Spintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility compared with the commercial THz emitters, and thus attracts great interest recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption—more than 50% energy is wasted and the conversion efficiency is thus limited. Here, a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency is theoretically proposed and experimentally demonstrated. The device, which consists of a metal–dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generation show one‐to‐one correspondence with the theoretical calculations. The strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter is achieved. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics.

Broadband metamaterials absorbers with high absorption, ultrathin thickness and easy configurations are in great demand for many potential applications. In this paper, we first analyse the coupling resonances in a Ti/Ge/Ti three-layer absorber, which can realise broadband absorption from 8 to 12 μm. Then we experimentally demonstrate two types of absorbers based on the Ti/Ge/Si3N4/Ti configuration. By taking advantage of coupling surface plasmon resonances and intrinsic absorption of lossy material Si3N4, the average absorptions of two types of absorbers achieve almost 95% from 8 to 14 μm (experiment result: 78% from 6.5 to 13.5 μm). In order to expand the absorption bandwidth, we further propose two Ti/Si/SiO2/Ti absorbers which can absorb 92% and 87% of ultra-broadband light in the 14–30 μm and 8–30 μm spectral range, respectively. Our findings establish general and systematic strategies for guiding the design of metamaterial absorbers with excellent broadband absorption and pave the way for enhancing the optical performance in applications of infrared thermal emitters, imaging and photodetectors.

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The possibility of simultaneous multiple wavelength lasing in various neodymium host crystals such as Nd:YAG, Nd:YLF, Nd:BEL, and Nd:YAP has been analyzed by the ‘‘oscillation condition.’’ It is shown that this kind of laser can be realized in all the described crystals in pulsed state, but only achieved in the Nd:YAP crystal in continuous-wave state. On the basis of the analyzed results, we have achieved cw simultaneous multiple wavelength lasing in Nd:YAP crystal for the first time at both 1.0795 and 1.3414 μm.

Two types of ultra–broadband long wavelength infrared (LWIR) absorbers with small period and super thin thickness are designed. The absorption with high absorptivity and large bandwidth is achieved through combined propagating and localized surfaced plasmon resonances. We first design a three-layer absorber with a Ti–Ge–Ti configuration, the period of the structure is only 1.4 µm (nearly 1/8 of the center wavelength), the thickness of its dielectric is only 0.5 µm (1/22 of the center wavelength), and the average absorption is 87.9% under normal incident from 8µm to 14µm. Furthermore, the four-layer absorber with a Ti–Ge–Si3N4–Ti configuration is designed to obtain more average absorption increasing to 94.5% from 8 µm to 14µm under normal incident, the period of the structure increases to 1.6 µm and the total thickness of dielectric increases to 0.6µm. The proposed absorber is polarization–independent and possesses a good tolerance of incident angle. We calculate that the average absorption of the four-layer absorber for both TE– and TM–modes still exceeds 90% up to an incident angle of θ = 40° (90.7% for TE–mode, 91.9% for TM–mode), and exceed 80% up to an incident angle of θ = 60° (80.2% for TE–mode, 82.1% for TM–mode).

This letter focuses on the fabrication and characterization of a terahertz detector integrated with a group of low pass filters and resonant antennas. The detector operates as a self-mixer on GaN/AlGaN high electron mobility transistor (HEMT). At room temperature, a strong dc photocurrent is produced with the aid of the antennas and filters. The responsivity of our HEMT device is estimated to be 53 mA/W and a noise equivalent power of 1 nW/Hz can be achieved at 300 K. In addition, the sensor properties of a similar HEMT detector without filter are tested as a comparison.

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