Nonlinear Optical Resonators Research Articles

Organic-inorganic interfaces and microcavities with hybrid excitons

We consider nanostructures consisting of both organic and inorganic materials, in particular, heterojunctions or coupled microcavities. Resonance interactions between Frenkel excitons and Wannier-Mott excitons (through the Coulomb dipole-dipole interaction at an interface or through the cavity photons in coupled microcavities) may bring about hybrid Frenkel-Wannier-Mott excitations. In such a situation, the properties of both molecular and covalent crystals conspire to overcome the basic limitations of the individual constituents. We show that the new hybrid states and their dispersion curves can be tailored to engineer the enhancement of resonant optical nonlinearities, fluorescence efficiency and relaxation processes.

Resonant third-order optical nonlinearity in the layered perovskite-type material (C 6H 13NH 3) 2PbI 4

The third-order nonlinear optical susceptibility, χ (3) (− ω; ω, − ω, ω), of the layered perovskite-type material (C 6H 13NH 3) 2PbI 4 is measured by a transient four-wave mixing technique using a 200-fs-pulse laser source. The maximum … χ (3)… value is 1.6 × 10 −6 esu at the lowest-exciton resonance at 8 K. Longitudinal and transverse relaxation times of the excitons are 7 ps and 0.2 ps, respectively.

Hybrid Frenkel-Wannier-Mott excitons at interfaces and in microcavities

We discuss the linear and nonlinear optical properties of organic-inorganic nanostructures (quantum wells and superlattices) brought about by resonance interactions between Frenkel excitons in organic QWs and Wannier-Mott excitons in semiconductor QWs. We show that such a coupling (Coulomb dipole-dipole at an interface and through the cavity photons in a microcavity) is responsible for the appearance of mixed Frenkel-Wannier excitations. We demonstrate that the new hybrid states and their dispersion curves can be tailored to engineer the enhancement of resonant optical nonlinearity, fluorescence efficiency and relaxation processes.

Moving spatial solitons in active nonlinear-optical resonators

We investigate spatial solitons in a resonator with a narrow-band gain element and a saturable absorber placed in Fourier-conjugated resonator planes. Solitons are stationary or move at discrete velocities depending on the resonator tuning. The modulus of the velocity of moving solitons is fixed, but the direction of their motion is arbitrary. Solitons compete in velocity space. The experiments are conducted on a photorefractive oscillator with bacteriorhodopsin saturable absorber. Observations agree well with solutions of a general order parameter equation for such resonators.

Resonant optical nonlinearity of conjugated polymers

When the energy of a pump wave is in resonance with the exciton creation energy, the electric susceptibility of a conjugated polymer in response to the probe wave is altered by the exciton gas. In this paper, we calculate the dependence of this change on the the exciton populations by the equation of motion (EOM) method. The magnitude of optical nonlinearity is also influenced by ambient temperature, by the extent of exciton wave functions, and by the strength of electron-electron interaction. All of these factors can be easily incorporated in the EOM approach systematically. Using the material parameters for polydiacetylene (PDA), the optical Kerr coefficient $n_2$ obtained is about $10^{-8} cm^2/W$, which is close to experimental value, and is four orders of magnitude larger than the value in nonresonant pump experiments.

One-photon resonant optical nonlinearity of soluble substituted group IV metallophthalocyanines

The synthesis and nonlinear optical characterization of highly soluble Ni-, Pd- and Pt-phthalocyanines is reported. When excited resonantly within the Q-band, the third-order nonlinearity is dominated by saturable absorption, observed using the z-scan technique. The imaginary part of the nonlinear susceptibility was observed at 665 nm. From a three-level model, ground-state recovery times in the picosecond regime were determined for all phthalocyanines studied. These fast recovery times were supported by time-resolved measurements using degenerate four-wave mixing in the phase-conjugate geometry.

Third-order resonant optical nonlinearity from trans-cis photoisomerization of an azo dye in a rigid matrix

We investigated the trans–cis photoisomerization of an azo dye in a rigid matrix and the resulting third-order resonant optical nonlinearity by means of the simple theoretical prediction of a two-energy-level system, thin-layer chromatography and H-NMR studies. A methylorange (MO), a methylred (MR), congored (CR) or a Disperse Red 1 (DR1) doped polyvinyl alcohol (PVA) or silica film was used as nonlinear optical material. The existence of equi-absorbing points, or isosbestic points in the absorbance spectrum change and the remarkable stationary transmittance to be independent of the action beam intensity enabled us to confirm the photoisomerization even in a rigid matrix. Then, we measured the third-order resonant optical nonlinearity of dichroism through the polarization absorbance spectrum measurement and determined the characteristic optical parameters of the photoisomerization in the film such as the quantum yields φT, φC, the thermal reaction constant K and the photoisomerization time constant by fitting the theoretical curve of the two-energy-level system to the observed temporal transmittance change after the action beam exposure of the MO/PVA film. The quantum yields were φ¯T=0.36 and φ¯C=0.38, respectively. The photoisomerization time constant of MO embedded in the PVA film was a few seconds. The thermal reaction constant K depended on the excitation beam intensity.

Nonlinear phenomena in organic multilayers

Abstract

Perturbation theory of nonlinear resonators with an application to Kerr-lens mode locking

The beam propagation in nonlinear optical systems and resonators is formulated by means of operators, and a perturbation method is used to solve, to the first order, the problem of calculation of the nonlinear losses caused by intracavity nonlinearities. Steady-state situations and weak nonlinearities are assumed, but no other restrictions concerning the type of nonlinearity and the resonator structure are imposed. The main result is a simple, closed-form expression of the resonator-loss perturbations that can be determined without the preliminary calculation of the nonlinear self-consistent cavity field, and it has a clear physical interpretation. As an example of an application, a formula for the nonlinear losses of resonators used for Kerr-lens mode locking is derived in the context of Gaussian modes. The result, which is valid for any type of resonator, is numerically illustrated in a few cases of practical relevance.

Formation and control of localized structures in nonlinear optical systems.

Diffractive effects in passive nonlinear optical resonators can lead to pattern-forming instabilities. When the pattern (in our case, a regular hexagonal lattice of intensity peaks) coexists with the homogeneous solution, soliton-like intensity peaks in the transverse plane can be excited. These solutions have the characteristics of localized structures and are highly degenerate with respect to the peak location. By injecting narrow laser pulses, it is possible to turn on such peaks at desired locations and to turn them off selectively. The conditions to ensure independence among the peaks are described as well. These features suggest the possibility of encoding optical information in the structure of the field profile. (c) 1996 American Institute of Physics.

Nonlinear optical fiber resonator: Optical fiber bistability

The linear and nonlinear charecteristics of verious optical fiber loop resonators have been processed uniformly using transmission matrix method. It is pointed that in nonlinear operation condition each of those optical fiber loop resonators may be used to make an all-optical fiber bistability device. The configuration, charecterictics and threshold of verious Er-doped fiber loop bistability devices have been calculated, analysised and compared, and the design principle of those devices has been given.

Weakly correlated exciton pair states in large quantum dots.

We present a calculation of the two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius, and identify a weakly correlated exciton pair state that has a large oscillator strength, increasing proportionately to the volume of the quantum dot. This state is shown to be responsible for the saturation of the size dependence of the resonant excitonic optical nonlinearity. It also provides a satisfactory understanding of the blueshift of the excited-state absorption in quantum dots. These results and the biexciton binding energy and oscillator strength are in good agreement with reported experimental results on CuCl.

Submilliwatt optical bistability in wafer fused vertical cavity at 1.55-μm wavelength

We report on low-power optical bistability in a vertical cavity structure at 1.55-/spl mu/m wavelength due to the bandgap resonant dispersive optical nonlinearity of InGaAsP. In this structure, a GaAs-AlAs Bragg reflector grown on GaAs and the nonlinear medium grown on InP are bonded through wafer-fusion, leading to a high-quality vertical cavity after deposition of a top dielectric mirror. This device shows interesting characteristics for optical switching applications at fiber communication wavelengths, such as a switching contrast higher than 8:1 in the reflective mode and a bistability threshold power as low as 0.6 mW. True steady-state memory effect is observed with continuous-wave input, and the device switching time is in the ns range.

ULTRAFAST OPTICAL TIME DEMULTIPLEXERS USING SEMICONDUCTOR OPTICAL AMPLIFIERS

The operating principle of the utilization of resonant optical nonlinearities in recently demonstrated ultra-fast all-optical interferometric switching devices is presented. Optimum switching windows expected from these devices are investigated with a simple theoretical analysis. The experimental results of these devices are presented along with the nonlinear optical properties of a semiconductor optical amplifier. The information from the experimental results of the nonlinear optical properties are used to address a problem associated with a saturation of the semiconductor optical amplifier by an incoming ultra-high bit rate data stream. We also discuss the advantages and disadvantages in different operation modes — same or different wavelengths for the control and data signals. Finally, we discuss the relative advantages and disadvantages among these devices.

Nonlinear optical and electroabsorption spectra of polydiacetylene crystals and films

Vibronic structure of nonlinear optical (NLO) coefficients is developed within the Condon approximation, displaced harmonic oscillators, and crude adiabatic states. The displacements of backbone modes of conjugated polymers are taken from vibrational data on the ground and 1B excited state. NLO resonances are modeled by three excitations and transition moments taken from Pariser–Parr–Pople (PPP) theory and optimized to polydiacetylene (PDA) spectra in crystals and films, with blue-shifted 1B exciton. The joint analysis of third-harmonic-generation, two-photon absorption, and nondegenerate four-wave-mixing spectra of PDA crystals and films shows weak two-photon absorption to 2A below 1B, leading to overlapping resonances in the THG spectrum, strong two-photon absorption to an nA state some 35% above 1B, and weak Raman resonances in nondegenerate FWM spectra. The full π-π* spectrum contributes to Stark shifts and field-induced transitions, as shown by PPP results for PDA oligomers. The Stark shift dominates high-resolution electroabsorption (EA) spectra of PDA crystals below 10 K. The close correspondence between EA and the first-derivative I′(ω) of the linear absorption above the 1B exciton in PDA crystals provides an experimental separation of vibrational and electronic contributions that limits any even-parity state in this 0.5 eV interval. An oscillator-strength sum rule is applied to the convergence of PDA oligomers with increasing length, N, and the crystal oscillator strengths are obtained without adjustable parameters. The sum rule for the 1B exciton implies large transition moments to higher-energy Ag states, whose locations in recent models are contrasted to PPP results. Joint analysis of NLO and EA spectra clarifies when a few electronic excitations are sufficient, distinguishes between vibrational and electronic contributions, and supports similar π-electron interactions in conjugated molecules and polymers.

Comparison of Sagnac and Mach–Zehnder ultrafast all-optical interferometric switches based on a semiconductor resonant optical nonlinearity

We present a theoretical analysis of recently demonstrated ultrafast all-optical interferometric switching devices (based on Sagnac and Mach-Zehnder interferometers) that use a large optical nonlinearity in a resonant regime. These devices achieve ~10-ps switching windows and do not require high-energy optical control pulses. We theoretically analyze and compare one Sagnac and two Mach-Zehnder switching configurations.

Nonlinear optical response of charge-transfer excitons at donor-acceptor interfaces

We discuss resonant and off-resonant optical nonlinearities of a system of charge-transfer excitons (CTEs) at a donor-acceptor (DA) interface. We consider an interface between organic materials for which the lowest electronic excitations are CTEs corresponding to displacement of an electron from donor to acceptor. Such photogenerated CTEs have large static electric dipoles perpendicular to the DA interface plane. This system exhibits strong resonant optical nonlinearities induced by the dipole-dipole repulsion among the CTEs. On account of this long-range interaction, the excitation intensity dependence of the Kerr nonlinearity depends on the two-dimensional density n of CTEs as n 3 2 , and the dependence of the CW nonlinear polarization on the laser field is beyond the usual expansion. The static electric field produced by the CTEs modifies the hyperpolarizabilities of nearby molecules, causing the intensity dependence of the nonlinear optical response to be stronger than usual.

Origin of the resonant optical Kerr nonlinearity in Cd(S, Se)-doped glasses and related topics

After having reported preliminary results related to saturation, we first theoretically consider the various mechanisms contributing to the resonant optical Kerr effect in Cd(S, Se)-doped glasses. We obtain the expression for the expected effective susceptibility in different possible cases. This nonlinearity is studied experimentally using optical-phase conjugation in the low-intensity regime. We show that, by time resolving the nonlinear response of such glasses having experienced various degrees of photodarkening, we can clearly assess the origin of the resonant optical Kerr effect in these materials. Usually, a combination of a fast free-carrier contribution due to particles without traps and of a slow trapped-carrier one due to particles with traps is observed. For the free-carrier contribution, induced absorption is observed to be almost as important as absorption saturation. We also report frequency-dependent measurements and discuss the change in absorption spectrum and the increase of the nonradiative decay rate that accompany darkening.

Second-harmonic-generation-induced optical bistability in prism or grating couplers

We deal with second-harmonic generation in chi((2)) nonlinear-optical resonators such as prism or grating couplers. The theoretical study is performed within the framework of a recently developed coupled-mode analysis leading to a set of equations governing the amplitudes of pump and second-harmonic frequency fields. We predict analytically that second-harmonic generation in prism or grating couplers may lead to optical bistability. We believe this to be the first demonstration of such an effect in chi((2)) optical resonators.

Resonant Optical Nonlinearity in Dye-Doped Mesogenic Media and Absorption Saturation

Abstract A detailed study of the transmittance of dye-doped LC-film in dependence on the excitation laser radiation intensity in different LC-phases (nematic and chiral liquid crystals) is presented. It should be underlined that these compounds are based on the same molecular basis (cyanobiphenyls and their derivatives). Transmittance of the investigated systems appeared to be independent on the light intensity in a wide power region. The observed effect could be associated with the dye re-absorption to a higher electron level with the “left” absorption equaling the fundamental one.