Abstract
Light with spatiotemporal orbital angular momentum (ST-OAM) is a recently discovered type of structured and localized electromagnetic field. This field carries characteristic space-time spiral phase structure and transverse intrinsic OAM. In this work, we present the generation and characterization of the second-harmonic of ST-OAM pulses. We uncovered the conservation of transverse OAM in a second-harmonic generation process, where the space-time topological charge of the fundamental field is doubled along with the optical frequency. Our experiment thus suggests a general ST-OAM nonlinear scaling rule - analogous to that in conventional OAM of light. Furthermore, we observe that the topology of a second-harmonic ST-OAM pulse can be modified by complex spatiotemporal astigmatism, giving rise to multiple phase singularities separated in space and time. Our study opens a new route for nonlinear conversion and scaling of light carrying ST-OAM with the potential for driving other secondary ST-OAM sources of electromagnetic fields and beyond.
Highlights
The orbital angular momentum (OAM) of light is a type of angular momentum associated with phase front vortices in the electromagnetic field[1,2]
We experimentally investigate the behavior of spatiotemporal orbital angular momentum (ST-OAM) pulses during the frequency up-conversion process of second-harmonic generation (SHG)
We investigate the effects of spatiotemporal astigmatism in SHG, which leads to non-conserved topological changes of the spiral phase structure, and the creation of multiple phase singularities separated in space-time
Summary
The orbital angular momentum (OAM) of light is a type of angular momentum associated with phase front vortices in the electromagnetic field[1,2]. For a propagating paraxial wave, the longitudinal OAM of light means that the OAM is parallel to the averaged wavevector and propagation direction of the beam. OAM can be intrinsic or extrinsic –-intrinsic OAM implies that the angular momentum is independent of the choice of the reference frame and can be described by an integer quantum number called the (spatial) topological charge l. The intrinsic longitudinal OAM (here referred to as conventional OAM) of light has an OAM of ħl per photon and a spiral phase structure e l , , surrounding a phase singularity in the x-y plane (see Fig. 1a). Most research efforts over the past three decades have focused on conventional OAM of light, which has impacted many important applications –– including optical tweezers, super resolution imaging, quantum and classical communication, and others3–5 ,9. Pulses with timevarying OAM, or self-torque, have been discovered[18]
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