Z-Scan studies of KYW, KYbW, KGW, and Ba(NO3)2 crystals

We present the studies of nonlinear refraction and nonlinear absorption in promising crystals which are extensively used in Raman lasers or as solid-state laser host materials: Ba(NO₃)₂, KGW, KYW, and KYbW. The single-beam z-scan technique with 1 ps laser pulses at 790 and 395 nm has been applied for the study. Nonlinear refraction-index intensity-coefficients and two-photon absorption coefficients have been determined for the crystals. The considerable enhancement of nonlinear refraction is observed in the crystals at 395 nm.     

Tailoring the nonlinear refractive index of fluoride-phosphate glasses for laser applications

In this paper we report on measurements of the nonlinear refractive index n₂ at 1047 nm of various fluoride-phosphate laser glasses relative to fused silica using degenerate four-wave mixing. We find good agreement with empirical estimates obtained from the d-line linear refractive index and the Abbe number for pure fluoride-phosphate glasses. Significant increase of n₂ is observed when adding sulfate and niobium oxide to the glass composition offering tailorable nonlinear properties for glasses employed in short pulse laser oscillators. Received: 27 December 1999 / Published online: 24 March 2000     

Nonlinear change in refractive index of Co2+:ZnSe at short-pulse single-beam 1.54-μm Z-scan probing

An experimental study and theoretical modeling of the nonlinear change in refractive index of a Co^{2 +}: ZnSe crystal at the short-pulse single-beam probing at the wavelength 1.54 μm is reported. In the experimental conditions of negligible contributions in the index non-linearity stemming from the Kerr-effect and inhomogeneous heating, the nonlinear change in refractive index in Co^{2 +}:ZnSe is shown to be caused by the resonant Co^{2 +} population-perturbation effect (i.e., by the Co^{2 +} ground-state absorption saturation). The Z-scan single-beam technique and novel theoretical approach addressing the resonant nonlinear refraction in a saturable doped medium are used, respectively, for an experimental and theoretical inspection of the phenomenon. For a set of Co^{2 +}:ZnSe samples with different concentrations of Co^{2 +} ions at the short-pulse (200 ns) mJ-range probing, we show that the maximal nonlinear change in refractive index is about of units of 10^{− 4} at the chosen wavelength.     

Nonlinear refractive index measurements in antimony–sulfide glass films using a single beam nonlinear image technique

The nonlinear refractive index, n₂, of films based on the new glass system Sb₂O₃–Sb₂S₃ was measured at 1064 nm with laser pulses of 15 ps, using a single-beam nonlinear image technique in presence of a phase object. The films were prepared from bulk glasses by RF-sputtering. A large value of n₂ = 3 × 10⁻¹⁵ m²/W, which is three orders of magnitude larger than for CS₂, was determined. The result shows the strong potential of antimony–sulfide glass films for integrated nonlinear optics.     

Determinations of the transient thermal lensing effect in metal cluster Polymer {WS4Cu4I2(bpe)3}n solution by the use of the Z-scan

The nonlinear optical (NLO) properties of a novel cluster Polymer {WS₄Cu₄I₂(bpe)₃}_n solution are studied by using Z-scan technique with laser pulses of 4.5 ns pulse-width at a wavelength of 532 nm. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermal effect. The thermal effect is mainly due to the strong nonlinear absorption. Numerical simulations obtained by solving simultaneously photo-acoustic and electromagnetic wave equations, agrees basically with experimental results.     

Linear and nonlinear optical properties of dye-doped KDP crystals: Effect of thermal treatment

Potassium dihydrogen phosphate crystals (KDP, KH₂PO₄) doped with the organic xylenol orange (XO) dye are grown, the XO concentration in the crystal matrix is about 10 ppm. The spectral and luminescent properties of nominally pure, dye-doped and dye-doped/annealed at 150 °C crystals (KDP, KDP:XO and KDP:XO_an) were measured. The annealing temperature effect on the degree of dye protonation in the crystal matrix is established. Analysis of the IR-absorption spectra reveals a strong interaction between the incorporated dye molecules and the hydrogen subsystem of the matrix. The nonlinear optical (NLO) properties of KDP, KDP:XO and KDP:XO_an crystals are studied within the self-action effect of picosecond laser pulses at 532 nm. The mechanism of photoinduced bleaching and the effects of laser beam self-focusing (in KDP) and self-defocusing (in KDP:XO and KDP:XO_an) are supposed to be due to resonance excitation of the subsystems of intrinsic defects and dye molecules, correspondingly. For KDP:XO_an it is shown that thermal annealing of intrinsic crystal defects leads to domination of more effective NLO response of the subsystem of dye molecules that is correlated with photoluminescence data.     

Nonlinear refraction and absorption in hetero- thiometallic planar clusters

The nonlinear absorption and refraction of the clusters [MoS₄Cu₄Br₂(py)₆] and [Et₄N]₂[MoS₄Cu₄(SCN)₄(2-pic)₄] have been investigated using the z-scan technique with a ns laser at 532 nm wavelength. They have the same planar ‘open’ structures and the same skeleton metal atoms; the only difference is that the former has halogen ligands while the latter possesses pseudo-halogen groups – SCN – as ligands. Alteration of nonlinear refractive index and enhancement of nonlinear absorption were found in these two clusters. A steady state model of excited state nonlinear refraction was proposed to explain this phenomenon. Received: 12 June 2001 / Revised version: 4 September 2001 / Published online: 29 November 2001     

Pulsed blue-light generation by the frequency doubling of the 4F3/2 to 4I9/2 transition in Nd:YAG and Nd:YAlO3

We report on the frequency doubling of Q-switchedNd:YAG and Nd:YAlO₃ lasers emitting at 946 and 930 nm, respectively (⁴F_3/2 to ⁴I_9/2 transition). The neodymium-doped laser host crystals were excited with a flashlamp-pumped Cr:LiSAF laser operating in a free-running mode. Blue-light pulses were obtained at both 473 nm (9 mJ, 25 ns FWHM) and 465 nm (4.4 mJ, 35 ns FWHM) by using a potassium niobate crystal as an extra-cavity frequency doubler. The second-harmonic generation conversion efficiencies reached 53% and 31%, respectively. Received: 23 June 1999 / Revised version: 8 August 1999 / Published online: 3 November 1999     

Multilayer Au/TiO2Composite Films with Ultrafast Third-Order Nonlinear Optical Properties

We report on the ultrafast third-order optical nonlinearity in multilayer Au/TiO2 composite films fabricated on quartz substrates by pulsed laser deposition technique. The linear optical properties of the films are determined and optical absorption peaks due to surface plasmon resonance of Au particles are observed at about 590hm. The third-order optical nonlinearities of the films are investigated by z-scan method using a femtosecond laser （50 fs） at the wavelength of 800 nm. The sample showed fast nonlinear optical responses with nonlinear absorption coefficient and nonlinear refractive index being -3.66 × 10^-10 m/W and -2.95 × 10^-17 m^2/W, respectively. The results also show that the nonlinear optical effects increase with the increasing Au concentration in the composite films.     

Third-Order Nonlinear Optical Properties of a Series of Polythiophenes

The third-order nonlinear optical properties of a series of polythiophenes are investigated with the Z-scan method under picosecond pulse laser irradiation at 532nm. The copolymers exhibit a good nonlinear response： large nonlinear refraction coefficients without nonlinear absorption. The signs of nonlinear refraction coefficients are positive, which are opposite to the negative signs of the polythiophene reported before. The mechanism accounting for the process of nonlinear refraction under pulse laser excitation is analyzed from the viewpoint of the electrondonor/acceptor units of polythiophenes. Moreover, changes of nonlinearity according to the lengths of main chains in polythiophene molecules are discussed.     

Femtosecond nonresonant degenerate four-wave mixing at atmospheric pressure and in a free jet

2 , N₂, and CO₂. For CO₂ additional experiments have been performed at reduced pressure and in a molecular beam. By delaying the probe pulse a periodic recovery of the DFWM signal is observed. The period of these transients can be assigned unambiguously to rotational Raman transitions of the ground state within the laser bandwidth. The decay of the transients yields the collisional dephasing of the Raman-induced polarization. At zero delay also optical-field-induced birefringence of electronic nature contributes to the signal. The different time scales of the Raman and electronic effects allow us to estimate their relative strength. Received: 3 August 1998 / Revised version: 21 October 1998 / Published online: 24 February 1999     

Nonlinear refraction of undoped and Fe-doped KTiOAsO4 crystals in the femtosecond regime

The third-order optical nonlinearities in undoped and Fe-doped KTA crystals have been measured using the Z-scan technique with femtosecond pulses at 780-nm wavelength. The nonlinear refractive index is determined to be 1.7×10⁻¹⁵ cm²/W and 0,9×10⁻¹⁵ cm²/W for undoped and Fe-doped KTA, respectively. No two-photon absorption occurs in these crystals. It is shown that doping with Fe₂O₃ could weaken refractive nonlinearity of KTA, suggesting that Fe:KTA will improve the performance of KTA in high-intensity femtosecond laser applications. In addition, the measured nonlinear index of refraction in KTA crystals is about five times lower than that predicted by the two-band theory. One of the reasons for the discrepancy is given as the applicable limit of the simple theory in which a two-parabolic band model has been assumed in the analysis. Received: 3 June 1999 / Published online: 20 October 1999     

Pulse width measurement of high-order harmonics by autocorrelation

Pulse widths of the 7th and 9th harmonics of a Ti:sapphire (TiS) laser have been measured by an autocorrelation technique using two-photon and three-photon ionization of He and Ar as nonlinear phenomena. The pulse widths of the 7th and 9th harmonics were 47 fs and 27 fs, respectively, where the pulse width of the fundamental pulse was 34 fs. The pulse width of the 9th harmonic depends strongly on the peak intensity of the fundamental pulse. These results can be explained by quantum theory by taking into account the temporal and spatial distribution of the fundamental beam. Received: 15 June 1999 / Revised version: 8 October 1999 / Published online: 2 February 2000     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

Narrow bandwidth operation of high-power broad-area diode laser using cascaded phase-conjugate injection locking

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO₃ are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection. Received: 18 November 1998 / Revised version: 29 January 1999 / Published online: 7 April 1999     

Highly efficient Raman conversion in O2 pumped by a seeded narrow band second-harmonic Nd:YAG laser

The first-Stokes conversion efficiency for a stimulated Raman scattering (SRS) is usually very low in gaseous oxygen media. In 3.0 Mpa O₂, a single longitudinal mode second harmonic Nd:YAG laser pump source gives a typical vibrational first-Stokes conversion efficiency of only 2.5%, In comparison, the accompanying stimulated Brillouin scattering (SBS) attains a reflectivity of 67%. However, by seeding an OPO beam into the Raman cavity, the first-Stokes photon conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5% in a 6.1 Mpa 41:59 O₂/ He mixture. This 54% efficiency was obtained for a seeder laser pulse-width less than one half that of pump laser (6.8 ns). A first-Stokes peak power conversion efficiency as high as 88% has been obtained when the pump and seeder pulse peaks coincide. So, we may expect a higher first-Stokes photon conversion efficiency if the seeder pulse-width can be made equal to or larger than that of the pump pulse. On the other hand, the beam quality of the first-Stokes in an O₂/ He mixture excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ and a pump laser repetition rate of 10 Hz, the thermal defocusing effect worsens the first-Stokes beam quality. This thermal defocusing effect is a result of the Raman heat release and could be eliminated by fast circulating and cooling the Raman gas medium.     

Nonlinear phase shift without cascaded second-order processes and third-order nonlinearity

A new mechanism for obtaining a nonlinear phase shift has been proposed and schemes are described for its implementation. As it is shown, the interference of two waves with intensity-dependent amplitude ratio coming from the second-harmonic generation should produce the nonlinear phase shift. The sign and amount of nonlinear distortion of a beam wavefront is dependent on the relative phase of the waves that is introduced by the phase element. The calculated value of n₂ ^eff exceeds that connected with cascaded quadratic nonlinearity, at the same conditions. Received: 1 November 1999 / Published online: 23 February 2000     

4-W and 23-ps pulses from a lamp-pumped Nd:YAG laser passively mode-locked by polarization switching in a KTP crystal

Passive mode-locking of a cw lamp-pumped Nd:YAG laser using nonlinear polarization switching in a type-II SHG crystal is reported. Light pulses with more than 5 W of average power and pulse duration shorter than 25 ps have been obtained at 1064 nm. Received: 29 January 1999 / Revised version: 24 March 1999 / Published online: 1 July 1999     

High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3

We have demonstrated an efficient high energy 2 μm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO₃ (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 μm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 μm.     

Novel glass-forming ferroelectric liquid-crystal material of high second-order nonlinearity

We report on a novel ferroelectric liquid-crystal material with an exceptionally large and stable nonlinear optical response. The material shows a phase transition from theSmC* mesophase to the glassy state near room temperature. This allowed us to state-freeze a stable nonlinear optically active configuration close to room temperature at 10° C for one day. The stored orientations can easily be forced to relax back by heating slightly above the glass-transition temperature. The material and the freezing process have been characterized in detail by analyzing the frequency-doubling process of the fundamental radiation of a Nd: YAG laser in the liquid crystal. Under favourable conditions, we obtained second-order susceptibilities as large as _zzz = 0.5 pm/V.