Detecting Phase and Polarization Singularities

Schematic of the designed structure under illumination of two OAM beams with different polarization states and topological charges. (Source: F. Feng et al.)

Optical singu­larities are key elements in modern optics and have been widely researched. In particular, phase and polari­zation singu­larities have been manipulated in various appli­cations, such as imaging and metrology, nonlinear optics, optical tweezers, sensing, quantum infor­mation, and optical communi­cation. In theory, both singu­larities can be detected simultaneously if one can detect the topo­logical charge and photon spin at the same time.

Several methods have been proposed to detect the topo­logical charge of the OAM in recent years, including holography, metasurfaces, optical trans­formation, and photonic circuits. However, these methods have drawbacks including the need to align the beam precisely with the structure, the need for complex detection processes, such as near-field micro­scopy, and the low diffraction effi­ciencies of some elements. These drawbacks strongly limit their appli­cations in new optical systems with optical fibres or integrated on-chip devices. Now, a team of scientists, led by Changjun Min, Xiaocong Yuan, Mike Somekh from Nano­photonics Research Center at the Shenzhen University and co-workers have developed an on-chip plasmonic spin-Hall nano­grating for simul­taneously detecting phase and polari­zation singu­larities.

They have designed a symmetry-breaking nano­grating structure first to unidirec­tionally launch the SPP wave according to the sign of the topological charge of the incident wave. The propa­gation angle of the generated SPP increases with the value of the topo­logical charge. The topological charge value of the incident beam can be accurately deter­mined by placing an output coupling grating on both sides of the nano­grating to couple the generated SPP wave to the far field and analysing the far-field optical micro­scopy image. Addi­tionally, a spin-Hall structure is integrated onto the nanograting so that the nano­grating can respond to the spin of the incident beam.

This combined structure direc­tionally couples the incident OAM beam to different positions depending on the polari­zation and topo­logical charge of the beam. It is proved experi­mentally that the structure detects the polari­zation singu­larity and phase singu­larity of the incident CVB beam simul­taneously. This device is very promising for achieving a highly compact photonic integrated circuit.

“We design a SPP based meta-surface which can detect simul­taneously phase and polari­zation singu­larities of the incident wave for two purposes in one: first, to rapidly and simul­taneously detect the phase and polari­zation singu­larities with single shot image; second, to enable optical communi­cation with photonics singu­larities of electro­magnetic waves,” the researchers explain. “This device is very promising for achieving a highly compact photonic integrated circuit. It has shown great potential in large-scale photonic inte­grated circuits and would benefit diverse appli­cations such as optical on-chip information processing and optical communi­cations. We are now trying to intergrate an addi­tional coupound phase modu­lation structure onto the device to cancel the diffrac­tion effect of the SPP wave during generation. This would further enhance the reso­lution and detection limit of the system,” they added.

The presented technique can be used for new generation of optical communi­cation. As photonic singu­larities are new degree of freedom which can carry far more information compare to what the intensity modu­lation, frequency modulation we use now. The tiny volum of the device and its capacity of information processing will open a new gate to modern communi­cation in both classic regime and quantum regime. (Source: CAS)

Reference: F. Feng et al.: On-chip plasmonic spin-Hall nanograting for simultaneously detecting phase and polarization singularities, Light Sci. Appl. 9, 95 (2020); DOI: 10.1038/s41377-020-0330-z

Link: Nanophotonics Research Center, Shenzhen Key Laboratory of Micro-Scale Optical Information Technology, Shenzhen University, Shenzhen, China

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