A new technique for the ultrafast imaging of nonluminous objects has been unveiled by Feng Chen and colleagues at Xi’an Jiaotong University in China. Their system captures up to 60 high-resolution images at a rate of almost 4 trillion frames per second by storing frames in overlapping subregions of a charge-couple device (CCD) array. The technique could soon be used to explore a variety of high-speed physical processes in unprecedented levels of detail.

Today’s fastest cameras use CCDs to capture the motions of molecules at speeds of over a trillion frames per second. This is done by temporarily storing consecutive image frames on separate subregions of the CCD, before moving the frames into longer-term storage. However, only a handful of consecutive frames can be captured in this way because the CCD array will quickly run out of space for new subregions – which cannot normally overlap.

In their study, Chen’s team introduce a technique called compressed ultrafast spectral-temporal (CUST) photography, which allows different subregions of a CCD to overlap. The technology comes in three modules. First, a “spectral-shaping” module selects a narrow band of wavelengths within a light pulse to be used for imaging. The pulse is then lengthened by a “pulse-stretching” module, which uses a series of lenses and diffraction gratings to create a pattern where higher frequencies lead the pulse, while lower frequencies follow behind. This ensures that the frequency of every part of the pulse corresponds with its time of arrival at the sensor – which allows images to be taken in quick succession.

Random patterns

Finally, the beam is broadened perpendicular to its direction of propagation, before hitting the CCD sensor, where specific frequencies are encoded into random binary patterns. These patterns are recorded as a series of compressed 2D images, which can then be stored in overlapping subregions of the CCD array. Later, the pattern associated with each frequency can be extracted from the array, allowing researchers to assemble a time-ordered video of the pulse.

Through CUST photography, Chen’s team could capture as many as 60 images at intervals of just 260 fs and at sub-nanometre resolution. To demonstrate these capabilities, they captured a video of a short, intense light pulse passing through a transparent solid, altering the refractive index of the material to reveal its location over time. They also took a video of a pulse leaving the same material, disappearing and being reflected by an unseen mirror, then reappearing. Finally, the researchers took a rapid series of images of a dye-filled letter A. Since each frame corresponded with a specific frequency, they could assemble a frequency spectrum of the dye.

The technology offers a new and advanced way to precisely record light propagation, reflection, and self-focusing in nonlinear media. The team says that CUST photography will also offer a simple way to measure high-speed physical processes including lattice vibrations, plasma dynamics, and chemical reactions.

CUST is described in Physical Review Letters.