When someone says, I am telling you a love story, you may feel, here we are, a cliché.

But what if that story happens in one trillionth of a second, that is where science comes into play.

Who are the bachelor and bachelorette?

They are electrons in negative and holes in positive. The Coulomb law makes them attractive.

But when there are too many photoexcited electrons and holes,

it just like a speed date in a hot room with overcrowded people,

The attraction between individuals gets screened and becomes weak.

High density of electrons and holes form a hot messy unbound state called plasma.

To find the true love before it is too late, the density of electron-hole must decay.

With fewer of them, you can almost hear the increase of their heart rate.

They start to form electron-hole pairs as a bound state.

With exciton as their new household name.

From unbound singles to bound pairs, one trillionth of a second,

is how long this transition believed to take in solid state.

You just heard I said “believed”. In fact, this transition I just described is a famous hypothesis in physics that still waits for more verifications.

As an experimental physicist at Berkeley Lab, I directly visualize and study the transition by making experimental movies.

But it happens so fast, how do I achieve my goal?

That is where ultrafast science can play the role.

I use two ultrashort laser pulses, one is called pump, and the other one is called probe.

The pump first comes to excite electrons and holes in a single-layer semiconductor.

The excitation is fairly strong, forming the plasma that is ready to go on.

The probe follows in a short time, to kick-off the electrons into a detection line,

so that I capture a picture of the electrons experiencing the transition at one instant moment.

I can further control this time with a precision of one thousand times shorter than a trillionth of a second.

By scanning the time step by step, I make a movie of the transition frame by frame.

My research results resolved a fundamental missing puzzle for fully understanding photoexcited semiconductors.

But for myself, the ultimate reward is to see, especially in the same journey with you, how compelling and romantic the nature of physics really looks like.