A ray-optic, Monte Carlo, description of a Gaussian beam waist – applied to reverse saturable absorption

The intensity distribution in a Gaussian beam is represented by a bundle of rays in which each ray has a random pointing error. The ray bundle is allowed to pass through a simple lens using the usual geometrical optics formulae and it is shown that the position of the focused beam waist agrees exactly with the usual Gaussian mode analysis. The interaction of the beam waist with a reverse saturable absorber (silicon napthalocyanaine) is modelled using a Monte Carlo method and a null absorption coefficient is introduced to increase the speed of the calculation. An f/5 geometry is considered and it is shown that the results from the ray model agree well with those obtained from standard split-step beam propagation techniques. Received: 23 July 1999/Revised version: 28 September 1999/Published online: 21 January 2000     

Photorefractive two-wave mixing in the presence of an incoherent beam

We present the numerical results of the photorefractive two-wave mixing in the presence of an incoherent beam by considering the influence of the ratios of the intensities and the light-excitation cross sections of the incoherent beam to the coherent beams on the space-charge field and the effective coupling coefficient. The results show that the incoherent beam can control the intensity of the coherent beams effectively. The experimental results agree with the theory well. One application is demonstrated to obtain the positive and negative coherent replicas of an incoherent image. The coherent images have a resolution of 80.6 line pairs/mm. Received: 2 March 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999     

Photorefractive two-beam coupling equations with multiple spatial–temporal features: an SVD approach

We analyze the evolution of multi-feature two-beam coupling, wherein each beam contains several spatial–temporal features (spatial patterns modulated by different signals), using a one-dimensional plane wave model to describe the evolution of paired components. This general scenario is of interest for analyzing signal-processing applications of photorefractives, such as source-separation by orthogonalization of source-modulated spatial patterns. We use singular-value decomposition (SVD) to express each beam as a simple superposition of modes that are both temporally uncorrelated and spatially orthogonal. We find a solution that is a natural matrix generalization of the scalar solution for simple two-beam coupling, and a test for its validity: the two operators that give the spatial overlap associated with the temporal basis signals in the two beams must commute. Equivalently, this means that the same set of signals must be modulating the SVD modes in the two beams. Then the SVD modes are preserved in the two-beam coupling evolution, with only their amplitudes changing. Dedicated to Prof. Dr. Eckard Kr?tzig on the occasion of his 60th birthday. Received: 10 November 1998 / Revised version: 13 January 1999 / Published online: 7 April 1999     

Laser ablation of phenylazide in an argon matrix: direct observation and chemical reactivity of ablated fragments

Ablation of pentafluorophenylazide (FPA) in an Ar matrix at 8–10 K was carried out upon irradiation with ns-pulsed UV lasers in a vacuum. The plume of ablated products was monitored by a time-resolved imaging/spectroscopic technique using a gated and intensified CCD camera system. A large amount of pentafluorophenylnitrene (FPN) having a high kinetic energy (≈6 eV) was ejected as fragments from the matrix film during ablation. A quantitative formation of triplet FPN from the photolysis of the FPA was observed by spectroscopic measurements in the IR and UV-visible regions, and was confirmed by a theoretical IR spectrum calculated according to density functional theory. A FPN beam is useful for chemical surface modification of organic materials, such as aromatic polyester and alkylthiol. A surface analysis of these materials by X-ray photoelectron spectroscopy and Fourier transform infrared reflection absorption spectroscopy showed that the FPN was immobilized onto the surface through chemical bonds. This technique for the chemical surface modification of materials is made possible by a pulsed beam of reactive fragments with a high density in the laser ablation process. Received: 14 June 1999 / Accepted: 18 June 1999 / Published online: 21 October 1999     

Permanent holographic recording in indium oxide thin films using 193 nm excimer laser radiation

Permanent holographic recording in sputtered indium oxide (InO_x) thin films is demonstrated, using ultraviolet radiation at 193 nm emitted by an ArF excimer laser. Steady-state refractive index changes of up to 5×10^-3 are calculated from the measured diffraction efficiency of a HeNe laser probe beam. The recorded gratings exhibit a dynamic behaviour that relaxes to a steady-state value that depends on the oxygen partial pressure used during growth and on the recording beam intensity. The observed behaviour is explained in terms of laser-induced structural changes. Received: 12 October 1998 / Accepted: 8 March 1999 / Published online: 4 August 1999     

High optical gain using counterpropagating beams in iron and terbium-doped photorefractive lithium niobate

Exceptionally high values of photorefractive gain coefficient of up to 100 cm^-1 have been observed in Fe-doped and Fe/Tb-doped crystals of photorefractive lithium niobate. It is believed that these are the highest observed coefficients of any crystalline photorefractive medium. Accurate measurements of gain coefficient have been possible for the first time by using a specially cut triangular crystal which allows the use of short interaction lengths and the complete elimination of surface reflections by having the beams incident at Brewster’s angle. Experimental results are consistent with a simple model of photorefractive beam coupling up until the onset of noise which is observed to deplete the pump for interaction lengths longer than 1 mm. Received: 28 September 1998 / Revised version: 8 January 1999 / Published online: 31 March 1999     

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave

Adiabatic focusing of cold atoms in a blue-detuned laser standing wave is analyzed. It is shown that using repulsive light forces that push atoms towards dark regions and thus minimizes heating, cold atoms can be adiabatically compressed by more than an order of magnitude to yield background-free sub-10-nm (rms) spots. The optimal parameters for the atomic lens and the maximal compression ratio are predicted using an analytic model and found to be in agreement with the exact results of our Monte Carlo simulations. A combined adiabatic-coherent scheme is proposed and shown to yield 8.8 nm spot size even for a thermal atomic beam. Received: 1 October 1999 / Revised version: 13 December 1999 / Published online: 5 April 2000     

Atom lithography with a cold, metastable neon beam

We study different aspects of atom lithography with metastable neon atoms. Proximity printing of stencil masks is used to test suitable resists that are sensitive to the internal energy of the atoms, including dodecanethiols on gold and octadecyltrichlorosilanes grown on a SiO₂ surface. As an example of patterning the atomic beam with laser light, we create parallel line structures on the surface with a periodicity of half the laser wavelength by locally de-exciting the atoms in a standing quenching wave. Received: 29 June 1999 / Revised version: 30 August 1999 / Published online: 10 November 1999     

Bright atomic beam by a temporal Zeeman acceleration

A novel method to produce a slow, monochromatic, and bright pulsed atomic beam from a magneto-optical trap by switching the magnetic field of the trap is proposed. A pulsed lithium atomic beam with a brightness of 1.1×10¹⁵ /sr s and a velocity of 13 m/s was produced as an experimental proof of this technique. The conversion efficiency from the trap into the atomic pulse was nearly 100%. Received: 22 October 1999 / Revised version: 15 November 1999 / Published online: 8 March 2000     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Self-pumped phase conjugation in Bi12TiO20 crystal without external cavity

We present an experimental study of the self-pumped phase-conjugate-mirror (SPPCM) recording in a Bi₁₂TiO₂₀ sample due to the partial internal reflection of the pump beam from the rear face. SPPCM has been recorded under an external alternating electric field of a square-wave form with response time equal to 170 s at the pump beam intensity of 520 mW/cm². The mechanism of the SPPCM formation is similar to that of photorefractive double-phase-conjugate mirrors. The most effective SPPCM is recorded at the much larger pump-beam-incident angle than it could be supposed from the typical angular dependence of the two-wave-mixing gain factor. Strong dependence of the gain distribution on the pump-beam-propagation angle is observed. It is found that the response time strongly depends on the preliminary history of the sample. Received: 11 August 1998 / Revised version: 14 January 1999 / Published online: 28 April 1999     

Neutron matter with a model interaction

An infinite system of neutrons interacting by a model pair potential is considered. We investigate a case when this potential is sufficiently strong attractive, so that its scattering length a tends to infinity, a . It appeared, that if the structure of the potential is simple enough, including no finite parameters, reliable evidences can be presented that such a system is completely unstable at any finite density. The incompressibility as a function of the density is negative, reaching zero value when the density tends to zero. If the potential contains a sufficiently strong repulsive core then the system possesses an equilibrium density. The main features of a theory describing such systems are considered. Received: 17 December 1999 / Accepted: 23 December 1999     

Erasure effect of the reading beam on the decay process of χ(2) in all-optical poling

We have studied the effect of the reading beam on the decay process of the photoinduced second-order susceptibility χ⁽²⁾ in both the guest-host system (disperse red 1-doped polymethyl methactrylate) and the side-chain system (disperse red 19-functionalized polyimide polymer). Both experimentally and theoretically, we have proved that the reading beam has the erasure effect on the decay process, that is, χ⁽²⁾ decays faster in the high intensity than in the low intensity of the reading beam. Furthermore, the relationship between the time constant τ_B of the slow component of the decay process and the intensity of the reading beam I_1.06 is derived from the theoretical equations, which is fitted well with the experimental results. Based on the fitting parameters, we also compare, quantitatively, the thermal-induced orientational diffusion of trans- and the quantum efficiency of trans-to-cis photoisomerization in the two systems. Received: 8 September 1999 / Revised version: 14 March 2000 / Published online: 16 August 2000     

Photochemical effects in the UV laser ablation of polymers: Implications for laser restoration of painted artworks

0 ∝t^1/2. The best results are expected for a circular top-hat beam shape. Received: 15 January 1999 / Accepted: 18 January 1999     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

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     

Visible upconversion lasers in praseodymium-ytterbium-doped fibers

We report on a ZBLAN-fiber-based praseodymium-ytterbium-doped upconversion fiber laser operating in the blue-green with diffraction-limited beam quality. cw output powers of more than 150 mW at 491 nm are achieved for several hours without degradation. The spectroscopic data of the active material and laser parameters including the amplitude noise are discussed. Received: 28 June 1999 / Revised version: 20 August 1999 / Published online: 20 October 1999     

Internal reflection mode scanning near-field optical microscopy with the tetrahedral tip on metallic samples

An internal reflection mode is introduced for scanning near-field optical microscopy (SNOM) with the tetrahedral tip. A beam of light is coupled into the tip and the light specularly reflected out of the tip is detected as a photosignal for SNOM. An auxiliary STM mode is used to control the distance during the scanning process and to record the topography of the sample simultaneously with the SNOM image. Images were obtained of different metallic samples which show a contrast in the order of 10% of the total reflected photosignal. In images of metallic samples an inverted contrast is consistently obtained compared to images previously obtained of comparable samples in a transmission mode. The contrast shows a pronounced dependence on the polarization of the incident beam with respect to the orientation of the edges of the tip. In the case of gold surfaces, the photosignal as a function of distance between the tip and the surface shows a pronounced peak in the near-field range of 0–20 nm which is tentatively attributed to the excitation of surface plasmons on the gold surface. The pronounced near-field effects and the strong contrast in the near-field images and the resolution well below 50 nm are an indication of a highly efficient coupling of the incident beam to a local excitation of the tip apex which is essential for the function of the tip as a probe for SNOM. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 21 October 1999     

Efficient magnetic guiding and deflection of atomic beams with moderate velocities

We have studied guidance and deflection of a beam of cesium atoms by a strong toroidal magnetic quadrupole field. The beam guide is made from permanent magnets sustaining a radial field gradient of 2.8 T/cm. Atoms with moderate longitudinal velocities ranging from 30 m/s to 70 m/s were inserted across the 10-mm-diameter aperture of a 24.5° arc with radius 300 mm. We have measured transmission and beam divergence and find good agreement with ray-tracing calculations and analytical estimates. The magnetic beam guide allows for 100% transmission of heavy atoms over large angles. Received: 9 November 1998 / Revised version: 6 April 1999 / Published online: 30 July 1999     

Atomic beam guide by a one-dimensional magneto-optical trap

An atomic beam has been collimated, compressed, and deflected simultaneously by an atomic beam guide based on an inclined one-dimensional magneto-optical trap (1D-MOT). Isotope-selected rubidium atoms were extracted from the naturally-mixed thermal atomic beam with this method. We could manipulate the transverse displacement of the deflected beam precisely by adjusting the current in the copper rods to generate the quadrupole magnetic field. We could extract more than 50% of the incident atoms as a deflection beam when we combined this deflection technique with the atomic deceleration using a broadband spectral light. Received: 10 December 1998 / Published online: 24 June 1999