Nonlinear optics in petawatt lasers
The development of laser systems of the petawatt power level opened up brilliant opportunities for experimental study of the interaction of ultraintense laser fields with gas and solid targets. Such experiments demand knowledge of exact parameters of laser radiation. The time profile of optical pulse intensity has a rather complex structure that is directly connected with the method of generation and amplification of optical radiation. Of particular interest are problems of controlling temporal characteristics of ultraintense laser fields, such as duration, time contrast, and polarization. The need of correcting time profile arises in many applied problems.
To control temporal characteristics of femtosecond laser pulses, second-harmonic generation (SHG) in nonlinear optical crystals may be used. The SHG process was discovered in 1961, but it is still poorly studied for femtosecond pulses. Conversion of high-power ultrashort laser radiation to the second harmonic requires additional allowance for the dispersion effects and the effects due to nonlinear cubic polarization, as well as prevention of small-scale self-focusing in the nonlinear element of the frequency doubler.
High efficiency (about 70%) second-harmonic generation of femtosecond pulses with an average intensity of ~1 TW/cm2 (at peak intensity up to 3–4 TW/cm2) and central wavelength of 910 nm was demonstrated in experiment. The pulse contrast at double frequency is equal to squared contrast of the original pulse.
Work is underway on further temporal compression of intense femtosecond pulses. To achieve this goal a method of using Kerr nonlinearity in ultrathin transmission optical elements for pulse phase self-modulation and additional spectrum broadening has been developed. It is intended to achieve time compression using chirped mirrors. This technique may be used for both primary and second harmonic radiation.
Nonlinear optical effects may be not only useful and employed to control parameters of laser radiation, but parasitic too. Self-focusing is a parasitic effect that limits radiation power. Recently, the IAP RAS researchers (E. A. Khazanov, S. Yu. Mironov, and V. V. Lozhkarev) proposed, calculated and confirmed experimentally the self-filtering method which allows removing spatial harmonic perturbations of the laser beam that have maximum gain, thus avoiding self focusing. The method works well for the laser radiation intensity of 1 TW/cm2 and higher.