TU Deflt-Terahertz generation from graphite

 (This work was performed by Gopakumar Ramakrishnan)

http://www.tnw.tudelft.nl/en/about-faculty/departments/imaging-physics/research/researchgroups/optics-research-group/research/thz-science-technology/thz-projects/thz-generation-from-graphite/

We illuminated graphite crystals with femtosecond laser pulses (wavelength centered around 800 nm, duration 50 fs) and found that they are capable of emitting THz pulses. This is somewhat unsual since the crystal symmetry of graphite prohibits second-order nonlinear optical processes. The crystals we used to measure the emission of THz pulses after illumination are so-called Highly Oriented Pyrolytic Graphite (HOPG) crystals. Rather than consisting of a single crystal, they consist of many microcrystallites with their c-axis oriented more or less in the same direction. An exaggerated illustration of this is shown below:

The degree of aligment of these crystallites is expresses in terms of the "Mosaic spread". Incidentally, other forms of graphite were also found to emit THz pulses upon illumination with femtosecond laser pulses, including pencil lead. A typical setup used to generate and detect the THz pulses is shown schematically here:

The crystal we used is a so-called rectangular cuboid crystal which we illuminated on all sides. Basically there are two kind of surfaces relevant here: the basal plane surfaces and the edge plane surfaces. The normal to the basal plane is parallel to the c-axis, whereas the normal to the edge plane is perpendicular to the c-axis. In the first experiments we used basal plane illumination like this:

This give rise to the emission of a THz pulses which look like this (left figure):

The figure on the right shows the dependence of the THz field amplitude on the pump power. At low powers, the dependence is clearly quadratic, indicative of a second-order nonlinear optical process. It's important to note that only a small signal is measured when the pump beam is perpendicularly incident on the sample. This is consistent with a picture in which the THz transient is generated by a current perpendicular to the basal plane surface (and thus parallel to the c-axis). To provide further evidence for this, we have also applied an in-plane magnetic field to the sample. If the idea about carrier transport along the c-axis is correct, a change in the direction of the current by the magnetic field should give rise to a change in the emitted THz field. In this case, the change is from "very little emission" to either a positive or a negative-going E-field.

The blue and the red curve on the right show the emitted THz electric field for two magnetic fields of opposite orientation. Clearly, reversing the magnetic field direction changes the polarity of the emitted THz pulse. This is not in contradiction with our earlier idea that after optical excitation, a current pulse along the c-axis is acquires an in-plane component due to the application of the magnetic field.
So then, do we understand everything? Well, not exactly. If we illuminate the edge plane surfaces, we also observe the emission of a THz pulse but in this case we can only speculate where this is coming from. One idea is that the THz pulse is emitted by carriers accelerated in static built-in potentials caused by stacking faults in the crystal, but this is speculation at this point. Finally, we already mentioned that pencil lead emits a THz pulse after illumination with a femtosecond laser pulse. In fact a pencil drawing on paper also emits THz light after illumination with a femtosecond laser pulse. This is illustrated in the drawing below where we plot the THz amplitude measured along a line across two lines drawn on paper: