High Optical Nonlinearity Research Articles

Synthesis of Tellurium Dioxide Nanoparticles by Spray Pyrolysis

Tellurite glasses (amorphous tellurium dioxide based materials) have two useful optical properties, high refractive index and high optical nonlinearity, that make them attractive for a range of applications. Unlike other metal oxide nanoparticles that are prepared in large quantities by vapor phase processes, Telluria nanoparticles have not been prepared by such methods. Here, we describe the vapor-phase synthesis of telluria nanoparticles from a dilute solution of telluric acid that is dispersed into fine droplets by an atomizer. These droplets are carried through a furnace, where they evaporate and where the precursors react to nucleate telluria particles. The nanoparticles were characterized by TEM, XPS, FTIR and XRD. The effects of operating parameters on nanoparticle composition, production rate and size were also studied. This laboratory-scale process is capable of producing milligram to gram quantities of amorphous telluria nanoparticles with primary particle diameters from 10 to 40 nm.

Simulation of interactions of high-intensity ultrashort pulses with dielectric filters

Modern table-top laser systems are capable of generating ultrashort optical pulses with sufficiently high intensity to induce nonlinear optical effects in many of the materials that are used in the construction of optical components. We discuss the interaction of such pulses with three types of dielectric filters: (a) dielectric stacks composed of a sequence of two dielectric layers with quarterwave optical thickness, (b) idealized rugate filters, i.e., filters with a refractive index profile that is sinusoidally modulated on the length scale of an optical wavelength, and (c) a rugate filter composed of two materials. We present finite difference time-domain (FDTD) computer simulations of optical pulse propagation through dielectric filters for pulses with widths in the range 5 to 100 fs and with peak intensities up to 10 TW/cm2. At low intensities the reflective properties of the dielectric filters determined using FDTD simulations are directly comparable to the results calculated using the characteristic matrix method, while at high intensities optical nonlinearity in the dielectric layers is responsible for a decrease in the reflectance of the filter and causes stretching and distortion of the reflected pulses.

Alkyl sulfone-containing optical polyimide for an efficient wavelength conversion

Several organic chromophore derivatives have been investigated to get a high optical nonlinearity and low optical loss for nonlinear optical devices. The pi-conjugation length and polar substituents of the chromophore are limited due to an optical propagation loss caused by the chromophore structure. A nitro group is inappropriate for the stilbene fused chromophore because of its strong polarizing power but sulfones exert a reasonable polarizability for the generation of a low optical propagation loss. Three side chain polyimides (PI-Dass, PI-Rdas and PI-Drdas) with sulfone chromophores exhibited optical propagation loss of ca. 3.2dB/cm at 0.78μm. According to the binding structure of the chromophore on the polymer, sulfone binding resulted in better optical nonlinearity than amino binding. A hyper branched structure contributes to the increment of the optical nonlinearity of the sulfone chromophore. Absorption spectrum analysis is performed with both solution and solid film of the optical polymer in order to examine the chromophore structure relating to the optical propagation loss.

Energy transfer between laser beams due to recording of optical axis gratings in liquid crystals

We have studied two-beam-coupling processes related to the generation of optical axis gratings in liquid crystals (LCs) due to the electromagnetic torque acting on the orientation of LCs in superimposed laser beams with spatially modulated polarization. Such gratings result in efficient coupling between the laser beams controlled by the frequency shift introduced between the beams at infrasound frequencies. Larger than 80% efficiency of energy transfer from a pump to a signal beam is reported at power density levels of the laser beams at 1-10 kW/cm2. The distinction between pump and signal beams is relative, since the sign of the frequency shift rather than the power ratio between the beams determines the direction of energy transfer. The experiments were performed for radiation at a 1.06 μm wavelength; high optical nonlinearity and low absorption make LCs suitable for use in visible and near-IR spectra, particularly in high-power laser systems. We have characterized the main fundamental features of the process of recording dynamic orientation gratings, plotting the gain spectra for several LCs and verifying the good quality of the amplified signal beam even for a strongly distorted pump. The obtained results set the groundwork for a variety of novel opportunities and applications that include combining high-power laser beams and wavelength division multiplexing at an ultranarrow interval of frequencies of the order of 1-100 Hz.

SYNTHESIS OF NOVEL CROSS-LINKED POLYURETHANE CONTAINING MODIFIED STILBENE AND SCHIFF BASE CHROMOPHORES FOR SECOND-ORDER NONLINEAR OPTICS

The cyano and nitro groups were chosen as acceptor groups, and the substituent amino or ether groups as donor groups to the matrix. Polyurethane (PU) was modified by glycerol to increase the content of chromophore and to improve the stability of the nonlinearity. T g and T m were raised and solubility, film-forming ability and other physical properties were improved. The average functional groups of the reactant can be adjusted to ≥ 2 with polygroups and monogroups mixture. These poled polymers show high second-order optical nonlinearity and would have potential application in frequency-doubling or electro-optical controlling devices.

Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis.

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Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

Highly polarized dithiafulvenes: synthesis and nonlinear optical properties

Push–pull dithiafulvenes with reduced bond length alternation (BLA) and high optical nonlinearities have been prepared. The interplay between the proaromaticity of the donor and the structural and optical properties of these merocyanines is discussed. The donor ability of dithiafulvenes can reach that of ferrocene or dialkylaminophenyl groups.

Evaluation of nonlinear optical polymers for second-harmonic generation: toward the balance of absorption and nonlinearity against intrinsic trade-off

Despite many advantages toward nonlinear optical (NLO) waveguide devices, NLO polymers have not been adopted successfully into practical wavelength converters due to their high absorption losses. Empirical and theoretical understandings about NLO susceptibility imply the fundamental trade-off between optical absorption and nonlinearity. Our theoretical analysis elucidates the effect of absorption losses on second-harmonic generation (SHG). We compare analytically maximum conversion efficiencies for several NLO polymers and found a convincing approach for the development of NLO polymers with the optimum combination of high optical nonlinearity and low material absorption, which leads us to realize efficient polymeric SHG devices.

PbS-Doped Mesostructured Silica Films with High Optical Nonlinearity

Mesostructured silica films have been doped with lead sulfide (PbS) nanocrystals by a two-step impregnation, using the mesopores as nanoreactors for PbS growth. Secondary ion mass spectrometry and transmission electron microscopy characterizations demonstrated nanocrystals growth inside the pores and throughout the film thickness, pointing out the feasibility of an impregnation-based synthesis made possible by the porous properties of mesoporous silica films. The mean particles diameter is 5 nm, which is compatible with the pore dimensions. The mesostructure order was retained after the growth of the nanocrystals, as pointed out by small-angle X-ray scattering measurements. Optical absorption and Z-scan measurements indicated that PbS nanoparticles show a quantum confinement effect, while the films are characterized by high nonlinearities of the optical response. The optical properties of this material can be usefully exploited in several nonlinear optical applications.

All-Optical Signal Processing Devices Based on Holey Fiber

Holey Fiber (HF) technology has progressed rapidly in recent years and has resulted in the development of a wide range of optical fibers with unique and highly useful optical properties including endlessly single-mode guidance, and high optical nonlinearity. In this paper the state-of-the-art HF technology for all-optical signal processing devices is reviewed from a perspective of possible application for telecommunications.

Single-mode Ag-As2Se3 strip-loaded waveguides for applications to all-optical devices

We report the fabrication and characterization of Ag-As2Se3 strip-loaded waveguides fabricated by the standard lift-off method and photodoping, where Ag-As2Se3 (i.e., Agx(As0.4Se0.6)100-x) is a very promising material for all-optical switching devices because of a high third-order optical nonlinearity. Absorption coefficient of the bulk glasses is also presented for different values of the Ag content x. The addition of Ag into an As2Se3 guiding layer increases its nonlinearity and eliminates or weakens a harmful photo-oxidation reaction of As2Se3 films. The measured propagation loss in the single-mode waveguides with the Ag content x=3 at.% was 0.5 dB/cm at a wavelength of 1.053 mum and was attributable to the absorption loss in the waveguide materials.

Shortest chemical bond length as a criterion for searching new noncentrosymmetric niobate and tantalate crystals with high optical nonlinearity

An empirical interrelationship between chemical bonding and acentric properties has been established for simple and binary niobates and tantalates. The fields of noncentrosymmetric niobates and tantalates have been defined on the plane of shortest oxide bond lengths. Criteria for searching new compounds promising for high acentric properties have been revealed.

ABSORPTION EFFECTS ON SECOND-HARMONIC GENERATION AND CASCADED WAVELENGTH CONVERSION IN NONLINEAR OPTICAL POLYMERS

Although nonlinear optical (NLO) polymers often exhibit very high optical nonlinearities, they have barely been applied to tentative NLO devices. The main reason lies in their high absorption losses, and this fundamental trade-off between optical nonlinearity and absorption, stems further advancement. Our theoretical analyses inform us that absorption effect is much less critical in cascaded wavelength conversion (CWC) than in second-harmonic generation (SHG), especially when absorption loss is very high at a given second-harmonic wavelength. Based on this result, we propose CWC as a more practical application of NLO polymers than SHG.

Localized hyperpolarizability approach to the origin of nonlinear optical properties inTeO2-based materials

We present an ab initio molecular orbital theoretical approach to the origin of nonlinear optical (NLO) properties in the $\mathrm{Te}{\mathrm{O}}_{2}$-based materials from the viewpoint of ``electron pairs'' assigned to the individual chemical bond. Localized static dipole moments, linear polarizabilities, and hyperpolarizabilities of the individual chemical bonds in the $\mathrm{Te}{\mathrm{O}}_{4}$ and $\mathrm{Te}{\mathrm{O}}_{3}$ structural units are calculated in terms of the localized molecular orbitals. While no significant difference is found between these structural units in both the static bond dipole moment and the linear polarizability, the hyperpolarizabilities exhibit large differences. The $\mathrm{Te}{\mathrm{O}}_{4}$ structural unit shows much higher second hyperpolarizability than the $\mathrm{Te}{\mathrm{O}}_{3}$ structural unit. It is shown that the lone pair of the electrons on Te atom has the very large nonlinear response property, which should cause the high third-order NLO efficiency of the $\mathrm{Te}{\mathrm{O}}_{2}$-based materials. Geometric dependency of the second hyperpolarizability is also discussed on the basis of the calculations on the several deformed structural units.

Thermal Stability Enhancement of Electrooptic Polymer Modulator

We synthesized a novel electrooptic (EO) polymer with good thermal stability and high optical nonlinearity. The EO polymer has a side-chain structure of a hyper-branched chromophore in polyetherimide. In order to improve the thermal stability, the EO polymer has been designed to have a high glass transition temperature (178/spl deg/C). The hyper-branched chromophores induced a large EO coefficient (35 pm/V at 1550 nm). An optical intensity modulator based on the EO polymer was demonstrated. The measured half-wave voltage was 8.5 V and the extinction ratio was 18 dB. The modulator showed good thermal stability at 125/spl deg/C.

Through space charge transfer and quadratic nonlinear optical (NLO) properties in alternated stacks of 2-amino-1,3-benzothiazole–ethylcoumarin-3-carboxylate charge transfer complexes: from the molecular to the bulk NLO response

The title compound is a charge transfer complex with an experimental low lying excitation located around 420 nm. It is used as a model compound for the investigation of a possible strategy towards NLO materials obtained by “through space” charge transfer. Its solid state structure (Pc space group) reveals non-centrosymmetric alternating chains of donor-acceptor entities, built from the two independent 2-amino-1,3-benzothiazole entities hydrogen-bonded in the crystal unit cell. Each independent chain gives rise to a one-dimensional charge transfer, and therefore a non-vanishing NLO response along the chain. The angle between the glide mirror and the charge transfer direction is close to the optimized value (61°) and leads to two extended lattices of high NLO capability over the whole extent of the crystal. Unfortunately, an angle of 175° is observed between the two lattices, because of the unexpected effect of the hydrogen bonding, thus cancelling most of the NLO response. The efficiency of the material is therefore limited to 0.15 times that of urea in second harmonic generation (SHG). Nevertheless, this investigation indicates that charge transfer complexes could be envisioned for an alternative approach to molecular materials with SHG capabilities.

Origin of high second- and third-order nonlinear optical response in ammonio/borato diphenylpolyene zwitterions: the remarkable role of polarized aromatic groups.

We present a quantum-chemical analysis of the molecular structure and second- and third-order polarizabilities in a series of promising nonlinear optical (NLO) chromophores, the zwitterionic ammonio/borato diphenylpolyenes, R3N+Ph(C=C)nPhB-R3, whose synthesis has been reported recently. The molecular geometries are obtained via MP2/6-31G optimization, while the NLO properties are calculated with the INDO Hamiltonian using the sum-over-states and finite-field real-space methods. The real-space approach allows the direct evaluation of the NLO-active segments of the molecules, while the sum-over-states results illustrate the virtual excitations and charge-transfer pathways that are essential in the NLO response. Both methods highlight the remarkable and unexpected result that it is the strongly polarized phenylene groups that play the key role in generating a high NLO response.

Two-photon optical absorption in PbO-SiO2 glasses

One- and two-photon absorption spectra in PbO-SiO2 glasses have been studied comparatively as a function of the PbO content. The two spectra show different composition dependence, which can be accounted for by taking related electronic wave functions and the densities-of-states into account. This interpretation can be extended to understand high optical nonlinearity in heavy-metal oxide glasses.

Synthesis and characterization of composition-spread (Sr,Ca)2CuO3 thin films with high third-order optical nonlinearity

The optical properties of composition-spread (SrxCa1−x)2CuO3(0⩽x⩽1) films showing high third-order optical nonlinearity are systematically investigated. Desired orientations and high crystallinity are obtained for the entire composition x by employing a composition-spread [(Sr2TiO4)x(LaSrAlO4)1−x] buffer layer and a temperature-gradient method in order to compensate the change in lattice mismatch and growth temperature depending on x. Linear absorption spectra and third-harmonic intensity with an exciting wavelength of 1.56 μm are measured as a function of x. As the Cu–O bond length increases with x, the charge-transfer gap decreases and the third-harmonic intensity increases, and both the charge-transfer gap and third-harmonic intensity change almost linearly with x between Ca2CuO3 and Sr2CuO3.