Adiabatic waveguide taper and on-chip wavelength demultiplexer are the key components of photonic integrated circuits. However, these two kinds of devices which designed by traditional semi-analytic methods or brute-force search methods usually have large size. Here, based on regularized digital metamaterials, we have designed, fabricated and characterized a twochannel focused wavelength demultiplexer with a footprint of 2.4 × 10 μm 2 . The designed demultiplexer can directly connect to a grating coupler under the absence of an adiabatic waveguide taper. The objective first method and modified steepest descent method are used to design the demultiplexer which splits 1520 nm and 1580 nm light from a 10-μm-wide input waveguide into two 0.48-μm-wide output waveguides. Experimental results show that the insertion loss of the upper (lower) channel of the demultiplexer is -1.77 dB (-2.10 dB) and the crosstalk is -25.17 dB (-12.14 dB). Besides, the simulation results indicate that the fabrication tolerance of our devices can reach ±20 nm in etching depth and ±10 nm in plane size changing. Benefit From the extensibility of our design method, we can design other types of ultra-compact 'focused' devices, like mode splitters, mode converters and power splitters, and we can also design devices with more complicated functionalities, for example, we have designed a three-channel focused wavelength demultiplexer.

No abstract

A hybrid metal-graphene metamaterial (MM) is reported to achieve the active control of the broadband plasmon-induced transparency (PIT) in THz region. The unit cell consists of one cut wire (CW), four U-shape resonators (USRs) and monolayer graphene sheets under the USRs. Via near-field coupling, broadband PIT can be produced through the interference between different modes. Based on different arrangements of graphene positions, not only can we achieve electrically switching the amplitude of broadband PIT, but also can realize modulating the bandwidth of the transparent window. Simultaneously, both the capability and region of slow light can be dynamically tunable. This work provides schemes to manipulate PIT with more degrees of freedom, which will find significant applications in multifunctional THz modulation.

A new structure is reported, which realizes the flat focusing by introducing the silicon subwavelength slits into the waveguide. The subwavelength silicon-air slits, with variable widths to match the phase compensation, makes possible to focus a plane wave. The flat lens proposed here demonstrates relatively high power gain at the focal point or two focal points. By using such a design, we demonstrate a grating coupler with an ultrashort taper of 22.5-μm to connect a 10-μm-wide input waveguide and a 0.5-μm-wide output waveguide, achieving a transmission up to nearly 95.4% numerically in the communication band. The length of which is one-twentieth of that for the traditional taper. To our best knowledge, this work is the first demonstration of an ultrashort taper based on flat lens, which significantly improves the integration of the photonics integrated circuits, and indicates an effective solution for potential applications in compactly integrated micro/nano optical devices.

Recently, phase-change materials (PCMs) have drawn more attention due to the dynamically tunable optical properties. Here, we investigate the active control of electromagnetically induced transparency (EIT) analogue based on terahertz (THz) metamaterials integrated with vanadium oxide (VO2). Utilizing the insulator-to-metal transition of VO2, the amplitude of EIT peak can be actively modulated with a significant modulation depth. Meanwhile the group delay within the transparent window can also be dynamically tuned, achieving the active control of slow light effect. Furthermore, we also introduce independently tunable transparent peaks as well as group delay based on a double-peak EIT with good tuning performance. Finally, based on broadband EIT, the active tuning of quality factor of the EIT peak is also realized. This work introduces active EIT control with more degree of freedom by employing VO2, and can find potential applications in future wireless and ultrafast THz communication systems as multi-channel filters, switches, spacers, logic gates and modulators. Keywords: terahertz metamaterials; phase-change materials; vanadium oxide; electromagnetically induced transparency 1. Introduction Over the past decades, Metamaterials (MMs) have been focused continually due to the capability to manipulate electromagnetic (EM) waves in an unnatural way [1]. By designing artificial meta-resonators of MMs and arranging them appropriately, MMs can tailor lightwaves in subwavelength scale, consequently providing optical responses with desirable properties. In recent years, MMs have come to the terahertz (THz) regime [2]. Located between infrared and microwave band, THz radiations have enjoyed a rise of interest and are promising for security scanning [3], future wireless communications as well as the sixth-generation (6G) networks [3-5]. Now, MMshave been regarded as ideal platforms to achieve chipscale THz devices, such as THz sources [6,7], modulators [8,9], sensors [10,11] and absorbers [12,13].Recently, electromagnetically induced transparency (EIT) analogue in THz MMs have attracted more attention [14][15][16]. EIT refers to a sharp transparent window within a broad absorption spectrum, which comes from the quantum interference between two distinct excitation pathways in the natural three-level atom system [17]. Due to the dispersion properties, EIT has potential applications in slow light, optical data storage, nonlinear process enhancement and signal processing [18]. Mimicking EIT in the classical system, MMs can reproduce such effect via the near-field coupling between bright and dark modes supported on meta-resonators [14]. Compared with the conventional EIT, which requires severe experimental conditions [18], MM-based EIT analogue is easier to be produced and more stable, therefore suitable for practical chipscale applications [15].At the same time, for THz communication systems, THz devices with active tunability are required [19]. Therefore, various active optical materials have been utilized to turn passive MMs...