When you want to measure something that happens incredibly fast, your measurement tool needs to be even faster – measuring a 100-meter dash with an hour glass isn’t going to give you the most accurate result. In the QMO Lab, we study how electrons move, which means most of the interesting stuff is happening over the course of just trillionths of a second. To measure these ultrafast processes, we use 200 femtosecond-long pulses of light to excite electrons. We do ‘pump-probe’ measurements, so-called because one initial pulse ‘pumps’ the system with energy and then, after a short, controlled time delay, a second pulse hits the system to ‘probe’ how it has changed. Measuring with different time delays lets us observe how fast electrons return to equilibrium after they’ve been pulsed, how quickly they can move between two points, and even how long they can live in excited states.
The core idea of pump-probe measurements is to split a pulsed beam in two (pictured in Figure 1 on the left). One beam travels a fixed length before arriving at the sample, while the other travels through a path with a delay stage. By using this moveable delay stage, we can control the time between when the pulses arrive with femtosecond precision. The delay and reference beams can then be recombined to carry out measurements where one beam scans in space around the other, where both beams scan together in space, or even measurements where both beams scan independently!
As mentioned earlier, it’s important to keep laser pulses as short as possible since they set the minimum time scale of a pump-probe measurement. Every time a pulsed beam encounters the surface of a mirror or lens, it broadens slightly, thus reducing the time(temporal) resolution of our measurement. Therefore, reducing the number of optics between the output of the laser and the sample is critical to getting the best measurements possible. A typical objective lens can have dozens of components which can lead to significant laser pulse broadening. To get around this problem, we use something called a graded-index (GRIN) lens. A GRIN lens is a single piece of glass made with an index of refraction that changes radially (a ray diagram is shown in Figure 2). Optically, a GRIN lens behaves just like a standard objective, but with only a single component, keeping the pulses as narrow as possible!
Figure 2: Ray diagram of a GRIN lens. (image from Thorlabs)