Nonlinear Optical Second Harmonic Generation Research Articles

Nonlinear optical diode effect in a magnetic Weyl semimetal

Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light.

Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX2 (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX2 for nonlinear optical device applications.

Nonlinear optical properties of lithium niobate crystals doped with alkaline earth and rare earth elements

Nonlinear optical properties of lithium niobate crystals doped with alkaline-earth (Zn2+) and rare-earth (Er3+) elements were quantitatively studied from the chemical bond viewpoint of crystal. The different influences of Zn and Er dopants on the dielectric response of LiNbO3 were compared. The value of nonlinear optical second-harmonic generation coefficients was determined mainly by the Li–O bonds both in LiNbO3:Zn and LiNbO3:Er crystals. We observed sharp changes in the second order nonlinear optical response of Zn-doped LiNbO3 single crystals at 1064 nm with increasing Zn concentration in the crystal. The calculated values of the nonlinear optical coefficients for the LiNbO3:Zn crystals exceed those for the LiNbO3:Er crystals. The LiNbO3:Zn crystal with dopant concentration 5.48 mol.% is the most suitable crystal for the second-harmonic conversion.

Intensity-dependent nonlinear optical properties in an asymmetric Gaussian potential quantum well-modulated by external fields

In this paper, the effects of external electric, magnetic and non-resonant intense laser fields on the nonlinear optical rectification (NOR), second-harmonic (SH), and third harmonic (TH) generation in a GaAs quantum well with asymmetrical Gaussian potential are theoretically investigated. Firstly, the energy eigenvalues and eigenfunctions of a single electron confined in the structure are obtained by using the diagonalization method within the framework of the effective-mass and parabolic band approaches. Then, using these energy eigenvalues and eigenfunctions, expressions derived within the compact density matrix approximation have been employed to calculate the coefficients of the nonlinear optical response in the structure. The obtained simulation results show that the influence of the external fields leads to significant changes in the coefficients of nonlinear optical rectification, second and third harmonic generation in the system. As a result, it has been seen that the amplitude and position of the peaks of nonlinear optical rectification, second and third harmonic coefficients can be controlled by changing the applied external fields.

High-performance quantum entanglement generation via cascaded second-order nonlinear processes

In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO3 (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e., second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC), to generate photon-pairs. Previously, the performance of the photon-pairs is contaminated by Raman noise photons. Here by fiber-integrating the PPLN waveguide with noise-rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.36 kHz and 3.51 kHz, respectively. Energy-time, frequency-bin, and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.71 ± 0.02 with two-photon interference visibility of 95.74 ± 0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56 ± 1.79% with a spatial quantum beating visibility of 96.85 ± 2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.60 ± 0.04 and quantum tomographic fidelity of 89.07 ± 4.35%. Our results provide a potential candidate for the quantum light source in quantum photonics.

Tailoring second-order nonlinear optical effects in coupled quantum dot-metallic nanosphere structures using the Purcell effect

We study the modification of second-order nonlinear optical processes, like nonlinear optical rectification and second-harmonic generation, of a quantum dot that is coupled to a metallic nanosphere. We show that the strong variation of the spontaneous decay rate of the quantum dot near the metallic nanosphere can lead to strong enhancement or suppression of the nonlinear optical rectification and second-harmonic generation coefficients depending on the distance of the quantum dot from the surface of the nanosphere and on the direction of the electric dipole moment of the quantum dot. The nonlinear optical coefficients are also strongly influenced by the pure dephasing rate of the quantum dot.

Effects of applied electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field

In this present study, the effects of electric and magnetic fields on the nonlinear optical rectification and second-harmonic generation in a graded quantum well under intense laser field have been investigated theoretically. The energy eigenvalues and their corresponding eigenfunctions are obtained by solving Schrodinger equation within the framework of effective mass approximation. The analytic expressions for the optical properties are calculated by the compact-density-matrix approach and iterative method. The numerical results are presented for a typical GaAs/Ga1−xAlxAs quantum well. The results show that the nonlinear optical rectification and second-harmonic generation coefficients are considerably affected by the electromagnetic fields and intense laser field.

Epithelial Ovarian Cancer Diagnosis of Second-Harmonic Generation Images: A Semiautomatic Collagen Fibers Quantification Protocol.

A vast number of human pathologic conditions are directly or indirectly related to tissular collagen structure remodeling. The nonlinear optical microscopy second-harmonic generation has become a powerful tool for imaging biological tissues with anisotropic hyperpolarized structures, such as collagen. During the past years, several quantification methods to analyze and evaluate these images have been developed. However, automated or semiautomated solutions are necessary to ensure objectivity and reproducibility of such analysis. This work describes automation and improvement methods for calculating the anisotropy (using fast Fourier transform analysis and the gray-level co-occurrence matrix). These were applied to analyze biopsy samples of human ovarian epithelial cancer at different stages of malignancy (mucinous, serous, mixed, and endometrial subtypes). The semiautomation procedure enabled us to design a diagnostic protocol that recognizes between healthy and pathologic tissues, as well as between different tumor types.

The nonlinear optical rectification and second harmonic generation in asymmetrical Gaussian potential quantum well: Effects of hydrostatic pressure, temperature and magnetic field

In the present work, the effects of hydrostatic pressure, temperature, and magnetic field on the nonlinear optical rectification (OR) and second-harmonic generation (SHG) in asymmetrical Gaussian potential quantum well (QW) have been investigated theoretically. Here, the expressions for the optical properties are calculated by the compact-density-matrix approach and iterative method. Simultaneously, the energy eigenvalues and their corresponding eigenfunctions have been obtained by using the finite difference method. The energy eigenvalues and the shape of the confined potential are modulated by the hydrostatic pressure, temperature, and magnetic field. So the results of a number of numerical experiments indicate that the nonlinear OR and SHG strongly depends on the hydrostatic pressure, temperature, and magnetic field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.

Nonlinear optical rectification and second-harmonic generation in a semi-parabolic quantum well under intense laser field: Effects of electric and magnetic fields

The effects of electric and magnetic fields on the nonlinear optical rectification and second harmonic generation coefficients related with intersubband transitions in a semi-parabolic quantum well under intense laser field are theoretically studied. The energy levels and corresponding wave functions are obtained by solving the conduction band Schrödinger-like equation in the parabolic approximation and the envelope function approach. Numerical calculations are presented for a typical GaAs/Ga1-xAlxAs quantum well. The results show that both the non-resonant intense laser field and the static external fields have significant influences on the magnitude and resonant peak energy positions of the coefficients under study.

Epi-detected quadruple-modal nonlinear optical microscopy for label-free imaging of the tooth

We present an epi-detected quadruple-modal nonlinear optical microscopic imaging technique (i.e., coherent anti-Stokes Raman scattering (CARS), second-harmonic generation (SHG), third-harmonic generation (THG), and two-photon excited fluorescence (TPEF)) based on a picosecond (ps) laser-pumped optical parametric oscillator system for label-free imaging of the tooth. We demonstrate that high contrast ps-CARS images covering both the fingerprint (500–1800 cm−1) and high-wavenumber (2500–3800 cm−1) regions can be acquired to uncover the distributions of mineral and organic biomaterials in the tooth, while high quality TPEF, SHG, and THG images of the tooth can also be acquired under ps laser excitation without damaging the samples. The quadruple-modal nonlinear microscopic images (CARS/SHG/THG/TPEF) acquired provide better understanding of morphological structures and biochemical/biomolecular distributions in the dentin, enamel, and the dentin-enamel junction of the tooth without labeling, facilitating optical diagnosis and characterization of the tooth in dentistry.

Crystal Growth, Optical Properties Measurement, and Theoretical Calculation of BPO4

Optically perfect single crystals of BPO4 (BPO) have been grown by the top-seeding flux method. The refractive indices are measured with the prism minimum-deviation method for the first time, on the basis of which the Sellmeier equation is obtained. The measured powder nonlinear optical second-harmonic generation effect is about twice that of KH2PO4 (KDP). Electronic structure, the linear refractive indices, and second-harmonic generation (SHG) coefficients of BPO are calculated on the basis of the plane-wave pseudopotential ab initio method. The calculation values are in line with the experimental data. The crystal is nonhygroscopic, chemically stable, and has a hardness of 6.5 Mohs.

Organic materials for second-harmonic generation: advances in relating structure to function

The relationships between molecular structure and the nonlinear optical phenomenon second-harmonic generation (SHG) are discussed. New-found relationships built up from basic structural axioms that were deduced in the 1970s and 1980s are the particular focus of this article, using structural results from X-ray and neutron-diffraction studies. The molecular and supramolecular manifestations of the SHG effect are borne out, although ways to optimize the effect on the molecular scale feature predominantly, since control of SHG on the supramolecular scale remains difficult given present limitations. The use of a variety of templates to generate head-to-tail oriented host-guest species thereby bypassing such limitations is described. The paper concludes with a look ahead at next generation 'octupolar' SHG-active compounds, the prediction of new series of SHG-active compounds via data-mining computational procedures, and developments in diffraction technology that may enable structural movies of a molecule to be captured during the SHG process. A practical assessment of the viability of organic SHG materials for industrial application is reviewed with a positive outcome, thus indicating a promising future for organic SHG materials.

Molecular motion analysis in nematic liquid-crystal cells using linear and nonlinear optical techniques

The molecular motion of nematic liquid-crystal (NLC) molecules in a NLC cell was analyzed upon the switching on, the switching off, and the sign reversal of an external electric field. Both time-resolved nonlinear optical second-harmonic generation, which is sensitive to induced molecular dipole moments, and time-resolved linear optical transmission measurement, which is sensitive to the birefringence of the NLC molecules, were used. The combination of these techniques allowed a detailed analysis of both the molecular reorientation in the external field (through transmission experiments) and the dynamics of the electrically induced molecular dipole moments (through second-harmonic generation), which are again responsible for the molecular reorientation. Different time constants for the linear and the nonlinear optical experiments that correspond to different physical processes were observed. As a new second-harmonic-generation technique, the method of time-resolved phase measurement is introduced to analyze the sign reversal of the susceptibility of the NLC molecules induced by the polarity reversal of an external electric field.

Molecular Beam Deposition of Organic Nonlinear Optical Materials

ABSTRACTThin films of the organic nonlinear optical materials 4‘-nitrobenzylidene-3-acetamino-4- methoxy-aniline (MNBA), and 2-cyclooctylamino-5-nitropyridine (COANP) have been deposited by organic molecular beam epitaxy. Homoepitaxial growth has been demonstrated for both materials. MNBA thin films were grown heteroepitaxially on lattice-matched ethylenediammonium terephthalate (EDT) substrates. Under optimum growth conditions, similar to those valid for homoepitaxial growth of the same material, i.e. at substrate temperatures of 80°C, and growth rates of 0.1 Å/s, highly oriented films were deposited in an island growth mode. Nonlinear optical second-harmonic generation experiments confirmed the macroscopic orientation of the deposited MNBA thin films. Growth experiments with MNBA and COANP on inorganic substrates lead to amorphous or polycrystalline films, depending primarily on the substrate temperature. A spontaneous recrystallization process, occurring in COANP thin films under ambiant conditions, led to the formation of macroscopic nonlinear optically active domain structures.

A high-temperature aromatic polyimide film displaying non-linear optical second-harmonic generation based on the approach of the poled guest–host system

A segmented rigid-rod polyimide has been synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2′-trifluoromethyl-4,4′-diaminobiphenyl (PFMB) in m-cresol at elevated temperature in a one-step Polymerization. This aromatic polyimide (6FDA-PFMB) is soluble in a variety of organic solvents. 6FDA-PFMB has a glass-transition temperature (Tg) of 344 °C and a high optical transmission above 375 nm. The refractive indices along the in-plane and the out-of-plane directions are different and show a negative birefringence. Both of the refractive indices decrease with increasing wavelength. When the non-linear optical guest molecule, 2-(p-nitrophenyl)-4,5-bis(p-hydroxyphenyl) oxazole (TNON) is doped into the 6FDA-PFMB matrix, phase separation does not occur below a 25% w/w concentration of TNON. The Tg of the guest-host system decreases with increasing TNON concentration. After corona poling, second-harmonic generation is retained up to 300 h at 80 °C. Even after the film is annealed at 200 °C for 0.5 h, a measurable fraction of the second-harmonic non-linearity is retained.