Second-order Nonlinear Optical Susceptibility Research Articles

Second-harmonic generation and linear electro-optical coefficients of SiC polytypes and nanotubes

The second-order nonlinear optical susceptibility $[{\ensuremath{\chi}}_{abc}^{(2)}]$ and linear electro-optical coefficient $({r}_{abc})$ of a large number of single-walled zigzag, armchair, and chiral silicon carbide (SiC) nanotubes (SiC NTs), as well as bulk SiC polytypes (2H-, 4H-, 6H-, and 3C-SiC) and single graphitic SiC sheet have been calculated from the first principles. The calculations are based on the density-functional theory in the local-density approximation where the highly accurate full-potential projector augmented-wave method was used. Both the zigzag and chiral SiC NTs are found to exhibit large second-order nonlinear optical behaviors with the ${\ensuremath{\chi}}_{abc}^{(2)}$ and ${r}_{abc}$ coefficients being up to ten times larger than that of bulk SiC polytypes and also being up to thirteen times larger than the counterparts of the corresponding BN NTs, indicating that SiC NTs are promising materials for nonlinear optical and optoelectric applications. The features in the spectra of ${\ensuremath{\chi}}_{abc}^{(2)}(\ensuremath{-}2\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega})$ of the SiC NTs are discussed in terms of the single- and two-photon resonances and also related to the features in the linear dielectric function $\ensuremath{\epsilon}(\ensuremath{\omega})$.

Photorefractive Performance of a Novel Multifunctional Inorganic−Organic Hybridized Nanocomposite Sensitized by CdS Nanoparticles

A novel multifunctional inorganic−organic photorefractive (PR) poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl-CdS nanocomposites with different molar ratios of CdS to poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl (PVNPAK) were synthesized via a postazo-coupling reaction and chemically hybridized approach, respectively. The nanocomposites are highly soluble and could be obtained as film-forming materials with appreciably high molecular weights and low glass transition temperature (Tg) due to the flexible spacers. The PVNPAK matrix possesses a highest-occupied molecular orbital value of about −5.36 eV determined from cyclic voltammetry. Second harmonic generation (SHG) could be observed in PVNPAK film without any poling procedure and 4.7 pm/V of effective second-order nonlinear optical susceptibility is obtained. The CdS particles as photosensitizers had a nanoscale size in PVNPAK adopting transmission electron microscopy. The improvement of interface quality between CdS and polymer matrix is responsible for efficient photoinduced charge generation efficiency in the nanocomposites. An asymmetric optical energy exchange between two beams on the polymer composites PVNPAK-CdS/ECZ has been found even without an external field in two-beam coupling (TBC) experiment, and the TBC gain and diffraction efficiency of 14.26 cm−1 and 3.4% for PVNPAK-5-CdS/ECZ, 16.43 cm−1 and 4.4% for PVNPAK-15-CdS/ECZ were measured at a 647.1 nm wavelength, respectively.

Calibration of the second-order nonlinear optical susceptibility of surface and bulk of glass

A two-beam second-harmonic generation technique is developed to calibrate the magnitude of the second-order nonlinear optical susceptibility components of surface and bulk (multipolar origin) of isotropic materials. The values obtained for fused silica calibrated against ChiXXX of crystalline quartz are chi parallel parallel perpendicular = 7.9(4), chi perpendicular parallel parallel (+)gamma = 3.8(4), parallel perpendicular perpendicular perpendicular(+)gamma = 59(4), and delta' = 7.8(4) in units of 10(-22) m(2)/V. Similar values are obtained for BK7 glass. An alternative way of calibration against ChiXYZ of quartz is demonstrated. The technique could also be extended to characterize the susceptibility tensor of crystals as a convenient alternative to the Maker-fringe technique.

Second-order nonlinear optical susceptibilities in asymmetric coupled quantumwells

The second harmonic generation (SHG) in the asymmetric coupled quantum wells(ACQWs) is studied theoretically for different widths of the right-well and the barrier. Theanalytical expression of the SHG susceptibility is deduced by using the compact densitymatrix approach and the iterative method. Numerical calculations are presented for typicalGaAs/AlxGa1−xAs ACQWs.The results show that the calculated SHG susceptibility in this coupled system can reach a magnitudeof 10−5 m V−1, 1–2 orders higher than that in single quantum systems. Moreover, the SHGsusceptibilities are not monotonic functions of the widths of the right-well and thebarrier, but have complex relationships with them. The calculated results alsoreveal that by adjusting the widths of the right-well and the barrier respectively, aset of optimal structural parameters can be found for obtaining a strong SHGsusceptibility.

Electronic structure, linear, nonlinear optical susceptibilities and birefringence of CuInX2(X = S, Se, Te) chalcopyrite-structure compounds

The electronic structure, linear and nonlinear optical properties have been calculated for CuInX2 (X=S, Se, Te) chalcopyrite-structure single crystals using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. We present results for band structure, density of states, and imaginary part of the frequency-dependent linear and nonlinear optical susceptibilities. We find that these crystals are semiconductors with direct band gaps. We have calculated the birefringence of these crystals. The birefringence is negative for CuInS2 and CuInSe2 while it is positive for CuInTe2 in agreement with the experimental data. Calculations are reported for the frequency-dependent complex second-order non-linear optical susceptibilities . The intra-band and inter-band contributions to the second harmonic generation increase when we replace S by Se and decrease when we replace Se by Te. We find that smaller energy band gap compounds have larger values of in agreement with the experimental data and previous theoretical calculations.

Linear and nonlinear optical susceptibilities for a novel borate oxide BaBiBO 4: Theory and experiment

The linear and nonlinear optical susceptibilities for different tensor components of BaBiBO 4 single crystals have been calculated using the full-potential linear augmented plane wave method. The results of these calculations are verified by our measurements of linear and nonlinear optical properties using Nd-YAG laser at fundamental wavelength 1064 nm. The calculated energy gap is in a good agreement with experimental energy gap data obtained by optical absorption. We present results for the imaginary and real parts of the frequency-dependent dielectric constant. The calculated birefringence of BaBiBO 4 is positive in agreement with the experimental data. Calculations are reported for the frequency-dependent complex part of second-order nonlinear optical susceptibilities χ ijk ( 2 ) ( ω ) . The linear and nonlinear optical susceptibilities are scissors corrected to match the value of the energy gap from the local density approximation calculations with the experimental value. The second harmonic generation efficiency of this compound is about five times larger than KDP (KH 2PO 4). It is crucial that we have obtained a large anisotropy of the second-order susceptibilities for three main second-order tensor components χ zzz ( 2 ) ( ω ) , χ xxz ( 2 ) ( ω ) , and χ yyz ( 2 ) ( ω ) both experimentally and theoretically. The possible origin of the obtained anisotropy is discussed within a framework of the energy band calculations.

Second-harmonic generation in collagen as a potential cancer diagnostic parameter

The fibrillar collagen network in tumor and normal tissues is different due to remodeling of the extracellular matrix during the malignant process. Collagen type I fibers have the crystalline and noncentrosymmetric properties required for generating the second-harmonic signal. The content and structure of collagen were studied by imaging the second-harmonic generation (SHG) signal in frozen sections from three tumor tissues, osteosarcoma, breast carcinoma, and melanoma, and were compared with corresponding normal tissues, bone/femur, breast, and dermis/skin. The collagen density was measured as the percentage of pixels containing SHG signal in tissue images, and material parameters such as the second-order nonlinear optical susceptibility given by the d22 coefficient and an empirical anisotropy parameter were used to characterize the collagen structure. Generally, normal tissues had much more collagen than tumor tissues. In tumor tissues, a cap of collagen was seen at the periphery, and further into the tumors, the distribution of collagen was sparse and heterogeneous. The difference in structure was reflected in the two times higher d22 coefficient and lower anisotropy values in normal tissues compared with tumor tissues. Together, the differences in the collagen content, distribution, and structure show that collagen signature is a promising diagnostic marker.

An ultra-high level second-order nonlinear optical susceptibility in strained asymmetric GaN–AlGaN–AlN quantum wells: Towards all-optical devices and systems

In this paper an asymmetric structure for enhancement of second-order nonlinear optical susceptibility in the strained asymmetric GaN–AlGaN–AlN quantum well (QW) is proposed. In this structure, the strain-induced spontaneous and piezoelectric effects have been taken into account, and the second-order optical susceptibility of the δ-doped step QW structure is analyzed by considering the Shrödinger–Poisson self-consistent for different Al mole fraction x, step position, and pump photon energy hω. The magnitude of the second-order susceptibility of the simulated results for the proposed structure show an enhancement more than 400 times compared traditional strained QWs. Our simulations show that with increasing well width in the case of constant step width, peak of the second-order susceptibility is increased as well as resonant wavelength is decreased. Also, it is shown that for a given well and step widths there is an optimum mole fraction corresponding to maximum second-order susceptibility. Finally in the case of constant well width and mole fraction of the step, with increasing step width the second-order susceptibility is decreased.

Large band-gap bowing inCu1−xAgxGaS2chalcopyrite semiconductors and its effect on optical parameters

The band-gap bowing in the ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ag}}_{x}\mathrm{Ga}{\mathrm{S}}_{2}$ chalcopyrite alloy is studied using the full-potential linearized muffin-tin orbital method combined with various structural models, including so-called special quasirandom structures. The calculations confirm a large band-gap bowing $(\ensuremath{\sim}0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ in agreement with recent experimental results. It is found that the large bowing in part arises from a nonlinear behavior of the $c∕a$ ratio with concentration. Layered structures are found to have similar bowing to special quasirandom structures. The nonlinear band-gap behavior also leads to a nonlinear behavior of the index of refraction and the second-order nonlinear optical susceptibility with concentration. The maximum ${\ensuremath{\chi}}^{(2)}$ is found for a 50% alloy and is calculated to be about $27\phantom{\rule{0.3em}{0ex}}\mathrm{pm}∕\mathrm{V}$.

Specific features in the band structure and linear and nonlinear optical susceptibilities ofLa2CaB10O19crystals

The electronic and optical properties have been calculated for monoclinic ${\mathrm{La}}_{2}\mathrm{Ca}{\mathrm{B}}_{10}{\mathrm{O}}_{19}$ (LCB) crystal using the state-of-the-art full potential linear augmented plane wave method. We present results for the band structure, density of states, birefringence, imaginary and real parts of the frequency dependent linear and nonlinear optical response. We have found that LCB is a semiconductor with an indirect energy band gap of about $4.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A simple scissor operator is applied to adjust the band energy gap from the calculations to match the experimental value $(5.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The calculated birefringence of the LCB crystal has positive sign in agreement with the experimental data. Calculations are reported for the frequency-dependent complex second-order nonlinear optical susceptibilities ${\ensuremath{\chi}}_{abc}^{(2)}(\ensuremath{\omega})$ up to $6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and for zero-frequency limit ${\ensuremath{\chi}}^{2}(0)$. LCB exhibits second harmonic generation efficiency about two times larger than KDP crystal $(\mathrm{K}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4})$.

Second-order nonlinear optical susceptibilities induced by built-in electric field in wurtzite nitride double quantum wells

Based on the density matrix method and the iterative treatment, the second-harmonic generation (SHG) susceptibility of a wurtzite nitride coupling quantum well (CQW) with strong built-in electric fields have been theoretically investigated. The effect of the band non-parabolicity effect has been taken into account. A typical wurtzite GaN/InxGa1–xN CQW are chosen to perform numerical calculations. The localized properties of the electronic ground state and the low-excited states in the system are analyzed in detail. The calculated SHG coefficients reach the order of magnitude of 10−7m/V, which is two-order larger than the corresponding values in wurtzite single quantum wells. Moreover, it is confirmed that the SHG coefficients are not monotonic functions of the well width, barrier width and the doped concentration of the CQW systems, but have complicated dependent relations on them. The reasons resulting in these characteristics can be attributed to the intense competition between the strong built-in electric field effect and the quantum size effect for the electronic confined situation in the wurtzite CQWs. The calculated results also show that a strong SHG effect can be realized in the nitride CQW by choosing a group of optimized structural parameters and doped fraction.

Study of thermal and optical properties of GeS 2–Ga 2S 3–Ag 2S chalcogenide glasses

Chalcogenide glasses in the GeS 2–Ga 2S 3–Ag 2S pseudo-ternary system were prepared by melt-quenching technique. The structural evolvement of the glasses was studied by Raman spectroscopy. The Raman results show that the addition of Ag 2S involves the breaking of [S 3Ge–GeS 3] ethane-like units and the formation of Ag–S ionic bonds. The results of differential scanning calorimetry (DSC) show that the glasses have relatively high-glass transition temperatures and good thermal stabilities. These GeS 2–Ga 2S 3–Ag 2S glasses have a wide range of transmission approximately from 0.50 to 12.5 μm. In addition, with the method of Maker fringe, SHG was observed in the 0.9GeS 2–0.05Ga 2S 3–0.05Ag 2S glass irradiated by an electron beam. The value of second-order nonlinear optical susceptibility d is as large as 6.6 pm/V, and the poling mechanism of electron beam irradiation was also discussed in this work.

Quantification of the second-order nonlinear susceptibility of collagen I using a laser scanning microscope

Characteristic changes in the organization of fibrillar collagen can potentially serve as an early diagnostic marker in various pathological processes. Tissue types containing collagen I can be probed by pulsed high-intensity laser radiation, thereby generating second harmonic light that provides information about the composition and structure at a microscopic level. A technique was developed to determine the essential second harmonic generation (SHG) parameters in a laser scanning microscope setup. A rat-tail tendon frozen section was rotated in the xy-plane with the pulsed laser light propagating along the z-axis. By analyzing the generated second harmonic light in the forward direction with parallel and crossed polarizer relative to the polarization of the excitation laser beam, the second-order nonlinear optical susceptibilities of the collagen fiber were determined. Systematic variations in SHG response between ordered and less ordered structures were recorded and evaluated. A 500 microm-thick z-cut lithiumniobate (LiNbO(3)) was used as reference. The method was applied on frozen sections of malignant melanoma and normal skin tissue. Significant differences were found in the values of d(22), indicating that this parameter has a potential role in differentiating between normal and pathological processes.

Polymorphism, crystal growth and characterization of an organic nonlinear optical material: DAPSH

We have investigated the crystal growth, structural and physical properties of the organic nonlinear optical salt trans-4′-(dimethylamino)-N-phenyl-4-stilbazolium hexafluorophosphate (DAPSH). The chromophore in this recently created material shows a first molecular hyperpolarizability considerably larger than that of the related cation in the well-studied salt DAST (trans-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate). Through recrystallization of DAPSH using various solvents and conditions we have obtained three different crystalline phases. Phase a of DAPSH has a large second-order nonlinear optical susceptibility χ2. It crystallizes from acetone solutions and grows as purple needles, which are non-centrosymmetric with the space group Cc being suitable for electro-optic applications. Phase b with the triclinic non-polar space group P appears as reddish-brown thin plates when grown from DMSO solutions. It shows large third-order nonlinear optical susceptibility χ3 and can be easily grown as thin films, making it a good candidate for all-optical switching applications. Phase c with the non-polar space group P21/m grows as bright orange plates when crystallized from (50 : 50 vol%) ethanol–acetone solutions. This material shows no significant bulk nonlinear optical activity.

SECOND-ORDER NONLINEAR OPTICAL SUSCEPTIBILITIES OF POLY[(HEXA-2,4-DIYNYLENE-1,6-DIOXY)DIBENZOATES] CONTAINING AZO CHROMOPHORE DISPERSE RED 19 BY ELECTROABSORPTION SPECTROSCOPY

The second-order nonlinear optical susceptibilities χ(2) of two isomeric polymers containing an azo dye, Disperse Red 19, were determined by the first-order electroabsorption spectroscopy (EAS), and compared with values previously obtained by SHG measurements. The para-polymer was found to have higher susceptibility than the corresponding meta-polymer. χ(2) were found to be 5–6 × 10-8 esu, which are comparable to those obtained by the Maker Fringe method.

Nonlinear optical, piezoelectric, and acoustic properties of SrB4O7

We have measured the second-order nonlinear optical susceptibilities of an SrB4O7 crystal and have determined its electromechanical coupling coefficients, piezoelectric charge and voltage constants, and bulk acoustic wave velocities by a resonance method using high-Q resonators. In addition, the dielectric permittivity of the crystal has been measured at 1 kHz.

Ultrafast Hot-Carrier Dynamics at Chemically Modified Ge Interfaces Probed by SHG

Time-resolved second-harmonic generation (SHG) was used to study the hot-carrier dynamics and nonlinear optical properties of S-terminated and Cl-terminated Ge(111) interfaces on the femtosecond time scale. The hot-carrier second-order nonlinear optical susceptibilities were determined to be 720 +/- 50 times greater than the valence-band second-order nonlinear optical susceptibilities for the Ge(111)-S system and 880 +/- 100 times greater in the Ge(111)-Cl system. Furthermore, the ground- and excited-state second-order nonlinear optical susceptibilities are suggested to be out of phase for Ge(111)-S and Ge(111)-Cl systems, leading to a pump-induced decrease in the SHG signal as opposed to the increase in the SHG signal observed in the Ge(111)-GeO2 system. Although the SHG response reaches a steady state in 415 +/- 90 fs in the Ge(111)-GeO2 system, a faster response is observed in the Ge(111)-S system, 220 +/- 85 fs, and in the Ge(111)-Cl system, 172 +/- 50 fs. This suggests significantly faster carrier cooling at the Ge(111)-Cl and Ge(111)-S interfaces, with significant implications for hot-carrier mediated device degradation, and migration to high-K dielectrics.

Retardation of orientation relaxation of azo-dye doped amorphous polymers upon all-optical poling

The orientation relaxation of azo-dyes upon all-optical poling was studied. Azo-dye doped amorphous polymers were poled for different durations by all-optical poling at several temperatures. A quasi-permanent macroscopic second-order nonlinear optical susceptibility χ (2) was inscribed in the dye-doped centrosymmetric polymer systems. Relaxation of the χ (2) was monitored by second harmonic generation. It shows that the polar orientation relaxation follows the duration of the poling process: the longer the poling, the slower the polar relaxation time. Causes of the orientation relaxation retardation are discussed.

Bistable molecular orientation of a fluorene derivative with an intramolecular charge transfer in a poly(methyl methacrylate) host

The orientation of bistable molecules incorporated in a polymer host is addressed and studied by means of all-optical poling. Molecular reorientation between two stable molecular forms is assigned as the origin of the optically induced second-order nonlinear optical susceptibility. We found a new non-photochromic photo-induced reaction scheme by which efficient all-optical poling is achieved.

Non-linear optical diagnostic of a-Si:H thin films deposited by RF-glow discharge

We have evidenced the high sensitivity of infra red-induced second harmonic generation (IR-ISHG) to the structural changes that occurred in amorphous hydrogenated silicon films (a-Si:H) prepared by Rf-glow discharge technique at different substrate temperatures and doping types. In every case, a maximal signal of the IR-induced SHG is achieved at a temperature of about 110K and pump–probe delaying time about 22–38ps. This one indicates a marked effect of doped subsystems in the observed non-linear optical effects. A tremendous effect of doping is established from a drastic change of the IR-induced SHG behavior presenting an anomaly at about 400MW/cm2 for a pumping power with wavelength 3.7μm. A minimum of the SHG is observed in that case for standard non-doped films. Note here that the doping type does not markedly affect the behavior of the second-order non-linear optical susceptibility. The thermo-annealing leads to a slight decrease of the effective second-order susceptibilities. More drastic changes are observed with doped samples for the pump-probe delaying time from about 39 till 24ps.