exhibit the giant oscillator strength near the band gap region, revealing the fact that alpha in the band gap region is enhanced significantly by the anomalous high oscillator strength. In high-energy optical transitions, however, the oscillator strength reduces sharply and the absorption properties are determined primarily by the joint density-of-state contribution. Based on DFT results, we show that the giant oscillator strength in the band edge region originates from a unique tetrahedral-bonding structure, with a negligible effect of constituent atoms.

High-Energy Optical Transitions and Optical Constants of CH3NH3PbI3 Measured by Spectroscopic Ellipsometry and Spectrophotometry

Optoelectronics based on metal halide perovskites (MHPs) have shown substantial promise, following more than a decade of research. For prime routes of commercialization such as tandem solar cells, ...

Unraveling local spin polarization of Zhang-Rice singlet in lightly hole-doped cuprates using high-energy optical conductivity

Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems.

Hole Cooling Is Much Faster than Electron Cooling in PbSe Quantum Dots.

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with processes such as carrier multiplication or hot charge extraction that may improve the light conversion efficiency of photovoltaic devices. Understanding charge carrier cooling is therefore of great interest. We investigate high-energy optical transitions in PbSe QDs using hyperspectral transient absorption spectroscopy. We observe bleaching of optical transitions involving higher valence and conduction bands upon band edge excitation. The kinetics of rise of the bleach of these transitions after a pump laser pulse allow us to monitor, for the first time, cooling of hot electrons and hot holes separately. Our results show that holes cool significantly faster than electrons in PbSe QDs. This is in contrast to the common assumption that electrons and holes behave similarly in Pb chalcogenide QDs and has important implications for the utilization of hot charge carriers in photovoltaic devices.

Birefringence and refractive indices of wurtzite GaN in the transparency range

Birefringence and anisotropic refractive indices of wurtzite GaN within the spectral range from 0.58 eV to 3.335 eV were determined combining optical retardation and spectroscopic ellipsometry measurements on a series of undoped m- and c-plane GaN bulk substrates grown by hydride vapor phase epitaxy. It is observable that the birefringence has a maximum close to the absorption edge and a weak broad minimum in near-IR range. A quantitative explanation of the whole data is given in terms of contributions to the optical response of GaN due to discrete excitons, Coulomb enhanced band-to-band optical transitions near the E0 critical point of the band structure, high-energy optical transitions, and infrared active optical phonon modes which are different for the ordinary and extraordinary waves both in magnitude and in spectral dependence.

Abnormal polarization switching phenomenon in a-plane AlxGa_1-xN

The optical polarization properties of a-plane AlxGa(1-)xN films have been investigated by polarization-dependent photoluminescence (PL). The degree of polarization decreased with increasing the Al composition, and the main optical polarization direction switched from ε ⊥ cto ε // c at about x = 0.07 due to the valence band switching, representing that the optical transition energy of ε // c is surpassing that of ε ⊥ c. However, with the Al composition larger than x = 0.1, the higher energy optical transitions of ε // cexhibited the stronger PL intensity, opposite to the normal situations that higher energy states commonly have weaker PL intensity than the lower energy states. We utilized the 6 × 6 k.p model and the lambertian-like radiation pattern assumption to explain this abnormal optical polarization switching behavior in the a-plane AlxGa(1-)xN layers and obtained good agreement with the experimental results.

Higher energy optical transitions in semiconducting carbon nanotubes

We have studied the high energy optical transitions of semiconducting single-walled carbonnanotubes using the nonorthogonal tight-binding model, which takes into account the excitoneffect. It is found from our calculations that the exciton’s binding energies for the high energyE33 and E44 transitions are large enough, indicating clearly they are also excitonic in nature. Moreimportantly, the logarithmic Kane–Mele correction, successful for the low energyE11 and E22 transitions, is now found to fail for describing the many-body effects in the higher energytransitions, which is consistent with the experimental result of Araujo et al (2007 Phys. Rev.Lett. 98 067401). Finally, it is interesting to find the possibility of a crossover effect between theE33 and E44 energies for certain mod 1 chiralities and the family behavior of the many-body corrections in theE33 and E44 transitions, both of which are well supported by the recent experiment of Haroz et al (2008Phys. Rev. B 77 125405).

Isolation of single-walled carbon nanotube enantiomers by density differentiation

Current methods of synthesizing single-walled carbon nanotubes (SWNTs) result in racemic mixtures that have impeded the study of left- and right-handed SWNTs. Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation. Enantiomer separation is enabled by the chiral surfactant sodium cholate, which discriminates between left- and right-handed SWNTs and thus induces subtle differences in their buoyant densities. This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm. In addition, circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs.

Advances in the application of modulation spectroscopy to vertical cavity structures

Modulated reflectance is a straightforward technique in which the reflectivity spectrum of a semiconductor is perturbed by an external periodic influence—most usefully a chopped laser beam, i.e. photo-modulated reflectance (PR). Since samples need no special mounting, can be studied in air at room-temperature and can be full-sized pre-fabrication wafers, PR is truly non-destructive. In contrast to conventional photo-luminescence, PR spectra are often replete with sharp, detailed derivative-like features arising from ground-state, and other possible higher-energy optical transitions within a sample, from which can be extracted material parameters crucial to successful and efficient device operation, such as compositions, layer thicknesses, built-in electric fields and band line-ups. The paper discusses the advances made by our group in the application and interpretation of modulation spectroscopy of vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes, which so far have defied analysis by the more conventional non-destructive techniques.

Lowest 4f-to-5d and Charge-Transfer Transitions of Rare-Earth Ions in LaPO4 and Related Host-Lattices

In order to study the high energy optical transitions i.e., 4f-to-5d (f-d) and charge-transfer (CT) spectra of trivalent rare earth (4fn-series, lanthanide) ions in crystal lattices, the luminescence excitation spectra of the series ions in LaPO4 host lattices are measured in the vacuum ultraviolet (VUV)-ultraviolet (UV) spectral region of 120–350 nm, and the CT-excitation spectra of Y2O2S and La2O2S are measured in a UV region. The f-d and CT bands are observed for the most 4fn-series ions in which the predicted transition energies are lower than the host lattice absorption edge, that is, the f-d bands for Ce3+, Pr3+, Sm3+, Tb3+ and Dy3+, and the CT band for Nd3+, Sm3+, Eu3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+. The energies of these observed peaks of f-d and CT bands agree fairly well with the values calculated by the method proposed in our previous report. The relationship between the energies of the CT transition and the absorption edge in various host lattices is discussed.

The lowest 4f-to-5d and charge-transfer transitions of rare earth ions in YPO 4 hosts

In order to study the high-energy optical transitions i.e. 4f-to-5d (f–d) and charge-transfer (CT) spectra of trivalent rare earth (4f n -series, lanthanide) ions in crystal lattices, the luminescence excitation spectra of the series ions in YPO 4 host lattices are measured in a VUV–UV spectral region of 120–350 nm. The f–d bands are observed with all of the series ions except for the ions with the transition energy higher than the host lattice absorption edge i.e. except for Gd 3+ and Yb 3+, and the CT bands are observed except for Ce 3+, Gd 3+ and Tb 3+ (Pm is unavailable). Jørgensen's formulation which has elucidated the systematic variation of the energies of the f–d and CT transitions through the 4f n -series is modified in a form of a single-electron scheme. The many-electron effects in the scheme are calculated and presented as a numerical table. The energies of the observed f–d and CT bands fit well with those calculated by the use of this table.

Formation of CdS nanocrystals in SiO 2 by ion implantation

We present a systematic study of the influence of ion dose and post-implantation annealing on the synthesis and growth of CdS nanocrystals in a SiO 2 matrix. Nanocrystals were obtained after implantation of monoenergetic Cd and S ions and subsequent annealing in a very wide range of annealing temperatures, T a. The average size, as determined from the blue shift of band gap E g, varied from 3.5–4.5 to 10 nm, depending on implantation and annealing parameters. For the highest dose, 10 17 ions/cm 2, the synthesis of CdS phase starts already during implantation. For T a above 700 °C, large nanocrystals (9–10 nm) prevail for all doses. High energy optical transitions, identified as the E 1A and E 1B transitions of hexagonal CdS, were also observed after annealings at higher temperature.

Optical Properties of as-Grown and Proton Irradiated ZnO

Conventional photoluminescence spectroscopy (PL) shows several high energy optical transitions within the region 3.072 to 3.4345 eV from as-grown ZnO excited with a 257 nm UV line. Performing inelastic light scattering experiments (ILS), we probed some previously reported modes. Proton implantation caused drastic changes in the excitonic region and generates a broad band centered at about 650 nm (1.9eV) In addition, we observed from both sides of the ZnO sample implantation-induced high-energy vibrational modes centered at about 1350 and 1600 cm -1 (0.167 and 0.198 eV, respectively).

Optical Properties of Doped Antiferromagnets

We investigate the consequences of an incommensurate magnetic order in doped La2-xSrxCuO4 using dynamical mean-field theory for the effective single-band model. The high-energy optical transitions are due to high-energy excitations across large Mott-Hubbard 'gap,' while low-energy excitations involve a pseudogap induced by the local spin order. The latter lead to a strong drop in the scattering rate at low ω, in qualitative agreement with the experimental data.(16 refs)

High quality ultrathin InAs/GaAs quantum wells grown by standard and low-temperature modulated-fluxes molecular beam epitaxy

We report on the optical study of ultrathin (1–3 monolayers) InAs quantum wells in GaAs. The samples have been grown either by standard molecular beam epitaxy (MBE) or low-temperature (350 °C) modulated-fluxes MBE. The latter technique enlarges the range of pseudomorphic deposition of strained InAs layers on GaAs. Both types of samples display sharp (down to 6 meV spectral width) and intense low-temperature excitonic photoluminescence, and a high in-plane homogeneity. Higher energy optical transitions have been observed in both absorption and photoluminescence excitation experiments, performed on multiple quantum well structures.

Faraday rotation in gadolinium orthochromite

Faraday rotation and linear birefringence of GdCrO3 were measured in the spectral range 0.8 to 2.0 μm at liquid nitrogen temperature. Good agreement is obtained between the experimental data and the theoretical values of rotation, calculated by the use of the formula describing Faraday rotation in the presence of the linear birefringence. The intrinsic Faraday rotation as a function of the wavelength is determined and it is probably due to higher-energy optical transitions.

Absolute Conduction- and Valence-Band Positions for Ge from an Anisotropic Model of Photoemission

An anisotropic direct-transition model for single-crystal semiconductors is shown to predict the direct-transition features seen in experimental photoemission spectra for Ge(111) for $h\ensuremath{\nu}\ensuremath{\lesssim}20$ eV. By comparing theory with experiment, all the conduction and valence bands at $L$ and $X$ within 1 Ry of the gap are determined. Comparison of experiment with current band models suggests that an \ensuremath{\sim} 10% self-energy correction may be needed to describe high-energy optical transitions.

Optical absorption in ion-bombarded magnetic garnet films

Optical absorption measurements are reported for the damaged surface layers produced by proton and neon bombardment of iron-garnet liquid-phase-epitaxial films. The excess optical absorption is shown to be an absorption tail on O(2p)→Fe(3d) charge-transfer bands centered near 4 eV with about 10% contributed by higher-energy optical transitions. The distribution of states for Ne bombardment is different from that for H bombardment, and in the case of Ne the state distribution depends on the dose. Saturation at a photon energy of 2.2 eV occurs at an absorption coefficient of ∼2×104 cm−1. The average damage density over the damaged layer thickness is observed to increase with decreasing H energy, in agreement with hard-bubble-suppression data and nuclear stopping power considerations. Based on optical absorption and hard-bubble-suppression results it is estimated that the dose window for hard bubble suppression corresponds roughly to an average disorder range from ∼3–30% amorphous (amorphous defined optically). Annealing studies show that no simple activation energy description applies to the damaged material. Deep states anneal out more rapidly than shallow states for both H and Ne bombardment.