Second-order Optical Processes Research Articles

Magnetic bulk photovoltaic effect as a probe of magnetic structures of mathrm{{EuSn_{2}As_{2}}}

The bulk photovoltaic effect (BPVE) is a second-order optical process in noncentrosymmetric materials that converts the light into DC currents. BPVE is classified into shift current and injection current according to the generation mechanisms and their dependence on the polarization of light is sensitive to the spatial and time-reversal symmetry of materials. In this work, we present a comprehensive study on the BPVE response of mathrm{EuSn_{2}As_{2}} with different magnetic structures through symmetry analysis and first-principles calculation. We demonstrate that the interlayer antiferromagnetic (AFM) mathrm{EuSn_{2}As_{2}} of even-layer breaks the inversion symmetry and has the second-order optical responses. Moreover, the bilayer AFM mathrm{EuSn_{2}As_{2}} not only displays distinct BPVE responses when magnetic moments align in different directions, but also shows symmetry-related responses in two phases which have mutually perpendicular in-plane magnetic moments. Due to the dependence of BPVE responses on the polarization of light and magnetic symmetry, these magnetic structures can be distinguished by the circular polarized light with well-designed experiments. Our work demonstrates the feasibility of the BPVE response as a tool to probe the magnetic structure.

Discrete frequency-bin entanglement generation via cascaded second-order nonlinear processes in Sagnac interferometer.

Discrete frequency-bin entanglement is an essential resource for applications in quantum information processing. In this Letter, we propose and demonstrate a scheme to generate discrete frequency-bin entanglement with a single piece of periodically poled lithium niobate waveguide in a modified Sagnac interferometer. Correlated two-photon states in both directions of the Sagnac interferometer are generated through cascaded second-order optical nonlinear processes. A relative phase difference between the two states is introduced by changing the polarization state of pump light, thus manipulating the two-photon state at the output of the Sagnac interferometer. The generated two-photon state is sent into a fiber polarization splitter, and then a pure discrete frequency-bin entangled two-photon state is obtained by setting the pump light. The frequency entanglement property is measured by a spatial quantum beating with a visibility of 96.0±6.1%. The density matrix is further obtained with a fidelity of 98.0±3.0% to the ideal state. Our demonstration provides a promising method for the generation of pure discrete frequency-bin entanglement at the telecom band, which is desired in quantum photonics.

Full-zone valley polarization landscape of finite-momentum exciton in transition metal dichalcogenide monolayers

In this study we present a theoretical investigation of the full-zone landscape of finite-momentum dark excitons in ${\mathrm{WSe}}_{2}$-MLs by solving the density-functional-theory (DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. The studies reveal the comprehensive valley-polarization landscape of finite-momentum exciton of ${\mathrm{WSe}}_{2}$ monolayer. Dictated by the crystal symmetry, the valley pseudospin texture over the extended exciton-momentum ${\mathbit{k}}_{\text{ex}}$ space exhibits rich structures, featured by the inherently full valley polarizations in the excitonic ${K}_{\text{ex}}$, ${K}_{\text{ex}}^{\ensuremath{'}}$, and ${Q}_{\text{ex},i}$ valleys and also by the contrasted valley depolarizations for the exciton states lying in the $\overline{{\mathrm{\ensuremath{\Gamma}}}_{\text{ex}}{M}_{\text{ex},i}}$ paths. Attractively, the superior valley polarizations of the intervalley dark excitons in ${\mathrm{WSe}}_{2}$-MLs are shown almost fully transferable to the optical polarization in the phonon-assisted photoluminescences because of the native suppression of exchange-induced depolarization in the second-order optical processes. The analysis of phonon-assisted photoluminescences accounts for the recently observed brightness, high degree of optical polarization, and long lifetime of the intervalley dark exciton states in tungsten-based TMD-MLs.

Ultrashort pulse compression method in bulk BBO crystal using electro-optically tunable cascaded second-order process: a theoretical analysis

We propose an electro-optically controllable cascaded second-order optical process that mimics as a third-order nonlinear process which is utilized for the compression of ultrashort pulses. Effective compression of optical pulses has been achieved by making efficient use of frequency doubling into an extraordinarily advantageous crystal like beta-barium-borate which is capable of generating shorter pulses within a few cycles. In accordance with simulation, it endeavors ≈ threefold compression in femtosecond pulse, i.e., 47 fs has been achieved by applying a voltage of ∓ 6.2 kV which delivers a phase mismatch (Δk) = ± 117.236 m−1 and effective cascaded refractive index (n2cascade) of ± 3.045 × 10–20 m2/W. This competent compression leads to an increase in the value of peak intensity from 50 to 288 GW/cm2. On account of the decent control in peak intensity, this promising methodology can be utilized for advanced scientific research, and also implemented easily in a large variety of applications in solid-state physics and medical field.

Raman-like resonant secondary emission causes valley coherence in CVD-grown monolayer MoS2

Monolayer transition metal dichalcogenides are promising materials for ``valleytronics.'' They have band gaps at energy-degenerate $K$ and ${K}^{\ensuremath{'}}$ valleys with opposite spins. Due to the lack of inversion symmetry, electron-hole pairs can be selectively created at $K$ or ${K}^{\ensuremath{'}}$ valleys by circularly polarized photons. In addition, linearly polarized light excitation creates the coherent superposition of exciton valley states, referred to as the generation of valley coherence. In this study we performed polarization resolved photoluminescence and resonant Raman spectroscopy of CVD-grown monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$. We found that the lowest exciton photoluminescence becomes polarized, indicating the effective generation of valley polarization and valley coherence due to the resonant effect, accompanied by a drastic change of the polarization selection rule of Raman scattering. These results were theoretically explained from the viewpoint of the selection rules of resonant Raman scattering. We conclude that the Raman-like resonant second-order optical process should be the main mechanism of valley coherence.

Compressive Broad-Band Hyperspectral Sum Frequency Generation Microscopy to Study Functionalized Surfaces.

A broad-band sum frequency generation microscope has been developed for the study of molecular monolayers on surfaces. Because sum frequency generation is a vibrational spectroscopy based on a second-order optical process, it is uniquely sensitive to detecting a molecule's vibrational fingerprints specifically at interfaces. In this microscope, a structured illumination beam generated by a spatial light modulator is used to irradiate the sample with a series of sparsifying pseudorandom patterns. The spectra associated with each pattern are then input into a reconstruction algorithm to compressively recover the full hyperspectral image cube. As a proof-of-principle, this system performed molecule-specific imaging of a microcontact-printed self-assembled monolayer of alkanethiolate on copper. This hyperspectral compressive imaging effectively recovered both spatial and spectral surface features with compression greater than 80%, meaning more than a 5-fold decrease in acquisition time compared to traditional methods.

Time Correlation Function Modeling of Third-Order Sum Frequency Vibrational Spectroscopy of a Charged Surface/Water Interface.

Sum frequency vibrational spectroscopy (SFVS), a second-order optical process, is interface-specific in the dipole approximation [Perry, A.; Neipert, C.; Moore, P.; Space, B. Chem. Rev. 2006, 106, 1234-1258; Richmond, G. L. Chem. Rev. 2002, 102, 2693-2724; Byrnes, S. J.; Geissler, P. L.; Shen, Y. R. Chem. Phys. Lett. 2011, 516, 115-124]. At charged interfaces, the experimentally detected signal is a combination of enhanced second-order and static-field-induced third-order contributions due to the existence of a static field. Evidence of the importance/relative magnitude of this third-order contribution is seen in the literature [Ong, S.; Zhao, X.; Eisenthal, K. B. Chem. Phys. Lett. 1992, 191, 327-335; Zhao, X.; Ong, S.; Eisenthal, K. B. Chem. Phys. Lett. 1993, 202, 513-520; Shen, Y. R. Appl. Phys. B: Laser Opt. 1999, 68, 295-300], but a molecularly detailed approach to separately calculating the second- and third-order contributions is difficult to construct. Recent work presented a novel molecular dynamics (MD)-based theory that provides a direct means to calculate the third-order contributions to SFVS spectra at charged interfaces [Neipert, C.; Space, B. J. Chem. Phys. 2006, 125, 224706], and a hyperpolarizability model for water was developed as a prerequisite to practical implementation [Neipert, C.; Space, B. Comput. Lett. 2007, 3, 431-440]. Here, these methods are applied to a highly abstracted/idealized silica/water interface, and the results are compared to experimental data for water at a fused quartz surface. The results suggest that such spectra have some quite general spectral features.

Effect of Nanostructures on Corona Poling Assisted Second Harmonic Generation in Hybrid Organic-Inorganic Films

Hybrid organic-inorganic films are attractive materials for an alternative manufacture of photonic devices based on non-linear second-order optical processes. However, their efficiency and stability, closely related to their chromophores non-centrosymmetric orientation, still need some improvements. The use of nanostructures in this kind of materials gives place to a change in the chromophores surrounding medium; it can be used to improve the efficiency and the stability of the chromophores orientation in the films. In this work we show that some nanostructures are able to improve significantly the organic-inorganic SiO2 films second-order non-linear optical responses. Two kinds of nanostructures were studied: some nanostructures were just immersed, but the other ones were spontaneously induced in the films. The immersed nanostructures were multi-walled carbon nanotubes and the spontaneously induced nanostructures were formed in the films by means of surfactants self-assembly. The films were synthesized by the sol-gel method and all of the films were doped with Disperse Red 1 (DR1) dipolar chromophores. The induced nanostructures were lamellar and hexagonal long-range ordered nanostructures, detected by X-ray diffraction (XRD). The second-order non-linear optical properties of the films were studied in resonance through Corona poling assisted second harmonic generation. The results show significant improvements, for some kinds of nanostructures, in the non-linear optical responses of the films. In particular, those films containing Triton X-100 showed, at a poling temperature of 120°C, an improved second-order non-linear coefficient d33 with respect to equivalent non-nanostructured films; the d33 value of those films, determined by the Maker fringes technique, was equal to 21.4 pm/V.

Full-Wave Analytical Solution of Second-Harmonic Generation in Metal Nanospheres

We present a full-wave analytical solution for the problem of second-harmonic generation from spherical nanoparticles. The sources of the second-harmonic radiation are represented through an effective nonlinear polarization. The solution is derived in the framework of the Mie theory by expanding the pump field, the nonlinear sources, and the second-harmonic fields in series of spherical vector wave functions. We use the proposed solution for studying the second-harmonic radiation generated from gold nanospheres as a function of the pump wavelength and the particle size, in the framework of the Rudnick–Stern model. We demonstrate the importance of high-order multipolar contributions to the second-harmonic radiated power. Moreover, we investigate the dependence of the p- and s-components of the second-harmonic radiation on the Rudnick–Stern parameters. This approach provides a rigorous methodology to understand second-order optical processes in nanostructured metals and to design novel nanoplasmonic devices in the nonlinear regime.

Generation of second harmonic radiation from sub-stoichiometric silicon nitride thin films

Enhancing second-order optical processes in Si-compatible materials is important for the demonstration of innovative functionalities and nonlinear optical devices integrated on a chip. Here, we demonstrate significantly enhanced Second-Harmonic Generation (SHG) by silicon-rich silicon nitride materials over a broad spectral range, and show a maximum conversion efficiency of 4.5 × 10−6 for sub-stoichiometric samples with 46 at. % silicon. The SHG process in silicon nitride thin films is systematically investigated over a range of material stoichiometry and thermal annealing conditions. These findings can enable the engineering of innovative Si-based devices for nonlinear signal processing and sensing applications on a Si platform.

Polarization-induced phase shift of ultrafast photocurrents

Shift and injection currents are known to occur for linearly and circularly polarized optical excitations of semiconductors, respectively. Here, we show with room-temperature experiments that for excitation of discrete transitions the frequency dynamics of the coherent polarization changes this phase rule significantly. For above-bandgap optical excitation of semiconductors two main components of all-optically induced photocurrents have been recognized and are known as shift and injection currents (1). These currents can be linked to a second-order optical nonlinear process associated with difference- frequency mixing. Microscopically, shift currents and injection currents result from a polar distribution of carriers in real and momentum space, respectively. They are also known as linear and circular photogalvanic currents, since they occur for optical excitation with linearly and circularly polarized light, respectively (1-3). Here, we show that the frequency dynamics of the coherent polarization resulting from excitation of discrete transitions is capable of significantly altering this phase rule such that shift and injection currents occur for both, linear and circular polarizations. The measurements are performed on exciton transitions in a (110)-oriented, 5-nm wide GaAs quantum well (QW) sample, with the x, y, and z directions being parallel to the (001), (1 0), and (110) crystallographic directions, respectively.

Theory of Quadrupole Contributions from Interface and Bulk in Second-Order Optical Processes

Abstract A unified treatment of the dipole and quadrupole contributions to the second-order optical response is developed. The concept of the effective susceptibility is expanded to incorporate both the surface and bulk contributions. This treatment allows us to quantitatively evaluate both surface and bulk contributions in experimental SFG/SHG spectra. The present theory clarifies a number of qualitative features in the quadrupole contribution, and is thereby utilized to settle recent confusions about the quadrupole and/or bulk effects on the surface nonlinear spectroscopy.

Semi-classical approximation for second-harmonic generation in nanoparticles

Second-harmonic generation by spherical nanoparticles is a non-local optical process that can also be viewed as the result of the nonlinear response of the a interface layer. The classical electrodynamic description, based e.g. on the nonlinear Mie theory, entails the knowledge of the dielectric function and the surface nonlinear optical susceptibility; both quantities are usually assumed to be predetermined, for instance from experiment. We propose here an approach based on the semi-classical approximation for the quantum sum-over-states expression that allows one to capture the second-order optical process from first principles. A key input is the electronic density, which can be obtained from effective single particle approaches such as density-functional theory in the local density implementation. We show that the resulting integral equations can be solved very efficiently rendering thus the treatment of macroscopic systems. As an illustration we present numerical results for the magic Na−2869 cluster.

Selection Rules for Light-Induced Magnetization of a Crystal with Threefold Symmetry: The Case of Antiferromagnetic NiO

We propose Raman-induced collinear difference-frequency generation (DFG) as a method to manipulate dynamical magnetization. When a fundamental beam propagates along a threefold rotational axis, this coherent second-order optical process is permitted by angular momentum conservation through the rotational analogue of the umklapp process. As a demonstration, we experimentally obtained polarization properties of collinear magnetic DFG along a [111] axis of a single crystal of antiferromagnetic NiO with micromultidomain structure, which excellently agreed with the theoretical prediction.

StrongK-edge Magnetic Circular Dichroism Observed in Photon-in–Photon-out Spectroscopy

A large enhancement of the x-ray magnetic circular dichroism is observed at the iron K absorption preedge of magnetite. This is achieved by performing resonant inelastic x-ray scattering (RIXS) experiments with a 2p hole in the final state of the second-order optical process. We measured and calculated the full 1s2p RIXS planes for opposite helicities of the incoming circularly polarized x rays. The crystal field multiplet calculations show that the enhancement arises from 2p-3d Coulomb repulsions and 2p and 3d spin-orbit coupling. The observed magnitude of the RIXS magnetic circular dichroism effect is ∼16%. This opens up new opportunities for a broad range of research fields allowing for truly bulk-sensitive, element-, and site-selective measurements of 3d transition metal magnetic moments and their ordering using hard x-ray photons.

Contribution of multielectron excitation to F 1s photoabsorption process in CaF 2 studied by soft X-ray absorption and emission spectroscopy

The photoexcited F K α X-ray intensity’s evolution was measured for CaF 2 with fine energy-step from the [1s2p] double excitation threshold to the saturation region, where square brackets indicate hole states. The observed evolution was directly compared with the X-ray absorption spectrum, and the contribution of multielectron excitation to the absorption spectrum was successfully resolved. The F-K fluorescence XANES spectra were calculated based on the coherent second-order optical processes for comparison.

The Green function of cavity polariton with radiative correction: a new description ofresonant second-order optical processes

A previously proposed Green function of cavity polariton associated with quantum wellexcitons in a distributed Bragg reflector cavity has been generalized to an arbitrarythree-dimensional cavity containing multilevel excitons. This Green function describes theelectromagnetic field, both inside and outside the cavity, caused by the cavitydielectrics and the linear polarization of excitons. The electromagnetic field inducedby any other polarizations can be calculated as a convolution with the Greenfunction. In this way, we can include in the Green function approach all the problemswhich deal with boundary conditions and the radiative shift/width of excitons.

Optical poling of oligoether acrylate photopolymers doped by 1- H-pyrazolo[3,4-b]quinolines derivative chromophores

Second harmonic generation excited by Nd:YAG laser ( λ=1.32 μ m) was observed during the investigation of second-order optical poling process in 1- H-pyrazolo[3,4-b]quinolines chromophores incorporated within the oligoether acrylate (OEA) photopolymer matrices. We have found that the observed SHG output is correlated well with the values of corresponding dipole moments. Due to incorporation of the chromophore into photopolymer matrices second-order microscopic hyperpolarizabilities of the composites substantially increase (up to 3.01 pm/V). The investigated composites possess long-lived photopolarized states and after about 10 5 laser pulses the output SHG output signal did not decrease below 75% compared to the SHG signal in the beginning of the treatment. Optimal conditions of the optical poling corresponded to power density of the doubled frequency beam about 0.024 GW/cm 2 and chromophore content about 5.2% (in weighting unites).

Magnetic circular dichroism and spin polarization in3d→4fresonant photoemission from Tb metal in the perpendicular geometry

We present combined experimental and theoretical results for the magnetic circular dichroism (MCD) in resonant $4f$ photoemission (RPE) from Tb metal in the perpendicular geometry at different photon energies across the whole of the ${M}_{4,5}$ resonance. The atomic calculations, which take into account the full multiplet structure and the coherent second-order optical process, give excellent agreement with new experimental results for the resonant photoemission decay. The angular dependence of the MCD-RPE and its strong spin polarization in the ${M}_{5}$ region are also reported. For a single-configuration state the variations in the spectral shape are found to be stronger as a function of angle than as a function of photon energy. Due to the presence of the multiplet structure in the intermediate state ${3d}^{9}{4f}^{9}$ the coherent second-order optical process is essential for a correct description of the RPE. While the analysis of the spectra in parallel geometry yields accurate values of the spin-orbit, Coulomb, and exchange interactions, only the angle dependent RPE provides the phase factors of the emitted photoelectron.

Magnetic circular dichroism of resonant x-ray emission spectroscopy for SmL3M4,5andL2M4in aSm21Co79amorphous alloy

The magnetic circular dichroism (MCD) of resonant x-ray emission spectroscopy (RXES) was studied for the Sm L 3 M 4 , 5 and L 2 M 4 optical processes in a ferromagnetic Sm 2 1 Co 7 9 amorphous alloy. The MCD of RXES (MCDRXES) originating from the optical process of 2p 6 3d 1 0 4f n →2p 5 3d 1 0 4f n + 1 →2p 6 3d 9 4f n + 1 which corresponds to dipolar decay after quadrupolar (E2) excitation, was obviously observed at the preedge region in addition to that of the normal dipolar (E I ) excitation. Moreover, the E2 contribution of the MCD has been separated into those for E2 excitation to the Sm 4f state with majority and minority spins. A theoretical calculation based on a formula of the coherent second-order optical process was carried out for all of the experimental MCDRXES spectra. In the calculation, the multiplet coupling effect and the enhancement of the 2p-5d dipole matrix elements proposed for the MCD of x-ray absorption were taken into account. The calculated MCDRXES spectra reproduced all of the experimental results very successfully. The present result provided an appropriate interpretation of the origins of the MCD effect, which comes from the E1 and the E2 contributions at both the L 3 and the L 2 edges.