Efficient intracavity second-harmonic generation at 1063 nm in a GdCOB crystal

We report an efficient intracavity second-harmonic generation (SHG) at 1063 nm in a non-linear optical crystal, GdCa₄O(BO₃)₃ (GdCOB), performed with a diode end pumped continuous-wave (CW) Nd:GdVO₄ laser. In the case of a laser with an a-cut 0.4 at % Nd:GdVO₄ crystal frequency-doubled with a GdCOB crystal cut for Type I frequency doubling. A CW SHG output power of 2.25 W has been obtained using a 15 mm long GCOB crystal. The optical conversion efficiency with respect to the incident pump power was 12.4%.     

Electro-optically tunable self-focusing and self-defocusing in KTP crystals by a cascaded second-order process

We report the transformation of a linear electro-optically tunable non-phase-matched second-order nonlinear process into a cascaded second-order nonlinear process in a bulk KTP crystal to generate the effect of electrooptically tunable Kerr-type nonlinearity. By applying an electric field on the x–y plane, parallel to the z-axis of the crystal, phase mismatch is created, which introduces a nonlinear phase shift between the launched and reconverted fundamental waves from the generated second harmonic wave. Due to the nonuniform radial intensity distribution of a Gaussian beam, a curvature will be introduced into the fundamental wavefront, which focuses or defocuses the incident beam while propagating through the crystal.     

Cascaded second-harmonic and sum-frequency generation of a CO2 laser by use of a single quasi-phase-matched GaAs crystal

Cascaded second-harmonic and sum-frequency generation were used to obtain the third harmonic of a CO(2) laser in a single quasi-phase-matched GaAs crystal. Both continuous-wave and pulsed regimes with a single-pass configuration were studied. The continuous case confirmed the good capability of the GaAs stack to bear high average power density. In the pulsed regime a 0.66% peak power conversion efficiency was achieved for the third harmonic when the laser was pumping at 8.2 MW/cm(2) , in fair agreement with theoretical predictions.     

Observation of ultraslow light propagation in a ruby crystal at room temperature

We have observed slow light propagation with a group velocity as low as 57.5+/-0.5 m/s at room temperature in a ruby crystal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneously broadened absorption profile of ruby. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. We observe slow light propagation both for Gaussian-shaped light pulses and for amplitude modulated optical beams in a system that is much simpler than those previously used for generating slow light.     

Entangled femtosecond free induction signals in a cadmium sulfide crystal at room temperature

Entangled free induction decay (EFID) femtosecond signals are experimentally observed for the first time at a wavelength of 790 nm in a cadmium sulfide (CdS) crystal in the two-photon absorption (TPA) regime upon excitation by two crossed (angle, 60°) laser beams. The sample emitted two EFID signals simultaneously in opposite directions. The signals were diffracted by nonequilibrium electric polarization gratings induced by two laser beams in accordance with the laws of energy and momentum conservation. The possibility of exciting EFID signals in the three-photon absorption regime is discussed.     

Enhanced second-harmonic generation by use of the photonic effect in a ferroelectric smectic C* liquid crystal

Special phase matching of second-harmonic generation (SHG) by use of the photonic effect was experimentally confirmed for an obliquely propagating fundamental beam in the helical smectic C>(*) liquid-crystal phase, as suggested by Belyakov [JETP Lett. 70, 811 (1999)]. The enhancement of SHG occurs when the second-harmonic wave is close to a photonic bandgap (half-pitch band) and consequently the fundamental wave is close to another photonic bandgap (full-pitch band). The enhanced signal was observed from both the transmitted and the reflected directions and was compared with our recently developed simulation.     

Inverted EPR signal from nitrogen defects in a synthetic diamond single crystal at room temperature

Electron paramagnetic resonance (EPR) in diamond single crystals was studied. The crystals were grown using apparatuses of the “split-sphere” type in a Ni-Fe-C system using the temperature gradient method with a subsequent high-temperature high-pressure treatment. It was found that, after the high-temperature high-pressure treatment of a diamond sample, the EPR signal from the lattice defects containing nitrogen atoms became inverted with the growth of the microwave power in an H₁₀₂ resonator. In a constant polarizing magnetic field, when the microwave power applied to the diamond was low, a resonance absorption by the nitrogen defects took place, whereas, when the microwave power was high, an emission was observed. The inversion of the EPR lines of a single nitrogen atom substituting for a carbon atom at a diamond lattice site could be caused by the presence of a nickel atom with an uncompensated magnetic moment at the adjacent tetrahedral interstitial site. In synthetic diamond crystals that were not subjected to high-temperature high-pressure treatment, the inversion of the EPR signal from nitrogen atoms (P1 centers, nitrogen in the C form) was absent.     

Parametric generation of light in arbitrary directions of a biaxial KTP crystal pumped by an Nd:YAG laser

The conditions for collinear phase matching in the parametric generation of light in arbitrary directions of a biaxial KTP crystal pumped by an Nd:YAG laser were investigated with the use of invariant expressions for the refractive indices of isonormal waves. The tuning curves, effective nonlinear coefficients, spectral and angular phase-matching widths were calculated. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 177–184, March–April, 1999.     

Widely tunable femtosecond soliton pulse generation at a 10-GHz repetition rate by use of the soliton self-frequency shift in photonic crystal fiber

We demonstrate a soliton self-frequency shift of approximately 120 nm in a fiber with 1.56-microm pulses generated at a 10-GHz repetition rate by an actively mode-locked laser. A highly nonlinear photonic crystal fiber with a length of only 12.6 m and a nonlinear coefficient of 62 W(-1) km(-1) is used to achieve such broadband operation. The wavelengths of the resulting sub-300-fs solitons can be tuned effectively by adjusting the input power. The maximum output power of the solitons exceeds 200 mW.     

Efficient second-harmonic generation at 384 nm in periodically poled lithium tantalate by use of a visible Yb--Er-seeded fiber source

We generated 56 mW of average power at 384 nm in periodically poled lithium tantalate by use of a visible fiber source based on a variable-pulse format seeded Yb-Er amplifier and frequency upconversion in periodically poled KTP. The feasibility of high-average-power, wide ultraviolet wavelength-range fiber-based sources is evaluated.     

Vanadium oxide thin films produced by magnetron sputtering from a V2O5 target at room temperature

Vanadium oxide thin films were grown by RF magnetron sputtering from a V₂O₅ target at room temperature, an alternative route of production of vanadium oxide thin films for infrared detector applications. The films were deposited on glass substrates, in an argon–oxygen atmosphere with an oxygen partial pressure from nominal 0% to 20% of the total pressure.X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS) analyses showed that the films were a mixture of several vanadium oxides (V₂O₅, VO₂, V₅O₉ and V₂O₃), which resulted in different colors, from yellow to black, depending on composition. The electrical resistivity varied from 1 mΩ cm to more than 500 Ω cm and the thermal coefficient of resistance (TCR), varied from −0.02 to −2.51% K⁻¹.Computational thermodynamics was used to simulate the phase diagram of the vanadium–oxygen system. Even if plasma processes are far from equilibrium, this diagram provides the range of oxygen pressures that lead to the growth of different vanadium oxide phases. These conditions were used in the present work.     

Decrease of superparamagnetic fraction at room temperature in ultrafine CoFe2O4 particles by Ag doping

Co_1???x Ag _x Fe₂O₄ nanoparticles have been prepared by the combustion route. The average crystallite sizes for compositions with x = 0 and 0.2 are found to be 36 and 33 nm respectively from the XRD line broadening. Compared to the pure CoFe₂O₄, Ag-doping reduces the intrinsic magnetization values (M, M _r), but enhances coercivity (H _c). Mössbauer spectra show two sextets, indicating occupancies of tetrahedral and octahedral sites by Fe^3?+?. Hyperfine fields of 505 and 477 kOe in pure CoFe₂O₄ have been found for octahedral and tetrahedral sites respectively at liquid nitrogen temperature. The hyperfine field decreases with Ag-doping which also corroborates the magnetization studies. EPR study confirms the room temperature ferromagnetic behavior for Co_1???x Ag _x Fe₂O₄ (x = 0.2). The room temperature Mössbauer studies on x?=?0.0 and 0.2 show the ferromagnetic sextets (95%) along with superparamagnetic doublet (5%). However, x = 0.6 sample shows the ferromagnetic sextets only at room temperature. Highly Ag doped samples could be useful for the fabrication of the high-density magnetic materials as well as magnetic drug delivery.     

Phase-matched second-harmonic generation at 440 nm in a LiNbO3 -Gd3Ga5O12 waveguide with a tunable stimulated Raman source

Using a tunable stimulated Raman source, we have observed second-harmonic generation in the blue in a sputtered LiNbO₃ film deposited on Gd₃Ga₅O₁₂. With the choice of Gd₃Ga₅O₁₂ as a substrate, it becomes possible to both phase-match a large range of fundamental wavelengths as well as have an excellent epitaxial quality surface. The SHG efficiency for the TM^ω₀ → TM^2ω₂ phase matched conversion process is estimated to be ～ 10^-4.     

Efficient intracavity second-harmonic generation at 930 nm in a BiB<Subscript>3</Subscript>O<Subscript>6</Subscript> crystal

It is reported that efficient continuous-wave (CW) blue laser generation at 465 nm in a BiB₃O₆ (BiBO) crystal at type-I phase matching direction performed with a diode-pumped Nd³⁺:YAlO₃ (Nd:YAP) laser. With incident pump power of 18.4 W, output power of 823 mW at 465 nm has been obtained using a 10 mm-long BiBO crystal. At the output power level of 823 mW, the output stability is better than 2.3%.     

Optimization of TiO_2/Cu/TiO_2 multilayers as a transparent composite electrode deposited by electron-beam evaporation at room temperature

Highly transparent indium-free composite electrodes of Ti O2/Cu/Ti O2 are deposited by electron-beam evaporation at room temperature. The effects of Cu thickness and annealing temperature on the electrical and optical properties of the multilayer film are investigated. The critical thickness of Cu mid-layer to form a continuous conducting layer is found to be 11 nm. The multilayer with a mid-Cu thickness of 11 nm is optimized to obtain a resistivity of 7.4×10-5Ω·cm and an average optical transmittance of 86% in the visible spectral range. The figure of merit of the Ti O2/Cu(11 nm)/Ti O2 multilayer annealed at 150ΩC reaches a minimum resistivity of 5.9×10-5Ω·cm and an average optical transmittance of 88% in the visible spectral range. The experimental results indicate that Ti O2/Cu/Ti O2 multilayers can be used as a transparent electrode for solar cell and other display applications.     

Single line high efficiency tunable fir generation by resonant four-wave mixing in CH3 pumped by a multiatmosphere tunable CO2 laser

By pumping CH₃F with a high pressure tunable TE-CO₂ laser, the resonant four-wave mixing process (RFWM) generates a very efficient tunable single line FIR emission at the Raman frequency. This result is strictly related to the spectroscopic structure of the CH₃F molecule. By means of this process, a tunable FIR emission on a 0.1 cm^–1 bandwidth 150 kW (8 mJ) single line, is obtained which can be used for many FIR multiphoton applications.     

Precise control over oxygen impurities in nano-crystalline silicon thin film processed with a low hydrogen dilution gas system at near room temperature

An atmosphere highly diluted with hydrogen is essential to increase the crystal fraction during formation of hydrogenated nano-crystalline (nc) or micro-crystalline (μc) silicon thin films via chemical vapor deposition (CVD). This hydrogen-rich process, however, hinders the ability for the material to find adequate use in micro-electronic devices due to contamination that results in oxygen-related problems such as donor-like doping, defect creation, or passivation. The use of neutral beam assisted chemical vapor deposition (NBaCVD), with a low hydrogen ratio (R = H₂/SiH₄) of 4, successfully deposits a highly-crystallized nc-silicon (HC nc-Si) thin film (TF) at near room temperature (<80 °C) and effectively reduces oxygen contamination by as much as 100 times when compared to conventional plasma enhanced CVD. During the formation of HC nc-Si TF via NBaCVD, energetic hydrogen atoms directly react with oxygen atoms near the surface of the nc-Si TF and remove the oxygen impurities. This is a completely different mechanism from the hydrogen-enhanced oxygen diffusion model. This technology meets the recent requirements of a high deposition rate and low temperature necessary for flexible electronics.     

Ultraslow Gaussian pulse propagation induced by a dispersive phase coupling in photorefractive bismuth silicon oxide crystals at room temperature

By use of the highly dispersive phase coupling effect in a photorefractive wave mixing process, we have observed ultraslow propagation of a single Gaussian light pulse with a group velocity ∼0.5 m/s in a photorefractive Bi₁₂SiO₂₀ crystal at room temperature. The ultraslow Gaussian pulse is amplified due to an intensity coupling effect but keeping its Gaussian profile with high fidelity. The group velocity of the Gaussian pulse can be controlled to a large extent. This technique is useful for controllable optical delay lines.