Ferroelectricity in antiferroelectric NaNbO3 crystal.

Sodium niobate (NaNbO3, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.

Femtosecond Yb:CaGdAlO4 thin-disk oscillator.

A mode-locked thin-disk laser based on Yb:CALGO is demonstrated for the first time. At an average output power of 28 W we obtained pulses with a duration of 300 fs and a pulse energy of 1.3 µJ. 197 fs pulses with 0.9 µJ of energy were achieved at an average output power of 20 W. The shortest pulse duration measured in our experiments was 135 fs with a spectrum centered at 1043 nm. The experiments also revealed a very broad tunability from 1032 to 1046 nm with sub-200 fs pulses.

Yb:CaGdAlO4 thin-disk laser.

We present the first demonstration of a Yb:CALGO thin-disk laser. In a slightly multimode configuration, we obtained up to 30 W of average power at a slope efficiency of 40% and an optical-to-optical efficiency of 32%. With a single-mode cavity, an average power of 25 W was achieved. A tuning range from 1018 to 1052 nm could be demonstrated by inserting a prism into the cavity. In the Q-switched regime, we obtained 1 mJ of pulse energy at a repetition rate of 100 Hz.

Instability of single-frequency operation in semilinear photorefractive coherent oscillators.

The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of the coupling strength at which the bifurcation occurs is a function of pump intensity ratio and cavity losses. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.

Periodically poled potassium niobate for second-harmonic generation at 463 nm.

We report on the fabrication and characterization of quasi-phase-matched potassium niobate crystals for second-harmonic generation. Periodic 30-mum -pitch antiparallel ferroelectric domains are fabricated by means of poling in an electrical field. Both birefrigence and periodic phase shift of the generated second harmonic contribute to phase matching when the d(31) nonlinear optical tensor element is used. 3.8 mW of second-harmonic radiation at 463 nm is generated by frequency doubling of the output of master-oscillator power-amplifier diode laser in a 5-mm-long crystal. The measured effective nonlinear coefficient is 3.7pm/V. The measured spectral acceptance bandwidth of 0.25 nm corresponds to the theoretical value.

Two-wave mixing in photorefractive BaTiO(3):Rh at 1.06 mum in the nanosecond regime.

We present two-beam coupling experiments in the nanosecond regime at 1.06 mum , using photorefractive BaTiO(3):Rh. The maximum observed exponential gain coefficient is 14.2 cm(-1) . No intensity-dependent electron-hole competition and no strong saturation of the photoionized charge carriers are observed for intensities of less than 20MW cm(-2) . The energy required for recording the photorefractive grating is not significantly different in the nanosecond and the cw regimes.

All-solid-state continuous-wave tunable blue-light source by intracavity doubling of a diode-pumped Cr:LiSAF laser.

We describe a diode-pumped cw Cr(3+):LiSrAlF(6) laser that produces 190-mW cw output at 860 nm. By frequency doubling in a KNbO(3) crystal we generate 13 mW of second-harmonic light tunable from 427 to 443 nm.

Characterization of KNbO(3) crystal for frequency doubling of a tunable pulsed near-infrared laser.

Using KNbO(3) crystals for second-harmonic generation in the blue spectral range, we have obtained conversion efficiencies as high as 53.7%. To our knowledge this value is the highest reported for a pulsed laser. A gainswitched nanosecond Ti:sapphire oscillator, amplified by a Cr:LiSrAlF(6) multipass amplifier, was used as the fundamental light source. The maximum energy obtained at the harmonic wavelength of 430 nm was 38 mJ for a bandwidth of 230 MHz.

High-quality self-pumped phase conjugation of nanosecond pulses at 532 nm using photorefractive BaTiO(3).

We implement a feedback loop oscillator, using a BaTiO(3) crystal that provides stable high-quality phase conjugation for nanosecond pulses, with an efficiency close to the theoretical maximum.

Efficient self-pumped phase conjugation at near-infrared wavelengths using cobalt-doped BaTiO(3).

Using BaTiO(3) crystals doped with cobalt, we have measured the reflectivity of self-pumped phase-conjugate mirrors operating in the total-internal-reflection geometry as a function of wavelength in the range between 633 and 933 nm. We find that for cobalt concentrations above 25 parts in 10(6), the reflectivity is higher than 50% for all the investigated wavelengths. At a wavelength of 800 nm the reflectivity is independent of incident power over a range of 12 to 600 mW.