IN LASER PRODUCED SPARKS 1. Introduction - Science Direct

Volume 70, number 1

OPTICS COMMUNICATIONS

1 February 1989

SECOND HARMONIC POLARIZATION AND CONVERSION EFFICIENCY IN LASER P R O D U C E D SPARKS D. BATANI 1, F. B I A N C O N I , A. G I U L I E T T I , D. G I U L I E T T I i and L. N O C E R A lstituto di Fisica Atomica e Molecolare, via Giardino, 7, 56100 Pisa, Italy

Received 18 July 1988

Experimental data are reported on second harmonic forward emission from sparks produced in helium by focused 1.06 ~tm radiation. The coto 2coconversion efficiencywas measured versus both laser power (ranging from 20 to 400 MW ) and gas density (from 6 to 300× 1017 atoms/cm3). Efficiency was found not to increase monotonically with laser power nor gas density, but having maxima for definite values of these parameters. Calculations in the far field approximation were used to interpret measurements of the polarization of the 202radiation.

1. Introduction The harmonic generation o f m o n o c h r o m a t i c light propagating in a non-linear medium has been strongly investigated since the invention o f the laser and nowadays important applications rely on this effect [ 1 ]. A special case o f a non-linear m e d i u m is a laserproduced plasma. The laser-plasma interaction can give rise to harmonic generation o f the laser field in this case the efficiency is generally low and the angular distribution strongly depends on interaction condition and its evolution [ 2 ]. Therefore the study of harmonic generation - mainly the second harmonic - in laser plasmas has no general applicative interest, but it can be useful as a diagnostics of the interaction itself. As long as the plasma is produced from solid surfaces, the region o f m a x i m u m conversion efficiency co to 20) is expected to lie nearby the critical layer (i.e. at electron density ne me co2/(4tee2) ) where the electron plasma frequency cope close to co makes easier to excite currents at frequency co with consequent harmonic multiplication. However, it has been shown both theoretically and experimentally [3-5,8,11] that even at a density much lower than the critical one, 2oa emission can be observed provided density gradients are present. Second harmonic emission is therefore a preferential Also at Dipartimento di Fisica, Universit/t di Pisa, Italy. 38

diagnostics for the plasma-radiation system to detect strong inhomogeneities. A m o n g the processes leading to such inhomogeneities, most important are selffocusing and filamentation o f the laser beam into the plasma. A few experimental works have given a direct evidence o f filamentation during the irradiation of solid targets by imaging the interaction region in second harmonic light [ 5-7 ]. We extended this useful diagnostics to the study of the interaction with low density plasmas produced in gases (laser sparks). In this condition density gradients are mainly directed perpendicularly to the laser beam; then it is theoretically expected that the second harmonic emission is restricted to small angles around the beam axis. This expectation was experimentally verified [ 8 ]. Also the measured value o f the conversion efficiency was in reasonable agreement with theory. We found a m a x i m u m efficiency for forward 2o2 generation of about 5 × 10-12 in terms of peak power. The extremely low level o f conversion is a considerable experimental problem; however an accurate spectral selection and temporal discrimination allowed us to use that faint emission as a diagnostic tool. Diagnostic results include time resolved images of the interaction zone with spatial resolution o f a few micrometers and temporal resolution of tens o f picoseconds. F r o m these images it was possible to have direct evidence for self-focusing and filamentation o f the laser beam interacting with 0 030-4018/89/$03.50 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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the plasma, phenomena previously observed and studied from the spatial re-distribution of the beam intensity (forward scattering at the fundamental frequency) and with interferometric methods [ 9,10 ]. The 209 sources resulted to be localized in a region around the geometrical focus of the focusing lens, with a depth of few millimeters and a diameter no larger than 0.1 mm. The diameter increased during the laser pulse rise at the mean rate of 5 × 105 cm/s, but also phases of decreasing diameter were observed, eventually repeated in the same event. That was a net evidence of whole beam self-focusing. Moreover micrometer sized substructures were visible, having a mean life of few nanoseconds [ 11 ]. It was reasonable to identify such structures with filaments generated by non-linear interaction. The study summarized above was performed into a plasma whose electron density was ne ~ 1019 c m - 3 . At the same density the m a x i m u m self-focusing activity was previously observed at intensity of about 1013 W / c m 2. Further measurements have shown that the angular distribution of the 2o9 emission has a minimum along the beam axis, as expected from the theory, and annular maxima typical of coherent processes of emission. Finally a general deep modulation of the 2o9 emission was observed on the scale of hundreds of picoseconds, as an amplification of weaker modulations already present in the laser pulse. Even though our aim was to carry out a suitable diagnostics to detect gradients into the plasma, the experimental data seemed to be interesting also from the point of view of the physics of second harmonic generation in a laser spark. The measurements here reported address to this general aspect. They include: measurements of conversion efficiency versus both gas density and laser power; measurements of second harmonic pulse length and analysis of the polarization of 2o9 emission.

2. Experimental

1 February 1989

beam profile quasi-gaussian with spatial modulations up to 15%; no hot spots; quasi-periodical time modulations on hundreds of picosecond time scale; linear polarization. Focusing: lens t"/8; focal spot radius in vacuum ro~ 25 ~tm. The measurements we report below refer to experimental data from a series of shots obtained in condition of good stability of the laser output, both in energy and shape. For each event we recorded signals related to both laser pulse ("o9" channel) and second harmonic pulse ("2oY' channel). The second harmonic light was collected forward into a cone whose aperture was about the same as the one of the lens focusing the laser light. Previous measurements had shown [ 8,11 ] that virtually all the 209 light is generated into this cone. Narrow interferential filters were used to reduce the background of plasma continuum around 532 nm to a few percents of the second harmonic signal. The two channels gave us simultaneously the time evolution, on the nanosecond scale, of power of both laser and of second harmonic light. A typical recording is given in fig. 1. Both 09 and 209 channels were carefully calibrated to have absolute power measurements. The atomic density na of the gas varied from 6 × 1017 to 3 × 1019 a t / c m 3, the laser peak power from 20 to 400 MW. The efficiency values we measured (apart from the case of power just above the breakdown threshold of the gas) were in the range between r/= 10 -12 and 10 -11. Here and in the follow-

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Details on the experimental set-up have been given in a previous paper [ 8 ]. The spark was produced by optical breakdown, focusing a neodymium laser beam into a helium filled cell. The experimental conditions were the following. Laser: wavelength 1.064 ~tm; pulse length fwhm 20 ns; peak power up to 500 MW;

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Fig. 1. Scopetraces of the laser pulse and second harmonic pulse. 1 div= 20 ns. 39

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ing, efficiency r/is the ratio between the peak values P:o~ and Po~ as derived immediately from oscilloscope values. This value is somewhat averaged by the time integration of detectors and scope. Actually from data obtained on 209 sources evolution using a streak camera [ 11 ], it appeared that locally (into micrometer sized regions) and for short time period (subnanosecond) the conversion efficiency rises to much higher levels. In fig. 2 we have plotted the 209 efficiency versus gas density for two fixed values of the laser power: 100 and 400 MW. Both plots show well defined maxima: at 100 MW the m a x i m u m efficiency is achieved between 1.0 and 1.5 × 10 ,9 a t / c m 3, while at 400 MW the peak shifts to lower gas density i.e. between 5.0 3

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and 8.3× 10 ,8 a t / c m 3. It is interesting to note that out from the peak the efficiency values were rather stable shot by shot, but around the maximum the data are considerably scattered, particularly in the case of higher power. This point will be discussed later. At lower power the efficiency showed a net step increase at low gas density, approximately corresponding to the known density threshold [ 12] for the breakdown in helium at 2 = 1 ~tm and I = 5 × 10' 2 W / cm 2, the intensity at the focal spot. In fig. 3 we have displayed 209 efficiency versus laser power at two fixed gas densities: 7 × 10 ~8 and 2.5)