Pattern selection and stabilization in an annular CO laser

The formation and stabilization of spatio-temporal patterns in an annular CO₂ laser is studied. We give experimental and numerical evidence of the role of a small spatial perturbation (consisting of a thin metallic wire inserted in the optical cavity) in the selection and stabilization of patterns. Received 15 December 1999     

Experimental study of a broad-band XeCl double-pass amplifier with SBS mirror

An oscillator-amplifier XeCl laser system has been used to experimentally investigate the effectiveness of a liquid stimulated Brillouin scattering (SBS) mirror for correcting the spatial aberrations of broad-band laser radiation in a double-pass amplifier. It has been found that the SBS amplifier performance is strongly dependent on the intensity I _p exciting the Brillouin medium. A good beam reconstruction has been attained at I _p1GW/cm², whereas highly aberrated output beams have been delivered by the SBS amplifier for I _p>1 GW/cm². By comparing the broad-band SBS amplifier performance to that of the same amplifier with a dielectrically coated flat mirror at one end, it has been found that the use of a SBS mirror is advantageous to obtain lower divergence output beams only for low energy pump beams (<1 mJ).     

Statistical properties for a class of nonlinear squeezed states

Theoretical investigations of dynamical behavior in optical parametric oscillators (OPO) have generally assumed that the cavity detunings of the interacting fields are controllable parameters. However, OPOs are known to experience mode hops, where the system jumps to the mode of lowest cavity detuning. We note that this phenomenon significantly limits the range of accessible detunings and thus may prevent instabilities predicted to occur above a minimum detuning from being evidenced experimentally. As a simple example among a number of instability mechanisms possibly affected by this limitation, we discuss the Hopf bifurcation leading to periodic behavior in the monomode mean-field model of a triply resonant OPO and show that it probably can be observed only in very specific setups.     

Electron impact single and double ionization of argon

Stable two-mode, and three-mode oscillations due to the spatial hole burning effect were observed experimentally with the increase of the pump power ratio in a laser-diode pumped sub-nanosecond microchip Cr,Yb:YAG self-Q-switched multimode laser. The stability of the output pulse trains was attributed to the mode coupling through antiphase dynamics between different modes. Modified multimode rate equations including the spatial hole-burning effect in the active medium and the non-linear absorption of the saturable absorber were proposed. Numerical simulations of the antiphase dynamics of such a laser were in good agreement with the experimental data, and the antiphase dynamics were explained by the evolution of the inversion population and the bleaching and recovery of the inversion population of the saturable absorber.     

Effects of nonlinear mode coupling on the chaotic dynamics of multimode lasers

The dynamics of individual modes of a cw multimode dye laser in a 2-mirror configuration has been studied theoretically and experimentally. Numerical solutions of the laser rate equations including nonlinear mode coupling exhibit chaotic and stochastic behavior in the regime of high and low power, respectively. Chaotic behavior due to mode coupling has been observed in the operating regime well above threshold. The mode dynamics in this case is characterized by a chaotic attractor with low dimension between 2 and 3. With decreasing laser power the dimension increases, suggesting stochastic behavior due to quantum noise in the limit of the laser threshold.On leave at 1. Institut für Experimentalphysik, Universität Hamburg, Fed. Rep. Germany     

High-order harmonic generation in nitrogen molecules with subpicosecond visible dye-laser pulses

An experimental study of high-order harmonic generation in nitrogen molecules (N₂) has been made using intense visible (616 nm) dye-laser pulses, where the harmonic radiation up to the 21st order is observed. The harmonic distribution represents a plateau that is preceded by an intensity minimum at the 7th order. The harmonic generation characteristics were atomic-like. It has been found that there are some similarities in the high-order harmonic generation characteristics for N₂ and Ar, including the highest-order harmonics, harmonic distributions, and the influence of the multiphoton ionization on the high-order harmonic generation. These similarities are reasonably attributed to the energetic correspondence of excited levels and ionization potentials. It is pointed out that the ac Stark shift of excited levels and ionization potentials plays an important role also in the high-order harmonic generation in N₂.     

Beam-size effect in time-delayed laser-induced double gratings

We have studied theoretically the effect of the probe-beam size on time-delayed laser-induced double gratings by assuming that the probe beam is described as a Gaussian beam. LettingE _s1 andE _s2 be the Four-Wave Mixing (FWM) signals originating from the diffraction of the probe beam by the gratings, the condition for the occurrence of the FWM signal modulation is that the divergence angle ofE _s1 andE _s2 is larger than half of the intersection angle between the propagation directions ofE _s1 andE _s2. We have also proposed methods to increase the modulation contrast.     

Thermal undulations of quasi-spherical vesicles stabilized by gravity

The classical treatment of quasi-spherical vesicle undulations has, in the present work, been reviewed and extended to systems, which are affected by a gravitational field caused by a density difference across the membrane. The effects have been studied by the use of perturbation theory leading to corrections to the mean shape and the fluctuation correlation matrix. These corrections have been included in an analytical expression for the flicker spectrum to probe how the experimentally accessible spectrum changes with gravity. The results are represented in terms of the gravitational parameter, g ₀ = ΔρgR ⁴/κ. The contributions from gravity are in most experimental situations small and thus negligible, but for values of g₀ above a certain limit, the perturbational corrections must be included. Expressions for the relative error on the flicker spectrum have been worked out, so that it is possible to define the regime where gravity is negligible. An upper limit of g₀ has also been identified, where the error in all modes of the flicker spectrum is significant due to distortion of the mean shape. Received 9 July 2002 and Received in final form 15 November 2002 RID="a" ID="a"e-mail: jonas@kemi.dtu.dk RID="b" ID="b"e-mail: ipsen@memphys.sdu.dk     

Cusp satellite bands in the spectrum of molecule

We report measurements and a theoretical explanation of the cusp-shaped satellite bands in the blue wing of the cesium D₂ resonance line which have been observed for the first time. The bands are identified as transitions where the upper state dissociates into the 6 ²P _3/2 + 6 ² S _1/2 atomic asymptote. The experiment has been performed using a standard absorption setup, computer controlled data acquisition and computer data processing. We have shown that the peculiar shape of the difference-potential curve is solely responsible for the spectrum containing the cusp-shaped satellite bands. The appearance of these satellite bands has been discussed and explained relating the theory of satellite bands to the catastrophe theory. The shape of the line wing and of the satellite bands have been calculated using the Fourier transform technique. To ensure a more stringent comparison between the experimental and the theoretical spectrum, we have analyzed and compared the derivatives of the measured and the calculated satellite band shape. On the contrary to the customary direct comparison between the measured and the calculated absorption coefficient, the derivative clearly shows all differences and resemblances between satellite band profiles. The degree of coincidence of the experimentally observed and the theoretically calculated satellite band shape can be used as an ultimate check on the assessment of the quality of potential-energy curves involved in the formation of satellite bands. Received: 1 October 1997 / Revised: 14 January 1998 / Accepted: 24 February 1998     

Pump and probe nonlinear processes: new modified sum rules from a simple oscillator model

The nonlinear oscillator model is useful to basically understand the most important properties of nonlinear optical processes. It has been shown to give the correct asymptotic behaviour and to provide the general features of harmonic generation to all orders, in particular dispersion relations and sum rules. We investigate the properties of pump and probe processes using this model, and study those cases where general theorems based on the holomorphic character of the Kubo response functions cannot be applied. We show that it is possible to derive new sum rules and new Kramers-Kr?nig relations for the two lowest moments of the real and of the imaginary part of the third order susceptibility and that new specific contributions become relevant as the intensity of the probe increases. Since the analytic properties of the susceptibility functions depend only upon the time causality of the system we are confident that these results are not model dependent and therefore have a general validity, provided one substitutes for the equilibrium values of the potential derivatives the density matrix expectation values of the corresponding operators. Received 25 January 1999 and Received in final form 26 April 1999     

Bistability and passive Q-switching of a CO2 laser with saturable absorber

The coexistence of bistability and repetitive passive Q-switching of a CO₂ laser with a gaseous saturable absorber has been investigated experimentally. With CH₃F as the saturable absorber bistability has been observed simultaneously with passive Q-switching, whereas with CH₃OH either one of these phenomena can only exist, as demonstrated for the first time. The measured features of both systems can be explained qualitatively by rate-equation theory.     

Cooperative frequency locking and tristability in a class-B laser

The locking and unlocking phenomena of the modes of the transverse familyq=1 in a CO₂ laser are investigated. The experimental results show three characteristic regions: a bistability between two helical waves of opposite handedness, a tristability among the two helicities of opposite handedness and an unlocked state, and the unlocked state. Near the locking threshold, oscillations which can be interpreted as due to the oscillations of the modal amplitudes and relative phase are also observed. These results are found to be consistent with solutions of the two-mode Maxwell-Bloch equations for class-B lasers.     

Light scattering with swollen hexagonal phases

We investigate, using quasi-elastic light scattering, some features of the long-wavelength, low-frequency modes of the hexagonal phase often encountered in the study of lyotropic (surfactant-solvent) systems. The hexagonal phase is swollen by an oil-based ferrofluid, allowing magnetically aligned samples to be prepared. We show experimentally the anisotropy of the two lowest-frequency modes. We develop a model which predicts that these slow modes are associated to particle diffusion and tube motion. With the help of microscopic as well as phenomenological analyses, we suggest that the latter presumably corresponds to a peristaltic mode. Confinement effects on the one-dimensional, Brownian diffusion of the colloids along the tube axis together with the coupling between the two modes are studied experimentally, varying the tube diameter to particle size ratio. Received 7 July 1999     

Modulation instability with incoherent white light in self-defocusing photorefractive crystal

We report on the first experimental observation of modulation instability and spontaneous pattern formation with incoherent white light emitted from an incandescent lamp in self-defocusing photorefractive LiNbO₃:Fe crystal. Experimental results show that the modulation instability of white light in self-defocusing medium is related to the input intensity, illumination time and the direction of the crystalline c-axis with respect to that of the lamp filament. At the illumination time t = 0 and the steady-state of MI, we give the spatial distribution of the optical Fourier power spectrum experimentally and the corresponding Fourier transformation of the output intensity numerically, and observe the emergence of the high frequency component along the c-axis.     

Quantum correlations close to a square pattern forming instability

We analyze the quantum fluctuations of the degenerate optical parametric oscillator close to an instability for the formation of a square pattern. While strong correlations between the fluctuations of the signal modes emitted at the critical wave number and with opposite wave vector are present both below and above threshold, no features signaling the square character of the pattern forming above threshold have been identified below threshold in the spatio-temporal second-order coherence. We also explore in which regimes a reduced few mode model gives meaningful results. Received 17 September 2002 / Received in final form 11 November 2002 Published online 26 February 2003     

Four-wave mixing of picosecond pulses in hollow fibers: expanding the possibilities of gas-phase analysis

Third-harmonic, difference-frequency, and sum-frequency generation processes in hollow fibers are experimentally studied with 30-ps pulses having an energy of several millijoules. The experimental dependence of the difference-frequency signal on the pressure of the gas filling the fiber agrees well with the results of calculations when the contribution of higher order waveguide modes is taken into consideration, thus indicating the importance of nonlinear-optical processes involving higher order waveguide modes of a hollow fiber. Hollow fibers are also shown to expand the possibilities of nonlinear-optical analysis of gases by allowing the generation of third-harmonic and sum-frequency signals, which vanish in the regime of tight focusing in a medium with normal dispersion. Received: 26 September 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

High-energy, fourth-harmonic generation using CO2 lasers

2 laser to 2.38 μm has been studied experimentally by using tandem AgGaSe₂ and ZnGeP₂ crystals. Up to 24 mJ per pulse of 2.38-μm radiation at a 10% overall conversion efficiency has been achieved with a mode-locked pulse format. A computer model was also developed and its predictions are in reasonable agreement with our experimental data. Received: 6 July 1997/Revised version: 8 December 1997     

The effect of an autoionizing state in the double-resonant four-wave sum mixing in Hg

Four-wave sum mixing (4-WSM) generation in Hg vapors, using the two-photon resonance 6¹ S ₀–8¹ S ₀ transition and near the 6p³ D ₁ autoionizing state is experimentally investigated. The wavelength dependence of the Hg atomic contribution to the wavevector mismatch and results related to the characteristics of the generated spectrum are presented. The effect on the spectrum profile by Hg number density, and the detuning from the two-photon resonance is studied along with the effect of the He buffer gas on the phase matching. Optimum conditions of operation are suggested.     

Well-confined Yb:GdVO4 laser waveguide formed by MeV C ion implantation

The planar waveguide in x-cut Yb:GdVO₄ crystal has been fabricated by 6.0 MeV carbon ion implantation with the fluence of 1 × 10¹⁴ ions/cm² at room temperature. The modes of the waveguide were measured by the prism-coupling method with the wavelength of 633 nm and 1539 nm, respectively. An enhanced ordinary refractive index region was formed with a width of about 4.0 μm beneath the sample surface to act as a waveguide structure. By performing a modal analysis on the observed transverse magnetic polarized modes, it was found that all the transverse magnetic polarized modes can be well-confined inside the waveguide. Strong Yb-related photoluminescence in Yb:GdVO₄ waveguide has been observed at room temperature, which reveals that it exhibits possible applications for integrated active photonic devices.