Femtosecond enhancement cavities¹ are key to applications including high-sensitivity linear^2,3,4 and nonlinear^5,6 gas spectroscopy, as well as efficient nonlinear optical frequency conversion^7,8,9,10. Yet, to date, the broadest simultaneously enhanced bandwidths amount to <20% of the central optical frequency^{8,9,11,12,13,14,15}. Here, we present an ultrabroadband femtosecond enhancement cavity comprising gold-coated mirrors and a wedged-diamond-plate input coupler, with an average finesse of 55 for optical frequencies below 40 THz and exceeding 40 in the 120–300 THz range. Resonant enhancement of a 50-MHz-repetition-rate offset-free frequency comb spanning 22–40 THz results in a waveform-stable ultrashort circulating pulse with a spectrum supporting a Fourier limit of 1.6 cycles, enabling time-domain electric-field-resolved spectroscopy of molecular samples with temporally separated excitation and molecular response¹⁶. The contrast between the two is improved by taking advantage of destructive interference at the input coupler. At an effective interaction length with a gas of up to 81 m, this concept promises parts-per-trillion-level ultrabroadband electric-field-resolved linear and nonlinear spectroscopy of impulsively excited molecular vibrations.

飞秒增强腔¹是应用的关键，包括高灵敏度线性^2,3,4和非线性^5,6气体光谱，以及高效的非线性光学频率转换^7,8,9,10。然而，迄今为止，最宽的同时增强带宽小于中心光频率的 20% ^{8,9,11,12,13,14,15}. 在这里，我们提出了一个超宽带飞秒增强腔，包括镀金反射镜和楔形金刚石板输入耦合器，对于 40 THz 以下的光频率，平均精度为 55，在 120-300 THz 范围内超过 40。跨越 22-40 THz 的 50 MHz 重复率无偏移频率梳的谐振增强导致波形稳定的超短循环脉冲，其频谱支持 1.6 个周期的傅里叶极限，实现时域电场分辨具有时间分离激发和分子响应的分子样品光谱学¹⁶. 通过利用输入耦合器处的相消干涉来改善两者之间的对比度。在与高达 81 m 的气体的有效相互作用长度下，这一概念有望对脉冲激发的分子振动进行万亿分之一级的超宽带电场分辨线性和非线性光谱。