Noncollinear Interaction Research Articles

Non-collinear upconversion of infrared light.

Two dimensional mid-infrared upconversion imaging provides unique spectral and spatial information showing good potential for mid-infrared spectroscopy and hyperspectral imaging. However, to extract spectral or spatial information from the upconverted images an elaborate model is needed, which includes non-collinear interaction. We derive here a general theory providing the far field of the upconverted light when two arbitrary fields interact inside a nonlinear crystal. Theoretical predictions are experimentally verified for incoherent radiation and subsequently applied to previously published data with good agreement.

Enhanced XUV pulse generation at 89 nm via nonlinear interaction of UV femtosecond filaments.

We demonstrated efficient generation of ultrafast extreme ultraviolet (XUV) pulses at 89 nm from an intense 267 nm UV femtosecond filamentation in argon gas. The XUV pulse generation efficiency was significantly enhanced through UV filament termination from the argon gas into the background vacuum that prevented backconversion of the generated third-harmonic XUV to the fundamental-wave UV pulses. This was further combined with noncollinear UV filament interaction that produced volume-plasma to pre-extract XUV pulses from the filament core, resulting in further XUV enhancement. The generated 89 nm XUV pulses reached what is so far, to our knowledge, the highest pulse energy of 190 nJ.

Characterization of in-situ terahertz detection by optical interaction in a periodically poled stoichiometric lithium tantalate nonlinear crystal

Terahertz waves are generated using a femtosecond laser pulse in a periodically poled stoichiometric lithium tantalate crystal and simultaneously detected via a non-collinear optical parametric interaction inside the same crystal. Real time up-conversion signal between the generated THz and an optic probe pulses is measured depending on the beam overlapped conditions using a general silicon-photodiode for the THz detection. The non-collinear geometry is to facilitate manipulated property of the position-dependent bandwidth at narrow and broad bandwidths of 45 GHz and 3.3 THz, respectively at the one crystal. Furthermore, an aperture effect at the detection part is characterized as the function of size and position owing to the spatial distribution of the frequency conversion signal and it is applied in optimization of the in-situ detection scheme.

Broadband parametric processes in χ(2) nonlinear photonic crystals.

We develop a general model, based on a (2+1)D unidirectional pulse propagation equation, for describing broadband noncollinear parametric interactions in 2D quadratic lattices. We apply it to the analysis of twin-beam optical parametric generation in hexagonally poled LiTaO3, gaining further insights into experimental observations.

Non-collinear interaction of photons with orbital angular momentum

We study the nonlinear interaction between two non-collinear light beams that carry orbital angular momentum (OAM). More specifically, two incident beams interact at an angle in a medium with a second order nonlinearity and thus generate a third, non-collinear beam at the second harmonic frequency that experiences a reduced conversion efficiency in comparison to that expected based on conventional phase-matching theory. This reduction scales with the input beam OAM and, differently from previous spiral bandwidth calculations, is due to a geometric effect whereby the input OAM is projected along the non-collinear interaction direction. The effect is relevant even at small interaction angles and is further complicated at large angles by a non-conservation of the total OAM in the nonlinear interaction. Experiments are performed under different conditions and are in excellent agreement with the theory. Our results have implications beyond the specific case studied here of second-harmonic generation, in particular for parametric down-conversion of photons or in general for phase-matched non-collinear interactions between beams with different OAM.

Possibilities of tomography system prototype using third-order acoustic nonlinear effects

Third-order nonlinear acoustical tomography is very important for medical diagnostics, because it will provide information on the new and unexplored quantity—third-order nonlinear acoustical parameter. A prototype of the tomography system for reconstructing the distributions of the acoustic nonlinear parameters is developed in our group on the basis of effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected signal at combination frequencies makes it possible to reconstruct the complete image of an object using three transmitters and one receiver. Third-order effects are born from two competing processes—the pure third order interaction (informational for diagnostic purposes) and the twofold interaction of the second order (interfering). To explore the application boundaries of the third-order nonlinear acoustical tomography, these two competing processes are considered in details. Two cases are compared. In the first case, the interaction takes place between two broadband coded signals and one monochromatic signal. In the second case, all three primary signals are broadband coded; then, amplitude of the interfering part of the signal is smaller. Moreover, in this case, the possibility of reconstructing the spatial distribution of the second and third-order nonlinear acoustical parameters arises due to the reciprocity principle generalized on the nonlinear scattering processes.

Nuclear spin polarization in a single quantum dot pumped by two laser beams

We theoretically investigate dynamic nuclear spin polarization in a self-assembled quantum dot pumped optically by two laser beams. With the assumption that a noncollinear interaction between the hole spin and nuclear spins leads to nuclear spin polarization, we find that both weak and strong nuclear spin polarizations can arise, depending on the intensities and central frequencies of the lasers. For weak nuclear spin polarization, we use a perturbation method to show that the distribution of the nuclear spin Overhauser field may become significantly narrower. Using Monte Carlo simulations to study a single quantum dot, we find that strong nuclear spin polarization can also be generated via appropriate optical pumping.

Tunable nonlinear beam shaping by non-collinear interactions

A method is proposed for nonlinear beam shaping, employing a non-collinear quasi phase-matched interaction in a crystal whose nonlinear coefficient is encoded by a computer generated hologram pattern. In this method the same axis is used for both satisfying the phase-matching requirements and encoding the holographic information, the result is a single shaped beam in the generated frequency. This allows to shape beams in one-dimension using a very simple method to fabricate patterned nonlinear crystals and to shape beams in two-dimensions with high conversion efficiency. The one-dimensional case is experimentally demonstrated by converting a fundamental Gaussian beam into Hermite-Gaussian beams at the second harmonic in a KTiOPO4 crystal. The two-dimensional case is demonstrated by generating Hermite-Gaussian and Laguerre-Gaussian beams in a stoichiometric lithium tantalate crystal. The suggested scheme enables broad wavelength tuning by simply tilting the crystal.

A prototype for a tomography system using third-order acoustic nonlinear effects

A prototype of a tomography system for reconstructing the distributions of acoustic nonlinear parameters is developed and manufactured on the basis of the effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected combination signal makes it possible to reconstruct the complete image of an object as a result of a single experiment using a small number of transducers, i.e., three radiators and one receiver. A mirror system is proposed, consisting of two coaxial conical acoustic mirrors that make it possible to transform the front of a wave from a cylindrical transducer into a homogeneous quasi-plane beam with a large width close to the real medical diagnostics requirements. Results of physical experiments are given.

Simultaneous collinear and non-collinear parametric generation in 1D single grating periodically poled lithium tantalate

We report a detailed study of multiple wavelengths optical parametric generation in a single grating one-dimensional periodically poled lithium tantalate crystal. Simultaneous collinear and non-collinear generations are observed around the pump collinear direction. Similar spectra are found in continuous spatial positions symmetrical to the collinear direction, with decreasing signal and increasing idler wavelengths, associated with decreasing powers when the far-field angle increases. A phase matching scheme is proposed to describe these interactions. Numerical simulations emphasize the agreement of our phase matching scheme with the experimental results. Single-pass gain and high pump power density are very likely at the origin of the simultaneous collinear and non-collinear QPM interactions.

Single-chain magnets: beyond the Glauber model

Single-chain magnets (SCMs) are one-dimensional (1D) slow-relaxing magnetic chains with interesting fundamental properties and applications, such as information storage. The mechanism underlying the slow relaxation of SCMs is Glauber dynamics, in which R. J. Glauber proposed about 50 years ago that the 1D correlation of ferromagnetically coupled Ising spins causes very slow magnetic relaxation at a finite temperature. Since the first experimental observation of SCM dynamics of a cobalt(II)–organic radical alternating chain in 2001, several SCM systems have been reported, supporting and expanding the Glauber model. In this review, we present the recent advances concerning SCMs with the main focus on SCM systems beyond the Ising limit, SCMs constructed with non-collinear or canted antiferromagnetic intrachain interactions, the relation between SCM dynamics and interchain interactions, and some SCMs with chirality, porosity, spin-crossover, photo-switchable states, and so on.

Influence of rare earth ion (Y3+) on the magnetic and dc electrical properties of high density nanocrystalline Mg[sbnd]Cd ferrites

The samples having general formula Mg1−xCdxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) with 5% Y3+ addition were prepared by oxalate co-precipitation technique. The characterization was carried by XRD, SEM and FTIR techniques. The XRD confirms the cubic spinel structure with orthoferrite (YFe2O3) secondary phase. Average crystallite size and grain size lies in the range (28.86–32.66nm and 0.37–0.69μm). The grain size is calculated by linear intercept method and lies in the range of 0.37–0.69μm. The FT-IR shows two absorption bands in the frequency range of 350–800cm−1. The addition of Y3+ for Fe3+ forms a secondary phase on the grain boundaries and increases the saturation magnetization with respect to that of pure MgCd ferrites. The saturation magnetization, magnetic moment and coercive field is found to increase with cadmium content up to x=0.4, obeying Neel's two sublattice model and decrease thereafter showing existence of non-collinear spin interaction. The saturation magnetization and coercivity shows size dependent behavior. Y–K type ordering is present in all samples except x=1. The DC electrical resistivity increases while the Curie temperature (Tc) decreases, with increase in cadmium content. The DC resistivity of Y3+ added MgCd ferrite is higher than that reported for pure samples and for ceramic method. Activation energy in paramagnetic region is higher than that in ferromagnetic region.

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements and the firstprinciple calculations on the Weberite structure Ca2Os2O7 single crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A \lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, small amount of itinerant carriers with short life time \tau are observed, which is gapped at 20 K with a direct gap of 0:24 eV . Our first principle calculations indicate that the anti-ferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t2g bands and push them away from Fermi level(EF). On the other hand, a non-collinear magnetic interaction is needed to push the Os(4c) bands away from EF, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and non-collinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

Electrical control of the Kondo effect in a helical edge liquid

Magnetic impurities affect the transport properties of the helical edge states of quantum spin Hall insulators by causing single-electron backscattering. We study such a system in the presence of a Rashba spin-orbit interaction induced by an external electric field, showing that this can be used to control the Kondo temperature, as well as the correction to the conductance due to the impurity. Surprisingly, for a strongly anisotropic electron-impurity spin exchange, Kondo screening may get obstructed by the presence of a non-collinear spin interaction mediated by the Rashba coupling. This challenges the expectation that the Kondo effect is stable against time-reversal invariant perturbations.

Collective nuclear stabilization in single quantum dots by noncollinear hyperfine interaction

We present a theory of efficient suppression of the collective nuclear spin fluctuation, which prolongs the electron spin coherence time through the noncollinear hyperfine interaction between the nuclear spins and the hole spin. This provides a general paradigm to combat decoherence by direct control of environmental noise, and a possible solution to the puzzling observation of symmetric broadening of the absorption spectra in two recent experiments [Xu et al., Nature (London) 459, 1105 (2009) and Latta et al., Nature Phys. 5, 758 (2009)].

Dynamic Nuclear Spin Polarization in the Resonant Laser Excitation of an InGaAs Quantum Dot

Resonant optical excitation of lowest-energy excitonic transitions in self-assembled quantum dots leads to nuclear spin polarization that is qualitatively different from the well-known optical orientation phenomena. By carrying out a comprehensive set of experiments, we demonstrate that nuclear spin polarization manifests itself in quantum dots subjected to finite external magnetic field as locking of the higher energy Zeeman transition to the driving laser field, as well as the avoidance of the resonance condition for the lower energy Zeeman branch. We interpret our findings on the basis of dynamic nuclear spin polarization originating from noncollinear hyperfine interaction and find excellent agreement between experiment and theory. Our results provide evidence for the significance of noncollinear hyperfine processes not only for nuclear spin diffusion and decay, but also for buildup dynamics of nuclear spin polarization in a coupled electron-nuclear spin system.

Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots

Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of five dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.

Problems in using nonlinear collinear processes in third-order acoustical tomography

The processes of acoustical generation of the third-order wave harmonic by nonlinear collinear interaction of the pure third order and the twofold interaction of the second order are considered. It is shown that, in tomography schemes that are based on collinear interaction of waves, the interfering signal considerably exceeds the useful signal that brings information on the acoustical nonlinear parameter of the third order already over short wave propagations. The solutions to the Burgers’ equation and the Riemann waves equation have been numerically modeled to verify theoretical results. It is concluded that, using the nonlinear third-order effects in diagnostics becomes possible only when the tomography schemes are based on the noncollinear interaction of waves.

Two-dimensional plasma grating by non-collinear femtosecond filament interaction in air

We experimentally demonstrated that two-dimensional (2D) plasma gratings could be generated in air by nonlinear interaction of three femtosecond filaments. The intensity interference of non-collinearly overlapped filaments was self-projected along a relatively long distance and accompanied with a wavelength-scale periodic change of the refractive index in the encircling air due to periodic plasma density modulation. The 2D plasma gratings supported 2D diffraction of the generated third-harmonic pulses. By using in-line time-resolved holographic imaging and time-delayed diffraction, the 2D plasma gratings were evidenced to last a few tens picoseconds after the excitation pulses.

Entanglement in finite spin rings with noncollinear Ising interaction

We investigate the entanglement properties of finite spin rings, with noncollinear Ising interaction between nearest neighbours. The orientations of the Ising axes are determined either by the spin position within the ring (model A) or by the direction of the bond (model B). In both cases, the considered spin Hamiltonians have a point group symmetry, rather than a translation invariance, as in spin rings with collinear Ising interaction. The ground state of these models exhibit remarkable entanglement properties, resembling GHZ-like states in the absence of an applied magnetic field (model B). Besides, the application of an homogeneous magnetic field allows to modify qualitatively the character of the ground state entanglement, switching from multipartite to pairwise quantum correlations (both models A and B).