Xiang Yin

Ultra-wideband microwave absorber by connecting multiple absorption bands of two different-sized hyperbolic metamaterial waveguide arrays

Microwave absorbers have important applications in various areas including stealth, camouflage, and antenna. Here, we have designed an ultra-broadband light absorber by integrating two different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has wide but different absorption bands due to broadband slow-light response, into a unit cell. Both the numerical and experimental results demonstrate that in such a design strategy, the low absorption bands between high absorption bands with a single-sized tapered HMM waveguide array can be effectively eliminated, resulting in a largely expanded absorption bandwidth ranging from 2.3 to 40 GHz. The presented ultra-broadband light absorber is also insensitive to polarization and robust against incident angle. Our results offer a further step in developing practical artificial electromagnetic absorbers, which will impact a broad range of applications at microwave frequencies.

Ultra-compact polarization beam splitter utilizing a graphene-based asymmetrical directional coupler.

A novel ultra-compact polarization beam splitter (PBS) utilizing an asymmetrical directional coupler with a combination of a silicon waveguide (SW) and a graphene multilayer embedded silicon waveguide (GMESW) has been proposed and investigated. The modal characteristics of the GMESW for the TM mode varies significantly, whereas that for the TE mode changes slightly with respect to the SW, inducing the launched TM mode to directly pass through the SW with little influence from the GMESW, while the TE mode undergoes a strong coupling and is transferred to the GMESW. A designed PBS with an 8.3 μm-long coupler and 200 nm-wide gap separation offers high extinction ratios (18.2 and 21.2 dB) and low insertion losses (0.16 and 0.36 dB) for the thru and cross ports, respectively. The presented PBS also presents the ability to work with variable splitting ratio power for the TM mode by varying the chemical potential of graphene, implying various applications in signal processing on a chip.

Ultrabroad terahertz bandpass filter by hyperbolic metamaterial waveguide.

We propose and demonstrate an ultrabroad terahertz (THz) bandpass filter (BPF) by integrating two different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has wide but different absorption and transmission bands, into a unit cell. With proper structural design of each HMM waveguide to control the absorption and transmission bands, we numerically demonstrate the designed BPF is capable of operating with a broad passband in the THz domain. A typical TM-polarized HMM BPF has a peak transmission of 37% at 3.3 THz with the passband bandwidth of 2.2 THz ranging from 2.97 to 5.17 THz. The co-designed three-dimensional HMM BPF also shows the capability of operating with independence to the polarization of incident light because of the structural symmetry and has sharp bandedge transitions of 22.6 and 17.6 dB/THz to the stop bands, respectively. The presented results here hold great promise for developing practical THz BPF with various applications in THz field.

Ultra-Broadband Super Light Absorber Based on Multi-Sized Tapered Hyperbolic Metamaterial Waveguide Arrays

We propose an ultra-broadband super light absorber by integrating different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has a different and wide absorption band due to broadband slow-light response, into a unit cell. We numerically demonstrate that such an absorber is superior to a single-sized HMM absorber in terms of absorption bandwidth, while maintaining a comparable absorption efficiency. A three different-sized HMM absorber presents the capability of working with an ultra-wide frequency band ranging from 1 to 30 THz, which is much larger than previously proposed absorbers working in the same spectral region. Such a design shows great promise for a broad range of applications such as thermal emitters, photovoltaics, optical-chemical energy harvesting, and stealth technology, where ultra-wideband absorption is in very high demand.

Plasmonic Rainbow Trapping by a Silica–Graphene–Silica on a Sloping Silicon Substrate

We give a proposal for plasmonic rainbow trapping based on a novel structure comprised of a silica-graphene-silica on a sloping silicon substrate, which, importantly, overcomes the intrinsic constraints that are required by metal/dielectric interface. As compared with previous plasmonic grating structures for rainbow trapping, the adiabatic control of the dispersion curve for the present one is achieved by gradually changing the equivalent permittivity of the graphene monolayer via the gap separation between the graphene monolayer and the silicon substrate. We attribute the rainbow trapping effect to the correlative dispersive relation between the slow plasmonic mode and the gap separation between the graphene monolayer and silicon substrate, which leads to the localization of light waves of different frequencies at different positions on the graphene surface. The group velocity can be reduced to be 1000 times smaller than light velocity in air, which is 1-2 smaller than that was previously reported in dielectric gratings-based plasmonic structures.

Ultra-Broadband TE-Pass Polarizer Using a Cascade of Multiple Few-Layer Graphene Embedded Silicon Waveguides

Considering that the variation of the modal characteristics for the TM mode is significantly larger than that for the TE mode when graphene is involved in a silicon-based waveguide, a silicon waveguide TE-pass polarizer with few-layer graphene embedded is proposed and demonstrated. Such a polarizer shows the performance merits including ultrahigh extinction ratio, low insertion loss for the TE mode, ultracompact footprint, and tunable bandwidth by dynamically varying the chemical potential of graphene with gate voltage. We further reveal that the operation bandwidth can be significantly enlarged by cascading multiple polarizers, each of which consists of a silicon waveguide embedded with few-layer graphene and is biased at different gate voltages. We numerically demonstrate that using a cascade of seven polarizers, it is capable of achieving 20-dB extinction ratio and ultralow insertion loss for the TE mode (<;0.13 dB) with bandwidth over 120 nm at telecommunication wavelengths.

Graphene-assisted ultra-compact polarization splitter and rotator with an extended bandwidth

The high refraction-index contrast between silicon and the surrounding cladding makes silicon-on-insulator devices highly polarization-dependent. However, it is greatly desirable for many applications to address the issue of polarization dependence in silicon photonics. Here, a novel ultra-compact polarization splitter and rotator (PSR), constructed with an asymmetrical directional coupler consisting of a rib silicon waveguide and a graphene-embedded rib silicon waveguide (GERSW), on a silicon-on-insulator platform is proposed and investigated. By taking advantage of the large modulation of the effective refractive index of the TE mode for the GERSW by tuning the chemical potential of graphene, the phase matching condition can be well satisfied over a wide spectral band. The presented result demonstrates that for a 7-layer-graphene-embedded PSR with a coupling length of 11.1 μm, a high TM-to-TE conversion efficiency (>−0.5 dB) can be achieved over a broad bandwidth from 1516 to 1602 nm.

Directional beaming of light from a subwavelength metal slit with phase-gradient metasurfaces

In this article, we demonstrate directional beaming of light from a metal nanoslit surrounded with phase-gradient metasurfaces on both sides. Distinct from the grating-based beaming structures, here the momentum mismatch between the surface wave and radiation wave is overcome by the phase-gradient metasurfaces. The deviation angle of the directional beam can be flexibly adjusted by appropriately arranging the phase-gradient of metasurfaces on each side of the nanoslit. The metasurface-based beaming structures also present the ability to operate with high diffraction efficiency and small divergence angle, implying various potential applications in nanophotonics.