The conversion of a photon’s frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material’s refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of ${\rm Re}\{{\varepsilon _r}\}\; = \;{0}$. We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZ wavelength. Remarkably, we show that these effects are a consequence of the slow propagation speed of pulses at the ENZ wavelength. Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ. Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.

中文翻译：

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ε接近零的材料的绝热频移：群速度的作用

长期以来，光子频率的转换一直是非线性光学的关键应用领域。已经讨论了材料折射率的缓慢时间变化如何导致脉冲频谱的绝热频移（AFS）。这样的刚性频谱变化具有相关的技术含义，例如在超快速信号处理中。在这里，我们研究了在接近零的ε（ENZ）材料中的AFS过程，并表明，如果在$ {\ rm Re} \ {{\ varepsilon _r} \附近工作，则可以在较短的长度内实现频移。 } \; = \; {0} $。我们还预测，如果折射率是由强烈的光脉冲引起的，则对于ENZ波长的泵浦而言，频移更为有效。值得注意的是，我们证明了这些影响是由于脉冲在ENZ波长处传播速度较慢的结果。我们的理论预测通过在ENZ处入射到铝氧化锌薄膜上的光脉冲的AFS获得的实验得到验证。我们的结果表明，在ENZ点附近工作的透明金属氧化物是未来频率转换方案的良好候选者。