Research progress on octave supercontinuum generation in solid medium

Abstract

When a short laser pulse passes through transparent medium, the spectrum may be broadened due to nonlinear optical effects, and a coherent octave supercontinuum may be generated under certain conditions. Such a supercontinuum may be compressed into a femtosecond few-cycle pulse, which has many applications in ultrafast optics and beyond. Spectral broadening has been achieved experimentally in gases, liquids, and solids. Current mainstream technique of supercontinuum generation is to send multi-cycle femtosecond pulses through inert-gas-filled hollow-core fibers. However, due to the limitation of the core diameter, the hollow-core fiber cannot work with high-energy laser pulses. With a much higher nonlinear index of refraction, solid-state material is naturally a more promising candidate for supercontinuum generation, but it is difficult to obtain a near-octave spectrum in one piece of solid without filamentation. The optical Kerr effect in solids triggers self-phase modulation (SPM) which induces desired spectral broadening as well as self-focusing, thus causing the laser intensity to rise drastically with substaintial multiphoton excitation and ionization leading to plasma formation. This behavior results in filamentation and optical breakdown, and eventually permanent damage to the material occurs if the laser pulse energy is high enough. Using a thin plate of dielectrics may minimize the effect of self-focusing-the beam exits from the nonlinear medium before it starts to shrink and causes damage. However, one thin plate does not provide enough nonlinear effect to generate a broad spectrum. To prevent disastrous self-focusing while achieving spectral broadening, using multiple Kerr elements has been proposed theoretically and demonstrated experimentally at microjoule to millijoule level. In such a configuration, a femtosecond laser pulse is being spectrally broadened via SPM in the thin plates, while self-focusing converges the beam in each plate but the focal spot is located outside the plate. Once the converging beam passes through its focal spot in air, the beam diverges and enters the next plate to repeat this process until the spectral broadening stops after several elements. Using this method, octave supercontinuum with energies at microjoule to millijoule level has been experimentally obtained in a spectral range covering near-ultraviolet to mid-infrared. In this paper, we review the development of supercontinuum generation in multiple thin solid plates, outline the principle of supercontinuum generation in this new type of thin solid medium, brief the experiments using this new method in recent years, and look into the prospects for its development.