Graphene-like two dimensional materials, such as WS2 and MoS2, are highly anisotropic layered compounds that have attracted growing interest from basic research to practical applications. Similar with MoS2, few-layer WS2 has remarkable physical properties. Here, we demonstrate for the first time that WS2 nanosheets exhibit ultrafast nonlinear saturable absorption property and high optical damage threshold. Soliton mode-locking operations are achieved separately in an erbium-doped fiber laser using two types of WS2-based saturable absorbers, one of which is fabricated by depositing WS2 nanosheets on a D-shaped fiber, while the other is synthesized by mixing WS2 solution with polyvinyl alcohol, and then evaporating them on a substrate. At the maximum pump power of 600 mW, two saturable absorbers can work stably at mode-locking state without damage, indicating that few-layer WS2 is a promising high-power flexible saturable absorber for ultrafast optics. Numerous applications may benefit from the ultrafast nonlinear features of WS2 nanosheets, such as high-power pulsed laser, materials processing, and frequency comb spectroscopy.

Metamaterials have attracted intensive research interest in recent years because their optical properties have a strong dependence on the geometry of metamaterial molecules rather than the material composition. [1][2][3] This feature has inspired the creation and tailoring of exotic properties, such as a negative refractive index, [ 4 , 5 ] perfect absorption, [ 6 ] and super lensing, [ 7 , 8 ] which are not readily available in nature. For many practical applications such as data storage [ 9 ] and optical switching, [ 10 ] switchable metamaterials that possess very different states are almost a necessity. [ 11 ] Most of the tunable metamaterials that have been demonstrated rely on tuning constituent materials or changing surrounding media by introducing natural materials with higher tunability, such as liquid crystals and phase changing materials. [12][13][14][15][16][17][18][19] However, this limits the choices of materials and becomes increasingly diffi cult to implement at higher frequencies. Moreover, the tuning range is usually too limited to achieve a switching effect between strikingly different states.A complementary approach is to mechanically reconfi gure the metamaterial molecules. [ 20 , 21 ] Micromachining technology has been developed for fabrication and actuation of micromechanical devices [22][23][24][25][26] with switching frequencies up to the GHz level. [ 27 ] An attempt was made to adjust the distance between several planar metamaterial layers in which effi cient transmission change was achieved but the tuning originated from a change in the layer structure rather than a change in metamaterial molecule. [ 22 ] Recently, another interesting work demonstrated the modifi cation of the optical properties of a metamaterial by reorienting the metamaterial molecules. [ 23 ] Inspired by these prior studies, we report the concept and design of switchable magnetic metamaterials by directly reshaping the metamaterial molecules using the micromachining technology and present working devices with switchable magnetic responses.The schematic diagram of the switchable magnetic metamaterial is shown in Figure 1 a. Each metamaterial molecule consists of two semi-square split rings. One is anchored on the substrate while the other can be moved by micromachined actuators. As a result, the gap between the split rings can be altered and thus the geometric shape of the metamaterial molecule can be changed. Figure 1 b-d illustrates the two semi-square spit rings in different states. In Figure 1 b, the two split rings are separated by a small gap, resulting in a geometric shape "[]". This is a typical split ring resonator. [ 28 ] For simple notation, this state is called the open-ring state. Figure 1 c,d show two extreme cases. In the former, the gap between the two split rings is closed and the actual metamaterial molecule becomes a closed ring in the "ٗ" shape. This is called the closed-ring state. In the latter, the movable ring is moved away until it touches the back side of the fi xed ring in the next metama...

A micromachined reconfigurable metamaterial is presented, whose unit cell consists of a pair of asymmetric split‐ring resonators (ASRRs); one is fixed to the substrate while the other is patterned on a movable frame. The reconfigurable metamaterial and the supporting structures (e.g., microactuators, anchors, supporting frames, etc.) are fabricated on a silicon‐on‐insulator wafer using deep reactive‐ion etching (DRIE). By adjusting the distance between the two ASRRs, the strength of dipole–dipole coupling can be tuned continuously using the micromachined actuators and this enables tailoring of the electromagnetic response. The reconfiguration of unit cells endows the micromachined reconfigurable metamaterials with unique merits such as electromagnetic response under normal incidence and wide tuning of resonant frequency (measured as 31% and 22% for transverse electric polarization and transverse magnetic polarization, respectively). The reconfiguration could also allow switching between the polarization‐dependent and polarization‐independent states. With these features, the micromachined reconfigurable metamaterials may find potential applications in transformation optics devices, sensors, intelligent detectors, tunable frequency‐selective surfaces, and spectral filters.

Transition-metal dichalcogenides, such as tungsten disulfide (WS2) and molybdenium disulfide (MoS2), are highly anisotropic layered materials and have attracted growing interest from basic research to practical applications due to their exotic physical property that may complement graphene and other semiconductor materials. WS2 nanosheets are found to exhibit broadband nonlinear saturable absorption property, and saturable absorbers (SAs) are fabricated by depositing WS2 nanosheets on side-polished fibers. Attributing to the weak evanescent field and long interaction length, the WS2 nanosheets are not exposed to large optical intensity, which allows the SA to work at the high-power regime. The SAs are used to mode lock erbium- and ytterbium-doped fiber lasers with normal dispersion, producing trains of dissipative soliton at 1.55 and 1.06 µm respectively. Simulations show that the bandgap of WS2 nanosheets decreases from 1.18 to 0.02 and 0.65 eV by introducing W and S defects respectively, which may contribute to the broadband saturable absorption property of the WS2.

A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated. Responsivity of 3.25 mA/W at 9.2 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K. The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength.