J. S. Bakos

Harmonic generation in plasmas of different density gradients

Abstract The effect of plasma scalelength on second and third harmonic generation in the interaction of a 700 fs, 248 nm laser pulse with solid targets was studied at nonrelativistic intensities. The density gradient was modified by a 700 fs prepulse 0–40 ps prior to the main pulse. The appropriately chosen prepulse leads to enhanced conversion to harmonics which is interpreted as a consequence of resonant absorption. There are differences in the effect of the prepulse for different materials and for different harmonic orders, which might open the possibility of selective harmonic generation.

Absolute measurement of velocity distribution of neutrals in sodium laser blow-off beam

Abstract Absolute density measurements of Na blow-off atomic beam were carried out with time of flight laser induced fluorescence. The preferentially used 1 μm Na layer was in situ evaporated on a glass plate. The velocity distribution of the blow-off neutrals was investigated as a function of the Nd laser fluence. The measurements showed that the blow-off beam contains two atomic packets with different propagation velocities in the laser range of 10–20 J/cm 2 and one atomic packet elsewhere. The faster packet has a maximum total number of atoms at 3 J/cm 2 and its velocity can be well fitted with a power function of the irradiance with v ∼ I 0.29 .

High-intensity narrow light pulse produced by self-focusing in laser spark

The shape of the laser light pulse transmitted through the spark produced in air by the same light pulse is investigated. A single‐mode and single‐frequency ruby laser is used. A narrow spike, overriding the amplitude of the original light pulse, is observed in the transmitted light. The spike is explained as the result of self‐focusing taking place in the plasma. The amplitude and the time of the appearance of the spike, i.e., of the self‐focused pulse, is investigated independently of the different experimental parameters. The light scattered by the spark is also observed; furthermore, clear experimental evidence of the self‐focusing taking place during the cascade ionization process is presented.

Population transfer in three-level Λ atoms with Doppler-broadened transition lines by a single frequency-chirped short laser pulse

We examine interaction of a single frequency-chirped laser pulse with three-level atoms that have a Λ configuration of levels. We show that it is possible to produce complete fast and robust population transfer of all atoms of the ensemble with Doppler-broadened transition lines from one ground state into the other ground state with negligibly small temporary population of the excited state by controlling the intensity of the laser pulse and the direction and speed of the frequency chirp.

Laser blow‐off plasma propagating in low‐pressure gas

The change of the properties of the plasma ball created in the laser blow‐off process is investigated in interaction with residual gas and buffer gas particles during its flight in space. The main process of the interaction is the collision leading to plasma particle loss. The center of mass velocity and the temperature of the ball weakly depend on the buffer gas density because the plasma is mainly collisionless.

Creation of a coherent superposition of quantum states by a single frequency-chirped short laser pulse

Traditional schemes for coherent population transfer or generation of coherent superposition states in multilevel atoms or molecules usually utilize two or more laser beams with radiation bandwidth smaller than the frequency interval between the working levels. We show the possibility of creation of the coherent superposition of three metastable states of a four-level atom with tripodlike level structure using a single short frequency-chirped laser pulse. The bandwidth of the pulse envelope (without chirp) must be comparable to or exceed the frequency distance between the two metastable levels. No appreciable excitation of the atom takes place during the creation of the coherent superposition state, thus diminishing significantly the effect of decoherence due to the spontaneous decay of the excited state. The proposed method of creation of superposition states is robust against variations in the laser pulse parameters. Since this method does not require maintaining steady resonance with the atomic transitions (owing to the frequency chirp of the laser pulse), it is effective both in homogeneously and inhomogeneously broadened media.

Investigation of laser blow-off atomic beams by electron impact excitation

Electron impact excitation in the edge region of the MT‐1 tokamak was used to measure the velocity distribution of a sodium laser blow‐off beam in a single shot. The different dependencies of the propagation velocity of the blow‐off neutral beam on the laser fluence at different sodium layer thicknesses reveal different regimes of the blow‐off process. In the case of thick targets laser ablation is the main accelerating mechanism giving a v∼I1/3 dependence. Thin targets, however, burn through during the laser pulse, and simple thermal heating process results in v∼I1/2 in good agreement with the experiments.

Investigation of transient acoustic waves in KDP electrooptic modulators

Piezoelectrically generated acoustic transients in longitudinal KDP modulator crystals were investigated, using a photographic method developed for visualizing the acoustic waves. The method gives the possiblity of following the processes of acoustic wave propagation, the reflections of waves at the side faces of the crystal and the wave diffusion caused by crystal imperfections.

Self-focusing of laser beam crossing a laser plasma

Abstract A crossed beam experiment was performed to clarify the mechanism of self-focusing in a laser produced spark. The plasma was created by one beam and self-focusing was observed in the weak probe beam which crossed the plasma. Experimental results show that the cause of self-focusing is the nonuniform heating mechanism.

Light pulse narrowing and power density increase in a laser-produced spark

Abstract The gaussian shape of a light pulse is distorted in a laser spark. A narrowing pulse appears with an amplitude enlarged by an order of magnitude due to self-focusing.