Brillouin-enhanced four-wave mixing (BE-FWM)—also known as Brillouin dynamic gratings—is an important nonlinear effect in photonics that couples four light waves by traveling acoustic waves. The effect has received much attention in the past few decades, especially for applications in fiber sensing, signal processing, and optical delay lines. Here, we report BE-FWM with optical chiral states (i.e., circular polarization and vortex states) in twisted photonic crystal fiber, by leveraging the topology-selective Brillouin effect. Phase-matching has the consequence that the traveling acoustic gratings created by circularly polarized vortex pump and Stokes in the stimulated Brillouin scattering can be used to modulate a frequency-shifted probe, where the pump/Stokes and probe have different circular polarization or topological charges. Based on our findings, we demonstrate cross-frequency selective information transfer and show that the information is transferred only when pump and probe have opposite circular polarization.

Optical vortex states-higher optical modes with helical phase progression and carrying orbital angular momentum-have been explored to increase the flexibility and capacity of optical fibres employed for example in mode-division-multiplexing, optical trapping and multimode imaging. A common requirement in such systems is high fidelity transfer of signals between different frequency bands and modes, which for vortex modes is not so straightforward. Here we report intervortex conversion between backward-propagating circularly polarised vortex modes at one wavelength, using chiral flexural phonons excited by chiral forward stimulated Brillouin scattering at a different wavelength. The experiment is carried out using chiral photonic crystal fibre, which robustly preserves circular polarisation states. The chiral acoustic wave, which has the geometry of a spinning single-spiral corkscrew, provides the orbital angular momentum necessary to conserve angular momentum between the coupled optical vortex modes. The results open up new opportunities for interband optical frequency conversion and the manipulation of vortex states in both classical and quantum regimes.

Photonic memory is an important building block to delay, route and buffer optical information, for instance in optical interconnects or for recurrent optical signal processing. Photonic-phononic memory based on stimulated Brillouin-Mandelstam scattering (SBS) has been demonstrated as a coherent optical storage approach with broad bandwidth, frequency selectivity and intrinsic nonreciprocity. Here, we experimentally demonstrated the storage of quadrature-phase encoded data at room temperature and at cryogenic temperatures. We store and retrieve the 2-bit states {00,01,10,11} encoded as optical pulses with the phases {0,pi/2,pi,3pi/2} - a quadrature phase shift keying (QPSK) signal. The 2-bit signals are retrieved from the acoustic domain with a global phase rotation of π, which is inherent in the process due to SBS. We also demonstrate full phase control over the retrieved data based on two different handles: by detuning slightly from the SBS resonance, or by changing the storage time in the memory scheme we can cover the full range [0,2pi). At a cryogenic temperature of 3.9 K, we have increased readout efficiency as well as gained access to longer storage times, which results in a detectable signal at 140 ns. All in all, the work sets the cornerstone for optoacoustic memory schemes with phase-encoded data

In physics experiments, mechanical and acoustic vibrations are often considered as disturbing noise and a nuisance. For example, the field of optomechanics came to life because of gravitation waves. To achieve the extreme sensitivity required to detect tiny distortions in spacetime caused by passing gravitational waves, it is crucial to overcome any noisy environments.

Optical neural networks have demonstrated their potential to overcome the computational bottleneck of modern digital electronics. However, their development towards high-performing computing alternatives is hindered by one of the optical neural networks’ key components: the activation function. Most of the reported activation functions rely on opto-electronic conversion, sacrificing the unique advantages of photonics, such as resource-efficient coherent and frequency-multiplexed information encoding. Here, we experimentally demonstrate a photonic nonlinear activation function based on stimulated Brillouin scattering. It is coherent and frequency selective and can be tuned all-optically to take LEAKYRELU, SIGMOID, and QUADRATIC shape. Our design compensates for the insertion loss automatically by providing net gain as high as 20 dB, paving the way for deep optical neural networks.