Abstract
We have built an imaging solution that allows us to
visualize propagation of light. The effective exposure time of each frame is
two trillionths of a second and the resultant visualization depicts the
movement of light at roughly half a trillion frames per second. Direct
recording of reflected or scattered light at such a frame rate with sufficient
brightness is nearly impossible. We use an indirect 'stroboscopic' method that
records millions of repeated measurements by careful scanning in time and
viewpoints. Then we rearrange the data to create a 'movie' of a nanosecond long
event.
The device has been developed by the MIT Media Lab’s Camera
Culture group in collaboration with Bawendi Lab in the Department of Chemistry
at MIT. A laser pulse that lasts less than one trillionth of a second is used
as a flash and the light returning from the scene is collected by a camera at a
rate equivalent to roughly half a trillion frames per second. However, due to
very short exposure times (roughly two trillionth of a second) and a narrow
field of view of the camera, the video is captured over several minutes by
repeated and periodic sampling.
The new technique, which we call Femto Photography, consists
of femtosecond laser illumination, picosecond-accurate detectors and
mathematical reconstruction techniques. Our light source is a Titanium Sapphire
laser that emits pulses at regular intervals every ~13 nanoseconds. These
pulses illuminate the scene, and also trigger our picosecond accurate streak
tube which captures the light returned from the scene. The streak camera has a
reasonable field of view in horizontal direction but very narrow (roughly
equivalent to one scan line) in vertical dimension. At every recording, we can
only record a '1D movie' of this narrow field of view. In the movie, we record
roughly 480 frames and each frame has a
roughly 1.71 picosecond exposure time. Through a system of mirrors, we orient
the view of the camera towards different parts of the object and capture a
movie for each view. We maintain a fixed delay between the laser pulse and our
movie starttime. Finally, our algorithm uses this captured data to compose a
single 2D movie of roughly 480 frames each with an effective exposure time of
1.71 picoseconds.