Intense light–matter interactions have revolutionized our ability to probe and manipulate quantum systems at sub-femtosecond timescales¹, opening routes to the all-optical control of electronic currents in solids at petahertz rates^2,3,4,5,6,7. Such control typically requires electric-field amplitudes in the range of almost volts per angstrom, when the voltage drop across a lattice site becomes comparable to the characteristic bandgap energies. In this regime, intense light–matter interaction induces notable modifications to the electronic and optical properties^8,9,10, dramatically modifying the crystal band structure. Yet, identifying and characterizing such modifications remain an outstanding problem. As the oscillating electric field changes within the driving field’s cycle, does the band structure follow and how can it be defined? Here we address this fundamental question, proposing all-optical spectroscopy to probe the laser-induced closing of the bandgap between adjacent conduction bands. Our work reveals the link between nonlinear light–matter interactions in strongly driven crystals and the sub-cycle modifications in their effective band structure.

强烈的光物质相互作用彻底改变了我们在亚飞秒时间尺度¹探测和操纵量子系统的能力，开辟了以拍赫兹速率^{2、3、4、5、6、7}对固体中的电子电流进行全光学控制的途径。当跨晶格位置的电压降变得与特征带隙能量相当时，这种控制通常需要在几乎每埃伏特范围内的电场幅度。在这种情况下，强烈的光-物质相互作用会引起电子和光学性质的显着改变^8,9,10，极大地改变了晶体能带结构。然而，识别和表征这种修饰仍然是一个突出的问题。当振荡电场在驱动场的周期内发生变化时，能带结构是否遵循以及如何定义？在这里，我们解决了这个基本问题，提出了全光学光谱法来探测激光诱导的相邻导带之间带隙的闭合。我们的工作揭示了强驱动晶体中的非线性光-物质相互作用与其有效能带结构中的子周期修改之间的联系。