The propagation of solitons along the interface between two dielectric nonlinear media was investigated theoretically extensively in the 1980s but never realized experimentally. Recently we predicted that the required small index differences between the media and hence solitons can be created at the interface between continuous and periodic discrete media consisting of arrays of weakly coupled waveguides. Our theoretical analysis has predicted the existence of stable solitons with power thresholds both in the centre and at the edge of the Brillouin zone. We have observed both of these discrete surface solitons with power thresholds in both Kerr and quadratically nonlinear media. Spatial solitons with fields in neighboring channels either in phase or pi out of phase with one another have been identified.
Discrete optical phenomena occur in one dimensional periodic arrays of parallel coupled waveguides. Coupling between
adjacent waveguides leads to a novel form of "discrete" diffraction, periodic dispersion relations involving multiple
bands, and contain regions of both normal, anomalous and zero diffraction. This in turn impacts many linear diffraction
phenomena, for example the Talbot effect. It also results in a diversity of phenomena in nonlinear optics including novel
types of spatial solitons, beam break-up also known as filamentation, and solitonic interactions. The fundamental
concepts are reviewed here and interesting examples of discrete optical phenomena discussed.
Discrete nonlinear optical systems exhibit unique properties unknown from wave propagation in bulk materials. Among them are the possibilities to form highly localized discrete solitons and the ability of a wide beam to propagate without diffraction and modulational instability across the array. The interaction between a highly localized discrete soliton and a non-diffracting beam has potential applications for all optical routing and switching. We present our results on the experimental investigation of this kind of beam interactions in a one-dimensional AlGaAs array at a wavelength of 1550 nm. A discrete soliton, almost completely confined to a single waveguide, was excited and the interaction with a wide beam of the same or orthogonal polarization was studied. We confirmed that the wide beam is able to drag the soliton over multiple waveguides towards itself while the soliton is able to maintain its original, highly confined shape. The outcome of the coherent interaction depends on the power of the wide beam and the relative phase between the two beams. This phase-dependence is due to linear interference in the case of co-polarized beams and due to four-wave mixing for orthogonally polarized beams.
In this work we report for the first time observation of AC photocurrents generated by photo-emf effect in high quality crystals of the conjugated polymer 2,4-hexadiyne-1,6-diol-bis-(para-toluene sulfonate) or polydiacetylene PTS (usually referred simply as PTS). We used illumination at λ=633nm where the absorption spectra has its peak. In principle, photocurrent at this wavelength should be small due to the large exciton binding energy, however we show that relative large photocurrents can be observed at modest electric fields (up 2000 V/cm). When the sample is illuminated with a periodically oscillating interference pattern it leads to spatial redistribution of charge carriers among traps. Spatial overlap of the resulting space-charge field with the oscillating photoconductivity pattern gives rise to an oscillating photocurrent (photo-emf signal). From photo-emf measurements, a Maxwell relaxation time of 0.17 sec at 6 W/cm2 and lifetime of carriers of 6 ms were estimated. The later was confirmed from photoconductivity measurements.
We report on linear optical properties of Disperse Red 1 (DR1) doped sol-gel planar and channel waveguides. The refractive index and optical propagation losses of the guiding layer were measured between 0.756 micrometers and 1.64 micrometers . In the telecommunications window, the attenuation is dominated by the overtones of the O-H bonds vibration bands. We also report on photobleached channel waveguides. Propagation losses were measured at 1.064 micrometers as a function of the waveguide width. Attenuation coefficients as low as 1 cm-1 in 4 to 8 micrometers wide channel waveguides were demonstrated. A good confinement of the light is observed in a 3 hours bleached sample. The suitability of plasma etching as an alternative technique for fabricating channel waveguides is demonstrated.
Anne-Claire Le Duff, Michael Canva, Yves Levy, Alain Brun, A. Galvan-Gonzalez, Tomas Pliska, George Stegeman, Robert Twieg, P. Chan, Khalid Lahlil, Frederic Chaput, Jean-Pierre Boilot
In chromophore doped materials, the trade-off between nonlinear coefficients and absorption is a critical issue. In particular, X(2):X(2) cascading based applications at 1.55 micrometers impose stringent conditions on the absorption at the second-harmonic wavelength. Transmission spectroscopy through a thin film does not provide sufficient resolution to measure the absorption coefficient on the red side of the main absorption band. However, assuming Voigt profiles, it is possible to extrapolate the absorption coefficient in the near infrared (NIR) from the main electronic resonance. We report on the dramatic impact of the host polymer matrix on the near infrared absorption of azobenzene chromophores. The effect is directly correlated to the host polymer glass transition temperature. The case of hybrid sol-gel material is also discussed. In particular, we present here an interesting correlation between the poling temperature and some spectroscopic properties. This correlation and thermo- spectroscopic measurements lead us to define an effective Tg in the sol-gel system. We also report on film thickness dependent absorption properties.
Doped polymers exhibit many attractive features for nonlinear optics. The performance demonstrated with some of these materials appears promising for application in real devices. However, the Achilles' heel of this class of chromophore-doped materials lies most certainly in their relatively modest chemical stability, especially their photostability. Our aim has been to quantify such side- effect phenomenon, systematically linked to the optical use of these materials. Photodegradation is a 2-step process: first, absorption, that may be characterized by (sigma) ((lambda) ), the absorption cross section, and, second, chemical reactivity from the induced excited-states, which may be quantified by B-1((lambda) ), the overall quantum efficiency of degradation. The photodegradation rate under a photon flux n is thus given by (tau) =B/((sigma) .n). We use the quantity C=B/(sigma) as a material figure of merit for photostability. Given long enough illumination times, C can always be measured. How precisely B is quantified is directly related to how precisely (sigma) is measured, which decreases dramatically as the wavelength of interest is shifted from the main absorption band towards the IR telecommunication spectral windows. Increasing future device lifetimes requires a simultaneous increase in the B parameter and a decrease in the loss due to the residual red-tail absorption. We report the systematic behavior that was found concerning the dependence of C on wavelength.
The crystal growth technique and associated optical characterizations of polydiacetylene PTS , poly bis(p-toluene sulfonate) of 2,4-hexadiyne-l,6-diol, are reviewed in this paper. The commonly observed defects such as twinning and cracking and their origins are analyzed. Two factors were found to significantly decrease the density of defects and improve the quality. First, the monomer crystal was grown reasonably quick consistent with single crystal growth to reduce polymerization. Second, the polymerization rate was reduced dramatically for the 10% to 90% conversion region for a smooth conversion by reducing the polymerization temperature. The absorption spectrum, typical Z-scans for measuring the optical nonlinearity, and SEM pictures of the surfaces are given as measures of the optical quality of the PTS crystals.
Semiconductors below the half band gap exhibit higher bond- electronic, off-resonant nonlinearities compare to silica fibers. They also exhibit low scattering losses, negligible two-photon absorption, and no linear absorption. Furthermore, due to the mature fabrication technology many complicated device designs can be easily fabricated. Thus, this material system offers unique opportunity to observe and test novel nonlinear phenomena and devices. In this paper, we will review the nonlinear optical properties of AlGaAs waveguides near the half band gap.
Poled polymers are an exciting class of materials for second-order and third-order-like (cascading) nonlinear optical applications at the telecom wavelengths around 1.3 and 1.55μm, as material parameters can be tailored for each specific application. In this survey, the most recent developments in the field of second-order nonlinear optical polymers for frequency conversion and cascading are briefly reviewed. The material parameters required for efficient frequency conversion are critically discussed together with the waveguide preparation, electrical poling and in-situ characterization of the achieved nonlinearity for various phase-matched device schemes. The state of the art in achieved second-harmonic generation conversion efficiencies is documented and predictions on the possible further development of the field are made.
S. Yilmaz, Werner Wirges, Walter Brinker, Simona Bauer-Gogonea, Stefan Bauer, Matthias Jaeger, George Stegeman, M. Ahlheim, M. Staehelin, B. Zysset, F. Lehr, Mart Diemeer, M. Flipse
Modal dispersion phase-matched second harmonic generation is demonstrated in polymer-based waveguides with a nonlinear optical core consisting of two side-chain polymers with different glass-transition temperatures. For an optimized overlap integral, a step like nonlinearity profile (chi(2)-inverted structure) is required across the core thickness. The chi(2)-inverted structure is achieved by two consecutive thermally assisted poling steps above and between the respective glass-transition temperatures, with an opposite poling field in the second poling step. The achieved chi(2)-inverted structure is monitored by in- situ electro-optic measurements and proved by electro-optic and second harmonic generation thermal analysis. Conversion efficiencies up to 7%/Wcm2 were achieved in first waveguide second-harmonic generation experiments.
A all-optical switching devices based on ultrafast bound- electronic nonlinearity have the potential to perform signal processing and routing at a speed in excess of 1 Thz. So far, AlGaAs operating at wavelength below half the band gap has been found to be one of the best materials for demonstrating and testing such devices and device concepts. A bright spatial soliton is a non-diffracting self-guided beam which exists due to an interplay between the self- focusing nonlinearity and the linear diffraction. In this talk, we will experimentally demonstrate all-optical switching based on spatial soliton dragging.
A novel switch is introduced which has the capability of interconnecting 1 input channel to N output channels in a single device, without crosstalk. It is based on the unique properties of spatial solitons which propagate without diffracting in space and create index channels which can be used to guide signals at the same or different frequencies. Angular scanning of the soliton channels is achieved by chirping the phase of the input wavefront. Some properties of the switch and the initial demonstration of soliton scanning in an AlGaAs planar waveguide at 1550 nm are discussed.
Second harmonic generation via counter propagating beams was demonstrated in poled polymer channel waveguides which were efficiently poled by in-plane geometry (d22 equals 153 pm/V). Although applications for high speed signal processing, such as pulse convolution and wavelength division multiplexing, are our primary object, it has competitive potential for frequency doublers against normal co-propagating geometries because of utilization of large resonant nonlinearities.
Based on cascading of second-order nonlinearities we propose novel schemes for all-optical signal processing exploiting the tensorial nature of the susceptibility. Waves of the same frequency and orthogonal polarizations can interact via the generated second-harmonic, with an indirect energy transfer and the consequent possibility of gain and transistor action, polarization switching, amplitude and phase modulation.
Photoelectron spectroscopy of coordinatively unsaturated organometallic anions can provide a means to probe the ground and low lying excited states of the corresponding neutral radicals. We report results for the early 3D transition metal monocarbonyls VCO and CrCO, and for the late metal complexes FeCO, CoCO and NiCO. Each spectrum displays a transition to the ground state of the neutral complex, and to an excited state whose spin multiplicity differs by two from that of the ground state. For a given complex, these states share nominally the same electron configuration but differ in the spin coupling of the metal 4s electron. There is a reversal in the state ordering as one proceeds across the transition series, from a high spin ground state for VCO (6(Sigma) +) and CrCO (7(Sigma) +) to a low spin ground state for FeCO (3(Sigma) -), CoCO (2(Delta) ) and NiCO (+1)(Sigma) )+). The measured state splittings and vibrational frequencies provide insight into the factors that determine the ordering and bonding properties of these states. Recent results for the linear H-M-CO isomers of Fe and Co are also reported.
Semiconductors and in particular AlGaAs operated at photon energies below half the band gap have proven over the last few years to be optimum materials for studying nonlinear guided phenomena, including ultrafast all-optical switching. Here we report experimental results on a range of characterization measurements and implementations of all-optical switching devices.
The third order susceptibility of a polydiacetylene polymer was investigated by three different nonlinear spectroscopic techniques in order to test two existing microscopic theories which predict the nonlinear response of such molecular systems. We previously found good agreement between a four essential state model and the THG spectra of (pi) -electron conjugated backbone polymers. However, when such a model was extended to two additional third order susceptibility spectra, poor agreement was found. In contrast the recently developed anharmonic oscillator model fitted well all four experimentally obtained spectra of the third order susceptibility.
We demonstrate that the Maker fringe technique can be applied to third order frequency mixing experiments designed to measure the electronic (chi) (3)(-(omega) 3;(omega) 1,(omega) 1,-(omega) 2) dispersion with (omega) 1 for a poly[3-tetradecylthiophene] thin film. Both the magnitude and the phase of the (chi) (3) were obtained. In the magnitude spectrum there was a strong three-photon resonance where the output frequency ((omega) 3equals2(omega) 1-(omega) 2) corresponds to the energy level of the first one photon state. We also found a peak which could be either a 2(omega) 1 or a (Delta) (omega) (equals(omega) 1-(omega) 2) resonance. Since this method is considered to be more sensitive to two-photon resonances that third harmonic generation, it can be used as a powerful tool to probe two photon states in the case of thin film polymer samples.
It is possible to define figures of merit which a nonlinear material must satisfy in order for it to be useful for all-optical switching in waveguides (as well as other nonlinear devices). Here we first discuss the physical basis for these figures of merit. Examples of materials in which these figures of merit both are and are not satisfied will be given. Finally the evolution of nonlinear directional couplers will be reviewed and the impact of figures of merit identified.
We demonstrate a new method for investigating the orientational distribution of the rod-like poly[bis(m-butoxyphenyl) silane] molecules in Langmuir-Blodgett (LB) films. The first two non-trivial in-plane orientational order parameters C2 and C4 could be easily deduced by measuring the independent tensor components of (chi) (3)(-(omega) 3;(omega) 1,(omega) 1,(omega) 2). Using this method, we show that the annealing process improves the molecular alignment along the dipping direction. We were also able to get information about the domain morphology of the film from the off-diagonal components of (chi) (3).
We demonstrate resonant third harmonic generation as a powerful tool for analyzing the quasi 2-D molecular orientational order in a Langmuir-Blodgett film of poly(bis-m- butoxyphenylsilane). By recording Maker-fringes at different angles between the polarization of the fundamental beam and the dipping direction, we were able to determine the order parameters *2> and *4> simultaneously. We found *2> equals 0.466 +/- 0.030 and *4> equal to -0.12 +/- 0.10.
We review a number of nonlinear guided-wave effects using gratings in indium antimonide planar
structures. In particular, we report on the experimental demonstrations of guided-wave optical bistability
and limiting in an input distributed coupler; butterfly bistability of an input-output grating-coupler
combination; all-optical tuning of a distributed feedback reflector; and enhancement of bistability effects
by the use of a counter-propagating feedback beam.
Strip-loading is a convenient and effective method to design and fabricate channel waveguides in multilayer active polymer
structures. Several micron wide strip-loaded poly(4-BCMU) waveguides are observed to be single mode at 1.06 im and
1.32 rim.
Based on the optical nonlinearity for poly(4-BCMU) measured by THG at 1.32 pm, an intensity of 650 MW/cm2 is
required to satisfy the optical phase shift material figure of merit, 4ir, for a directional coupler with 1 cm interaction
length. To estimate the optical power for directional coupler operation, we initially assume no index saturation effects,
negligible one or two photon absorption, waveguide propagation losses less than 1 dB/cm, and unity coupling efficiency.
40 W is the lowest estimated peak pulse power of operation for a device with 6 .tm2 area waveguides. Actual coupling
efficiencies of light incident on our 6 pj2 ea waveguides are between 30 % and 40 %. The propagation losses for our
sixip-Ioaded waveguides are between 4 dB/cm (i'M) and 9dB/cm (TE). The guided wave intensities are at least ten fold
lower than the input intensities. Incident peak pulse powers of at least several hundred watts will be required for 1 cm
long (lirectional coupler device operation. Initial interferometry results suggest that average power dependent thermal
phase shifts dominate peak power dependent electronic phase shifts at duty cycles approaching unity. Polymer
superlattices are considered as advanced nonlinear optical materials.
We report on the optical characterization of poly-4BCMU slab waveguides, and on the fabrication of efficient
grating guided-wave couplers using this material. Thin film samples were obtained, by spin coating,
from a cyclopentanone solution. Grating couplers were used to measure the film refractive indices and
thicknesses. Waveguide attenuation was obtained from measurements of the variation in scattering with propagation
distance in the visible and near infrared. These parameters allowed us to design, fabricate and test a
grating coupler with a coupling efficiency of 45% at 1 .064 pm.
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