Martin Luttmann
Light beams, just like massive objects, can carry angular momentum. The spin of photons, as well as their orbital angular momentum (OAM), take integer values, and so does the total optical angular momentum.
In this presentation, we will first see that things get more complicated when both the SAM and the OAM are simultaneously present in a light beam, for instance when polarization and phase both vary in space. Such beams obey certain types of rotational symmetry, and are eigenstates of a “generalized angular momentum” (GAM). Remarkably, the GAM of light can take half-integer values. Here, we report the results of an experiment where intense infrared GAM beam were used to drive high harmonic generation. By implementing novel angular momentum measurement techniques in the extreme-UV, we find experimentally that the harmonic of order q carries a GAM equal to q times that of the driver, while its SAM and OAM are ill-defined. We conclude that, in some situations, the GAM is the “good” quantum number, efficiently describing the nonlinear interaction. Such ultrafast EUV beams with fractional angular momentum could provide original tools to explore new kinds of dichroism in molecules or magnetic systems.
The second part of this talk will focus on ongoing experiments aiming at using the spin or the OAM of terahertz (THz) light to coherently control states of matter. A first experiment drives chiral phonons with resonant, circularly polarized light pulses, and probes the emergence of a real magnetic field around the sample. A second experiment aims at using THz pulses with OAM to control the magnetic flux quantization in superconductors. These experiments may allow us to better understand how artifical gauge fields translate into real magnetic fields, and how, and with what timescale, magnetic flux quantization can be disrupted or re-established optically. Such results would be crucial for the developpmeent of future magnetic memories and superconducting qubits.