A silicon detector is like photographic film, but instead of light, it senses charged particles. When a particle flies through the silicon, it knocks out electrons, and we see a signal. But how do you look inside the detector itself and see where the electric field is strong and where it's weak? One way is to use a laser. A short laser pulse is focused to a tiny spot inside the silicon, creating an electron-hole pair. This process is called two-photon absorption: two laser photons hit the same spot simultaneously and combine their energy, like two weak hammer blows adding up to one strong one.

The problem is that the generated charges don't stay put. The electric field inside the detector pulls them apart, and the signal spreads across several neighboring pixels of the chip. Usually, researchers picked the brightest pixel or the earliest arrival time, but that led to systematic errors: the picture shifted, as if the photo were taken with a shaky hand. The authors of the new work took a different approach. They combine several triggered pixels into one event if they overlap, and determine the event time from the pixel with the largest charge. This method allows reconstructing a precise 3D grid of measurements (voxels, like cubes in a computer game) even when the laser and detector run without a shared clock.

The method was tested on a real Timepix3 chip, which records each charge arrival with sub-nanosecond precision. It turned out that the new algorithm reconstructs the position and time of events more accurately than older methods. This matters not only for particle physics but also for medical detectors, where silicon sensors help see X-rays or monitor radiation dose. If we can see the electric field inside the sensor, we can better calibrate it and make images cleaner.