Ivan Biaggio

Materials constants of KNbO3 relevant for electro‐ and acousto‐optics

The elastic, piezoelectric, dielectric, elasto‐optic, and electro‐optic tensors have been determined by numerically evaluating the measurements published until now and using the additional measurements presented in this work. The dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1. The complete set of parameters consisting of the low‐frequency clamped dielectric constants eSij, refractive indices, elastic stiffness constants at constant electric field CEijkl, piezoelectric coefficients eijk, elasto‐optic tensor at constant electric field pEijkl, and clamped electro‐optic coefficients rSijk is used to calculate effective electro‐optic coefficients and effective dielectric constants that have to be used in photorefractive experiments where the elastic deformations associated with a periodic space‐charge field have to be considered.

A new class of organic donor-acceptor molecules with large third-order optical nonlinearities

Donor-acceptor molecules with 4-(dimethylamino)phenyl donor and 1,1,4,4-tetracyanobuta-1,3-diene acceptor moieties are readily prepared by short, high-yielding routes. The quite small chromophores are characterised by X-ray crystallography and feature intense intramolecular charge-transfer bands, substantial quinoid character in the donor rings, reversible electrochemical reductions and oxidations and powerful third-order optical nonlinearities.

Silicon Organic Hybrid Technology—A Platform for Practical Nonlinear Optics

A cost-effective route to build electrically as well as optically controlled modulators in silicon photonics is reviewed. The technology enables modulation at bit rates beyond 100 Gbit/s. This platform relies on the well-established silicon-based complementary metal-oxide-semiconductor processing technology for fabricating silicon-on-insulator (SOI) waveguides, while an organic cladding layer adds the required nonlinearity. The strength of this hybrid technology is discussed, and two key devices in communications are exemplarily regarded in more detail. The first device demonstrates demultiplexing of a 120 Gbit/s signal by means of four-wave mixing in a slot-waveguide that has been filled with a highly nonlinear chi(3)-organic material. The second device is a 100 Gbit/s/1 V electrooptic modulator based on a slow-light SOI photonic crystal covered with a chi(2) -nonlinear organic material.

Refractive indices of orthorhombic KNbO 3 . I. Dispersion and temperature dependence

We have measured the refractive indices of potassium niobate in the orthorhombic phase at six different temperatures between 22 and 180°C in the wavelength range 400–3400 nm. Sellmeier coefficients are determined by using a two-term oscillator model with an IR correction term. The temperature-dependent spontaneous polarization and the quadratic polarization optical coefficients are obtained from the index data. For practical purposes the temperature dependence can be well approximated by a polynomial expansion of degree 2.

Refractive indices of orthorhombic KNbO 3 . II. Phase-matching configurations for nonlinear-optical interactions

Based on temperature-dependent refractive-index data of orthorhombic KNbO3 [ J. Opt. Soc. Am. B9, 380 ( 1992)], we analyze various nonlinear-optical second-order interactions. Phase-matching curves as a function of wavelength, propagation direction, and temperature are given. Both type I and type II phase matching are found to be possible in KNbO3 for second-harmonic generation and for sum-frequency generation and optical parametric oscillation. To demonstrate the precision of the calculations based on the new refractive-index data, we compare experimentally measured values with calculated values of acceptance angles, phase-matching angles, and phase-matching temperatures for some specific cases. A temperature-tuned optical parametric oscillator employing KNbO3 is demonstrated. Good agreement is found between calculated and experimental results.

Optical properties of highly nonlinear silicon-organic hybrid (SOH) waveguide geometries

Geometry, nonlinearity, dispersion and two-photon absorption figure of merit of three basic silicon-organic hybrid waveguide designs are compared. Four-wave mixing and heterodyne pump-probe measurements show that all designs achieve high nonlinearities. The fundamental limitation of two-photon absorption in silicon is overcome using silicon-organic hybrid integration, with a five-fold improvement for the figure of merit (FOM). The value of FOM = 2.19 measured for silicon-compatible nonlinear slot waveguides is the highest value published.

Property Tuning in Charge‐Transfer Chromophores by Systematic Modulation of the Spacer between Donor and Acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.

Extended conjugation and donor-acceptor substitution to improve the third-order optical nonlinearity of small molecules

The authors use donor-acceptor substitution to increase the third-order polarizability γ of small cyanoethynylethene molecules by lowering the energy corresponding to the first optical transition. They identify a weak power-law dependence for γ depending on the number of conjugated electrons that separate the donor and acceptors (N1.5), which is the result of the two competing contributions that depend on the conjugation length: the energy separation of the ground and first excited states and the strength of the transition dipole matrix elements. The molecules are highly efficient with respect to their small masses and fall within a factor of 50 of the theoretical limit for centrosymmetric molecules, putting them among the best molecules measured to date.

Highly efficient third-order optical nonlinearities in donor-substituted cyanoethynylethene molecules.

We investigated the third-order nonlinear optical properties of several donor-substituted cyanoethynylethene molecules in the zero-frequency limit. We observed nonlinearities that are extraordinarily large relative to the small molecular mass of these molecules and that are within a factor of 50 from the fundamental limit. At a wavelength of 1.5 microm, the rotational average of the third-order molecular polarizability is 53 +/- 13 x 10(-48) m5 V(-2) (3.8 x 10(-33) esu) for the best molecule, which corresponds to 1.7 x 10(-48) m5 V(-2) per delocalized electron. The high nonlinear efficiency of these molecules is due to the compact two-dimensional conjugated system and the effective donor-acceptor substitution patterns.

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.