Time-resolved electron microscopy aims to track nanoscale excitations and dynamic states of matter at a temporal resolution ultimately reaching the attosecond regime. Periodically time-varying fields in an illuminated specimen cause free-electron inelastic scattering, which enables the spectroscopic imaging of near-field intensities. However, access to the evolution of nanoscale fields and structures within the cycle of light requires sensitivity to the optical phase. Here we introduce free-electron homodyne detection as a universally applicable approach to electron microscopy of phase-resolved optical responses at high spatiotemporal resolution. In this scheme, a phase-controlled reference interaction serves as the local oscillator to extract arbitrary sample-induced modulations of a free-electron wavefunction. We demonstrate this principle through the phase-resolved imaging of plasmonic fields with few-nanometre spatial and sub-cycle temporal resolutions. Due to its sensitivity to both phase- and amplitude-modulated electron beams, free-electron homodyne detection measurements will be able to detect and amplify weak signals stemming from a wide variety of microscopic origins, including linear and nonlinear optical polarizations, atomic and molecular resonances, and attosecond-modulated structure factors.

时间分辨电子显微镜旨在以最终达到阿秒状态的时间分辨率跟踪纳米级激发和物质的动态状态。照明样本中周期性随时间变化的场会引起自由电子非弹性散射，从而实现近场强度的光谱成像。然而，了解光周期内纳米级场和结构的演化需要对光学相位敏感。在这里，我们介绍自由电子零差检测作为一种普遍适用的高时空分辨率相位分辨光学响应电子显微镜方法。在该方案中，相控参考相互作用充当本地振荡器，以提取自由电子波函数的任意样本引起的调制。我们通过具有几纳米空间和子周期时间分辨率的等离子体场的相位分辨成像证明了这一原理。由于其对调相和调幅电子束的敏感性，自由电子零差检测测量将能够检测和放大来自各种微观来源的微弱信号，包括线性和非线性光学偏振、原子和分子共振，以及阿秒调制的结构因子。