Parity-forbidden Transitions Research Articles

Effect of sensitizing Cr3+ ion on optical parameters of Nd-doped yttrium calcium oxyborate crystals

Pure, Nd doped, and Nd–Cr bidoped yttrium calcium oxyborate Yca4O(BO3)3 (YCOB), NdxY1−xCa4O(BO3)3 (NYCOB) with x = 0.86, and Cr:NdxY1−xCa4O(BO3)3 (Cr:NYCOB) with x = 0.45 and a small amount of Cr3+ ions crystallized with a fluorapatite-type structure in the monoclinic system. The unit cell constants were a = 0.8076(7), b = 1.6020(10), and c = 0.3527(2) nm with the angle β of 101.23° for the NYCOB and z = 2. The measured absorption spectra of NYCOB and Cr:NYCOB were compared to Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. The experiments showed that the Cr3+ ions were introduced into the crystalline lattice as a sensitizer to absorb the excitation energy and transfer it to the Nd3+ ions in the lattice structure.

Electric-field-induced optical second-harmonic generation and nonlinear optical rectification in semiconducting carbon nanotubes

We have calculated the nonlinear susceptibility for the optical second-harmonic generation (SHG) from a bundle of aligned single-walled carbon nanotubes (SWCNs) of the `zig-zag' type subjected to a constant electric field E 0 , parallel to their axis. The breakdown of inversion symmetry caused by the electric field is accompanied by the occurrence of the parity-forbidden two-photon transitions between the valence- and conduction-band edges. As a result, the third-order nonlinear-susceptibility χ (3) spectrum for SHG clearly demonstrates two peaks strongly distinguished on intensity: one is the two-photon resonance exactly at the pump photon energy ℏ ω equal to one half of the band gap Δ g ; the other – small peak – sits at ℏ ω=Δ g , which corresponds to the one-photon interband transitions. It is found that the intensity of both peaks grows sharply with an increase of the SWCN radius. The susceptibility χ (3) relevant to the effect of nonlinear optical rectification in SWCNs has also been calculated. It is shown that near the fundamental absorption edge, the electric voltage appearing on the ends of SWCNs due to the optical rectification effect sharply changes its polarity. For a bundle of identical (25,0) SWCNs, 3 μm in length and placed in an electric field E 0∼10 4 V cm −1, the optical rectification voltage is found to be about 0.35 μV under excitation of the sample by a continuous laser with a radiation intensity of 30 mW cm −2, which may be of practical importance for mid-IR signal processing.

Optical Parameters of Nd-Doped Gadolinium Aluminium Tetraborate Crystals

Nd-doped gadolinium aluminium tetraborate, Nd x Gd 1-x Al 3 (BO 3 ) 4 (NGAB), crystallizes with x = 0.1 in the trigonal system with point group 32, conforming space group R32. They were grown using flux method, of large sizes and good optical quality. The unit cell constants were a = 0.93057 (6) and c = 0.72481 (3) nm, and z = 1. The measured absorption of the NGAB is compared to Judd-Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. A measured band gap of the eigentransition was about 4.00 eV. The fitted intensity parameters Ω t (t = 2.4, and 6) were 2.446 × 10 -20 , 2.266 × 10 -20 and 2.598 × 10 -20 cm 2 . Both the emission properties of the sub-stable 4 F 3/2 state with the level lifetime of 506.20 μs and a fluorescence spectrum corresponding to the transition 4 F 32 → 4 I 11/2 were studied. The results indicate the pass band of laser for potential applications in practice.

Optical Parameters of Nd-Doped Gadolinium Aluminium Tetraborate Crystals

Nd-doped gadolinium aluminium tetraborate, NdxGd1-xAl3(BO3)4 (NGAB), crystallizes with x = 0.1 in the trigonal system with point group 32, conforming space group R32. They were grown using flux method, of large sizes and good optical quality. The unit cell constants were a = 0.93057 (6) and c = 0.72481 (3) nm, and z = 1. The measured absorption of the NGAB is compared to Judd-Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. A measured band gap of the eigentransition was about 4.00 eV. The fitted intensity parameters Ot (t = 2, 4, and 6) were 2.446 × 10-20, 2.266 × 10-20 and 2.598 × 10-20 cm2. Both the emission properties of the sub-stable 4F3/2 state with the level lifetime of 506.20 μs and a fluorescence spectrum corresponding to the transition 4F3/2 → 4I11/2 were studied. The results indicate the pass band of laser for potential applications in practice.

Raman spectra and laser properties of Yb-doped yttrium orthovanadate crystals

Yb-doped yttrium orthovanadate YbxY1−xVO4 (YYV), with x=0.02, crystallizes with a zircon-type structure in the tetragonal system, conforming to the space group I41/amd. The lattice constants are a=0.7122(5) and c=0.6291(3) nm at room temperature. The primitive cell contains four formula units. The Raman spectra and crystallography show no distortion of the VO4 and Y/YbO8 groups. The crystal field is similar to that of the YVO4 crystal and is suitable for the dopant Yb3+ ions. A strong absorption at 975 nm arises from the F27/2 to F25/2 transition; with a bandwidth of 70 nm it may provide a useful pump band for InGaAs diode lasers. This band is about seven times broader than that of Nd-doped yttrium aluminum garnet and more than three times as broad as that of Nd-doped yttrium orthovanadate. The measured absorption of the YYV is compared to Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. The fluorescence spectrum reveals a ground state splitting of ΔE=463 cm−1. The fitted fluorescence lifetime is 1.18 ms at the wavelength of 1.0 μm. The calculated absorption cross section is 1.38×10−20 cm2. The order of magnitude of the emission cross section is estimated as 10−21 cm2. In Yb-doped yttrium aluminum garnet, the fluorescence lifetime is about 1.16–1.30 ms and the absorption cross section is 1.5×10−20 cm2. In Nd-doped yttrium orthovanadate, the fluorescence lifetime is 115 μs and the emission cross section is 10.7×10−19 cm2. A suitable concentration of the Yb3+ ions doped in the YbxY1−xVO4 crystals should modify the optical and the potential laser properties of the crystals.

Optical spectroscopy of arsenic- and silver-containing sol-gelcoatings

Sol-gel silica coatings doped with 1 mol% silver and/or 1 mol% arsenic oxide have been investigated by photoluminescence (PL) and optical absorption (OA) spectroscopy. The presence of Ag+ ions in the silica host has been monitored by recording a luminescence peak located between 320 and 330 nm upon excitation with 228 nm light, whereas the formation of small particles of metallic silver has been assessed by recording the absorption band centred at about 405 nm. The luminescence peak has been related to the d10d9 s parity-forbidden transitions in Ag+, which are partially allowed by odd-phonon assistance. On the other hand, the absorption peak at about 405 nm arises from the well known surface-plasmon resonance of silver particles. Coating densification under various atmospheres gives rise to significant effects on the PL and OA spectra. Results indicate that, after coating densification in air, most of the silver appears as Ag+ ions, in contrast to coating densification under a 90% N2-10% H2 atmosphere, which favours the formation of small particles of metallic silver. The presence of arsenic oxide in the silver coatings densified in air has been found to improve the stabilization of Ag+ ions, so that partially prevents the formation of colloidal silver under reducing atmospheres.

Observation of absorption bands due to the parity-forbidden transitions of ions in crystals

Optical absorption and magnetic circular dichroism spectra of ions in crystals have been measured. The fine structure of the absorption bands due to the and transitions in the spin- and parity-forbidden intraconfigurational electronic transition has been studied at different temperatures. By comparison with the one-photon and two-photon excitation spectra for the luminescence due to the transition, it is shown that the fine structure consists of the absorption peaks due to three kinds of ion which substitutes ions but have different site symmetries, i.e. the cubic, tetragonal and trigonal symmetries.

Parity- and spin-forbidden optical transitions ofCr+3inGdAlO3

In this paper, the electric-dipole transition induced by exchange is used to explain the strong dependence of spin- and parity-forbidden transitions of ${\mathrm{Cr}}^{+3}$ on the sublattice magnetization of an antiferromagnet host.

Optical properties of impurities in crystals

The optical properties of impurities in single crystals of the perovskite-like are studied. The optical absorption spectra consist of two bands at 192 and 213 nm, which are attributed to parity-forbidden transitions by analogy with the alkali halides. Under optical excitation in each of the absorptions, only one photoluminescence band is observed at 243 nm, which is thermally enhanced. The temperature dependence of its decay time is similar to that found for alkali halides, and therefore its behaviour is analogously attributed to two emitting levels, the lowest one being metastable. Measurements of the thermally stimulated luminescence, and of its spectral distribution show that for two kinds of emission mechanism coexist, one due to the deexcitation of ions, and the second caused by intrinsic deexcitation of the host lattice. The features of the optical behaviour of in crystals are compared with those known of for in other systems.

Polar CdSe nanocrystals: Implications for electronic structure

We report direct spectroscopic evidence for parity-forbidden electronic transitions in CdSe nanocrystals and relate it through perturbation theory to an internal electric field. This field is consistent with an estimated spontaneous polarization of Ps=−0.6 μC/cm2 for CdSe. The observed mixing of odd and even band edge hole states by this symmetry breaking internal electric field indicates that current theories of nanocrystal electronic structure, which assume inversion symmetry, require substantial revision.

Excitation and emission thermal shifts in ABF3:Mn2+ perovskites: coupling with impurity vibrational modes

The thermal shifts undergone by the first moment of the 6A1g(S) to 4T1g(G) excitation band and the associated emission band of Mn2+-doped ABF3 perovskites are investigated in the 9-300 K temperature range. It is found that these shifts are similar for the whole series and have average values of +150 and +450 cm-1 for excitation and emission, respectively. Both the sign and the magnitude of these different thermal shifts are explained in terms of (i) the phonon assistance mechanism required to gain intensity of the parity-forbidden transitions, (ii) the quadratic electron-phonon coupling and (iii) thermal expansion effects. To achieve this analysis a previous discussion upon the nature of the vibrational modes seen in the optical spectra is carried out. It is stressed that the impurity vibrational mode displaying h(cross) omega g=570 cm-1 in the emission spectrum of KMgF3:Mn2+ exhibits a value of 540 cm-1 in the corresponding excitation spectrum. This situation, which is also found for other modes seen in the optical spectra of KMgF3:Mn2+, indicates that the mode (though associated with the LO3 branch of KMgF3) is not a pure mode of the lattice but displays a kind of resonant character. As a salient feature the calculated thermal shifts are based on the experimental shifts experienced by the frequencies of the optical and acoustic modes on going from the ground 6A1g to the excited 4T1g state of MnF64-. At variance with findings for the R lines in Cr3+ and V2+, it is clearly demonstrated that the explicit and implicit contributions to the thermal shift of the zero-phonon line in MnF64- are similar and both induce red shifts upon heating. Moreover the present analysis reveals that the explicit contribution to the thermal shift undergone by the zero-phonon line of KMgF3:Mn2+ is mainly dominated by the odd-parity low-energy modes. The calculated thermal shifts reproduce reasonably well the experimental data.

Magnetoluminescence studies in GaAs-AlxGa1-xAs single heterojunctions: Observation of parity-forbidden Landau-level transitions.

We have observed a number of allowed and parity-forbidden Landau-level transitions in a modulation-doped GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single heterojunction structure using photoluminescence-excitation spectroscopy at 2 K. The GaAs layer in this structure is 5000 \AA{} thick. From the allowed Landau-level transitions we determine the reduced mass of the electron-hole pair to be 0.07${\mathit{m}}_{\mathit{e}}$. Using a heavy-hole mass of 0.45${\mathit{m}}_{\mathit{e}}$, we determine the average value of the electron effective mass to be 0.084${\mathit{m}}_{\mathit{e}}$. From the parity-forbidden transitions where the hole Landau level is the same but the electron Landau levels are different, we determine the average value of the electron mass to be 0.085${\mathit{m}}_{\mathit{e}}$, in excellent agreement with the value determined from the allowed transitions.

Symmetry adaptation techniques in n-photon absorption spectroscopy

We present some recent progress achieved in the application of symmetry adaptation techniques to n-photon absorption spectroscopy of rare earth ions in finite symmetry. More specifically, this work is concerned with the determination of the intensity of n-photon transitions between Stark levels (rather than J levels) of an ion in an environment with a given symmetry. The role of symmetry is taken into account through the use of initial and final state vectors characterized by irreducible representations of the (double) group for the ion site symmetry. Two distinct situations are considered: the case of parity-allowed n-photon transitions (as e.g. intra-configurational transitions between Stark levels of the 4f N configuration for n even) and the case of parity-forbidden n-photon transitions (as e.g. inter-configurational transitions from Stark levels of the 4f N configuration to Stark levels of the 4f N - 15d configuration for n even).

Spectra of one-dimensional excitons in polysilanes with various backbone conformations.

Linear and nonlinear optical spectra (one-photon and two-photon absorption, photoluminescence, and electroabsorption spectra) have been investigated for solid thin films of polysilanes with a variety of conformations, such as trans-planar, alternating trans-gauche (TGTG' type), 7/3 helical, and disordered backbone structures. Optical spectra have revealed features of one-dimensional exciton states characteristic of the respective conformations. In particular, the lowest allowed and second parity-forbidden (or two-photon allowed) transitions can be assigned to the ground ${(}^{1}$${\mathit{B}}_{1\mathit{u}}$) and first excited ${(}^{1}$${\mathit{A}}_{\mathit{g}}$) states of one-dimensional Wannier-type exciton, respectively. Furthermore, the higher excited exciton states and subsequent band-to-band transitions show up in the electroabsorption spectra for polysilanes with various conformations of the Si backbones. Quantitative analysis based on a microscopic theory for the one-dimensional excitons is also presented.

Electric field dependence of allowed and forbidden transitions in In 0.53Ga 0.47As/In 0.52Al 0.48As single quantum wells by room temperature modulation spectroscopy

Room temperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250 Å thick In 0.53Ga 0.47As/In 0.52Al 0.48As single quantum well system as a function of an externally applied electric field. We observe parity forbidden transitions, involving conduction band states with quantum numbers up to n = 5, which become more pronounced at high electric fields. The ground state and the forbidden transitions showed a significant redshift due to the quantum confined Stark effect. Despite the appreciable Stark shift, the rather large, field-induced linewidth broadening and the relatively low electric field at which the ground state exciton is ionized, poses limitations on using this wide quantum well system for electro-optic applications.

Electric-field dependence of interband transitions in In0.53Ga0.47As/In0.52Al0.48As single quantum wells by room-temperature electrotransmittance

Room-temperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250-Å-thick In0.53Ga0.47As/In0.52Al0.48As single-quantum-well system as a function of an externally applied electric field. Parity forbidden transitions, involving conduction-band states with quantum numbers up to n=5, which become more pronounced at high electric fields were observed. The ground-state and the forbidden transitions showed a significant red shift due to the quantum confined Stark effect. A comparison with previously reported results on thinner InGaAs/InAlAs quantum wells indicated that the wide-well sample exhibits the largest shift, as expected from theory. Despite the appreciable Stark shift, the rather large, field-induced linewidth broadening and the relatively low electric field at which the ground-state exciton is ionized poses limitations on using this wide-quantum-well system for electro-optic applications.

Some forbidden lines of platinum

A detailed examination of a high-resolution Fourier-transform spectrum of a platinum hollow-cathode discharge in the near infrared has yielded one Pt i and four Pt ii parity-forbidden magnetic dipole transitions. Theoretical calculations and analysis of the hyperfine structure are used to confirm these assignments, and applications to astrophysics and lasers are suggested.

Accurate energy levels for singly ionized platinum (Pt ii)

New observations of the spectrum of Pt ii have been made with hollow-cathode lamps. The region from 1032 to 4101 Å was observed photographically with a 10.7-m normal-incidence spectrograph. The region from 2245 to 5223 Å was observed with a Fourier-transform spectrometer. Wavelength measurements were made for 558 lines. The uncertainties vary from 0.0005 to 0.004 Å. From these measurements and three parity-forbidden transitions in the infrared, accurate values were determined for 28 even and 72 odd energy levels of Pt ii.

Parity-Forbidden Transitions of Deoxyheme

The parity-forbidden transitions of deoxyheme via the A 2 u and E u modes are investigated. The possible transitions are 5 B 2 g → 5 E g , 5 E g → 5 A 1 g , 5 E g → 5 B 1 g , 1 A 1 g → 1 E g (8062cm -1 ) and 1 A 1 g → 1 E g (20902 cm -1 ). The variations of the oscillator strengths with respect to temperatures are calculated for these transitions. The theoretically obtained optical spectra are fairly well agree with the experimerntal ones observed by Kobayashi and Yanagawa.

Effects of vibronic coupling on the EPR spectra of copper(II) doped K 2ZnF 4

The single crystal g-values of ≈≈ 1% Cu 2+ doped into tetragonal K 2ZnF 4 measured over a temperature range between 4 and 295 K are reported. The results are interpreted in terms of a predominantly d z 2 ground state wavefunction for the CuF 4− 6 guest species, with a small admixture of d x 2- y 2 caused by vibronic coupling. To a first approximation both the magnitudes and temperature dependence of the g-values may be described using a model directly analogous to that conventionally used to represent the temperature dependence of the intensity of parity-forbidden electronic transitions. The generality of this model has been investigated by carrying out numerical calculations of the vibronic wavefunctions and energy levels, and values of the ligand-field and warping parameters of the “Mexican-hat” potential surface of the copper(II) guest complex have been derived. The results reported for several other systems where vibronic coupling had been thought to influence EPR parameters are also discussed using the present model.