Modern-day lasers can create extremely short light pulses, which can be utilized for various applications, from researching materials to therapeutic diagnostics. For this reason, it is essential to gauge the shape of the laser light wave with high accuracy.
This usually requires a large, complex experimental setup. But, not anymore! Now, this can be done using a tiny crystal with a diameter of less than one millimeter.
Prof. Joachim Burgdörfer from the Institute of Theoretical Physics at the TU Wien said, “Extremely short light pulses with a duration in the order of femtoseconds (10-15 seconds) were investigated. In order to create an image of such light waves, they must be made to interact with electrons. The reaction of the electrons to the electric field of the laser gives us very precise information about the shape of the light pulse.”
Measuring infrared laser pulse requires the addition of a much shorter laser pulse with a wavelength in the X-ray range. Both pulses are sent through a gas. The X-ray pulse ionizes individual atoms, electrons are released, which are then accelerated by the electric field of the infrared laser pulse. The motion of the electrons is recorded, and if the experiment is carried out many times with different time shifts between the two pulses, the shape of the infrared laser pulse can eventually be reconstructed.
Prof. Christoph Lemell (TU Vienna) said, “The experimental effort required for this method is very high. A complicated experimental setup is needed, with vacuum systems, many optical elements, and detectors.”
In order to bypass such limitations, scientists came with an idea of measuring light pulses not in gas but in a solid.
Isabella Floss (TU Vienna) said, “In a gas, you have to ionize atoms first to get free electrons. In a solid, it is sufficient to give the electrons enough energy so that they can move through the solid, driven by the laser field. This generates an electric current which can be directly measured.”
Scientists used tiny crystals of silicon oxide with several hundred micrometers for this purpose. They are hit by two different laser pulses: The pulse, which is to be investigated, can have any wavelength ranging from ultraviolet light and visible colors to long-wave infrared. While this laser pulse penetrates the crystal, another infrared pulse is fired at the target.
Joachim Burgdörfer said, “This second pulse is so strong that non-linear effects in the material can change the energy state of the electrons so that they become mobile. This happens at a very specific point in time, which can be tuned and controlled very precisely.”
As soon as the electrons can move through the crystal, they are accelerated by the electric field of the first beam. This produces an electric current, which is measured directly at the crystal. This signal contains precise information about the shape of the light pulse.
Scientists studied the effect theoretically and analyzed in computer simulations.
Christoph Lemell said, “Thanks to the close cooperation between theory and experiment, we have been able to show that the new method works very well, over a large frequency range, from ultraviolet to infrared.”
“The waveform of light pulses can now be measured much more easily than before, with the help of such a much simpler and more compact setup.”
The new method opens up many interesting applications: It should be possible to characterize novel materials precisely, to answer fundamental physical questions about the interaction of light and matter, and even to analyze complex molecules—for example, to reliably and quickly detect diseases by examining tiny blood samples.
The study is published in the journal Nature Communications.