Ultra-compact electro-optic phase modulator based on a lithium niobate topological slow light waveguide.

Electro-optic modulators (EOMs) are essential devices of optical communications and quantum computing systems. In particular, ultra-compact EOMs are necessary for highly integrated photonic chips. Thin film lithium niobate materials are a promising platform for designing highly efficient EOMs. However, EOMs based on conventional waveguide structures are at a millimeter scale and challenging to scale down further, greatly hindering the capability of on-chip integration. Here, we design an EOM based on lithium niobate valley photonic crystal (VPC) structures for the first time. Due to the high effective refractive index introduced by the strong slow light effect, the EOM can achieve an ultra-compact size of 4 µm×14 µm with a half-wave voltage of 1.4 V. The EOM has a high transmittance of 0.87 in the 1068 nm because of the unique spin-valley locking effect in VPC structures. The design is fully compatible with current nanofabrication technology and immune to fabrication defects. Therefore, it opens a new possibility in designing lithium niobate electro-optic modulators and will find broad applications in optical communication and quantum photonic devices.

Thermal tunable silicon valley photonic crystal ring resonators at the telecommunication wavelength.

Tunable ring resonators are essential devices in integrated circuits. Compared to conventional ring resonators, valley photonic crystal (VPC) ring resonators have a compact design and high quality factor (Q-factor), attracting broad attention. However, tunable VPC ring resonators haven't been demonstrated. Here we theoretically demonstrate the first tunable VPC ring resonator in the telecommunication wavelength region, the resonance peaks of which are tuned by controlling the temperature based on the thermal-optic effect of silicon. The design is ultracompact (12.05 µm by 10.44 µm), with a high Q-factor of 1281.00. By tuning the temperature from 100 K to 750 K, the phase modulation can reach 7.70 π, and the adjustment efficiency is 0.062 nm/K. Since thermal tuning has been broadly applied in silicon photonics, our design can be readily applied in integrated photonic circuits and will find broad applications. Furthermore, our work opens new possibilities and deepens the understanding of designing novel tunable VPC photonic devices.

Design of wavelength division multiplexing devices based on tunable edge states of valley photonic crystals.

Wavelength division multiplexing (WDM) devices are key photonic integrated circuit (PIC) elements. Conventional WDM devices based on silicon waveguides and photonic crystals have limited transmittance due to the high loss introduced by the strong backward scattering from defects. In addition, it is challenging to reduce the footprint of those devices. Here we theoretically demonstrate a WDM device in the telecommunication range based on all-dielectric silicon topological valley photonic crystal (VPC) structures. We tune its effective refractive index by tuning the physical parameters of the lattice in the silicon substrate, which can continuously tune the operating wavelength range of the topological edge states, which allows the designing of WDM devices with different channels. The WDM device has two channels (1475 nm-1530 nm and 1583 nm-1637 nm), with contrast ratios of 29.6 dB and 35.3 dB, respectively. We demonstrated highly efficient devices for multiplexing and demultiplexing in a WDM system. The principle of manipulating the working bandwidth of the topological edge states can be generally applied in designing different integratable photonic devices. Thus, it will find broad applications.

Unidirectional transmission of visible region topological edge states in hexagonal boron nitride valley photonic crystals.

Here we theoretically design valley photonic crystals (VPCs) based on two-dimensional (2D) hexagonal boron nitride (hBN) materials, which are able to support topological edge states in the visible region. The edge states can achieve spin-dependent unidirectional transmission with a high forward transmittance up to 0.96 and a transmission contrast of 0.99. We further study the effect of refractive index on transmittance and bandwidth, and it is found that with the increase of refractive index, both transmittance and bandwidth increased accordingly. This study opens new possibilities in designing unidirectional transmission devices in the visible region and will find broad applications.

Filtering characteristics of 1D photonic crystal with Gaussian film thickness perturbation.

Conventional theoretical and numerical studies on photonic crystal, which often does not consider the film thickness error during the experimental preparation process, will meet a large deviation between the experiment and the simulation. The filtering characteristics of one-dimensional (1D) photonic crystals with random film thickness errors (modeled as the Gaussian distribution) are systematically investigated by statistical method and numerical simulations. By studying the influence of the deviation of film thickness and the period number on the filter characteristics, it shows that the forbidden bandwidth is reduced to 80.27% of the intrinsic energy band when the film thickness deviation is σ=0.25a. Furthermore, we found that introduction of a slight disturbance of the film thickness (σ=0.01a) to photonic crystal will broaden the forbidden bandwidth to 100.49%. The proposed photonic crystal model with film thickness deviation can reduce the error between experiment and theory, which can be used for designing broadband photonic bandgaps. These structures have potential applications such as light-matter interactions, ultra-small filters, and photonic chips.

Integrated nanophotonic optical diodes designed by genetic algorithms.

Integrable nanophotodiode devices have attracted much research interest in recent years because of their potential applications in all-optical computing and optical communication systems. We propose a new optical diode design scheme. We use genetic algorithms (GAs) to design an optical diode, which has a device footprint of only 2.5×2.5µm2. These devices designed by GA have the ability to achieve high-efficiency unidirectional transmission. Simulations show the forward transmission efficiency can reach higher than 65% for a Gaussian beam between the wavelengths of 1400 and 1600 nm, and the peak transmission efficiency reaches 75%. The transmission contrast at the design wavelength between 1500 and 1600 nm is higher than 90%, which meets the requirements of high unidirectionality, wide operational bandwidth, and small scale. The devices have more advantages for optical diodes compared with structures designed by photonic crystals and gratings. The application of this scheme provides a new idea for the design and research of all-optical diodes in the field of optical communication.

Asymmetric universal invisible gateway.

Previous invisible gateways are mainly based on super-scattering effect, which can only work for the perfect electric conductor (PEC) wall, while no further exploration is conducted for the walls made of other materials (i.e., the actual wall is not PEC). In this study, we design an asymmetric universal invisible gateway by transformation optics, which is versatile for applying arbitrary materials as wall materials. In addition, its unique asymmetric structure leads to the difference of the detection results when the relative position of the detection source and the invisible gateway changes: one side can only see a complete wall (no gateway) and the other side can detect the gateway in the middle of the wall. This research advances a new step for the specific application of invisible gateway.

Switchable broadband metamaterial absorber/reflector based on vanadium dioxide rings.

A new broadband tunable metamaterial absorber based on different radii of vanadium dioxide (VO2) rings loaded on the dielectric layer is designed. According to the insulator-to-metal phase transition characteristics of VO2 under thermal excitation, the dynamic adjustment of the absorption by the external temperature is achieved. The simulation results demonstrate that when VO2 is in its metal phase at high temperature, an absorption greater than 90% in the bandwidth range of 2.64-7 THz can be obtained and its relative bandwidth is reached to 90.5%. However, the absorption rate in the same frequency range is always lower than 2.3% when VO2 is in the insulator phase at low temperature, which means that the absorber can be used as a perfect reflector. The maximum tunable range of the proposed absorber can be realized from below 2.3% to nearly 100%. We further analyze and discuss the equivalent impedance and electric field distribution of the absorber and clarify the adjustment mechanism of the absorption performance of the VO2 ring. In addition, a multireflection interference theory is also investigated to quantitatively explain the physical absorption mechanism. Such a tunable broadband absorber based on temperature control has great potential to be applied to sensors, thermophotovoltaics, and wireless communication.

Asymmetric transmission of light waves in a photonic crystal waveguide heterostructure with complete bandgaps.

Here, we theoretically present an on-chip nanophotonic asymmetric transmission device (ATD) based on the photonic crystal (PhC) waveguide structure with complete photonic bandgaps (CPBGs). The ATD comprises two-dimensional silica and germanium PhCs with CPBGs, within which line defects are introduced to create highly efficient waveguides to achieve high forward transmittance. In the meantime, the total internal reflection principle is applied to block the backward incidence, achieving asymmetric transmission. We optimize the design of the PhCs and the waveguide structure by scanning different structure parameters. The optimized ATD shows a high forward transmittance of 0.581 and contrast ratio of 0.989 at the wavelength of 1582 nm for TE mode. The results deepen the understanding and open up the new possibility in designing novel ATDs. The on-chip ATD will find broad applications in optical communications and quantum computing.

Broadband and wide-angle light absorption of organic solar cells based on multiple-depths metal grating.

A silver grating containing three grooves with different depths in one period was proposed as the back electrode for improving light absorption in organic solar cells. We found that the broadband absorption enhancement of the active layer covering the visible and near-infrared bands can be obtained due to the excitation of surface plasmon resonance and the multiple resonances of cavity mode. The integrated absorption efficiency of the proposed structure under TM polarization between 350 nm to 900 nm is 57.4%, with consideration of the weight of AM 1.5G solar spectrum, and is increased by 13.4% with respect to the equivalent planar device. Besides, the wide-angle absorption in proposed structure can be observed in the range from 0 to 50 degrees. These findings are of great importance for rationally designing composite nanostructures of metal gratings-based absorbers for sensing and photon-detecting applications.

Tunable dual-channel filter based on the photonic crystal with air defects.

We propose a tuning filter containing two channels by inserting a defect layer (Air/Si/Air/Si/Air) into a one-dimensional photonic crystal of Si/SiO2, which is on the symmetry of the defect. Two transmission peaks (1528.98 and 1564.74 nm) appear in the optical communication S-band and C-band, and the transmittance of these two channels is up to 100%. In addition, this design realizes multi-channel filtering to process large dynamic range or multiple independent signals in the near-infrared band by changing the structure. The tuning range will be enlarged, and the channels can be moved in this range through the easy control of air thickness and incident angle.