Ultrafast events require imaging modalities that operate on ultrafast timescales, with frame rates greater than 100 million frames per second, to peer into the underlying physics driving the phenomena. Traditionally, ultrafast laser pulses (with pulse duration of a 100 thousandth of a billionth of a second) are used to probe these events using pump-probe methods. In a pump- probe method, data from a single time slice is captured by carefully timing the initiation of the event with the light pulse that probes the event. Then, by generating many events and changing the timing between the initiation of the event and the probing light pulse, the full evolution of the event can be captured. This requires, however, that each event be identical, which is often not the case for ultrafast events such as dynamically evolving plasmas, laser-induced damage, and irreversible photo-chemical reactions. In the case of nonrepeatable processes, single-shot ultrafast imaging methods must be used. While many single-shot ultrafast imaging methods have been recently developed, few have been made to image the phase and amplitude of the dynamically evolving object, which is critical in many ultrafast phenomena such as electrostatic discharge. Recently, Barolaketal., developed a new ultrafast imaging methodology that provides quantitative information of both the phase and the amplitude of the dynamically evolving object.
In the article, Barolak et al. present their novel imaging method called Ultrafast Time-Resolved Imaging via Multiplexed Ptychography (or UTIMP) which combines the power of ptychography with ultrafast laser sources. In UTIMP, a single ultrafast light pulse is split into a series of temporally separated pulses. Each pulse then gets split up into a 2D grid of beamlets which scatter off the dynamically evolving object at shifted transverse locations on the object. Far field diffraction pattern intensities from each beamlet are then collected on a camera. Diffraction data from different pulses within the temporally separated pulse train, incoherently combine on the camera, and an image for each time slice is then algorithmically reconstructed using a multiplexed ptychographic reconstruction algorithm. This generates an ultrafast motion picture of the dynamically evolving event in both phase and amplitude. The frame rate for UTIMP is set by the temporal spacing of the probe pulses, giving usa maximum theoretically achievable frame rate in the 100’s of trillions of frames a second which is on par with the fastest single-event imaging system in the world.
UTIMP was experimentally realized by imaging conduction band electron dynamics in ZnSe from a two-photon absorption event. When probed at near visible wavelength, conduction band electrons only interact with light by changing the velocity of the light based on the conduction band electron density. Since the dynamically evolving event does not absorb light, the event must be imaged quantitatively in phase. For this experimental demonstration, 4 time slices were reconstructed from a single event to show the formation of the conduction band electron density in the ZnSe crystal and the relaxation of the electrons back to the valence band over the following nanoseconds. The results of this experiment were verified with a nonlinear pulse propagation simulation, which calculates the expected conduction band electron density as a function of input pulse energy.
UTIMP represents a major step forward in single-shot ultrafast imaging. Bringing the powers of ptychography to the ultrafast regime, exciting new possibilities are capable by leveraging the substantial amount of work that’s been done to extend ptychography to higher dimensional imaging modalities. By combining these massively multiplexed methods with UTIMP, it should be possible to generate ultrafast motion pictures of a single dynamically evolving event in 3D, with multiple illumination wavelengths, or with multiple polarization states.
