A theater spotlight keeps the actor in its beam no matter where they step. The terahertz radar we are about to describe does the same with a short light pulse reflected off a moving object.

Terahertz waves sit between microwaves and infrared light. They pass through clothing and plastic but do not damage tissue, so they are being explored for security screening and medical diagnostics. The classic method measures the shape of a short pulse reflected from a sample. The trouble is that any shift of the sample moves the pulse in time, and it jumps out of the narrow observation window. Previously, that meant one thing: the target had to stay still.

The Japanese physicists built their setup from two terahertz lasers. The pulse repetition frequencies are tightly linked, but one laser can be phase-shifted to move the time window. Instead of slowly scanning the whole range, the system rapidly oscillates the delay within a narrow window of 61 picoseconds and makes up to 154 full measurements per second. The position of the pulse peak within the window is measured in real time. If the peak shifts because of sample motion, a feedback loop adjusts the phase of the second laser and brings the peak back to the center.

In this way, they managed to keep the pulse in the window while a mirror moved at a speed of 4 centimeters per second over a distance of 12 centimeters. That is 26 times the width of the window itself. The precision of determining the peak position was 10 femtoseconds per measurement, and with averaging over 0.44 seconds it improved to 1.5 femtoseconds, corresponding to a displacement of 225 nanometers. The maximum tracking speed was estimated at 10.8 centimeters per second.

In a model experiment, a gold mirror on a vibration motor mimicked breathing and heartbeat: the oscillation amplitudes were 2.1 millimeters and 0.21 millimeters, and the frequencies were 0.25 and 1.2 hertz. The system recovered these weak motions against a background of linear movement up to 10 centimeters. The spectrum of the recovered signal clearly showed both frequencies.

Previously, terahertz spectroscopy required a stationary sample, otherwise the pulse would jump out of the measurement window. Now the window itself follows a target that shifts by tens of centimeters, with precision down to fractions of a micron. This was achieved with a pair of lasers with a programmable frequency comb: one rapidly scans a narrow window, and the second, driven by a feedback signal, shifts that window to keep the peak centered.

The spotlight that turns on its own to follow the actor now works in the world of terahertz waves as well. The next step is to speed up the turning and learn to track an object moving in a plane. Then contactless monitoring of breathing and heartbeat in patients, or quality control on an assembly line, will become routine practice.