Most conventional methods of ultrafast photography utilize evenly spaced intervals in which images are taken, but new developments have shown just how much can be left out of the story, especially when multiple timescales are involved.
Ultrafast events, such as a laser pulse or shock-wave emission, can be captured using ultrafast imaging. Conventional methods of imaging operate on microsecond (millionths of a second) or millisecond (thousandths of a second) timescales and usually have a set, evenly spaced — or mostly evenly spaced — interval in which images are taken. While this can work to capture some characteristics of the ultrafast event, not every important and observable dynamic happens at neatly spaced intervals. To address this, a team led by Associate Professor Keiichi Nakagawa of the Department of Bioengineering at the University of Tokyo developed a method to capture these unseen instances by focusing their limited image captures on the times during which dynamic changes take place, using logarithmic ultrafast photography and band-resolved individual delay generation (BRIDGE).
Logarithmic ultrafast photography is a process that distributes a set number of single-shot frames across a period of time, timed to correspond to moments of characteristic importance of the target occurrence.The team developed this technique to better capture instances otherwise unseen due to their speed of reaction and the shortcomings of conventional techniques.
The concept of logarithmic ultrafast photography is implemented in sequentially timed all-optical mapping photography, or STAMP, a high image-quality and ultrafast operation, using BRIDGE. BRIDGE extracts wavelength bands sequentially and allows for the independent adjustability of these bands while maintaining its ability to translate to a specific instance in time during the dynamic process. Within this process is a finite number of frames that can be placed within the femtosecond (quadrillionths of a second) to nanosecond (billionths of a second) timescales.
With BRIDGE, timing control is accessible across timescales and can enhance what normally occurs too quickly to be observed.
"We wanted to find a way to connect these very different timescales in a single measurement. In other words, we wanted to see both the first instance and what follows, without having to choose one or the other," said Keitaro Shimada, a postdoctoral researcher in the Nakagawa Group and lead author of the study.
By customizing the intervals between frames when taking ultrafast images, researchers are able to slow down key moments that might otherwise be missed if taken at regular intervals, allowing the beginning of an ultrafast event to be captured in addition to what follows immediately afterwards, in one shot. This technique allows for 11 frames to capture the key instances across five orders of magnitude in time, which can be equated to approximately 260 frames-per-second exposure. Conventional single-shot ultrafast imaging most often involves imaging at a uniform interval within a selected window of time, giving most of its credit to a singular moment captured during the process instead of the entire journey.
"We are particularly interested in phenomena that are difficult to reproduce exactly. In those cases, recording the early and later stages of the same event can reveal relationships that might be lost when a sequence is reconstructed from separate experiments," said Nakagawa, a co-author and principal investigator of the study.
The interest of the researchers continues to lie upon learning more about the unknown: short-lived phenomena whose peak moments are elusive, or the untold connection between different phases of the same physical process that have yet to be seen due to the timescale differential. Uses for this technique are vast, though researchers propose most of their use to be in the fields of laser processing, material dynamics, plasma and shock-wave research, and even biomedical applications.
