Showing papers in "Journal of The Optical Society of America B-optical Physics in 1992"

Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging

Abstract: We present a polarization-sensitive optical coherence-domain reflectometer capable of characterizing the phase retardation between orthogonal linear polarization modes at each reflection point in a birefringent sample. The device is insensitive to the rotation of the sample in the plane perpendicular to ranging. Phase measurement accuracy is ±0.86°, but the reflectometer can distinguish local variations in birefringence as small as 0.05° with a distance resolution of 10.8 μm and a dynamic range of 90 dB. Birefringence-sensitive ranging in a wave plate, an electro-optic modulator, and a calf coronary artery is demonstrated.

Determination of bound-electronic and free-carrier nonlinearities in ZnSe, GaAs, CdTe, and ZnTe

Abstract: We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 μm and in ZnSe at 1.06 and 0.53 μm, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient σr,), the two-photon absorption coefficient β, and the bound electronic nonlinear refractive index n2. The real and imaginary parts of the third-order susceptibility (i.e., n2 and β, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n2 changes from positive to negative as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n2 in solids based on Kramers–Kronig transformations [ Phys. Rev. Lett.65, 96 ( 1990); IEEE J. Quantum Electron.27, 1296 ( 1991)]. We find that the values of σr are in agreement with simple band-filling models.

Explicit asymptotic formulas for the positions, widths, and strengths of resonances in Mie scattering

Abstract: Explicit asymptotic formulas are derived for the positions, widths, and strengths of the morphology-dependent resonances in Mie scattering. These formulas are compared with numerical data and found to be highly accurate, especially for the low-order resonances most relevant to nonlinear processes. They permit the interpretation of experimental data on light scattering from microdroplets without resorting to the full apparatus of the Mie scattering formalism.

Nonlinear optical properties of carbon-black suspensions (ink)

Abstract: We performed a series of experiments on suspensions of carbon particles in liquids (ink) and carbon particles deposited on glass to determine the mechanisms for the observed optical-limiting behavior. Both materials show reduced transmittance for increasing fluence (energy per unit area). We found that nonlinear scattering dominates the transmissive losses and that the limiting is fluence dependent, so that limiters based on black ink are effective for nanosecond pulses but not for picosecond pulses. Additionally, the nonlinear scattering and the limiting behavior cease after repeated irradiation. For the liquid, flowing eliminates this effect. All the data obtained are consistent with a model of direct heating of the microscopic-sized carbon particles by linear absorption with subsequent optical breakdown initiated by thermally ionized carriers. A simple calculation gives temperatures higher than the sublimation temperature at the onset of limiting. Emission spectra measurements show singly ionized carbon emission lines with a hot blackbody background emission consistent with temperatures of ≃4000 K. A rapid expansion of the microscopic plasmas generated by the breakdown will effectively scatter further input light. Indeed, in time-resolved experiments the trailing portion of the pulse is most heavily scattered. The time-resolved transmittance of a weak cw probe beam also follows the temporal dependence of the singly ionized carbon emission (≃102 ns). We directly monitored the expansion of the scattering centers by angularly resolving the scattered light for different input fluences and fitting to Mie scattering theory. Since the carbon is black and the microplasmas are initiated by linear absorption, the limiting is extremely broadband. Within the context of this model we discuss the limitations and optimization of ink-based optical limiters.

Dispersive perturbations of solitons of the nonlinear Schrödinger equation

Abstract: A useful analysis of dispersive (radiative) perturbations of solitons of the nonlinear Schrodinger equation is developed. With reference to the propagation of optical solitons in glass fibers, the analysis is used to treat the collision of a low-intensity wave packet with a soliton, the radiation field created by the local perturbation of a soliton, and finally that created by a spatially periodic perturbation of the parameters of the fiber, or equivalently by a periodic variation in gain and loss that averages to zero. Perturbations whose wavelength is short compared with the soliton period produce exponentially small radiation fields as a result of the need for phase matching.

Wave breaking in nonlinear-optical fibers

Abstract: An analytical investigation is made of the interplay between dispersion and nonlinearity in the creation of wave breaking in optical fibers. Wave breaking is found to involve two independent processes: (a) overtaking of different parts of the pulse and (b) nonlinear generation of new frequencies during overtaking. Analytical predictions for the distance of wave breaking are obtained and found to be in good agreement with numerical results.

Nondegenerate two-photon absorption in zinc blende semiconductors

Abstract: An algorithm is presented for the calculation of the nondegenerate two-photon absorption coefficient by using second-order perturbation theory and a Kane band-structure model, including the effects of nonparabolicity and nonzone-center wave functions. The polarization dependence is included by correctly accounting for the symmetry of the electronic wave functions. A comparison is made with degenerate two-photon absorption data in various zinc blende semiconductors, and excellent agreement is found without the use of fitting parameters. Comparisons are also made with nondegenerate two-photon absorption spectra measured in ZnSe and ZnS by using a picosecond continuum and with some polarization-dependent measurements obtained by a two-color Z-scan measurement.

Spatial and temporal transformation of femtosecond laser pulses by lenses and lens systems

Abstract: A Fourier-optical analysis of the transformation of ultrashort light pulses by lenses is given. Inserting the material dispersion up to second order, we find a coupling between the temporal, spectral, and spatial properties of the light pulse. In general, this coupling leads to a drastic increase in pulse duration and width of the spatial intensity distribution in the focal plane of the lens, which can be avoided with the use of achromatic lens doublets. The results are discussed for Gaussian-shaped input pulses.

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

Abstract: 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.

Photorefractive quantum wells: Transverse Franz-Keldysh geometry

Abstract: The photorefractive properties of semi-insulating AlGaAs–GaAs multiple quantum wells are described for the transverse Franz–Keldysh geometry with the electric field in the plane of the quantum wells. Combining the strong electroabsorption of quantum-confined excitons with the high resistivity of semi-insulating quantum wells yields large nonlinear optical sensitivities. The photorefractive quantum wells have effective nonlinear optical sensitivities of n2 ≈ 103 cm2/W and α2/α0 ≈ 104 cm2/W for applied fields of 10 kV/cm. Photorefractive gains approaching 1000 cm−1 have been observed in two-wave mixing under dc electric fields and stationary fringes. The transverse Franz–Keldysh geometry retains the general transport properties and behavior of conventional bulk photorefractive materials. The resonant excitation of free electrons and holes in the quantum wells leads to novel behavior associated with electron–hole competition. We demonstrate that under resonant excitation of electrons and holes the device resolution is fundamentally limited by diffusion lengths but is insensitive to long drift lengths.

Controlling transverse-wave interactions in nonlinear optics - generation and interaction of spatiotemporal structures

Abstract: It is shown that a new type of instability of a light field in a dissipative medium (spatiotemporal instability, which causes the generation of new types of nonlinear light wave) can be observed by controlling the spatial scale and the topology of the transverse interactions of light fields in a medium with cubic nonlinearity. The excitation conditions for optical reverberators, rotating helical waves, and various dissipative structures are experimentally determined. Transformations and interactions of the structures lead to optical turbulence in both space and time. Physical interpretation of these phenomena is based on the parabolic equation for the nonlinear phase shift. It is found that this theoretical model allows one not only to obtain the excitation conditions but to investigate thoroughly such phenomena as hysteresis and nonlinear interactions of structures.

Detection and identification of single molecules in solution

Abstract: We have extended our recent experiments in the detection of single fluorescent molecules in solution to the exploration of spectroscopy at the single-molecule level. As a first step we have developed a technique that can efficiently distinguish between two species of dye molecules on the basis of differences in their emission spectra. We have also demonstrated that another spectroscopic property, fluorescence lifetime, can be accurately determined at the single-molecule level. Spectroscopic properties can be used to identify fluorescent molecules and to reveal static or slowly varying aspects of the microenvironment of each molecule, thereby yielding information unavailable from bulk studies.

Radiation force in the magneto-optical trap

Abstract: An overview of the theory of the magneto-optical trap is presented, along with measurements of the effect of an imbalance in the intensities of the trapping beams. This investigation tests the theory of the spring constant of the trap and confirms that the confining force at the center of the trap results from an induced orientation of the atomic ground state. The experimental results give the magnitude of this force, which has not yet been calculated accurately. We calculate the radiation field in the three-dimensional molasses, finding that the relative time phase of the orthogonal standing waves is significant, and we give some insight into the phenomenon of interference fringes when the beams are misaligned. We also discuss the limitation of the trapped atomic density resulting from photon scattering within the cloud, predicting that densities above 1013 atoms/cm3 could be achieved in a trap operating at low saturation of the atomic transition. Finally, we briefly consider collisional loss at low densities, finding an especially large contribution from resonant dipole–dipole scattering.

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

Abstract: 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.

Thermal quenching of fluorescence in chromium-doped fluoride laser crystals

Abstract: The temperature dependence of the fluorescence lifetime of chromium ions in the fluorides LiBaAlF6, LiCaAlF6, LiSrAlF6, and LiSr0.8Ca0.2AlF6 is reported. In each crystal the lifetime at low temperatures is nearly constant. Above a temperature that depends on the host the lifetime drops rapidly with increasing temperature. This strong decrease is attributed to nonradiative decay processes. From the experimental data the radiative decay rate, the thermal quenching activation energy, and the nonradiative decay rate are determined. Excellent agreement with the Mott model is then demonstrated by using these parameters.

Calculation of the trapping force in a strongly focused laser beam

Abstract: We present what is, to the best of our knowledge, the first detailed calculation of the reverse trapping force acting on a dielectric sphere when it is illuminated by a strongly focused laser beam. The calculation is carried out within the geometrical optics approximation. The phenomenon of laser trapping was discovered experimentally by Ashkin et al. [ Opt. Lett.11, 288 ( 1986)] and is of great practical interest in view of the possibility it offers for freely manipulating biological particles, such as viruses and bacteria, in a nondestructive manner. We support our calculations by a qualitative experiment that clearly shows the accessibility of the trapping effect in practice. We use, as an experimental improvement, an objective with a central field stop producing a conical dark field. This enhances the relative contribution from high-N.A. illumination and makes it easier to achieve optical trapping.

Dyes in modified polymers:, problems of photostability and conversion efficiency at high intensities

Abstract: We present the results of the investigation of modified polymethyl methacrylate doped with xanthene-series dyes, phthalocyanine, and chelates in the regimes of laser generation and saturated absorption. The mechanisms responsible for the lasing and the bleaching efficiencies and for the dyes’ photodestruction are discussed. Experimental results are presented that confirm the appropriateness of the mechanisms and methods suggested for increasing the conversion efficiency and photostability. The results obtained lead us to formulate the basic principles for choosing the composition and the methods for synthesis of dye-doped polymer materials for high-intensity laser optics.

Quadratic nonlinear-optical properties of a new transparent and highly efficient organic–inorganic crystal: 2-amino-5-nitropyridinium-dihydrogen phosphate (2A5NPDP)

Abstract: Linear and quadratic nonlinear-optical (NLO) properties of a new organic–inorganic crystal, 2-amino-5-nitro-pyridinium-dihydrogen phosphate (2A5NPDP), are reported. 2A5NPDP was designed to display high NLO efficiency by incorporating highly polarizable organic molecules arranged in a polar order between phosphate polyanion sheets. The material is transparent in the visible and the near-IR (from 0.42 to 2 μm) with favorable phase-matching (PM) conditions for second-harmonic generation in the 1-μm region. Crystal nonlinear coefficients responsible for PM were determined by the Maker fringe method, leading at 1.06 μm to d15 ≅ 7.2 pm/V and d24 ≅ 1.3 pm/V. The measured crystal coefficients significantly depart from oriented gas model calculations based on molecular hyperpolarizability values measured in an aprotic solution, thus suggesting important crystal-field effects and protonation contributions. Thermal sensitivity of the PM angle at 1.34 μm is found to be 155″/°C, which is considerably larger than typical values of inorganic NLO crystals and opens the way to efficient thermal tuning of phase-matched processes.

Phase retrieval approach for coherent anti-Stokes Raman scattering spectrum analysis

Abstract: A noniterative phase retrieval method for coherent anti-Stokes Raman scattering spectrum analysis is presented. This method facilitates the computation of the real and imaginary parts of the effective third-order susceptibility χ when only its modulus is known. One obtains this result by approximating the squared modulus |χ|2 by the maximum entropy model and using some a priori information on χ.

Spectral properties of lasing microdroplets

Abstract: The spectral behavior of 15.3-μm-diameter Rhodamine 6G in water solution droplets was studied. Microdroplet lasing is known to occur simultaneously at many discrete wavelengths, each corresponding to one of many possible spherical cavity resonances. We show that lasing takes place on several mode orders at once. Modes of a given order were found to form a bell-shaped spectral cluster of typically 4–6 resonance lines having consecutive principal mode numbers. Clusters of different mode orders appear somewhat displaced spectrally from one another, with lowest-order clusters shifted to the red. This multiplicity of lasing modes is accounted for by spatial hole-burning effects. The relative lasing intensities of the differing mode orders are explained by an output coupling theory that considers the gain enhancement that is due to cavity quantum electrodynamic effects. An upper limit of 108 for the Q of a nondegenerate cavity mode was estimated from the data.

Parametric and Raman amplification in birefringent fibers

Abstract: We derive the gain for mixed Stokes and anti-Stokes waves propagating in a birefringent fiber in the presence of an intense pump wave, under the combined action of parametric four-photon mixing and Raman scattering, for different types of phase matching. We discuss the conditions for the suppression or the enhancement of the Raman gain as a function of the linear mismatch and of the sideband detuning, owing to coupling between Stokes and anti-Stokes components of the growing wave. We confirm the analytical predictions of the linearized analysis with the numerical solution of the coupled nonlinear Schrodinger equations, including the contribution of the tensorial Raman response of the fiber.

Impulsive stimulated Raman scattering experiments in the polariton regime

Abstract: Femtosecond time-resolved impulsive stimulated Raman scattering (ISRS) experiments on optic phonon–polariton modes are discussd. The effects of polariton dispersion and propagation are treated. The results provide guidance of accurate analysis of ISRS data from polar phonons.

Optical absorption and stimulated emission of neodymium in yttrium lithium fluoride

Abstract: A spectroscopic investigation of Nd3+ in yttrium lithium fluoride was performed. Spectrally and orientationally resolved cross sections for the 4F3/2 − 4I11/2 and 4F3/2 − 4I9/2 transitions are presented. We applied the Judd–Ofelt theory to measured absorption spectra to determine the orientation-averaged intensity parameters Ω2 = 0.362 × 10− 20 cm2, Ω4 = 4.02 × 10− 20 cm2, and Ω6 = 4.84 × 10− 20 cm2. Using these intensity parameters, we predicted the radiative lifetime of the metastable 4F3/2 state to be 525 μS, in excellent agreement with measured 4F3/2 decay signatures. Finally, absorption cross-section data are presented that will be of interest to the laser designer.

Optical spectra and kinetics of single impurity molecules in a polymer: spectral diffusion and persistent spectral hole burning

Abstract: With high-efficiency fluorescence excitation techniques optical spectra of single impurity molecules of perylene in a polyethylene matrix can be obtained at 1.5 K. Analysis of such spectra shows a variety of spectral diffusion effects, including fast (<2 s) resonance frequency changes on the 1–100-MHz scale, which lead to a range of apparent linewidths, as well as discontinuous jumps in the resonance frequency of 10–1000 MHz on a longer time scale. In addition, light-induced changes in the resonance frequency of a single molecule (persistent spectral hole burning) have been conclusively observed by showing that the burning time decreases with increased laser power. Surprisingly, hole-burned single molecules often spontaneously return to the original frequency in 1–100 s. Measurements of the burning time for a large number of hole-burning events for the same single molecule yield an exponential burn-time distribution, which is the first direct measurement to our knowledge of the stochastic kinetics of a single molecule. Analysis of the signal-to-noise function appropriate to these experiments gives the conditions under which other systems may permit single-molecule detection: strong absorption, high fluorescence yield, weak bottlenecks in the optical pumping process, and low hole-burning quantum efficiency.

Investigation of the dependence of degenerate four-wave mixing line intensities on transition dipole moment

Abstract: We report an investigation of the dependence on transition dipole moment of isolated line intensities in degenerate four-wave mixing in the A2Σ+ (υ′ = 0) ← X2Π (υ″ = 0) band for NO. For spectrally integrated intensities we found that the dependence was well described by a power law μx, where μ is the one-photon transition dipole moment. As a result of saturation, the exponent x depends on laser intensity. The observations are in reasonable agreement with a simple two-level model [ Opt. Lett.294 ( 1978);Opt. Lett.3, 205 ( 1978)], which predicts limiting dependences of μ8 and μ3 for low and high intensities, respectively. At intermediate laser intensities the model is consistent with an observed rapid decrease in the exponent with increasing laser intensity, followed by a plateau extending to high intensities.

Continuous-wave infrared laser spectrometer based on difference frequency generation in AgGaS 2 for high-resolution spectroscopy

Abstract: A high-resolution cw spectrometer based on difference frequency generation (DFG) in a 20-mm-long AgGaS2 crystal pumped by two stabilized single-frequency cw dye–Ti:sapphire lasers is described. Experiments performed with a Rhodamine 6G and DCM dye laser combination pumped by a 20-W argon laser are reported. Informed radiation is generated from 7 to 9 μm with an ultranarrow linewidth (<0.5 MHz) and an output power 1 μW by making use of 90° type I phase matching. The performance of the DFG laser spectrometer is evaluated by using a portion of the ν2 band of NH3 near 1177 cm−1.

Effect of the Raman part of the nonlinear refractive index on propagation of ultrashort optical pulses in fibers

Abstract: For ultrashort optical pulses propagating in silica optical fibers, the Raman contribution to the effective nonlinear refractive index decreases for pulse widths less than ~100 fsec. By numerical simulation, we show that this contribution not only decreases, but becomes negative, for pulse widths less than ~30 fs. This negative contribution can be understood in both time and frequency domains and comes from the finite time delay inherent in the Raman response. We expect that these features of the Raman contribution to the nonlinear refractive index will be especially important in high-nonlinearity glasses.

Focusing limits of a terawatt laser in an underdense plasma

Abstract: The focusing of an intense 1053-nm, 1-ps laser pulse in an argon gas vapor has been studied. Experimental results are reported for different gas pressures and incident laser powers. In this paper we show that defocusing can occur and that it is sensitive both to the incident laser power and to the argon gas pressure in the experimental chamber. It is suggested that such a phenomenon is due to the refractive index of the plasma created by optical-field-induced ionization of the gas in the leading edge of the laser pulse. We show that it is not possible to obtain simultaneously a high focused intensity (>1017 W/cm2) and a high electron density (>1019 cm−3).

Variation of the Verdet constant with temperature of terbium gallium garnet

Abstract: A measurement of the variation of the Verdet constant of terbium gallium garnet as a function of temperature was made for visible and near-infrared wavelengths. By combining results of the angle of rotation presented here with previously published absolute measurements, we determined the Verdet constant as a function of wavelength for wavelengths extending from about 0.5 to 1.1 μm. Data correlated well with paramagnetic models. Such information is necessary to determine the angle of rotation as well as the variation of the extinction ratio of a Faraday isolator with temperature.

Modulation and filtering control of soliton transmission

Abstract: Two methods of controlling the temporal diffusion of solitons in long-distance transmission are investigated theoretically. One, pursued experimentally by Nakazawa et al. [ Electron. Lett.27, 1270 ( 1991)] uses amplitude modulation synchronized with the bit rate. The other method, first proposed recently, uses filtering. The latter is better adapted to wavelength division multiplexing. It is shown that the so-called Gordon-Haus limit of soliton propagation can be extended significantly.