Second Harmonic Generation Studies Research Articles

Solvent-Dependent Delamination, Restacking, and Ferroelectric Behavior in a New Charge-Separated Layered Compound: [NH4 ][Ag3 (C9 H5 NO4 S)2 (C13 H14 N2 )2 ]⋅8 H2 O.

A new anionic coordination polymer, [NH4 ][Ag3 (C9 H5 NO4 S)2 (C13 H14 N2 )2 ]⋅8 H2 O, with a two-dimensional structure, has been synthesized by a reaction between silver nitrate, 8-hydroxyquinoline-5-sulfonic acid (HQS), and 4,4'-trimethylene dipyridine (TMDP). The compound stabilizes in a noncentrosymmetric space group, and the lattice water molecules and the charge-compensating [NH4 ]+ group occupy the inter-lamellar spaces. The lattice water molecules can be fully removed and reinserted, which is accompanied by a crystalline-amorphous-crystalline transformation. This transformation resembles the collapse/delamination and restacking of the layers. To the best of our knowledge, this is the first observation of delamination and restacking in an inorganic coordination polymer that contains silver. The presence of a natural dipole (the anionic framework and cationic ammonium ions) along with the noncentrosymmetric space group gives rise to the room-temperature ferroelectric behavior of the compound. The ferroelectric behavior is also water-dependent and exhibits a ferroelectric-paraelectric transformation. The temperature-dependent dielectric measurements indicate that the ferroelectric/ paraelectric transformation occurs at 320 K. This transformation has also been investigated by using in-situ IR spectroscopy and PXRD studies. The second-harmonic generation (SHG) study indicated values that are comparable to some of the known SHG solids, such as potassium dihydrogen phosphate (KDP) and urea.

A study on the l-lysine-iodic acid: semi organic non linear optical single crystals for electro-optic applications

Non linear optical single crystals of l-lysine-iodic acid (LLI) of dimensions upto 24 × 14 × 5 mm3 have been grown successfully by slow evaporation technique from aqueous solution. The crystal structure of the grown material was solved by single crystal X-ray diffraction analysis and it was found that the LLI crystal belongs to monocinic system with space group P21. Functional groups of the grown crystal were identified by Fourier Transform Infrared (FTIR) spectral analysis. The UV–Vis spectral analysis was carried out to measure the transparent range of the LLI crystal which is nearly 85% and the band gap energy is found to be 5.51 eV. Thermal stability and decomposition temperature of LLI crystal was found by means of TGA and DTA analyses. The mechanical behavior of the grown crystal has been employed using Vicker’s micro hardness technique. The Second Harmonic Generation (SHG) efficiency of the crystal was investigated and it was found to be 3.2 times of KDP. The particle size dependent SHG studies of LLI crystals were performed using Nd:YAG laser. The laser damage threshold value of LLI crystal is found to be 8.54 GW/cm2. Dielectric study indicates the reasonable dielectric constant and low dielectric loss of LLI crystal which are essential properties to develop optoelectronic devices. The ac and dc electrical conductivity measurements were carried out at various temperatures. Photoconductivity study exhibits the negative photoconductivity nature of the LLI crystal and the results are discussed for the first time.

Magnetization-induced effects in second harmonic generation under the lattice plasmon resonance excitation.

We present an experimental study of optical second harmonic generation (SHG) from arrays of nanostructures exhibiting collective plasmon resonances in the visible spectral range. Gold nanoparticles with the lateral diameter of 100 nm are packed in a square lattice with the period of 400 nm and are covered by a 90 nm thick iron garnet layer. We show an enhancement of SHG in the spectral vicinity of the lattice surface plasmon resonance. It is demonstrated that application of the external DC magnetic field results in a spectral shift of the SHG maximum up to 5 nm, accompanied by a pronounced phase modulation of the second harmonic wave. This spectral shift significantly prevails over the analogous linear magneto-optical effect and is explained by the interference of resonant and nonresonant SHG contributions of various nature.

On second harmonic generation and multiphoton-absorption induced luminescence from laser-reshaped silver nanoparticles embedded in glass

Spherical silver nanoparticles (NPs) of 30 nm diameter embedded in soda-lime glass were uniformly reshaped (elongated) after irradiation by a linearly polarised 250 fs pulsed laser operating within the NPs’ surface plasmon resonance band. We observed second harmonic generation (SHG) and multiphoton-absorption-induced luminescence (MAIL) in the embedded laser-reshaped NPs upon picosecond (10 ps) pulsed laser excitation at 1064 nm. A complementary study of SHG and MAIL was conducted in soda-lime glass containing embedded, mechanically-reshaped silver NPs of a similar elongation ratio (aspect ratio) to the laser-reshaped NPs. This supports the notion that the observed difference in SHG and MAIL in the studied nanocomposite systems is due to the shape modification mechanism. The discrete dipole approximation method was used to assess the absorption and scattering cross-sections of the reshaped NPs with different elongation ratios.

Growth, structural, UV–visible, SHG, mechanical and dielectric studies of bis-thiourea zinc chloride doped KDP crystal for NLO device applications

Optically transparent 11 × 10 × 4 mm3 bis-thiourea zinc chloride (BTZC) doped potassium dihydrogen phosphate (KDP) crystal has been grown by slow evaporation solution technique. The cell parameters of the grown crystal have been determined by single crystal X-ray diffraction analysis. The incorporation of BTZC in KDP crystal has been qualitatively analysed by FT-IR spectral analysis. The optical transparency and vital optical constants of BTZC doped KDP single crystals have been evaluated in the range of 200–900 nm. The mechanical behaviour of pure and doped KDP crystals has been investigated under the Vickers microhardness studies. The dielectric parameters of grown crystal have been investigated within the frequency range of 10–100 KHz. In Kurtz–Perry powder test, the second harmonic generation (SHG) efficiency of BTZC doped KDP crystal is found to be 1.65 times that of KDP material.

Beyond Single-Wavelength SHG Measurements: Spectrally-Resolved SHG Studies of Tetraphosphonate Ester Coordination Polymers

Powder second-harmonic generation (SHG) efficiencies are usually measured at single wavelengths. In the present work, we provide a proof of concept of spectrally resolved powder SHG measured for a newly obtained series of three non-centrosymmetric coordination polymers (CPs). CPs are constructed from tetrahedral linker-tetraphenylmethane-based tetraphosphonate octaethyl ester and cobalt(II) ions of mixed, octahedral (Oh), and tetrahedral (Td), geometries and different sets of donors (CoO6 vs CoX3O). Isostructurality of the obtained materials allowed for the determination of anion-dependent tunability of SHG optical spectra and their relationship with solid-state absorption spectra.

Comparative Study of Second-Harmonic Generation from Epsilon-Near-Zero Indium Tin Oxide and Titanium Nitride Nanolayers Excited in the Near-Infrared Spectral Range

We perform a comparative study of second-harmonic generation (SHG) from indium tin oxide (ITO) and from titanium nitride (TiN) nanolayers excited in the near-infrared spectrum. Both materials are compatible with Si technology and are candidate platforms for integrated nonlinear optics. In this work, we fabricate ITO samples with an e-near-zero (ENZ) condition, which can be continuously tailored in the 1150–1670 nm spectral range, and TiN samples with a metallic behavior in the same spectral range. For the ITO nanolayers, we observe tunability and enhancement of the SHG intensity when the samples are excited at their respective ENZ condition, in agreement with the electromagnetic modeling and analogous to its third-harmonic generation studied earlier. On the other hand, we show that the SHG efficiency of TiN nanolayers is lower by a factor of 50. We determine experimentally that the dominant component of the second-order susceptibility for our best ITO nanolayer is χzzz(2ω) = 0.18 pm V–1, and we theoretica...

Angle noncritical phase-matched second-harmonic generation in the monoclinic crystal BaGa₄Se₇.

We performed a complete study of angle noncritical phase-matched second-harmonic generation in BaGa4Se7 in the mid-infrared region. We measured the dielectric frame orientation and showed that it does not rotate as a function of wavelength. We determined the phase-matching wavelengths and the associated spectral and angular acceptances along the x and y axes of the dielectric frame. We also estimated the magnitude of the nonlinear coefficients involved.

Nonlinear multiple scattering of acoustic waves by a layer of bubbles

We present a theoretical and experimental study of the acoustic second-harmonic generation by a single layer of bubbles. This simple system allows us to investigate the subtle interplay between nonlinear effects and multiple scattering. A perturbative model is shown to give an excellent agreement with the experimental measurements, and we demonstrate the existence of an optimal concentration of bubbles, for which the harmonic generation is maximum. The potential of bubble screens as efficient subwavelength acoustic nonlinear sources is discussed.

Optically stimulated optical effects in the ZnO:Tm nanorods

We have performed the studies of photoinduced second harmonic generation in the ZnO nanorods doped by Thulium with different content (0.1; 0.2;0.5% of Tm). The illumination was performed with the Er:glass laser emitting at 1540nm wavelength of the 10ns laser and the output second harmonic generation (SHG) was studied. The AFM data analysis was performed. The role of the rare earth Tm ions on the morphology and the optically stimulated SHG is discussed. The SHG yield was found to be strongly dependent on Thulium content and Root Mean Square roughness of the studied layers.

Intense and tunable second-harmonic generation in biased bilayer graphene

The centrosymmetric two-dimensional material bilayer graphene (BLG) does not show dipole-allowed second-harmonic generation (SHG) in its pristine form. However, the symmetry can be broken by applying an electric field perpendicular to the layer. Here, we present a theoretical study of SHG from biased BLG. We show that the sheet second-harmonic susceptibility reaches very large values of several hundred ${\mathrm{nm}}^{2}$/V in the midinfrared region. The SHG is tunable depending on the strength of the electric field. Furthermore, a strong, tunable double resonance appears in the spectrum. We believe that this study could spark interest in the nonlinear optical properties of biased BLG.

Modal method for second harmonic generation in nanostructures

A modal method for the study of second harmonic (SH) generation in nanostructures is depicted both for plane waves and focused beam illumination. It is based on the B-spline modal method, and relies on the undepleted pump approximation. The nonlinear polarization induced by an incident plane wave or a focused beam generates a source term at the doubled frequency. The latter is divided into a finite number of sub-sources, and the SH field is subsequently computed by integration of these sub-source contributions.

Second-harmonic generation studies of implantation defects depth profile in hydrogen implanted garnet film

We demonstrate that complete depth profile of structural defects induced by ion implantation, with spatially separated lattice strain and implant ion contributions can be obtained with the use of second-harmonic generation (SHG) effect. The SHG effect was measured in transmission geometry for (111)-oriented garnet films implanted with H2+ ions of 60 keV energy and 1.5 × 1016 cm−2 dose, and etched to different depths along implantation range. Extremely strong sensitivity of the measured SHG intensity patterns in implanted samples on angle of incidence was found. The integral d¯ and differential d¯′ depth profiles of nonlinear optical susceptibility tensor were obtained from the experiment with the use of a phenomenological model developed for crystals of C3v symmetry. Direct correlation between perpendicular strain-related part of d¯′ profile and the induced effective magnetic anisotropy field is shown. Prominent peak in d¯′ located much deeper than the strain profile range was found, and its origin as related to the effect induced by hydrogen presence in the garnet crystal structure was interpreted.

Second harmonic generation by propagation of a p-polarized obliquely incident laser beam in underdense plasma

An analytical study of second harmonic generation due to interaction an intense, p-polarized laser beam propagating obliquely in homogeneous underdense plasma, in the mildly relativistic regime, has been presented. The efficiency of the second harmonic radiation as well as its detuning length has been obtained and their variation with the angle of incidence is analyzed. It is shown that, for a given plasma electron density, the second harmonic efficiency increases with the angle of incidence while the detuning length decreases. The second harmonic amplitude vanishes at normal incidence of the laser beam.

Molecular Organization of Crystalline β-Carotene in Carrots Determined with Polarization-Dependent Second and Third Harmonic Generation Microscopy

Polarization-in, polarization-out (PIPO) second harmonic generation (SHG) and third harmonic generation (THG) microscopy was used to study the crystalline organization of β-carotene molecules within individual aggregates contained in the chromoplasts of orange carrots in vivo. Multimodal PIPO SHG and PIPO THG studies of the aggregates revealed one dominant SHG and THG dipole signifying that β-carotene molecules are oriented along a single axis. Three-dimensional visualization of the orientation of β-carotene molecules with respect to the aggregate axis was also performed with both microscopy modalities and revealed organization of the aggregates as ribbon-like structures consisting of twists and folds. Therefore, PIPO SHG and PIPO THG microscopy provides information on the crystalline organization and the orientation of ordered biological structures in vivo where multimodal polarization dependent SHG and THG investigations are particularly advantageous as both noncentrosymmetric and centrosymmetric crystalline organizations can be probed.

Experimental and simulation study of the wavelength dependent second harmonic generation of collagen in scattering tissues

We report on the wavelength dependence of second harmonic generation (SHG) of collagen in scattering tissues over the wavelength range of 800-1200 nm. The study incorporates inclusion of the molecular hyperpolarizability β of collagen and optical scattering, both of which are wavelength dependent. Using 3D SHG imaging and Monte Carlo simulations, we find the wavelength dependence of β is not well described by a two-state model based on known absorption bands. We further find that longer wavelength excitation is inefficient as the reduction in scattering is overcome by the decreased β far from resonance and the optimal excitation is within the 800-900 nm range. The impact is larger for backward collected SHG.

Structural, photoinduced optical effects and third-order nonlinear optical studies on Mn doped and Mn–Al codoped ZnO thin films under continuous wave laser irradiation

We report the photoinduced optical effects, third-order nonlinearity, and optical power limiting of Mn doped and Mn–Al codoped ZnO thin films. The thin films were prepared by the spray pyrolysis technique. The structural properties of the deposited films were examined by x-ray diffraction studies. Z-scan measurements were conducted to evaluate the nonlinear optical parameters using a He–Ne laser operating in continuous wave mode at 633 nm wavelength. The present study reveals that the introduction of Mn and Mn–Al into ZnO leads to significant changes in the third-order nonlinear susceptibility. Photoinduced second-harmonic generation studies also reveal the dependence of the nonlinear optical properties on the Mn content. Switching over from saturable absorption to reverse saturable absorption was observed. The optical limiting studies show that the films possess a lower limiting threshold and clamping level, which is essential for use in eye and sensor protection applications. Hence, the Mn doped and Mn–Al codoped ZnO thin films investigated here emerge as promising candidates for future optoelectronic and photonic device applications such as optical power limiters.

Mode control and mode conversion in nonlinear aluminum nitride waveguides

While single-mode waveguides are commonly used in integrated photonic circuits, emerging applications in nonlinear and quantum optics rely fundamentally on interactions between modes of different order. Here we propose several methods to evaluate the modal composition of both externally and device-internally excited guided waves and discuss a technique for efficient excitation of arbitrary modes. The applicability of these methods is verified in photonic circuits based on aluminum nitride. We control modal excitation through suitably engineered grating couplers and are able to perform a detailed study of waveguide-internal second harmonic generation. Efficient and broadband power conversion between orthogonal polarizations is realized within an asymmetric directional coupler to demonstrate selective excitation of arbitrary higher-order modes. Our approach holds promise for applications in nonlinear optics and frequency up/down-mixing in a chipscale framework.

Nonlinear Optical Bioinspired Peptide Nanostructures

A nonlinear optical effect of a second harmonic generation (SHG) was first observed in quartz and then found in many inorganic materials that have an asymmetric crystalline structure. Second‐order nonlinear responses were also found in organic and biomaterials and are exploited for fundamental studies of helical and chiral biological molecules, such as proteins and amyloid fibrils. Another class of biomaterials is man‐made bioinspired nanostructures, which are composed of chemically synthesized biomolecules and are self‐assembled into supramolecular nanofibrils. Most of these materials have an asymmetric crystalline structure and possess ferroelectric and related phenomena. Here, the SHG effect is studied in bio‐organic peptide nanostructures of different morphologies and symmetries. These nanostructures are self‐assembled in different solvents from peptide precursors with a variable number of phenylalanine (F) amino acid units. A pronounced SHG response is detected in FFF‐nanobelts, FF‐nanotubes, and FFF‐nanospheres. SHG and Raman spectroscopy studies during phase transformation in FF‐nanotubes allow the definition of the intermolecular bonds responsible for SHG. Using two‐photon optical microscopy, orientational molecular ordering in aligned peptide supramolecular structures is found by adapting a generic model developed earlier for diverse biological protein fibrils. Efficient optical frequency conversion from NIR to green and blue light is demonstrated, as well as an effect of nonlinear optical waveguiding. These results suggest these bioinspired nanostructures as promising for a new generation of nonlinear optical nanomaterias, which can be integrated into nanophotonic devices.

β-BaTeMo2O9 microcrystals as promising optically operated materials

Studies of optical second harmonic generation (SHG) at fundamental wavelength of 1064 nm under photoinducing treatment of monoclinic piezoelectric β-BaTeMo2O9 (β-BTMO) were done. Continuous wave (CW) lasers generating at 808 and 1040 nm were used as photoinducing sources. The investigations were performed for the β-BTMO microcrystalline powder samples with grain sizes varying within the 25–300 μm range. We showed that depending on the microcrystallites size, the photoinduced changes of the SHG were substantially different depending on number of defects which were controlled by positron annihilation. The photoinduced SHG efficiency was substantially higher for more defective crystallites. The processes are completely reversible; however, their photoinduced time kinetics is very sensitive to the wavelength of the photoinducing CW laser beam. The possible reasons for the observed differences are discussed within a framework of intrinsic defect trapping levels and their interactions with phonon subsystem.