Tom Efthimiopoulos

Four-wave mixing and parametric four-wave mixing near the 4P–4S transition of the potassium atom

Potassium 4S1/2–6S1/2 two-photon excitation initiates the emission of several internally generated photons. For the first time two emission lines, one close to and one below the potassium 4P3/2 level, are reported for low pumping intensity. Radiation emitted below the 4P3/2 level is due to a parametric four-wave mixing process that uses the photons emitted at the 5P3/2–4S1/2 transition and a two-step four-wave mixing process generates the line emitted close to the 4P3/2 level.

Stimulated emissions and quantum interference in potassium atom-laser interaction

The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P–4S transition under the two-photon 4S1/2 –6S1/2 excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.

Cascade emission and four‐wave mixing parametric processes in potassium

Strong stimulated cascade emission and coherent four‐wave parametric radiations are observed. By proper observation of the excitation spectra, the cascade emission and the coherent four‐wave mixing parametric processes can be identified. The effect of the buffer gas pressure on the intensity and the pulse compression of the emitted radiation were investigated. Cascade inversionless coherent radiation emission with absorption reduction and third order nonlinear susceptibility enhancement due to the presence of a strong internally generated field are discussed for the 4p–4s strong emission observed.

Study of the emission in K under two-photon excitation

It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.

UV laser beam/YBCO target interaction for optimum preablation for the growth of YBCO thin films

The morphological and compositional changes which take place during the laser irradiation of YBCO targets are discussed. The understanding of the evolution of target composition, will lead to better control of film quality. In the case stated here, a XeCl excimer laser was used to irradiate rotated and also non-rotated YBa2Cu3O7-x targets. The modified surface of these targets was systematically studied by scanning electron microscopy and energy dispersive x-ray microanalysis. It was observed that some crucial deposition parameters, such as the number of pulses and fluence has a strong effect on the roughening of the target. A comparative study was done on the derived High-Tc superconducting thin films. It was shown that the conditions during the preablation procedure which affect the target surface morphology correlate closely with the appearance of laser droplets on the film surface.

Ablation rate and threshold flux of cartilage irradiated by a XeCl excimer laser

Results are presented related to the ablation of animal cartilage by a XeCl laser beam (308 nm). In particular the minimum energy flux for ablation and the ablation rate as a function of the energy flux were measured. The effect of the pulse repetition rate on the cartilage temperature and the influence of a flowing cooling gas were also investigated. A new method, based on the observation of the acoustic wave produced during the ablation process and the delay of arrival of the wave to the back of the sample with respect to the laser pulse, was introduced to measure the ablation threshold and the on line ablation rate. This method was compared with other methods that use either a profilometer or the perforation of the sample to measure the ablation rate. It is shown that the acoustic method can provide accurate, on line information of the ablation rate and threshold. It is also shown that the ablation rate is reduced considerably along the depth of the sample.

Advanced-technology laser-aided air pollution monitoring in Athens: the Greek differential absorption lidar

This paper describes the needs for establishing a mobile laser laboratory (LIDAR) for air pollution monitoring in the Athens area. It also gives the specifications of the laser unit of the LIDAR system and the various studies to be performed in Athens area.

Experimental evidence of quasi-absorptionless type of resonant sum-mixing process

The theoretical and experimental investigation of the absorption correlation in the sum-mixing generation in Hg vapor was performed. The absorption correlation equals to 0.90 was observed for the first time. The corresponding increasing of the absorption length is as much as 10 times.

Photo-dissociated HgBr2 for mode-locking excimer lasers

It is shown that a two-photon dissociation of HgBr2 molecular vapor can be used as a non-linear fast opening passive mode locking technique for excimer laser. Pulses below our resolution limit of 400 ps and with a modulation better than 85% are reported.

Wave mixing and quantum interference effect in potassium atoms

Experimental results are presented related to the wave mixing in K. The influence of the medium number density, the incident radiation intensity and the detuning from resonance on the spectral, spatial and temporal characteristics of the radiation emitted near the 5P-4S transition is investigated for the 4S1/2 - 6S1/2 two-photon excitation. The different processes involved and the quantum interference is examined.