Frequency Up-Conversion Photon-Type Terahertz Imager

Abstract

Terahertz imaging has many important potential applications. Due to the failure of Si readout integrated circuits (ROICs) and the thermal mismatch between the photo-detector arrays and the ROICs at temperatures below 40 K, there are big technical challenges to construct terahertz photo-type focal plane arrays. In this work, we report pixel-less photo-type terahertz imagers based on the frequency up-conversion technique. The devices are composed of terahertz quantum-well photo-detectors (QWPs) and near-infrared (NIR) light emitting diodes (LEDs) which are grown in sequence on the same substrates using molecular beam epitaxy. In such an integrated QWP-LED device, photocurrent in the QWP drives the LED to emit NIR light. By optimizing the structural parameters of the QWP-LED, the QWP part and the LED part both work well. The maximum values of the internal and external energy up-conversion efficiencies are around 20% and 0.5%. A laser spot of a homemade terahertz quantum cascade laser is imaged by the QWP-LED together with a commercial Si camera. The pixel-less imaging results show that the image blurring induced by the transverse spreading of photocurrent is negligible. The demonstrated pixel-less imaging opens a new way to realize high performance terahertz imaging devices.Terahertz imaging has many potential applications, such as deep space exploration, imaging of biological tissues, mail screening and fingerprint identification, and situation awareness in fire disasters 1 . Terahertz focal plane arrays (FPAs) are key components for real time terahertz imaging systems 2 . Several different FPAs working in the terahertz regime have been developed in recent years based on the phase change of VO x , the complementary metal-oxide-semiconductor transistor (CMOS) technology, and the pyroelectric effect of LiTaO 3 crystal 2,3 . However, photo-type terahertz FPAs are still absent now. Compared to the thermal and other FPAs, photo-type FPAs have advantages of high detection sensitivity, short response time, large linear response range, and high damage threshold 2 .In mid-and far-infrared spectral regimes, FPAs are constructed by flip-chip bonding photo-detector arrays to commercial Si readout integrated circuits (ROICs) 2 . HgCdTe and quantum well infrared photo-detector (QWIP) FPAs are in the dominant position in mid-and far-infrared regimes 4 . Terahertz photo-detectors based on inter-subband transition in one-and three-dimensional quantum confined semiconductor structures were demonstrated in the past decade 2,4 . GaAs/(Al,Ga)As quantum well photo-detectors (QWPs) operating in terahertz band were realized by Liu et al. 5 . At temperatures below 30 K, background-limited infrared performance (BLIP) is reached for terahertz QWPs in 3.0-7.0 THz 6 . Due to the high quality of molecular beam epitaxy (MBE) grown GaAs/(Al,Ga)As multi-layer structure and the mature III-V fabrication processing technique, it is possible to fabricate terahertz QWPs with uniform detection performance in a large area scale. However, there are two t...

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

“…The imaging quality can also be evaluated by the factor of the signal to noise (FSNR) of the HIWIP-LED device. FSNR is characterized by the ratio of R / J bg 18 , in which R is the calibrated responsivity and the J bg is the background current density. The calculated FSNR and responsivity are shown in Fig.…”

“…The connection of hybrid GaAs-based FPA-readout integrated circuit (ROIC) imaging device will be destroyed by repeated heating and cooling because of the severe thermal mismatch between the GaAs and Si. The dead pixels increase with the temperature decreasing 18 .…”

“…And the cost would be greatly reduced at the same time. THz pixelless imaging based on the integrated QWP and LED is recently demonstrated to exhibit a promising application potential 18 . However, the 45° edge coupled geometry was adopted to realize the optical coupling, which caused the imaging spot of the quantum cascade laser (QCL) elongated in one direction.…”

Direct detection of a fast modulated terahertz light with a spectrally matched quantum‐well photodetector

“…The QCL, operating at 6.8 K, is fixed to a copper heat sink and mounted onto the cold finger of continuous‐flow liquid‐helium cryostat. The receiver is a GaAs/Al 0.019 Ga 0.981 As‐based terahertz QWP, with a similar active region structure used in [9], a size of 0.8 × 0.8 mm, and a 45‐degree incident facet, is cooled down to 4.5 K using another continuous‐flow liquid‐helium cryostat. The peak response frequency of the detector is about 4.3 THz, which is spectrally matched to the terahertz QCL.…”

Theoretical Investigation on Microcavity Coupler for Terahertz Quantum-Well Infrared Photodetectors

“…Advantages of single-element terahertz QWIPs, such as high sensitivity and fast response speed, have been demonstrated [4][5][6]. The prototype terahertz pixel-less imager based on QWIP-LED (light emission diode) frequency up-converter has been realized [7]. Because of the high-quality GaAs/AlGaAs multi quantum wells (MQWs) and mature fabrication techniques [2], it is expected that large-scale terahertz QWIP focal plane arrays will be fabricated under the boost of terahertz imaging applications in the near future.…”