Yuanhui Wen

Winding light beams along elliptical helical trajectories

Conventional caustic methods in real or Fourier space produced accelerating optical beams only with convex trajectories. We developed a superposition caustic method capable of winding light beams along nonconvex trajectories. We ascertain this method by constructing a one-dimensional (1D) accelerating beam moving along a sinusoidal trajectory, and subsequently extending to two-dimensional (2D) accelerating beams along arbitrarily elliptical helical trajectories. We experimentally implemented the method with a compact and robust integrated optics approach by fabricating micro-optical structures on quartz glass plates to perform the spatial phase and amplitude modulation to the incident light, generating beam trajectories highly consistent with prediction. The theoretical and implementation methods can in principle be extended to the construction of accelerating beams with a wide variety of nonconvex trajectories, thereby opening up a route of manipulating light beams for fundamental research and practical applications.

Design and optimization of optical modulators based on graphene-on-silicon nitride microring resonators

In order to overcome the challenge of obtaining high modulation depth due to weak graphene–light interaction, a graphene-on-silicon nitride (SiNx) microring resonator based on graphenes gate-tunable optical conductivity is proposed and studied. Geometrical parameters of graphene-on-SiNx waveguide are systematically analyzed and optimized, yielding a loss tunability of 0.04 dB μm−1 and an effective index variation of 0.0022. We explicitly study the interaction between graphene and a 40 μm-radius microring resonator, where electro-absorptive and electro-refractive modulation are both taken into account. By choosing appropriate graphene coverage and coupling coefficient, a high modulation depth of over 40 dB with large fabrication tolerance is obtained.

Tailoring accelerating beams in phase space

An appropriate wave-front design will enable light fields that propagate along arbitrary trajectories, thus forming accelerating beams in free space. Previous strategies for designing such accelerating beams rely mainly on caustic methods, which start from diffraction integrals and deal only with two-dimensional fields. Here we introduce an alternate perspective to construct accelerating beams in phase space by designing the corresponding Wigner distribution function (WDF). We find that such a WDF-based method is capable of providing both the initial field distribution and the angular spectrum in need by projecting the WDF into the real space and the Fourier space, respectively. Moreover, this approach applies to the construction of both two- and three-dimensional fields, greatly generalizing previous caustic methods. It may therefore open a new route for construction of highly tailored accelerating beams and facilitate applications ranging from particle manipulation and trapping to optical routing as well as material processing.

Cascaded metasurface structures

Metasurfaces are well known as a single-layer subwavelength structure with superior capability in wavefront shaping, and cascaded metasurfaces with multi-layer structures resulting in compact optical devices will further improve the performance and also bring in new functionalities, such as image correction, optical transformation and processing. However, in order to realize such cascaded metasurfaces, a key issue is to achieve high precision alignment of the structures in each layer usually with large distance in between. Here we propose and experimentally confirm two schemes based on optical alignment and electron-beam alignment respectively to align the frontside and backside of a wafer in order to realize cascaded metasurfaces on these two sides. Except for cascaded metasurface structures, these approaches may also have a wide range of applications in need of other cascaded structures for specific functionality.

Highly adjustable helical beam: design and propagation characteristics

Light fields with extraordinary propagation behaviours such as nondiffracting and self-bending are useful in optical delivery for energy, information, and even objects. A kind of helical beams is constructed here based on the caustic method. With appropriate design, the main lobe of these helical beams can be both well-confined and almost nondiffracting while moving along a helix with its radius, period, the number of rotations and main lobes highly adjustable. In addition, the main lobe contains almost half of the optical power and the peak intensity fluctuates below 15% during propagation. These promising characteristics may enable a variety of potential applications based on these beams.

Generation of Accelerating Light Beams Based on Integrated Photonic Approaches

Previous methods for the generation of accelerating beams are mostly based on bulky phase-only spatial light modulator. Here we propose an experimental scheme for the generation of accelerating beams by fabricating a quartz plate capable of realizing both phase and amplitude modulation. The phase modulation is implemented by aligned 16-level etching into the quartz plate, while the amplitude modulation is realized by controlling the duty ratio of the partial metal coverage locally. Based on this approach, a sinusoidal accelerating beam with multiple bending is generated for demonstration and propagates along a sinusoidal trajectory as expected.