New parametric emissions in diatomic sodium molecules

By excitation of sodium vapor with an ultra-violet laser, tunable coherent emissions around 688 and 770 nm generated from diatomic sodium molecules have been observed for the first time. We demonstrate that the emission around 688 nm is due to an axially phase-matched four-wave-mixing process involving cascade emissions of 2.54 µm and 910 nm radiation from sodium dimers.     

Q-switched Yb 3+ :YVO 4 laser with Raman self-conversion

We report the efficient continuous-wave (CW) and Q-switched laser operation of a diode-pumped Yb:YVO₄ laser. A CW output power of 1 W with a slope efficiency of 59% with respect to absorbed pump power was demonstrated. Passively Q-switched with a Cr⁴⁺:YAG saturable absorber, a Yb:YVO₄ laser with Raman conversion was demonstrated. Q-switched 18.7- J pulses with a pulse duration of 17 ns and a peak power up to 1 kW were obtained at 1018-nm fundamental wavelength and 3.6- J pulses with a pulse duration of 6 ns and a peak power of about 0.6 kW were obtained at 1119.5-nm first-Stokes wavelength.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2

A simple experimental technique is presented capable of separating the contribution of purely optical Kerr effects from that of thermo-optical effects in the nonlinear response of materials under high-repetition-rate laser irradiation. The technique has been realized by combining the single-beam Z-scan method with the single-beam thermal lens measurement method. We demonstrate this technique by analysing the nonlinear response at 770 nm of CS₂ which exhibits cumulative thermal effects when irradiated by very intense femtosecond laser pulses at a 76-MHz repetition rate. Received: 3 November 1998 / Revised version: 4 January 1999 / Published online: 2 June 1999     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Frequency degenerate oscillation in semilinear photorefractive oscillator with reflection gratings

The oscillation intensity of a semilinear photorefractive oscillator with reflection gratings is calculated for different cavity losses, different pump intensity ratios and different crystal orientations. Distinct from an oscillator with transmission gratings, this oscillator always has a soft excitation in the vicinity of the threshold. The theoretical results are in reasonable agreement with that of experimental studies with BaTiO₃:Co as a photorefractive crystal.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Mechanism transition of self-pumped phase conjugation in KTa1 −xNbxO3:Fe crystals

Mechanism transitions of Self-Pumped Phase Conjugation (SPPC) with wavelength and doping concentration are observed in KTN:Fe (KTa_{1 –x} Nb _x O₃:Fe with _x = 0.48) crystals. The SPPC mechanism in KTN: Fe (0.4 wt. %) crystal transforms from Stimulated Photorefractive Backscattering and Four-Wave Mixing (SPB-FWM) to cat (or total internal reflection) as the wavelength increases from 514.5 nm to 620 nm. SPPC at 514.5 nm is formed with the cat mechanism in a 0.2 wt. % doped KTN:Fe crystal, while with the SPB-FWM mechanism in a 0.4 wt. % doped one. These mechanism transitions are discussed with respect to the dependence of the backscattering gain coefficient of the crystals on wavelength and doping concentration.     

Tunable UV radiation at short wavelengths (188–240 nm) generated by sum-frequency mixing in lithium borate

Sum-frequency mixing (₃=₁+₂) of UV laser radiation (₁=266 nm and 213 nm) and tunable coherent infrared light (₂=1.2–2.6 m) in lithium borate (LBO) generates radiation at short wavelengths (₃=188–242 nm). The UV radiation at ₁ is produced by the fourth and fifth harmonic of a pulsed Nd-YAG laser. The infrared light is generated with an optical parametric oscillator of beta barium borate. The phase-matching angle is measured as function of ₃ and compared with calculated values. For UV laser radiation at shorter wavelengths (173 nm₁213 nm) the calculations predict an extension of the tuning range of the sum-frequency generated at ₃ to wavelengths as short as the LBO transmission cutoff at 160 nm.     

Phase-matched third-harmonic generation of Nd: Glass-laser picosecond pulses in a new cyanine-dye solution

The phase-matched direct tripling of picosecond light pulses of a mode-locked Nd: glass laser in a new cyanine dye PMC is studied. The solvents trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) are applied. The S ₀–S ₁ absorption peak of the dye is around =480 nm and the absorption cross section at the third-harmonic wavelength of ₃=351.3 nm is only ₃1×10^–19 cm². Phase-matching occurred at concentrations of C_PM=0.0874 mol/dm³ in HFIP and 0.1088 mol/dm³ in TFE. A third-harmonic energy conversion efficiency of _E0.01 was achieved at a pump-laser peak intensity of I _0L2.5×10¹¹ W/cm² in a 5 mm long sample of PMC in TFE. The conversion efficiency is limited by destruction of phase-matching due to the intensity-dependent nonlinear refractive index of the dye solutions.     

Tunable wavelength conversion between picosecond pulses using cascaded second-order nonlinearity in LiNbO3 waveguides

Tunable wavelength conversion between picosecond pulses is demonstrated by exploiting cascaded secondorder nonlinearity in periodically poled LiNbO₃ waveguides when the pump pulse with 40-GHz repetition rate and 7.5-ps pulse width is adopted. No external continuous-wave input is required in the proposed wavelength converter. The converted signal wavelength can be tuned from 1519 to 1562.6 nm as the lasing wavelength is changed from 1534.5 to 1572.1 nm.This revised version was published online in August 2005 with a corrected cover date.     

Self-action effects in ionization and fragmentation of toluene by femtosecond laser pulses

Occurrence and influence of several self-action effects, induced in the entrance window of a time-of-flight interaction chamber, on the ion and fragment yields of the toluene molecule in the focus of a Ti:sapphire laser pulse are analyzed. Experimental data are obtained using different focusing geometries and are compared with results from numerical simulations using a nonlinear pulse propagation model as well as those from the strong-field S-matrix theory. It is shown that (a) self-focusing leads to a displacement of the focus while amplified small perturbations of the input beam result in hot spots in the spatial distribution and in an energy-dependent focus area, (b) small self-phase modulation changes the form of the spectrum and broadens it slightly. A strong increase of the toluene fragmentation appears to correlate to linear and nonlinear changes in the focal spot area, while the scenario of excitation of Raman modes proposed in an earlier publication is ruled out as the dominant effect.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Laser through-hole drilling and laser cutting in teflon

We report a novel technique for laser high-speed drilling and cutting in teflon films. The new laser drilling surpasses the conventional techniques in simplicity, throughput and spatial resolution. The laser cutting and drilling process consists of three simple steps. First, a thin absorbing layer (in this case 300 Å of gold) is deposited on the teflon to allow for laser absorption. Second, the drilling is performed by pulsed-laser irradiation at the rate of one hole per pulse. The irradiation process does not completely open the holes in which debris still remain. Third, the ultrasonic cleaning in water is used to remove the modified and weakly bound material inside the drilled holes, leaving behind 50 m diameter through holes in 25 m thick teflon sheets. The drilling process-window is well mapped. The cutting process is obtained by fast scanning the laser beam at laser powers above a threshold value. This new technique is desirable for packaging because of its drilling speed as high as 60 000 holes per minute, its fast cutting and its low laser equipment cost.     

Intensity clamping and re-focusing of intense femtosecond laser pulses in nitrogen molecular gas

We present results of measurements of fluorescence spectra due to the interaction of a Ti:sapphire laser pulse with N₂ molecules at different gas pressures and pulse energies. The analysis of the data together with the results of numerical simulations, using a propagation model, reveal signatures of the phenomena of intensity clamping and of re-focusing of the laser pulse at high gas pressure. The laser pulse energy for intensity clamping as a function of the gas pressure is determined. Received: 21 May 2001 / Revised version: 10 July 2001 / Published online: 19 September 2001     

A microchip-laser-pumped DFB-polymer-dye laser

A miniaturized, high repetition rate, picosecond all solid state photo-induced distributed feedback (DFB) polymer-dye laser is described by applying a passively Q-switched and frequency-doubled Cr⁴⁺:Nd³⁺:YAG-microchip laser (pulse width Δτ=540 ps, repetition rate ν=3 kHz, pump energy E_pump=0.15 μJ) as a pump source. A poly-methylmethacrylate film doped with rhodamine B dye serves as active medium. The DFB-laser pulses are temporally and spectrally characterized, and the stability of the thin polymer/dye film at high repetition rates is analyzed. The shortest DFB-laser pulses obtained have a duration of 11 ps. After the emission of 350000 pulses the intensity of the DFB-laser output has decreased by a factor of two and the pulse duration has increased by a factor of 1.2. For single DFB-laser pulses of 20-ps duration the spectral bandwidth is measured to be Δλ=0.03 nm, which is only 0.005 nm above the calculated Fourier limit assuming a Gaussian profile for the temporal shape of the pulses. Coarse wavelength tuning of the DFB laser between 590 and 619 nm is done by turning the prism. Additionally, a fine tuning of the DFB-polymer-laser wavelength is achieved by changing the temperature of the polymer/dye layer (=-0.05 nm/°C) in the range from 20 to 40 °C. Received: 1 March 2001 / Revised version: 23 May 2001 / Published online: 18 July 2001     

Stimulated IR- and vacuum-UV emission following two-photon-excitation of molecular hydrogen using an ArF laser

Hydrogen molecules are excited by two 193 nm-photons [X¹ _g ⁺ E,F¹ _g ⁺ ;Q₀₂(1)] generated with a commerical oscillator-amplifier ArF laser. Stimulated emission is subsequently observed in the IR (=752.4 nm to 836.6 nm;E, FB) and in the VUV (=134.0 nm to 160.9 nm;BX) with a VUV conversion efficiency of 0.2%. The rotational fine-structure, the saturation behaviour, pump depletion as well as SRS-onset on and off two-photon resonance are investigated.     

Dependence of spectrum on pump-signal angle in BBO-I noncollinear optical-parametric chirped-pulse amplification

A BBO-I noncollinear optical-parametric chirped-pulse amplifier pumped by a Q-switched and frequency-doubled Nd:YAG laser has been demonstrated. At an optimal pump-signal angle, the temporally stretched chirped signal pulses with bandwidth of 36 nm (FWHM) were amplified without any distortion in spectrum. The gain bandwidth was very sensitive to the pump-signal angle. Variation of this angle by about 1.5 mrad may significantly reduce the amplified bandwidth and result in a significant distortion in the amplified spectrum. Received: 5 October 2000 / Revised version: 13 June 2001 / Published online: 19 September 2001     

Analysis of ionic fragments from 308 nm photoablation of polyimide

The ionic products from excimer laser photoablation (=308 nm) of polyimide (Kapton) film have been studied as a function of fluence. Large ion masses up to about 900a.m.u. are easily observed, the mass distribution depending strongly on the fluence. Velocities of the emitted particles lie between 1400 and 10 000 ms^–1, again dependent on the fluence. A mechanism to explain the high velocities is suggested consisting of ionisation of the surface polymer molecules followed by a Coulomb explosion combined with expansion of the high density gas formed by the photoablation.     

Comparative studies of absorptance behaviour of alkaline-earth fluorides at 193 nm and 157 nm

Absorptance losses in MgF₂, CaF₂ and BaF₂ during 193-nm (DUV) and 157-nm (VUV) irradiation are investigated by employing a high-resolution laser calorimetric technique which allows the determination of both single- and two-photon absorptance at energy densities up to 110 mJ/cm². A strong wavelength dependence of the DUV and VUV absorption characteristics is observed: while effective two-photon absorption takes place at 193 nm, either no similar effect at all (in the case of BaF₂) or only a very minor effect (CaF₂) is observed at 157 nm. A first explanation for this absorption behaviour is given, implying the energetic band structure of CaF₂. In addition it is shown that, due to the strong nonlinear dependency, above a critical energy density the absorptance at 193 nm can exceed the absorptance at 157 nm. Furthermore, different single- and two-photon absorption coefficients are determined for different CaF₂ samples at 193 nm, indicating a two-step absorption mechanism. In addition, laser-induced aging is found in a MgF₂ sample at 193 nm, but not at 157 nm. Received: 21 June 2001 / Revised version: 2 November 2001 / Published online: 7 February 2002     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

Optical bistability in fluorescein dyes

Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (1 m) doped polymeric films. At concentrations larger than 10^–5 mole/l dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 10⁵ l/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^–18 cm²/molecule, while that for the dye monomer molecule is 7.6×10^–17 cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system.Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^–4 mole/l fluorescein than in 10^–6 mole/l fluorescein in which dimer formation is not that efficient.In disodium fluorescein no significant dimer formation is observed even at 10^–3 mole/l dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.     

Femtosecond Kerr ellipsometry in polydiacetylene solutions: Two-photon effects

We describe a pump-probe femtosecond Kerr experiment in polydiacetylene solutions (4BCMU, yellow form). The Kerr ellipsometry technique permits a separate determination of the real and of the imaginary part of anisotropic index changes. The wavelength (620 nm) of the pump pulses lies in the transparency region of polydiacetylene while the probe pulses have a broad white spectrum. The largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum with 4×10^–3 cm/MW is observed close to 4 eV. The two-photon excited states have a life time close to 12 ps and they present a broad absorption band in the 2 eV range.