Watching photons on the fly

Video recording of ultrafast phenomena such as dynamic events in molecular biology would transform our understanding of a range of phenomena. However, using a detector array based on CCD or CMOS technologies is fundamentally limited by the sensor's on-chip storage and data transfer speed. To get around this problem, the most practical approach is to use a streak camera. In this ultrafast imaging device, the incident light first passes through a narrow entrance slit (usually 50μm wide) and is imaged onto the photocathode of a streak tube. Here, the incident light is converted to photoelectrons, which are accelerated by an accelerating mesh. A pair of electrodes then applies a sweeping (i.e., time-varying) voltage along the axis perpendicular to the device's entrance slit. Because of this sweeping voltage, electrons arriving at different times are deflected to different spatial positions, and these electrons are then multiplied by a microchannel plate. They subsequently bombard a phosphor screen and are converted back into light. The phosphor screen is imaged to a CCD, which records the image. However, the resultant image is normally 1D: only a single line of the scene can be seen at a time. Acquiring a 2D image requires mechanical scanning across the entire field of view, which poses severe restrictions on the recordable scenes because the event itself must be repetitive.

Previous approaches to enable 2D ultrafast imaging of nonrepetitive events include sequentially timed all-optical mapping photography and parallel streak imaging using a tilted lenslet array.^{1, 2} However, most of them either rely on active illumination or suffer from significant throughput loss. To overcome these limitations, we have developed a new computational ultrafast imaging method, referred to as compressed ultrafast photography (CUP), which can capture 2D dynamic scenes at up to 100 billion frames per second.³ Akin to a conventional photographic camera, CUP is receive-only, thereby allowing high-speed video recording of a variety of luminescent—such as fluorescent or bioluminescent—objects.