Second Harmonic Generation Values Research Articles

Role of interlayer coupling in second harmonic generation in bilayer transition metal dichalcogenides

Little is known about the role of weak interlayer coupling in the second harmonic generation (SHG) effects of two-dimensional van der Waals (vdW) systems. In this article, taking homo-bilayer $MoS_2/MoS_2$ and hetero-bilayer $MoS_2/MoSe_2$ as typical examples, we have systemically investigated their SHG susceptibilities as a function of interlayer hopping strength using first-principles calculations. For the SHG at zero frequency limit of both $MoS_2/MoS_2$ and $MoS_2/MoSe_2$, although the increase of tint can increase the intensities of interlayer optical transitions (IOT), the increased band repulsion around G point can eventually decrease their SHG values; the larger the tint, the smaller the SHG response. For the SHG spectra of $MoS_2/MoSe_2$ in the low photon-energy region, opposite to the $MoS_2/MoS_2$, their peak values are very sensitive to the variable tint, due to the strong $t_{int}$-dependent IOT dominating in the band edge; the larger the tint, the larger the SHG. For the SHG of $MoS_2/MoS_2$ in the high photon-energy region, comparing to the $MoS_2/MoSe_2$, their peak values will decrease in a much more noticeable way as the $t_{int}$ increases, due to the larger reduction of band nesting effect. Our study not only can successfully explain the puzzling experimental observations for the different SHG responses in different bilayer transition-metal dichalcogenides under variable tint, but also may provide a general understanding for designing controllable the SHG effects in the vdW systems.

Pd and octahedra do not get along: Square planar [PdS4] units in non-centrosymmetric La6PdSi2S14

Non-linear optical (NLO) materials require a balance of high second-harmonic generation (SHG) signal and laser damage threshold (LDT), as well as phase matchable behavior. Herein, we report a new member of the (RE)6(TM)x(Tt)2Q14 family of compounds, La6PdSi2S14, which, unlike all other reported TM analogues crystallizing in hexagonal P63 space group, crystallizes in the non-centrosymmetric monoclinic P21 space group. The crystal structure contains chains of edge-sharing distorted square planar [PdS4] units. The square-planar coordination of Pd in La6PdSi2S14 exhibits remarkable NLO properties with high SHG (3.7 × AgGaS2) and LDT (3 × AgGaS2) values as well as phase matchability. This shows the promise of novel materials with distorted structural motifs for enhanced NLO properties. Upon formation of bimetallic chiral sulfides containing both Cu and Pd, Cu occupies the opposite faces of the octahedra forming [CuS3] units while Pd can be stabilized in the center of PdS6 octahedra in the hexagonal P63 crystal structure of La6Pd0.5CuSi2S14. This suggests that it is possible to form mixed metal systems which could further enhance NLO properties by incorporation of additional structural distortions.

Analyzing role of relaxation time on second harmonic generation and optical dielectric function of impurity doped quantum dots under the aegis of noise

Present study monitors the influence of relaxation time (τ) on second harmonic generation (SHG) and total optical dielectric function (TODF) of impurity containing quantum dot (QD) under the stewardship of Gaussian white noise. Two distinct roadways viz. ‘additive’ and ‘multiplicative’ have been exploited for the entrance of noise to the system. Both in absence and presence of noise the SHG and TODF peak positions do not exhibit any shift with increase in τ. The SHG peak values display monotonic behavior with the variation of τ. However, the TODF peak values delicately depend on variation of τ being guided by the presence of noise and also on its mode of introduction. Presence of additive (multiplicative) noise causes enhancement (suppression) of SHG and TODF peak values as τ varies over a range. The findings appear to be crucial in the fabrication of QD-based opto-electronic devices where role of noise deserves to be properly addressed.

Estimation of the Rashba Strength from Second Harmonic Generation in 2D and 3D Hybrid Organic–Inorganic Perovskites

The Rashba effect (RE), caused by concurrent spin–orbit coupling and structural inversion asymmetry (SIA), has been invoked to explain outstanding optoelectronic properties of hybrid organic inorganic perovskites (HOIPs). The SIA in HOIPs also manifests itself in second harmonic generation (SHG). Here, we establish a quantitative relation between the RE strength and the low-frequency SHG value in two-dimensional (2D) and three-dimensional (3D) HOIPs using a newly developed effective-mass model. The calculated SHG, which exhibits one- and two-photon resonances between the valence and conduction bands, approaches a finite value at low frequencies. Using the measured SHG values in literature, the estimated RE strength is 10–3 eV A for 3D CH3NH3PbI3 and 4 × 10–2 eV A for 2D (C4H9NH3)2PbI4. Our results facilitate a simple and reliable determination of the RE strengths of 2D and 3D HOIPs and help quantify the impact of RE on various properties in HOIPs.

Growth, structural, optical, thermal, laser damage threshold and theoretical investigations of organic nonlinear optical 2-aminopyridinium 4-nitrophenolate 4-nitrophenol (2AP4N) single crystal

The good quality organic 2-aminopyridinium 4-nitrophenolate 4-nitrophenol (2AP4N) single crystals with the dimension of 30 × 5 × 5 mm3 have been grown by slow evaporation solution technique (SEST) at ambient temperature within the period of 30 days using methanol as solvent. Initially, the structure of grown 2AP4N single crystal was confirmed by the single crystal X-ray diffraction (SXRD). Intermolecular interactions of 2AP4N molecule were analyzed by Hirshfeld surface analysis. The grown crystal was studied by powder X-ray diffraction (PXRD) measurement, it has sharp peaks which indicates good crystallinity. The presence of various functional groups have been confirmed by FTIR and FT-Raman spectra analysis. The optical quality (transparency) of the grown crystal was studied by UV–Vis NIR spectral analysis and it has good optical transparency in the visible and near IR regions with the cut-off wavelength of 470 nm. The energy values of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been calculated. The density of states (DOS) spectra was used to study the bonding, anti-bonding and non-bonding interactions. The Mulliken charge distribution was used to confirm the sign and magnitude of charge of each atom. The distribution of charge and its related properties were analyzed by the molecular electrostatic potential (MEP). The natural bonding orbital (NBO) theory was used to analyse the inter-intra molecular interactions of 2AP4N. First-order hyperpolarizability (βtotal) of 2AP4N molecule was found to be 1.071−29 esu, which is 28.6 times that of urea. Photoluminescence measurement reveals that the 2AP4N crystal has very high emission at 500 nm. The thermal stability of grown crystal was found to be 90 °C. The dislocation density was analyzed and it was confirmed to possess less defects. The laser damage threshold (LDT) energy has been measured by using Nd: YAG laser (532 nm). The efficiency of second harmonic generation (SHG) of grown 2AP4N was evaluated by Kurtz-Perry powder technique. The SHG efficiency of 2AP4N was found to be 4.5 times that of standard KDP material. The high SHG values of 2AP4N crystals may be more favorable for nonlinear optical (NLO) device applications.

Novel borate CsZn2B3O7 single crystal with large efficient second harmonic generation in deep-ultraviolet spectral range

The linear and nonlinear optical susceptibility dispersion of CsZn2B3O7 single crystal are comprehensively investigated for a bulk structure in form of single crystal taking into account the influence of the packing structural units. The calculation highlights that the BO3 structural units packing is the main source for the large birefringence in CsZn2B3O7 due to high anisotropic electron distribution, and, hence, it affects the macroscopic second harmonic generation (SHG) coefficients. The large SHG is due to hyperpolarizablity formed by ZnO4 tetrahedra and co-parallel BO3 triangle groups. The absorption edge of CsZn2B3O7 occurs at λ = 218 nm and the optical band gap is estimated to be 5.68 eV that is in good agreement with the experimental data (5.69 eV). Therefore, CsZn2B3O7 is expected to produce a coherent radiation in deep-ultraviolet (DUV) region with SHG of about 1.5 × KDP (1.5 × 0.39 p.m./V = 0.585 p.m./V) that agrees with the measurements. This work is aimed at the report of reliable SHG value and the details of the NLO tensor for bulk CsZn2B3O7 single crystal.

Gamma irradiation effects on crystalline and optical properties of pure and doped Potassium Hydrogen Phthalate (KHP) single crystals

The undoped and urea doped semi-organic nonlinear optical Potassium Hydrogen Phthalate (KHP) crystals were grown by solution growth slow evaporation technique at room temperature. Grown crystals were irradiated with gamma rays using 60Co radioactive source. Grown crystals were irradiated with different dose from 100krad to 5Mrad. Irradiated and unirradiated crystals were subjected to powder x-ray diffraction and lattice parameters were calculated. It was also observed, slight colour change of crystals with increase in the dose of irradiation. The functional groups of irradiated undoped and urea doped KHP crystals were identified from FTIR spectral studies. The UV–vis spectrum reveals an increase in the optical absorption with increase in gamma dose in undoped and doped KHP. From Tauc the plot optical band gap is estimated and found to decrease with increase in gamma dose. Second harmonic generation (SHG) efficiency was confirmed by Kurtz-Perry powder method and SHG values decreases with increase in radiation dose.

(Invited) Non-Destructive Characterization of Dielectric - Semiconductor Interfaces by Second Harmonic Generation

Here we present the characterization of dielectric - semiconductor interfaces using optical second harmonic generation (SHG). The technique is contactless, which makes it useful for non-destructive monitoring of ultra-thin dielectrics. The generated second harmonic is sensitive to material parameters and charges near interfaces. The signal exhibits time dependence due to charging of traps with carriers generated by the incident laser during the measurement. Quantitative models are applied to the time dependent SHG signals in the Al2O3 stacks, which provide trapping kinetics information, and thus Al2O3 film quality. Al2O3 films are commonly used for passivating Si in photovoltaic stacks. To further demonstrate that time dependent behavior is due to changes in the electric field across the semiconductor/dielectric interface, additional systems are presented for comparison: SiNx on silicon, passivated and non-passivated silicon-on-insulator substrates. Results confirm the value of SHG as a quality control method for multiple thin film dielectrics.

LiMoO3(IO3), a novel molybdenyl iodate with strong second-order optical nonlinearity

The linear and nonlinear optical susceptibilities of molybdenyl iodate LiMoO3(IO3) have been calculated and compared with the available experimental data. The calculated linear optical properties reveal that there exists a considerable anisotropy thereby favoring an enhanced phase matching conditions necessary for observation of the second harmonic generation (SHG) and optical parametric oscillation (OPO). It has been found that LiMoO3(IO3) posses positive birefringence and negative uniaxial anisotropy. The molybdenyl iodate with 2D−Mo−O sheets capped by IO3− group is the main source of the SHG due to the electronic transition from the lone-pair electrons of oxygen atoms (O−2p) to Mo−4d and I−5p states. Moreover, the presence of the alkali metals can give further strength to the SHG. We would like to mention that the dipole moments in MoO6 octahedra and IO3− group arranged in a way which strengthens the SHG. Our calculated value of the dominant component of the SHG agrees well with the experimental value obtained by Chen's group, which is four times larger than that of the well known KDP (KH2PO4). Also we found that it is equal to one half of the experimental value of the well known KTiOPO4 (KTP) single crystals. Further, from the obtained SHG values of the dominant components we have calculated the microscopic first hyperpolarizability at static limit and at λ = 1064 nm.

Second-Harmonic Generation in Vertically Coupled InAs/GaAs Quantum Dots with a Gaussian Potential Distribution: Combined Effects of Electromagnetic Fields, Pressure, and Temperature

Simulation of quantum dots (QD) is usually performed on the basis of abrupt changes between neighboring materials. In practice, it is not possible to construct such QD because in a real structure the interface between two adjacent materials is not a step. In the work discussed in this paper, vertically coupled InAs/GaAs quantum dots (VCQD) with a non-abrupt change between two neighboring materials are considered. A potential function in the form of a Gaussian distribution was used to show this effect. We also focused on studying the effect of structure, applied electric (F) and magnetic (B) fields, pressure (P), and temperature (T) on second-harmonic generation (SHG). The analytical expression for SHG was investigated theoretically by use of the density matrix approach, the effective mass, and the finite-difference method (FDM). It was found that the major resonant peak value of SHG is a non-monotonic function of the barrier width (L B). Moreover, the major resonant peak of SHG is blue-shifted (red-shifted) and its magnitude increases (decreases) monotonically with increasing temperature (pressure). The results obtained also show that the magnitude and position of the resonant peaks of SHG are affected by changes in external conditions, for example applied electric and magnetic fields, structural dimensions of the coupled QD system, and relaxation time (T 0). Calculations also show that SHG in a VCQD structure with a non-abrupt potential change can be controlled and optimized by appropriate choice of structural dimensions and the external conditions mentioned above.

Analysis on linear and nonlinear optical properties of two Bisphenols with DFT approach: A comparative study

Density functional theory (DFT) method with B3LYP functional has been employed for the polarizability (α) and first order hyperpolarizability (β) calculations of two Bisphenols (4,4′ thiodiphenol and 4,4′ sulfonyldiphenol). Correlation between the dipole moment, polarizability and charge distribution is analyzed extensively. The calculated octupolar over dipolar contribution ratio parameter ρ from the hyperpolarizability components gives rise to macroscopic second harmonic generation (SHG) phenomenon identifying with the dipole nature of 4,4′ thiodiphenol and the tendency towards the octupole nature of 4,4′ sulfonyldiphenol molecule. Discussions on the geometrical structures of both Bisphenol molecules have been extended based on the comparison of optimized gas phase with solid state geometrical parameters to explain the high coplanarity in 4,4′ sulfonyldiphenol molecule which leads to the extended π-conjugation. Theoretically predicted large value of SHG in 4,4′ sulfonyldiphenol and zero value in case of 4,4′ thiodiphenol are confirmed with Kurtz–Perry Powder technique.

Octupolar molecules for nonlinear optics: from molecular design to crystals and films with large second-harmonic generation.

We summarize the nonlinear optical (NLO) properties of octupolar molecules, crystals, and films developed in our laboratory. We present the design strategy, structure-property relationship, and second-order NLO properties of 1,3,5-trinitro- and 1,3,5-tricyano-2,4,6-tris(p-diethylaminostyryl)benzene (TTB) derivatives, TTB crystals, and films prepared by free-casting TTB in poly(methyl methacrylate) (PMMA). The first hyperpolarizability of TTB was fivefold larger than that of the dipolar analogue. Moreover, the TTB crystal showed unprecedentedly large second-harmonic generation (SHG). While TTB crystal films (20 wt% TTB/PMMA) on various substrates showed appreciable SHG values, the cylinder film exhibited much larger SHG values and large electro-optic (EO) coefficients. The large SHG values and EO coefficients, as well as the high thermal stability of the cylinder film, will make it a potential candidate for NLO device applications.

Experimental and theoretical debate on efficient second harmonic generation in Bis (Cinnamic acid): Hexamine cocrystal

Second harmonic generation (SHG) in Bis (Cinnamic acid): Hexamine cocrystal was extensively analyzed through charge transfer (CT). The CT interactions through hydrogen bonding were well established with the aid of vibrational analysis and Natural Bond Orbital (NBO) analysis. The retentivity of coplanar nature of the cinnamic acid in the cocrystal was confirmed through UV–Visible spectroscopy and supported by Raman studies. Structural analysis indicated the quinoidal character of the given material presenting a high SHG efficiency. The first order hyperpolarizability value was calculated theoretically by density functional theory (DFT) and Hartree–Fock (HF) methods in support for the large value of SHG.

Second Harmonic Generation and Hyperpolarizabilities of the Double-Cubane Compound [Sb7S8Br2](AlCl4)3: Chalcogenide in Ionic Liquids

Because noncentrosymmetric [Sb7S8Br2](AlCl4)3 single crystals possess a wide optical transparency region, it is a promising material for nonlinear optical applications. We have calculated the dispersion of linear and nonlinear optical susceptibilities including optical second harmonic generation (SHG) using a relaxed geometry. We find that the fundamental optical absorption edge situated at about 2.03 eV is in excellent agreement with the experimental data. Calculations of ε(2)(xx)(ω), ε(2)(yy)(ω), and ε(2)(zz)(ω) tensor components of the frequency-dependent dielectric function are presented. The single crystal possesses a considerable anisotropy of linear optical susceptibilities, which usually favors an enhanced phase matching conditions necessary to observe SHG and optical parametric oscillator (OPO) effects. Our calculations show that, in [Sb7S8Br2](AlCl4)3, |χ123(2)(ω)| is the principal tensor component having the highest value of SHG at zero frequency limit as well as at 1.165 eV (λ = 1064 nm) laser wavelength generation. The microscopic second-order hyperpolarizability, β123, of the dominant SHG component is calculated at the static limit and at λ = 1064 nm.

Synthesis, growth of single crystals and their characterization: l-Alaninium maleate (LAM)

In recent days amino acid single crystals are gaining importance due to good optical behavior. The title compound consists of l-alanine; an amino acid and maleic acid; an aromatic organic compound. It has been successfully synthesized and the single crystal has been grown by slow evaporation solution growth. The grown single crystal has been characterized by powder X-ray diffraction (PXRD) analysis and FTIR measurements. The crystalline perfection was examined by high-resolution X-ray diffraction (HRXRD) technique and found that the specimen quality is quite good. Optical behavior has been assessed by UV–Vis analysis and found that there is no absorption in the entire visible region. The relative second harmonic generation (SHG) efficiency measurement reveals that the incorporation of maleic acid to l-alanine leads to increase its value of SHG. The mechanical behavior and the thermal analysis have been carried out using Vickers microhardness and TG/DTA measurements respectively. Its hyperpolarizability was estimated by density functional theory (DFT).

All-Optical Poling Properties of the Host-Guest Adulterant Thin-Films

The poling conditions were studied in AOP process by second harmonic generation (SHG) for host-guest adulterant thin-films. The guest was 4’-Nitro-4-dimethylaminoazobenzene and host was PMMA. The experimental results showed that the optimal poling temperature was about room temperature, the optimal ratio of light intensity of fundamental signal (ω) and its second harmonic signal (2ω) during AOP was approximately 100, the thin films had the same rule that the saturated value of SHG depended on the ratio. In addition, we obtained a suitable thickness around 2200nm that the stronger SHG after poling was generated.

Second harmonic generation of a new nonlinear optical material l-valine hydrobromide

Synthesis and single-crystal growth of a new nonlinear optical crystal l-valine hydrobromide (VHBr) is reported here. Low-temperature solution growth method was employed to grow bulk single crystal of VHBr. The grown crystals were characterized by single-crystal X-ray diffraction analysis, Fourier transform infrared, UV–Vis–NIR spectral studies and dielectric measurement. Thermal stability of VHBr has been studied by DTA–TG and DSC analysis. DSC studies reveal the possibility of ferro–para phase transition in VHBr at 118 °C, which is further confirmed by dielectric measurement with respect to temperature. Kurtz powder second harmonic generation (SHG) measurement confirms the nonlinear optical (NLO) property of the grown crystal and SHG value is comparable to urea. Morphology of the grown crystal is also determined by using crystallographic data.

New Hybrid Materials Made from Dipolar Fe(II) Arylacetylide Complexes Exhibiting Temperature-Dependent Second Harmonic Generation Properties

We show in this contribution that new hybrid materials which are active for second harmonic generation (SHG) can be obtained easily from dipolar organoiron(II) NLO-phores like (η2-dppe)(η5-C5Me5)Fe(C≡C-4-C6H4X) (1; X = NO2 and 2; X = C(O)H; dppe = 1,2-(diphenylphosphino)ethane). These new materials are formed upon UV irradiation of dispersions of 1 or 2 in oligoether-acrylates (OEAs) in the presence of an electrostatic dc field. Monitoring the evolution of the second-order susceptibility of these mixtures during the photosolidification process reveals the appearance of a growing noncentrosymmetric arrangement of the organometallic chromophores. Using an optimized experimental protocol, we find solid samples can be isolated which present sizable second-order optical susceptibilities; an SHG value around 3 pm/V is obtained with the samples containing the nitro-functionalized complex 1, while less intense SHG responses were obtained with dispersions of 2. These results are briefly discussed in the light of D...

Phonon-Assisted Second Harmonic Generation in As1-xBixTe3−CaBr2−PbBr2 Glasses

We have found experimentally the IR-induced second harmonic generation (SHG) in glasses possessing different degrees of electron-phonon interactions. For the investigations, we have chosen As(1)(-)(x)()Bi(x)Te(3)-CaBr(2)-PbBr(2) (0 < x <1) glasses. General formalism is based on consideration of fifth-order nonlinear optical susceptibility. The effect is observed in the middle IR region (spectral range 0.92-10.5 microm) where the value of the electronic energy gap is commensurable to the energies of actual quasi-phonons participating in the anharmonic (non-centrosymmetric) electron-phonon interactions. Varying the As/Bi ratio allows us to operate by the degree of electron-phonon anharmonicity in a wide spectral range. The second harmonic generation (SHG) output signal shows a correlation with IR-induced anharmonic phonon modes within the 1.5-4.8 microm spectral range. A maximum value of SHG is achieved at pump-probe delaying times of about 12.5-20 ps, which are typical for relaxation of the anharmonic electron-quasi-phonon subsystem. The maximally achieved value of the phonon-assisted optical susceptibility was about 6 x 10(-38) m(4)/V(4). The SHG signal was saturated for the IR pump power densities of about 1.73 GW/cm(2), corresponding to output SHG signals of about 9.8 x 10(-4) with respect to the fundamental ones. By varying the degree of electron-phonon anharmonicity and changing content of glasses, it was unambiguously shown that the IR-induced SHG signal correlates well with changes of oscillator strengths of IR-induced anharmonic phonon modes.

Preparation of novel optical-grade metanitroaniline and polymethylmethacrylate-coated single crystals and their optical properties

Preparation and optical properties of metanitroaniline (m-NA) and polymethylmethacrylate (PMMA)-coated m-NA single crystals were investigated. In this report, we developed relatively large-sized m-NA single crystals in methyl ethyl ketone (MEK) using solution growth techniques. The average size of the single crystals achieved is about 10×5×5 mm. The developed crystals were further coated with laboratory-developed optical-grade PMMA to study the effect of polymer coating on the second harmonic generation (SHG) properties of the single crystals. m-NA single crystals were characterized using DSC, FTIR and XRD measurement techniques. Optical characterization was done using UV-visible spectroscopy. Morphology was studied using scanning electron microscopy (SEM). Improved nonlinear optical (NLO) properties corresponding to SHG intensity values were also recorded for these coated and uncoated single crystals. PMMA-coated m-NA single crystal surfaces showed high SHG values.