Nonlinear optical memory for manipulation of orbital angular momentum of light

Torsion-tunable OAM mode generator based on oxyhydrogen-flame fabricated helical long-period fiber grating

We demonstrate a class of all-fiber torsion-tunable orbital angular momentum (OAM) mode generators based on oxyhydrogen-flame fabricated helical long-period fiber gratings (HLPFGs). The 1-order and 3-order OAM modes are excited based on the HLPFGs inscribed in the single-mode fiber (SMF) and six-mode fiber (6MF), respectively. Theoretical analysis reveals that the twisting can result a resonant wavelength shift of the HLPFG, which means that the OAM modes can also be excited at various wavelength by simply applying a twist rate on the HLPFG. Experiments are carried out to characterize the torsional tunability of the OAM modes, and the results show that the 1-order and 3-order OAM modes can be excited at various wavelength of ∼1564 - 1585 nm and ∼1552 - 1574 nm, respectively, when the torsion angle varied from -360° to 360°, which is consistent with the theoretical analysis. Therefore, the HLPFG can be a candidate for all-fiber wavelength tunable OAM mode generator.

Ultraslow vortex four-wave mixing via multiphoton quantum interference

Orbital angular momentum (OAM) light is nowadays an intriguing resource in classical and quantum optics due to the richness of physical properties it shows in interaction with matter. A key ingredient needed to exploit the full potential of OAM light is the control of quantum interference, a crucial resource in fields like quantum communication and quantum optics. Here, we study the vortex four-wave mixing (FWM) via multi-photon quantum interference in an ultraslow propagation regime. We find that the structured information can be manipulated via two-photon detuning and three photon detuning, which manifests itself as a spatial modulation. The detailed explanations based on the dispersion relation are given, which are in good agreement with our simulations. Furthermore, in order to clearly show the modulated mechanism, we perform the interference between the FWM field and a same-frequency Gaussian beam. It is found that the interference patterns are also manipulated by adjusting the multi-photon detunings. This work may have some potential applications in quantum control based on OAM light.

Optical angular momentum and atoms

Any coherent interaction of light and atoms needs to conserve energy, linear momentum and angular momentum. What happens to an atom's angular momentum if it encounters light that carries orbital angular momentum (OAM)? This is a particularly intriguing question as the angular momentum of atoms is quantized, incorporating the intrinsic spin angular momentum of the individual electrons as well as the OAM associated with their spatial distribution. In addition, a mechanical angular momentum can arise from the rotation of the entire atom, which for very cold atoms is also quantized. Atoms therefore allow us to probe and access the quantum properties of light's OAM, aiding our fundamental understanding of light-matter interactions, and moreover, allowing us to construct OAM-based applications, including quantum memories, frequency converters for shaped light and OAM-based sensors.This article is part of the themed issue 'Optical orbital angular momentum'.

Narrow band amplification of light carrying orbital angular momentum

We report on the amplification of an optical vortex beam carrying orbital angular momentum via induced narrow Raman gain in an ensemble of cold cesium atoms. A 20% single-pass Raman gain of a weak vortex signal field is observed with a spectral width of order of 1 MHz, much smaller than the natural width, demonstrating that the amplification process preserves the phase structure of the vortex beam. The gain is observed in the degenerated two-level system associated with the hyperfine transition 6S_1/2(F = 3) ↔ 6P_3/2(F' = 2) of cesium. Our experimental observations are explained with a simple theoretical model based on a three-level Λ system interacting coherently with the weak Laguerre-Gauss field and a strong coupling field, including an incoherent pumping rate between the two degenerate ground-states.

Arithmetic with optical topological charges in stepwise-excited Rb vapor

We report on experimentally observed addition, subtraction, and cancellation of orbital angular momentum (OAM) in the process of parametric four-wave mixing that results in frequency up- and down-converted emission in Rb vapor. Specific features of OAM transfer from resonant laser fields with different optical topological charges to the spatially and temporally coherent blue light (CBL) have been considered. We have observed the conservation of OAM in nonlinear wave mixing in a wide range of experimental conditions, including a noncollinear geometry of the applied laser beams, and furthermore, that the CBL accumulates the total OAM of the applied laser light. Spectral and power dependences of vortex and plane wavefront blue light beams have been compared.