Co-doped Cr Research Articles

Room Temperature Ferromagnetism and Enhanced Band Gap Values in N Co-doped Cr: CdS Nanoparticles for Spintronic Applications

Room Temperature Ferromagnetism and Enhanced Band Gap Values in N Co-doped Cr: CdS Nanoparticles for Spintronic Applications

Enhanced Thermoelectric Properties of Codoped Cr2Se3: The Distinct Roles of Transition Metals and S.

Pristine Cr2Se3 is a narrow-band gap semiconductor but with an inferior ZT value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of the Cr2Se3 material by optimizing carrier concentration, suppressing the bipolar thermal conductivity, and reducing the lattice thermal conductivity simultaneously. First, the effect of different dopants (Nb, Ni, and Mn) on the phase composition and thermoelectric transport properties of M2 xCr2-2 xSe3 (M = Nb, Ni, and Mn; x = 0-0.02) compounds are systematically investigated. The roles of those dopants are distinct. Mn-doped samples show superior thermoelectric properties in comparison with those of other-element-doped samples. Doping with Mn significantly increases the carrier concentration, accompanied with a suppression of the intrinsic excitation and a reduction of both the bipolar thermal conductivity and the lattice thermal conductivity of Cr2Se3. To further reduce the thermal conductivity, we have synthesized Mn and S codoped Mn0.04Cr1.96Se3-3 xS3 x ( x = 0-0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum ZT value of Mn0.04Cr1.96Se2.7S0.3 reaches 0.33 at 823 K. Compared with the pristine Cr2Se3 sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperatures by 200 K.

Crystal growth and optical properties of Cr 3+, Er 3+, RE 3+: Gd 3Ga 5O 12 (RE=Tm, Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr 3+, Er 3+, and RE 3+ (RE=Tm, Ho, Eu) doped Gd 3Ga 5O 12 crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr 3+ ion highly increases the absorption pump power and the three kinds of co-doped RE 3+ ions depopulate the Er: 4I 13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ∼3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.

Luminescence properties of highly Cr co-doped Nd:YAG powder produced by sol–gel method

We examined luminescence properties of highly chromium co-doped Nd:YAG powders fabricated by the sol–gel method. Though the maximum luminescence intensity of Cr:YAG was at 3.0 mol% of Cr 3+ concentration, we found that the enhancement effect on Nd:YAG by co-doping Cr 3+ was continued until 6.0 mol% of Cr 3+ concentration. This phenomenon is well-explained by a simple energy transfer model derived from the rate equations between Cr 3+ and Nd 3+. The enhancement of sensitization for Nd 3+ activator by high co-doped Cr 3+ is very effective for making a miniaturized or thin film-type solar laser.

Numerical solution and experiment of a self-Q-switched 946 nm Cr,Nd:YAG laser

A LD-pumped self-Q-switched 946 nm laser by using a co-doped Cr,Nd:YAG crystal as a gain medium as well as a saturable absorber is studied. The 946 nm self-Q-switched rate equations of co-doped crystal are solved numerically by Runge-Kutta method directly. The important parameters of lasers, such as average output power, threshold pump power, pulse width, pulses repetition rate, and optimal transmission of output coupler are obtained numerically. Experimentally, the maximum average output power up to 2.61 W, corresponding to a slope efficiency of 23.43%, was obtained in a simple and compact linear cavity. The optical-to-optical efficiency is 17.3% and the peak power is 7.57 kW with repetition rate of 23.78 kHz and pulse width of 14.5 ns. In the experiments, a high-quality fundamental transverse mode can be preserved in a large range of incident pump power. The numerical results of the 946 nm self-Q-switched Cr,Nd:YAG laser are in good agreement with the experimental results.

Codoped Cr and W rutile nanosized powders obtained by pyrolysis of triethanolamine complexes

Polycrystalline, rutile phase, nanosized mixed transition metal oxide, with compositions Cr 2 x W x Ti 1−3 x O 2 ( x = 0.05, 0.1, 0.15), have been prepared through pyrolysis of aqueous precursor solutions containing stoichiometric amounts of ammonium dichromate, dimethylammonium tungstate, titanium oxalate and optimum amounts of TEA. The rutile phased oxide solid solutions obtained on calcination of the precursors at 800 °C have been characterized by XRD, TG-DTA, BET surface area measurement, UV–visible spectroscopy and TEM. Substitution of Cr(III) for Ti(IV) ions in the rutile lattice causes colouring effect in the solid solutions while the W(VI) ions are incorporated for charge compensation. The nanosized powders of the solid solutions exhibit buff colour and their average particle sizes and specific surface areas are found to range between 30–60 nm and 56–68 m 2/g, respectively. Various TiO 2-based mixed oxides of variant colours can be prepared through the developed chemical process.

Timing jitter reduction and single-frequency operation in an acousto-optic Q-switched Cr,Nd:YAG laser

A laser-diode-pumped watt-level single-frequency combined actively and passively Q-switched (CAPQ) laser with a wavelength of 1064 nm that simultaneously uses a codoped Cr(4+),Nd(3+):YAG crystal as the gain medium and mode selector is presented. The optimum repetition period of the CAPQ laser is approximately 1.5 times the natural repetition period of the passively Q-switched laser. The averaged timing jitter is approximately 400 ns in the CAPQ laser and the repetition rate of the CAPQ laser is experimentally shown to reach a time instability of 0.2% over 10(6) pulses. The output is a stable single frequency with a linewidth of 400 MHz at 20 W pump power.

Performance of diode-end-pumped Cr4+, Nd3+:YAG self-Q-switched and Nd:YAG/Cr4+:YAG diffusion bonded lasers

Abstract Self-passively Q-switching of a diode-pumped Cr,Nd:YAG, where the Cr4+ is used as a saturable absorber for the 1064 nm laser emission is reported. The maximum average output power was obtained using an output coupler of R=86%. The self-Q-switched diode pumped laser yielded 1.86-W average output power with low threshold pumping power (≈1.7-W), average slope efficiency of ≈34%, pulse duration of about 14–16 ns, and modulation frequency ranging from 2.4 to 73 kHz, depending on the input pumping power. These results are the highest reported for self-Q-switched lasers. Higher slope efficiency (42%) and shorter Q-switched pulses were obtained for a Q-switched Nd:YAG/Cr4+:YAG diffusion bonded laser. A comparison of the codoped Cr,Nd:YAG laser performance, with that of a diffusion bonded laser is reported and analyzed.

Cr,Nd:YAG self-Q-switched laser with high efficiency output

The high efficient laser performance of self-Q-switched laser in the co-doped Cr 4+,Nd 3+:YAG microchip with 1.8 mm thickness was demonstrated. The slope efficiency is varied with the reflectivity of output coupler at 1064 nm , and the highest slope efficiency of 26% was obtained for 95% reflectivity of output coupler at 1064 nm . The pulse width, the single pulse energy and the pulse repetition rate for different reflectivity of the output couplers were measured, and the experimental results agree with the numerical calculations of the passively Q-switched rate equations. This can lead to develop the diode laser pumped monolithic self-Q-switched solid-state microchip lasers, especially for the intracavity frequency-doubled solid-state microchip lasers.

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO3, congruent Cr:LiNbO3 and congruent Cr,Mg:LiNbO3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr3+ impurity ions that preferentially occupy Li+ sites (CrLi) in the Cr:LiNbO3 crystals. Low-field centers related to Cr3+ substitution of Nb5+ (CrNb) occur in addition to CrLi in co-doped Cr,Mg:LiNbO3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4 T 2 state, resulting in its crossing with the 2 E state of Cr3+. This level-crossing effect was observed for the dominant Cr3+ Li and Cr3+ Nb centers at pressures that correlate well with estimations based on the 4 T 2-2 Eenergy gap (230 cm-1 and 1160 cm-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm-1/kbar, respectively). We also studied inhomogeneous broadeningof the 2 E?4 A 2transitions at ambient pressure for the minor high-field “defect” Cr3+ Li centers in congruent LiNbO3. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr3+ ions in the LiNbO3 lattice.

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO3 and of congruent Cr, Mg: LiNbO3 were studied. Cr3+ impurity ions preferentially occupy Li+ sites (CrLi) in the LiNbO3 crystal lattice, while Cr3+ ions substituting for Nb5+ ions (CrNb) occur in addition to CrLi centers in codoped Cr, Mg: LiNbO3 crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr3+ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the 4 T 2 state resulting in crossing with the 2 E state, the replacement of the broad band 4 T 2 → 4 A 2 emission by a narrow R-line emission 2 E → 4 A 2 occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr 3+Li and Cr 3+Nb centers at pressures which correlated well with estimations based on the 4 T 2-2 E energy gap (230 and 1160 cm−1) and on the rate of their pressure-induced change (14.35 and 11.4 cm−1/kbar, respectively).

Multicolor-emitting thin-film electroluminescent devices using Ga2O3 phosphors co-doped with Mn and Cr

High-luminance multicolor-emitting thin-film electroluminescent (TFEL) devices have been developed using a new Mn- and Cr-co-doped Ga2O3 phosphor with variations of both Mn and Cr from 0 to a content of 20 at. %. Ga2O3:Mn,Cr TFEL devices driven at 1 kHz exhibited a change from green to red in emission color, maintaining luminances above 100 cd/m2, when the co-doped Cr content was varied from 0 to 20 at. % under a constant Mn content of 0.3 at. % doping. In addition, the emission color of a Ga2O3:Mn,Cr TFEL device co-doped with a Mn content of 0.3 at. % and a Cr content of 3 at. % changed from green to red with an increase of the applied voltage.

Efficient Cr,Nd:Gd3Sc2Ga3O12 laser at 1.06 μm pumped by visible GaInP/AlGaInP laser diodes

The 1.06 μm Nd transition in co-doped Cr,Nd:Gd3Sc2Ga3O12 (Cr,Nd:GSGG) is obtained by diode pumping Cr3+ at 670 nm and is shown to produce efficient, low-threshold laser operation. Both cw and long-pulse diode pumping were demonstrated, with pump power levels as high as 300 mW cw and 1 W pulsed. The lowest threshold power measured was 938 μW, and the highest output power obtained was 43 mW cw and 173 mW pulsed. The best slope efficiency obtained was 42.1%, 78% of the theoretical maximum. Loss measurements indicate a value of 0.4% cm−1.