Time-Resolved Microscopy of Near-Infrared to Visible Waveforms

Abstract

Laser pulses, since their first demonstration in 1960, have reached an unprecedented level of temporal confinement: light fields on a femtosecond timescale are widely available and routine in modern experiments. Nowadays, ultrashort few-cycle electric fields can be utilized to generate even shorter, attosecond pulses, establishing a new direction of optics called attosecond physics. Such short light localization provides incredibly high temporal resolution, enabling direct observation of sub-cycle dynamics of the interaction of the fundamental waveform with matter in pump-probe measurements. Generation of attosecond pulses is an extremely sophisticated task, involving highly nonlinear processes and requiring a vacuum environment. Therefore, free-space methods that allow for the reconstruction of the time-varying field oscillations and direct performance of experiments with attosecond temporal precision are being developed. As the electric field is a function of time and space, the time-varying component is not always able to fully characterize the pulse. The large bandwidth associated with ultrashort pulses can be a reason for the formation of harmful spatio-temporal distortions. They often lead to a significant peak intensity reduction and unexpected experimental outcomes. The lack of direct access to the spatio-temporal evolution of near-infrared and visible few-cycle pulses is indeed of great concern. A potential spatio-temporal metrology technique can not only detect various distortions but also probe the properties of spatially inhomogeneous samples, extending the field-resolved spectroscopic toolbox to include spatial dimensions. This dissertation aims to advance a revolutionary metrology approach for absolute space-time characterization of electric fields by extending its capacity to the near-infrared and visible spectral regions. Its application in microscopy yields detection of subwavelength-localized structures in a wide-field geometry and extraction of spatio-temporal light-matter interaction with sub-cycle temporal resolution. This work is based on a well-established technique for complete field reconstruction, referred to as electro-optic sampling. The concept of electro-optic sampling relies on a phase-stable test field to be sampled that is coincident with an ultrashort probe pulse in an electro-optic crystal. Their nonlinear interaction induces a polarization rotation of the probe pulse that is sensitive to the strength and sign of the test electric field at a given instant. By varying the time delay between the pulses and employing dedicated instruments to read out the polarization rotation, it is possible to completely reconstruct the test field. Such instruments typically average over the spatial variations of the polarization rotation, yielding a single temporal waveform. Therefore, the assumption about homogeneous spatial distribution of the investigated field has to be made in these measurements. Alternatively, the dependence of the polarization rotation on the spatial coordinates in electro-optic sampling can be recorded using a standard imaging system. In this case, absolute spatio-temporal field information about the test electric field including the carrier envelope phase can be measured. We refer to this technique as electro-optic imaging. The optical scheme for electro-optic imaging was first introduced with relatively long pulses, in the terahertz spectral range (0.1-10 THz). In the present thesis, we dramatically extend the detection limits of electro-optic imaging towards shorter wavelengths, as low as 670 nm (450 THz). For the first time, the imaging technique is applied to demonstrate the full spatio-temporal reconstruction of few-cycle pulses in the near-infrared and visible regimes. Arbitrary spatio-temporal distortions of the laser pulses are detected and analyzed by converting time-dependent field snapshots into a hyperspectral image. Direct access to spatio-temporal dynamics of the electric field is utilized to investigate innovative metasurface optical devices and their incredible control over light properties. Metasurface optics allow diffraction-limited performance to be realized without cumbersome optical designs. The imaging apparatus can open a new door to comprehensive stu-dies of absolute space-time light confinement after the interaction of metasurface optics with an incident broadband field. It is particularly intriguing that not only the far-field but also near-field radiation can be accessed with electro-optic imaging in real time. This has been demonstrated in terahertz range, where microscopic samples placed directly on a thin electro-optic crystal were imaged with subwavelength resolution. A proof-of-concept of near-field detection in the near-infra-red range is shown by utilizing field enhancement with spherical microparticles. The imaging apparatus presented in this dissertation is expected to enrich the tools of attosecond metrology by including spatial dimensions. Additionally, this field-resolved microscopy method opens a novel path towards wide-field hyperspectral and label-free imaging with subwavelength resolution for applications in nanoscience and biology.