| 1. |
- Baev, Alexander, et al.
(author)
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Upconverted lasing based on many-photon absorption : an all dynamic description
- 2004
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 21:2, s. 384-396
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Journal article (peer-reviewed)abstract
- A theory is developed for the propagation through a nonlinear medium of strong pump and amplifiedspontaneous-emission pulses. The theory is based on a solution of the density matrix equations that aims at providing an adequate treatment of the nonlinear polarization of the material without addressing the Taylor expansion over the powers of intensity. The theory has been applied for modeling of three-photon absorption induced upconverted stimulated emission of organic molecules in solvents. Numerical results are presented for the organic chromophore 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4'-(6-hydroxyhexyl sulfonyl) stilbene dissolved in dimethyl sulfoxide. The results are in good agreement with available experimental results.
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| 2. |
- Boudrioua, A., et al.
(author)
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Nonlinear optical properties of (H+,He+)-implanted planar waveguides in z-cut lithium niobate : annealing effect
- 2001
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 18:12, s. 1832-1840
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Journal article (peer-reviewed)abstract
- Reflection second-harmonic generation from the polished waveguide end face was used to investigate the nonlinear properties of LiNbO3-implanted waveguides fabricated by use of 2-MeV He+ and 1.5-Mev H+ beams. Results were compared with waveguides obtained by protonic exchange in benzoic acid, In contrast to the exchanged sample where the nonlinearity is strongly reduced, the implanted samples showed that the guiding region presents rather the same response as the substrate. The area where the optical barrier is located showed a strongly enhanced second-harmonic signal that was likely to be due to structural modifications in this area. Moreover, the investigation of the annealing effect showed strong interaction of protons with the lattice compared with that of He+ ions.
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| 3. |
- Ferrini, R., et al.
(author)
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Radiation losses in planar photonic crystals : two-dimensional representation of hole depth and shape by an imaginary dielectric constant
- 2003
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 20:3, s. 469-478
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Journal article (peer-reviewed)abstract
- Waveguide modes in two-dimensional (2-D) photonic crystals (PhCs) deeply etched through monomode slab waveguides, e.g., AlGaAs/GaAs, GaAs/AlOx, or InP/GaInAsP, suffer from radiation losses that are strongly affected by the air hole depth and shape. The issue of three-dimensional (3-D) out-of-plane losses is addressed analytically by means of an incoherent approximation. Assuming separability both for the dielectric map and for the electric field, this approach is valid for defects such as in-plane microcavities, PhC-based waveguides, bends and couplers. Out-of-plane scattering is translated into an effective imaginary index in the air holes, so that 3-D losses can be cast in a simple 2-D calculation. The case of cylindroconical holes is treated, and the validity of this approach is experimentally confirmed by transmission measurements through simple PhC slabs.
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| 4. |
- Fokine, Michael
(author)
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Formation of thermally stable chemical composition gratings in optical fibers
- 2002
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 19:8, s. 1759-1765
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Journal article (peer-reviewed)abstract
- Experimental results and a discussion of possible chemical pathways in the formation of thermally stable chemical composition gratings in optical fibers are presented. Gratings are formed through high-temperature treatment of UV-exposed hydrogen-loaded fibers. The final refractive-index modulation is ascribed to variations in fluorine concentration attained by periodically increased diffusion of fluorine. The mechanism behind this increase is the formation of mobile hydrogen fluoride from chemical reactions of fluorine and UV-induced hydroxyl, which occur with the spatial periodicity of the UV pattern. A hydroxyl-assisted increase in fluorine diffusion has been verified by time-of-flight secondary-ion mass spectroscopy. Formation of ultrastable grating by periodic variation of oxygen concentration through diffusion of molecular water is also discussed.
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| 5. |
- Gel'mukhanov, Faris, et al.
(author)
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Dynamics of two-photon absorption by molecules and solutions
- 2002
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 19:5, s. 937-945
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Journal article (peer-reviewed)abstract
- A dynamic density-matrix based theory of two-photon absorption (TPA) of molecules and solutions is presented. The theory highlights the influence of pulse duration, dephasing, and resonant conditions on the final TPA cross section as well as that of saturation, including a hierarchy of saturation intensities. A breakdown of the conventional identification of TPA with coherent one-step TPA is predicted for the long-pulse regime in which incoherent two-step TPA can even dominate the coherent one-step TPA process. The major role of the solvent is to enhance the off-resonant contributions to TPA furnished by collisional dephasing.
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| 6. |
- Hall, Björn, 1973, et al.
(author)
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Pump-probe collision in optical fibers : Four-wave-mixing induced frequency jump
- 2001
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In: Journal of the Optical Society of America. B, Optical physics. - : Optical Society of America (OSA). - 0740-3224 .- 1520-8540. ; 18:11, s. 1652-1658
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Journal article (peer-reviewed)abstract
- An investigation is made of ultrafast pump-probe pulse collisions near the zero-dispersion wavelength in an optical single-mode fiber. A steplike probe frequency shift is observed when the pump power is gradually increased. The magnitude of this frequency jump is shown to depend on the phase difference between the pulses. This new effect is investigated numerically and experimentally and is attributed to four-wave mixing.
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| 7. |
- Laine, T. A., et al.
(author)
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Self-guided waves and exact solutions of the nonlinear Helmholtz equation
- 2000
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 17:5, s. 751-757
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Journal article (peer-reviewed)abstract
- The paraxial wave theory is known to lead to inaccurate predictions in self-focusing of optical beams. The nonlinear Helmholtz equation describes more accurately wave propagation in dispersive, spatially local, Kerr-type media. We derive rigorous bright and dark solutions to the nonlinear Helmholtz equation in a full three-dimensional form. These expressions are new and unique. The solutions are obtained with a multidimensional extension of the (paraxial) nonlinear Schrodinger equation. We also establish energy conservation laws for both nonlinear wave equations in terms of spatial currents. Our results give insight, for example, into the diffraction and breakup of tightly confined nonlinear fields.
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| 8. |
- Laine, T. A., et al.
(author)
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Thin-layer theory of nonlinear fields
- 2000
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 17:10, s. 1711-1715
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Journal article (peer-reviewed)abstract
- We derive a local nonlinear thin-layer theory for electromagnetic fields that propagate in layered structures of isotropic, dispersive, and spatially local Kerr media. By use of an ansatz of plane waves together with a thin-layer approximation, the two-dimensional Kerr-Maxwell equation is rigorously solved within a very thin slab, and the characteristic matrix of the nonlinear medium is determined. The theory makes use of periodicity and allows a direct calculation of the nonlinear field throughout the structure on the basis of the transmitted field. The method is applied in the two polarizations, TE and TM, and is illustrated with a numerical example. The nonlinear thin-layer technique provides a simple and accurate analytical theory that includes multiple plane-wave incident fields and takes rigorously into account all nonlinear interactions of these waves.
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| 9. |
- Laine, T. A., et al.
(author)
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Variational analysis and angular bistability in layered nonlinear Kerr media with phase conjugation
- 2000
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 17:6, s. 977-984
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Journal article (peer-reviewed)abstract
- The calculus of variations is applied to electromagnetic fields in a layered nonlinear structure supporting a guided wave. The system also includes a phase-conjugate mirror (PCM). By introducing a variational dimension and using a collection of plane waves as a trial function, we approximate the exact solution of the nonlinear Kerr-Maxwell equation. The formalism is new, and it involves the nonlinear interference of multiple plane waves. A simple analytical expression for the nonlinear field in the presence of the PCM is derived, and the fact that the scattered intensities may become bistable when the angle of incidence is varied is demonstrated. In particular, our theory predicts the angular bistability in the backscattering direction, where the effect of the guided waves is subtle. Our numerical results are also in good agreement with other theoretical approaches and with the experimental data.
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| 10. |
- Malomed, B. A., et al.
(author)
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Propagation of an optical pulse in a fiber link with random-dispersion management
- 2001
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In: Journal of the Optical Society of America. B, Optical physics. - : Optical Society of America (OSA). - 0740-3224 .- 1520-8540. ; 18:9, s. 1243-1251
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Journal article (peer-reviewed)abstract
- A model of a long optical communication line consisting of alternating segments with anomalous and normal dispersion, whose lengths are picked randomly from a certain interval, is considered. As the first stage of the analysis, we calculate small changes in parameters of a quasi-Gaussian pulse passing a double-segment cell by means of the variational approximation (VA) and we approximate the evolution of the pulse passing many cells by smoothed ordinary differential equations with random coefficients, which are solved numerically. Next we perform systematic direct simulations of the model. Simulations reveal slow long-scale dynamics of the pulse, frequently in the form of long-period oscillations of its width. It is thus found that the soliton is most stable in the case of zero path-average dispersion (PAD), less stable in the case of anomalous PAD, and least stable in the case of normal PAD. The soliton's stability also strongly depends on its energy, the soliton with low energy being much more robust than its high energy counterpart.
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| 11. |
- Myrén, Niklas, et al.
(author)
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In-fiber electrode lithography
- 2004
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 21:12, s. 2085-2088
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Journal article (peer-reviewed)abstract
- The fabrication of meter-long continuous internal fiber electrodes is achieved through deposition of a silver film inside a twin-hole fiber. Photolithography of the electrodes with 5-μm resolution inside the fiber is demonstrated by point-by-point side exposure to 0.53-μm radiation through the unharmed acrylate coating, causing laser ablation. A proof-of-principle experiment demonstrates the creation of a phase-matched structure for frequency doubling.
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| 12. |
- Pitois, C., et al.
(author)
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Absorption and scattering in low-loss polymer optical waveguides
- 2001
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 18:7, s. 908-912
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Journal article (peer-reviewed)abstract
- The absolute absorption spectra of low-loss optical waveguides, together with their intrinsic and extrinsic scatterings, were measured in the near infrared. Photothermal deflection spectroscopy was used to measure the full absorption spectra of a series of fluorinated cross-linked polymers. Assignment of-CH3,-CH2-, and -OH overtones as well as of combinations of overtones were made by use of the theory of anharmonic vibrations based on a Morse potential for local modes. Details of the molecular potential are given. The total attenuation in slab waveguides made with these polymers was measured by a prism-coupling technique and compared with the absolute absorption. Losses that are due to the material itself and those that are due to the processing are quantified.
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| 13. |
- Qiu, Min, et al.
(author)
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Optimal design of a two-dimensional photonic crystal of square lattice with a large complete two-dimensional bandgap
- 2000
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 17:6, s. 1027-1030
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Journal article (peer-reviewed)abstract
- Optimal design of a two-dimensional photonic crystal with a square lattice of air holes in GaAs is considered. It is shown how a maximum complete two-dimensional band gap is obtained by optimally connecting the dielectric rods with veins. The complete two-dimensional bandgap of our optimal design reaches Delta omega = 0.0762(2 pi c/a) (a is the lattice constant).
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| 14. |
- Thoss, A., et al.
(author)
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Kilohertz sources of hard x rays and fast ions with femtosecond laser plasmas
- 2003
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In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 20:1, s. 224-228
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Journal article (peer-reviewed)abstract
- We demonstrate a new, stable, kilohertz femtosecond laser plasma source of hard-x-ray continuum and K-alpha emission that uses a microscopic liquid jet target that is continuous and debris free. Plasmas produced by ultrashort (50-fs) intense laser pulses from a fine (10-30-mum diameter) liquid Ga jet emit bright 9.3- and 10.3-keV K-alpha and K-beta lines superimposed on a multikilovolt bremmstrahlung continuum. Kilohertz femtosecond x-ray sources will find many applications in time-resolved x-ray diffraction and microscopy studies. As high-intensity lasers become more compact and operate at increasingly high repetition-rates, they require a target configuration that is both repeatable from shot to shot and debris free. Our target provides a pristine, unperturbed filament surface at rates >100 kHz. A number of liquid metal targets are considered. We show the hard-x-ray spectrum described above. The source was generated by a 50-fs-duration, 1-kHz, 2-W, high-intensity Ti:sapphire laser. Using the same technology, we also generate forward-going sub-mega-electron-volt (sub-MeV) protons from a 10-mum liquid water target at 1-kHz repetition rates. Kilohertz sources of high-energy ions will find many applications in time-resolved particle interaction studies and will lead to efficient generation of short-lived isotopes for use in nuclear medicine and other applications. The protons were detected with CR-39 track detectors in both the forward and the backward directions up to energies of similar to500 keV. As the intensity of compact high-repetition-rate lasers sources increases, we can expect improvements in the energy, conversion efficiency, and directionality to occur. The effect of these developments is discussed. As compact, high-repetition-rate femtosecond laser technology reaches focused intensities of similar to10(19) W/cm(2), many new applications of high-repetition-rate hard-x-ray and MeV ion sources will become practical.
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| 15. |
- Tillman, K. A., et al.
(author)
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Low-threshold, high-repetition-frequency femtosecond optical parametric oscillator based on chirped-pulse frequency conversion
- 2003
-
In: Journal of the Optical Society of America. B, Optical physics. - 0740-3224 .- 1520-8540. ; 20:6, s. 1309-1316
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Journal article (peer-reviewed)abstract
- We report a quasi-phase-matched optical parametric oscillator that incorporates a chirped nonlinear crystal and uses prechirped pulses matched to the crystal chirp to improve the conversion efficiency and reduce the operational threshold. A 20-mm crystal of aperiodically poled KTiOPO4 is phase matched to stretched Ti:sapphire pump pulses. The Ti:sapphire laser produces 104-MHz output pulses at 850 nm that are stretched from 190 to 900 fs with an average output power of 750 mW The system has demonstrated a pump depletion of more than 80%, a signal slope efficiency of 35%, and a threshold of 14.4 mW The cavity showed tuning from 1194 to 1455 nm. over a length range of 130 mum. The approach described demonstrates the potential of using chirped-pulse-chirped-crystal quasi-phase matching in long nonlinear crystals as a method to reduce ultrafast optical parametric oscillator thresholds.
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| 16. |
- Wei, CH, et al.
(author)
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Polarization dependence of light intensity distribution near a nanometric aluminum slit
- 2004
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In: Optical Society of America. Journal B: Optical Physics. - 0740-3224. ; 21:5, s. 1005-1012
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Journal article (peer-reviewed)abstract
- The near-field radiation pattern of a long thin slit (with a width much smaller than the excitation wavelength) in a uniform aluminum surface was measured and modeled by numerical computation. In particular, the interplay between the incident light polarization and the slit width is found to play an essential role in the near-field profile on the back side of the nanoslits. Two-dimensional finite-difference time-domain computer simulations were performed to calculate the near-field intensity profile for different slit widths and metal thicknesses. This method will allow the optimization of three-dimensional near-field radiation patterns for a variety of near-field molecular scanning schemes. (C) 2004 Optical Society of America.
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