Terahertz generation from ZnTe optically pumped above and below the bandgap.

We report on the generation of THz waves through optical rectification in ZnTe of femtosecond laser pulses whose photon energy is tuned from below to above the ZnTe bandgap energy. The THz signal exhibits a pronounced peak at the bandgap energy, at THz frequencies for which losses in ZnTe remain small. This peak is likely due to the resonance of the ZnTe nonlinear susceptibility in the vicinity of the bandgap.

Two-photon-absorption enhanced terahertz generation from KTP optically pumped in the visible-to-UV range.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν1 (A1g) mode.

Observation of terahertz beam diffraction by fabrics.

The structure of most of fabrics is an almost periodic network of interlaced yarns or threads, whose periodicity is of the order of the terahertz wavelength. We report here on the diffraction of terahertz electromagnetic waves by the yarn network in common cloths. In the case of linen, this effect could lead to overestimating (by a factor as large as 3) the absorption when determined from the classical terahertz time-domain experiment. Our results are confirmed by a HFSS numerical simulation.

Simultaneous passively Q-switched dual-wavelength solid-state laser working at 1065 and 1066 nm.

A passively Q-switched dual-wavelength solid-state laser is presented. The two wavelengths are emitted by two different crystals in order to avoid gain competition, and the synchronization between the pulses is obtained by external triggering of the saturable absorber. Sum frequency mixing is demonstrated, proving the interest of this source for terahertz generation in the 0.3-0.4 THz range through difference frequency generation.

Highly efficient broadband double-sided Fresnel lens for THz range.

Modern passive THz setups require effective optical elements with a large numerical aperture. Here we propose a new type of the optical element for THz applications, which is a broadband double-sided Fresnel-like lens with an optimized thickness. The optimization is performed to obtain a very low attenuation, low material cost, and small weight in the element media. It also provides achromatic properties for the assumed wavelength range. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Off-axis metallic diffractive lens for terahertz beams.

A diffractive optical element for off-axis focusing of terahertz radiation is presented. It was designed in a nonparaxial regime and manufactured in a metal slab by laser cutting of curved stripes. The optical function of the structure includes focusing and deflecting the illuminating beam of a chosen frequency in a particular place. Therefore, the element acts as both a spatial and a spectral filter; hence it is especially suitable for separating the terahertz signal from a broadband thermal load in passive detection devices. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.

Pulsed blue laser at 491 nm by nonlinear cavity dumping.

A nonlinear cavity dumping process is applied for the first time to generate kW peak power pulses at 491 nm. The system is based on efficient sum-frequency mixing of 1063 nm and 912 nm radiations in a BiBO nonlinear crystal placed inside a Nd:GdVO4 laser oscillator with a high finesse cavity at 912 nm. The nonlinear cavity dumping process is triggered by high peak power nanosecond pulses from a 1063 nm Q-switched Nd:GdVO4 laser operating at 10 kHz. To reach the kW range at 491 nm a key point is to Q-switch the high finesse 912 nm cavity instead of continuous wave operation. Thus, the peak power (9.3 kW for 3 ns pulses) and the average power (280 mW) obtained at 491 nm are 14 times higher than the one obtained when the 912 nm laser operated in continuous wave.

Thermal conductivity measurements of laser crystals by infrared thermography. Application to Nd:doped crystals.

We present a thermal conductivity measurement method for laser crystals based on thermal mapping of the crystal face by an infrared camera. Those measurements are performed under end-pumping of the laser crystal and during laser operation. The calculation of the fraction of pump power converted into heat is therefore simplified, and it is possible to link easily the temperature in the crystal to the thermal conductivity. We demonstrate the efficiency of this measurement method with a Nd:YAG crystal, before using it to compare Nd:YVO(4) and Nd:GdVO(4) crystals.

491 nm generation by sum-frequency mixing of diode pumped neodymium lasers.

We present three different dual-wavelength laser architectures to obtain cw blue radiations. They are based on diode-pumped Nd-doped crystals lasing on the 4F3/2-4I11/2 and 4F3/2-4I9/2 transitions. Blue radiations were achieved by intracavity sum frequency operation in a BiB3O6 crystal. We report a maximum output blue power of 303 mW at 491 nm for a pump power of 10 W.

1064 nm Nd:YVO4 laser intracavity pumped at 912 nm and sum-frequency mixing for an emission at 491 nm.

We present for the first time a Nd:YVO(4) laser emitting at 1064 nm intracavity pumped at 912 nm by a Nd:GdVO(4) laser. We carried out a model to design the system properly, and laser performance was experimentally investigated. Intracavity sum-frequency mixing at 912 and 1064 nm was then realized in a BiBO crystal to reach the blue range. We obtained a cw output power of 155 mW at 491 nm with a pump laser diode emitting 20 W at 808 nm.

Diode-pumped Nd:YAG laser emitting at 899 nm and below.

We present what is, to the best of our knowledge, the first diode-pumped Nd:YAG laser emitting at 899 nm and below, based on the (4)F(3/2) - (4)I(9/2) transition, generally used for a 946 nm emission. A power of 630 mW at 899 nm has been achieved in cw operation and 16 mW at 884 nm with a fiber-coupled laser diode emitting 9 W at 808 nm. Intracavity second-harmonic generation in cw mode has also been demonstrated with a power of 100 mW at 450 nm by using a LiB(3)O(5) nonlinear crystal.