Optics Communications97 (1993) 54-58 North-Holland OPTICS COMMUNICATIONS Characterization of short pulses by self-diffraction in liquid crystals t3. Cipparrone, D. Duca, A. Mazzulla, C. Umeton Dipartimento di Fisica, Universitit della Calabria, 87036 Rende (CS), Italy and F. Simoni Dipartimento di Science Fisiche, Universith di Napoli, Piazza& Tecchio 80, 80125 Napoli, Italy Received 16 October 1992 We present measurements of picosecondlaser pulses performedby exploitingthe "self-diffraction" effectin two kinds of liquid crystallinematerials:nematic liquid crystals (NLC) and polymerdispersed liquid crystal (PDLC). Results are in good agreement with those obtained by standard second harmonic generationtechniques. The method exhibits severaladvantages,and the use of PDLC samples, which enable low noise measurements, makes the "self-diffraction" technique competitive with the standard ones. 1. Introduction In the last years, the availability of ultrashort laser pulses in a wide wavelength range has stimulated the study and development of appropriate techniques for characterizing the output pulses. Second harmonic generation (SHG) in nonlinear crystals [1,2] is the well known, standard one, but in spite of a femto- second accuracy, it needs different high-quality non- linear crystals to cover a wide wavelength range. A different technique has been developed which involves the arising of a dynamical grating in non- linear media and yields the same information as SHG [3-10]. It exploits the diffraction of light from the transient grating which is formed by the interference of two pulses, obtained by beam-splitting the pulse to be measured and introducing a variable delay be- tween them. Different geometries can be used ac- cordingly to the choice of the probe beam: it can be represented by the output of a different source or by one of the two pulses which produce the grating; in this last case, the phenomenon is called "self-dif- fraction" [ 3,6 ]. The diffraction efficiency is related to the short pulses fourth-order coherence function and can provide both the pulse duration Zp and the coherence time zc [ 8 ]. This "self-diffraction" tech- nique has been successfully used with semiconductor films and dyes to measure rp and zc of picosecond pulses [3,9] and successively it has been also used with slowly responding, photorefractive media to re- veal chirp and self phase-modulation effects [ 11]. The method exhibits several advantages if compared with the SHG one: it is extremely broadband and background free (no filtering of beams is necessary) and alignment is easy. Some difficulty can be rep- resented however by an asymmetry in the detected signal which is due to the influence of the orienta- tional relaxation time of the dye molecules or to the carrier lifetime in the semiconductor film. In fact they | are both in the range Tf~,~-~ 100 ps, and the reliable value of rr must be known for a correct calculation of the quantities of interest [4,8]. The problem can be overcome by using handy, inexpensive broad-band media with very long relaxation times. In this paper we present measurements of the time duration Zp of the picosecond, Fourier transform (zp ~ To) pulses from a modelocked Nd: YAG laser, performed by self-diffraction" in two kinds of liquid 54 0030-4018/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.