Broadband and photovoltaic THz/IR response in the GaAs-based ratchet photodetector.

High-performance broadband infrared (IR)/terahertz (THz) detection is crucial in many optoelectronic applications. However, the spectral response range of semiconductor-based photodetectors is limited by the bandgaps. This paper proposes a ratchet structure based on the GaAs/AlxGa1-xAs heterojunction, where the quasi-stationary hot hole distribution and intravalence band absorption from light or heavy hole states to the split-off band overcome the bandgap limit, ensuring an ultrabroadband photoresponse from near-IR to THz region (4 to 300 THz). The peak responsivity of the proposed structure can reach 7.3 A/W, which is five orders of magnitude higher than that of the existing broadband photon-type detector. Because of the ratchet effect, the proposed photodetector has a bias-tunable photoresponse characteristic and can operate in the photovoltaic mode with a broad photocurrent spectrum (18 to 300 THz). This work not only demonstrates a broadband photon-type THz/IR photodetector but also provides a method to study the light-responsive ratchet.

Strong Terahertz Absorption of Monolayer Graphene Embedded into a Microcavity.

Terahertz reflection behaviors of metallic-grating-dielectric-metal (MGDM) microcavity with a monolayer graphene embedded into the dielectric layer are theoretically investigated. A tunable wideband reflection dip at about the Fabry-Pérot resonant frequency of the structure is found. The reflectance at the dip frequency can be electrically tuned in the range of 96.5% and 8.8%. Because of the subwavelength distance between the metallic grating and the monolayer graphene, both of the evanescent grating slit waveguide modes and the evanescent Rayleigh modes play key roles in the strong absorption by the graphene layer. The dependence of reflection behaviors on the carrier scattering rate of graphene is analyzed. A prototype MGDM-graphene structure is fabricated to verify the theoretical analysis. Our investigations are helpful for the developments of electrically controlled terahertz modulators, switches, and reconfigurable antennas based on the MGDM-graphene structures.

Terahertz quantum cascade lasers with sampled lateral gratings for single mode operation.

In this paper, we presented single mode terahertz quantum cascade lasers (THz QCLs) with sampled lateral grating emitting approximately 3.4 THz. Due to strong mode selection, the implementation of sampled lateral grating on THz QCL ridges can result in stable single longitudinal mode emission with a side-mode suppression ratio larger than 20 dB. The measured peak power of the grating laser is improved by about 11.8% compared to the power of devices with uniform distributed feedback gratings. Furthermore, the far-field pattern of the presented device is uninfluenced by grating structures.

Broadband THz to NIR up-converter for photon-type THz imaging.

High performance terahertz imaging devices have drawn wide attention due to their significant application in healthcare, security of food and medicine, and nondestructive inspection, as well as national security applications. Here we demonstrate a broadband terahertz photon-type up-conversion imaging device, operating around the liquid helium temperature, based on the gallium arsenide homojunction interfacial workfunction internal photoemission (HIWIP)-detector-LED up-converter and silicon CCD. Such an imaging device achieves broadband response in 4.2-20 THz and can absorb the normal incident light. The peak responsivity is 0.5 AW-1. The light emitting diode leads to a 72.5% external quantum efficiency improvement compared with the one widely used in conventional up-conversion devices. A peak up-conversion efficiency of 1.14 × 10-2 is realized and the optimal noise equivalent power is 29.1 pWHz-1/2. The up-conversion imaging for a 1000 K blackbody pin-hole is demonstrated. This work provides a different imaging scheme in the terahertz band.