295 research outputs found
1.55-ÎŒm mode-locked quantum-dot lasers with 300âMHz frequency tuning range
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 106, 031114 (2015) and may be found at https://doi.org/10.1063/1.4906451.Passive mode-locking of two-section quantum-dot mode-locked lasers grown by metalorganic vapor phase epitaxy on InP is reported. 1250-ÎŒm long lasers exhibit a wide tuning range of 300âMHz around the fundamental mode-locking frequency of 33.48 GHz. The frequency tuning is achieved by varying the reverse bias of the saturable absorber from 0 to â2.2âV and the gain section current from 90 to 280âmA. 3âdB optical spectra width of 6â7ânm leads to ex-facet optical pulses with full-width half-maximum down to 3.7 ps. Single-section quantum-dot mode-locked lasers show 0.8 ps broad optical pulses after external fiber-based compression. Injection current tuning from 70 to 300âmA leads to 30âMHz frequency tuning.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeEC/FP7/EU/264687/Postgraduate Research on Photonics as an Enabling Technology/PROPHE
Behaviour of the V<sup>2+</sup> Ground State in Tetrahedrally Coordinated IIâVI Semiconductors
The ground state of the lattice-neutral charge state of vanadium in tetrahedrally coordinated IIâVI materials is a 4T1 term and is hence subject to a Jahn-Teller (JT) effect. In this paper photo-luminescence and EPR investigations on V2+ (3d3) in ZnS, ZnSe, ZnTe, and CdTe are combined with Tanabe-Sugano calculations including JT interaction. For ZnS the no-phonon structure is explained by a tunnelling splitting
Vanadium centers in ZnTe crystals. II. Electron paramagnetic resonance
Four V-related electron-paramagnetic-resonance (EPR) spectra are observed in Bridgman-grown ZnTe doped with vanadium. Two of them are attributed to the charge states VZn3+(A+) and VZn2+(A0) of the isolated V impurity. For the ionized donor, VZn3+(A+), the spectrum reveals the typical behavior of the expected 3A2(F) ground state in tetrahedral symmetry. The incorporation on a cation lattice site could be proved by the resolved superhyperfine interaction with four Te ions. The second spectrum showing triclinic symmetry and S=3/2 is interpreted as the neutral donor state VZn2+(A0). The origin of the triclinic distortion of the cubic (Td) crystal field could be a static Jahn-Teller effect. The two additionally observed EPR spectra are attributed to nearest-neighbor V-related defect pairs. The spectrum of the first one, V2+Zn-YTe, shows trigonal symmetry and can be explained by the S=3/2 manifold of an orbital singlet ground state. An associated defect "YTe" is responsible for the trigonal distortion of the tetrahedral crystal field of V2+Zn. The spectrum of the second pair defect also shows trigonal symmetry and can be described by S=1/2. The ground-state manifold implies a VZn3+âXTe pair as the most probable origin of this spectrum. The S=1/2 ground state is produced by a dominating isotropic exchange interaction coupling the S=1 ground-state manifold of V3+Zn to an assumed S=1/2 ground state of "XTe" in antiferromagnetic orientation. The nature of the associated defects "YTe" and "XTe" remains unknown for both pairs since no hyperfine structure has been observed, but most probably acceptorlike defects are involved
Vanadium centers in ZnTe crystals. I. Optical properties
In ZnTe:V bulk crystals with nominal vanadium concentrations between 1000 and 7000 ppm three vanadium-ion states V+, V2+, and V3+ were found in low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5E(D)â5T2(D) transition of V+(d4) at 3401 cmâ1 (0.422 eV), for 4T2(F)â4T1(F) of V2+(d3) at 4056 cmâ1 (0.503 eV), and for 3T2(F)â3A2(F) of V3+(d2) at 4726 cmâ1 (0.586 eV). The energies of the internal transitions are reduced with respect to the corresponding transitions in ZnS:V and ZnSe:V. The respective excitation spectra display, in addition to broad charge-transfer bands, higher excited levels of the individual charge states. Crystal-field calculations of the detected transition energies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in which the donor level (V2+/V3+) is situated 12 500 cmâ1 (1.55 eV) below the conduction-band edge and the acceptor level (V2+/V+) 9400 cmâ1 (1.17 eV) above the valence-band edge. The dynamical behavior of the three infrared lurainescence bands was measured. Decay time constants of 43 ÎŒs (V+), 120 ÎŒs (V2+), and 420 ÎŒs (V3+) were found. Electron-paramagnetic-resonance (EPR) results measured on the same samples are presented in an accompanying paper and confirm the optical detection of isolated substitutional V2+(d3) and V3+(d2) ions. Relations between the EPR and optical results are discussed
Hyperfine structure of antiprotonic helium revealed by a laser-microwave-laser resonance method
Using a newly developed laser-microwave-laser resonance method, we observed a
pair of microwave transitions between hyperfine levels of the
state of antiprotonic helium. This experiment confirms the quadruplet hyperfine
structure due to the interaction of the antiproton orbital angular momentum,
the electron spin and the antiproton spin as predicted by Bakalov and Korobov.
The measured frequencies of GHz and
GHz agree with recent theoretical
calculations on a level of .Comment: 4 pages, 4 figures, 1 tabl
First observation of two hyperfine transitions in antiprotonic He-3
We report on the first experimental results for microwave spectroscopy of the
hyperfine structure of antiprotonic He-3. Due to the helium nuclear spin,
antiprotonic He-3 has a more complex hyperfine structure than antiprotonic He-4
which has already been studied before. Thus a comparison between theoretical
calculations and the experimental results will provide a more stringent test of
the three-body quantum electrodynamics (QED) theory. Two out of four
super-super-hyperfine (SSHF) transition lines of the (n,L)=(36,34) state were
observed. The measured frequencies of the individual transitions are
11.12559(14) GHz and 11.15839(18) GHz, less than 1 MHz higher than the current
theoretical values, but still within their estimated errors. Although the
experimental uncertainty for the difference of these frequencies is still very
large as compared to that of theory, its measured value agrees with theoretical
calculations. This difference is crucial to be determined because it is
proportional to the magnetic moment of the antiproton.Comment: 8 pages, 6 figures, just published (online so far) in Physics Letters
Theoretical models of ferromagnetic III-V semiconductors
Recent materials research has advanced the maximum ferromagnetic transition
temperature in semiconductors containing magnetic elements toward room
temperature. Reaching this goal would make information technology applications
of these materials likely. In this article we briefly review the status of work
over the past five years which has attempted to achieve a theoretical
understanding of these complex magnetic systems. The basic microscopic origins
of ferromagnetism in the (III,Mn)V compounds that have the highest transition
temperatures appear to be well understood, and efficient computation methods
have been developed which are able to model their magnetic, transport, and
optical properties. However many questions remain.Comment: 4 pages, 4 figures, review, to be published in Curr. Appl. Phy
Preliminary Results from Recent Measurements of the Antiprotonic Helium Hyperfine Structure
We report on preliminary results from a systematic study of the hyperfine
(HF) structure of antiprotonic helium. This precise measurement which was
commenced in 2006, has now been completed. Our initial analysis shows no
apparent density or power dependence and therefore the results can be averaged.
The statistical error of the observable M1 transitions is a factor of 60
smaller than that of three body quantum electrodynamic (QED) calculations,
while their difference has been resolved to a precision comparable to theory (a
factor of 10 better than our first measurement). This difference is sensitive
to the antiproton magnetic moment and agreement between theory and experiment
would lead to an increased precision of this parameter, thus providing a test
of CPT invariance.Comment: 6 pages, 4 figure
Structure-activity correlations for BrĂžnsted acid, Lewis Acid, and photocatalyzed reactions of exfoliated crystalline niobium oxides
Exfoliated crystalline niobium oxides that contain exposed but interconnected NbO6 octahedra with different degrees of structural distortion and defects are known to catalyze BrĂžnsted acid (BA), Lewis acid (LA), and photocatalytic (PC) reactions efficiently but their structureâactivity relationships are far from clear. Here, three exfoliated niobium oxides, namely, HSr2Nb3O10, HCa2Nb3O10, and HNb3O8, are synthesized, characterized extensively, and tested for selected BA, LA, and PC reactions. The structural origin for BA is associated mainly with acidic hydroxyl groups of edge-shared NbO6 octahedra as proton donors; that of LA is associated with the vacant band position of Nb5+ to receive electron pairs from substrate; and that of PC is associated with the terminal Nb=O of NbO6 octahedra for photon capture and charge transfer to long-lived surface adsorbed substrate complex through associated oxygen vacancies in close proximity. It is believed that an understanding of the structureâactivity relationships could lead to the tailored design of NbOx catalysts for industrially important reactions
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