Junjie Du

Experimental demonstration of in-plane negative-angle refraction with an array of silicon nanoposts.

Controlling an optical beam is fundamental in optics. Recently, unique manipulation of optical wavefronts has been successfully demonstrated by metasurfaces. However, these artificially engineered nanostructures have thus far been limited to operate on light beams propagating out-of-plane. The in-plane operation is critical for on-chip photonic applications. Here, we demonstrate an anomalous negative-angle refraction of a light beam propagating along the plane, by designing a thin dielectric array of silicon nanoposts. The circularly polarized dipoles induced by the high-permittivity nanoposts at the scattering resonance significantly shape the wavefront of the light beam and bend it anomalously. The unique capability of a thin line of the nanoposts for manipulating in-plane wavefronts makes the device extremely compact. The low loss all-dielectric structure is compatible with complementary metal-oxide semiconductor technologies, offering an effective solution for in-plane beam steering and routing for on-chip photonics.

Novel silicon-on-insulator grating couplers based on CMOS poly-silicon gate layer

Grating couplers are widely investigated as a coupling interface between silicon-on-insulator waveguides and optical fibers. In this work, novel grating couplers based on strip poly-Si are proposed. This structure utilizes the poly-Si gate layer of the CMOS MOSFETs, and thus enables grating couplers integrated with CMOS circuits without adding any additional masks and process steps. Simulation results show that a coupling efficiency over 60% can be achieved between silicon-on-insulator waveguides and fibers.

Tailoring Optical Gradient Force and Optical Scattering and Absorption Force

The introduction of the concept of gradient force and scattering and absorption force is an important milestone in optical trapping. However the profiles of these forces are usually unknown, even for standard setups. Here, we successfully calculated them analytically via multipole expansion and numerically via Mie theory and fast Fourier transform. The former provides physical insight, while the latter is highly accurate and efficient. A recipe to create truly conservative energy landscapes is presented. These may open up qualitatively new features in optical manipulation.

Calculating gradient force and scattering force in optical tweezers using fourier transform

Division of optical force into gradient force and scattering force is a decades long problem. The concepts of “gradient force” and “scattering force” are very commonly applied in the interpretation of optical trapping simulation and experiment, but no one has seen their true face except for a few special cases. Such a division is important because the gradient force is responsible for optical trapping, while scattering force is responsible for particle transportation. All previous studies either has unjustified approximations or consider particle small (dipole) or large (geometrical optics) compare to the wavelength of light. Here, we present a rigorous, accurate, and efficient method to separate the gradient force and the scattering force using Fourier Transformation. This approach can, in principles, handle spherical particles of arbitrary size and composition, as long as the total optical force can be computed by, for example, Mie scattering theory plus Maxwell stress tensor. Particle of different sizes and compositions trapped by an optical tweezers are considered, which includes dipolar/Mie sized particle made up of dielectric or metal. Not surprisingly, the numerical aperture and aberration also play an important role. We fully tested the accuracy and stability of this approach: it is sufficiently accurate as long as a sufficiently large unit cell is employed such that the optical force in the cell boundary can be treated as zero.

Broadband In-Plane Light Bending With a Doublet Silicon Nanopost Array

The in-plane tailoring of light propagation is significant in on-chip optical interconnections. Recently, an in-plane negative-angle refraction was realized with a thin line of silicon nanoposts, which are at resonance in the first angular momentum channel. This advancement is different from metasurface research, which mostly focuses on out-of-plane operations. In this paper, we experimentally demonstrate that a thin array of doublet silicon nanoposts, in which each unit comprises two tangent nanoposts functioning as an upright interface, remarkably improve efficiency for molding light. The designed upright interface exhibits a broadband response featuring high efficiency in a negative-angle light bending in the wavelength of 1480-1600 nm. The broadband, compactness, low loss, and complementary metal-oxide semiconductor (CMOS) compatibility enable the use of the subwavelength array as an alternative component for on-chip optical control.

Steering and tuning of on-chip optical beams

We report on-chip structures based on ordered dielectric nano-particle chain which can be used to steer and tune optical beams. With different elaborately designed structures, the optical beam can transmit in a negative direction, or totally reflected beyond the normal incidence with a subwavelength nanorod chain. The mechanism of the phenomenon is believed to be due to the symmetry of resonant modes in the dielectric nanoparticles. With the low-loss feature and the ultra-compact characteristic, this structure may find applications in photonic circuits.