Noncollinear Phase Research Articles

Broadband amplification in non-linear crystals using controlled angular dispersion of signal beam

Recent advances in high intensity laser technology, such as the optical parametric chirped pulse amplification technique, allow the amplification of broadband pulses to high energies, by parametric interaction in a non-linear crystal. In this article, angular dispersion of the signal beam inside the non-linear media is added to a common setup to improve the non-collinear phase matching range. A comparative study of the performance of BBO, LBO and KDP using this geometry is undertaken. Computer simulations show that both BBO and LBO are excellent broadband performers. Although the maximum bandwidth of KDP does not broaden, we show how to avoid its severe narrowing below the degeneracy wavelength.

Integrated-grating-induced control of second-harmonic beams in frequency-doubling crystals

We report a novel type of integrated nonlinear photonic device for controlling the generation of several second-harmonic beams. Two-dimensional diffraction gratings are recorded with femtosecond laser pulses at the entrance surface of a frequency-doubling crystal. This periodic spatial modulation of the material surface induces noncollinear propagation of the fundamental input signal in the crystal. By slightly changing the angle of incidence of the seed beam, collinear and noncollinear phase matching can be achieved between different diffraction orders, in this way allowing the efficient generation of several second-harmonic beams.

Dynamic recording of χ(2) holograms with multifrequency object and reference waves

The transformation properties of images formed by inertialess dynamic χ(2) holograms recorded by multifrequency reference and object beams are studied theoretically and experimentally. The regular features of the spatial localization and scales of holographic images obtained with different frequency combinations of the reference and object image-forming beams are ascertained. It is shown that, in the general case of an arbitrary variation in the ratio of these frequencies, the reconstructed images of an object point source are localized on a segment of a straight line, which we referred to as the locris. The slope of the locris does not depend on the frequency ratio of the reference and object beams, and its endpoints are specified by the positions of the reference and object sources. The possibility of obtaining of achromatic holographic images for noncollinear multifrequency interactions is demonstrated. The conditions for the noncollinear phase matching of interacting waves in a KTP crystal, as well as for simultaneous attainment of phase matching for selected frequency combinations, are determined. The important role of controlling the shape of wavefronts in experiments with frequency conversion of radiation in the reference beam by stimulated Raman scattering of light is ascertained.

Compensation of group velocity mismatching of three pulses in femtosecond optical parametric amplification

For avoiding adverse effect arising from group velocity mismatching in parametri c amplification, a new method is presented, in which we combine pulse front slan t and noncollinear phase matching to completely compensate group velocity mismat ching of three pulses, in femtosecond optical parametric amplification (OPA). Th e change of the phase matching angle, the pulse front slant angle and the noncol linear angle with the signal wavelength are calculated, when group velocity matc hing of three pulses is achieved in BBO crystal type_Ⅰ and typeⅡ phase matching configurations. The effect of group velocity matching of three pulses on the spatial walk_off length, the parametric gain and the parametric bandwidth is analyzed. The results indicate that, using this method can make matching of gro up velocities of three pulses, when tuned continuously in both BBO type_I and Ⅱ phase matching femtosecond OPA, and thus the effective interaction length of th ree pulses is increased considerably. This calculation method and the analysis r esults provided a theoretical guidance for obtaining parametric pulses with high er gain and shorter duration.

Optimized basis sets for the collinear and non-collinear phases of iron

Systematic implementations of density functional calculations of magnetic materials, basedon atomic orbitals basis sets, are scarce. We have implemented in one such codethe ability to compute non-collinear arrangements of the spin moments in theGGA approximation, including spiral structures. We have also made a thoroughstudy of the degree of accuracy of the energy with the size of the basis and theextent of the orbitals. We have tested our results for the different phases of bulkiron as well as for small clusters of this element. We specifically show how therelative stability of the different competing states changes with the degree ofcompleteness of the basis, and present the minimal set which provides reliable results.

Phase separation in La0.67Ca0.33Mn0.9Fe0.1O3: a Mössbauer study

Mössbauer spectroscopy of La0.67Ca0.33Mn0.9Fe0.1O3 shows phase separation above and below the magnetic transition temperatureTC. These phase separations are found to be independent and related todifferent phenomena. The relative volumes of the two phases above and belowTC are also very different. Mössbauer spectra aboveTC show that the phase splitting is due to different lattice distortions around Fe in the two phases.The paramagnetic component showing large distortion disappears when the magnetichyperfine splitting appears on lowering the temperature. The phase splitting belowTC is due to a difference in the nature of magnetic ordering and the degree of covalence of themagnetic ions in the two phases. DC magnetization shows that the spin ordering in the twophases is collinear and non-collinear, respectively. At low temperatures, the non-collinearphase converts into the collinear phase when the applied magnetic field exceeds a certainvalue. When the field is withdrawn, the non-collinear phase is not fully regained.

COUPLED CLUSTER TREATMENTS OF QUANTUM MAGNETS: TWO EXAMPLES

In this article we study the ground-state properties of two square-lattice Heisenberg quantum spin models with competing bonds using a high-order coupled cluster treatment. The first model is a spin-half model with competing nearest-neighbour bonds with and without frustration. We discuss the influence of quantum fluctuations on the ground-state phase diagram and in particular on the nature of the zero-temperature phase transitions from phases with collinear magnetic order at small frustration to phases with noncollinear spiral order at large frustration. The second model is a highly frustrated ferrimagnet, which contains one sublattice (A) entirely populated with spin-one spins and an other sublattice (B) entirely populated with spin-half spins. Sublattice A sites are nearest-neighbours to sublattice B sites and vice versa and frustration is introduced by next-nearest-neighbour bonds. The model shows two collinear ordered phases and a noncollinear phase in which (classically) the spin-one spins are allowed to cant at an angle. Both examples show that the coupled-cluster method is able to describe the zero-temperature transitions well and provides a consistent description of collinear, noncollinear, and disordered phases, for cases in which other standard techniques (e.g. the quantum Monte Carlo technique for spin systems which are frustrated) are not applicable.

Femtosecond optical parametric generation of noncollinear phase matching for a biaxial crystal.

In an optical parametric generation of a femtosecond pulse for a biaxial crystal, the interaction of three waves can be used as a model of noncollinear phase matching in which the group velocities of the interacting pulses are suitably linked to each other. For satisfaction of group-velocity matching, the tunable parametric generation of femtosecond pulses must use noncollinear phase matching. We consider three conditions of group-velocity matching for femtosecond pulses. Signal and idler pulses can be obtained when the coupled-wave equations, including the group-velocity mismatch and group-velocity dispersion effects, are solved. A Fourier method is an effective method for solving the equations, and from the solution of the equations the relation between duration of output pulses and wavelengths can be obtained. In a comparison of collinear and noncollinear matches, when the latter is group-velocity matched, the duration of its outpulses are smaller, and the outpulses can be continually tuned from the visible to the mid-infrared.

Understanding the complex metallic element Mn. I. Crystalline and noncollinear magnetic structure of α-Mn

Manganese is an element with outstanding structural and magnetic properties. While most metallic elements adopt a simple crystal structure and order magnetically---if at all---in a simple ferromagnetic or antiferromagnetic configuration, the stable phase of manganese at ambient conditions, paramagnetic $\ensuremath{\alpha}\ensuremath{-}\mathrm{Mn},$ adopts a complex crystal structure with 58 atoms in the cubic cell. At a N\'eel temperature of ${T}_{N}=95\mathrm{K},$ a transition to a complex noncollinear antiferromagnetic phase takes place. The magnetic phase transition is coupled to a tetragonal distortion of the crystalline structure. In this paper we present an ab initio spin-density functional study of the structural and magnetic properties of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Mn}.$ It is shown that the strange properties of Mn arise from conflicting tendencies to simultaneously maximize according to Hund's rule the magnetic spin moment and the bond strength, as expected for a half-filled d band. Short interatomic distances produced by strong bonding tend to quench magnetism. The crystal structure of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Mn}$ is essentially a consequence of these conflicting tendencies---it may be considered as a topologically close-packed intermetallic compound formed by strongly magnetic (MnI, MnII) and weakly magnetic (MnIII) or even nearly nonmagnetic (MnIV) atoms. The noncollinear magnetic structure is due to the fact that the MnIV atoms arranged on triangular faces of the coordination polyhedra are not entirely nonmagnetic---their frustrated antiferromagnetic coupling leads to the formation of a local spin structure reminiscent of the N\'eel structure of a frustrated triangular antiferromagnet. Consequently, also the other magnetic moments are rotated out of their collinear orientation. The calculated crystalline and magnetic structures are in good agreement with experiment. However, it is suggested that the magnetism leads to a splitting of the crystallographically inequivalent sites into a larger number of magnetic subgroups than deduced from the magnetic neutron diffraction data, but in accordance with NMR experiments. In a companion paper, the properties of the other polymorphs of Mn and their relative stability will be discussed.

Frequency-tunable terahertz wave generation via excitation of phonon-polaritons in GaP

High-power, wide-frequency-tunable terahertz waves were generated based on difference-frequency generation in GaP crystals with small-angle noncollinear phase matching. The tunable frequency range was as wide as 0.5–7 THz, and the peak power remained high, near 100 mW, over most of the frequency region. The tuning properties were well described by the dispersion relationship for the phonon-polariton mode of GaP up to 6 THz. We measured the spectra of crystal polyethylene and crystal quartz with high resolution using this THz-wave source.

Frequency-tunable high-power terahertz wave generation from GaP

A frequency-tunable terahertz wave was generated from GaP crystals using an optical parametric oscillator as the pump source and a YAG laser (1.064 μm) as the signal source. By tuning the very small angle, θin, between the pump and signal light beam directions, tunable terahertz waves over the frequency range from 0.5 to 3 THz were obtained. The THz frequency changed almost linearly with the angle θin. The precise noncollinear phase matching condition is discussed. The pulsed peak power of the THz wave was as high as 480 mW at 1.3 THz.

The existence of a stable noncollinear phase in a Heisenberg model with complex structure

We have analyzed the properties of a noncollinear magnetic phase obtained in the mean-field analysis of the model of two coupled Heisenberg subsystems. The domain of its existence and stability is narrow and depends on the ratio between the averaged over nearest neighbours microscopic exchange parameters.

Pressure-induced antiferromagnetism in TbPt and TbPt 0.6 Cu 0.4 : a neutron-diffraction study

We present the evolution of the magnetic structures of TbPt and TbPt0.6Cu0.4 under pressures up to 65 kbar. The first compound is ferromagnetic non-collinear -CxFz, while in the Cu-diluted one an almost antiferromagnetic (AF) behaviour appears, corresponding to a complex structure in which the -CxFz phase coexists with an incommensurate one. We have found that pressure favours a general AF behaviour, which manifests as a clear increasing of the AF component in the non-collinear phase, and in the evolution to a pure AF arrangement of the moments in the unit cell for the incommensurate one.

Focus wave modes in optical parametric generators

It is shown that in the paraxial approximation the conditions for noncollinear phase matching for three-wave parametric interaction in a nonlinear crystal coincide with the conditions for generation of focus wave modes. As a result, the creation of broadband localized optical fields (pulsed beams without diffraction and dispersion spreading) inside the crystals of parametric generators is feasible.

Effect of pressure and Mn substitution on magnetic ordering of Ce 2 Fe 17-x Mn x ( x=0,1 )

The magnetic ordering of Ce2Fe17 and Ce2Fe16Mn at ambient and under high pressures up to 5 kbar was studied by neutron diffraction in the temperature range 2–300 K. The existence of collinear ferromagnetic (below 94 K) and incommensurate helical antiferromagnetic structures with a propagation vector τ1=(0,0,τ1z) was found at ambient pressure in Ce2Fe17. A helical structure with τ2=(0,0,τ2z) exists below 200 K down to low temperatures in Ce2Fe16Mn. Application of pressure results in the suppression of the ferromagnetic phase in Ce2Fe17 by P>3 kbar. A new pressure-induced incommensurate antiferromagnetic phase appears at T 3 kbarit transforms to a non-collinear phase, which can be described using a distorted elliptical spiral model.

Surface roughness dependence of noncollinear phase matching

Spontaneous noncollinear second harmonic generation in potassium dihydrogen phosphate (KDP) crystals was studied theoretically and experimentally as a function of the scattering centres on the surface of the crystal. A model that explains the main features of the second harmonic generated beam was developed in terms of the surface roughness defined by the size of the dispersing grains. It predicts, for the first time, the intensity distribution of the hollow cones obtained in noncollinear phase matching.

Broadband infrared generation with noncollinear optical parametric processes on periodically poled LiNbO_3

Broadband signal and idler generation based on the spectral retracing behavior in noncollinear phase matching of optical parametric generation in periodically poled LiNbO(3) (PPLN) is reported. Using PPLN of 29.5-mum quasi-phase-matching period and a Q-switched Nd:YAG laser as a pump, we obtained a broad signal spectrum from 1.66 to 1.96 mum and corresponding idler wavelengths from 2.328 to 2.963 mum. The experimental results were consistent with theoretical predictions. Circular and elliptical pump beams were also compared.

Phase matching analysis of noncollinear optical parametric process in nonlinear anisotropic crystals

Phase matching for noncollinear optical parametric generation is investigated. All the possible phase matching configurations and existence conditions for general noncollinear three-wave mixing interactions are derived for propagation within the crystal principal planes. Numerical expressions for the critical phase matching angles are presented wherever possible. Finally, as an application of these expressions, several numerical calculations of the phase matching angles for general noncollinear phase matched optical parametric amplification in the nonlinear-optical crystals such as β-BaB 2O 4, LiB 3O 5 and KTiOPO 4 are completed, and the results are graphically presented.

Temporal solitons in second-harmonic generation with a noncollinear phase-mismatching scheme

A noncollinear second-harmonic-generation scheme that includes two gratings and a nonlinear optical crystal generates temporal solitons with a noncollinear phase mismatch and frequency-chirped laser pulses. At 180-fs pulse duration, 25-GW/cm2 fundamental intensity, -7647.3-m(-1) wave-vector mismatch, 66-fs delay time, and +/-3.07163 x 10(25) s(-2) frequency-chirp rates, temporal solitons with durations from 139 to 155 fs and Gaussian shapes can be obtained. The corresponding conversion efficiency is greater than 40%.

Crosscorrelation measurements of ultrashort visible pulses: comparison between nonlinear crystals and SiC photodiodes

Crosscorrelation (CC) measurements of tunable sub-20 fs visible pulses from noncollinearly phase matched optical parametric amplifiers are reported. It is found that even 25 and 100 μm thick BBO crystals used as the nonlinear medium possess too small an acceptance bandwidth. For vanishing chirp of the pulses or linear chirp of the same sign, the resulting error in the observed CC width is not very severe, however, for opposing chirp errors larger than a factor of 2 are demonstrated. In contrast, measurements utilizing two-photon photoconductivity in SiC photodiodes are found to be simple and very reliable. The SiC photodiode is well suited for both interferometric autocorrelation measurements on the 10 fs time scale and for CC measurements in the visible.