Adventures In Time And Space 10: Light In Slow Motion

A research team at the Massachusetts Institute of Technology created an imaging system that allows them to capture light at fast enough speeds to show it traveling in slow motion down the length of a one-liter soda bottle and reflecting back again. The MIT Media Lab’s Camera Culture Group, led by Project Director Ramesh Raskar, collaborated with the lab of Moungi Bawendi of the MIT Department of Chemistry, since fast chemical reactions also occur within a similar timescale, that of femtoseconds (one quadrillionth of a second), where atoms within reactant molecules are rearranging themselves to form new product molecules. The team’s system collects visual data at a rate of half a trillion exposures per second.

The new system is called Femto Photography and it consists of illumination bursts from a titanium sapphire laser lasting femtoseconds (1 x 10-15 of a second), image captures from detectors lasting picoseconds (1 x 10-12 of a second), and mathematical reconstruction techniques to put it all together.

The exposure time of each image frame is 2 picoseconds, or two trillionths (2 x 10-12) of a second, and the resultant video shows the movement of light at roughly half a trillion frames per second. Since it is nearly impossible to capture images at such a fast frame rate, the system uses a stroboscopic method where a laser pulse lasting less than 1 picosecond or one trillionth (1 x 10-12) of a second is used as flash. The returning light is then collected by a camera that records half a trillion frames per second.

However, due to the very short exposure times of 2 picoseconds, millions of repeated measurements need to be collected over several minutes and then rearranged using a reconstruction algorithm to create a video of the event lasting a nanosecond, or one billionth (1 x 10-9) of a second. In a nanosecond, light travels about 30 centimeters or 12 inches, about the length of a one-liter soda bottle. For comparison, the blink of a human eye is about 0.4 second or 400 milliseconds, 400 millionths (400 x 10-3) of a second. Thus, an eye blink is nine orders of magnitude slower than what the imaging system can capture on video.

Beyond educational and artistic purposes, the MIT team hopes to use the new imaging system for research into understanding ultrafast processes, into analyzing industrial faults and material properties, and as a type of medical “ultrasound with light.”