Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Chaos in photovoltaic HgCdTe detectors under laser irradiation

Chaos in photovoltaic HgCdTe detectors irradiated by a low-power stable continuous-wave laser is found in our experiment. The detectors give a chaotic output in the laser irradiation power density range a little higher than their saturation threshold. Chaos is diagnosed by calculating the power spectrum and the Lyapunov exponent with experimental data of the detectors’ output signals vs. time. We conclude with a qualitative discussion of the possible causes of the observed chaotic behavior. Received: 3 April 2002 / Revised version: 8 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. (Fax: +86-0731/4514127, E-mail: maliqin01@263.net)     

Nonlinear side effects of fs pulses inside corneal tissue during photodisruption

In order to evaluate the potential for refractive surgery, fs laser pulses of 150-fs pulse duration were used to process corneal tissue of dead and living animal eyes. By focusing the laser radiation down to spot sizes of several microns, very precise cuts could be achieved inside the treated cornea, accompanied with minimum collateral damage to the tissue by thermal or mechanical effects. During histo-pathological analysis by light and transmission electron microscopy considerable side effects of fs photodisruption were found. Due to the high intensities at the focal region several nonlinear effects occurred. Self-focusing, photodissociation, UV-light production were observed, leading to streak formation inside the cornea. Received: 12 October 2001 / Revised version: 13 February 2002 / Published online: 14 March 2002     

Third-order nonlinear optical response of Au-core CdS-shell composite nanoparticles embedded in BaTiO<Subscript>3</Subscript> thin films

Au-core CdS-shell composite nanoparticles were synthesized by a direct self-assembly process and integrated into BaTiO₃ thin films. Characterization by transmission electron microscopy showed that the average diameter of these composite nanoparticles was about 8 nm. Using the femtosecond time-resolved optical Kerr effect method, we investigated the third-order nonlinear optical response of the Au@CdS nanoparticles embedded in the BaTiO₃ thin films at a wavelength of 800 nm. An ultrafast nonlinear response and a large effective third-order nonlinear susceptibility of χ⁽³⁾=7.7×10^-11 esu were observed. We attributed the enhancement of the third-order optical nonlinearity to a localized electric field effect originating from the core-shell structure under off-surface-plasmon resonance conditions. Received: 13 May 2002 / Revised version: 23 October 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/6510-4949, E-mail: sxqian@fudan.ac.cn     

Artifacts in femtosecond transient absorption spectroscopy

The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed. Received: 29 May 2001 / Final version: 15 October 2001 / Published online: 29 November 2001     

Transverse effects and mode couplings in OPOS

00 pump field. The corresponding differential equations for the time evolution of the self-consistent mode amplitudes are derived for typical cavity geometries including the propagation integrals within the crystal. We discuss the new physical effects introduced by these couplings in some typical cases. This includes a change of the OPO threshold and stable simultaneous oscillation of several mode pairs with fixed relative phases as well as oscillating self-pulsing type solutions. Received: 12 January 1998/Accepted: 11 February 1998     

Surface relief gratings in photoaddressable polymers generated by cw holography

Surface relief gratings (SRGs) are a widely observed phenomenon in holographic experiments with photoaddressable polymers. This article proposes a model which is able to explain the observed effects in cw experiments. It is based on forces in an electrical field which are caused by the inhomogeneous spatial distribution of the refractive index and the permittivity in the illuminated sample. Received: 11 September 2000 / Revised version: 5 December 2000 / Published online: 21 February 2001     

Modulational instability in one-dimensional saturable waveguide arrays: Comparison with Kerr nonlinearity

Discrete modulational instability within the first band of uniform one-dimensional waveguide arrays possessing a saturable self-defocusing nonlinearity is investigated in detail within the coupled mode approach. Explicit analytical results for both the threshold and the maximal gain of instability are compared with the corresponding data from waveguide arrays exhibiting Kerr nonlinearity. We find that saturation bounds the interval of existence of discrete modulational instability, stabilizes the frequency region of perturbations around ±π/2 and decreases both gain and critical spatial frequency of perturbations.     

Numerical analysis of the spatial behaviour of nanosecond optical parametric oscillators of beta-barium borate

We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M ²) on pump energy. An important result is the strong increase of the M ² value for pump energies exceeding 1–2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the M ² value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation. Received: 1 April 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999     

Steering one-dimensional odd dark beams of finite length

This paper presents arguments for the existence and dynamics of one-dimensional odd dark beams of finite length in bulk Kerr nonlinear media. Their characteristic mixed edge–screw phase dislocations force them to steer in space. The modulational stability of these dark beams and one possible interaction scheme are discussed. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Reverse-symmetry waveguides: theory and fabrication

We present an extensive theoretical analysis of reverse-symmetry waveguides with special focus on their potential application as sensor components in aqueous media and demonstrate a novel method for fabrication of such waveguides. The principle of reverse symmetry is based on making the refractive index of the waveguide substrate less than the refractive index of the medium covering the waveguiding film (n_water=1.33). This is opposed to the conventional waveguide geometry, where the substrate is usually glass or polymers with refractive indices of ≈1.5. The reverse configuration has the advantage of deeper penetration of the evanescent electromagnetic field into the cover medium, theoretically permitting higher sensitivity to analytes compared to traditional waveguide designs. We present calculated sensitivities and probing depths of conventional and reverse-symmetry waveguides and describe schemes for easy implementation of reverse symmetry. Polymer waveguides are demonstrated to be candidates for cheap, mass-producible reverse-symmetry sensor modules. The grating-coupled waveguiding films of controlled thickness are produced by soft lithography. The resulting films are combined with air-grooved polymer supports to form freestanding single-material polymer waveguides of reverse symmetry capable of guiding light. Received: 20 December 2001 / Published online: 14 March 2002     

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Quasi-stable propagation of vortices and soliton clusters in saturable Kerr media with square-root nonlinearity

Analytical and numerical investigation of the propagation of optical beams in Kerr-like saturable photorefractive media is carried out, utilizing a novel model for the local isotropic part of the space-charge field generated in the medium. Using a variational technique, optimal propagation parameters for the most stable propagation of otherwise unstable single Gaussian, single vortex, and optical soliton cluster beams are determined. Analogy between a ring of identical weakly overlapping solitons and a vortex of the same topological charge is explored.     

Asymmetric ultra-short pulse splitting measured in air using FROG

Frequency-resolved optical gating is used to measure non-linear ultra-short pulse propagation in air over a distance of up to 4.35 m. During spatial collapse of the pulse, both pulse steepening and asymmetric pulse splitting are observed to occur at a power level of about 14 GW and at a propagation distance less than the diffraction length. Received: 10 April 2002 / Published online: 12 July 2002     

Wavelength dependence of nonlinear absorption in a bis-phthalocyanine studied using the Z-scan technique

The wavelength dependence of the nonlinear absorption of a bis-phthalocyanine, Nd(Pc)₂, dissolved in dimethyl formamide was studied in the rising part of the Q-band using the open aperture Z-scan technique, to determine the wavelength region over which the nonlinear absorption changes from reverse saturable absorption to saturable absorption. It was found that the sample could be used as a reverse saturable absorber, and hence as an optical limiter, up to a wavelength of about 604 nm. The imaginary part of the third order susceptibility was also calculated for these wavelengths. Resonant enhancement of the imaginary part of the third order susceptibility was clearly observed. Received: 26 April 2002 / Revised version: 30 June 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +91-484/576-714, E-mail: kpu@cusat.ac.in RID="**" ID="**"Present address: Optical Sciences Center, University of Arizona, Tucson, USA     

Threshold of oscillation in a ring-loop phase conjugator as a second-order optical phase transition

The threshold behavior is studied for a ring-loop coherent oscillator with a photorefractive strontium barium niobate (SBN) sample. A soft onset of oscillation is revealed and critical slowing down is observed in the temporal development of light-induced scattering below the threshold, thus pointing to a similarity with a second-order phase transition. Received: 10 July 2000 / Published online: 20 September 2000     

Quasi-two-dimensional dark spatial solitons and generation of mixed phase dislocations

This paper presents experimental evidence that orthogonally crossed dark soliton stripes form quasi-two-dimensional spatial solitons with a soliton constant equal to that of singly charged optical vortices. Besides the pairs of oppositely charged optical vortex solitons, the snake instability of the dark formation at moderate saturation is found to lead to generation of steering mixed edge–screw phase dislocations with zero total topological charges. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Composite spatial solitons in a saturable nonlinear bulk medium

We review the generation of the recently predicted multi-component spatial optical solitons in a saturable nonlinear bulk medium. We present numerical simulations for an effectively isotropic model and experimental results for a set of different combinations of a Gaussian beam co-propagating incoherently with a beam of a more complex internal structure, such as a higher order transverse laser mode. We discuss the different formation processes and the general properties of a variety of different dipole-mode composite solitons and expand our investigations to the generation of a quadrupole-mode composite soliton. Received: 1 December 2000 / Revised version: 12 January 2001 / Published online: 21 March 2001     

Absolute absorber quantification in turbid media at small source–detector separations

The aim of our studies was to develop a method to determine the absorption coefficient of a turbid medium in a reflection geometry with small source–detector separations. Therefore, the time-integrated microscopic Beer–Lambert law (MBL) was modified in order to obtain the absolute absorption coefficient from mean time of flight and dc-intensity measurements. The new technique was evaluated using turbid phantoms having varying scattering (μ_s ^′ between 0.2 and 2.2 mm^-1) and absorbing properties (μ_a between 0.04 and 0.14 mm^-1), comparable to many biological tissues at various source–detector separations between 3 and 11 mm. The measurements were performed at a wavelength of 1064 nm. We found that this new method was able to determine the absolute absorption coefficient of the selected phantoms with a standard error of less than 0.005 mm^-1 over the range of optical properties investigated. Received: 23 November 2001 / Revised version: 28 February 2002 / Published online: 14 May 2002