基于固体介质的倍频程连续光谱产生的研究进展

超快激光经过透明介质时由于非线性作用光谱会得到展宽,甚至能够产生超过一个倍频程的相干超连续光谱,这样的光源能够压缩得到几个甚至单个光周期的超短脉冲,并在现代超快科学的各个领域得到了广泛应用.实验中已经在气体、液体和固体中都观测到了光谱的展宽,目前较为成熟的方法是使用充满惰性气体的空芯光纤和具有高非线性效应的固体材料展宽光谱.但空芯光纤由于芯径限制无法用于高能量激光脉冲的光谱展宽,而固体材料又容易被高功率密度的脉冲激光损坏.随着激光技术的发展其脉冲能量不断提高,一种新的、利用多片薄固体介质实现光谱展宽的方式被提出.多片薄的非线性介质可以实现光谱展宽的逐片累积,而且避免了激光在介质中因自聚焦产生过高功率密度带来的损坏.目前使用这种方法已经在实验上得到了近毫焦尔量级的倍频程光谱,覆盖了近紫外到中红外的整个区域,并实现了脉冲压缩.本文简要回顾了超快激光在固体中光谱展宽的发展历程,概述了新型薄片固态介质产生超连续光谱的原理,对近年来使用此新方法的实验进行了简要分析,并对其发展前景进行了展望.

Research progress on octave supercontinuum generation in solid medium

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.