Observation Of Second-harmonic Generation Research Articles

Potassium trihydrogen dioxalate dihydrate (KTO) nonlinear optical (NLO) single crystal for optical limiting application

In nonlinear optics, optical power limiting (OPL) compounds are known to be an important class of materials because it has the ability to protect the optical sensor components as well as human eyes from the abrupt intense laser power. Here, we have grown such optical limiting material so called potassium trihydrogen dioxalate dihydrate (KTO) single crystal from the aqueous solution by slow cooling method and other aspects such as recrystallization, solubility, nucleation rate, metastable zonewidth and induction period were analysed. Single crystal X-ray diffraction analysis results in triclinic crystal system with centrosymmetric space group P 1¯ of KTO single crystal. The electronic band structure shows that KTO single crystal exhibits an insulating nature with band-gap energy of 3.436 eV (DFT).The strong observation of second harmonic generation (SHG) signal of 79 mV output from the centrosymmetric system is quite surprising and the efficiency of it is also almost equivalent to that of the standard KDP material. And this signal may be attributed due to the dehydration of the crystal under laser irradiation (centrosymmetric to non-centrosymmetric) and the third order nonlinear χ(3) is found to be 1.82 × 10−6 esu. Apart from that, the material shows optical limiting behaviour at which it deviates from the linear response at 3.25 mW of input fluence (nonlinear origin) which further confirms that all nonlinear material will always be coupled with both linear and nonlinear property. In addition, optical, mechanical and dielectric properties of KTO single crystal are discussed in detail.

Strong second-harmonic generation by sublattice polarization in non-uniformly strained monolayer graphene

Despite the potential of graphene for building a variety of quantum photonic devices, its centrosymmetric nature forbids the observation of second harmonic generation (SHG) for developing second-order nonlinear devices. To activate SHG in graphene, extensive research efforts have been directed towards disrupting graphene’s inversion symmetry using external stimuli like electric fields. However, these methods fail to engineer graphene’s lattice symmetry, which is the root cause of the forbidden SHG. Here, we harness strain engineering to directly manipulate graphene’s lattice arrangement and induce sublattice polarization to activate SHG. Surprisingly, the SHG signal is boosted 50-fold at low temperatures, which can be explained by resonant transitions between strain-induced pseudo-Landau levels. The second-order susceptibility of strained graphene is found to be larger than that of hexagonal boron nitride with intrinsic broken inversion symmetry. Our demonstration of strong SHG in strained graphene offers promising possibilities for developing high-efficiency nonlinear devices for integrated quantum circuits.

Second-harmonic generation in a high-index doped silica micro-ring resonator.

We report the first, to the best of our knowledge, observation of second-harmonic generation (SHG) in a high-index doped silica micro-ring resonator, due to the symmetry-breaking-induced χ(2) at the core and cladding interface of the waveguide. The generated SH power is shown to have quadratic dependence on the in-cavity power of the fundamental pump at around 1550 nm. The pumping wavelength sweep method is adopted to fulfill the phase-matching condition for maximum conversion efficiency of SHG. This work offers a new approach to generate a visible source for the visible-light integrated optical platform from infrared-visible light conversion.

Nonlinear feature of phase matched Lamb waves in solid plate

The measurement of second harmonic Lamb waves is investigated as a promising feature extraction technique to characterize material nonlinearity in plate-like structures. In general, the effect of second harmonic generation can be weak, due to the dispersive nature of Lamb waves. The phase matched Lamb modes are explored to generate the second harmonics with a cumulative growth effect. Among all phase matched Lamb modes, it is of particular interest to find the most suitable Lamb modes for improving the efficiency of second harmonic generation and reception. Analytical expressions and experimental observations of second harmonic generation of Lamb waves in an isotropic plate with weak nonlinearity, are presented in this paper. Nonlinear parameters of Lamb waves are derived in this paper for the first time as a function of signal frequency, wave mode and geometric parameters of waveguides. Nonlinear features of various phase matched Lamb modes are discussed for the comparison of efficiency of second harmonic generation. The experimental results are found to be consistent with the theoretical predictions in a qualitative manner. This study shows that physics based feature selection is essential for efficient generation and reception of second harmonic Lamb waves.

Observation of second harmonic generation in doped polymeric carbon monoxide

• Novel material produced by X-ray irradiation of an oxalate salt pressurized to ∼3 GPa. • Material exhibits second harmonic generation. • Rugged material able to withstand focused laser beams may find utility as an optical sensor and nonlinear optical mixer. Second harmonic generation (SHG) experiments were conducted on samples of a novel material: doped polymeric CO (dp-CO) that were synthesized via synchrotron X-ray irradiation of SrC 2 O 4 and high pressure. The goal of the study was to ascertain if the novel material is rugged enough to handle high levels of radiation to enable its possible use as future sensors or optoelectronic devices. Three samples were tested. Two of the smaller samples that were synthesized in a diamond anvil cell (DAC) and recovered at ambient conditions rapidly decomposed/degraded in the presence of the strong 3.6 × 10 9 mW/cm 2 intensity optical beam. The larger sample that was synthesized in a Paris-Edinburgh cell at 3 GPa after 1.5 h of hard X-ray white beam irradiation and also recovered to ambient conditions was not damaged by a 6.1 × 10 9 mW/cm 2 strength optical beam and exhibited second harmonic generation. This lends credence that this novel material (when synthesized under select conditions) may offer utility as a rugged radiation hardened and easy to manufacture nonlinear optical device.

On the second harmonic generation activity in centrosymmetric crystals

The observation of second harmonic generation (SHG) signals, which is forbidden in centrosymmetric crystals, should necessitate clarification of their causes. The authors of a recent paper (Sivaraman et al Molecular crystals & Liquid crystals 656 (2017) 153-168) report to have grown a so-called crystal of diglycine zinc dipicrate which, in fact, is tetraaquabis(glycine)zinc(II) dipicrate dihydrate. This compound despite being centrosymmetric, was reported to exhibit a strong second harmonic response viz. 1.07 times of potassium dihydrogen phosphate. Based on a reinvestigation, we prove that the observation of SHG is due to dehydration of the crystal under laser irradiation.

Observation of second-harmonic generation in silicon nitride waveguides through bulk nonlinearities.

We present experimental results on the observation of a bulk second-order nonlinear susceptibility, derived from both free-space and integrated measurements, in silicon nitride. Phase-matching is achieved through dispersion engineering of the waveguide cross-section, independently revealing multiple components of the nonlinear susceptibility, namely χ(2) yyy = 0.14 ± 0.08 pm/V and χ(2) xxy = 0.30 ± 0.18 pm/V. Additionally, we show how the second-harmonic signal may be tuned through the application of bias voltages across silicon nitride. The material properties measured here are anticipated to allow for the realization of new nanophotonic devices in CMOS-compatible silicon nitride waveguides, adding to their viability for telecommunication, data communication, and optical signal processing applications.

Second harmonic generation in a KNbO3 nanorod and its detection by using a near-field scanning optical microscope

We report on an observation of second harmonic generation in an individual KNbO3 nanorod by using a near-field scanning optical microscope. The second harmonic is successfully generated by irradiating with a femtosecond laser having center wavelengths of 1200, 1100, and 972 nm. Such a second harmonic yield shows a clear dependence on the incident laser polarization, where maximum yield is obtained when the incident laser polarization is parallel to the long axis of an individual nanorod. A spatially-resolved second harmonic image shows a bright spot at the edge of the nanorod, which is attributed to the elaborated intensity of both fundamental laser light and second harmonic light inside the nanowire owing to cavity-mode formation.

Model-based nonlinear guided wave approach for nondestructive evaluation in solid media

The measurement of acoustic nonlinear response of guided wave propagation has been explored as a promising tool for early detection of micro-damages. Considering the high sensitivity of the nonlinear ultrasonic approach and the great advantages of the guided wave techniques, the nonlinear guided wave techniques have drawn significant attention. Frequency tuning and mode selection play a critical role for practical nonlinear guided wave testing. Among all the guided wave modes, it is of particular interest to find the more suitable modes for the improvement of efficiency. Analytical expressions and experimental observations of second harmonic generation of guided waves in an isotropic plate, are presented in this talk. Nonlinear parameters of guided waves are newly derived as a function of wave mode, geometric information of waveguides and frequency. Nonlinear features of various phase matched modes are discussed for the comparison of efficiency of second harmonic generation. The experimental results are consistent with the theoretical predictions. This study shows that physically based feature selection is essential for efficient generation of second harmonic guided waves. The feasibility of Lamb waves mixing for nonlinear ultrasonic test is also discussed in this work.

Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1−xFex)nO3n+1−δ Ruddlesden-Popper superstructures

Bulk crystals and thin films of PbTi1−xFexO3−δ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti1−xFex)nO3n+1−δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that and . The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3n+1−δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

Ba2YFeO5.5: A Ferromagnetic Pyroelectric Phase Prepared by Topochemical Oxidation.

Reaction of the anion-deficient, cation-ordered perovskite phase Ba2YFeO5 with 80 atm of oxygen pressure at 410 °C results in the formation of the Fe4+ phase Ba2YFeO5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba2YFeO5 (space group P21/n), to yield a noncentrosymmetric (NCS) phase Ba2YFeO5.5 (space group Pb21m (No. 26), a = 12.1320(2) A, b = 6.0606(1) A, c = 8.0956(1) A, V = 595.257(2) A3) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba2YFeO5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation ...

Femtosecond Carrier Dynamics and Nonlinear Effects in Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are complicated unipolar semiconductor devices based on intersubband transitions and resonant tunneling. In this study, femtosecond mid-infrared (Mid-IR) pulses are employed to investigate the nature of carrier transport through the active and injector regions of a room temperature, pulse biased ultrastrong coupling design QCL. Despite the low average power (<;1 mW) of femtosecond Mid-IR pulses, the efficient coupling of these pulses into the QCL waveguide made the study of nonlinear effects in QCLs possible. Biased just below threshold, we observed ultrafast gain recovery within the first 200 fs mainly contributed by electrons resonant tunneling through a much thinner injector barrier than that of conventional designed QCLs, which overcomes the interface-roughness-induced detuning of resonant tunneling. Oscillation or overshooting within the first picosecond is caused by electron relaxation from continuum region excited by strong pump beam, as well as coherent electron tunneling transport from injector to active region. The former feature is supported by the observation of second harmonic generation (SHG) with emission of λ≈2.2 μm pulses and measured positive photoconductivity. The transport of electrons through the injector region contributes to a slower gain recovery. A much longer recovery (hundreds of picoseconds) can be explained as electrons are depleted from upper stages down to lower stages in real space.

Focus issue introduction: nonlinear optics

It is now fifty years since the original observation of second harmonic generation ushered in the field of nonlinear optics, close on the heels of the invention of the laser. This feature issue celebrates this anniversary with papers that span the range from new nonlinear optical materials, through the increasingly novel methods that have been developed for phase matching, to emerging areas such as nonlinear metamaterials and plasmonic enhancement of optical properties. It is clear that the next fifty years of nonlinear optics will witness a proliferation of new applications with increasing technological impact.

Focus issue introduction: nonlinear optics

It is now fifty years since the original observation of second harmonic generation ushered in the field of nonlinear optics, close on the heels of the invention of the laser. This feature issue celebrates this anniversary with papers that span the range from new nonlinear optical materials, through the increasingly novel methods that have been developed for phase matching, to emerging areas such as nonlinear metamaterials and plasmonic enhancement of optical properties. It is clear that the next fifty years of nonlinear optics will witness a proliferation of new applications with increasing technological impact.

Nonlinear Optical Processes in Optically Trapped InP Nanowires

We report on the observation of nonlinear optical excitation and related photoluminescence from single InP semiconductor nanowires held in suspension using a gradient force optical tweezers. Photoexcitation of free carriers is achieved through absorption of infrared (1.17 eV) photons from the trapping source via a combination of two- and three-photon processes. This was confirmed by power-dependent photoluminescence measurements. Marked differences in spectral features are noted between nonlinear optical excitation and direct excitation and are related to band-filling effects. Direct observation of second harmonic generation in trapped InP nanowires confirms the presence of nonlinear optical processes.

Glycine glycinium picrate—Reinvestigation of the structure and vibrational spectra

The crystal of diglycine picrate (glycine glycinum picrate) has been obtained from an aqueous solution containing stoichiometric quantities of the components. The species crystallizes in the monoclinic system (space group P2 1/ c). The crystal structure was determined with high accuracy, IR and Raman spectra are discussed and compared with previous results, and the molecular structure is presented. It was shown that crystals of diglycine picrate obtained from the solution containing equimolar quantities may contain picric acid as impurity, which is the reason for the previously reported observation of second harmonic generation in this centrosymmetric crystal. With this example we want to point out the risk of misinterpretation of SHG signals in general.

Spontaneous Formation of Polar Liquid Crystal in Lyotropic Solution of Helical Poly(γ-Benzyl Glutamate)

In this study, we are concerned with the important characteristic in biological polypeptide helices, polarity. Through the observation of second harmonic generation (SHG) effect, we first show that a polar packing structure is spontaneously formed in solid films of the typical helical polypeptide, poly(γ-benzyl L-glutamate) (PBLG) which were cast from the form B solvents. Secondly, we present the spontaneous formation of polar nematic phase in lyotropic liquid crystal (LC) solutions of PBLG and poly(γ-benzyl D-glutamate) (PBDG) mixture and the polar-to-nonpolar transition upon increasing the temperature or decreasing the concentration of the solutions. The huge dipole moment of PBG molecule resulting from the accumulation of residue dipole component is attributable to the unusual formation of the polar nematic phase. The decisive evidence of polar liquid crystal open a new field on LCs, and possess the potentiality of the self-organization into the polar packing which may serve as electric devices in living systems.

Phase matching in monolithic Bragg reflection waveguides

We report what is believed to be the first observation of second-harmonic generation by type I phase matching the bulk chi(xyz)((2))(d(14)) nonlinear coefficient using Bragg reflection waveguides. Second-harmonic power of 0.7 microW was observed for a pump wavelength of 1587.8 nm with an average power of 25.2 mW and a pulse width of approximately 2 ps at a repetition rate of 75.6 MHz. An order of magnitude enhancement between the phase-matched and un-phase-matched second-harmonic conversion efficiency has been observed. Conversion efficiency at the phase-matched wavelengths was 0.001%. The bandwidth of the second harmonic was found to be equal to 0.43 nm, agreeing with the theoretical predictions.

Observation of second-harmonic generation in an oriented glassy nematic phase of a closo-decaborane derivative

The optical second-harmonic generation (SHG) and the linear optical properties were investigated in a planar cell containing aligned nonchiral nematic derivative of B10H102−: 1-(4-heptyloxypyridin-1-yl)-10-(4-pentyl-1-thiacyclohexyl)-closo-decaborane (HPTD) as a function of temperature. The compound showed a low level of the SHG signal in the polycrystalline state, and no nonlinear optical effect was detected in the fluid nematic state (N) either in the presence or absence of an orienting electric field. When frozen, the resulting thermodynamically unstable nematic glassy phase (glassy N) unexpectedly exhibited the second-order nonlinear optical properties, implying a partial polar order in the arrangement of the in-plane-aligned molecules in a liquid-crystal cell. The efficiency of the SHG was analyzed for two polarization configurations yielding the main component of the second-order nonlinear susceptibility χxxx(2)=2.3±0.3pmV−1 along the nematic director x. A comparison of the experimental susceptibility with a hypothetical one estimated from the vectorial component of the molecular hyperpolarizability showed a relatively low degree of the spontaneous polar order. The dispersion of the principal refractive indices in the glassy state of HPTD was derived from dichroic absorption spectra using an interference fringe method and fitted with a Sellmeier-type equation. At 527nm the birefringence of HPTD is Δn=0.31.

Measuring the electric charge in cloud droplets by use of second-harmonic generation

We report on the observation of second-harmonic generation (SHG) from electrically charged microdroplets while they are excited with femtosecond laser pulses. The intense SHG emission results from chi(3) coupling between two photons of the incident laser pulse and the electrostatic field induced by the surface charges. For Iinc= 2 x 10(12) W/cm2, we estimate the second-harmonic emission to be ISHG= 2.5 x 10(3) photons per droplet and per pulse. The possibility of using SHG to measure remotely the electric charge of water droplets in thunderclouds is discussed.